Dennis A. Priebat, MD

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the gastrointestinal (GI) tract and arise from the interstitial cells of Cajal of the myenteric plexus. These tumors are rare, with about 1 case per 100,000 persons diagnosed in the United States annually, but may be incidentally discovered in up to 1 in 5 autopsy specimens of older adults.^1,2 Epidemiologic risk factors include increasing age, with a peak incidence between age 60 and 65 years, male gender, black race, and non-Hispanic white ethnicity. Germline predisposition can also increase the risk of developing GISTs; molecular drivers of GIST include gain-of-function mutations in the KIT proto-oncogene and platelet-derived growth factor receptor α (PDGFRA) gene, which both encode structurally similar tyrosine kinase receptors; germline mutations of succinate dehydrogenase (SDH) subunit genes; and mutations associated with neurofibromatosis type 1.

GISTs most commonly involve the stomach, followed by the small intestine, but can arise anywhere within the GI tract (esophagus, colon, rectum, and anus). They can also develop outside the GI tract, arising from the mesentery, omentum, and retroperitoneum. The majority of cases are localized or locoregional, whereas about 20% are metastatic at presentation.¹ GISTs can occur in children, adolescents, and young adults. Pediatric GISTs represent a distinct subset marked by female predominance and gastric origin, are often multifocal, can sometimes have lymph node involvement, and typically lack mutations in the KIT and PDGFRA genes.

This review is the first of 2 articles focusing on the diagnosis and management of GISTs. Here, we review the evaluation and diagnosis of GISTs along with management of localized disease. Management of advanced disease is reviewed in a separate article.

Case Presentation

A 64-year-old African American man with progressive iron deficiency and abdominal discomfort undergoes upper and lower endoscopy and is found to have a bulging mass within his abdominal cavity. He undergoes a computed tomography (CT) evaluation of the chest, abdomen, and pelvis with contrast, which reveals the presence of a 10-cm gastric mass, with no other lesions identified. He undergoes surgical resection of the mass and presents for review of his pathology and to discuss his treatment plan.

What histopathologic features are consistent with GIST?

What factors are used for risk stratification and to predict likelihood of recurrence?

Clinical Presentation and Diagnosis

Most patients present with symptoms of overt or occult GI bleeding or abdominal discomfort, but a significant proportion of GISTs are discovered incidentally. Lymph node involvement is not typical, except for GISTs occurring in children and/or with rare syndromes. Most syndromic GISTs are multifocal and multicentric. After surgical resection, GISTs usually recur or metastasize within the abdominal cavity, including the omentum, peritoneum, or liver. These tumors rarely spread to the lungs, brain, or bones; when tumor spread does occur, it tends to be in heavily pre-treated patients with advanced disease who have been on multiple lines of therapy for a long duration of time.

The diagnosis usually can be made by histopathology. Specimens can be obtained by endoscopic ultrasound (EUS)– or CT-guided methods, the latter of which carries a very small risk of contamination from percutaneous biopsy. In terms of morphology, GISTs can be spindle cell, epithelioid, or mixed neoplasms. Epithelioid tumors are more commonly seen in the stomach and are often PDGFRA-mutated or SDH-deficient. The differential diagnosis includes other soft-tissue GI wall tumors such as leiomyosarcomas/leiomyomas, germ cell tumors, lymphomas, fibromatosis, and neuroendocrine and neurogenic tumors. A unique feature of GISTs that differentiates them from leiomyomas is near universal expression of CD117 by immunohistochemistry (IHC); this characteristic has allowed pathologists and providers to accurately distinguish true GISTs from other GI mesenchymal tumors.³ Recently, DOG1 (discovered on GIST1) immunoreactivity has been found to be helpful in identifying patients with CD117-negative GISTs. Initially identified through gene expression analysis of GISTs, DOG1 IHC can identify the common mutant c-Kit-driven CD117-positive GISTs as well as the rare CD117-negative GISTs, which are often driven by mutated PDGFRA.⁴ Importantly, IHC for KIT and DOG1 are not surrogates for mutational status, nor are they predictive of tyrosine kinase inhibitor (TKI) sensitivity. If IHC of a tumor specimen is CD117- and DOG1-negative, the specimen can be sent for KIT and PDGFRA mutational analysis to confirm the diagnosis. If analysis reveals that these genes are wild-type, then IHC staining for SDH B (SDHB) should follow to assess for an SDH-deficient GIST (negative staining).

Risk Stratification for Recurrence

The clinical behavior of GISTs can be variable. Some are indolent, while others behave more aggressively, with a greater malignant potential and a higher propensity to recur and metastasize. Clinical and pathologic features can provide important prognostic information that allows providers to risk-stratify patients. Various institutions have assessed prognostic variables for GISTs. In 2001, the National Institutes of Health (NIH) held a GIST workshop that proposed an approach to estimating metastatic risk based on tumor size and mitotic index (NIH or Fletcher criteria).⁵ Joensuu et al later proposed a modification of the NIH risk classification to include tumor location and tumor rupture (modified NIH criteria or Joensuu criteria).^6-8 Similarly, the Armed Forces Institute of Pathology (AFIP) identified tumor site as a prognostic factor, with gastric GISTs having the best prognosis (AFIP-Miettinen criteria).^9-11 Tabular schemes were designed which stratified patients into discrete groups with ranges for mitotic rate and tumor size. Nomograms for ease of use were then constructed utilizing a bimodal mitotic rate and included tumor site and size.¹² Finally, contour maps were developed, which have the advantage of evaluating mitotic rate and tumor size as continuous nonlinear variables and also include tumor site and rupture (associated with a high risk of peritoneal metastasis) separately, further improving risk assessment. These contour maps have been validated against pooled data from 10 series (2560 patients).¹³ High-risk features identified from these studies include tumor location, size, mitotic rate and tumor rupture and are now used for deciding on the use of adjuvant imatinib and as requirements to enter clinical trials assessing adjuvant therapy for resected GISTs.

Case Continued

The patient’s operative and pathology reports indicate that the tumor is a spindle cell neoplasm of the stomach that is positive for CD117, DOG1, and CD34 and negative for smooth muscle actin and S-100, consistent with a diagnosis of GIST. Resection margins are negative. There are 10 mitoses per 50 high-power fields (HPF). Per the operative report, there was no intraoperative or intraperitoneal tumor rupture. Thus, while his GIST was gastric, which generally has a more favorable prognosis, the tumor harbors high-risk features based on its size and mitotic index.

What further testing should be requested?

Molecular Alterations

It is recommended that a mutational analysis be performed as part of the diagnostic work-up of all GISTs.¹⁴ Mutational analysis can provide prognostic and predictive information for sensitivity to imatinib and should be considered standard of care. It may also be useful for confirming a GIST diagnosis, or, if negative, lead to further evaluation with an IHC stain for SDHB. The c-Kit receptor is a member of the tyrosine kinase family and, through direct interactions with stem cell factor (SCF), can upregulate the PI3K/AKT/mTOR, Ras/Raf/MEK/ERK, and JAK-STAT pathways, resulting in transcription and translation of genes that enhance cell growth and survival.¹⁵ The cell of origin of GISTs, the interstitial cells of Cajal, are dependent on the SCF–c-Kit interaction for development.¹⁶ Likewise, the large majority of GISTs (about 70%) are driven by upregulation and constitutive activation of c-Kit, which is normally autoinhibited. About 80% of KIT mutations involve exon 11; these GISTs are most often associated with a gastric location and are associated with a favorable recurrence-free survival (RFS) rate with surgery alone.¹⁷KIT exon 9 mutations are much less common, encompassing only about 10% of GIST KIT mutations, and GISTs with these mutations are more likely to arise from the small bowel.¹⁷

About 8% of GISTs harbor gain-of-function PDGFRA driver mutations rendering constitutively active PDGFRA.¹⁸PDGFRA mutations are mutually exclusive from KIT mutations, and PDGFRA-mutated tumors most often occur in the stomach. PDGFRA mutations generally are associated with a lower mitotic rate and gastric location. Identification of the PDGFRA D842V mutation on exon 18, which is the most common, is important, as it is associated with imatinib resistance, and these patients should not be offered imatinib.¹⁹

Several other mutations associated with GISTs outside of the KIT and PDGFRA spectrum have been identified. About 10% of GISTs are wildtype for KIT and PDGFRA, and not all KIT/PDGFRA-wildtype GISTs are imatinib-sensitive and/or respond to other TKIs.¹⁸ These tumors may harbor aberrations in SDH and NF1, or less commonly, BRAF V600E, FGFR, and NTRK.^20,21 SDH subunits B, C and D play a role in the Krebs cycle and electron transport chain. Germline mutations in these SDH subunits can result in the Carney-Stratakis syndrome characterized by the dyad of multifocal GISTs and multicentric paragangliomas.²² This syndrome is most likely to manifest in the pediatric or young adult population. In contradistinction is the Carney triad, which is associated with acquired loss of function of the SDHC gene due to promoter hypermethylation. This syndrome classically occurs in young women and is characterized by an indolent-behaving triad of multicentric GISTs, non-adrenal paragangliomas, and pulmonary chondromas.²³ Like PDGFRA D842V–mutated GISTs, SDH-deficient and NF1-associated GISTs are considered imatinib resistant, and these patients should not be offered imatinib therapy.¹⁴

Case Continued

The patient’s GIST is found to harbor a KIT exon 11 single codon deletion. He appears anxious and asks to have everything done to prevent his GIST from coming back and to improve his lifespan.

Management

A multidisciplinary team approach to the management of all GISTs is essential and includes input from radiology, gastroenterology, pathology, medical and surgical oncology, nuclear medicine, and nursing.

Surgical Resection

Small esophagogastric and duodenal GISTs ≤ 2 cm can be asymptomatic and managed with serial endoscopic surveillance, typically every 6 to 12 months, with biopsies if the tumors increase in size. GISTs larger than 2 cm require surgical resection, with resection of the full pseudocapsule and an R0 resection, if possible, since larger GISTs carry a higher risk of growth and recurrence. If an R0 resection would lead to significant morbidity or functional sequelae, an R1 may suffice. Rectal GISTs are an exception, where microscopic margins have been shown to be associated with an increased risk of local failure.²⁴ It is important to explore the abdomen thoroughly for peritoneal, rectovaginal, and vesicular implants and metastasis to the liver. A formal lymph node dissection is not necessary because lymph nodes are rarely involved and should only be removed when clinically suspicious. Tumor rupture must be avoided. A laparoscopic approach should only be considered for smaller tumors, since there is a risk of tumor rupture with larger tumors.¹⁴

When is adjuvant imatinib indicated?

Adjuvant Imatinib

Among patients with local or locally advanced GISTs, the risk of death from recurrence with surgery alone can be high, with a historical 5-year overall survival (OS) of about 35%.²⁵ As a result, multiple studies have assessed the benefit of adjuvant imatinib, which is now considered standard of care for patients with imatinib-sensitive, high-risk GISTs. In addition to inhibiting BCR-ABL, imatinib mesylate inhibits multiple other receptor tyrosine kinases, including PDGFR, SCF and c-Kit. As a result, imatinib has demonstrated in vitro inhibition of cell proliferation and apoptosis and clinical activity against GISTs expressing CD117.²⁶ Importantly, adjuvant imatinib should only be offered to patients with imatinib-sensitive mutations, such as KIT exon 11 and KIT exon 9 mutations. Adjuvant imatinib should not be offered to patients with imatinib-insensitive mutations such as PDGFR 842V, NF1, or BRAF-related or SDH-deficient GISTs.

The ACOSOG Z9000 was the first study of adjuvant imatinib in patients with resected GISTs.²⁵ This was a single-arm, phase 2 study involving 106 patients with surgically resected GISTs deemed high-risk for recurrence, defined as size > 10 cm, tumor rupture, or up to 4 peritoneal implants. Patients were treated with imatinib 400 mg daily for 1 year. The primary and secondary endpoints were OS and RFS, respectively. Long-term follow-up of this study demonstrated 1-, 3-, and 5-year OS of 99%, 97%, and 83%, and 1-, 3-, and 5-year RFS of 96%, 60%, and 40%, which compared favorably with historical controls. In a multivariable analysis, increasing tumor size, small bowel location, KIT exon 9 mutation, high mitotic rate, and older age were independent risk factors for a poor RFS.²⁵ It is important to note that the benefit of adjuvant imatinib waned after discontinuation of therapy, creating a rationale to study adjuvant imatinib for longer periods of time.

As a result of the promising phase 2 data, ACOSOG opened a phase 3 randomized trial (Z9001) comparing 1 year of adjuvant imatinib to placebo among patients with surgically resected GISTs that were > 3 cm in size and that stained positive for CD117 on pathology. The trial accrued 713 patients and was stopped early at a planned interim analysis, which revealed a 1-year RFS of 98% for imatinib versus 83% for placebo (hazard ratio [HR], 0.35; P < 0.001). The 1-year OS did not differ between the 2 arms (92.2% vs 99.7%; HR, 0.66; P = 0.47).²⁷ When comparing the 2 arms, imatinib was associated with a higher RFS among patients with a KIT exon 11 deletion, but not among patients with other KIT mutation types, PDGFRA mutations, or who were KIT/PDGFRA wildtype.²⁸ Imatinib was granted approval by the US Food and Drug Administration (FDA) for the adjuvant treatment of high-risk GISTs based on the results of the ACOSOG Z9001 trial.

The EORTC 62024 study was a randomized placebo-controlled trial assessing the benefit of 2 years of adjuvant imatinib.²⁹ Patients had to be considered intermediate or high risk per the 2002 NIH consensus classification to be eligible. The trial enrolled 918 patients. The 5-year OS rate, the original primary endpoint, did not differ between the 2 groups (100% vs 99%). The 3-year and 5-year RFS rates, secondary endpoints, were significantly longer among patients treated with imatinib (84% vs 66% and 69% vs 63%, respectively). Again, it was noted that the benefit of imatinib waned over time after treatment discontinuation.

The Scandinavian Sarcoma Group (SSG XVIII) trial was a prospective randomized phase 3 trial that compared 3 years versus 1 year of adjuvant imatinib.³⁰ Patients had to be enrolled within 12 weeks of the postoperative period and had to have GISTs that were CD117-positive and with a high estimated risk of recurrence, per the modified NIH consensus criteria (size > 10 cm, > 10 mitoses per 50 HPF, diameter > 5 cm with mitotic count > 5, or tumor rupture before or at surgery). Three years of adjuvant imatinib was associated with a 54% reduction in the hazard for recurrence at 5 years (65.6% vs 47.9%; HR, 0.46; P < 0.001) and a 55% reduction in the hazard for death at 5 years (OS 92% vs 81.7%; HR, 0.45; P = 0.02). Based on the results of this study, the FDA granted approval for the use of 3 years of adjuvant imatinib in patients with high-risk resected GISTs.

The observation that a longer duration of adjuvant imatinib was associated with superior RFS and OS led to studies to further explore longer durations of adjuvant imatinib. The PERSIST-5 (Postresection Evaluation of Recurrence-free Survival for Gastrointestinal Stromal Tumors With 5 Years of Adjuvant Imatinib) was a multicenter, single-arm, phase 2 prospective study of adjuvant imatinib with a primary endpoint of RFS after 5 years.³¹ Patients had to have an intermediate or high risk of recurrence, which included GISTs at any site > 2 cm with > 5 mitoses per 50 HPF or nongastric GISTs that were ≥ 5 cm. With 91 patients enrolled, the estimated 5-year RFS was 90% and the OS was 95%. Of note, about half of the patients stopped treatment early due to a variety of reasons, including patient choice or adverse events. Importantly, there were no recurrences in patients with imatinib-sensitive mutations while on therapy. We know that in patients at high risk of relapse, adjuvant imatinib delays recurrence and improves survival, but whether any patients are cured, or their survival curves are just shifted to the right, is unknown. Only longer follow-up of existing studies, and the results of newer trials utilizing longer durations of adjuvant treatment, will help to determine the real value of adjuvant therapy for GIST patients.³² Based on this study, it would be reasonable to discuss a longer duration of imatinib with patients deemed to be at very high risk of recurrence and who are tolerating therapy well. We are awaiting the data from the randomized phase 3 Scandinavian Sarcoma Group XII trial comparing 5 years versus 3 years of adjuvant imatinib therapy, and from the French ImadGIST trial of adjuvant imatinib for 3 versus 6 years. A summary of the aforementioned key adjuvant trials is shown in the Table.

When imatinib is commenced, careful monitoring for treatment toxicities and drug interactions should ensue in order to improve compliance. Dose density should be maintained if possible, as retrospective studies suggest suboptimal plasma levels are associated with a worse outcome.³³

When should neoadjuvant imatinib be considered?

Neoadjuvant Imatinib

Neoadjuvant imatinib should be considered for patients requiring total gastrectomy, esophagectomy, or abdominoperineal resection of the rectum in order to reduce tumor size, limit subsequent surgical morbidity, mitigate tumor bleeding and rupture, and aid with organ preservation. Patients with rectal GISTs that may otherwise warrant an abdominoperineal resection should be offered a trial of imatinib in the neoadjuvant setting. There is no evidence for the use of any other TKI aside from imatinib in the neoadjuvant or adjuvant setting. With neoadjuvant imatinib, it is difficult to accurately assess the mitotic rate in the resected tumor specimen.

The RTOG 0132/ACRIN 6665 trial was a prospective phase 2 study evaluating the efficacy of imatinib 600 mg daily in the perioperative setting.³⁴ The trial enrolled 50 patients, 30 with primary GISTs (group A) and 22 with recurrent metastatic GISTs (group B). Based on data from the metastatic setting revealing a time to treatment response of about 2.5 months, patients were treated with 8 to 12 weeks of preoperative imatinib followed by 2 years of adjuvant imatinib. Imatinib was stopped 24 hours preoperatively and resumed as soon as possible postoperatively. In group A, 7% of patients achieved a partial response (PR), 83% achieved stable disease, and 2-year progression-free survival (PFS) and OS were 83% and 93%, respectively. In group B, 4.5% of patients achieved a PR, 91% achieved stable disease, and 4.5% experienced progressive disease in the preoperative period; the 2-year PFS and OS were 77% and 91%, respectively. The results of this trial demonstrated the feasibility of using perioperative imatinib with minimal effects on surgical outcomes and set the rationale to use neoadjuvant imatinib in select patients with borderline resectable or rectal GISTs. Another EORTC pooled analysis from 10 sarcoma centers revealed that after a median of 10 months of neoadjuvant imatinib, 83.2% of patients achieved an R0 resection and only 1% progressed during treatment.³⁵ After a median follow-up of 46 months, the 5-year disease-free survival and OS were 65% and 87%, respectively.

Mutational testing should be performed beforehand to ensure the tumor is imatinib-sensitive. If a KIT exon 9 mutation is identified, then 400 mg twice daily should be considered (given the benefit seen with 800 mg imatinib for advanced GIST patients), although there are no studies to confirm this practice. Neoadjuvant imatinib is recommended for a total of 6 to 12 months to ensure maximal tumor debulking, but with very close monitoring and surgical input for disease resistance and growth.¹⁴ Imatinib should be stopped 1 to 2 days preoperatively and resumed once the patient has recovered from surgery for a total of 3 years (pre-/postoperatively combined). Neoadjuvant therapy has been shown to be safe and effective, but there have been no randomized trials to assess survival.

What is appropriate surveillance for resected GISTs?

Surveillance

There have been no randomized studies to guide the management of surveillance after surgical resection and adjuvant therapy. There is no known optimal follow-up schedule, but several have been proposed.^13,36 Among high-risk patients, it is suggested to image every 3 to 6 months during adjuvant therapy, followed by every 3 months for 2 years after discontinuing therapy, then every 6 months for another 3 years and annually thereafter for an additional 5 years. High-risk patients usually relapse within 1 to 3 years after finishing adjuvant therapy, while low-risk patients can relapse later given that their disease can be slower growing. It has been recommended that low-risk patients undergo imaging every 6 months for 5 years, with follow-up individualized thereafter. Very-low-risk patients may not require more than annual imaging. Because most relapses occur within the peritoneum or liver, imaging should encompass the abdomen and pelvis. Surveillance imaging usually consists of CT scans of the abdomen and pelvis. MRI scans can be utilized for patients at lower risk or who are out several years in order to avoid excess radiation exposure. MRI is also specifically helpful for rectal and esophageal lesions. Chest CT or chest radiograph and bone scan are not routinely required for follow-up.

Case Conclusion

The patient receives adjuvant imatinib and experiences grade 2 myalgias, periorbital edema, and macrocytic anemia, which result in imatinib discontinuation after 3 years of treatment. He is seen every 3 to 6 months and a contrast CT abdomen and pelvis is obtained every 6 months for 5 years. During this 5-year follow-up period, he does not have any clinical or radiographic evidence of disease recurrence.

Further follow-up of this patient is presented in the second article in this 2-part review of management of GISTs.

Key Points

• GISTs are the most common mesenchymal neoplasms of the GI tract and can occasionally occur in extragastrointestinal locations as well.
• GISTs encompass a heterogeneous family of tumor subsets with different natural histories, mutations, and TKI responsiveness.
• Surgery is the mainstay of treatment for localized GISTs, with cure rates greater than 50%.
• For very small (< 2 cm) esophagogastric GISTs, endoscopic ultrasound evaluation and follow-up is recommended.
• For tumors ≥ 2 cm, biopsy and excision is the standard approach.
• For localized GISTs, complete surgical resection (R0) is standard treatment, with no lymphadenectomy for clinically negative lymph nodes.
• Mutational analysis should be considered standard of practice. It can be helpful for confirming the diagnosis and can be predictive and prognostic in determining specific TKI therapy and dose.
• Adjuvant imatinib at a dose of 400 mg for 3 years is standard of care for GISTs that are at high risk of relapse and are imatinib-sensitive, and it is the only TKI approved for adjuvant therapy. Patients with PDGFRA D842V, NF1, BRAF or SDH-deficient GISTs should not receive adjuvant imatinib therapy.
• Neoadjuvant therapy can be utilized for sites where extensive resection would lead to significant morbidity. It should be given for 6 to 12 months, but patients need to be monitored closely for tumor growth.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the gastrointestinal (GI) tract and arise from the interstitial cells of Cajal of the myenteric plexus. These tumors are rare, with about 1 case per 100,000 persons diagnosed in the United States annually, but may be incidentally discovered in up to 1 in 5 autopsy specimens of older adults.^1,2 Epidemiologic risk factors include increasing age, with a peak incidence between age 60 and 65 years, male gender, black race, and non-Hispanic white ethnicity. Germline predisposition can also increase the risk of developing GISTs; molecular drivers of GIST include gain-of-function mutations in the KIT proto-oncogene and platelet-derived growth factor receptor α (PDGFRA) gene, which both encode structurally similar tyrosine kinase receptors; germline mutations of succinate dehydrogenase (SDH) subunit genes; and mutations associated with neurofibromatosis type 1.

GISTs most commonly involve the stomach, followed by the small intestine, but can arise anywhere within the GI tract (esophagus, colon, rectum, and anus). They can also develop outside the GI tract, arising from the mesentery, omentum, and retroperitoneum. The majority of cases are localized or locoregional, whereas about 20% are metastatic at presentation.¹ GISTs can occur in children, adolescents, and young adults. Pediatric GISTs represent a distinct subset marked by female predominance and gastric origin, are often multifocal, can sometimes have lymph node involvement, and typically lack mutations in the KIT and PDGFRA genes.

This review is the first of 2 articles focusing on the diagnosis and management of GISTs. Here, we review the evaluation and diagnosis of GISTs along with management of localized disease. Management of advanced disease is reviewed in a separate article.

Case Presentation

A 64-year-old African American man with progressive iron deficiency and abdominal discomfort undergoes upper and lower endoscopy and is found to have a bulging mass within his abdominal cavity. He undergoes a computed tomography (CT) evaluation of the chest, abdomen, and pelvis with contrast, which reveals the presence of a 10-cm gastric mass, with no other lesions identified. He undergoes surgical resection of the mass and presents for review of his pathology and to discuss his treatment plan.

What histopathologic features are consistent with GIST?

What factors are used for risk stratification and to predict likelihood of recurrence?

Clinical Presentation and Diagnosis

Most patients present with symptoms of overt or occult GI bleeding or abdominal discomfort, but a significant proportion of GISTs are discovered incidentally. Lymph node involvement is not typical, except for GISTs occurring in children and/or with rare syndromes. Most syndromic GISTs are multifocal and multicentric. After surgical resection, GISTs usually recur or metastasize within the abdominal cavity, including the omentum, peritoneum, or liver. These tumors rarely spread to the lungs, brain, or bones; when tumor spread does occur, it tends to be in heavily pre-treated patients with advanced disease who have been on multiple lines of therapy for a long duration of time.

The diagnosis usually can be made by histopathology. Specimens can be obtained by endoscopic ultrasound (EUS)– or CT-guided methods, the latter of which carries a very small risk of contamination from percutaneous biopsy. In terms of morphology, GISTs can be spindle cell, epithelioid, or mixed neoplasms. Epithelioid tumors are more commonly seen in the stomach and are often PDGFRA-mutated or SDH-deficient. The differential diagnosis includes other soft-tissue GI wall tumors such as leiomyosarcomas/leiomyomas, germ cell tumors, lymphomas, fibromatosis, and neuroendocrine and neurogenic tumors. A unique feature of GISTs that differentiates them from leiomyomas is near universal expression of CD117 by immunohistochemistry (IHC); this characteristic has allowed pathologists and providers to accurately distinguish true GISTs from other GI mesenchymal tumors.³ Recently, DOG1 (discovered on GIST1) immunoreactivity has been found to be helpful in identifying patients with CD117-negative GISTs. Initially identified through gene expression analysis of GISTs, DOG1 IHC can identify the common mutant c-Kit-driven CD117-positive GISTs as well as the rare CD117-negative GISTs, which are often driven by mutated PDGFRA.⁴ Importantly, IHC for KIT and DOG1 are not surrogates for mutational status, nor are they predictive of tyrosine kinase inhibitor (TKI) sensitivity. If IHC of a tumor specimen is CD117- and DOG1-negative, the specimen can be sent for KIT and PDGFRA mutational analysis to confirm the diagnosis. If analysis reveals that these genes are wild-type, then IHC staining for SDH B (SDHB) should follow to assess for an SDH-deficient GIST (negative staining).

Risk Stratification for Recurrence

The clinical behavior of GISTs can be variable. Some are indolent, while others behave more aggressively, with a greater malignant potential and a higher propensity to recur and metastasize. Clinical and pathologic features can provide important prognostic information that allows providers to risk-stratify patients. Various institutions have assessed prognostic variables for GISTs. In 2001, the National Institutes of Health (NIH) held a GIST workshop that proposed an approach to estimating metastatic risk based on tumor size and mitotic index (NIH or Fletcher criteria).⁵ Joensuu et al later proposed a modification of the NIH risk classification to include tumor location and tumor rupture (modified NIH criteria or Joensuu criteria).^6-8 Similarly, the Armed Forces Institute of Pathology (AFIP) identified tumor site as a prognostic factor, with gastric GISTs having the best prognosis (AFIP-Miettinen criteria).^9-11 Tabular schemes were designed which stratified patients into discrete groups with ranges for mitotic rate and tumor size. Nomograms for ease of use were then constructed utilizing a bimodal mitotic rate and included tumor site and size.¹² Finally, contour maps were developed, which have the advantage of evaluating mitotic rate and tumor size as continuous nonlinear variables and also include tumor site and rupture (associated with a high risk of peritoneal metastasis) separately, further improving risk assessment. These contour maps have been validated against pooled data from 10 series (2560 patients).¹³ High-risk features identified from these studies include tumor location, size, mitotic rate and tumor rupture and are now used for deciding on the use of adjuvant imatinib and as requirements to enter clinical trials assessing adjuvant therapy for resected GISTs.

Case Continued

The patient’s operative and pathology reports indicate that the tumor is a spindle cell neoplasm of the stomach that is positive for CD117, DOG1, and CD34 and negative for smooth muscle actin and S-100, consistent with a diagnosis of GIST. Resection margins are negative. There are 10 mitoses per 50 high-power fields (HPF). Per the operative report, there was no intraoperative or intraperitoneal tumor rupture. Thus, while his GIST was gastric, which generally has a more favorable prognosis, the tumor harbors high-risk features based on its size and mitotic index.

What further testing should be requested?

Molecular Alterations

It is recommended that a mutational analysis be performed as part of the diagnostic work-up of all GISTs.¹⁴ Mutational analysis can provide prognostic and predictive information for sensitivity to imatinib and should be considered standard of care. It may also be useful for confirming a GIST diagnosis, or, if negative, lead to further evaluation with an IHC stain for SDHB. The c-Kit receptor is a member of the tyrosine kinase family and, through direct interactions with stem cell factor (SCF), can upregulate the PI3K/AKT/mTOR, Ras/Raf/MEK/ERK, and JAK-STAT pathways, resulting in transcription and translation of genes that enhance cell growth and survival.¹⁵ The cell of origin of GISTs, the interstitial cells of Cajal, are dependent on the SCF–c-Kit interaction for development.¹⁶ Likewise, the large majority of GISTs (about 70%) are driven by upregulation and constitutive activation of c-Kit, which is normally autoinhibited. About 80% of KIT mutations involve exon 11; these GISTs are most often associated with a gastric location and are associated with a favorable recurrence-free survival (RFS) rate with surgery alone.¹⁷KIT exon 9 mutations are much less common, encompassing only about 10% of GIST KIT mutations, and GISTs with these mutations are more likely to arise from the small bowel.¹⁷

About 8% of GISTs harbor gain-of-function PDGFRA driver mutations rendering constitutively active PDGFRA.¹⁸PDGFRA mutations are mutually exclusive from KIT mutations, and PDGFRA-mutated tumors most often occur in the stomach. PDGFRA mutations generally are associated with a lower mitotic rate and gastric location. Identification of the PDGFRA D842V mutation on exon 18, which is the most common, is important, as it is associated with imatinib resistance, and these patients should not be offered imatinib.¹⁹

Several other mutations associated with GISTs outside of the KIT and PDGFRA spectrum have been identified. About 10% of GISTs are wildtype for KIT and PDGFRA, and not all KIT/PDGFRA-wildtype GISTs are imatinib-sensitive and/or respond to other TKIs.¹⁸ These tumors may harbor aberrations in SDH and NF1, or less commonly, BRAF V600E, FGFR, and NTRK.^20,21 SDH subunits B, C and D play a role in the Krebs cycle and electron transport chain. Germline mutations in these SDH subunits can result in the Carney-Stratakis syndrome characterized by the dyad of multifocal GISTs and multicentric paragangliomas.²² This syndrome is most likely to manifest in the pediatric or young adult population. In contradistinction is the Carney triad, which is associated with acquired loss of function of the SDHC gene due to promoter hypermethylation. This syndrome classically occurs in young women and is characterized by an indolent-behaving triad of multicentric GISTs, non-adrenal paragangliomas, and pulmonary chondromas.²³ Like PDGFRA D842V–mutated GISTs, SDH-deficient and NF1-associated GISTs are considered imatinib resistant, and these patients should not be offered imatinib therapy.¹⁴

Case Continued

The patient’s GIST is found to harbor a KIT exon 11 single codon deletion. He appears anxious and asks to have everything done to prevent his GIST from coming back and to improve his lifespan.

Management

A multidisciplinary team approach to the management of all GISTs is essential and includes input from radiology, gastroenterology, pathology, medical and surgical oncology, nuclear medicine, and nursing.

Surgical Resection

Small esophagogastric and duodenal GISTs ≤ 2 cm can be asymptomatic and managed with serial endoscopic surveillance, typically every 6 to 12 months, with biopsies if the tumors increase in size. GISTs larger than 2 cm require surgical resection, with resection of the full pseudocapsule and an R0 resection, if possible, since larger GISTs carry a higher risk of growth and recurrence. If an R0 resection would lead to significant morbidity or functional sequelae, an R1 may suffice. Rectal GISTs are an exception, where microscopic margins have been shown to be associated with an increased risk of local failure.²⁴ It is important to explore the abdomen thoroughly for peritoneal, rectovaginal, and vesicular implants and metastasis to the liver. A formal lymph node dissection is not necessary because lymph nodes are rarely involved and should only be removed when clinically suspicious. Tumor rupture must be avoided. A laparoscopic approach should only be considered for smaller tumors, since there is a risk of tumor rupture with larger tumors.¹⁴

When is adjuvant imatinib indicated?

Adjuvant Imatinib

Among patients with local or locally advanced GISTs, the risk of death from recurrence with surgery alone can be high, with a historical 5-year overall survival (OS) of about 35%.²⁵ As a result, multiple studies have assessed the benefit of adjuvant imatinib, which is now considered standard of care for patients with imatinib-sensitive, high-risk GISTs. In addition to inhibiting BCR-ABL, imatinib mesylate inhibits multiple other receptor tyrosine kinases, including PDGFR, SCF and c-Kit. As a result, imatinib has demonstrated in vitro inhibition of cell proliferation and apoptosis and clinical activity against GISTs expressing CD117.²⁶ Importantly, adjuvant imatinib should only be offered to patients with imatinib-sensitive mutations, such as KIT exon 11 and KIT exon 9 mutations. Adjuvant imatinib should not be offered to patients with imatinib-insensitive mutations such as PDGFR 842V, NF1, or BRAF-related or SDH-deficient GISTs.

The ACOSOG Z9000 was the first study of adjuvant imatinib in patients with resected GISTs.²⁵ This was a single-arm, phase 2 study involving 106 patients with surgically resected GISTs deemed high-risk for recurrence, defined as size > 10 cm, tumor rupture, or up to 4 peritoneal implants. Patients were treated with imatinib 400 mg daily for 1 year. The primary and secondary endpoints were OS and RFS, respectively. Long-term follow-up of this study demonstrated 1-, 3-, and 5-year OS of 99%, 97%, and 83%, and 1-, 3-, and 5-year RFS of 96%, 60%, and 40%, which compared favorably with historical controls. In a multivariable analysis, increasing tumor size, small bowel location, KIT exon 9 mutation, high mitotic rate, and older age were independent risk factors for a poor RFS.²⁵ It is important to note that the benefit of adjuvant imatinib waned after discontinuation of therapy, creating a rationale to study adjuvant imatinib for longer periods of time.

As a result of the promising phase 2 data, ACOSOG opened a phase 3 randomized trial (Z9001) comparing 1 year of adjuvant imatinib to placebo among patients with surgically resected GISTs that were > 3 cm in size and that stained positive for CD117 on pathology. The trial accrued 713 patients and was stopped early at a planned interim analysis, which revealed a 1-year RFS of 98% for imatinib versus 83% for placebo (hazard ratio [HR], 0.35; P < 0.001). The 1-year OS did not differ between the 2 arms (92.2% vs 99.7%; HR, 0.66; P = 0.47).²⁷ When comparing the 2 arms, imatinib was associated with a higher RFS among patients with a KIT exon 11 deletion, but not among patients with other KIT mutation types, PDGFRA mutations, or who were KIT/PDGFRA wildtype.²⁸ Imatinib was granted approval by the US Food and Drug Administration (FDA) for the adjuvant treatment of high-risk GISTs based on the results of the ACOSOG Z9001 trial.

The EORTC 62024 study was a randomized placebo-controlled trial assessing the benefit of 2 years of adjuvant imatinib.²⁹ Patients had to be considered intermediate or high risk per the 2002 NIH consensus classification to be eligible. The trial enrolled 918 patients. The 5-year OS rate, the original primary endpoint, did not differ between the 2 groups (100% vs 99%). The 3-year and 5-year RFS rates, secondary endpoints, were significantly longer among patients treated with imatinib (84% vs 66% and 69% vs 63%, respectively). Again, it was noted that the benefit of imatinib waned over time after treatment discontinuation.

The Scandinavian Sarcoma Group (SSG XVIII) trial was a prospective randomized phase 3 trial that compared 3 years versus 1 year of adjuvant imatinib.³⁰ Patients had to be enrolled within 12 weeks of the postoperative period and had to have GISTs that were CD117-positive and with a high estimated risk of recurrence, per the modified NIH consensus criteria (size > 10 cm, > 10 mitoses per 50 HPF, diameter > 5 cm with mitotic count > 5, or tumor rupture before or at surgery). Three years of adjuvant imatinib was associated with a 54% reduction in the hazard for recurrence at 5 years (65.6% vs 47.9%; HR, 0.46; P < 0.001) and a 55% reduction in the hazard for death at 5 years (OS 92% vs 81.7%; HR, 0.45; P = 0.02). Based on the results of this study, the FDA granted approval for the use of 3 years of adjuvant imatinib in patients with high-risk resected GISTs.

The observation that a longer duration of adjuvant imatinib was associated with superior RFS and OS led to studies to further explore longer durations of adjuvant imatinib. The PERSIST-5 (Postresection Evaluation of Recurrence-free Survival for Gastrointestinal Stromal Tumors With 5 Years of Adjuvant Imatinib) was a multicenter, single-arm, phase 2 prospective study of adjuvant imatinib with a primary endpoint of RFS after 5 years.³¹ Patients had to have an intermediate or high risk of recurrence, which included GISTs at any site > 2 cm with > 5 mitoses per 50 HPF or nongastric GISTs that were ≥ 5 cm. With 91 patients enrolled, the estimated 5-year RFS was 90% and the OS was 95%. Of note, about half of the patients stopped treatment early due to a variety of reasons, including patient choice or adverse events. Importantly, there were no recurrences in patients with imatinib-sensitive mutations while on therapy. We know that in patients at high risk of relapse, adjuvant imatinib delays recurrence and improves survival, but whether any patients are cured, or their survival curves are just shifted to the right, is unknown. Only longer follow-up of existing studies, and the results of newer trials utilizing longer durations of adjuvant treatment, will help to determine the real value of adjuvant therapy for GIST patients.³² Based on this study, it would be reasonable to discuss a longer duration of imatinib with patients deemed to be at very high risk of recurrence and who are tolerating therapy well. We are awaiting the data from the randomized phase 3 Scandinavian Sarcoma Group XII trial comparing 5 years versus 3 years of adjuvant imatinib therapy, and from the French ImadGIST trial of adjuvant imatinib for 3 versus 6 years. A summary of the aforementioned key adjuvant trials is shown in the Table.

When imatinib is commenced, careful monitoring for treatment toxicities and drug interactions should ensue in order to improve compliance. Dose density should be maintained if possible, as retrospective studies suggest suboptimal plasma levels are associated with a worse outcome.³³

When should neoadjuvant imatinib be considered?

Neoadjuvant Imatinib

Neoadjuvant imatinib should be considered for patients requiring total gastrectomy, esophagectomy, or abdominoperineal resection of the rectum in order to reduce tumor size, limit subsequent surgical morbidity, mitigate tumor bleeding and rupture, and aid with organ preservation. Patients with rectal GISTs that may otherwise warrant an abdominoperineal resection should be offered a trial of imatinib in the neoadjuvant setting. There is no evidence for the use of any other TKI aside from imatinib in the neoadjuvant or adjuvant setting. With neoadjuvant imatinib, it is difficult to accurately assess the mitotic rate in the resected tumor specimen.

The RTOG 0132/ACRIN 6665 trial was a prospective phase 2 study evaluating the efficacy of imatinib 600 mg daily in the perioperative setting.³⁴ The trial enrolled 50 patients, 30 with primary GISTs (group A) and 22 with recurrent metastatic GISTs (group B). Based on data from the metastatic setting revealing a time to treatment response of about 2.5 months, patients were treated with 8 to 12 weeks of preoperative imatinib followed by 2 years of adjuvant imatinib. Imatinib was stopped 24 hours preoperatively and resumed as soon as possible postoperatively. In group A, 7% of patients achieved a partial response (PR), 83% achieved stable disease, and 2-year progression-free survival (PFS) and OS were 83% and 93%, respectively. In group B, 4.5% of patients achieved a PR, 91% achieved stable disease, and 4.5% experienced progressive disease in the preoperative period; the 2-year PFS and OS were 77% and 91%, respectively. The results of this trial demonstrated the feasibility of using perioperative imatinib with minimal effects on surgical outcomes and set the rationale to use neoadjuvant imatinib in select patients with borderline resectable or rectal GISTs. Another EORTC pooled analysis from 10 sarcoma centers revealed that after a median of 10 months of neoadjuvant imatinib, 83.2% of patients achieved an R0 resection and only 1% progressed during treatment.³⁵ After a median follow-up of 46 months, the 5-year disease-free survival and OS were 65% and 87%, respectively.

Mutational testing should be performed beforehand to ensure the tumor is imatinib-sensitive. If a KIT exon 9 mutation is identified, then 400 mg twice daily should be considered (given the benefit seen with 800 mg imatinib for advanced GIST patients), although there are no studies to confirm this practice. Neoadjuvant imatinib is recommended for a total of 6 to 12 months to ensure maximal tumor debulking, but with very close monitoring and surgical input for disease resistance and growth.¹⁴ Imatinib should be stopped 1 to 2 days preoperatively and resumed once the patient has recovered from surgery for a total of 3 years (pre-/postoperatively combined). Neoadjuvant therapy has been shown to be safe and effective, but there have been no randomized trials to assess survival.

What is appropriate surveillance for resected GISTs?

Surveillance

There have been no randomized studies to guide the management of surveillance after surgical resection and adjuvant therapy. There is no known optimal follow-up schedule, but several have been proposed.^13,36 Among high-risk patients, it is suggested to image every 3 to 6 months during adjuvant therapy, followed by every 3 months for 2 years after discontinuing therapy, then every 6 months for another 3 years and annually thereafter for an additional 5 years. High-risk patients usually relapse within 1 to 3 years after finishing adjuvant therapy, while low-risk patients can relapse later given that their disease can be slower growing. It has been recommended that low-risk patients undergo imaging every 6 months for 5 years, with follow-up individualized thereafter. Very-low-risk patients may not require more than annual imaging. Because most relapses occur within the peritoneum or liver, imaging should encompass the abdomen and pelvis. Surveillance imaging usually consists of CT scans of the abdomen and pelvis. MRI scans can be utilized for patients at lower risk or who are out several years in order to avoid excess radiation exposure. MRI is also specifically helpful for rectal and esophageal lesions. Chest CT or chest radiograph and bone scan are not routinely required for follow-up.

Case Conclusion

The patient receives adjuvant imatinib and experiences grade 2 myalgias, periorbital edema, and macrocytic anemia, which result in imatinib discontinuation after 3 years of treatment. He is seen every 3 to 6 months and a contrast CT abdomen and pelvis is obtained every 6 months for 5 years. During this 5-year follow-up period, he does not have any clinical or radiographic evidence of disease recurrence.

Further follow-up of this patient is presented in the second article in this 2-part review of management of GISTs.

Key Points

• GISTs are the most common mesenchymal neoplasms of the GI tract and can occasionally occur in extragastrointestinal locations as well.
• GISTs encompass a heterogeneous family of tumor subsets with different natural histories, mutations, and TKI responsiveness.
• Surgery is the mainstay of treatment for localized GISTs, with cure rates greater than 50%.
• For very small (< 2 cm) esophagogastric GISTs, endoscopic ultrasound evaluation and follow-up is recommended.
• For tumors ≥ 2 cm, biopsy and excision is the standard approach.
• For localized GISTs, complete surgical resection (R0) is standard treatment, with no lymphadenectomy for clinically negative lymph nodes.
• Mutational analysis should be considered standard of practice. It can be helpful for confirming the diagnosis and can be predictive and prognostic in determining specific TKI therapy and dose.
• Adjuvant imatinib at a dose of 400 mg for 3 years is standard of care for GISTs that are at high risk of relapse and are imatinib-sensitive, and it is the only TKI approved for adjuvant therapy. Patients with PDGFRA D842V, NF1, BRAF or SDH-deficient GISTs should not receive adjuvant imatinib therapy.
• Neoadjuvant therapy can be utilized for sites where extensive resection would lead to significant morbidity. It should be given for 6 to 12 months, but patients need to be monitored closely for tumor growth.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the gastrointestinal (GI) tract and arise from the interstitial cells of Cajal of the myenteric plexus. These tumors are rare, with about 1 case per 100,000 persons diagnosed in the United States annually, but may be incidentally discovered in up to 1 in 5 autopsy specimens of older adults.^1,2 Epidemiologic risk factors include increasing age, with a peak incidence between age 60 and 65 years, male gender, black race, and non-Hispanic white ethnicity. Germline predisposition can also increase the risk of developing GISTs; molecular drivers of GIST include gain-of-function mutations in the KIT proto-oncogene and platelet-derived growth factor receptor α (PDGFRA) gene, which both encode structurally similar tyrosine kinase receptors; germline mutations of succinate dehydrogenase (SDH) subunit genes; and mutations associated with neurofibromatosis type 1.

GISTs most commonly involve the stomach, followed by the small intestine, but can arise anywhere within the GI tract (esophagus, colon, rectum, and anus). They can also develop outside the GI tract, arising from the mesentery, omentum, and retroperitoneum. The majority of cases are localized or locoregional, whereas about 20% are metastatic at presentation.¹ GISTs can occur in children, adolescents, and young adults. Pediatric GISTs represent a distinct subset marked by female predominance and gastric origin, are often multifocal, can sometimes have lymph node involvement, and typically lack mutations in the KIT and PDGFRA genes.

This review is the first of 2 articles focusing on the diagnosis and management of GISTs. Here, we review the evaluation and diagnosis of GISTs along with management of localized disease. Management of advanced disease is reviewed in a separate article.

Case Presentation

A 64-year-old African American man with progressive iron deficiency and abdominal discomfort undergoes upper and lower endoscopy and is found to have a bulging mass within his abdominal cavity. He undergoes a computed tomography (CT) evaluation of the chest, abdomen, and pelvis with contrast, which reveals the presence of a 10-cm gastric mass, with no other lesions identified. He undergoes surgical resection of the mass and presents for review of his pathology and to discuss his treatment plan.

What histopathologic features are consistent with GIST?

What factors are used for risk stratification and to predict likelihood of recurrence?

Clinical Presentation and Diagnosis

Most patients present with symptoms of overt or occult GI bleeding or abdominal discomfort, but a significant proportion of GISTs are discovered incidentally. Lymph node involvement is not typical, except for GISTs occurring in children and/or with rare syndromes. Most syndromic GISTs are multifocal and multicentric. After surgical resection, GISTs usually recur or metastasize within the abdominal cavity, including the omentum, peritoneum, or liver. These tumors rarely spread to the lungs, brain, or bones; when tumor spread does occur, it tends to be in heavily pre-treated patients with advanced disease who have been on multiple lines of therapy for a long duration of time.

The diagnosis usually can be made by histopathology. Specimens can be obtained by endoscopic ultrasound (EUS)– or CT-guided methods, the latter of which carries a very small risk of contamination from percutaneous biopsy. In terms of morphology, GISTs can be spindle cell, epithelioid, or mixed neoplasms. Epithelioid tumors are more commonly seen in the stomach and are often PDGFRA-mutated or SDH-deficient. The differential diagnosis includes other soft-tissue GI wall tumors such as leiomyosarcomas/leiomyomas, germ cell tumors, lymphomas, fibromatosis, and neuroendocrine and neurogenic tumors. A unique feature of GISTs that differentiates them from leiomyomas is near universal expression of CD117 by immunohistochemistry (IHC); this characteristic has allowed pathologists and providers to accurately distinguish true GISTs from other GI mesenchymal tumors.³ Recently, DOG1 (discovered on GIST1) immunoreactivity has been found to be helpful in identifying patients with CD117-negative GISTs. Initially identified through gene expression analysis of GISTs, DOG1 IHC can identify the common mutant c-Kit-driven CD117-positive GISTs as well as the rare CD117-negative GISTs, which are often driven by mutated PDGFRA.⁴ Importantly, IHC for KIT and DOG1 are not surrogates for mutational status, nor are they predictive of tyrosine kinase inhibitor (TKI) sensitivity. If IHC of a tumor specimen is CD117- and DOG1-negative, the specimen can be sent for KIT and PDGFRA mutational analysis to confirm the diagnosis. If analysis reveals that these genes are wild-type, then IHC staining for SDH B (SDHB) should follow to assess for an SDH-deficient GIST (negative staining).

Risk Stratification for Recurrence

The clinical behavior of GISTs can be variable. Some are indolent, while others behave more aggressively, with a greater malignant potential and a higher propensity to recur and metastasize. Clinical and pathologic features can provide important prognostic information that allows providers to risk-stratify patients. Various institutions have assessed prognostic variables for GISTs. In 2001, the National Institutes of Health (NIH) held a GIST workshop that proposed an approach to estimating metastatic risk based on tumor size and mitotic index (NIH or Fletcher criteria).⁵ Joensuu et al later proposed a modification of the NIH risk classification to include tumor location and tumor rupture (modified NIH criteria or Joensuu criteria).^6-8 Similarly, the Armed Forces Institute of Pathology (AFIP) identified tumor site as a prognostic factor, with gastric GISTs having the best prognosis (AFIP-Miettinen criteria).^9-11 Tabular schemes were designed which stratified patients into discrete groups with ranges for mitotic rate and tumor size. Nomograms for ease of use were then constructed utilizing a bimodal mitotic rate and included tumor site and size.¹² Finally, contour maps were developed, which have the advantage of evaluating mitotic rate and tumor size as continuous nonlinear variables and also include tumor site and rupture (associated with a high risk of peritoneal metastasis) separately, further improving risk assessment. These contour maps have been validated against pooled data from 10 series (2560 patients).¹³ High-risk features identified from these studies include tumor location, size, mitotic rate and tumor rupture and are now used for deciding on the use of adjuvant imatinib and as requirements to enter clinical trials assessing adjuvant therapy for resected GISTs.

Case Continued

The patient’s operative and pathology reports indicate that the tumor is a spindle cell neoplasm of the stomach that is positive for CD117, DOG1, and CD34 and negative for smooth muscle actin and S-100, consistent with a diagnosis of GIST. Resection margins are negative. There are 10 mitoses per 50 high-power fields (HPF). Per the operative report, there was no intraoperative or intraperitoneal tumor rupture. Thus, while his GIST was gastric, which generally has a more favorable prognosis, the tumor harbors high-risk features based on its size and mitotic index.

What further testing should be requested?

Molecular Alterations

It is recommended that a mutational analysis be performed as part of the diagnostic work-up of all GISTs.¹⁴ Mutational analysis can provide prognostic and predictive information for sensitivity to imatinib and should be considered standard of care. It may also be useful for confirming a GIST diagnosis, or, if negative, lead to further evaluation with an IHC stain for SDHB. The c-Kit receptor is a member of the tyrosine kinase family and, through direct interactions with stem cell factor (SCF), can upregulate the PI3K/AKT/mTOR, Ras/Raf/MEK/ERK, and JAK-STAT pathways, resulting in transcription and translation of genes that enhance cell growth and survival.¹⁵ The cell of origin of GISTs, the interstitial cells of Cajal, are dependent on the SCF–c-Kit interaction for development.¹⁶ Likewise, the large majority of GISTs (about 70%) are driven by upregulation and constitutive activation of c-Kit, which is normally autoinhibited. About 80% of KIT mutations involve exon 11; these GISTs are most often associated with a gastric location and are associated with a favorable recurrence-free survival (RFS) rate with surgery alone.¹⁷KIT exon 9 mutations are much less common, encompassing only about 10% of GIST KIT mutations, and GISTs with these mutations are more likely to arise from the small bowel.¹⁷

About 8% of GISTs harbor gain-of-function PDGFRA driver mutations rendering constitutively active PDGFRA.¹⁸PDGFRA mutations are mutually exclusive from KIT mutations, and PDGFRA-mutated tumors most often occur in the stomach. PDGFRA mutations generally are associated with a lower mitotic rate and gastric location. Identification of the PDGFRA D842V mutation on exon 18, which is the most common, is important, as it is associated with imatinib resistance, and these patients should not be offered imatinib.¹⁹

Several other mutations associated with GISTs outside of the KIT and PDGFRA spectrum have been identified. About 10% of GISTs are wildtype for KIT and PDGFRA, and not all KIT/PDGFRA-wildtype GISTs are imatinib-sensitive and/or respond to other TKIs.¹⁸ These tumors may harbor aberrations in SDH and NF1, or less commonly, BRAF V600E, FGFR, and NTRK.^20,21 SDH subunits B, C and D play a role in the Krebs cycle and electron transport chain. Germline mutations in these SDH subunits can result in the Carney-Stratakis syndrome characterized by the dyad of multifocal GISTs and multicentric paragangliomas.²² This syndrome is most likely to manifest in the pediatric or young adult population. In contradistinction is the Carney triad, which is associated with acquired loss of function of the SDHC gene due to promoter hypermethylation. This syndrome classically occurs in young women and is characterized by an indolent-behaving triad of multicentric GISTs, non-adrenal paragangliomas, and pulmonary chondromas.²³ Like PDGFRA D842V–mutated GISTs, SDH-deficient and NF1-associated GISTs are considered imatinib resistant, and these patients should not be offered imatinib therapy.¹⁴

Case Continued

The patient’s GIST is found to harbor a KIT exon 11 single codon deletion. He appears anxious and asks to have everything done to prevent his GIST from coming back and to improve his lifespan.

Management

A multidisciplinary team approach to the management of all GISTs is essential and includes input from radiology, gastroenterology, pathology, medical and surgical oncology, nuclear medicine, and nursing.

Surgical Resection

Small esophagogastric and duodenal GISTs ≤ 2 cm can be asymptomatic and managed with serial endoscopic surveillance, typically every 6 to 12 months, with biopsies if the tumors increase in size. GISTs larger than 2 cm require surgical resection, with resection of the full pseudocapsule and an R0 resection, if possible, since larger GISTs carry a higher risk of growth and recurrence. If an R0 resection would lead to significant morbidity or functional sequelae, an R1 may suffice. Rectal GISTs are an exception, where microscopic margins have been shown to be associated with an increased risk of local failure.²⁴ It is important to explore the abdomen thoroughly for peritoneal, rectovaginal, and vesicular implants and metastasis to the liver. A formal lymph node dissection is not necessary because lymph nodes are rarely involved and should only be removed when clinically suspicious. Tumor rupture must be avoided. A laparoscopic approach should only be considered for smaller tumors, since there is a risk of tumor rupture with larger tumors.¹⁴

When is adjuvant imatinib indicated?

Adjuvant Imatinib

Among patients with local or locally advanced GISTs, the risk of death from recurrence with surgery alone can be high, with a historical 5-year overall survival (OS) of about 35%.²⁵ As a result, multiple studies have assessed the benefit of adjuvant imatinib, which is now considered standard of care for patients with imatinib-sensitive, high-risk GISTs. In addition to inhibiting BCR-ABL, imatinib mesylate inhibits multiple other receptor tyrosine kinases, including PDGFR, SCF and c-Kit. As a result, imatinib has demonstrated in vitro inhibition of cell proliferation and apoptosis and clinical activity against GISTs expressing CD117.²⁶ Importantly, adjuvant imatinib should only be offered to patients with imatinib-sensitive mutations, such as KIT exon 11 and KIT exon 9 mutations. Adjuvant imatinib should not be offered to patients with imatinib-insensitive mutations such as PDGFR 842V, NF1, or BRAF-related or SDH-deficient GISTs.

The ACOSOG Z9000 was the first study of adjuvant imatinib in patients with resected GISTs.²⁵ This was a single-arm, phase 2 study involving 106 patients with surgically resected GISTs deemed high-risk for recurrence, defined as size > 10 cm, tumor rupture, or up to 4 peritoneal implants. Patients were treated with imatinib 400 mg daily for 1 year. The primary and secondary endpoints were OS and RFS, respectively. Long-term follow-up of this study demonstrated 1-, 3-, and 5-year OS of 99%, 97%, and 83%, and 1-, 3-, and 5-year RFS of 96%, 60%, and 40%, which compared favorably with historical controls. In a multivariable analysis, increasing tumor size, small bowel location, KIT exon 9 mutation, high mitotic rate, and older age were independent risk factors for a poor RFS.²⁵ It is important to note that the benefit of adjuvant imatinib waned after discontinuation of therapy, creating a rationale to study adjuvant imatinib for longer periods of time.

As a result of the promising phase 2 data, ACOSOG opened a phase 3 randomized trial (Z9001) comparing 1 year of adjuvant imatinib to placebo among patients with surgically resected GISTs that were > 3 cm in size and that stained positive for CD117 on pathology. The trial accrued 713 patients and was stopped early at a planned interim analysis, which revealed a 1-year RFS of 98% for imatinib versus 83% for placebo (hazard ratio [HR], 0.35; P < 0.001). The 1-year OS did not differ between the 2 arms (92.2% vs 99.7%; HR, 0.66; P = 0.47).²⁷ When comparing the 2 arms, imatinib was associated with a higher RFS among patients with a KIT exon 11 deletion, but not among patients with other KIT mutation types, PDGFRA mutations, or who were KIT/PDGFRA wildtype.²⁸ Imatinib was granted approval by the US Food and Drug Administration (FDA) for the adjuvant treatment of high-risk GISTs based on the results of the ACOSOG Z9001 trial.

The EORTC 62024 study was a randomized placebo-controlled trial assessing the benefit of 2 years of adjuvant imatinib.²⁹ Patients had to be considered intermediate or high risk per the 2002 NIH consensus classification to be eligible. The trial enrolled 918 patients. The 5-year OS rate, the original primary endpoint, did not differ between the 2 groups (100% vs 99%). The 3-year and 5-year RFS rates, secondary endpoints, were significantly longer among patients treated with imatinib (84% vs 66% and 69% vs 63%, respectively). Again, it was noted that the benefit of imatinib waned over time after treatment discontinuation.

The Scandinavian Sarcoma Group (SSG XVIII) trial was a prospective randomized phase 3 trial that compared 3 years versus 1 year of adjuvant imatinib.³⁰ Patients had to be enrolled within 12 weeks of the postoperative period and had to have GISTs that were CD117-positive and with a high estimated risk of recurrence, per the modified NIH consensus criteria (size > 10 cm, > 10 mitoses per 50 HPF, diameter > 5 cm with mitotic count > 5, or tumor rupture before or at surgery). Three years of adjuvant imatinib was associated with a 54% reduction in the hazard for recurrence at 5 years (65.6% vs 47.9%; HR, 0.46; P < 0.001) and a 55% reduction in the hazard for death at 5 years (OS 92% vs 81.7%; HR, 0.45; P = 0.02). Based on the results of this study, the FDA granted approval for the use of 3 years of adjuvant imatinib in patients with high-risk resected GISTs.

The observation that a longer duration of adjuvant imatinib was associated with superior RFS and OS led to studies to further explore longer durations of adjuvant imatinib. The PERSIST-5 (Postresection Evaluation of Recurrence-free Survival for Gastrointestinal Stromal Tumors With 5 Years of Adjuvant Imatinib) was a multicenter, single-arm, phase 2 prospective study of adjuvant imatinib with a primary endpoint of RFS after 5 years.³¹ Patients had to have an intermediate or high risk of recurrence, which included GISTs at any site > 2 cm with > 5 mitoses per 50 HPF or nongastric GISTs that were ≥ 5 cm. With 91 patients enrolled, the estimated 5-year RFS was 90% and the OS was 95%. Of note, about half of the patients stopped treatment early due to a variety of reasons, including patient choice or adverse events. Importantly, there were no recurrences in patients with imatinib-sensitive mutations while on therapy. We know that in patients at high risk of relapse, adjuvant imatinib delays recurrence and improves survival, but whether any patients are cured, or their survival curves are just shifted to the right, is unknown. Only longer follow-up of existing studies, and the results of newer trials utilizing longer durations of adjuvant treatment, will help to determine the real value of adjuvant therapy for GIST patients.³² Based on this study, it would be reasonable to discuss a longer duration of imatinib with patients deemed to be at very high risk of recurrence and who are tolerating therapy well. We are awaiting the data from the randomized phase 3 Scandinavian Sarcoma Group XII trial comparing 5 years versus 3 years of adjuvant imatinib therapy, and from the French ImadGIST trial of adjuvant imatinib for 3 versus 6 years. A summary of the aforementioned key adjuvant trials is shown in the Table.

When imatinib is commenced, careful monitoring for treatment toxicities and drug interactions should ensue in order to improve compliance. Dose density should be maintained if possible, as retrospective studies suggest suboptimal plasma levels are associated with a worse outcome.³³

When should neoadjuvant imatinib be considered?

Neoadjuvant Imatinib

Neoadjuvant imatinib should be considered for patients requiring total gastrectomy, esophagectomy, or abdominoperineal resection of the rectum in order to reduce tumor size, limit subsequent surgical morbidity, mitigate tumor bleeding and rupture, and aid with organ preservation. Patients with rectal GISTs that may otherwise warrant an abdominoperineal resection should be offered a trial of imatinib in the neoadjuvant setting. There is no evidence for the use of any other TKI aside from imatinib in the neoadjuvant or adjuvant setting. With neoadjuvant imatinib, it is difficult to accurately assess the mitotic rate in the resected tumor specimen.

The RTOG 0132/ACRIN 6665 trial was a prospective phase 2 study evaluating the efficacy of imatinib 600 mg daily in the perioperative setting.³⁴ The trial enrolled 50 patients, 30 with primary GISTs (group A) and 22 with recurrent metastatic GISTs (group B). Based on data from the metastatic setting revealing a time to treatment response of about 2.5 months, patients were treated with 8 to 12 weeks of preoperative imatinib followed by 2 years of adjuvant imatinib. Imatinib was stopped 24 hours preoperatively and resumed as soon as possible postoperatively. In group A, 7% of patients achieved a partial response (PR), 83% achieved stable disease, and 2-year progression-free survival (PFS) and OS were 83% and 93%, respectively. In group B, 4.5% of patients achieved a PR, 91% achieved stable disease, and 4.5% experienced progressive disease in the preoperative period; the 2-year PFS and OS were 77% and 91%, respectively. The results of this trial demonstrated the feasibility of using perioperative imatinib with minimal effects on surgical outcomes and set the rationale to use neoadjuvant imatinib in select patients with borderline resectable or rectal GISTs. Another EORTC pooled analysis from 10 sarcoma centers revealed that after a median of 10 months of neoadjuvant imatinib, 83.2% of patients achieved an R0 resection and only 1% progressed during treatment.³⁵ After a median follow-up of 46 months, the 5-year disease-free survival and OS were 65% and 87%, respectively.

Mutational testing should be performed beforehand to ensure the tumor is imatinib-sensitive. If a KIT exon 9 mutation is identified, then 400 mg twice daily should be considered (given the benefit seen with 800 mg imatinib for advanced GIST patients), although there are no studies to confirm this practice. Neoadjuvant imatinib is recommended for a total of 6 to 12 months to ensure maximal tumor debulking, but with very close monitoring and surgical input for disease resistance and growth.¹⁴ Imatinib should be stopped 1 to 2 days preoperatively and resumed once the patient has recovered from surgery for a total of 3 years (pre-/postoperatively combined). Neoadjuvant therapy has been shown to be safe and effective, but there have been no randomized trials to assess survival.

What is appropriate surveillance for resected GISTs?

Surveillance

There have been no randomized studies to guide the management of surveillance after surgical resection and adjuvant therapy. There is no known optimal follow-up schedule, but several have been proposed.^13,36 Among high-risk patients, it is suggested to image every 3 to 6 months during adjuvant therapy, followed by every 3 months for 2 years after discontinuing therapy, then every 6 months for another 3 years and annually thereafter for an additional 5 years. High-risk patients usually relapse within 1 to 3 years after finishing adjuvant therapy, while low-risk patients can relapse later given that their disease can be slower growing. It has been recommended that low-risk patients undergo imaging every 6 months for 5 years, with follow-up individualized thereafter. Very-low-risk patients may not require more than annual imaging. Because most relapses occur within the peritoneum or liver, imaging should encompass the abdomen and pelvis. Surveillance imaging usually consists of CT scans of the abdomen and pelvis. MRI scans can be utilized for patients at lower risk or who are out several years in order to avoid excess radiation exposure. MRI is also specifically helpful for rectal and esophageal lesions. Chest CT or chest radiograph and bone scan are not routinely required for follow-up.

Case Conclusion

The patient receives adjuvant imatinib and experiences grade 2 myalgias, periorbital edema, and macrocytic anemia, which result in imatinib discontinuation after 3 years of treatment. He is seen every 3 to 6 months and a contrast CT abdomen and pelvis is obtained every 6 months for 5 years. During this 5-year follow-up period, he does not have any clinical or radiographic evidence of disease recurrence.

Further follow-up of this patient is presented in the second article in this 2-part review of management of GISTs.

Key Points

• GISTs are the most common mesenchymal neoplasms of the GI tract and can occasionally occur in extragastrointestinal locations as well.
• GISTs encompass a heterogeneous family of tumor subsets with different natural histories, mutations, and TKI responsiveness.
• Surgery is the mainstay of treatment for localized GISTs, with cure rates greater than 50%.
• For very small (< 2 cm) esophagogastric GISTs, endoscopic ultrasound evaluation and follow-up is recommended.
• For tumors ≥ 2 cm, biopsy and excision is the standard approach.
• For localized GISTs, complete surgical resection (R0) is standard treatment, with no lymphadenectomy for clinically negative lymph nodes.
• Mutational analysis should be considered standard of practice. It can be helpful for confirming the diagnosis and can be predictive and prognostic in determining specific TKI therapy and dose.
• Adjuvant imatinib at a dose of 400 mg for 3 years is standard of care for GISTs that are at high risk of relapse and are imatinib-sensitive, and it is the only TKI approved for adjuvant therapy. Patients with PDGFRA D842V, NF1, BRAF or SDH-deficient GISTs should not receive adjuvant imatinib therapy.
• Neoadjuvant therapy can be utilized for sites where extensive resection would lead to significant morbidity. It should be given for 6 to 12 months, but patients need to be monitored closely for tumor growth.

Most advanced gastrointestinal stromal tumors (GISTs) are due to a recurrence of localized disease, with only a small minority presenting with metastatic disease.¹ Compared with chemotherapy, tyrosine kinase inhibitors (TKIs) have significantly improved the natural history of the disease, with median overall survival (OS) increasing from less than 1 year to about 5 years and approximately 1 in 5 patients achieving long-term survival.² In addition, newer drugs in development and in clinical trials appear promising and have the potential to improve outcomes even further. This article reviews current evidence on options for treating metastatic or recurrent GISTs and GISTs that have progressed following initial therapy. The evaluation and diagnosis of GIST along with management of localized disease are reviewed in a separate article.

Case Presentation

A 64-year-old African American man underwent surgical resection of a 10-cm gastric mass, which pathology reported was positive for CD117, DOG1, and CD34 and negative for smooth muscle actin and S-100, consistent with a diagnosis of GIST. There were 10 mitoses per 50 HPF, and there was no intraoperative or intraperitoneal tumor rupture. The patient was treated with adjuvant imatinib, which was discontinued after 3 years due to grade 2 myalgias, periorbital edema, and macrocytic anemia. Surveillance included office visits every 3 to 6 months and a contrast CT abdomen and pelvis every 6 months. For the past 5 years, he has not had any clinical or radiographic evidence of disease recurrence. New imaging reveals multiple liver metastases and peritoneal implants. He feels fatigued and has lost about 10 lb since his last visit. He is 5 years out from his initial diagnosis and 2 years out from last receiving imatinib. His original tumor harbored a KIT exon 11 deletion.

What treatment should you recommend now?

Imatinib for Advanced GISTs

Before the first report of the efficacy of imatinib for metastatic GISTs in 2002, patients with advanced unresectable or metastatic GISTs were routinely treated with doxorubicin-based chemotherapy regimens, which were largely ineffective, with response rates (RRs) of around 5% and a median overall survival (OS) of less than 1 year.^3,4 In 2002 a landmark phase 2 study revealed imatinib’s significant efficacy profile in advanced or metastatic GISTs, resulting in its approval by the US Food and Drug Administration (FDA).⁵ In this study, 147 patients with CD117-positive GISTs were randomly assigned to receive daily imatinib 400 mg or 600 mg for up to 36 months. The RRs were similar between the 2 groups (68.5% vs 67.6%), with a median time to response of 12 weeks and median duration of response of 118 days. Results of this study were much more favorable when compared to doxorubicin, rendering imatinib the new standard of care for advanced GISTs. A long-term follow-up of this study after a median of 63 months confirmed near identical RRs, progression-free survival (PFS), and median survival of 57 months among the 2 groups.⁶

Imatinib Daily Dosing

Although 400 mg of daily imatinib proved to be efficacious, it was unclear if a dose-response relationship existed for imatinib. An EORTC phase 2 study demonstrated a benefit of using a higher dose of imatinib at 400 mg twice daily, producing a RR of 71% (4% complete , 67% partial) and 1-year PFS of 73%, which appeared favorable compared with once-daily dosing and set the framework for larger phase 3 studies.⁷ Two phase 3 studies compared imatinib 400 mg once daily versus twice daily (until disease progression or unacceptable toxicity) among patients with CD117-positive advanced or metastatic GISTs. These studies were eventually combined into a meta-analysis (metaGIST) to compare RR, PFS and OS between the treatment groups. Both studies allowed cross-over to the 800 mg dose for patients who progressed on 400 mg daily.

The first study, conducted jointly by the EORTC, Italian Sarcoma Group, and Australasian Gastro-Intestinal Trials Group (EU-AUS),⁸ randomly assigned 946 patients to 400 mg once daily or twice daily. There were no differences in response rates between the groups, but the twice-daily group had a predicted 18% reduction in the hazard for progression compared with the once-daily group (estimated HR, 0.82; P = 0.026), which came at the expense of greater toxicities warranting dose reductions (60%) and treatment interruptions (64%). Cross-over to high-dose imatinib was feasible and safe, producing a partial response in 2%, stable disease in 27%, and a median PFS of 81 days. The second study was an intergroup study conducted jointly by SWOG, CALGB, NCI-C, and ECOG (S0033, US-CDN), with a nearly identical study design as the EU-AUS trial.⁹ The trial enrolled 746 patients. After a median follow up of 4.5 years, the median PFS and OS were not statistically different (18 vs 20 months and 55 vs 51 months, respectively). There were also no differences in response rates. One third of patients initially placed on the once-daily arm who crossed over after progression achieved a treatment response or stable disease.

The combined EU-AUS and US-CDN analysis (metaGIST) included 1640 patients with a median age of 60 years and 58% of whom were men; 818 and 822 patients were assigned to the 400 mg and 800 mg total daily doses, respectively.¹⁰ The median follow-up was 37.5 months. There were no differences in OS (49 vs 48.7 months), median PFS (18.9 vs 23.2 months), or overall response rates (51.4% vs 53.9%). Patients who had crossed over (n = 347) to the 800 mg total daily dose arm had a 7.7-month average PFS while on the higher daily dose. An analysis was performed on 377 patients in the EU-AUS trial assessing the impact of mutational status on clinical outcomes among imatinib-treated patients. KIT exon 9 activating mutations were found to be a significant independent prognostic factor for death when compared with KIT exon 11 mutations. However, the adverse prognostic value of KIT exon 9 mutations was partially overcome with higher doses of imatinib, as those who received 800 mg total had a significantly better PFS, with a 61% relative risk reduction, than those who received 400 mg. Altogether, it was concluded that imatinib 400 mg once daily should be the standard-of-care first-line treatment for advanced or metastatic GISTs, unless a KIT exon 9 mutation is present, in which case imatinib 800 mg should be considered, if 400 mg is well tolerated. In addition, patients treated with frontline imatinib at 400 mg once daily, if tolerated well, should be considered for imatinib 800 mg upon progression of disease.

Despite there being problems with secondary resistance, significant progress has occurred in the treatment of metastatic disease over a short period of time. Prior to 2000, median OS for patients with metastatic GISTs was 9 months. With the introduction of imatinib and other TKIs, the median OS has increased to 5 years, with an estimated 10-year OS rate of approximately 20%.²

Since at this point, imatinib was a well-established standard of care for advanced GISTs, it was questioned whether imatinib therapy could be interrupted. At this time, treatment interruption in a stop-and-go fashion was deemed feasible in other metastatic solid tumors such as colorectal cancer (OPTIMOX1).¹¹ The BFR French trial showed that stopping imatinib therapy in patients who had a response or stable disease after 1, 3, or 5 years was generally followed by relatively rapid tumor progression (approximately 50% of patients within 6 months), even when tumors were previously removed.¹² Therefore, it is recommended that treatment in the metastatic setting should be continued indefinitely, unless there is disease progression. Hence, unlike with colorectal cancer or chronic myelogenous leukemia, as of now there is no role for imatinib interruption in metastatic GISTs.

Case Continued

The patient is started on imatinib 400 mg daily, and overall he tolerates therapy well. Interval CT imaging reveals a treatment response. Two years later, imaging reveals an increase in the tumor size and density with a new nodule present within a preexisting mass. There are no clinical trials in the area.

What defines tumor progression?

Disease Progression

When GISTs are responding to treatment, on imaging the tumors can become more cystic and less dense but with an increase in size. In addition, tumor progression may not always be associated with increased size—increased density of the tumor or a nodule within a mass that may indicate progression. If CT imaging is equivocal for progression, positron emission tomography (PET) can play a role in identifying true progression. It is critically important that tumor size and density are carefully assessed when performing interval imaging. Of note, radiofrequency ablation, cryotherapy, or chemoembolization can be used for symptomatic liver metastases or oligometastatic disease. When evaluating for progression, one needs to ask patients about compliance (ie, maintaining dose intensity related to side effects of therapy as well as the financial burden of treatment—copay toxicity).

What are mechanisms of secondary imatinib resistance?

Imatinib resistance can be subtle in patients with GISTs, manifesting with new nodular, enhancing foci enclosed within a preexisting mass (resistant clonal nodule), or can be clinically or radiographically overt.¹³ Imatinib resistance occurs through multiple mechanisms including acquisition of secondary activating KIT mutations in the intracellular ATP-binding domain (exons 13 and 14) and the activation loop (exons 17 and 18).¹⁴

What are the treatment options for this patient?

Second-line Therapy

Sunitinib malate is a multitargeted TKI that not only targets c-Kit and PDGFRA, but also has anti-angiogenic activity through inhibition of vascular endothelial growth factor receptors (VEGFR). Sunitinib gained FDA approval for the second-line treatment of advanced GISTs based on an international double-blind trial that randomized 312 patients with imatinib-resistant metastatic GISTs in a 2:1 fashion to receive sunitinib 50 mg daily for 4 weeks on and 2 weeks off or placebo.^15,16 The trial was unblinded early at the planned interim analysis, which revealed a marked benefit, producing a 66% reduction in the hazard risk of progression (27.3 vs 6.4 weeks, HR, 0.33; P < 0.001). The most common treatment-related adverse events were fatigue, diarrhea, skin discoloration, nausea, and hand-foot syndrome. Another open-label phase 2 study assessed a continuous dosing schema of sunitinib 37.5 mg daily, which has been shown to be effective with less toxicity.¹⁷ Among the 60 patients enrolled, the primary endpoint of clinical benefit rate at 24 weeks was reached in 53%, which consisted of 13% partial responses and 40% stable disease. Most toxicities were grade 1 or 2 and easily manageable through standard interventions. This has been recommended as an alternative to the initial scheduled regimen.¹⁸ Part of sunitinib’s success is its activity against GISTs harboring secondary KIT exon 13 and 14 mutations, and possibly its anti-angiogenic activity.¹⁹ Sunitinib is particularly efficacious among GISTs harboring KIT exon 9 mutations.

Patients who have progressed on prior imatinib and sunitinib can receive third-line regorafenib, a multi-TKI that differs chemically from sorafenib by a fluorouracil group (fluoro-sorafenib). FDA approval of regorafenib was based on the phase 3 GRID (GIST Regorafenib In progressive Disease) multicenter international trial.²⁰ This trial randomly assigned 199 patients in a 2:1 fashion to receive regorafenib 160 mg daily for 21 days out of 28-day cycles plus best supportive care (BSC) versus placebo plus BSC. Cross-over was allowed. Regorafenib significantly reduced the hazard risk of progression by 73% compared with placebo (4.8 vs 0.9 months; HR, 0.27; P < 0.001). There was no difference in OS, which may be because of cross-over (median OS, 17.4 months in both arms). As a result, regorafenib is now considered standard third-line treatment for patients with metastatic GISTs. It has a less favorable toxicity profile than imatinib, with hand-foot syndrome, transaminitis, hypertension and fatigue being the most common treatment toxicities. In order to avoid noncompliance, it is recommended to start at 80 mg and carefully titrate upwards to the 160 mg dose.

A list of landmark studies for advanced GISTs is provided in Table 1.

A summary of FDA-approved drugs for treating GISTs is provided in Table 2.

Clinical Trials

Clinical trial enrollment should be considered for all patients with advanced or unresectable GISTs throughout their treatment continuum. Owing to significant advances in genomic profiling through next-generation sequencing, multiple driver mutations have recently been identified, and targeted therapies are being explored in clinical trials.²¹ For example, the neurotrophic receptor tyrosine kinase (NTRK) gene appears to be mutated in a small number of advanced GISTs, and these can respond to the highly selective TRK inhibitor larotrectinib.²² Additionally, ongoing studies are assessing immunotherapies for sporadic GISTs and treatment for familial GISTs (Table 3). Some notable studies include those assessing the efficacy of agents that target KIT and PDGFR secondary mutations, including avapritinib (BLU-285) and DCC-2618, MEK inhibitors, and the multi-kinase inhibitor crenolanib for GISTs harboring the imatinib-resistant PDGFRA D842V mutation. There are also studies utilizing checkpoint inhibitors alone or in combination with imatinib.

Case Conclusion

Given the patient’s progression on imatinib, he is started on second-line sunitinib malate. He experiences grade 1 fatigue and hand-foot syndrome, which are managed supportively. After he has been on sunitinib for approximately 8 months, his disease progresses. He subsequently undergoes genomic profiling of his tumor and starts BLU-285 on a clinical trial.

Key Points

• For advanced and metastatic disease, TKIs have substantially improved the prognosis of KIT mutated GISTs, with 3 FDA-approved drugs: imatinib, sunitinib, and regorafenib. Imatinib 400 mg is the standard-of-care frontline therapy for locally advanced, unresectable, or metastatic imatinib-sensitive GISTs. If a patient has a KIT exon 9 mutation and 400 mg is well-tolerated, increasing to 800 mg is recommended. Imatinib should be continued indefinitely unless there is intolerance, a specific patient request for interruption, or progression of disease.
• When there is progression of disease in a patient with a sensitive mutation on 400 mg of imatinib, the dose can be increased to 800 mg.
• For patients who are imatinib-intolerant or have progression, standard second line is sunitinib.
• For patients who further progress or are sunitinib-intolerant, regorafenib is the standard third-line treatment.
• There needs to be close attention to side effects, drug and food interactions, and patient copay costs in order to maintain patient compliance while on TKI therapy.
• There are still major limitations in the systemic treatment of GISTs marked by their inherent genetic heterogeneity and secondary resistance. Continued translational and clinical research is needed in order to improve treatment for patients who develop secondary resistance or who have less common primary resistant mutations. Patients are encouraged to participate in clinical trials of new therapies.

Summary

GISTs are the most common mesenchymal tumors of the GI tract. They comprise an expanding landscape of tumors that are heterogenous in terms of natural history, mutations, and response to systemic treatments. The mainstay of treatment for localized GISTs is surgical resection followed by at least 3-years of adjuvant imatinib for patients with high-risk features who are imatinib-sensitive. Patients with GISTs harboring resistance mutations such as PDGFRA D842V or with SDH-deficient or NF1-associated GISTs should not receive adjuvant imatinib. Patients with more advanced GISTs and/or in difficult to resect sites harboring a sensitive mutation can be considered for neoadjuvant imatinib. Those with metastatic GISTs can receive first-, second-, and third-line imatinib, sunitinib, or regorafenib, respectively. Clinical trial enrollment should be encouraged for patients whose GISTs harbor primary imatinib-resistant mutations, and those with advanced or unresectable GISTs with secondary resistance.

Most advanced gastrointestinal stromal tumors (GISTs) are due to a recurrence of localized disease, with only a small minority presenting with metastatic disease.¹ Compared with chemotherapy, tyrosine kinase inhibitors (TKIs) have significantly improved the natural history of the disease, with median overall survival (OS) increasing from less than 1 year to about 5 years and approximately 1 in 5 patients achieving long-term survival.² In addition, newer drugs in development and in clinical trials appear promising and have the potential to improve outcomes even further. This article reviews current evidence on options for treating metastatic or recurrent GISTs and GISTs that have progressed following initial therapy. The evaluation and diagnosis of GIST along with management of localized disease are reviewed in a separate article.

Case Presentation

A 64-year-old African American man underwent surgical resection of a 10-cm gastric mass, which pathology reported was positive for CD117, DOG1, and CD34 and negative for smooth muscle actin and S-100, consistent with a diagnosis of GIST. There were 10 mitoses per 50 HPF, and there was no intraoperative or intraperitoneal tumor rupture. The patient was treated with adjuvant imatinib, which was discontinued after 3 years due to grade 2 myalgias, periorbital edema, and macrocytic anemia. Surveillance included office visits every 3 to 6 months and a contrast CT abdomen and pelvis every 6 months. For the past 5 years, he has not had any clinical or radiographic evidence of disease recurrence. New imaging reveals multiple liver metastases and peritoneal implants. He feels fatigued and has lost about 10 lb since his last visit. He is 5 years out from his initial diagnosis and 2 years out from last receiving imatinib. His original tumor harbored a KIT exon 11 deletion.

What treatment should you recommend now?

Imatinib for Advanced GISTs

Before the first report of the efficacy of imatinib for metastatic GISTs in 2002, patients with advanced unresectable or metastatic GISTs were routinely treated with doxorubicin-based chemotherapy regimens, which were largely ineffective, with response rates (RRs) of around 5% and a median overall survival (OS) of less than 1 year.^3,4 In 2002 a landmark phase 2 study revealed imatinib’s significant efficacy profile in advanced or metastatic GISTs, resulting in its approval by the US Food and Drug Administration (FDA).⁵ In this study, 147 patients with CD117-positive GISTs were randomly assigned to receive daily imatinib 400 mg or 600 mg for up to 36 months. The RRs were similar between the 2 groups (68.5% vs 67.6%), with a median time to response of 12 weeks and median duration of response of 118 days. Results of this study were much more favorable when compared to doxorubicin, rendering imatinib the new standard of care for advanced GISTs. A long-term follow-up of this study after a median of 63 months confirmed near identical RRs, progression-free survival (PFS), and median survival of 57 months among the 2 groups.⁶

Imatinib Daily Dosing

Although 400 mg of daily imatinib proved to be efficacious, it was unclear if a dose-response relationship existed for imatinib. An EORTC phase 2 study demonstrated a benefit of using a higher dose of imatinib at 400 mg twice daily, producing a RR of 71% (4% complete , 67% partial) and 1-year PFS of 73%, which appeared favorable compared with once-daily dosing and set the framework for larger phase 3 studies.⁷ Two phase 3 studies compared imatinib 400 mg once daily versus twice daily (until disease progression or unacceptable toxicity) among patients with CD117-positive advanced or metastatic GISTs. These studies were eventually combined into a meta-analysis (metaGIST) to compare RR, PFS and OS between the treatment groups. Both studies allowed cross-over to the 800 mg dose for patients who progressed on 400 mg daily.

The first study, conducted jointly by the EORTC, Italian Sarcoma Group, and Australasian Gastro-Intestinal Trials Group (EU-AUS),⁸ randomly assigned 946 patients to 400 mg once daily or twice daily. There were no differences in response rates between the groups, but the twice-daily group had a predicted 18% reduction in the hazard for progression compared with the once-daily group (estimated HR, 0.82; P = 0.026), which came at the expense of greater toxicities warranting dose reductions (60%) and treatment interruptions (64%). Cross-over to high-dose imatinib was feasible and safe, producing a partial response in 2%, stable disease in 27%, and a median PFS of 81 days. The second study was an intergroup study conducted jointly by SWOG, CALGB, NCI-C, and ECOG (S0033, US-CDN), with a nearly identical study design as the EU-AUS trial.⁹ The trial enrolled 746 patients. After a median follow up of 4.5 years, the median PFS and OS were not statistically different (18 vs 20 months and 55 vs 51 months, respectively). There were also no differences in response rates. One third of patients initially placed on the once-daily arm who crossed over after progression achieved a treatment response or stable disease.

The combined EU-AUS and US-CDN analysis (metaGIST) included 1640 patients with a median age of 60 years and 58% of whom were men; 818 and 822 patients were assigned to the 400 mg and 800 mg total daily doses, respectively.¹⁰ The median follow-up was 37.5 months. There were no differences in OS (49 vs 48.7 months), median PFS (18.9 vs 23.2 months), or overall response rates (51.4% vs 53.9%). Patients who had crossed over (n = 347) to the 800 mg total daily dose arm had a 7.7-month average PFS while on the higher daily dose. An analysis was performed on 377 patients in the EU-AUS trial assessing the impact of mutational status on clinical outcomes among imatinib-treated patients. KIT exon 9 activating mutations were found to be a significant independent prognostic factor for death when compared with KIT exon 11 mutations. However, the adverse prognostic value of KIT exon 9 mutations was partially overcome with higher doses of imatinib, as those who received 800 mg total had a significantly better PFS, with a 61% relative risk reduction, than those who received 400 mg. Altogether, it was concluded that imatinib 400 mg once daily should be the standard-of-care first-line treatment for advanced or metastatic GISTs, unless a KIT exon 9 mutation is present, in which case imatinib 800 mg should be considered, if 400 mg is well tolerated. In addition, patients treated with frontline imatinib at 400 mg once daily, if tolerated well, should be considered for imatinib 800 mg upon progression of disease.

Despite there being problems with secondary resistance, significant progress has occurred in the treatment of metastatic disease over a short period of time. Prior to 2000, median OS for patients with metastatic GISTs was 9 months. With the introduction of imatinib and other TKIs, the median OS has increased to 5 years, with an estimated 10-year OS rate of approximately 20%.²

Since at this point, imatinib was a well-established standard of care for advanced GISTs, it was questioned whether imatinib therapy could be interrupted. At this time, treatment interruption in a stop-and-go fashion was deemed feasible in other metastatic solid tumors such as colorectal cancer (OPTIMOX1).¹¹ The BFR French trial showed that stopping imatinib therapy in patients who had a response or stable disease after 1, 3, or 5 years was generally followed by relatively rapid tumor progression (approximately 50% of patients within 6 months), even when tumors were previously removed.¹² Therefore, it is recommended that treatment in the metastatic setting should be continued indefinitely, unless there is disease progression. Hence, unlike with colorectal cancer or chronic myelogenous leukemia, as of now there is no role for imatinib interruption in metastatic GISTs.

Case Continued

The patient is started on imatinib 400 mg daily, and overall he tolerates therapy well. Interval CT imaging reveals a treatment response. Two years later, imaging reveals an increase in the tumor size and density with a new nodule present within a preexisting mass. There are no clinical trials in the area.

What defines tumor progression?

Disease Progression

When GISTs are responding to treatment, on imaging the tumors can become more cystic and less dense but with an increase in size. In addition, tumor progression may not always be associated with increased size—increased density of the tumor or a nodule within a mass that may indicate progression. If CT imaging is equivocal for progression, positron emission tomography (PET) can play a role in identifying true progression. It is critically important that tumor size and density are carefully assessed when performing interval imaging. Of note, radiofrequency ablation, cryotherapy, or chemoembolization can be used for symptomatic liver metastases or oligometastatic disease. When evaluating for progression, one needs to ask patients about compliance (ie, maintaining dose intensity related to side effects of therapy as well as the financial burden of treatment—copay toxicity).

What are mechanisms of secondary imatinib resistance?

Imatinib resistance can be subtle in patients with GISTs, manifesting with new nodular, enhancing foci enclosed within a preexisting mass (resistant clonal nodule), or can be clinically or radiographically overt.¹³ Imatinib resistance occurs through multiple mechanisms including acquisition of secondary activating KIT mutations in the intracellular ATP-binding domain (exons 13 and 14) and the activation loop (exons 17 and 18).¹⁴

What are the treatment options for this patient?

Second-line Therapy

Sunitinib malate is a multitargeted TKI that not only targets c-Kit and PDGFRA, but also has anti-angiogenic activity through inhibition of vascular endothelial growth factor receptors (VEGFR). Sunitinib gained FDA approval for the second-line treatment of advanced GISTs based on an international double-blind trial that randomized 312 patients with imatinib-resistant metastatic GISTs in a 2:1 fashion to receive sunitinib 50 mg daily for 4 weeks on and 2 weeks off or placebo.^15,16 The trial was unblinded early at the planned interim analysis, which revealed a marked benefit, producing a 66% reduction in the hazard risk of progression (27.3 vs 6.4 weeks, HR, 0.33; P < 0.001). The most common treatment-related adverse events were fatigue, diarrhea, skin discoloration, nausea, and hand-foot syndrome. Another open-label phase 2 study assessed a continuous dosing schema of sunitinib 37.5 mg daily, which has been shown to be effective with less toxicity.¹⁷ Among the 60 patients enrolled, the primary endpoint of clinical benefit rate at 24 weeks was reached in 53%, which consisted of 13% partial responses and 40% stable disease. Most toxicities were grade 1 or 2 and easily manageable through standard interventions. This has been recommended as an alternative to the initial scheduled regimen.¹⁸ Part of sunitinib’s success is its activity against GISTs harboring secondary KIT exon 13 and 14 mutations, and possibly its anti-angiogenic activity.¹⁹ Sunitinib is particularly efficacious among GISTs harboring KIT exon 9 mutations.

Patients who have progressed on prior imatinib and sunitinib can receive third-line regorafenib, a multi-TKI that differs chemically from sorafenib by a fluorouracil group (fluoro-sorafenib). FDA approval of regorafenib was based on the phase 3 GRID (GIST Regorafenib In progressive Disease) multicenter international trial.²⁰ This trial randomly assigned 199 patients in a 2:1 fashion to receive regorafenib 160 mg daily for 21 days out of 28-day cycles plus best supportive care (BSC) versus placebo plus BSC. Cross-over was allowed. Regorafenib significantly reduced the hazard risk of progression by 73% compared with placebo (4.8 vs 0.9 months; HR, 0.27; P < 0.001). There was no difference in OS, which may be because of cross-over (median OS, 17.4 months in both arms). As a result, regorafenib is now considered standard third-line treatment for patients with metastatic GISTs. It has a less favorable toxicity profile than imatinib, with hand-foot syndrome, transaminitis, hypertension and fatigue being the most common treatment toxicities. In order to avoid noncompliance, it is recommended to start at 80 mg and carefully titrate upwards to the 160 mg dose.

A list of landmark studies for advanced GISTs is provided in Table 1.

A summary of FDA-approved drugs for treating GISTs is provided in Table 2.

Clinical Trials

Clinical trial enrollment should be considered for all patients with advanced or unresectable GISTs throughout their treatment continuum. Owing to significant advances in genomic profiling through next-generation sequencing, multiple driver mutations have recently been identified, and targeted therapies are being explored in clinical trials.²¹ For example, the neurotrophic receptor tyrosine kinase (NTRK) gene appears to be mutated in a small number of advanced GISTs, and these can respond to the highly selective TRK inhibitor larotrectinib.²² Additionally, ongoing studies are assessing immunotherapies for sporadic GISTs and treatment for familial GISTs (Table 3). Some notable studies include those assessing the efficacy of agents that target KIT and PDGFR secondary mutations, including avapritinib (BLU-285) and DCC-2618, MEK inhibitors, and the multi-kinase inhibitor crenolanib for GISTs harboring the imatinib-resistant PDGFRA D842V mutation. There are also studies utilizing checkpoint inhibitors alone or in combination with imatinib.

Case Conclusion

Given the patient’s progression on imatinib, he is started on second-line sunitinib malate. He experiences grade 1 fatigue and hand-foot syndrome, which are managed supportively. After he has been on sunitinib for approximately 8 months, his disease progresses. He subsequently undergoes genomic profiling of his tumor and starts BLU-285 on a clinical trial.

Key Points

• For advanced and metastatic disease, TKIs have substantially improved the prognosis of KIT mutated GISTs, with 3 FDA-approved drugs: imatinib, sunitinib, and regorafenib. Imatinib 400 mg is the standard-of-care frontline therapy for locally advanced, unresectable, or metastatic imatinib-sensitive GISTs. If a patient has a KIT exon 9 mutation and 400 mg is well-tolerated, increasing to 800 mg is recommended. Imatinib should be continued indefinitely unless there is intolerance, a specific patient request for interruption, or progression of disease.
• When there is progression of disease in a patient with a sensitive mutation on 400 mg of imatinib, the dose can be increased to 800 mg.
• For patients who are imatinib-intolerant or have progression, standard second line is sunitinib.
• For patients who further progress or are sunitinib-intolerant, regorafenib is the standard third-line treatment.
• There needs to be close attention to side effects, drug and food interactions, and patient copay costs in order to maintain patient compliance while on TKI therapy.
• There are still major limitations in the systemic treatment of GISTs marked by their inherent genetic heterogeneity and secondary resistance. Continued translational and clinical research is needed in order to improve treatment for patients who develop secondary resistance or who have less common primary resistant mutations. Patients are encouraged to participate in clinical trials of new therapies.

Summary

GISTs are the most common mesenchymal tumors of the GI tract. They comprise an expanding landscape of tumors that are heterogenous in terms of natural history, mutations, and response to systemic treatments. The mainstay of treatment for localized GISTs is surgical resection followed by at least 3-years of adjuvant imatinib for patients with high-risk features who are imatinib-sensitive. Patients with GISTs harboring resistance mutations such as PDGFRA D842V or with SDH-deficient or NF1-associated GISTs should not receive adjuvant imatinib. Patients with more advanced GISTs and/or in difficult to resect sites harboring a sensitive mutation can be considered for neoadjuvant imatinib. Those with metastatic GISTs can receive first-, second-, and third-line imatinib, sunitinib, or regorafenib, respectively. Clinical trial enrollment should be encouraged for patients whose GISTs harbor primary imatinib-resistant mutations, and those with advanced or unresectable GISTs with secondary resistance.

Most advanced gastrointestinal stromal tumors (GISTs) are due to a recurrence of localized disease, with only a small minority presenting with metastatic disease.¹ Compared with chemotherapy, tyrosine kinase inhibitors (TKIs) have significantly improved the natural history of the disease, with median overall survival (OS) increasing from less than 1 year to about 5 years and approximately 1 in 5 patients achieving long-term survival.² In addition, newer drugs in development and in clinical trials appear promising and have the potential to improve outcomes even further. This article reviews current evidence on options for treating metastatic or recurrent GISTs and GISTs that have progressed following initial therapy. The evaluation and diagnosis of GIST along with management of localized disease are reviewed in a separate article.

Case Presentation

A 64-year-old African American man underwent surgical resection of a 10-cm gastric mass, which pathology reported was positive for CD117, DOG1, and CD34 and negative for smooth muscle actin and S-100, consistent with a diagnosis of GIST. There were 10 mitoses per 50 HPF, and there was no intraoperative or intraperitoneal tumor rupture. The patient was treated with adjuvant imatinib, which was discontinued after 3 years due to grade 2 myalgias, periorbital edema, and macrocytic anemia. Surveillance included office visits every 3 to 6 months and a contrast CT abdomen and pelvis every 6 months. For the past 5 years, he has not had any clinical or radiographic evidence of disease recurrence. New imaging reveals multiple liver metastases and peritoneal implants. He feels fatigued and has lost about 10 lb since his last visit. He is 5 years out from his initial diagnosis and 2 years out from last receiving imatinib. His original tumor harbored a KIT exon 11 deletion.

What treatment should you recommend now?

Imatinib for Advanced GISTs

Before the first report of the efficacy of imatinib for metastatic GISTs in 2002, patients with advanced unresectable or metastatic GISTs were routinely treated with doxorubicin-based chemotherapy regimens, which were largely ineffective, with response rates (RRs) of around 5% and a median overall survival (OS) of less than 1 year.^3,4 In 2002 a landmark phase 2 study revealed imatinib’s significant efficacy profile in advanced or metastatic GISTs, resulting in its approval by the US Food and Drug Administration (FDA).⁵ In this study, 147 patients with CD117-positive GISTs were randomly assigned to receive daily imatinib 400 mg or 600 mg for up to 36 months. The RRs were similar between the 2 groups (68.5% vs 67.6%), with a median time to response of 12 weeks and median duration of response of 118 days. Results of this study were much more favorable when compared to doxorubicin, rendering imatinib the new standard of care for advanced GISTs. A long-term follow-up of this study after a median of 63 months confirmed near identical RRs, progression-free survival (PFS), and median survival of 57 months among the 2 groups.⁶

Imatinib Daily Dosing

Although 400 mg of daily imatinib proved to be efficacious, it was unclear if a dose-response relationship existed for imatinib. An EORTC phase 2 study demonstrated a benefit of using a higher dose of imatinib at 400 mg twice daily, producing a RR of 71% (4% complete , 67% partial) and 1-year PFS of 73%, which appeared favorable compared with once-daily dosing and set the framework for larger phase 3 studies.⁷ Two phase 3 studies compared imatinib 400 mg once daily versus twice daily (until disease progression or unacceptable toxicity) among patients with CD117-positive advanced or metastatic GISTs. These studies were eventually combined into a meta-analysis (metaGIST) to compare RR, PFS and OS between the treatment groups. Both studies allowed cross-over to the 800 mg dose for patients who progressed on 400 mg daily.

The first study, conducted jointly by the EORTC, Italian Sarcoma Group, and Australasian Gastro-Intestinal Trials Group (EU-AUS),⁸ randomly assigned 946 patients to 400 mg once daily or twice daily. There were no differences in response rates between the groups, but the twice-daily group had a predicted 18% reduction in the hazard for progression compared with the once-daily group (estimated HR, 0.82; P = 0.026), which came at the expense of greater toxicities warranting dose reductions (60%) and treatment interruptions (64%). Cross-over to high-dose imatinib was feasible and safe, producing a partial response in 2%, stable disease in 27%, and a median PFS of 81 days. The second study was an intergroup study conducted jointly by SWOG, CALGB, NCI-C, and ECOG (S0033, US-CDN), with a nearly identical study design as the EU-AUS trial.⁹ The trial enrolled 746 patients. After a median follow up of 4.5 years, the median PFS and OS were not statistically different (18 vs 20 months and 55 vs 51 months, respectively). There were also no differences in response rates. One third of patients initially placed on the once-daily arm who crossed over after progression achieved a treatment response or stable disease.

The combined EU-AUS and US-CDN analysis (metaGIST) included 1640 patients with a median age of 60 years and 58% of whom were men; 818 and 822 patients were assigned to the 400 mg and 800 mg total daily doses, respectively.¹⁰ The median follow-up was 37.5 months. There were no differences in OS (49 vs 48.7 months), median PFS (18.9 vs 23.2 months), or overall response rates (51.4% vs 53.9%). Patients who had crossed over (n = 347) to the 800 mg total daily dose arm had a 7.7-month average PFS while on the higher daily dose. An analysis was performed on 377 patients in the EU-AUS trial assessing the impact of mutational status on clinical outcomes among imatinib-treated patients. KIT exon 9 activating mutations were found to be a significant independent prognostic factor for death when compared with KIT exon 11 mutations. However, the adverse prognostic value of KIT exon 9 mutations was partially overcome with higher doses of imatinib, as those who received 800 mg total had a significantly better PFS, with a 61% relative risk reduction, than those who received 400 mg. Altogether, it was concluded that imatinib 400 mg once daily should be the standard-of-care first-line treatment for advanced or metastatic GISTs, unless a KIT exon 9 mutation is present, in which case imatinib 800 mg should be considered, if 400 mg is well tolerated. In addition, patients treated with frontline imatinib at 400 mg once daily, if tolerated well, should be considered for imatinib 800 mg upon progression of disease.

Despite there being problems with secondary resistance, significant progress has occurred in the treatment of metastatic disease over a short period of time. Prior to 2000, median OS for patients with metastatic GISTs was 9 months. With the introduction of imatinib and other TKIs, the median OS has increased to 5 years, with an estimated 10-year OS rate of approximately 20%.²

Since at this point, imatinib was a well-established standard of care for advanced GISTs, it was questioned whether imatinib therapy could be interrupted. At this time, treatment interruption in a stop-and-go fashion was deemed feasible in other metastatic solid tumors such as colorectal cancer (OPTIMOX1).¹¹ The BFR French trial showed that stopping imatinib therapy in patients who had a response or stable disease after 1, 3, or 5 years was generally followed by relatively rapid tumor progression (approximately 50% of patients within 6 months), even when tumors were previously removed.¹² Therefore, it is recommended that treatment in the metastatic setting should be continued indefinitely, unless there is disease progression. Hence, unlike with colorectal cancer or chronic myelogenous leukemia, as of now there is no role for imatinib interruption in metastatic GISTs.

Case Continued

The patient is started on imatinib 400 mg daily, and overall he tolerates therapy well. Interval CT imaging reveals a treatment response. Two years later, imaging reveals an increase in the tumor size and density with a new nodule present within a preexisting mass. There are no clinical trials in the area.

What defines tumor progression?

Disease Progression

When GISTs are responding to treatment, on imaging the tumors can become more cystic and less dense but with an increase in size. In addition, tumor progression may not always be associated with increased size—increased density of the tumor or a nodule within a mass that may indicate progression. If CT imaging is equivocal for progression, positron emission tomography (PET) can play a role in identifying true progression. It is critically important that tumor size and density are carefully assessed when performing interval imaging. Of note, radiofrequency ablation, cryotherapy, or chemoembolization can be used for symptomatic liver metastases or oligometastatic disease. When evaluating for progression, one needs to ask patients about compliance (ie, maintaining dose intensity related to side effects of therapy as well as the financial burden of treatment—copay toxicity).

What are mechanisms of secondary imatinib resistance?

Imatinib resistance can be subtle in patients with GISTs, manifesting with new nodular, enhancing foci enclosed within a preexisting mass (resistant clonal nodule), or can be clinically or radiographically overt.¹³ Imatinib resistance occurs through multiple mechanisms including acquisition of secondary activating KIT mutations in the intracellular ATP-binding domain (exons 13 and 14) and the activation loop (exons 17 and 18).¹⁴

What are the treatment options for this patient?

Second-line Therapy

Sunitinib malate is a multitargeted TKI that not only targets c-Kit and PDGFRA, but also has anti-angiogenic activity through inhibition of vascular endothelial growth factor receptors (VEGFR). Sunitinib gained FDA approval for the second-line treatment of advanced GISTs based on an international double-blind trial that randomized 312 patients with imatinib-resistant metastatic GISTs in a 2:1 fashion to receive sunitinib 50 mg daily for 4 weeks on and 2 weeks off or placebo.^15,16 The trial was unblinded early at the planned interim analysis, which revealed a marked benefit, producing a 66% reduction in the hazard risk of progression (27.3 vs 6.4 weeks, HR, 0.33; P < 0.001). The most common treatment-related adverse events were fatigue, diarrhea, skin discoloration, nausea, and hand-foot syndrome. Another open-label phase 2 study assessed a continuous dosing schema of sunitinib 37.5 mg daily, which has been shown to be effective with less toxicity.¹⁷ Among the 60 patients enrolled, the primary endpoint of clinical benefit rate at 24 weeks was reached in 53%, which consisted of 13% partial responses and 40% stable disease. Most toxicities were grade 1 or 2 and easily manageable through standard interventions. This has been recommended as an alternative to the initial scheduled regimen.¹⁸ Part of sunitinib’s success is its activity against GISTs harboring secondary KIT exon 13 and 14 mutations, and possibly its anti-angiogenic activity.¹⁹ Sunitinib is particularly efficacious among GISTs harboring KIT exon 9 mutations.

Patients who have progressed on prior imatinib and sunitinib can receive third-line regorafenib, a multi-TKI that differs chemically from sorafenib by a fluorouracil group (fluoro-sorafenib). FDA approval of regorafenib was based on the phase 3 GRID (GIST Regorafenib In progressive Disease) multicenter international trial.²⁰ This trial randomly assigned 199 patients in a 2:1 fashion to receive regorafenib 160 mg daily for 21 days out of 28-day cycles plus best supportive care (BSC) versus placebo plus BSC. Cross-over was allowed. Regorafenib significantly reduced the hazard risk of progression by 73% compared with placebo (4.8 vs 0.9 months; HR, 0.27; P < 0.001). There was no difference in OS, which may be because of cross-over (median OS, 17.4 months in both arms). As a result, regorafenib is now considered standard third-line treatment for patients with metastatic GISTs. It has a less favorable toxicity profile than imatinib, with hand-foot syndrome, transaminitis, hypertension and fatigue being the most common treatment toxicities. In order to avoid noncompliance, it is recommended to start at 80 mg and carefully titrate upwards to the 160 mg dose.

A list of landmark studies for advanced GISTs is provided in Table 1.

A summary of FDA-approved drugs for treating GISTs is provided in Table 2.

Clinical Trials

Clinical trial enrollment should be considered for all patients with advanced or unresectable GISTs throughout their treatment continuum. Owing to significant advances in genomic profiling through next-generation sequencing, multiple driver mutations have recently been identified, and targeted therapies are being explored in clinical trials.²¹ For example, the neurotrophic receptor tyrosine kinase (NTRK) gene appears to be mutated in a small number of advanced GISTs, and these can respond to the highly selective TRK inhibitor larotrectinib.²² Additionally, ongoing studies are assessing immunotherapies for sporadic GISTs and treatment for familial GISTs (Table 3). Some notable studies include those assessing the efficacy of agents that target KIT and PDGFR secondary mutations, including avapritinib (BLU-285) and DCC-2618, MEK inhibitors, and the multi-kinase inhibitor crenolanib for GISTs harboring the imatinib-resistant PDGFRA D842V mutation. There are also studies utilizing checkpoint inhibitors alone or in combination with imatinib.

Case Conclusion

Given the patient’s progression on imatinib, he is started on second-line sunitinib malate. He experiences grade 1 fatigue and hand-foot syndrome, which are managed supportively. After he has been on sunitinib for approximately 8 months, his disease progresses. He subsequently undergoes genomic profiling of his tumor and starts BLU-285 on a clinical trial.

Key Points

• For advanced and metastatic disease, TKIs have substantially improved the prognosis of KIT mutated GISTs, with 3 FDA-approved drugs: imatinib, sunitinib, and regorafenib. Imatinib 400 mg is the standard-of-care frontline therapy for locally advanced, unresectable, or metastatic imatinib-sensitive GISTs. If a patient has a KIT exon 9 mutation and 400 mg is well-tolerated, increasing to 800 mg is recommended. Imatinib should be continued indefinitely unless there is intolerance, a specific patient request for interruption, or progression of disease.
• When there is progression of disease in a patient with a sensitive mutation on 400 mg of imatinib, the dose can be increased to 800 mg.
• For patients who are imatinib-intolerant or have progression, standard second line is sunitinib.
• For patients who further progress or are sunitinib-intolerant, regorafenib is the standard third-line treatment.
• There needs to be close attention to side effects, drug and food interactions, and patient copay costs in order to maintain patient compliance while on TKI therapy.
• There are still major limitations in the systemic treatment of GISTs marked by their inherent genetic heterogeneity and secondary resistance. Continued translational and clinical research is needed in order to improve treatment for patients who develop secondary resistance or who have less common primary resistant mutations. Patients are encouraged to participate in clinical trials of new therapies.

Summary

GISTs are the most common mesenchymal tumors of the GI tract. They comprise an expanding landscape of tumors that are heterogenous in terms of natural history, mutations, and response to systemic treatments. The mainstay of treatment for localized GISTs is surgical resection followed by at least 3-years of adjuvant imatinib for patients with high-risk features who are imatinib-sensitive. Patients with GISTs harboring resistance mutations such as PDGFRA D842V or with SDH-deficient or NF1-associated GISTs should not receive adjuvant imatinib. Patients with more advanced GISTs and/or in difficult to resect sites harboring a sensitive mutation can be considered for neoadjuvant imatinib. Those with metastatic GISTs can receive first-, second-, and third-line imatinib, sunitinib, or regorafenib, respectively. Clinical trial enrollment should be encouraged for patients whose GISTs harbor primary imatinib-resistant mutations, and those with advanced or unresectable GISTs with secondary resistance.