Nationwide, approximately 1,000 adults are diagnosed with systemic mastocytosis annually. This rare disease is a myeloid neoplasm with a highly variable phenotypic expression, in which abnormal mast cells proliferate and infiltrate organs and tissues. It swings widely from a nonadvanced form, composed of indolent or smoldering disease, to advanced disease that progresses to leukemia in 6% of cases.
More than 80% of systemic mastocytosis is driven by the KIT D816V mutation. Along with a host of other rare KIT mutations, KIT D816V activates KIT-receptor tyrosine kinase to trigger mast cell proliferation.
Dr. Gotlib could not be contacted for an interview. However, there are many good reasons to identify patients with systemic mastocytosis, according to Attilio Orazi, MD, professor and chair of the department of pathology at Texas Tech University, El Paso. The chief reason is that the patient may be in grave peril.
“The degree of heterogeneity is amazing. ... There’s very indolent [disease], which is really not a big deal. And then you have a disease in which you’re dead in 3 months,” Dr. Orazi said. “So you run the gamut between an indolent, no-problem cutaneous disease to a very nasty systemic, aggressive leukemia-like neoplasm.”
Since 2001, the diagnosis of mastocytosis has been guided by the World Health Organization Classification of Tumours, or “Blue Book.” In 2022, Dr. Orazi along with 137 other senior experts, most of whom were involved in past editions of the Blue Book, published their own version: The International Consensus Classification of Myeloid Neoplasms and Acute Leukemias (the ICC 2022).
In September 2021, this group of specialists held a virtual/in-person advisory committee meeting at the University of Chicago to create the document. One factor in their decision to go it alone, Dr. Orazi said, was that WHO decided to proceed with the fifth edition of the Blue Book using its own internal editorial group without convening an advisory committee, despite repeated requests to do so.
ICC 2022 divides advanced systemic mastocytosis into three subtypes: aggressive systemic mastocytosis (ASM), systemic mastocytosis with an associated hematologic neoplasm (SM-AHN), and mast cell leukemia (MCL). Median survival is 3.5 years for patients with ASM, 2 years for those with SM-AHN and as low as 2 months for MCL.
The second key reason to increase awareness of mastocytosis among physicians, said Dr. Orazi, is that patients falling through the net are likely to be ambulatory, and their presentation can be “a little confusing.”
Patients with indolent disease are relatively straightforward to recognize, explained Dr. Orazi. Similarly, very sick patients with SM-AHN or MCL are easily recognized by hem-oncs.
“But if you see a patient in an ambulatory setting, in your clinic or whatever, and you’re suspicious, then you need to decide [how] you’re going to investigate that patient further,” he said, Dr. Orazi noted the next step is not always obvious, especially for primary-practice or internal medicine physicians likely to be unfamiliar with such a rare disease.
A practice survey published in 2022 by other researchers backed up Dr. Orazi’s remarks. The study found that community/solo-practice physicians were less likely to have tested systemic mastocytosis patients for KIT816V mutation than academic/specialty physicians (58% vs. 80%; P = .004; n = 111). Clinicians treating these patients estimated that it took an average of 8.5 months for a “typical” patient to receive the diagnosis from the time of symptom onset.
The research was headed by Ruben Mesa, MD, director of University of Texas Health, San Antonio, and funded by Blueprint Medicines, the manufacturer of avapritinib (Ayvakit), a new drug for the disease.
Dr. Orazi urged clinicians to have a high degree of suspicion for mastocytosis in a patient who walks into the clinic with any combination of the following: urticarial-type skin manifestations, especially if persistent into adulthood; history of undue reaction to an insect sting; a big spleen in a patient with a history of cutaneous flushing or rash; chronic diarrhea, especially if a biopsy has shown “too many mast cells” in the lamina propria of the small bowel; and positivity for KIT816V mutation.
Dr. Orazi stressed that the majority of patients will have indolent disease, but for the few patients for whom immediate treatment is essential, “the distinction between indolent and aggressive [disease] is really very, very important.”
Patients with advanced systemic mastocytosis can now be effectively treated, following the arrival of midostaurin (Rydapt, Tauritmo) and avapritinib.
Midostaurin, a multikinase/KIT inhibitor, was approved by the Food and Drug Administration in 2017 for the treatment of advanced systemic mastocytosis (ASM, SM-AHN, and MCL). Avapritinib, a selective kinase inhibitor of KIT816V and platelet-derived growth factor receptor alpha as well as multiple KIT exon 11, 11/17 and 17 mutants, gained the same indication in June 2021.
As with all rare diseases, it is challenging to obtain accurate numbers on how many patients are affected by systemic mastocytosis. The first population-based study of the disorder, presented at the 2018 annual meeting of the American Society of Hematology, used the Surveillance, Epidemiology, and End Results database from 2000 to 2014 to estimate incidence at 0.046 per 10,000, which translates to 1,050 new adult cases per year. The study data have never been published in full.
How many of these cases are advanced disease? There are no U.S. data but extrapolating from a Danish registry study that found 82% of systemic mastocytosis cases to be indolent disease, the incidence of advanced systemic mastocytosis in the United States could be as low as 200 adults a year.
This information, in turn, suggests that identifying more patients with advanced disease would not only benefit those patients but would also benefit clinical trial investigators who are seeking the proverbial needle in the haystack.
Nationwide, five clinical trials are recruiting individuals with advanced systemic mastocytosis, collectively looking for 352 patients in the United States. Two of the studies focus on mast-cell activation (NCT0544944) and cutaneous mastocytoses (NCT04846348). Two trials in a range of hematological malignancies are testing bispecific antibodies flotetuzumab and MGD024 (both from Macrogenics; NCT04681105, NCT05362773).
Apex, a phase 2 study of tyrosine-kinase inhibitor bezuclastinib (a Cogent hopeful), is specifically focusing on advanced disease. Dr. Gotlib and coinvestigators are aiming for 140 participants.
As a pathologist, Dr. Orazi said he find mastocytosis fascinating because he believes he has “a truly useful role,” contrasting with some other hematological diseases in which the molecular profile rules.
“Pathology plays a major role here,” he explained, “because you have to correlate what you see at the microscope with the full clinical picture, selected laboratory tests such as CBC and serum tryptase, and molecular results. You often need integration through a pathologist to put all the pieces together.
“It’s easier to treat once you know exactly what disease you’re dealing with and whether it is an aggressive or indolent subtype,” Dr. Orazi concluded.
Dr. Orazi disclosed no conflicts of interest. Dr. Gotlib has disclosed ties with Blueprint Medicines, Deciphera, Incyte, and Kartos Therapeutics, and has led committees for Blueprint Medicine’s EXPLORER and PATHFINDER studies, Deciphera’s Study Steering Committee for ripretinib in AdvSM, and the Central Response Review Committee for the phase 2 study of bezuclastinib in AdvSM.