Liquid Biopsies in a Veteran Patient Population With Advanced Prostate and Lung Non-Small Cell Carcinomas: A New Paradigm and Unique Challenge in Personalized Medicine

Discussion

The veteran patient population is a unique cohort due to its distinct demographic characteristics with a high volume of cancer cases diagnosed each year. According to data from VA Central Cancer Registry (VACCR), the most frequently diagnosed cancers are prostate (29%) and lung (18%).⁶

Liquid biopsy is a novel, promising technology that uses ctDNA and circulating tumor cells in peripheral blood for detecting genetic alterations through next generation sequencing.^7-9 The advent of this minimally invasive, revolutionary technology has been a breakthrough in the field of precision oncology for prognosis, to monitor treatment response or resistance to therapy and further personalize cancer therapy.^9,10

Comprehensive genomic profiling by liquid biopsy has many advantages over the molecular studies performed on tissue biopsy. Due to the tumor heterogeneity, tissue samples may not represent the full profile of the tumor genomics of cancer, while liquid biopsy has full presentation of the disease.^11,12 Many times, tissue biopsy may be limited by a sample size that precludes full genetic profiling in addition to higher total cost, potential technical issues during processing, and possible side effects of the biopsy procedure.^7,13 Additionally, as the tumor progresses, new driver mutations other than the ones previously detected on the primary tissue may emerge, which can confer resistance to the existing therapy.^7,13

Advanced prostatic and lung carcinomas with biochemical, distant organ, or bony progression harbor unique signature genetic mutations indicating poor prognosis, lack of response or resistance to the existing therapy, and high risk of relapse.^14,15 Some of the unique characteristics of the veteran patient population include a more aged patient population multiple comorbidities, higher frequency of > 1 type of cancer, advanced cancer stage at presentation, and specific risks factors such as exposure to Agent Orange in veterans who served during the Vietnam War era.^16,17 We studied the utility of liquid biopsy in cancer care, including type and incidence of genomic alterations associated with advanced prostate and lung cancers, in this unique patient population.

The amount of cell-free DNA (cfDNA), also known as ctDNA varies widely in cancer patients. Some of the factors associated with low concentration of cfDNA are disease stage, intervening therapy, proliferation rates, and tumor vascularization.^18,19 In the peripheral blood, of the total cfDNA, fractions of cfDNA varies from 0.01 to 90%.^18,19 All samples containing ≥ 20 ng cfDNA (20 - 100 ng) were subjected to the hybrid capture-based NGS FoundationACT assay.²⁰ In our study, 2 specimens did not meet the minimum criteria of adequacy (20 ng cfDNA); however, the overall adequacy rate for the detection of mutation, irrespective of the cancer type was 29 of 31 (93.5%) with only 2 inadequate samples. This rate is higher than the rate reported in the literature, which is about 70%.²⁰

Significant differences were encountered in the incidence of DNA damage repair genes including Tp53 mutations when compared with those in the general patient population (Table 2). According to recent National Comprehensive Cancer Network (NCCN) guidelines, all prostate cancers should be screened for DDR gene mutations as these genes are common in aggressive prostate cancers and strongly associated with poor outcomes and shortened survival. Due to relatively high frequency of DDR gene mutations in advanced prostatic cancers, liquid biopsy in patients with these advanced stage prostate cancers may be a useful tool in clinical decision making and exploring targeted therapy.²⁰

We also evaluated the frequency of the most commonly occurring genetic mutations, Tp53 in advanced solid malignancies, especially advanced prostate and NSCLC. Previous studies have reported Tp53 mutation in association with risk factors (carcinogens) of cancer and have been a surrogate marker of poor survival or lack of response of therapy.²⁵ Knowledge of Tp53 mutation is crucial for closer disease monitoring, preparing the patient for rapid progression, and encouraging the physician to prepare future lines of therapy.^25-27 Although Tp53 mutation varies with histologic type and tissue of origin, Beltran and colleagues reported it in 30 to 40% of tumors, while Robles and colleagues reported about 40 to 42% incidence.^25,27

Our study showed notable mutations in 23 of 29 adequate cases. Further, our study showed a high frequency of mutated Tp53 in 65.2% of combined advanced prostate and NSCLC cases. We then correlated cases of Vietnam War-era veterans with risk potential of Agent Orange exposure and Tp53 mutation. We found 7 of 15 Vietnam War-era veterans were positive for Tp53 mutations irrespective of the cancer type. The high incidence of Tp53 mutations in advanced prostate and lung carcinomas in the veteran patient population makes this tumor marker an aspiration not only as a surrogate of aggressive disease and tumor progression, but also as a key marker for targeted therapy in advanced prostate and lung cancers with loss of Tp53 function (Figure 3).

Mutations and amplifications in the AR gene are fundamental to progression of prostate cancer associated with advanced, hormone-refractory prostate cancer with the potential for targeted therapy with AR inhibitors. In our study, AR amplification was detected in 4 of 21 (19%) advanced prostate cancer cases, which is significantly lower than the 30 to 50% previously reported in the literature.^28-32 Neither AR amplification or mutation was noted in advanced NSCLC in our study as previously reported in literature by Brennan and colleagues and Wang and colleagues.^33-35 This is significant as it provides a pathway for future studies to focus on additional driver mutations for targeted therapies in advanced prostate carcinoma. To date, AR gene mutation does not play a role for personalized therapy in advanced NSCLC. Perhaps, a large cohort study with longitudinal analysis is needed for absolutely ruling out the possibility of personalized medicine in advanced lung cancer using this biomarker.