Role and rationale for molecular testing in advanced prostate cancer

Dr. Concepcion is chief clinical urologist officer, Integra Connect, West Palm Beach, FL, and clinical associate professor of urology, Vanderbilt University School of Medicine, Nashville, TN.

Prostate cancer is a clinically heterogenous disease with variability in progression once diagnosed, ranging from the very indolent cases that may require no therapy to patients who present with de novo metastasis. In 2019, there were approximately 174,650 newly diagnosed prostate cancer cases in the United States and a cancer-specific mortality of 31,620 directly attributable to the disease or 5.2% of all cancer deaths.¹

A number of newer therapies (all mechanistically different) and treatment regimens have been approved for the management of both patients with metastatic castration-sensitive prostate cancer (mCSPC) and metastatic castration-resistant prostate cancer (mCRPC). A unique dynamic progressive model estimates the incidence of these two subsets may approach 42,970 patients in 2020.²

Unfortunately, despite the availability of superior agents, optimal sequences or a combination of these oncolytics have yet to determined, as there are no predictive biomarkers to inform the provider what is the most ideal initial line of therapy (LOT) and as patients progress, what will be the most appropriate next LOT. What makes this situation even more challenging is that these newer therapies, as well as those that we anticipate will be approved in 2020 and beyond, are targeted for molecular drivers of prostate cancer. For the patient with mCSPC or mCRPC, how can we best determine the initial and subsequent LOTs, given the limitations of the monotherapy registration trials?

A number of key genomic mutations have been consistently identified in patients with prostate cancer (hormone naive and mCRPC). These mutations include gene fusion/chromosomal rearrangements (TMPRSS2-ERG), androgen receptor (AR) amplification, inactivation of tumor suppressor genes (PTEN/PI3-K/AKT/mTOR, TP53, Rb1), and oncogene activation (c-myc, RAS-RAF).³ More significantly, defects in DNA repair appear to be central in increasing one’s susceptibility to malignant transformation.

Germline vs. somatic mutations

It is critical to patient management that we determine whether these mutations are inherited (germline) or acquired (somatic). Germline mutations are changes in DNA that are present in the patient’s reproductive cells (sperm or ovum) and are thus passed from generation to generation and will be identified in every cell of the body. Therefore, germline testing can be conducted with just a swab from the mouth, saliva, or blood from the patient. There are many companies in the United States that currently offer germline testing.

It is paramount that in order to obtain the most comprehensive analysis and report, genetic testing through next-generation sequencing in a diagnostic laboratory is mandatory. This type of testing should be compared with many of the direct-to-consumer tests that are currently marketed to patients. The testing platforms deployed by many of these companies are much less robust and often include a very limited number of known genetic mutations in their panels.

For example, thousands of identified BRCA mutations have been identified, but only a handful may be tested in some of these direct-to-consumer testing kits. This situation can lead to an unacceptable number of studies with false-negative results and should not be used for clinical decision-making.

Acquired or somatic mutations can be defined as any alteration in DNA that occurs after conception. These can occur in any cell of the body (except the reproductive cells) and usually arise as a result of exogenous or environmental exposures, such as tobacco smoking or UV radiation. Therefore, somatic testing requires next-generation sequencing of cells extracted from the tumor itself and cannot be performed by using a sample of saliva or blood.

Pritchard and colleagues were among the first to demonstrate the value of assessing inherited genetic changes in prostate cancer. Among 692 patients with metastatic prostate cancer, they examined the prevalence of mutations in 20 DNA repair genes.⁴ Mutations were identified in 82 men (11.8%) with significant geographic heterogeneity, even among these recognized cancer centers (prevalence of 8.8% in patients treated at the University of Washington and 18.5% in those treated at Memorial Sloan Kettering), potentially reflecting referral biases. Subsequently, Castro et al found a prevalence of germline DNA damage repair gene mutations of 16.2% in patients with mCRPC.⁵

Unlike other disease states in which commonly identified germline mutations may be actionable, actionable germline mutations are relatively uncommon in patients with prostate cancer. Nicolosi and colleagues found that actionable mutations were identified in 1.74% of their study cohort with a diverse patient population.⁶ In previous analyses, Robinson et al reported clinically actionable pathogenic germline mutations in 8% of 150 patients with mCRPC, in contrast to clinically actionable aberrations in the AR in 63% and aberrations in other cancer-related genes in 65% of patients.⁷ It is likely not surprising that actionable underlying germline mutations would be more common in a cohort with more advanced prostate cancer.

In patients with regional or metastatic prostate cancer, somatic tumor testing may also be considered on the basis of the observation that nearly 90% of men have potentially actionable mutations at the tumor level, whereas only a relatively small proportion of men would have actionable germline mutations (approximately 9% of patients with mCRPC, according to the National Comprehensive Cancer Network). In these patients, testing may be undertaken for somatic homologous recombination repair (HRR) gene mutations (eg, BRCA1, BRCA2, ATM, PALB2, FANCA, RAD51D, and CHEK2) and for microsatellite instability (MSI) or mismatch repair (MMR).⁷

In patients with advanced prostate cancer, identification of underlying germline mutations may guide treatment selection to determine the most appropriate next LOT, especially in those who have progressed through multiple lines of prior therapy, including AR signaling agents. Patients with identified MSI-high status, defects in DNA MMR genes, or CDK12 biallelic loss may respond to checkpoint inhibition therapy.⁸

Pembrolizumab (KEYTRUDA), an FDA-approved PD-1 inhibitor, is the first immunotherapy to win approval in a tumor-agnostic manner and not based on organ type. Further, patients with mutations in HRR genes (including BRCA1/2, CHEK2, and genes that cause Fanconi anemia) may be better suited for treatment with PARP inhibitors, many of which are in ongoing phase III trials with expected approval in 2020.

Finally, patients with DNA repair defects may have increased sensitivity to platinum-based chemotherapeutics.⁹ Given the uncertainty regarding optimal treatment selection and pending approval of current agents in trial, the National Comprehensive Cancer Network prostate cancer guideline panel recommends clinical trial enrollment for all men with prostate cancer and identified DNA repair gene mutations. In addition, somatic testing for specific gene variants may be undertaken.

For the most part, this approach is used in patients with advanced disease with the goal of identifying specific actionable targets. For example, mutations in HRR or MMR genes and identification of MSI-high versus MSI-low status may suggest certain treatments are more likely to be beneficial.

In addition to genetic testing of tumor tissue, assessing circulating tumor cells may offer important information. For example, testing of AR variant status can be performed using circulating tumor cells and may be predictive of disease.

Generally, genetic testing yields results that are unambiguous and will show that a gene mutation is present or absent. However, the reporting of the significance and association of that mutation relative to a disease state can be quite variable. Given that the coding sequence for a particular gene has been defined and the sequencing machines are fairly similar, what is considered “positive” or deleterious versus “negative” or favorable/no mutation relative to risk of disease is predicated on the number of patients tested.

As noted, a number of genes have been identified as associated with an increased susceptibility risk for prostate cancer. Multigene panels are becoming used more often, but the makeup of these panels is not uniform. A recent analysis looking at various commercially available multigene panels shows that the average number of genes tested is 12 (range, 4-16). BRCA1/BRCA2 are included in all the panels, but 20% did not include HOXB13 or MMR genes.¹⁰

The clinical experience and number of patients tested with BRCA1/2 is more extensive compared with other genes. More and more mutations continue to be discovered, but the significance to the patient has yet to be determined until even further samples are processed. These discoveries, classified as variants of unknown significance (VUS), represent a gray area in which there is a change in the genetic sequence; however, it is still unknown whether this change is associated with a deleterious or favorable prognosis. Among women with breast cancer, detection of a VUS is more common than identification of known pathogenic variants.¹¹ Although ongoing work seeks to better delineate the importance of these VUS, the involvement of a genetic counselor is key to helping patients navigate this uncertain situation.

Conclusion

Urologists will need to incorporate comprehensive genomic testing, just as we embraced PSA testing back in the 1990s. A recent survey conducted among 52 single-specialty independent urology community practices identified the following three issues related to incorporation and development of a comprehensive testing program¹²:

• medical/legal liability for unaddressed identified mutations

• reimbursement concerns and cost of testing

• complexity and time involved to enter a complete family history and pedigree into the electronic health record.

None of these considerations, however, is insurmountable if the practice has a commitment to enhance and deliver precision medicine for our patients with prostate cancer.

References

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