Olaparib is the first Health Canada-approved agent in metastatic prostate cancer to use a companion diagnostic to identify alterations in BRCA1, BRCA2, or ATM. As olaparib is introduced, clinicians must learn to access and interpret germline and somatic next-generation sequencing (NGS) results, and how to manage affected patients who appear to have distinct clinical features. The traditional model of referring patients to a hereditary cancer clinic (HCC) for germline testing is likely impractical in this disease, as the metastatic prostate cancer patient population would be overwhelming. Alternate approaches to this are clinician-ordered genetic testing (so-called “mainstreaming”), out-of-pocket payment for third-party private company genetic testing, or germline testing done in conjunction with somatic testing, particularly cell free circulating tumor DNA (ctDNA).
Germline testing alone is not sufficient for identifying Olaparib-eligible patients, as less than half of BRCA1, BRCA2, or ATM alterations are germline in origin, but it is critically important to identify family members who are carriers so that risk-reduction measures can be undertaken. Somatic testing is not widely available in Canada, but some patients can access it through research protocols or by paying out-of-pocket. Somatic testing can be performed on archival or fresh solid tissue biopsy samples, or through whole blood samples to access plasma-derived circulating tumor DNA (ctDNA). Both testing approaches have relative advantages and disadvantages, but neither may be informative in all patients and, therefore, ideal somatic NGS pathways should provide options for both tissue and ctDNA testing.
We advocate that clinicians begin discussions with their provincial lab formularies, HCC, and molecular pathology labs to highlight the importance of germline and somatic testing in this population and identify pathways for patient access. While olaparib has approval for use in BRCA1, BRCA2, and ATM-altered mCRPC, emerging evidence suggests that PARP inhibitors have variable activity in these three genes, with BRCA2 alterations appearing to be the most responsive. Retrospective and prospective series have reported varying outcomes to standard of care therapies, such as ARATs and taxane-based chemotherapy, in metastatic castration-resistant prostate cancer (mCRPC) patients with DNA damage repair (DDR) gene alterations, such as BRCA2. In the absence of high-level evidence showing a lack of benefit, we believe this patient population should still be considered for these treatments.
In addition, platinum-based chemotherapy appears to have activity in DDR gene-altered mCRPC and should be considered another option when access to olaparib is not possible.
At present, there is no evidence to support an optimal treatment sequence in this patient population, therefore, physician and patient preferences will need to be taken into consideration when selecting therapies. As olaparib and other PARP inhibitors are tested in different disease states and in combination with other therapies, we will likely see a more refined approach to use of these agents and management of this new biomarker-defined patient population.
Circulating Tumor DNA Abundance and Potential Utility in De Novo Metastatic Prostate Cancer