The unselected PROPEL trial showed a broad population benefit, but regulators ultimately restricted its PARP+ARPI approval to BRCA-mutated patients. This aligns with the MAGNITUDE trial, which used prospective selection and halted its non-biomarker arm for futility, validating the necessity of pre-planned genomic stratification.

In prostate cancer, BRCA2 mutations typically involve complete gene loss, making them a significant oncogenic event. In contrast, BRCA1 mutations are often "passenger" mutations without complete loss of function, leading to reduced benefit from PARP inhibitors. This differs from breast and ovarian cancers, where both are highly significant.

Patients with BRCA-mutated prostate cancer who progress on first-line PARP inhibitors have very poor options. Standard therapies like docetaxel are largely ineffective, and emerging data suggests cross-resistance with Lutetium, creating a significant unmet clinical need for novel approaches.

When a patient has a BRCA2 mutation, clinicians on the panel view it as such a dominant predictive biomarker that they would prioritize a PARP inhibitor-based triplet regimen. This single genetic finding often outweighs other clinical factors or even the potential addition of docetaxel in treatment decisions.

Not all DNA damage repair gene alterations create PARP inhibitor sensitivity. Clinical data from multiple trials (TRITON, PROfound, TALAPRO-2) consistently shows that while BRCA1/2 mutations confer significant benefit, alterations in genes like ATM and CHEK2, which are not core to homologous recombination repair (HRR), do not.

Three 2025 trials (AMPLITUDE, PSMA-addition, CAPItello) introduced personalized therapy for metastatic hormone-sensitive prostate cancer. However, significant benefits were confined to narrow subgroups, like BRCA-mutated patients. This suggests future success depends on even more stringent patient selection, not broader application of targeted agents.

For a newly diagnosed metastatic prostate cancer patient, an effective strategy is to initiate ADT alone while immediately ordering NGS testing. Waiting a few weeks for the genetic results before adding an ARPI allows for a more informed treatment choice, such as selecting a PARP inhibitor combination for a patient with a BRCA2 mutation.

A unique three-arm trial allowing crossover between single-agent PARP inhibitors, AR inhibitors, and a combination showed superior outcomes for the upfront combination. This suggests that "saving" a therapy for later is a suboptimal strategy for this biomarker-selected patient population.

Experts advise using PARP inhibitors at the earliest opportunity for patients with BRCA mutations. As prostate cancer advances, it develops additional drivers of disease and intrinsic resistance, which can render targeted therapies like PARP inhibitors less effective if they are reserved for later lines of treatment.