Biomarker-guided PARP inhibition shows durable benefit in hormone-sensitive prostate cancer

The TALAPRO-3 trial clinical trial published at the end of May in the New England Journal of Medicine found that adding the PARP inhibitor talazoparib to enzalutamide significantly extended progression-free survival in men with metastatic hormone-sensitive prostate cancer who carry alterations in homologous recombination repair genes, offering new evidence that biomarker-driven treatment decisions can meaningfully change the disease trajectory earlier in the cancer's course.

Neeraj Agarwal, M.D., a professor of medicine at the Huntsman Cancer Institute at the University of Utah, and team enrolled 599 patients whose tumors were screened for alterations across a 12-gene homologous recombination repair panel that included ATM, ATF, BRCA1, BRAC2, CDK12, CHEK2, FANCA, MLH1, MRE11A, NBN, PALB2 and RAD51C. Eligible patients were men 18 years of age or older (20 years of age or older in Japan) with histologically or cytologically confirmed adenocarcinoma of the prostate who had alterations in one or more homologous recombination repair genes. Eligibility criteria included metastatic disease documented by a positive bone scan for bone disease or computed tomography or magnetic resonance imaging (MRI) for soft-tissue disease and an Eastern Cooperative Oncology Group performance-status score of 0 or 1 (on a scale of 0 to 5, with higher scores indicating greater disability). Patients with metastatic APMS prostate cancer could have received up to 3 months of androgen-deprivation therapy with or without an approved androgen-receptor pathway inhibitor. After a protocol amendment in September 2021, patients who had received previous docetaxel therapy could not enroll. Patients who had not undergone bilateral orchiectomy must have been receiving ongoing androgen-deprivation therapy with a gonadotropin-releasing hormone agonist or antagonist. All the patients provided written informed consent.

At three years, 77% of patients receiving talazoparib plus enzalutamide remained free from imaging-based disease progression, compared with 56% in the placebo plus enzalutamide group, a hazard ratio of 0.48 that the researchers called statistically and clinically significant.

Until now, evidence supporting PARP inhibitor use in prostate cancer was largely confined to the castration-resistant setting, a later, more treatment-refractory stage of the disease. TALAPRO-3 was designed to test whether moving this strategy earlier, into what investigators now call androgen pathway modulation-sensitive, or APMS, disease, would benefit the substantial proportion of patients whose tumors harbor DNA repair defects.

That proportion is not small. The authors note that homologous recombination repair gene alterations are found in approximately 29% of men with metastatic hormone-sensitive prostate cancer and are associated with worse outcomes regardless of disease volume or treatment regimen. By targeting this molecularly defined population up front, investigators sought to intervene before resistance mechanisms emerge.

One of the trial's most clinically significant findings was the breadth of benefit observed across the biomarker landscape. While patients with BRCA1 or BRCA2 alterations showed the greatest improvement, a hazard ratio of 0.37 for disease progression or death, patients with non-BRCA alterations also derived meaningful benefit, with a hazard ratio of 0.57.

Exploratory analyses suggested particularly notable signals in patients with CDK12 alterations, where the hazard ratio was 0.28, and ATM alterations, where it was 0.43. The authors cautioned that small patient numbers in individual gene subgroups limit firm conclusions, but the directional consistency across genes reinforces the case for comprehensive molecular profiling rather than BRCA-only testing.

Agarwal and team were direct about the policy implications. Identifying patients who carry these alterations requires upfront molecular testing, an area where clinical practice has lagged behind the science. The investigators argued that TALAPRO-3, alongside emerging data from the AMPLITUDE trial evaluating niraparib plus abiraterone in the same setting, underscores the importance of routine genomic testing in men newly diagnosed with metastatic prostate cancer.

"These findings highlight the importance of molecular testing in men with metastatic APMS prostate cancer," Agarwal and his co-authors wrote, noting that biomarker-driven strategies could be deployed earlier in the disease course.

One drawback of the combination is what appears to be a meaningful toxicity burden. Grade 3 or higher adverse events were nearly twice as common in the talazoparib group as in the control group, 81% versus 44%. Anemia was the dominant concern, occurring at grade 3 or higher in 51% of patients receiving talazoparib, though the authors noted that proactive dose management allowed most patients to remain on treatment.