Treatment Outcomes and Tumor Loss of Heterozygosity in Germline DNA Repair-deficient Prostate Cancer

Background: Germline mutations in DNA repair genes were recently reported in 8-12% of patients with metastatic castration-resistant prostate cancer (mCRPC). It is unknown whether these mutations associate with differential response to androgen receptor (AR)-directed therapy.

Objective: To determine the clinical response of mCRPC patients with germline DNA repair defects to AR-directed therapies and to establish whether biallelic DNA repair gene loss is detectable in matched circulating tumor DNA (ctDNA).

Design, Setting, And Participants: We recruited 319 mCRPC patients and performed targeted germline sequencing of 22 DNA repair genes. In patients with deleterious germline mutations, plasma cell-free DNA was also sequenced.

Outcome Measurements And Statistical Analysis: Prostate-specific antigen response and progression were assessed in relation to initial androgen deprivation therapy (ADT) and subsequent therapy for mCRPC using Kaplan-Meier analysis.

Results And Limitations: Of the 319 patients, 24 (7.5%) had deleterious germline mutations, with BRCA2 (n=16) being the most frequent. Patients (n=22) with mutations in genes linked to homologous recombination were heterogeneous at initial presentation but, after starting ADT, progressed to mCRPC with a median time of 11.8 mo (95% confidence interval [CI] 5.1-18.4). The median time to prostate-specific antigen progression on first-line AR-targeted therapy in the mCRPC setting was 3.3 mo (95% CI 2.7-3.9). Ten out of 11 evaluable patients with germline BRCA2 mutations had somatic deletion of the intact allele in ctDNA. A limitation of this study is absence of a formal control cohort for comparison of clinical outcomes.

Conclusions: Patients with mCRPC who have germline DNA repair defects exhibit attenuated responses to AR-targeted therapy. Biallelic gene loss was robustly detected in ctDNA, suggesting that this patient subset could be prioritized for therapies exploiting defective DNA repair using a liquid biopsy.

Patient Summary: Patients with metastatic prostate cancer and germline DNA repair defects exhibit a poor response to standard hormonal therapies, but may be prioritized for potentially more effective therapies using a blood test.