# Emergence of BRCA Reversion Mutations in Prostate Cancer Prior to PARP Inhibitor Exposure: Clinical and Therapeutic Implications

## TL;DR

This study finds that BRCA reversion mutations can occur in prostate cancer patients before they receive PARP inhibitors, often after chemotherapy or radiation, which may lead to resistance.

## Contribution

The study reveals that BRCA reversion mutations can emerge in prostate cancer prior to PARP inhibitor treatment, suggesting DNA-damaging therapies may promote resistance.

## Key findings

- BRCA reversion mutations were detected in 30% of patients without prior PARP inhibitor exposure.
- Patients with prior chemotherapy had shorter PARP inhibitor response durations compared to chemotherapy-naïve patients.
- DNA-damaging therapies like chemotherapy and radiation may promote BRCA reversion mutations and primary resistance.

## Abstract

Inactivating BRCA1/2 mutations confer sensitivity to poly(ADP‐ribose) polymerase inhibitors (PARPi) in prostate cancer (PCA). However, secondary BRCA1/2 reversion mutations (BRCArev) can restore BRCA function and mediate acquired PARPi resistance. While BRCArev typically arise under PARPi selective pressure, their occurrence in PARPi‐naïve settings remains incompletely understood. We sought to characterize the frequency, clinical context, and therapeutic correlates of BRCArev detection in men with advanced PCA, including those without prior PARPi exposure.

We restrospectively analyzed clinical liquid biopsy results from men with PCA using FoundationOneLiquid CDx between January and December 2023. BRCArev were defined as sequence alterations predicted to restore open reading frame of BRCA1 or BRCA2 harboring pathogenic inactivating variants. Clinical and treatment histories, including chemotherapy and PARPi exposure, were abstracted from medical records.

Over a one‐year period, we identified 10 PCA patients with inactivating BRCA1/2 alterations, BRCArev and available treatment history. BRCArev were detected in three of 10 (30%) patients who had not received prior PARPi therapy. All three PARP‐naïve patients had previously received chemotherapy (platinum or docetaxel) and radiation, and each exhibited multiple BRCArev events. The remaining 7 patients (70%) had prior olaparib exposure. Among these, the duration of PARPi response was longer in chemo‐naïve patients compared with previously treated with chemotherapy (median: 22 vs. 7.5 months, respectively).

BRCArev can emerge in PCA in the absence of prior PARPi therapy, particularly following exposure to cytotoxic chemotherapy and radiation. These findings support that DNA‐damaging therapies may promote BRCArev formation, potentially predisposing to primary PARPi resistance. Early integration of PARPi therapy before chemotherapy may enhance clinical benefit and circumvent emergence of primary resistance.