Background: PARP inhibitors (PARPi) are approved for the treatment of metastatic prostate cancer (mPC) associated to various DNA damage repair (DDR) gene mutations; but clinical benefit differs among patients. Biomarkers of homologous recombination repair (HRR) deficiency may help refine patient stratification for a more precise therapy selection. We report an exploratory analysis from the TOPARP-B phase II clinical trial of olaparib in mPC (NCT01682772), investigating the predictive value of an HRR function assay detecting RAD51 foci by immunofluorescence in tumor biopsies. Design: We analyzed formalin-fixed paraffin-embedded (FFPE) primary or metastatic biopsies from mPC patients treated with olaparib in the clinical trial. We evaluated baseline HRR function based on detection of RAD51 and γH2AX foci in geminin-positive tumor cells by immunofluorescence (IF). All samples were scored by two trained readers blinded to genomic and clinical data. Samples were considered HRR deficient (HRD) when RAD51 scores were low, pre-defined as <10% tumor cells presenting ≥5 RAD51 foci/cell. The association of the RAD51 score, response to olaparib and survival (radiographic progression-free survival, rPFS, and overall survival, OS) was analyzed by Chi-Square and log-rank tests. Results: RAD51 and γH2AX were successfully scored in 52 cases, in the same biopsies previously used for NGS in the clinical trial. All tumors showed abundant DNA damage (γH2AX scores >40%). The intra-class correlation score (ICC) between the two blinded readers was 0.88. Overall, 22 of 52 (42%) cases were considered as HRD based on low RAD51 scores. Response rate (based on the composite RECIST/PSA/CTC trial criteria) was 15/22 (68%) vs 7/30 (23%) for patients with low vs high RAD51 scores (p=0.001). Patients with low RAD51 scores also had longer rPFS (median 9.3 vs 2.9 months p=0.002) and overall survival (median 17.4 vs 9.5 months, p=0.05) from initiation of olaparib. All 16/16 cases with BRCA1/2 alterations were identified as RAD51 low. For patients with PALB2 mutations, 2/2 patients with biallelic loss showed RAD51 low scores and responded to olaparib, whereas 2/2 patients with monoallelic PALB2 mutations showed RAD51 high scores and did not respond to olaparib. Mutations in ATM and CDK12 did not associate with low RAD51. Indeed, 10/11 ATM-mutated and 8/10 CDK12-mutated tumors presented high RAD51 scores; RECIST/PSA responses were observed in two patients with ATM mutations and high RAD51 scores. Conclusion: A RAD51-based IF assay performed on FFPE biopsies can detect prostate cancers with deficient HRR function, including BRCA1/2 and biallelic PALB2 mutated cases, and may be useful for patient stratification for PARP inhibitor treatment in prostate cancer. Further validation of the assay in larger cohorts is warranted.

Citation Format: Sara Arce-Gallego, Alba Llop-Guevara, Suzanne Carreira, Nuria Porta, Roberta Fasani, Diletta Bianchini, George Seed, Pasquale Rescigno, Alec Paschalis, Claudia Bertan, Chloe Baker, Jane Goodall, Susana Miranda, Ruth Riisnaes, Ines Figueiredo, Ana Ferreira, Rita Pereira, Bora Gurel, Daniel Nava Rodrigues, Wei Yuan, Jan Rekowski, Emma Hall, Violeta Serra, Johann S. de Bono, Joaquin Mateo. A homologous recombination repair (HRR) functional assay to stratify patients with metastatic prostate cancer for PARP inhibitor treatment in the TOPARP-B clinical trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT161.