2O Association of RAD51 with homologous recombination deficiency (HRD) and clinical outcomes in untreated triple-negative breast cancer (TNBC): Analysis of the GeparSixto randomized clinical trial

Abstract

Current genetic and genomic tests that measure HRD show limited predictive value. We compare the performance of a functional HRD test based on scoring RAD51 nuclear foci with genetic/genomic HRD tests, and assess its capacity to select patients (pts) with primary TNBC sensitive to platinum-based neoadjuvant chemotherapy (NACT). A retrospective, blinded analysis from the GeparSixto randomized trial was conducted on TNBC pts who received neoadjuvant paclitaxel plus non-pegylated liposomal doxorubicin (Myocet®) and bevacizumab (PM) or PM plus carboplatin (PMCb). Functional HRD biomarkers (RAD51, BRCA1 and yH2AX nuclear foci) were quantified in formalin-fixed paraffin-embedded (FFPE) tumor samples on a tissue microarray format (TMA). Concordance analyses were performed between the RAD51 score and tumor BRCA (tBRCA) status or genomic HRD score (myChoice® CDx). Associations with clinical outcomes were studied by logistic (pathological complete response, pCR) and Cox (disease-free survival, DFS) regression models. Functional HRD was predefined as a RAD51 score ≤10% (RAD51-low). RAD51, BRCA1 and yH2AX were successfully scored in 133/200 TMA cores (67%). Functional HRD by RAD51-low was evidenced in 81/133 tumors (62%). The RAD51 test identified 93% of tBRCA-mutated tumors and 45% of the non-tBRCA mutant cases as functional HRD. The concordance between RAD51 and genomic HRD was 87% (95%CI 79-93%). In pts with RAD51-high tumors, the pCR rate was similar between treatment arms (PMCb 31% vs PM 39%, odds ratio (OR) 0.71, 0.23-2.24, p=0.561). Pts with RAD51-low tumors benefited from PMCb (66% vs 33%, OR 3.96, 1.56-10.05, p=0.004; interaction test p=0.023). The addition of Cb showed a trend towards better DFS in both RAD51-high (hazard ratio (HR) 0.40, 0.12-1.29, p=0.125) and RAD51-low (HR 0.45, 0.16-1.25, p=0.124) groups. The RAD51 test is highly concordant with tBRCA mutation and genomic HRD. RAD51 independently predicts clinical benefit from adding Cb to NACT in TNBC. Our results support further development to incorporate RAD51-testing in the clinical decision making.