Association of plasma DNA repair deficient (DRD) status with clinical outcome of metastatic castration-resistant prostate cancer (mCRPC) patients (pts) treated with abiraterone acetate (AA) plus prednisone/dexamethasone (+P/D)

5066 Background: Somatic DRD deficiency in 15-30% of mCRPC pts have been observed and inhibition of enzyme poly ADP ribose polymerase (PARP) could prove beneficial. We aimed to define DRD status using plasma from pts treated on AA and evaluate associations with prospectively-collected outcome measures. Methods: Plasma DNA samples (128 baseline [BL], 134 cycle 2 day 1 [C2D1], 46 progression [PROG]) from chemotherapy-naïve mCRPC pts in a phase 2 study (NCT01867710) evaluating AA+P/D were subjected to custom target-capture next-generation sequencing. DRD assay was optimized and validated to detect pathogenic point mutations, small insertions/deletions, and copy number alternations (DRD+) in 8 DRD genes: BRCA1, BRCA2, FANCA, ATM, CHEK2, HDAC2, BRIP1, and PALB2. Analysis for genomic aberrations was a secondary exploratory objective. Associations with overall survival (OS), progression-free survival (PFS), and radiographic PFS (rPFS) were assessed using Cox regression models and Kaplan Meier analyses. Results: 11.7% of BL and 17.4% of PROG were DRD+. Bi-allelic was observed in 73.3% of BL DRD+ samples. Shorter PFS was observed in BL DRD+ vs DRD- (5.3 vs 15.5 mo; HR: 2.32; 95%CI:1.39-4.28; P < 0.002). Median PFS for BL DRD biallelic + vs DRD biallelic- was 5.1 vs 15.4 mo (HR: 2.49; 95% CI: 1.23-4.38; P < 0.0095). For multivariate analysis using DRD+, ALP, and LDH as covariates, DRD+ (HR: 2.1; 95% CI: 1.18-3.75; P < 0.012) and high ALP (HR: 1.66; 95% CI: 1.08-2.56; P < 0.021) were strongly associated with worse PFS. Median OS for BL DRD+ vs DRD- was 28.8 vs 41.3 mo (HR: 1.67; 95%CI:0.88-3.18; P = 0.116). Median rPFS for BL DRD+ vs DRD- was 16.2 vs 20.9 mo (HR: 1.64; 95%CI:0.83-3.21; P = 0.152). Of 39 Pts with BL, C2D1 and PROG samples, 3 were DRD+ (7.7%) at all 3 timepoints, 3 (7.7%) only at BL, 3 (7.7%) only at PROG (bi-allelic), 2 (5.1%) had extra deletion at PROG. Conclusions: Patients with mCRPC harboring DRD+ have worse outcomes with AA and represent a population with an unmet medical need.