Phase II study of niraparib in patients with metastatic castration-resistant prostate cancer (mCRPC) and biallelic DNA-repair gene defects (DRD): Preliminary results of GALAHAD.

202 Background: New therapies are needed for patients (pts) with mCRPC progressing after androgen-receptor signaling inhibitors (ARSIs) and taxane therapies. Niraparib is a once daily highly selective inhibitor of poly (ADP-ribose) polymerase (PARP-1 and 2). Methods: GALAHAD is an ongoing open label Ph 2 study assessing niraparib (300 mg daily) in pts with DRD progressing on/after ARSIs and taxane chemotherapy. Using a validated plasma assay, DRD status was defined as pathogenic mutations (including homozygous deletions) of BRCA1/2, ATM, FANCA, PALB2, CHEK2, BRIP1 or HDAC2. Composite response rate (RR) was defined as an objective response by RECIST 1.1 for measurable disease, circulating tumor cell (CTC) conversion to < 5/7.5 mL blood or prostate-specific antigen (PSA) decline of ≥50% (PSA 50 ). Here, preliminary data on RR and adverse events (AEs) are reported in pts with biallelic DRD. Results: As of 10 Sep 2018, 123 pts with mCRPC and DRD were enrolled, of whom 39 had biallelic DRD (23 BRCA1/2). The median follow-up was 5.7 mo (2.0–23.7). Table depicts RRs for pts with biallelic DRD by BRCA status. Composite and objective RRs were 65% and 38% in pts with biallelic BRCA1/2, respectively. 3/8 pts (38% [2/5 BRCA1/2 and 1/3 non-BRCA]) with measurable visceral metastases showed objective response. Among the 20 biallelic responders, the duration of treatment (tx) has exceeded 4 mo in 13 pts and 6 mo in 8 pts; 14 pts remain on tx. The most common grade 3/4 hematologic AEs were anemia (25%) and thrombocytopenia (15%) (manageable by dose reduction/interruption). The most common grade 3/4 nonhematologic AEs were asthenia (6%) and hypertension (5%). Conclusions: These results suggest niraparib has compelling activity as monotherapy for pts with treatment-resistant mCRPC, particularly those with biallelic BRCA1/2 identified by a blood assay. Clinical trial information: NCT02854436. [Table: see text]