Abstract 2476: Preclinical assessment of the PARP inhibitor rucaparib in homologous recombination deficient prostate cancer models

Abstract

Abstract Rucaparib (CO-338) is an oral small molecule inhibitor of the poly(ADP-ribose) polymerase (PARP) enzymes PARP-1, PARP-2 and PARP-3 that is being developed for cancer patients with evidence of homologous recombination (HR) deficiency, including BRCA1 and BRCA2 mutations. PARP inhibitors have demonstrated clinical activity in BRCA1, BRCA2, and ATM mutant prostate cancer patients (Mateo et al. N Engl J Med. 2015;373:1697-708); however, limited preclinical validation of PARP inhibitors in prostate cancer has been reported to date, since no BRCA mutant nor HR deficient prostate cell lines have been identified. In this study, we modelled the functional inactivation of DNA repair genes using siRNA and CRISPR/Cas9, and examined the impact on PARP inhibition, in the DU145 and PC-3 prostate cancer cell lines. These cell lines were chosen based on transfection efficiency, lack of obvious genetic defects in DNA repair pathways, and baseline insensitivity to rucaparib (IC50 > 10 µM). A panel of 26 DNA repair genes were individually knocked down by siRNA, and changes to rucaparib potency were examined in a 7-day cell viability assay. The knockdown of BRCA1 or BRCA2 increased rucaparib cytotoxicity in both prostate cell lines. For example, the IC50 of rucaparib in DU145 and PC-3 cells with reduced BRCA2 levels was 275 and 297 nM, representing a 36- and 37-fold increase in rucaparib sensitivity, respectively. In addition to BRCA1 and BRCA2, the knockdown of several other genes including BARD1, CDK12, FANCA, PALB2, RAD51, RAD51C, RAD51D, and RAD54L increased rucaparib efficacy ≥ 2-fold in at least 1 cell line examined. To further investigate HR deficiency in prostate cancer, BRCA2 was knocked out in the human 22Rv1 prostate cancer cell line using CRISPR/Cas9. Clone 1-50 displayed biallelic BRCA2 inactivation due to the introduction of a deleterious premature stop codon, and immunoblot analysis verified lack of the full-length BRCA2 protein. The IC50 of rucaparib in the parental and clone 1-50 cells was 9,504 and 287 nM, respectively, demonstrating a 33-fold increase in rucaparib cytotoxicity in this BRCA2 deficient model. Androgen receptor (AR) signaling and proliferation were also examined in clone 1-50. In contrast to published reports (Schiewer et al. Cancer Discov. 2012;2:1134-49), no impact of PARP inhibition on dihydrotestosterone (DHT) induced gene expression was observed in the parental 22Rv1 and BRCA2 deficient clone 1-50 cell lines. In vivo model development and efficacy studies with the 22Rv1 and clone 1-50 cell lines are ongoing. Taken together, these findings support the hypothesis that deficiencies in BRCA1 and BRCA2, as well as additional HR genes, may sensitize prostate cancer cells to rucaparib. A phase 2 study investigating rucaparib in patients with metastatic castration-resistant prostate cancer and HR gene deficiency (TRITON2) is ongoing (NCT02952534). Citation Format: Minh Nguyen, Andrew D. Simmons, Thomas C. Harding. Preclinical assessment of the PARP inhibitor rucaparib in homologous recombination deficient prostate cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2476. doi:10.1158/1538-7445.AM2017-2476