Abstract 2475: In vitro and in vivo assessment of the mechanism of action of the PARP inhibitor rucaparib
Authors: Robillard Liliane , Nguyen Minh , Harding Thomas C. , Simmons Andrew D. 
Published: July 2017

Background: Rucaparib (CO-338) is an oral small molecule inhibitor of poly (ADP-ribose) polymerase (PARP) that is being developed for patients with BRCA1 and BRCA2 mutated and homologous recombination deficient ovarian cancer (Swisher et al., 2016; Lancet Oncol. Epub). The mechanism of action (MOA) of rucaparib was characterized in the studies reported here.

Methods: Enzymatic and cellular reporter assay profiling were performed by BPS Biosciences. Rucaparib potency was assessed using 6-day cell viability assays (CellTiter-Glo). In vivo PK/PD and efficacy studies in the MDA-MB-436 (BRCA1 mutant) and the HBCx-17 (BRCA2 mutant) TNBC models were performed at Crown Biosciences and XenTech, respectively.

Results: In biochemical assays rucaparib inhibited PARP-1, PARP-2 and PARP-3, with IC50 values of 0.8, 0.5, and 28 nM, respectively. Rucaparib weakly inhibited PARP5a (tankyrase 1[TNKS1]) and PARP5b (tankyrase 2[TNKS2]) with an IC50 of 796 and 486 nM; however rucaparib did not inhibit TNKS1/2 in a cellular reporter assay evaluating Wnt pathway signaling (IC50 &gt;10 µM). Rucaparib cytotoxicity was evaluated in the BRCA1 mutant (UWB1.289) and wild-type (UWB1.289+BRCA1) isogenic pair, and a panel (n=26) of breast and ovarian cancer cell lines. The UWB1.289 cell line was more sensitive to rucaparib (IC50 = 375 nM) than the UWB1.289+BRCA1 cell line (IC50 = 5430 nM). Rucaparib also decreased the cell viability of several BRCA1 and BRCA2 mutant ovarian and breast cancer cell lines. Mechanistic analyses in the UWB1.289 cell line demonstrated a dose dependent decrease in poly (ADP) ribosylation (PAR; IC50 = 2.8 nM), and an increase in DNA damage and apoptosis. Taken together, these results are consistent with the concept of synthetic lethality, where simultaneous inhibition of multiple DNA repair pathways results in irreparable DNA damage and cell death. In vivo PK/PD and efficacy studies assessed the MOA and activity of rucaparib in the MDA-MD-436 (BRCA1 mutant) xenograft model. In a PK/PD study, dose dependent inhibition of PAR was observed in the tumors of mice treated with 15, 50 and 150 mg/kg BID rucaparib, resulting in statistically significant (p &lt; 0.05) inhibition of 45%, 86% and 96%, respectively. An inverse and dose dependent correlation between PAR and rucaparib levels in the plasma and tumor was observed, and the levels of rucaparib in the tumor were consistently higher (&gt; 2-fold) than the levels of rucaparib measured in plasma. Dose dependent and statistically significant tumor growth inhibition was observed in both subcutaneous and orthotopic xenograft studies evaluating the anti-tumor activity of rucaparib in the MDA-MB-436 model. Rucaparib also demonstrated potent activity in the BRCA2 mutant HBCx-17 patient-derived xenograft model.

Conclusion: These results demonstrate that rucaparib is a potent and selective inhibitor of PARP1, PARP-2, and PARP-3 with in vitro and in vivo activity in BRCA1/2 deficient tumors.

Citation Format: Liliane Robillard, Minh Nguyen, Thomas C. Harding, Andrew D. Simmons. In vitro and in vivo assessment of the mechanism of action of the PARP inhibitor rucaparib [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 2475. doi:10.1158/1538-7445.AM2017-2475