Poly (ADP-ribose) polymerase (PARP) inhibitors are an emerging class of potential anti-cancer therapeutics currently being explored in patients with tumors harboring BRCA1 or BRCA2 mutations. Inhibition of

PARP increases DNA damage that cannot be repaired in BRCA1/2 mutant tumor cells and will therefore result in cell death.

Proof-of-concept trials have demonstrated the clinical benefit of PARP inhibition in BRCA1/2 -associated tumors (Audeh et al. 2010; Fong et al. 2010). Rucaparib (CO-338, AG014699, PF-01367338) is a potent, orally available PARP1 and PARP2 inhibitor (Ki = 1.4 and

0.17 nM, respectively) currently being investigated in phase 1/2 trials both as

a monotherapy and in combination with chemotherapy. Here, we investigated the preclinical efficacy of rucaparib as single agent and in combination with other targeted agents in cell lines and xenograft models and correlated anti-tumor response to HR status. Sensitivity to rucaparib was determined in vitro in a panel of cancer cell lines using i) a six day CellTiter-Glo viability assay and ii) a 14-18 day clonogenic assay. DNA damage and repair was assessed by γH2AX and RAD51 foci formation as determined by immunofluorescence. Cell lines were deemed HR-proficient if RAD51 foci were ≥2-fold

24 hours post DNA damage. In vivo anti-tumor activity of rucaparib as a single agent was assessed in the

MDA-MB-436 breast cancer xenograft model. We tested a wide range of cancer cell lines including breast, ovarian, prostate and small cell lung cancer lines for RAD51 foci formation and sensitivity to rucaparib in vitro. Cell lines sensitive to rucaparib were generally deficient in RAD51 foci formation. For example, the BRCA1 mutated ovarian cell line UWB1.289 had a rucaparib IC50 of 0.375μM and was HR deficient. Re-introduction of a wild-type BRCA1 allele into the

UWB1.289 cell line restored HR proficiency and increased the IC50

14-fold to 5.4μM. Results in vitro, correlated with xenograft response to rucaparib in vivo. Treatment of mice bearing BRCA1 mutated MDA-MB-436 breast tumor xenografts resulted in tumor regression corresponding to the low IC50 of 0.091μM to rucaparib in vitro and a HR deficient phenotype as assessed by RAD51. Assessing rucaparib in combination with several PI3K pathway inhibitors including Pan-PI3K, TORC1/2 and PI3K/mTOR inhibitors showed additive or synergistic anti-tumor efficacy depending on the specific compound examined. Our findings demonstrate that cell lines deficient in their HR DNA repair pathway are sensitive to rucaparib treatment. Investigation of rucaparib sensitivity in tumors harboring mutations in additional HR pathway genes is merited and could expand the utility of rucaparib beyond BRCA 1/2. Combination therapy approaches employing PI3K inhibitors with rucaparib may be of particular promise.

Citation Format: Liliane Robillard, Thomas C. Harding, Annette O. Walter. Preclinical efficacyof the PARP inhibitor rucaparib (AG014699/PF-01367338) as a monotherapy and incombination with PI3K inhibition. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3349. doi:10.1158/1538-7445.AM2013-3349