Abstract P3-11-12: Epigenetic modulation to deepen and prolong the response to PARP inhibitors

Background: BRCA1 and BRCA2 are key mediators of DNA damage repair response including the homologous recombination repair (HRR) pathway. Mutations in these genes predispose women to various cancers including breast, ovarian and pancreatic cancers. A new class of drugs, the poly (ADP-ribose) polymerase inhibitors (PARPi), were designed to leverage impaired DNA repair in BRCA-mutated tumors and are FDA approved for various cancers (breast, ovarian). Efforts to combine PARPi with cytotoxic agents show increased efficacy but overlapping toxicities hamper their tolerability. Further, the short duration of response to PARPi monotherapy in diseases like breast cancer has inspired the search for PARPi combinations with other agents including epigenetic modifiers. To overcome these hurdles, we explored synergistic interactions between PARPi and DNA methyltransferases inhibitors (DNMTi) such as decitabine, a cytidine analog. When incorporated in newly synthesized DNA strands during S phase, these agents will trigger a covalent protein-DNA complex formation causing cell cycle arrest and cell death. Material & Methods: BRCA wild-type (WT) and BRCA-mutated breast cancer cell lines were used for in vitro analyses: SUM149PT and HCC-3153 (BRCA1-mutated), MDA-MB-231 and BT-549 (BRCA-WT), UCRP231A (a CRISPR/Cas9 engineered BRCA2-mutated from parental MDA-MB-231), and MCF10A (a BRCA-WT non-tumorigenic breast cell line). The combined effect of talazoparib (PARPi) and decitabine (DNMTi) was analyzed using Combenefit® software to determine their synergistic interaction. Cell cycle analyses, dead/alive cells ratio analyses, as well as pH2AX levels were evaluated. In vivo analyses included xenograft models with SUM149PT as well as the parental BRCA-WT MDA-MB-231 and BRCA2-mutated MDA-MB-231 (UCRP231A) cells in immunodeficient NOD.Cg-Rag1tm1Mom Il2rgtm1Wjl/SzJ mice. Results: Our preclinical data in BRCA-deficient breast cancer cell lines, demonstrated a synergistic inhibition of cell growth and enhancement of cell death at concentrations of talazoparib and decitabine where each agent individually had minimal efficacy. In cells with intact HRR pathways, the drug combination showed an enhanced inhibition of cell growth but was not synergistic in inducing cell death compared to BRCA deficient cell lines. This difference was further manifested by different cell cycle profiles: non-BRCA mutated cells arrested in S-phase while BRCA-mutated cells progressed through S-phase without arrest and entered cell death from G2/M. In vivo mouse data showed a significant synergistic interaction between the two drugs without enhanced toxicity. Conclusion: The ability to induce a synergistic interaction between talazoparib and decitabine at low drug concentrations present a promising therapeutic strategy for patients with HRR-deficient tumors. Ongoing experiments are being conducted to generate additional engineered BRCA- and other HRR-deficient cell lines (ATM, CHEK2 and others) through CRISPR/Cas9 DNA editing system and further delineate the differential mechanism of interaction between the HRR-deficient and HRR-intact cell lines in preparation for clinical testing. Citation Format: Romain Pacaud, Mallika Dhawan, Scott Thomas, Elysia Roche, Jose Garcia, Nela Pawlowska, Pamela Munster. Epigenetic modulation to deepen and prolong the response to PARP inhibitors [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-11-12.