Abstract B055: Characterization of highly selective and CNS-penetrant PARP1 inhibitors
Authors: Cowley Phillip M , McGuinness Barry E , Campbell Gillian M , Wise Alan 
Published: December 2023

Abstract
First-generation PARP1/2 inhibitors are FDA-approved for cancers such as breast, ovarian prostate and pancreas and provide significant therapeutic benefit to patients whose tumors harbor BRCA1 or BRCA2 mutations or homologous recombination deficiency (HRD). However, their use is associated with hematological toxicities that narrow their therapeutic index and restrict their application, particularly in combination with standard-of-care chemotherapy. All four FDA-approved PARP1 inhibitors are largely non-selective for the closely related enzyme PARP2, inhibition of which has been shown to drive hematotoxicity. Hence, the development of second-generation molecules highly selective for PARP1 over PARP2 offers a significant opportunity to 1) dramatically enhance therapeutic index, 2) enable additional precision medicine/combination approaches with chemotherapy, radiotherapy, immunotherapy and targeted agents and 3) expand the addressable patient population to those whose tumors harbour additional DDR defects. Utilising X-ray crystallography and structure-based drug design, we describe the characterization of exquisitely selective PARP1 inhibitors. These molecules demonstrate high potency in multiple biochemical and cell-based assays, including viability and colony forming unit read-outs in BRCA-deficient cancer cells, whilst sparing HR-proficient cancer cell lines and primary cells. The molecules are potent PARP1-DNA trappers and exhibit high selectivity across the PARP family and wider secondary pharmacology targets. The molecules potentiate the activity of ATR inhibitors when tested in ATM-deficient cancer cell lines. This chemical series generally exhibits highly desirable physico-chemical and in vitro ADME properties, which translate to high oral bioavailability and low clearance in rodent PK studies. Exemplar molecules also demonstrate high rat CNS penetrance using in vivo microdialysis. Lead molecules yield deep and durable dose-dependent anti-tumor efficacy in a variety of BRCAm xenograft models with responses continuing after cessation of treatment. The compounds demonstrate strong PK/PD relationships which correlate with anti-tumor efficacy using inhibition of tumor PARylation as a PD read-out. Strikingly, the molecules demonstrate compelling anti-tumor efficacy in BRCA-deficient xenograft models that are known to be largely resistant to FDA-approved non-selective PARP1/2 inhibitors. Taken together, these data predict low once daily therapeutic oral dosing based on a highly favorable pre-clinical PK profile and potent in vivo efficacy, with the potential to demonstrate improved efficacy and tolerability compared to marketed PARP inhibitors. In summary, we describe the characterization of next generation potent and selective CNS-penetrant PARP1 inhibitors. These molecules have the potential for an improved safety profile and greater combination potential with standard-of-care and targeted anti-cancer therapies.
Citation Format: Phillip M Cowley, Barry E McGuinness, Gillian M Campbell, Alan Wise. Characterization of highly selective and CNS-penetrant PARP1 inhibitors [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B055.