Abstract P081: In vitro activity and efficacy of novel dual PARP-HDAC inhibitors

Introduction: Inhibition of poly adenosine diphosphate-ribose polymerase (PARP) is an effective treatment strategy against tumors with homologous recombination (HR) DNA repair deficiencies. Combination treatments utilizing PARP inhibitors (PARPi) in concert with inhibitors of key DNA repair or PARPi-resistance pathways, may expand the utility of PARPi beyond cancers harboring germ-line HR deficiencies. Acetylation and deacetylation of histones is an important regulatory event in the DNA damage response. Combining histone deacetylation inhibitors (HDACi) and PARPi have sensitized PARPi-resistant cells to treatment; however, combination regimens often require sequential administration to manage overlapping toxicities and accommodate diverse pharmacokinetics, constituting a significant limitation of these strategies in a clinical setting. Here, we test the activity of kt-3000 series, a novel bi-functional class of small molecules with dual PARP and HDAC inhibitor activity in HR-proficient cancer cells where PARPi have historically lacked single-agent activity. Methods: PARP enzyme activity was measured using the Trevigen Universal Colorimetric PARP Assay Kit. HDAC activity was measured using nuclear extracts of treated cells and the HDAC Fluorometric Activity Assay Kit from Cayman Chemical, as per the manufacturer’s protocol. PARP activity was measured by fluorescence in C41 cells following compound treatment for 2 hours and PARP-activation with 1M H₂O₂ by staining cells with an anti-PARP Ab followed by a FITC-coupled secondary antibodies. Cell survival EC50 values were obtained by treating cells with a range of inhibitor concentrations, then quantifying cell confluency after 72-hours of treatment based on images taken with an Incucyte S3 system. Results: kt-3000 series compounds are potent inhibitors of PARP and HDAC with IC50 values for PARP enzyme activity in the low nM range, comparable to those of FDA-approved PARP inhibitors (olaparib, rucaparib, niraparib, and talazoparib) and IC50 values for the inhibition of HDAC activity comparable to FDA-approved HDAC inhibitors (panabinostat, belinostat, and vorinostat). Fluorescent assays of PARP activity in C41 cells also resulted in IC50 values in the nM range and cell survival EC50 values of the dual inhibitors are comparable to FDA-approved PARP inhibitors alone and in similar range with FDA-approved HDAC inhibitors alone. Conclusion: Our novel dual PARP-HDAC inhibitors show potent inhibition of PARP activity in vitro comparable to FDA-approved PARP inhibitors and also show potent inhibition of HDAC activity. The potency and activity of kt-3000 series compounds exhibit potential superiority to FDA-approved PARP inhibitors and HDAC inhibitors, in formulation as a single molecule. Development of these multi-target inhibitors will target unmet medical needs in the treatment of HR-proficient cancer types with dysregulation of histone deacetylation. Citation Format: Sarah Truong, Fariba Ghaidi, Louise Ramos, Jay Joshi, Dennis Brown, Neil Sankar, John Langlands, Jeffrey Bacha, Wang Shen, Mads Daugaard. In vitro activity and efficacy of novel dual PARP-HDAC inhibitors [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P081.