Systematic high-throughput combination drug screen to enhance poly (ADP-ribose) polymerase (PARP) inhibitor efficacy in ovarian cancer.
Authors: Foster Katherine , Dasari Santosh K. , Powell Reid T. , Nguyen Nghi D. , Sobieski Mary , Stephan Clifford , Bahl Ashwani , Yap Timothy A. , Jazaeri Amir A. , Westin Shannon Neville , Sood Anil K 
Published: June 2023

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Background: PARP inhibitors currently offer the greatest benefit to patients with loss-of-function BRCA1 or 2 mutations or other causes of homologous recombination deficiency (HRD). We hypothesized that combination therapy, possibly by augmenting or inducing an HRD phenotype, would enhance the efficacy of PARP inhibition in epithelial ovarian cancer. Methods: We performed a high-throughput screen of an informer set containing FDA-approved drugs, clinical candidates, and small-molecule probes to identify synergistic partners with olaparib in representative HRD and non-HRD human epithelial ovarian cancer cells. Single agent activity was described by area under the curve of the Hafner growth rate index (AUC_GRI) and synergy was ranked by cumulative difference from Bliss (ΔBliss). Analysis of the drug screen was performed using algorithms developed by the Gulf Coast Consortia’s Combinatorial Drug Discovery Program at the Texas A&M Institute of Bioscience and Technology. MTT assays were then independently performed to verify and characterize synergy between CT7439, an agent selected following the drug screen, and olaparib. MTT data were analyzed using the SynergyFinder Plus web application. Results: In an initial single agent screen, we identified 62 compounds with a mean AUC_GRI <0.75, indicating inhibitory activity. Prioritizing more potent agents and those of mechanistic interest, we selected 40 hits to validate and advance to a combination screen. Diverse mechanisms of action were represented among the hits, including many small molecule inhibitors of various cell cycle pathways. Following a combination screen with olaparib, the compounds selected for final validation were: AZD 7762 (CHK1/2 inhibitor), dinaciclib (pan-CDK inhibitor), onalespib (HSP90 inhibitor), and SN-38 (topoisomerase 1 inhibitor). The ΔBliss for these agents are presented in the table. Due to mechanistic rationale and concern for overlapping toxicities among the other candidates, we focused further study on CDK inhibitors. In MTT assays, we identified robust synergy between olaparib and CT7439, a next-generation oral CDK12/13 inhibitor, with a mean Bliss synergy score of 1.31 and scores as high as 22.03 at certain concentrations in the non-HRD model OVCAR-5. These findings were recapitulated in the HRD cell lines OVCAR-4 (mean Bliss synergy score 1.56) and OVCAR-8 (mean Bliss synergy score 5.69). Results from an in vivo model will be presented as well. Conclusions: Pairing a PARP inhibitor and CDK inhibitor, a strategy identified by a high-throughput drug screen and known to impair homologous recombination, resulted in robust synergy in both HRD and non-HRD ovarian cancer models. Clinical translation of this combination may enhance the efficacy of PARP inhibitors in patients with ovarian cancer. [Table: see text]