Abstract 822: Combination of carboplatin and CHK1 inhibition to overcome platinum resistance in high grade serous ovarian cancer

Abstract Platinum based chemotherapy is the most effective treatment for high grade serous ovarian cancer (HGSOC). Resistance in the recurrent setting is a poor prognostic indicator with few treatment options. CHK1 inhibitors have been studied in HGSOC in preclinical and early phase trials alone and in combination with cytotoxic chemotherapy, but have not been studied in combination with platinum to our knowledge. We used the CHK1 inhibitor, prexasertib, as a strategy to overcome platinum resistance by promoting cell cycle progression, limiting repair of platinum induced DNA damage. Sensitivity to carboplatin and prexasertib alone and in combination were measured in a panel of pre-clinical HGSOC cell lines. DNA damage was detected using the Repair Assisted Damage Detection method in untreated and treated cells for 24h with carboplatin and prexasertib alone and in combination. An enzymatic cocktail of UDG, FPG, T4PDG, Endo IV, Endo VIII, and AAG was used to extract DNA damaged lesions. Damaged lesions were labeled for detection with polymerase and a Digoxigenin labeled dUTP. Samples were stained for anti-DIG and anti-pCHK1 S317, imaged on a confocal microscope, and analyzed in Nikon Elements software. Expression of pCHK1 S317 was confirmed via Western Blot. A synergistic effect was observed in all cell lines with the combination of carboplatin and prexasertib. In ES2 cells, carboplatin alone resulted in 15.4% cell death and increased to 72.5% (p<0.001) and 86.8% (p<0.0001) with the addition of prexasertib 5nM and 10nM, respectively. In SKOV3 cells, carboplatin resulted in 38.2% cell death and increased to 80.0% (p=0.0002) and 86.6% (p<0.0001). In OvCar4 cells, carboplatin resulted in 53.2% cell death and increased to 90.6% (p=0.0001) and 92.4% (p<0.0001). In OV90 cells, carboplatin resulted in 25.3% cell death and increased to 56.6% (p<0.0001) and 60.1% (p<0.0001). In OvCar8 cells, carboplatin resulted in 29.3% cell death and increased to 91.9% (p<0.0001) and 95.4% (p<0.0001). DNA damage increased in ES2 cells with prexasertib 10 nM + carboplatin (1.4-fold increase, p=0.03), SKOV3 with prexasertib 5 nM + carboplatin (257.0-fold increase, p=0.002), OvCar4 with prexasertib 10 nM + carboplatin (1.4 fold increase, p=0.03) and OvCar8 with prexasertib 10 nM + carboplatin (2.0-fold increase, p=0.0003). There was no difference in DNA damage in OV90 cells. pCHK1 S317 was increased significantly in all 5 cell lines treated with combination therapy, consistent with known phosphorylation of CHK1 at S317 by ATR in response to DNA damage and inhibition of p-CHK1 auto-phosphorylation by prexasertib leading to inactivation. The CHK1 inhibitor, prexasertib, demonstrated a synergistic effect when used in combination with carboplatin, correlating with an expected increase in DNA damage and p-CHK1 S317. The use of CHK1 inhibitors with platinum may be a new treatment strategy in platinum resistant HGSOC. Citation Format: Mackenzie Cummings, Jaden Williams, Jennifer Pierce, Nathaniel Jones, Kevin Lee. Combination of carboplatin and CHK1 inhibition to overcome platinum resistance in high grade serous ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 822.