Abstract A035: 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 chemotherapy, but not 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 was used to extract DNA damaged lesions, which were labeled for detection and imaged on a confocal microscope. Expression of pCHK1 S317 was confirmed via Western Blot. Combinations were assessed in vivo with an OVCAR8 tumor model. All cell lines were fairly resistant to single agent (SA) carboplatin with IC50s ranging from 44- >100 mM. In the OvCar8 cell line, 29.3% cell death was observed with SA carboplatin and increased to 91.9% (p<0.0001) and 95.4% (p<0.0001) with the addition of 5 nM and 10 nM prexasterib, respectively. Similarly, combination therapy also resulted in a synergistic effect in the ES2, SKOV3, OvCar4, and OV90 cell lines. 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 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. In the OVCAR8 mouse model, carboplatin (50 mg/kg once weekly x 5 weeks) showed no significant difference compared to control. Prexasertib alone (8 mg/kg twice daily [KL1] x 3 days followed by 4 days off x 5 weeks) rendered stable disease starting from treatment initiation with tumor volume significantly different from control (P < 0.001). However, the combination of carboplatin + prexasertib induced tumor regression with tumor volumes significantly different from both control (P < 0.0001) and prexasertib alone (P < 0.05). The CHK1 inhibitor, prexasertib, demonstrated a synergistic effect when used in combination with carboplatin both in vitro and in a mouse tumor model, 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 Y. 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 AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl):Abstract nr A035.