Abstract B058: Targeting mediator kinase CDK8/19 potentiates chemotherapeutic responses, reverses tumor growth, and prolongs survival from ovarian clear cell carcinoma

Abstract Ovarian clear cell carcinomas (OCCC) are a rare histological subtype of epithelial ovarian cancer with known treatment resistance, a feature associated with transcriptional reprogramming. CDK8 and CDK19 Mediator Kinases, which are isoforms of the Mediator Kinase – enzymatic components of a regulatory module that binds to the transcriptional Mediator complex –and not a part of the overall transcription machinery, have been previously reported to modify oncogenic transcription factors. Inhibition of the kinase activity of CDK8/19 was shown to prevent and reverse chemoresistance in other cancers with two CDK8/19 inhibitors currently in clinical trials for non–ovarian cancers. Based on the unmet need for OCCCs, we assessed CDK8/19 expression in a panel of OCCC primary and metastatic tissues and utilized cell line models for investigation into the therapeutic potential of CDK8/19 inhibitors in preventing tumorigenesis and synergizing with existing chemotherapeutics. The synergistic effects of SNX631-6, a preclinical CDK8/19 inhibitor drug candidate, were tested in vitro using three clear cell (ES2, TOV-21-G, RMG1) and one non-clear cell (SKOV3) ovarian carcinoma cell lines. A sub-inhibitory concentration 50 (IC50) concentration of SNX631-6 (500 nM) strongly potentiated effects of cisplatin, carboplatin, and paclitaxel. Additionally, cells demonstrating platinum resistance reverted to platinum sensitivity when exposed to the same concentration of SNX631-6. In vivo antitumor effects and tolerability of oral SNX631-6 were assessed in Foxn1nu homozygous female mice injected subcutaneously with ES2+luc cells. Mice were treated with a vehicle (n=10), intraperitoneal (IP) cisplatin (2 mg/kg) twice weekly (n=9), SNX631-6 medicated diet (330 ppm) orally ad libitum (n=10), or IP cisplatin combined with SNX631 diet (n=10) until tumors reached a predetermined critical volume of 2500mm3. In ES2+luc injected mice, significant tumor growth inhibition relative to vehicle occurred in mice treated with a combination of IP cisplatin and SNX631-6 (p0. 001) or SNX631-6 alone (p0. 01). Additionally, the use of IP cisplatin and SNX631-6 significantly delayed time-to-critical-volume compared to vehicle (p=0. 0001). SNX631-6 alone and in combination with IP cisplatin was well tolerated in all mice based on body weight measurements. In summary, SNX631-6 potentiated platinum responses in vitro and demonstrated strong in vivo antitumor effects, including tumor regression, when used as monotherapy and in combination with platinum-based therapy. These data suggest that therapeutically achievable doses of SNX631-6 may provide clinical benefit, prevent tumorigenesis, and reverse platinum resistance in patients with OCCC. Citation Format: Wade Barton, Asha Kumari, Gary School, Zachary Mack, Liz Macias Quintero, Karthik Rangavajhula, Rebecca Arend, Mengqian Chen, Eugenia V. Broude, Karthikeyan Mythreye. Targeting mediator kinase CDK8/19 potentiates chemotherapeutic responses, reverses tumor growth, and prolongs survival from ovarian clear cell carcinoma abstract. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84 (5 Suppl₂): Abstract nr B058.