Targeting Mediator Kinase Cyclin-Dependent Kinases 8/19 Potentiates Chemotherapeutic Responses, Reverses Tumor Growth, and Prolongs Survival from Ovarian Clear Cell Carcinoma

Abstract

Background/objective: Ovarian clear cell carcinomas (OCCCs) are a rare histological subtype of epithelial ovarian cancer characterized by resistance to platinum-based therapy. CDK8/19, a component of the regulatory CDK module associated with Mediator complex, has been implicated in transcriptional reprogramming and drug resistance in various solid tumors. Our study aimed to investigate the therapeutic potential of CDK8/19 kinase inhibition using selective inhibitors SNX631 and SNX631-6 in OCCC treatment, both as monotherapy and in combination with standard chemotherapeutics.

Methods: CDK8 and Ki67 levels were evaluated via immunohistochemistry in benign, primary, and metastatic ovarian cancer tissues. The efficacy of SNX631 alone and in combination with cisplatin or paclitaxel was assessed in OCCC cell lines (ES-2, TOV-21-G, RMG-1). In vivo evaluation utilized xenograft models with subcutaneous and intraperitoneal delivery of the OCCC ES2 cells and oral delivery of SNX631-6, with the monitoring of tumor growth, metastatic spread, and survival.

Results: CDK8 protein levels were elevated in OC tissues, particularly in OCCC primary and metastatic lesions compared to benign tissue. While CDK8/19 inhibition showed limited effects on in vitro cell proliferation, SNX631-6 demonstrated significant antitumor and anti-metastatic activity in vivo. Notably, SNX631-6 enhanced the efficacy of cisplatin, substantially inhibiting tumor growth and extending overall survival.

Conclusions: Therapeutically achievable doses of CDK8/19 inhibitors may provide clinical benefit for OCCC patients by inhibiting tumor growth and reversing platinum resistance, potentially addressing a critical treatment challenge in this rare ovarian cancer subtype.

Keywords: clear cell ovarian carcinoma; cyclin; ovarian cancer.

Figure 3

Combining standard-of-care chemotherapy with CDK8/19 inhibition enhances chemotherapeutic response in ovarian cancer cell lines. (A–D) Seven-day dose–response curves for indicated OC cells treated with increasing concentrations of carboplatin (0–50 µM), cisplatin (0–50 µM), or taxol (0–100 nM) alone or in combination with 500 nM SNX631. Cell viability data are expressed as percentage SRB measurements relative to vehicle-treated cells. IC₅₀ values represent n = 6 experiments with six replicates per drug concentration. Combination IC₅₀ values are shown in the lower left corner. (E) Seven-day dose–response curves for SKOV-3 CDK8/19 dKO cells combined with indicated chemotherapeutics. (F–H) A quantitation of the IC₅₀ change between (F) carboplatin, (G) cisplatin, or (H) taxol monotherapy and combination therapy across indicated OC lines. (Mean SEM (n = 6)). *, p < 0.05; **, p < 0.01; ***, p < 0.001, unpaired t-test.

Figure 1

CDK8/19 protein expression is highest in both metastatic and primary clear cell ovarian carcinoma. (A) (i) Sample information on the tissue microarrays. (ii) The mean fluorescence intensity across all cores stained for CDK8, with each patient analyzed in duplicate cores. (B) (i) Representative images of CDK8 and Ki67 showing the distribution pattern in a subset of cores from the same tissue arrays as in (A). Images from each section were adjusted to optimally show the labeling pattern, independent of signal intensity. (ii) The colocalization of CDK8 and Ki67 in six representative OCCC sections. Pearson’s coefficient displays the overall correlation between the two markers and Manders’ M2 displays the fraction of Ki67+ areas that are also CDK8+. (C) Western blots from listed cell lines assessing the baseline expression of CDK8, MED12, Cyclin C (CCNC), and ARID1A. Quantitative differences in ARID1A normalized to vinculin are included. (D) Heatmap comparing CDK8, CCNC, and MED12 expression from available public data (www.Depmap.com) for ovarian clear cell carcinoma cell models. (Mean SEM, ***, p < 0.001, ****, p < 0.0001). Original western blots are presented in Figure S8.

Figure 2

CDK8/19 inhibition and/or loss is not independently cytotoxic to ovarian cancer cell lines. (A–D) Seven-day dose–response curves for indicated OC cells treated with increasing concentrations of SNX631 (0–3 µM), carboplatin (0–50 µM), cisplatin (0–50 µM), or taxol (0–100 nM) alone. Cell viability data are expressed as percentage SRB measurements relative to vehicle-treated cells. Inhibitory concentration 50 values (IC₅₀) represent n = 6 experiments, with six replicates per drug concentration, and are shown in the lower left corner. (E) (i) Seven-day dose–response curve in SKOV-3 CDK8/19 dKO cells treated with increasing concentrations of SNX631. (ii) Western blot of CDK8 and CDK19 in SKOV-3 CRISPR dKO cells. Original western blots are presented in Figure S8.

Figure 4

CDK8/19 inhibition monotherapy and in combination with chemotherapy inhibited in vivo tumor growth and metastasis and prolonged survival. (A) Subcutaneous (s.c.) tumor volume changes for ES2-luc NU/J xenografts treated with control (PBS, n = 9), cisplatin 2mg/kg twice weekly (n = 10), oral SNX631-6 (n = 10), or cisplatin 2 mg/kg twice weekly + SNX631-6 combination (n = 10) with (B) time-to-event of end-point tumor volume. (C) Luminescence in i.p. cavity over time of intraperitoneal tumor growth of ES2-luc cells in NU/J mice treated as described (inset: representative luminescence images of mice at indicated days before significant fluid/ascites accumulation). (D) Time-to-event as defined by survival endpoints for injected mice in respective treatment groups. (E–H) Representative images of Ki67 labeling in subcutaneous (E,F) and (G,H) intraperitoneal ovarian tumors. Proliferative index normalized to total nuclei from 10 random fields calculated for N = 3 mice per group and presented in (F,H). (Mean SEM, (n = 3); **, p < 0.01; ***, p < 0.001, ****, p < 0.0001; one-way ANOVA followed by Tukey’s multiple comparison).