SR-4835, a CDK12 inhibitor, enhances the antitumor efficacy of sunitinib in human renal cell carcinoma through suppression of the ATR-associated DNA damage response pathway: an in vitro and in vivo study

Background Sunitinib remains an important therapeutic option for advanced renal cell carcinoma (RCC), but intrinsic or acquired resistance substantially limits its clinical benefit. Cyclin-dependent kinase 12 (CDK12) is a transcription-associated kinase that maintains Ser2 phosphorylation of the RNA polymerase II C-terminal domain and supports transcription of genes involved in the DNA damage response (DDR). We investigated the clinical relevance of CDK12 in RCC and evaluated whether pharmacologic CDK12 inhibition with SR-4835 could enhance the antitumor activity of sunitinib. Methods CDK12 expression was assessed by immunohistochemistry in human RCC specimens and correlated with clinicopathological features, overall survival, and response to sunitinib. The antitumor effects of SR-4835, alone or in combination with sunitinib, were examined in four human RCC cell lines by assessing cell viability, apoptosis, clonogenicity, and DNA damage, along with immunoblotting, RNA sequencing, and quantitative real-time PCR. Combination effects were quantified using the Chou–Talalay method. The in vivo antitumor efficacy of SR-4835, alone or in combination with sunitinib, was further evaluated in RCC xenograft models. Results CDK12 was overexpressed in RCC tumors and was associated with advanced stage, poorer overall survival, and poor clinical response to sunitinib. SR-4835 reduced cell viability and clonogenic growth and induced caspase-dependent apoptosis in RCC cells. Mechanistically, SR-4835 decreased RNA polymerase II Ser2 phosphorylation and downregulated DDR-related programs, including ATM and ATR, at both the transcript and protein levels. SR-4835 also suppressed AKT phosphorylation and BCL-2 expression while increasing p53, p21, and p27. Notably, SR-4835 significantly enhanced sunitinib-induced cytotoxicity and apoptosis, with synergistic effects observed across all four RCC cell lines (combination index < 1). Combined treatment further increased DNA damage and more effectively disrupted ATR-associated DDR signaling. In vivo, SR-4835 combined with sunitinib produced greater tumor growth inhibition than either monotherapy without significant body weight loss. Conclusions CDK12 is a clinically relevant therapeutic target in RCC. Pharmacologic inhibition of CDK12 by SR-4835 exerts antitumor activity and sensitizes RCC cells to sunitinib, at least in part through suppression of the RNA polymerase II Ser2–ATM/ATR DDR axis and attenuation of AKT–BCL-2 survival signaling. These findings support CDK12 inhibition as a promising strategy to improve sunitinib efficacy in RCC.