Abstract P4-15-13: CDK8 inhibition potentiates anti-ER and anti-HER2 therapies in breast cancer

CDK8, along with its paralog CDK19, is a cyclin dependent kinase which, in contrast to other members of the CDK family does not regulate cell cycle progression. CDK8 acts as a pleiotropic transcription regulator potentiating the induction of transcription by several transcription factors. Immunohistochemical staining of breast tissue arrays and bioinformatics analysis of gene expression microarray data of breast cancer patients revealed that CDK8 is overexpressed in breast cancer and that higher CDK8 expression correlates with the failure of systemic therapy. Small-molecule selective inhibitors of CDK8 and CDK19 (Senexin A and Senexin B) inhibited the mitogenic effects of estrogen and estrogen-dependent transcription in estrogen receptor (ER)+ breast cancer cell lines. CDK8/19 inhibitors had a cytostatic effect on different ER+ cell lines, and this growth inhibition was synergistic with the effect of the anti-estrogen fulvestrant, particularly in ER+ cell lines resistant to estrogen deprivation. Some of the ER+ cell lines sensitive to CDK8/19 inhibition also express HER2, and therefore we tested CDK8/19 inhibitors in combination with the HER2 and EGFR tyrosine kinase inhibitor lapatinib and an anti-HER2 monoclonal antibody, a biosimilar of trastuzumab. CDK8/19 inhibition produced a synergistic decrease in cell growth with both HER2 inhibitors; this effect was especially pronounced with a trastuzumab biosimilar. Surprisingly, the synergistic effect with HER2 inhibitors was observed in both ER+ HER2+ and ER-HER2+ cell lines, suggesting an effect on a HER2-complementing molecular target other than ER. Interestingly, CDK8/19 inhibition also synergized with trastuzumab biosimilar in a breast cancer cell line that exhibits innate resistance to trastuzumab, suggesting that CDK8/19 inhibition can overcome trastuzumab resistance in breast cancer. These results suggest that combining anti-estrogen and anti-CDK8 therapy may be more effective than conventional hormone therapy for ER positive breast cancer and that combining anti-HER2 and anti-CDK8 therapy is a rational potential treatment for HER2+ breast cancer, regardless of ER status or sensitivity to trastuzumab. Citation Format: Martina S McDermott, Chang-uk Lim, Mengqian Chen, Alexander Chumanevich, James F Catroppo, Balazs Gyorffy, David Oliver, Igor B Roninson, Eugenia V Broude. CDK8 inhibition potentiates anti-ER and anti-HER2 therapies in breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-15-13.