Abstract 3025: Discovery of preclinical development candidate inhibitors of the mediator complex-associated kinases CDK8 and CDK19 and evaluation of their therapeutic potential

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Abstract Background The Mediator complex-associated kinases CDK8 and CDK19 are cyclin C-dependent enzymes that, with MED12 and MED13, form the kinase module of the Mediator complex. CDK8 expression correlates with activation of β-catenin in colon and gastric cancers and has also been associated with increased mortality in colorectal, breast and ovarian cancers. CDK8 is located in a region of chromosome 13 known to undergo copy number gain in ∼60% of colorectal cancers and inducible shRNA-mediated knockdown of CDK8 protein reduces the growth of colorectal cancer human tumor xenograft animal models harboring CDK8 gene amplification. Results Here we report the discovery and evaluation of CCT251545, a potent, selective and orally bioavailable small molecule chemical probe for CDK8 and CDK19 that we identified from a cell-based WNT pathway screen [1]. We also report a structure-based design approach to the discovery of CCT251921, a potent, selective and orally bioavailable inhibitor of CDK8, with equipotent affinity for CDK19, that has optimised pharmacokinetic and pharmaceutical properties suitable for preclinical development. Furthermore, we describe the discovery of MSC2530818, a structurally differentiated back-up candidate with equivalent pharmacological profile to CCT251921, from a high throughput screen versus CDK8 and subsequent structure-based design. Taking advantage of these two structurally distinct and highly selective dual CDK8/19 modulators we were able to reliably define on-target effects of targeting both CDK8 and CDK19 in the cellular context and in in vivo animal models. We describe gene expression profiles resulting from dual inhibition of CDK8 and CDK19 to demonstrate robust modulation of WNT signalling and additional pathways, including stress and immune response, consistent with the multiple contexts in which Mediator complex is known to regulate gene transcription. We show that both CCT251921 and MSC2530818 exhibit potent cell-based and in vivo inhibition of STAT1SER727 phosphorylation, a target engagement biomarker of CDK8 inhibition, and further demonstrate in vivo antiproliferative activity in human tumour xenograft animal models of colorectal cancer and acute myeloid leukaemia at exposures where pharmacodynamics biomarker modulation is evident. Recent observations suggest CDK8 as a novel anticancer therapeutic target; here we will disclose, for the first time, comprehensive preclinical efficacy, toleration and safety findings for both CCT251921 and MSC2530818 which will inform on the potential for dual CDK8/19 inhibition in the clinical setting.