Abstract 129: Assessing the mechanism and therapeutic potential of modulators of the human mediator complex-associated protein kinases CDK8 and CDK19
Authors: Clarke Paul A. , Ortiz-Ruiz Maria-Jesus , Poele Robert Te , Adeniji-Popoola Olajumoke , Box Gary , Esdar Christina , Ewan Kenneth , Gowan Sharon , Brandon Alexis De Haven , Hewitt Phllip , Kaufmann Wolfgang , Mallinger Aurelie , Raynaud Florence , Rohdich Felix , Schiemann Kai , Simon Stephanie , Schneider Richard , Valenti Melanie , Blagg Julian , Dale Trevor , Eccles Suzanne , Workman Paul , Wienke Dirk Wienke Dirk 
Published: July 2017

Abstract
Mediator-associated protein kinases CDK8 and CDK19 are context-dependent drivers or suppressors of tumorigenesis. Their inhibition is predicted to have pleiotropic effects, but it is unclear whether this will impact on the clinical utility of CDK8/19 inhibitors. We identified two structurally differentiated chemical series, suitable for exploring their function. In addition to tools that fulfil the criteria set out for chemical probes, the lead compounds from each series, CCT251921 and MSC2530818, had optimal pharmacological and pharmaceutical properties making them suitable for preclinical studies. Having potent, highly selective, orally bioavailable exemplar compounds from these series in hand, we were well positioned to investigate the therapeutic potential of dual CDK8/19 inhibition. The compounds exhibited modest anti-tumor activity in colorectal cancer cell line xenograft models with modulation of p-STAT1SER727, a target engagement biomarker, and altered gene expression profiles, including super-enhancer regulated gene expression, consistent with the inhibition of CDK8/19. In PDX-derived cell cultures we observed inhibition of soft-agar growth in cells derived from different tumor types. However, we only detected significant antitumour activity in 1 of 6 colorectal PDX models tested in vivo, and one example of sensitization to standard of care chemotherapy, despite showing inhibition of p-STAT1SER727. Acute myeloid leukemia cells were the most sensitive cancer type in the PDX panel with therapeutic potency seen in systemic and sub-cutaneous models. Significantly, the compounds impacted on stem cell biology. In a bone progenitor model we saw dose-responsive activation and inhibition of markers of bone matrix and bone deposition that was distinct from WNT blockade. Treatment of a diverse collection of normal cell co-culture models detected a unique response profile consistent with stimulation of an immune/inflammatory response. In vivo treatment of a genetically engineered mouse model expressing oncogenic beta-catenin shifted cells within hyperplastic intestinal crypts from a stem cell to a transit amplifying phenotype. Finally, in pre-clinical tolerability studies we observed a similar, widespread adverse safety profile at therapeutically relevant exposures for both CCT251921 and MSC2530818. At the concentrations tested we detected &gt;80% inhibition of p-STAT1SER727 and increased IL-12 plasma levels. Since the observed pathological effects were generated with two potent, highly selective, but structurally distinct compounds, we conclude that the adverse consequences of treatment are the direct result of inhibition of CDK8 and/or CDK19. The serious and complex nature of the toxicity observed indicates that the clinical development of either series of CDK8/19 modulators, or other chemotypes with similar profiles, will be extremely challenging.
Citation Format: Paul A. Clarke, Maria-Jesus Ortiz-Ruiz, Robert Te Poele, Olajumoke Adeniji-Popoola, Gary Box, Christina Esdar, Kenneth Ewan, Sharon Gowan, Alexis De Haven Brandon, Phllip Hewitt, Wolfgang Kaufmann, Aurelie Mallinger, Florence Raynaud, Felix Rohdich, Kai Schiemann, Stephanie Simon, Richard Schneider, Melanie Valenti, Julian Blagg, Trevor Dale, Suzanne Eccles, Paul Workman, Dirk Wienke Dirk Wienke. Assessing the mechanism and therapeutic potential of modulators of the human mediator complex-associated protein kinases CDK8 and CDK19 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 129. doi:10.1158/1538-7445.AM2017-129