The Wnt signalling pathway is frequently deregulated in cancer due to mutations; indeed 80% of colon cancers have defects in the APC gene leading to high levels of β-catenin. ¹ Despite the importance of Wnt signalling, it is only recently that small molecule inhibitors have been identified and progressed towards clinical trials. However, such inhibitors commonly act upstream of Wnt activating mutations. With this in mind, we sought small molecules that block Wnt signalling downstream of β-catenin. We have screened a diverse small molecule chemical library against a cell-based luciferase reporter assay to identify small molecule inhibitors of Wnt signalling. ² Medicinal chemistry optimisation of a singleton trisubstituted pyridine resulted in the discovery of CCT251545, a potent, selective and orally bioavailable small molecule inhibitor of Wnt signalling. ³ Knowledge of the structure activity relationship of this series of compounds enabled the design of probes for identification of the biochemical targets. Using SILAC-based quantitative mass spectrometry, we identified the Mediator complex-associated protein kinases CDK8 and CDK19 as the primary targets. ⁴ Protein X-ray crystallography studies of CCT251545 in complex with CDK8/cyclin C enabled the design of improved CDK8/19-selective compounds. Metabolic stability and aqueous solubility of the chemical probe CCT251545 were optimised in order to facilitate further in vivo evaluation of CDK8/19 pharmacology and progression into preclinical studies. This led to the discovery of CCT251921, a potent, highly selective and orally bioavailable dual CDK8/19 ligand with improved pharmacokinetic and pharmaceutical properties. ⁵ Scaffold hopping and biochemical HTS versus CDK8 led to additional compound series with a Type I binding mode and resulted in structurally differentiated back-up candidate (MSC2530818) with equivalent pharmacological profile to CCT251921. ^6-8 CCT251921 and MSC2530818, were profiled in in vivo efficacy and safety studies to investigate the therapeutic potential of dual CDK8/19 modulation with two structurally differentiated compounds. ^{9 1}Kahn, M. Nat. Rev. Drug Discovery 2014, 13, 513-532 ²Ewan, K.; Pajak, B. et al Cancer Res. 2010, 70, 5963-5973 ³Mallinger, A.; Crumpler, S. et al J. Med. Chem. 2015, 58, 1717-1735 ⁴Dale, T.; Clarke, P. A. et al Nat. Chem. Biol. 2015, 11, 973-980 ⁵Mallinger, A.; Schiemann K. et al J. Med. Chem. 2016, 59, 1078-1101 ⁶Mallinger, A.; Schiemann, K. et al ACS Med. Chem. Lett. 2016, 7, 573-578 ⁷Schiemann, K.; Mallinger, A. et al Bioorg. Med. Chem. Lett. 2016, 26, 1443-1451 ⁸Czodrowski, P.; Mallinger, A. et al J. Med. Chem. 2016, 59, 9337-9349 ⁹Clarke; P.A., Ortiz-Ruiz; M.-J. et al eLife, http://dx.doi.org/10.7554/eLife.20722