Discovery and pharmacological characterization of a novel series of highly selective inhibitors of cyclin‐dependent kinases 4 and 6 as anticancer agents

Background and Purpose Cyclin D‐dependent kinases 4 and 6 (CDK4/6) are crucial regulators of the G1 to S phase transition of the cell cycle and are actively pursued as therapeutic targets in cancer. We sought to discover a novel series of orally bioavailable and highly selective small molecule inhibitors of CDK4/6. Experimental Approach The discovery of pharmacological inhibitors and optimization for potency, selectivity and drug properties were achieved by iterative chemical synthesis, biochemical screening against a panel of kinases, cell‐based assays measuring cellular viability, cell cycle distribution, induction of apoptosis and the level of retinoblastoma tumour suppressor protein (Rb) phosphorylation and E2 factor (E2F)‐regulated gene expression and in vitro biopharmaceutical and in vivo pharmacokinetic profiling. Key Results We discovered several lead compounds that displayed >1000‐fold selectivity for CDK4/6 over other members of the CDK family. The lead compounds, 82 , 91 and 95 , potently inhibited the growth of cancer cells by inducing G1 arrest with a concomitant reduction in the phosphorylation of Rb at S780 and in E2F‐regulated gene expression. With a remarkable selectivity for CDK4 over 369 human protein kinases, 91 was identified as a highly potent and orally bioavailable drug candidate. Conclusions and Implications We have identified unique and new inhibitors of CDK4/6 as potential drug candidates. Compound 91 represents an ideal candidate for further development as targeted cancer therapy.