A pharmacological approach to inhibition of cyclin-dependent kinases 4 and 6 (Cdk4/6) using highly selective small molecule inhibitors has the potential to provide novel cancer therapies for clinical use. Achieving high levels of selectivity for Cdk4/6, versus other ATP-dependent kinases, presents a significant challenge. The pyrido[2,3-d]pyrimidin-7-one template provides an effective platform for the inhibition of a broad cross-section of kinases, including Cdks. It is now demonstrated that the modification of pyrido[2,3-d]pyrimidin-7-ones to include a 2-aminopyridine side chain at the C2-position provides inhibitors with exquisite selectivity for Cdk4/6 in vitro. This selectivity profile is recapitulated in cells where the most selective inhibitors create a G(1) block at concentrations up to 100-fold the IC(50) for cell proliferation. On the basis of its selectivity profile and pharmacokinetic profile, compound 43 (PD 0332991) was identified as a drug candidate for the treatment of cancer.

Inhibition of the cell cycle kinase, cyclin-dependent kinase-4 (Cdk4), is expected to provide an effective method for the treatment of proliferative diseases such as cancer. The pyrido[2,3-d]pyrimidin-7-one template has been identified previously as a privileged structure for the inhibition of ATP-dependent kinases, and good potency against Cdks has been reported for representative examples. Obtaining selectivity for individual Cdk enzymes, particularly Cdk4, has been challenging. Here, we report that the introduction of a methyl substituent at the C-5 position of the pyrido[2,3-d]pyrimidin-7-one template is sufficient to confer excellent selectivity for Cdk4 vs other Cdks and representative tyrosine kinases. Further optimization led to the identification of highly potent and selective inhibitors of Cdk4 that exhibit potent antiproliferative activity against human tumor cells in vitro. The most selective Cdk4 inhibitors were evaluated for antitumor activity against MDA-MB-435 human breast carcinoma xenografts in mice.