Selectivity and potency of cyclin-dependent kinase inhibitors

Members of the cyclin-dependent kinase (CDK) family play key roles in various cellular processes. There are 11 members of the CDK family known till now. CDKs are activated by forming noncovalent complexes with cyclins such as A-, B-, C-, D- (D1, D2, and D3), and E-type cyclins. Each isozyme of this family is responsible for particular aspects (cell signaling, transcription, etc) of the cell cycle, and some of the CDK isozymes are specific to certain kinds of tissues. Aberrant expression and overexpression of these kinases are evidenced in many disease conditions. Inhibition of isozymes of CDKs specifically can yield beneficiary treatment modalities with minimum side effects. More than 80 3-dimensional structures of CDK2, CDK5, and CDK6 complexed with inhibitors have been published. This review provides an understanding of the structural aspects of CDK isozymes and binding modes of various known CDK inhibitors so that these kinases can be better targeted for drug discovery and design. The amino acid residues that constitute the cyclin binding region, the substrate binding region, and the area around the adenosine triphosphate (ATP) binding site have been compared for CDK isozymes. Those amino acids at the ATP binding site that could be used to improve the potency and subtype specificity have been described.