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Cyclin-dependent protein kinases (CDKs) are key regulators of the cell division cycle, whose various checkpoints proliferating cells must traverse. Since CDK deregulation, either through direct or indirect means, is found in most cancer cells, pharmacological CDK inhibition has become an attractive strategy towards mechanism-based and non-genotoxic therapies in oncology. Over the last decade, discovery and lead optimisation efforts have provided a wealth of potential drug candidate molecules capable of inhibiting CDKs, blocking cell-cycle progression, modulating transcription and inducing apoptosis selectively in cancer cells. However, only few such agents have as yet reached clinical evaluation. Here, the preclinical and clinical results obtained so far with flavopiridol (L868275, HMR1275; Aventis), 7-hydroxystaurosporine (UCN-01, KW-2401; Kyowa Hakko Kogyo) and roscovitine (R-roscovitine, CYC202; Cyclacel) are summarised. Furthermore, the potential for monotherapy and applications in combination with existing drugs are discussed.

Cyclin-dependent kinases (CDKs) have long been known to be the main facilitators of the cell proliferation cycle. However, they also play important roles in the regulation of the RNA polymerase II transcription cycle. Cancer cells display aberrant cell cycle regulation to gain proliferative advantages and they also appear to have an exaggerated dependence on RNA polymerase II transcriptional activity to sustain pro-survival and antiapoptotic signalling. A picture is now starting to emerge that both the cell-cycle and transcriptional functions of CDKs can be exploited pharmacologically with CDK inhibitors that possess appropriate selectivity profiles. In this article, recent advances into these mechanistic insights and how they can guide clinical development in terms of choice of indication are reviewed, as well as combinations with existing chemotherapies. An overview is also given of recent clinical trial results with the lead CDK inhibitor drug candidates seliciclib (CYC202, (R)-roscovitine; Cyclacel) and alvocidib (flavopiridol; Aventis-NCI), as well as the development of other clinical entries and advanced preclinical compounds. The discussion focuses on oncology, but we point out recent results with CDK inhibitors in virology and nephrology.