Modulating cell cycle: current applications and prospects for future drug development.

The cell cycle is a highly conserved and ordered set of events, culminating in cell growth and division. It is tightly controlled by many regulatory mechanisms that either permit or restrain its progression. The main families of regulatory proteins that play key roles in controlling cell cycle progression are the cyclins, the cyclin dependent kinases (Cdks), their substrate proteins, the Cdk inhibitors (CKI) and the tumor suppressor gene products, p53 and pRb. Many cell cycle control genes, when deregulated, can cause cells that are not dividing to enter the cell cycle and begin to proliferate leading to cancer development. They do so by interfacing with the basic cell cycle regulatory machinery to activate cell cycle entry. There is at present much optimism about the possibility of finding anticancer drug treatment strategies that modulate cell cycle regulatory molecules. Candidate targets for such strategies include crucial cell cycle molecules involved in G(1) to S phase or G(2) to M phase transition. This review will outline the basic regulatory machinery responsible for catalyzing cell cycle entry and describe the latest advances made in the field of cell cycle regulation. The basis of targeting the cell cycle particularly the Cdks as an approach to developing novel, specific and perhaps more effective anticancer treatments will be discussed. Examples of novel cell cycle-targeting agents that are in, or are close to being in clinical trials will be provided.