[Development of new antitumor reagents which block the cell cycle progression]

Most tumor cells have abnormalities in the cell-cycle regulation system. Cyclin-dependent kinases (cdk) are major regulatory molecules for the cell cycle control. Therefore, cell cycle inhibitors, especially cdk-inhibitors, may be a promising new type of antitumor drug. In this review, low-molecular-weight cell-cycle inhibitors isolated from mainly microorganisms are described. As the activation of cdk is caused by the association with cyclins as well as by their phosphorylation and dephosphorylation, kinase inhibitors and phosphatase inhibitors are also candidates for cell cycle inhibitors.