Cyclin-dependent kinases (CDKs) are serine/threonine kinases whose activity depends on a regulatory subunit- a cyclin. CDKs play important roles in the control of cell division and modulate transcription in response to several extra- and intracellular cues. Specifically, CDKs phosphorylate their substrates by transferring phosphate groups from ATP to specific stretches of amino acids in the substrates. Different types of eukaryotic cells contain different types and numbers of CDKs. Several drug discovery programs have produced potent small-molecule Cdk inhibitors. Additionally, several compounds that inhibit Cdk activity are currently in clinical trials, including flavopiridol, CYC-202, UCN-01 , and BMS-387032. Most of these compounds, however, inhibit multiple Cdks, with Cdk2 being a particularly common target.

Joshiet al has described Riviciclib, a potent Cdk inhibitor. It shows 40-fold selectivity toward Cdk4-D1, compared with Cdk2-E. Riviciclib is more selective Cdk4-D1, Cdk1-B, and Cdk9-T1 with IC50 less than 100 nM and less selective for non-Cdk kinases. It showed potent antiproliferative effects against various human cancer cell lines, with an IC50 ranging from 300 to 800 nM. Besides, Riviciclib produced potent inhibition of Cdk4-D1 activity that was found to be competitive with ATP and not with retinoblastoma protein. The compound also induced apoptosis in human promyelocytic leukemia (HL-60) cells, as evidenced by the induction of caspase-3 and DNA ladder studies. These data suggest that Riviciclib has the potential to be developed as an anti-Cdk chemotherapeutic agent. It also showed excellent in vivo activity against murine and human tumor models of colon and lung carcinoma without toxic side effects.

Joshi KS, et al. P276-00, a novel cyclin-dependent inhibitor induces G1-G2 arrest, shows antitumor activity on cisplatin-resistant cells and significant in vivo efficacy in tumor models.Mol Cancer Ther. 2007 Mar;6(3):926-34.