Roscovitine is an oral 2,6,9 trisubstituted purine analog currently under phase II investigation, which competes with ATP for the catalytic binding site on CDK2 with a demonstrated antitumor activity in many human cancer models and a nice toxicity profile. One of the most prominent effects of the drug is the inhibition of CDK2/cyclin E complexes, which causes a decrease in inhibitor licensed Rb phosphorylation and a consequent inacti vation of E2F family members, thus leading to cyclin transcriptional downregulation Inhibitors,Modulators,Libraries and ultimately to cell cycle arrest. This strong transcriptional depression of most of the cell cycle related cyclins further enforces the drugs inhibitory effect on CDK/cyclin complexes. Furthermore, Roscovitine has been shown to downregu late several other genes involved in a wide spectrum of cellular functions, probably as a result of partial CDK7/cyclin H and CDK9/cyclin T inhibition.

In addition, whole genome ChIP on chip analysis recently mapped E2F transcription factor family members to the promoters of many more genes than were traditionally associated with the cell cycle, suggesting an alterna tive mechanism to explain these transcriptional effects. We investigated the effects that Roscovitine may have on cyclin A1 transcription as one of Inhibitors,Modulators,Libraries the possible mechanisms through which CDK2 inhibition may curb DNA DSB repair activity. The promoter of the cyclin A1 gene, CCNA1 is not E2F dependent and, consis tently, increasing doses of Inhibitors,Modulators,Libraries Roscovitine did not repress cyclin A1 basal transcription levels in contrast to cyclins A2, B, D and E.

However, we demonstrated that Roscov itine at doses preferentially inhibiting CDK2 but not CDK7 and 9 completely abolished cyclin A1 DNA damage induced upregulation, thus suggesting that resi dual CDK2 activity is required for cyclin A1 upregula tion. In addition Roscovitine co administered with doxorubicin was able to largely modify the patterns of cell cycle phase distribution Inhibitors,Modulators,Libraries in comparison to doxorubi cin only treatment. This resulted in an augmented S phase and consequently in an increased expression of cyclin A2. The combined treatment thus resulted Inhibitors,Modulators,Libraries in the complete inversion of the doxorubicin induced switch between cyclin A1 and cyclin A2. Roscovitine, alone or under DNA damaging condi tions, was able to diminish cyclin A1 protein levels as well.

Such transcriptional and post transcriptional repression was observed in different NSCLC, prostate and breast cancer cell lines and we propose that this potentiates and synergizes the Roscovitine mediated CDK2 inhibition thus Oligomycin A buy resulting in a significant decrease of cellular NHEJ ability. In fact, we observed that combi nation treatment led to an increase in DNA DSBs and overall DNA damage over time, further substantiating, not only the importance of CDK inhibitors in combina tion therapy but also the role of CDKs in DNA repair mechanisms.