In Vitro Activity of a Novel Small Molecule Cyclin Dependent Kinase Inhibitor, CYC202 (seliciclib or R-Roscovitine), in Multiple Myeloma (MM).

Perturbation of cell cycle is central to tumorigenesis, resulting in unrestricted cell proliferation. Cyclin dependent kinase (CDK) inhibitors have the potential to induce cell cycle arrest, followed by apoptosis, in cancer cells. CYC202 (seliciclib or R-roscovitine; Cyclacel, Dundee, UK) is a potent inhibitor of CDKs currently undergoing phase II clinical testing. In vitro data has shown this tri-substituted purine to have greatest potency against CDK2/cyclin E, although it also inhibits CDK2/cyclin A, CDK7/ cyclin H, and CDK9/cyclin T. It has been tested against a broad range of tumor cell lines, ultimately inducing cell cycle changes and apoptosis. This orally bio-available compound has also induced regression of human tumor xenografts in nude mice, prompting its clinical testing. Phase I studies have demonstrated favorable pharmacokinetics and toxicity profiles, and phase II trials are currently ongoing in combination with gemcitabine /cisplatin for non-small cell lung cancer and as a monotherapy in B-cell malignancies. We have previously shown that MM cell lines demonstrate relatively low levels of p21^WAF1, suggesting that CDKs are constitutively activated, thereby promoting uncontrolled cell cycle regulation, growth, and proliferation. Conversely, inhibiting CDK activity may therefore trigger MM cell growth inhibition. Here we studied the in vitro activity of CYC202 in MM cells. Our data demonstrates that CYC202 has potent cytotoxic effects against MM cells that are both sensitive (MM1.S, RPMI 8226, U266, H929) and resistant (MM1.R, Dox-40, LR5, MR 20) to conventional chemotherapy. MM cell line cytotoxicity, as evidenced by MTT assays, is noted at 24 hours, with IC₅₀ ranging from 25–50 mmol. In contrast, this dose was not toxic to normal peripheral blood mononuclear cells. Cell cycle analysis demonstrated an increase (35–50%) in MM cells in sub-G1 phase at 24 hrs induced by CYC202 (25 mmol), suggesting that CYC202 triggers apoptosis. Caspase-8 and poly ADP-ribose polymerase (PARP) cleavage, evidenced by western blot analysis further confirmed apoptosis of MM cells. Importantly, CYC202 triggered a rapid down-regulation of MCL-1, a known anti-apoptotic protein in MM. Treatment of MM cells with CYC202 also resulted in decreased phosphorylation of retinoblastoma protein. Protein expression of certain CDKs, specifically CDC2, CDK4, and CDK6, were also down-regulated after treatment. Ongoing studies are delineating the specific signaling cascades affected by CYC202 treatment. These studies provide the pre-clinical basis for a clinical trial of CYC202, either alone or in combination with other agents to improve patient outcome in MM.