CYC202 (seliciclib or R-Roscovitine), a small molecule cyclin dependent kinase inhibitor, overcomes drug resistance via down-regulation of Mcl-1 in multiple myeloma (MM)

Abstract

6532 Background: Cell cycle is positively regulated by a family of proteins called cyclin dependent kinases (CDKs). CDK inhibitors have the potential to induce apoptosis in cancer cells. CYC202 is a potent inhibitor of CDKs currently undergoing phase II clinical testing in lung cancer, breast cancer, and B cell hematologic malignancies. Methods: We studied the in vitro activity of CYC202 in multiple myeloma (MM). MM cell lines and patient derived cells were used. Co-culture systems with patient derived bone marrow stromal cells (BMSCs) were used to recapitulate the in vivo host-tumor cell/BM interaction. Studies of cell cycle and apoptosis were performed and molecular mechanisms of drug activity were delineated. Results: Our data demonstrates that CYC202 has potent cytotoxicity against MM cells that are both sensitive (MM1.S, RPMI 8226, U266, OPM2) and resistant (MM1.R, Dox-40, LR5, MR 20) to conventional chemotherapy as well as primary MM patient cells. Cell cycle analysis demonstrated an increase in the apoptotic fraction of MM cells. Importantly, CYC202 triggered a rapid down-regulation of Mcl-1 transcription and protein expression independent of caspase cleavage. Co-culture experiments with BMSCs resulted in increased Mcl-1 expression associated with STAT3 phosphorylation, both of which were inhibited in a time and dose dependent manner with CYC202. To further characterize the molecular mechanism of Mcl-1 inhibition in our co-culture system, we demonstrate inhibition of interleukin-6 (IL-6) transcription and adhesion-mediated production of IL-6. Mcl-1 expression was only in part blocked by neutralizing antibody to IL-6, suggesting other possible mechanisms of Mcl-1 modulation by CYC202. CYC202 with Bortezomib and Doxorubicin demonstrated strong synergism in vitro, suggesting a role for combination therapies in MM. Conclusions: Our data demonstrates a molecular mechanism of activity of CYC202 in MM. Importantly, CYC202 overcomes drug resistance and is active even in the presence of BMSCs. These data provide the pre-clinical rationale for testing CYC202, alone and in combination, to treat patients with relapsed/refractory MM. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Cyclacel Ltd.