Novel cyclin dependent kinase (CDK) inhibitors induce down regulation of Mcl-1 and apoptosis in multiple myeloma cells

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Multiple myeloma is a disease of malignant B-cells that have extended survival and a low proliferation rate which accumulate in the bone marrow causing osteolytic bone lesions. Treatment with current chemotherapeutic agents initially reduces tumour burden but the disease remains incurable, with the average patient survival time being three years. Therefore, there is an urgent need for novel therapies ideally based on an understanding of the biology of the disease. One area of interest is that of cyclin dependent kinases that regulate two main cellular processes essential for malignant cell survival; cell cycle progression and transcription. These kinases are good cellular targets for anti-neoplastic drugs. Seliciclib (CYC202, R-roscovitine) is a CDK inhibitor, currently in clinical trials, that principally targets CDK2, CDK7 and CDK9 and shows potent activity against multiple myeloma cells (MacCallum et al., 2005. Cancer Res. 65 pp5399). Selicliclib acts in part by inhibiting the kinases that phosphorylate the C-terminal domain (CTD) of RNA polymerase II resulting in the inhibition of transcription and down regulation of mRNAs and proteins with short half-lives. One such protein is the anti-apoptotic Mcl-1 that is essential for multiple myeloma cell survival. This study characterises three novel CDK inhibitors, two tri-substituted purines and a 2-anilino-4-(thiazol-5-yl)-pyrimidine that have greater potency in both in vitro kinase assays and against cells in culture. Upon treatment of multiple myeloma cells, all three compounds rapidly inhibit phosphorylation of the CTD of RNA polymerase II at phospho-serine 2, accompanied by an equivalent effect on gene expression to that seen with seliciclib treatment. These changes include down regulation of Mcl-1 at both the mRNA and protein level resulting in induction of apoptosis; detectable by PARP cleavage and TUNEL. The results demonstrate that these CDK inhibitors work by a similar mechanism to seliciclib but with increased potency. Furthermore, they provide scientific rationale for their continued pre-clinical development.