Cyclin dependent kinases (CDKs) and their cyclin complexes play a crucial role in cell cycle control and transcriptional regulation. In multiple myeloma (MM), the abnormal activation of different CDKs and their cyclin partners, especially CDK4/cyclin D1 and CDK6/Cyclin D2, mediate uncontrolled cell cycle progression. Therefore CDKs represent promising novel therapeutic targets for MM. Additionally the cytokine dependent PI3K/Akt signaling pathway mediates growth, survival, drug resistance, migration and cell cycle regulation in MM. Activated Akt in turn phosphorylates downstream target molecules like glycogen synthase kinase (GSK)-3 β impacting growth and survival. Here we investigated the preclinical activity of a novel small-molecule multi-CDK inhibitor, AT7519 in MM. In vitro kinase assays demonstrated more potent inhibition of CDK 1, 2, 4, 5 and 9 compared to CDK 3, 6, and 7. AT7519 also demonstrated potent inhibitory activity against GSK-3 β. No significant inhibitory effects against other kinases were observed. We next investigated the growth inhibitory effect of AT7519 on MM cell lines. Maximal cytotoxicity was observed in 48 hour culture with IC50 values ranging from 0.5μM (MM.1S, U266) to 4 μM (MM1R). AT7519 was also effective against primary tumor cells from MM patients with no significant cytotoxicity noted in peripheral blood mononuclear cells from healthy volunteers. To delineate the underlying mechanism of cytotoxicity induced by AT7519, cell cycle analysis using PI staining in MM.1S cell line was performed. No significant accumulation of cells in a particular phase of cell cycle was noted; however, AT7519 showed an increased sub-G1 population, indicative of apoptosis, which was confirmed by Annexin V+PI+ staining and associated with caspase-8-9 and -3 cleavage. Importantly, we found that AT7519 markedly inhibited phosphorylation (serine 2 and serine 5 sites) of the carboxyl terminal domain of RNA polymerase II (RNA pol II) within 6 hours of treatment. Non-cell cycle CDKs including CDK9 are responsible for phosphorylation and activation of RNA pol II. Similarly, AT7519 also inhibited phosphorylation of GSK-3β while no significant effects on CDK expression levels were evident at early time points. To investigate whether there was a correlation between inhibition of phosphorylation of GSK-3β and RNA pol II, MM.1S cells were cultured with α-amanitin, a specific inhibitor of RNA pol II. Although phosphorylation of RNA pol II was significantly inhibited, phosphorylation of GSK-3β was not altered by amanitin (10 μM for up to 24 hours). These results suggest that GSK-3β and RNA pol II dephosphorylation at serine 2 and serine 5 may be two independent mechanisms by which AT7519 induces apoptosis in MM cells. Ongoing studies are confirming the role of GSK-3 β in AT7519 induced cytotoxicity of MM cells. Finally, the in vivo efficacy of AT7519 was examined using a xenograft mouse model of human MM. Mice treated with AT7519 demonstrated slower tumor growth compared to the control group without adverse effects. Moreover, AT7519 resulted in a significant prolongation in median overall survival in treated mice (40 days in the treatment group versus 27.5 days in the control cohort, p = 0.0324). In conclusion, these results show significant anti-MM activity of AT7519, and provide the rationale for its clinical evaluation in MM.

Disclosures: Squires:Astex Therapeutics: Employment. Anderson:Celgene: Speakers Bureau; Millennium Pharmaceuticals: Speakers Bureau; Novartis: Speakers Bureau. Raje:Millennium Pharmaceuticals: Speakers Bureau; Celgene: Speakers Bureau; Novartis: Speakers Bureau.

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