Abstract 4630: The pan-PIM inhibitor PIM447 enhances the antitumor activity of lenalidomide in multiple myeloma cells via synergistic inhibition of c-MYC

Introduction: Immunomodulatory drugs (IMiDs) thalidomide, lenalidomide and pomalidomide have significantly contributed to improved overall survivals in multiple myeloma (MM) patients. However, cure remains an elusive goal for the vast majority of MM patients and novel combination regimens are needed. Recently, the mechanism of action of IMiDs was identified to be inducing degradation of two MM transcription factors, IZKF1 and IZKF3. IKZF1/IKZF3 regulates the expression of oncogene c-MYC via another transcription factor-interferon regulatory factor 4 (IRF4). This new finding provides an opportunity to study synergistic combinations of IMiDs with targeted therapies. PIM447 (formerly LGH447, Novartis Pharmaceuticals) is a pan-PIM kinase inhibitor that is in clinical development for the treatment of patients with MM and other hematological malignancies. Based on the observation that several MM lines are sensitive to both IMiD and PIM447, we evaluated the combination effects of PIM447 and lenalidomide in 9 MM cell lines. Methods: An 8×8 serial dilution dose matrix of PIM447 and lenalidomide were tested in a panel of 9 MM cell lines. RT-PCR and immunoblotting were performed in selected lines to define possible mechanisms of combination activity. Results: Combination activity (synergy score>2) was observed in 4 out of the 9 MM cell lines tested, with MM1-S and NCI-H929 showing the highest synergy scores. Combination activity was only noted in the presence of PIM447 single agent activity and appeared to correlate with PIM2 protein expression. In both MM1-S and NCI-H929 cells, the combination led to increased apoptosis as measured by PARP cleavage. PIM447 did not affect the levels of various targets of lenalidomide, including IKZF1, IKZF3 and IRF4, and lenalidomide did not affect PIM downstream effectors such as phospho-S6RP. Alterations involving c-MYC are amongst the most frequent genetic features found in MM, and the combination of PIM447 and lenalidomide resulted in greater reduction of c-MYC expression. In both MM1-S and NCI-H929 cells, c-MYC knockdown has previously been shown to inhibit cell proliferation, suggesting the combination activity might derive from the observed reduction of c-MYC expression. The mechanisms by which c-MYC expression were synergistically decreased by the combination may involve both enhanced downregulation of c-MYC transcripts and accelerated degradation of c-MYC protein. Conclusions: The pan-PIM kinase inhibitor PIM447 enhanced the