Combined Carfilzomib and Selective PI3Kδ Inhibition (TGR1202) Results In Enhanced Myeloma Cell Apoptosis

Introduction The PI3K signaling pathway plays a vital role in regulating cell growth, proliferation and survival in multiple myeloma (MM) as well as in many other cancers. TGR-1202, an isoform-specific PI3Kδ inhibitor, with efficacy in preclinical models of hematologic malignancies, is currently in Phase I clinical development. In multiple myeloma, PI3K signaling appears to be very important for many extracellular signals, yet inhibition with -pan PI3K inhibitors have not exhibited significant activity. However, literature reports indicate that there are several MM cell lines that express PI3K -δ, and do appear to have differential sensitivity to specific isoform inhibition as opposed to pan PI3K inhibition. In this report, we sought to evaluate the effects of TGR-1202 alone and in combination with the proteasome inhibitor carfilzomib, with the intent of further understanding the mechanism of action and evaluating the impact of the combination. Methods Human myeloma cell lines (MM.1S, MY5, RPMI8226, U266, KMS11, ARH-77, OPM1, OPM2, LP1, JJN3 and L363) were treated with TGR-1202 alone, carfilzomib alone, or with the combination of TGR-1202 and carfilzomib. Annexin V/PI staining and Western blot were used to identify the cellular and molecular sequelae of the combination. Result 10 µM TGR-1202 alone did not cause significant cell death in the MM cell lines tested at 48 hours. When cells were treated with the combination of TGR-1202 and 3 nM carfilzomib, we observed enhanced apoptosis in all of the tested cell lines. In the U266 cell line, 3 nM carfilzomib and 10 µM TGR-1202 induced 16% and 14% cell apoptosis respectively. In the combination treatment, apoptosis increased to 75%. To explore the molecular mechanisms underlying the combination, we used a Western blot assay to evaluate the impact of the combination on the mTOR signaling pathway, a known reciprocal feedback loop when PI3K is blocked. TGR-1202 alone did not have an obvious effect on the mTOR signaling pathway, yet combining TGR-1202 with carfilzomib, significantly inhibited phospho-mTOR, suggesting total pathway blockade. Conclusion The combination of TGR-1202 with carfilzomib induces synergistic apoptosis in MM cell lines. Preliminary data suggests that this occurs through blockade of the entire reciprocal feedback loop of mTOR activation. Additional data from primary patient samples and underlying mechanisms will be presented at the meeting. These findings support the rationale for future clinical studies of TGR-1202, a selective PI3K-δ inhibitor in combination with the proteasome inhibitor carfilzomib. Disclosures: Boise: Onyx: Consultancy. Lonial: Millennium: Consultancy; Celgene: Consultancy; Novartis: Consultancy; BMS: Consultancy; Sanofi: Consultancy; Onyx: Consultancy.