Abstract 376: Combination of duvelisib with either ibrutinib or dexamethasone prevents mTOR-dependent feedback in aggressive B-cell lymphoma cell lines

Abstract

Abstract Background Duvelisib (IPI-145), an oral dual inhibitor of PI3K-δ and PI3K-γ, is in clinical development for the treatment of hematological malignancies. We previously reported on a high throughput combination screen evaluating duvelisib in combination with standard of care and emerging agents in lymphoma cell lines. Results showed dexamethasone and ibrutinib have significant synergy with duvelisib in selected diffuse large B-cell and follicular lymphoma cell lines. Based on these results we sought to better understand the mechanism of this synergy. Methods Duvelisib alone and in combination with dexamethasone or ibrutinib was evaluated in DoHH2 and SU-DHL4 lymphoma cell lines and xenograft models. Cellular signaling was measured by western blot and flow cytometry using phospho-specific antibodies. In vitro, specific inhibitors of ERK (SCH-772984), pan-PI3K (GDC-0941), PI3K-mTOR (PF-04691502), mTORC1 (Rapamycin), and mTORC 1/2 (AZD-8055) were used to evaluate the contribution of these pathways to potential feedback loops. Apoptosis was measured by caspase 3/7 activation, PARP cleavage, and by western blotting and qRT-PCR for pro- and anti-apoptotic Bcl-2 family members after duvelisib or combination treatment. Results Western blotting revealed that cell lines treated with duvelisib alone showed inhibition of phosphorylated (p)AKT at serine 473 only out to 12 hours, with mTORC2 dependent re-phosphorylation of AKT evident at 24 hours. Combination with dexamethasone or ibrutinib prevented this reactivation and also inhibited downstream signaling effectors pPRAS40 and pS6. Combination of duvelisib with dexamethasone also significantly reduced p-4EBP1, a regulator of cap dependent translation initiation, leading to the decreased presence of c-MYC 6 hours after treatment. Enhanced caspase 3/7 activation was seen in both cell lines 4-6 hours post combination treatment compared to monotherapies and was accompanied by a decrease in anti-apoptotic MCL-1 protein and an increase in pro-apoptotic BIM protein. These changes correlated to the appearance of cleaved PARP and eventual cell death. In support of the in vitro studies, in vivo studies with xenografts generated from these cell lines revealed that duvelisib in combination with either dexamethasone or ibrutinib led to greater tumor growth inhibition compared to single agent administration. Conclusions Synergy between duvelisib and dexamethasone or ibrutinib was observed in aggressive lymphoma cell lines. Mechanistic studies revealed a combined effect on apoptotic pathway family members and inhibition of mTOR signaling, which blocked both reactivation of pAKT and translation initiation. These data provide a rationale for exploring these combinations clinically and suggest that suppression of mTOR-driven survival signaling may be an important mechanism for the observed combination synergy. Citation Format: Kerry White, Erin Murphy, Kerrie Faia, Jennifer Proctor, Nicole Kosmider, Melissa Pink, Stanley Goldstein, Karen McGovern, Jeffery L. Kutok. Combination of duvelisib with either ibrutinib or dexamethasone prevents mTOR-dependent feedback in aggressive B-cell lymphoma cell lines. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 376.