Methylnaltrexone potentiates the antiangiogenic effects of the mTOR inhibitor, temsirolimus

Abstract

e14636 Background: Angiogenesis is important in cancer growth and metastasis. Recent therapeutic interventions cancer, include drugs such as temsirolimus that target both tumor growth and angiogenesis including the mammalian target of rapamycin (mTOR) inhibitor. Previously, we have demonstrated that the peripheral opioid antagonist methylnaltrexone (MNTX) potentiates the effects of bevacizumab and 5FU on endothelial cell (EC) migration and proliferation. Since mTOR inhibitor therapy is also associated with significant side effects, we examined potential adjunctive agents that could reduce the therapeutic dose and improve the therapeutic window for this approach. Methods: We determined the effect(s) of MNTX, naloxone and/or temsirolimus on VEGF-induced human pulmonary microvascular EC proliferation and migration assays. We used siRNA and specific inhibitors to analyze Src, Akt and mTOR regulation. Results: MNTX inhibited EC VEGF-induced proliferation and migration with an IC50 of ∼100 nM. Adding 10 nM MNTX to EC shifted the IC50 of temsirolimus inhibition of VEGF-induced proliferation from ∼10 nM to ∼1 nM. Further, adding 10 nM MNTX shifted the IC50 of temsirolimus on inhibition of EC migration from ∼50 nM to ∼10 nM. These synergistic effects were not observed with naltrexone, a tertiary mu opioid receptor antagonist. On a mechanistic level, we observed that treatment of human EC with MNTX, but not naltrexone, increased protein tyrosine phosphatase (PTP) activity which was independent of mu opioid receptor expression. Treatment of human EC with the PTP inhibitor, 3,4-Dephostatin, inhibited both the synergy between MNTX and temsirolimus and increased VEGF-induced tyrosine phosphorylation of Src with enhanced PI3 kinase and mTOR complex 2-dependent phosphorylation of Akt and subsequent activation of mTOR complex 1 (temsirolimus target), while silencing Src, Akt or mTOR complex 2 components blocked VEGF-induced angiogenic events. Conclusions: Synergy of MNTX with mTOR inhibitors may have therapeutic implications which await clinical proof-of-concept. [Table: see text]