Synergistic effects of methylnaltrexone with 5-fluorouracil and bevacizumab on inhibition of vascular endothelial growth factor-induced angiogenesis

Many patients with cancer receive combinations of drug treatments that include 5-fluorouracil (5-FU) and bevacizumab. Therapeutic doses of 5-FU are often associated with unwanted side effects, and bevacizumab is costly. Therefore, we explored potential agents that can reduce the therapeutic concentration of these drugs. Our data indicate that methylnaltrexone (MNTX), a peripheral antagonist of the μ-opioid receptor, exerts a synergistic effect with 5-FU and bevacizumab on inhibition of vascular endothelial growth factor (VEGF)-induced human pulmonary microvascular endothelial cell (EC) proliferation and migration, two key components in cancer-associated angiogenesis. MNTX inhibited EC proliferation with an IC₅₀ of ∼100 nmol/L. Adding 100 nmol/L MNTX to EC shifted the IC₅₀ of 5-FU from ∼5 μmol/L to ∼7 nmol/L. Further, adding 50 ng/mL MNTX shifted the IC₅₀ of bevacizumab on inhibition of EC migration from ∼25 to ∼6 ng/mL. These synergistic effects were not observed with naltrexone, a tertiary μ-opioid receptor antagonist. On a mechanistic level, we observed that treatment of human EC with MNTX, but not naltrexone, increased receptor protein tyrosine phosphatase μ activity, which was independent of μ-opioid receptor expression. Silencing receptor protein tyrosine phosphatase μ expression (small interfering RNA) in human EC inhibited both synergy between MNTX and bevacizumab or 5-FU and increased VEGF-induced tyrosine phosphorylation of Src and p190 RhoGAP with enhanced activation of Akt and the actin cytoskeletal regulatory protein, RhoA, whereas silencing Src, Akt, or RhoA blocked VEGF-induced angiogenic events. Therefore, addition of MNTX could potentially lower the therapeutic doses of 5-FU and bevacizumab, which could improve index.