Enhancement of vascular targeting by inhibitors of nitric oxide synthase

Abstract Purpose: This study investigates the enhancement of the vascular targeting activity of the tubulin-binding agent combretastatin A4 phosphate (CA4P) by various inhibitors of nitric oxide synthases. Methods and Materials: The syngeneic tumors CaNT and SaS growing in CBA mice were used for this study. Reduction in perfused vascular volume was measured by injection of Hoechst 33342 24 h after drug administration. Necrosis (hematoxylin and eosin stain) was assessed also at 24 h after treatment. Combretastatin A4 phosphate was synthesized by a modification of the published procedure and the nitric oxide synthase inhibitors L -NNA, L -NMMA, L -NIO, L -NIL, S -MTC, S -EIT, AMP, AMT, and L -TC, obtained from commercial sources. Results: A statistically significant augmentation of the reduction in perfused vascular volume by CA4P in the CaNT tumor was observed with L -NNA, AMP, and AMT. An increase in CA4P-induced necrosis in the same tumor achieved significance with L -NNA, L -NMMA, L -NIL, and AMT. CA4P induced little necrosis in the SaS tumor, but combination with the inhibitors L -NNA, L -NMMA, L -NIO, S -EIT, and L -TC was effective. Conclusions: Augmentation of CA4P activity by nitric oxide synthase inhibitors of different structural classes supports a nitric oxide-related mechanism for this effect. L -NNA was the most effective inhibitor studied.