Nitric Oxide Synthase Inhibition Enhances the Tumor Vascular-Damaging Effects of Combretastatin A-4 3-O-Phosphate at Clinically Relevant Doses

Purpose: The therapeutic potential of combining the prototype tumor vascular-disrupting agent combretastatin A-4 3-O phosphate (CA-4-P) with systemic nitric oxide synthase (NOS) inhibition was; investigated preclinically. Experimental Design: Vascular response (uptake of I-125-labeled iodoantipyrine; laser Doppler flowmetry) and tumor response (histologic necrosis; cytotoxicity and growth delay) were determined. Results: Inducible NOS selective inhibitors had no effect on blood flow in the P22 rat sarcoma. In contrast, the non-isoform-specific NOS inhibitor N-omega-nitro-L-arginine (L-NNA; 1 and 10 mg/kg i.v. or chronic 0.1 or 0.3 mg/mL in drinking water) decreased the P22 blood flow rate selectively down to 36% of control at 1 hour but did not induce tumor necrosis at 24 hours. CA-4-P, at clinically relevant doses, decreased the P22 blood flow rate down to 6% of control at 1 hour for 3 mg/kg but with no necrosis induction. However, L-NNA administration enhanced both CA-4-P-induced tumor vascular resistance at 1 hour (chronic L-NNA administration) and necrosis at 24 hours, with 45% or 80% necrosis for 3 and 10 mg/kg CA-4-P, respectively. Bolus L-NNA given 3 hours after CA-4-P was the most effective cytotoxic schedule in the CaNT mouse mammary carcinoma, implicating a particular enhancement by L-NNA of the downstream consequences of CA-4-P treatment. Repeated dosing Of L-NNA with CA-4-P produced enhanced growth delay over either treatment alone in P22, CaNT, and spontaneous T138 mouse mammary tumors, which represented a true therapeutic enhancement. Conclusions: The combination of NOS inhibition with CA-4-P is a promising approach for targeting tumor vasculature, with relevance for similar vascular-disrupting agents in development.