1400W is a potent, selective inhibitor of iNOS. Using purified human iNOS, nNOS, and eNOS, 1400W exhibits Ki values of 7 nM, 2 µM, and 50 µM, respectively. In rat aortic rings it is at least 1,000-fold more potent against iNOS compared to eNOS. In mice bearing a murine mammary carcinoma, continuous infusion of 1400W at a rate of 10 mg/kg for 6 days decreased tumor weight approximately 50% compared to controls.