Cell-permeable. A slow, tight binding and highly selective inhibitor of iNOS (inducible nitric oxide synthase/NOS II) (Kd ≤ 7 nM). In contrast, inhibition of nNOS and eNOS is weaker and rapidly reversible with Kᵢ values of 2 and 50 µM respectively. Inhibits tumor growth in vivo. Prevents circulatory failure but not organ injury induced by endotoxin. Displays protective effects in oxygen-deprived brain slices.