Substituted N-Phenylisothioureas: Potent Inhibitors of Human Nitric Oxide Synthase with Neuronal Isoform Selectivity

S-Ethyl N-phenylisothiourea (4) has been found to be a potent inhibitor of both the human constitutive and inducible isoforms of nitric oxide synthase. A series of substituted N-phenylisothiourea analogues was synthesized to investigate the structure−activity relationship of this class of inhibitor. Each analogue was evaluated for human isoform selectivity. One analogue, S-ethyl N-[4-(trifluoromethyl)phenyl]isothiourea (39), exhibited 115-fold and 29-fold selectivity for the neuronal isoform versus the inducible and endothelial derived constitutive isoforms, respectively. Studies have shown the substituted N-phenylisothiourea 39 binds competitively with l-arginine.