INHIBITION OF NITRIC-OXIDE SYNTHASE WITH PYRAZOLE-1-CARBOXAMIDINE ANDRELATED-COMPOUNDS

Guanidines, amidines, S-alkylisothioureas, and other compounds contain ing the amidine function (-C(=H)NH2) have been described as inhibitors of the generation of nitric oxide (NO) by NO synthase (NOS). Here we report on the inhibition of the activity of NOS isoforms by compounds in which the amidine function is attached to a nitrogen of 1,2-diazo h eterocycles to form N-carboxamidines and related compounds. 1H-Pyrazol e 1-carboxamidine HCl (PCA) inhibited the activity of purified inducib le NOS (iNOS), endothelial NOS (eNOS), and neuronal NOS (nNOS) isoform s to a similar extent (IC50 = 0.2 mu M). 3-Methyl-PCA and 4-methyl-PCA showed reduced potencies, but a preference for iNOS [IC50 = 5 and 2.4 mu M, respectively; cf. N-G-methyl-L-arginine (NMA) IC50 = 6 mu M]. I nhibition of purified iNOS by PCAs could be reversed completely by exc ess L-arginine, while their inhibition of NO production by stimulated RAW macrophages could be reversed by transfer to a drug-free medium. T his suggests a competitive mode of inhibition. PCA caused potent conce ntration-dependent inhibition of the acetylcholine-induced, endotheliu m-dependent relaxations of precontracted rat thoracic aorta (IC50 = 30 mu M). 4-Methyl-PCA inhibited the relaxations only at greater than or equal to 300 mu M. In contrast, 4-methyl-PCA was more effective than both PCA and NMA in restoring the ex vivo contractility of aortic ring s taken from lipopolysaccharide-treated rats. PCA and NMA, but not 4-m ethyl-PCA, caused marked increases in mean arterial pressure when admi nistered i.v. to anesthetized rats. In conclusion, PCA and related com pounds caused potent inhibition of NOS. Substitution of the pyrazole r ing reduced potency, but improved selectivity towards iNOS as exemplif ied by 4-methyl-PCA. (C) 1997 Elsevier Science Inc.