Inhibition of neuronal nitric oxide synthase by 4-amino pteridine derivatives: Structure-activity relationship of antagonists of (6R)-5,6,7,8-tetrahydrobiopterin cofactor

The family of nitric oxide synthases (NOS) catalyzes the conversion of L-ar ginine to L-citrulline and nitric oxide (NO), an important cellular messeng er molecule which has been implicated in the pathophysiology of septic shoc k and inflammatory and neurodegenerative disease states. NOS can be maximal ly activated by the ubiquitous cofactor, (6R)-5,6,7,8-tetrahydrobiopterin ( H(4)Bip), and antagonists of H(4)Bip may be of therapeutic importance to in hibit pathologically high NO formation. The 4-amino substituted analogue of H(4)Bip was reported to be a potent NOS inhibit-or. Therefore, we develope d a series of novel 4-amino pteridine derivatives, antipterins, to pharmaco logically target the neuronal isoform of nitric oxide synthase (NOS-I). To functionally characterize the pterin/anti-pterin interaction and establish a structure-activity relationship (SAR), we systematically altered the subs tituents in the 2-, 4-, 5-, 6-, and 7-position of the pteridine nucleus. Va rying the substitution pattern in the 2-, 5-, and 7-position resulted in no significant inhibitory effect on enzyme activity. In contrast, bulky subst ituents in the B-position, such as phenyl, markedly increased the inhibitor y potency of the reduced 4-amino-5,6,7,8-tetrahydropteridines, possibly as a consequence of hydrophobic interactions within NOS-I. However, this was n ot the case for the aromatic 4-amino pteridines. Interestingly, chemical mo dification of the 4-amino substituent by dialkyl/diaralkylation together wi th 6-arylation of the aromatic 2,4-diamino pteridine resulted in potent and efficacious inhibitors of NOS-I, suggesting possible hydrophilic and hydro phobic interactions within NOS-I. This SAR agrees with (a) the recently pub lished crystal structure of the oxygenase domain of the inducible NOS isofo rm (NOS-II) and (b) the comparative molecular field analysis of selected NO S-I inhibitors, which resulted in a 3D-QSAR model of the pterin binding sit e interactions. Further optimization should be possible when the full lengt h structure of NOS-I becomes available.