Exploration of the Active Site of Neuronal Nitric Oxide Synthase by the Design and Synthesis of Pyrrolidinomethyl 2-Aminopyridine Derivatives

Neuronal nitric oxide synthase (nNOS) represents an important therapeutic target for the prevention of brain injury and the treatment of various neurodegenerative disorders. A series of trans-substituted amino pyrrolidinomethyl 2-aminopyridine derivatives (8-34) was designed and synthesized. A structure activity relationship analysis led to the discovery of low nanomolar nNOS inhibitors ((+/-)-32 and (+/-)-34) with more than 1000-fold selectivity for nNOS over eNOS. Four enantiomerically pure isomers of 3'-[2 ''-(3 ''-fluorophenethylamino)ethoxy]pyrrolidin-4'-yl}methyl}-4-methylpyridin-2-amine (4) also were synthesized. It was found that (3'R,4'R)-4 can induce enzyme elasticity to generate a new "hot spot" for ligand binding. The inhibitor adopts a unique binding mode, the same as that observed for (3'R,4'R)-3'-[2 ''-(3'''-fluorophenethylamino)ethylamino]pyrrolidin-4'-yl}methyl}-4-methylpyridin-2-amine ((3'R,4'R)-3) (J. Am. Chem. Soc. 2010, 132 (15), 5437-5442). On the basis of structure-activity relationships of 8-34 and different binding conformations of the cis and trans isomers of 3 and 4, critical structural requirements of the NOS active site for ligand binding are revealed.