Inhibition of neuronal nitric oxide synthase (nNOS) is a promising therapeutic approach to treat neurodegenerative diseases. Recently, we have achieved considerable progress in improving the potency and isoform selectivity of human nNOS inhibitors bearing a 2-aminopyridine scaffold. However, these inhibitors still suffered from too low cell membrane permeability to enter into CNS drug development. We report herein our studies to improve permeability of nNOS inhibitors as measured by both PAMPA-BBB and Caco-2 assays. The most permeable compound (12) in this study still preserves excellent potency with human nNOS (Ki = 30 nM) and very high selectivity over other NOS isoforms, especially human eNOS (hnNOS/heNOS = 2799, the highest hnNOS/heNOS ratio we have obtained to date). X-ray crystallographic analysis reveals that 12 adopts a similar binding mode in both rat and human nNOS, in which the 2-aminopyridine and the fluorobenzene linker form crucial hydrogen bonds with glutamate and tyrosine residues, respectively.

Effective delivery of therapeutic drugs into the human brain is one of the most challenging tasks in central nervous system drug development because of the blood-brain barrier (BBB). To overcome the BBB, both passive permeability and efflux transporter liability of a compound must be addressed. Herein, we report our optimization related to BBB penetration of neuronal nitric oxide synthase (nNOS) inhibitors toward the development of new drugs for neurodegenerative diseases. Various approaches, including enhancing lipophilicity and rigidity of new inhibitors and modulating the p K_a of amino groups, have been employed. In addition to determining inhibitor potency and selectivity, crystal structures of most newly designed compounds complexed to various nitric oxide synthase isoforms have been determined. We have discovered a new analogue (21), which exhibits not only excellent potency ( K_i < 30 nM) in nNOS inhibition but also a significantly low P-glycoprotein and breast-cancer-resistant protein substrate liability as indicated by an efflux ratio of 0.8 in the Caco-2 bidirectional assay.