Heterocyclic analogues of l-citrulline as inhibitors of the isoforms of nitric oxide synthase (NOS) and identification of Nδ-(4,5-dihydrothiazol-2-yl)ornithine as a potent inhibitor Original Research Article

l-Thiocitrulline is a known potent inhibitor of several isoforms of nitric oxide synthase (NOS). To explore the structure–activity relationships (SARs) for this molecule in more depth than has previously been reported, three analogues substituted at the sulphur of the isothioureas have been synthesised. In two of these, the S-substituent was ‘tied back’ sterically by cyclisation to the nitrogen remote from the amino-acid unit. Nδ-(4,5-Dihydrothiazol-2-yl)ornithine was identified as an inhibitor of rat inducible and constitutive isoforms of NOS and of a constitutive NOS derived from a human tumour xenograft. Analogous Nδ-(thiazol-2-yl)ornithines were less active, whereas the corresponding Nδ-(oxazol-2-yl)ornithine and Nδ-(pyrimidin-2-yl)ornithine failed completely to inhibit NOS. A new efficient preparation of the critical synthetic intermediate, Nα-Boc-thiocitrulline t-butyl ester, has been developed. Further exploration of the SAR with 2-amino-5-(heterocyclylthio)pentanoic acids (synthesised from 2-(Boc-amino)-5-bromopentanoic acid t-butyl ester), with N-(4-aminobutyl)thiourea and with 2-(4-aminobutylamino)-4,5-dihydrothiazole enabled refinement of our previous model for binding of the substrate, l-arginine, and the inhibitors to NOS.