The discovery of novel, potent and highly selective inhibitors of inducible nitric oxide synthase (iNOS).

By careful analysis of experimental X-ray ligand crystallographic protein data across several inhibitor series we have discovered a novel, potent and selective series of iNOS inhibitors exemplified by compound 8.

Identification of iNOS inhibitors using InteraX.

Inducible nitric oxide synthase (iNOS) is active as a homodimer. A cell-based assay suitable for high-throughput screening (HTS) was generated to identify inhibitors of iNOS dimerization using the InteraX enzyme complementation technology of Applied Biosystems. The cells contain 2 chimeric proteins of complementing deletion mutants of beta-galactosidase, each fused to the oxygenase domain of human iNOS. The assay was characterized using known iNOS dimerization inhibitors, which gave a decrease in beta-galactosidase activity. Surprisingly, the assay was also able to identify compounds that have the same profile as known inhibitors of fully formed dimeric iNOS by causing an increase in beta-galactosidase activity. The iNOS InteraX assay was used to screen approximately 800,000 compounds in a 384-well format. After hit confirmation, 3359 compounds were taken forward for full IC50 determination in InteraX and cytotoxicity assays. Of these compounds 40.5% were confirmed as greater than 10-fold more active in InteraX compared to a cytotoxicity assay and were classified as potential iNOS dimerization inhibitors as they did not inhibit beta-galactosidase alone. In the same primary screen, 901 compounds gave a significant increase in beta-galactosidase activity. Many of these were known inhibitors of iNOS. After IC50 determination in InteraX and cytotoxicity assays, 182 novel compounds remained as potential arginine-competitive inhibitors of dimeric iNOS.

Anchored plasticity opens doors for selective inhibitor design in nitric oxide synthase.

Nitric oxide synthase (NOS) enzymes synthesize nitric oxide, a signal for vasodilatation and neurotransmission at low concentrations and a defensive cytotoxin at higher concentrations. The high active site conservation among all three NOS isozymes hinders the design of selective NOS inhibitors to treat inflammation, arthritis, stroke, septic shock and cancer. Our crystal structures and mutagenesis results identified an isozyme-specific induced-fit binding mode linking a cascade of conformational changes to a new specificity pocket. Plasticity of an isozyme-specific triad of distant second- and third-shell residues modulates conformational changes of invariant first-shell residues to determine inhibitor selectivity. To design potent and selective NOS inhibitors, we developed the anchored plasticity approach: anchor an inhibitor core in a conserved binding pocket, then extend rigid bulky substituents toward remote specificity pockets, which become accessible upon conformational changes of flexible residues. This approach exemplifies general principles for the design of selective enzyme inhibitors that overcome strong active site conservation.

Selective inhibitors of inducible nitric oxide synthase: potential agents for the treatment of inflammatory diseases?

Nitric Oxide (NO) is widely recognized as an important messenger and effector molecule in a variety of biological systems. There is strong evidence from animal models that elevated or lowered NO levels are associated with a variety of pathological states. In nature, NO is synthesised from the amino acid l-arginine by a small family of closely related oxygenase enzymes: the nitric oxide synthases (NOS). A number of studies in animals have associated excessive NO production by one of these enzymes--the inducible NOS isoform (iNOS or NOS-II)--with acute and chronic inflammation in model systems and have also demonstrated that administration of NOS inhibitors can produce beneficial effects. Regrettably, however, the relatively poor potency, selectivity and pharmacokinetic (ADME) profiles of the available inhibitors have so far precluded a convincing demonstration of their efficacy in the clinic. This review will describe the current state of knowledge of the structure and function of NOS and the various approaches that are being followed in the search for truly selective NOS inhibitors as therapeutic agents for inflammatory diseases.

2-aminopyridines as highly selective inducible nitric oxide synthase inhibitors. Differential binding modes dependent on nitrogen substitution.

4-Methylaminopyridine (4-MAP) (5) is a potent but nonselective nitric oxide synthase (NOS) inhibitor. While simple N-methylation in this series results in poor activity, more elaborate N-substitution such as with 4-piperidine carbamate or amide results in potent and selective inducible NOS inhibition. Evidently, a flipping of the pyridine ring between these new inhibitors allows the piperidine to interact with different residues and confer excellent selectivity.

1,2-Dihydro-4-quinazolinamines: potent, highly selective inhibitors of inducible nitric oxide synthase which show antiinflammatory activity in vivo.

The discovery of a novel class of nitric oxide synthase (NOS) inhibitors, 2-substituted 1,2-dihydro-4-quinazolinamines, and the related 4'-aminospiro[piperidine-4,2'(1'H)-quinazolin]-4'-amines is described. Members of both series exhibit nanomolar potency and high selectivity for the inducible isoform of the enzyme (i-NOS) relative to the constitutive isoforms in vitro. Efficacy in acute and chronic animal models of inflammatory disease following oral administration has also been demonstrated using these compounds.

Conformational changes in nitric oxide synthases induced by chlorzoxazone and nitroindazoles: crystallographic and computational analyses of inhibitor potency.

Nitric oxide is a key signaling molecule in many biological processes, making regulation of nitric oxide levels highly desirable for human medicine and for advancing our understanding of basic physiology. Designing inhibitors to specifically target one of the three nitric oxide synthase (NOS) isozymes that form nitric oxide from the L-Arg substrate poses a significant challenge due to the overwhelmingly conserved active sites. We report here 10 new X-ray crystallographic structures of inducible and endothelial NOS oxygenase domains cocrystallized with chlorzoxazone and four nitroindazoles: 5-nitroindazole, 6-nitroindazole, 7-nitroindazole, and 3-bromo-7-nitroindazole. Each of these bicyclic aromatic inhibitors has only one hydrogen bond donor and therefore cannot form the bidentate hydrogen bonds that the L-Arg substrate makes with Glu371. Instead, all of these inhibitors induce a conformational change in Glu371, creating an active site with altered molecular recognition properties. The cost of this conformational change is approximately 1-2 kcal, based on our measured constants for inhibitor binding to the wild-type and E371A mutant proteins. These inhibitors derive affinity by pi-stacking above the heme and replacing both intramolecular (Glu371-Met368) and intermolecular (substrate-Trp366) hydrogen bonds to the beta-sheet architecture underlying the active site. When bound to NOS, high-affinity inhibitors in this class are planar, whereas weaker inhibitors are nonplanar. Isozyme differences were observed in the pterin cofactor site, the heme propionate, and inhibitor positions. Computational docking predictions match the crystallographic results, including the Glu371 conformational change and inhibitor-binding orientations, and support a combined crystallographic and computational approach to isozyme-specific NOS inhibitor analysis and design.