In Vivo Characterization of the Novel Imidazopyridine BYK191023 [2-[2-(4-Methoxy-pyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine], a Potent and Highly Selective Inhibitor of Inducible Nitric-Oxide Synthase

Inducible nitric oxide synthase: Regulation, structure, and inhibition

A considerable number of human diseases have an inflammatory component, and a key mediator of immune activation and inflammation is inducible nitric oxide synthase (iNOS), which produces nitric oxide (NO) from l-arginine. Overexpressed or dysregulated iNOS has been implicated in numerous pathologies including sepsis, cancer, neurodegeneration, and various types of pain. Extensive knowledge has been accumulated about the roles iNOS plays in different tissues and organs. Additionally, X-ray crystal and cryogenic electron microscopy structures have shed new insights on the structure and regulation of this enzyme. Many potent iNOS inhibitors with high selectivity over related NOS isoforms, neuronal NOS, and endothelial NOS, have been discovered, and these drugs have shown promise in animal models of endotoxemia, inflammatory and neuropathic pain, arthritis, and other disorders. A major issue in iNOS inhibitor development is that promising results in animal studies have not translated to humans; there are no iNOS inhibitors approved for human use. In addition to assay limitations, both the dual modalities of iNOS and NO in disease states (ie, protective vs harmful effects) and the different roles and localizations of NOS isoforms create challenges for therapeutic intervention. This review summarizes the structure, function, and regulation of iNOS, with focus on the development of iNOS inhibitors (historical and recent). A better understanding of iNOS' complex functions is necessary before specific drug candidates can be identified for classical indications such as sepsis, heart failure, and pain; however, newer promising indications for iNOS inhibition, such as depression, neurodegenerative disorders, and epilepsy, have been discovered.

The role of dimethylarginine dimethylaminohydrolase (DDAH) in pulmonary fibrosis

Pulmonary fibrosis is a devastating and progressive parenchymal lung disease with an extremely poor prognosis. Patients suffering from idiopathic pulmonary fibrosis (IPF) display a compromised lung function alongside pathophysiological features such as highly increased production of extracellular matrix, alveolar epithelial cell dysfunction, and disordered fibroproliferation - features that are due to a dysregulated response to alveolar injury. Under pathophysiological conditions of IPF, abnormally high concentrations of nitric oxide (NO) are found, likely a result of increased activity of the inducible nitric oxide synthase (NOS2), giving rise to products that contribute to fibrosis development. It is known that pharmacological inhibition or knockdown of NOS2 reduces pulmonary fibrosis, suggesting a role for NOS inhibitors in the treatment of fibrosis. Recent reports identified a critical enzyme, dimethylarginine dimethylaminohydrolase (DDAH), which is exceedingly active in patients suffering from IPF and in mice treated with bleomycin. An up-regulation of DDAH was observed in primary alveolar epithelial type II (ATII) cells from mice and patients with pulmonary fibrosis, where it co-localizes with NOS2. DDAH is a key enzyme that breaks down an endogenous inhibitor of NOS, asymmetric dimethylarginine (ADMA), by metabolizing it to l-citrulline and dimethylamine. DDAH was shown to modulate key fibrotic signalling cascades, and inhibition of this enzyme attenuated many features of the disease in in vivo experiments, suggesting a possible new therapeutic strategy for the treatment of patients suffering from IPF.

Erythropoietin improves skeletal muscle microcirculation through the activation of eNOS in a mouse sepsis model

BACKGROUND

Sepsis and septic shock remain the major causes of morbidity and mortality in intensive care units. One mechanism that leads to organ failure is microcirculatory dysfunction. Erythropoietin (EPO) is a glycoprotein produced by the kidney that primarily regulates erythropoiesis, but it also can exert hemodynamic, anti-inflammatory, and tissue protective effects. We previously reported that administration of EPO to septic mice improves mouse skeletal muscle capillary perfusion and tissue bioenergetics. The objective of this study was to explore the potential mechanism(s) involved.

METHODS

Sepsis was induced by intraperitoneal (i.p.) injection of a fecal suspension (12.5 g in 0.5 saline/mouse) in mice. At 18 hours after sepsis induction, a single dose of rHuEPO (400 U/kg) was given to the mice. Mouse capillary perfusion density and nicotinamide adenine dinucleotide (NADH) fluorescence in skeletal muscle were observed using intravital microscopy. Endothelial cells derived from the skeletal muscle were treated with rHuEPO (5 U/mL) and endothelial nitric oxide synthase (eNOS) activation and activity were assessed.

RESULTS

Septic mice had decreased capillary perfusion density and increased tissue NADH fluorescence indicating impaired tissue bioenergetics, whereas animals treated with rHuEPO demonstrated an improvement in capillary perfusion density and decreased skeletal muscle NADH fluorescence. The beneficial effect of rHuEPO did not occur in septic mice treated with l-NAME (an NOS inhibitor, 20 mg/kg) or mice genetically deficient in eNOS. Treatment of endothelial cells with rHuEPO resulted in activation of eNOS as indicated by increased eNOS phosphorylation and NO production.

CONCLUSIONS

Our results suggest that eNOS plays an important role in mediating the beneficial effect of rHuEPO on microcirculation in this septic mouse model.

Novel nanomolar imidazo[4,5-b]pyridines as selective nitric oxide synthase (iNOS) inhibitors: SAR and structural insights

Inducible arginine oxidation and subsequent NO production by correspondent synthase (iNOS) are important cellular answers to proinflammatory signals. Prolonged NO production has been proved in higher organisms to cause stroke or septic shock. Several classes of potent NOS inhibitors have been reported, most of them targeting the arginine binding site of the oxygenase domain. Here we disclose the SAR and the rational design of potent and selective iNOS inhibitors which may be useful as anti-inflammatory drugs.

Effects of a selective iNOS inhibitor versus norepinephrine in the treatment of septic shock

Inhibition of NOS is not beneficial in septic shock; selective inhibition of the inducible form (iNOS) may represent a better option. We compared the effects of the selective iNOS inhibitor BYK191023 with those of norepinephrine (NE) in a sheep model of septic shock. Twenty-four anesthetized, mechanically ventilated ewes received 1.5 g/kg body weight of feces into the abdominal cavity to induce sepsis. Animals were randomized into three groups (each n = 8): NE-only, BYK-only, and NE + BYK. The sublingual microcirculation was evaluated with sidestream dark-field videomicroscopy. MAP was higher in the NE + BYK group than in the other groups, but there were no significant differences in cardiac index or systemic vascular resistance. Mean pulmonary arterial pressure was lower in BYK-treated animals than in the NE-only group. PaO2/FiO2 was higher and lactate concentration lower in the BYK groups than in the NE-only group. Mesenteric blood flow was higher in BYK groups than in the NE-only group. Renal blood flow was higher in the NE + BYK group than in the other groups. Functional capillary density and proportion of perfused vessels were higher in the BYK groups than in the NE-only group 18 h after induction of peritonitis. Survival times were similar in the three groups. In this model of peritonitis, selective iNOS inhibition had more beneficial effects than NE on pulmonary artery pressures, gas exchange, mesenteric blood flow, microcirculation, and lactate concentration. Combination of this selective iNOS inhibitor with NE allowed a higher arterial pressure and renal blood flow to be maintained.

Inhibition of inducible nitric oxide synthase in respiratory diseases

Nitric oxide (NO) is a key physiological mediator and disturbed regulation of NO release is associated with the pathophysiology of almost all inflammatory diseases. A multitude of inhibitors of NOSs (nitric oxide synthases) have been developed, initially with low or even no selectivity against the constitutively expressed NOS isoforms, eNOS (endothelial NOS) and nNOS (neuronal NOS). In the meanwhile these efforts yielded potent and highly selective iNOS (inducible NOS) inhibitors. Moreover, iNOS inhibitors have been shown to exert beneficial anti-inflammatory effects in a wide variety of acute and chronic animal models of inflammation. In the present mini-review, we summarize some of our current knowledge of inhibitors of the iNOS isoenzyme, their biochemical properties and efficacy in animal models of pulmonary diseases and in human disease itself. Moreover, the potential benefit of iNOS inhibition in animal models of COPD (chronic obstructive pulmonary disease), such as cigarette smoke-induced pulmonary inflammation, has not been explicitly studied so far. In this context, we demonstrated recently that both a semi-selective iNOS inhibitor {L-NIL [N6-(1-iminoethyl)-L-lysine hydrochloride]} and highly selective iNOS inhibitors (GW274150 and BYK402750) potently diminished inflammation in a cigarette smoke mouse model mimicking certain aspects of human COPD. Therefore, despite the disappointing results from recent asthma trials, iNOS inhibition could still be of therapeutic utility in COPD, a concept which needs to be challenged and validated in human disease.

Therapeutic effects of erythropoietin in murine models of endotoxin shock

OBJECTIVE

Erythropoietin has recently emerged as a cytoprotective cytokine, which possesses the ability to protect many tissues, including the brain, heart, and kidneys, against ischemia or traumatic injury. We investigated the therapeutic effects of erythropoietin in a murine model of endotoxin shock.

DESIGN

Prospective, randomized study.

SETTING

University-based research laboratory.

SUBJECTS

Male BALB/c mice.

INTERVENTIONS

Mice intraperitoneally received either lipopolysaccharide (LPS) from Escherichia coli or vehicle. Erythropoietin was administered at a dose of 1000 IU/kg subcutaneously at different time points after LPS administration. We also investigated the effect of erythropoietin on the development of septic shock caused by cecal perforation.

MEASUREMENTS AND MAIN RESULTS

Treatment of mice with erythropoietin, within 2 hours after LPS administration, improved the mortality rate. Treatment of cecal perforated mice with erythropoietin extended survival by 12 hours, but all animals died by 72 hours in both groups. Erythropoietin attenuated apoptosis in the lungs, liver, small intestine, thymus, and spleen, as assessed by terminal deoxynucleotidyl transferase-mediated nucleotide nick-end labeling staining, active caspase-3 immunostaining and immunoblotting, and measurements of caspase-3/7 activity. Erythropoietin also reduced inducible nitric oxide synthase expression, nitric oxide production, peroxynitrite formation, and tissue hypoxia. In contrast, erythropoietin did not affect the degree of LPS-induced inflammation, as assessed by measurements of blood levels of interleukin-1beta, interleukin-6, tumor necrosis factor-alpha, growth-related oncogene/keratinocyte-derived cytokine, and high mobility group box 1, the phosphorylation levels of nuclear factor kappaB, and the number of neutrophils infiltrating the lungs and the liver.

CONCLUSIONS

The results of the study demonstrate that administration of a large dose of erythropoietin after induction of experimental endotoxemia improved survival and that the beneficial effects of erythropoietin were associated with inhibition of apoptosis, nitric oxide production, and tissue hypoxia, without alterations in inflammatory responses.

BYK191023 (2-[2-(4-methoxy-pyridin-2-yl)-ethyl]-3h-imidazo[4,5-b]pyridine) is an NADPH- and time-dependent irreversible inhibitor of inducible nitric-oxide synthase

Imidazopyridine derivates were recently shown to be a novel class of selective and arginine-competitive inhibitors of inducible nitric-oxide synthase (iNOS), and 2-[2-(4-methoxypyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine (BYK191023) was found to have very high selectivity in enzymatic and cellular models ( Mol Pharmacol 69: 328-337, 2006 ). Here, we show that BYK191023 irreversibly inactivates murine iNOS in an NADPH- and time-dependent manner, whereas it acts only as a reversible l-arginine-competitive inhibitor in the absence of NADPH or during anaerobic preincubation. Time-dependent irreversible inhibition by BYK191023 could also be demonstrated in intact cells using the RAW macrophage or iNOS-overexpressing human embryonic kidney 293 cell lines. The mechanism of BYK191023 inhibition in the presence of NADPH was studied using spectral, kinetic, chromatographic, and radioligand binding methods. BYK191023-bound iNOS was spectrally indistinguishable from l-arginine-bound iNOS, pointing to an interaction of BYK191023 with the catalytic center of the enzyme. [(3)H]BYK191023 was recovered quantitatively from irreversibly inactivated iNOS, and no inhibitor metabolite was detected by high-performance liquid chromatography (HPLC). Size exclusion chromatography revealed only about 20% iNOS dissociation into monomers. Furthermore, HPLC and spectrophotometric analysis showed that the irreversible inhibition was associated with loss of heme from iNOS and a reduced ability to form the distinctive ferrous heme-CO complex (cytochrome P450). Thus, enzyme inactivation is mainly caused by heme loss, and it occurs in the inhibitor-bound enzyme in the presence of electron flux from NADPH.