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.

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.

Impaired interdomain communication in mitochondrial Hsp70 results in the loss of inward-directed translocation force.

The essential mitochondrial Hsp70 (mtHsp70) is required for the import of mitochondrial preproteins into the matrix compartment. The translocation-specific activity of mtHsp70 is coordinated by its interaction with specific partner proteins, forming the import motor complex that provides the energy for unfolding and complete translocation of precursor polypeptide chains. A major biochemical characteristic of Hsp70-type chaperones is their nucleotide-regulated affinity to polypeptide substrates. To study the role of this allosteric regulation in the course of preprotein translocation, we have generated specific mtHsp70 mutations located within or close to the interface between the nucleotide-binding and the substrate-binding domains. Mitochondria isolated from the mtHsp70 mutants displayed severely reduced import efficiencies in vitro. Two of the mutants exhibited strong growth defects in vivo and were significantly impaired in the generation of an inward-directed, ATP-dependent import force on precursor proteins in transit. The biochemical properties of these two mutant proteins were consistent with defects in the transfer of conformational signals to the substrate-binding domain, resulting in a prolonged and enhanced interaction with imported substrate proteins. Furthermore, interference with the allosteric mechanism resulted in defects of translocation-specific partner protein interaction. We conclude that even a partial disruption of the interdomain communication in the mtHsp70 chaperone results in an almost complete breakdown of its translocation-driving properties.

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.

Montelukast exerts no acute direct effect on NO synthases.

The cysteinyl leukotrienes (CysLTs) LTC(4), LTD(4) and LTE(4) are potent proinflammatory lipid mediators that play a central role in inflammation, contraction and remodelling of airways observed in asthmatics. Montelukast, a competitive inhibitor of the cysteinyl leukotriene-1 (CysLT(1)) receptor attenuates asthmatic airway inflammation, contraction and remodelling. As a number of studies have shown that montelukast reduced exhaled nitric oxide (NO) levels, a marker of inflammation that correlates with the severity of asthma, we investigated whether or not a direct inhibition of NO synthase (NOS) by montelukast takes place. In an ex vivo rat lung perfusion and ventilation model the NOS-dependent vasodilation effect after lipopolysaccharide (LPS) infusion was assessed with and without montelukast. Functional organ bath studies using isolated aortic rings from the same species aimed to assess effects of montelukast on the inducible and endothelial NOS isoenzymes (i- and eNOS) as well as on iNOS expression. Neuronal NOS (nNOS) was assessed by field stimulated rabbit corpus cavernosum, and isolated human iNOS enzyme activity was assessed for potential inhibition. Montelukast failed to cause vasoconstriction in LPS challenged rat lung, or to inhibit i- and eNOS activity as well as iNOS expression in aortic rings from the same species. Also the assays for nNOS in rabbit corpus cavernosum and on isolated human iNOS enzyme gave no evidence for a direct inhibition by montelukast in physiological and supraphysiological concentrations up to 10(-4)M. We therefore conclude that montelukast has no acute NOS inhibitor action. Its effect on exhaled NO is therefore probably indirectly mediated by a modulation of the asthmatic airway inflammation.

Mitochondrial protein sorting: differentiation of beta-barrel assembly by Tom7-mediated segregation of Mdm10.

The mitochondrial outer membrane contains two distinct machineries for protein import and protein sorting that function in a sequential manner: the general translocase of the outer membrane (TOM complex) and the sorting and assembly machinery (SAM complex), which is dedicated to beta-barrel proteins. The SAM(core) complex consists of three subunits, Sam35, Sam37, and Sam50, that can associate with a fourth subunit, the morphology component Mdm10, to form the SAM(holo) complex. Whereas the SAM(core) complex is required for the biogenesis of all beta-barrel proteins, Mdm10 and the SAM(holo) complex play a selective role in beta-barrel biogenesis by promoting assembly of Tom40 but not of porin. We report that Tom7, a conserved subunit of the TOM complex, functions in an antagonistic manner to Mdm10 in biogenesis of Tom40 and porin. We show that Tom7 promotes segregation of Mdm10 from the SAM(holo) complex into a low molecular mass form. Upon deletion of Tom7, the fraction of Mdm10 in the SAM(holo) complex is significantly increased, explaining the opposing functions of Tom7 and Mdm10 in beta-barrel sorting. Thus the role of Tom7 is not limited to the TOM complex. Tom7 functions in mitochondrial protein biogenesis by a new mechanism, segregation of a sorting component, leading to a differentiation of beta-barrel assembly.

The novel imidazopyridine 2-[2-(4-methoxy-pyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine (BYK191023) is a highly selective inhibitor of the inducible nitric-oxide synthase.

We have identified imidazopyridine derivatives as a novel class of NO synthase inhibitors with high selectivity for the inducible isoform. 2-[2-(4-Methoxy-pyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine (BYK191023) showed half-maximal inhibition of crudely purified human inducible (iNOS), neuronal (nNOS), and endothelial (eNOS) NO synthases at 86 nM, 17 microM, and 162 microM, respectively. Inhibition of inducible NO synthase was competitive with l-arginine, pointing to an interaction of BYK191023 with the catalytic center of the enzyme. In radioligand and surface plasmon resonance experiments, BYK191023 exhibited an affinity for iNOS, nNOS, and eNOS of 450 nM, 30 microM, and >500 microM, respectively. Inhibition of cellular nitrate/nitrite synthesis in RAW, rat mesangium, and human embryonic kidney 293 cells after iNOS induction showed 40- to 100-fold higher IC(50) values than at the isolated enzyme, in agreement with the much higher l-arginine concentrations in cell culture media and inside intact cells. BYK191023 did not show any toxicity in various rodent and human cell lines up to high micromolar concentrations. The inhibitory potency of BYK191023 was tested in isolated organ models of iNOS (lipopolysaccharide-treated and phenylephrine-precontracted rat aorta; IC(50) = 7 microM), eNOS (arecaidine propargyl ester-induced relaxation of phenylephrine-precontracted rat aorta; IC(50) > 100 microM), and nNOS (field-stimulated relaxation of phenylephrine-precontracted rabbit corpus cavernosum; IC(50) > 100 microM). These data confirm the high selectivity of BYK191023 for iNOS over eNOS and nNOS found at isolated enzymes. In summary, we have identified a new highly selective iNOS inhibitor structurally unrelated to known compounds and l-arginine. BYK191023 is a valuable tool for the investigation of iNOS-mediated effects in vitro and in vivo.

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.

Excessive release of nitric oxide from inducible nitric-oxide synthase (iNOS) has been postulated to contribute to pathology in a number of inflammatory diseases. We recently identified imidazopyridine derivatives as a novel class of potent nitricoxide synthase inhibitors with high selectivity for the inducible isoform. In the present study, we tested the in vivo potency of BYK191023 [2-[2-(4-methoxy-pyridin-2-yl)-ethyl]-3H-imidazo-[4,5-b]pyridine], a selected member of this inhibitor class, in three different rat models of lipopolysaccharide-induced systemic inflammation. Delayed administration of BYK191023 dose-dependently suppressed the lipopolysaccharide-induced increase in plasma nitrate/nitrite (NO(x)) levels with an ED(50) of 14.9 micromol/kg/h. In a model of systemic hypotension following high-dose lipopolysaccharide challenge, curative administration of BYK191023 at a dose that inhibited 83% of the NO(x) increase completely prevented the gradual decrease in mean arterial blood pressure observed in vehicle-treated control animals. The vasopressor effect was specific for endotoxemic animals since BYK191023 did not affect blood pressure in saline-challenged controls. In addition, in a model of lipopolysaccharide-induced vascular hyporesponsiveness, BYK191023 infusion partially restored normal blood pressure responses to norepinephrine and sodium nitroprusside via an l-arginine competitive mechanism. Taken together, BYK191023 is a member of a novel class of highly isoform-selective iNOS inhibitors with promising in vivo activity suitable for mechanistic studies on the role of selective iNOS inhibition as well as clinical development.

The putative helical lid of the Hsp70 peptide-binding domain is required for efficient preprotein translocation into mitochondria.

The mitochondrial Hsp70 (Ssc1) is an essential component of the preprotein import machinery, responsible for the unfolding and movement of polypeptide chains through the mitochondrial membranes into the matrix. Here, we have analyzed the role of the carboxy-terminal variable domain during the protein translocation reaction. This segment is thought to form an alpha-helical lid over the substrate binding site. Truncated Ssc1 molecules lacking parts or all of the lid region showed reduced binding to substrate proteins but were able to interact with the co-chaperone Mge1 and the inner membrane anchor Tim44. Deletions of the complete lid resulted in a lethal phenotype in vivo, caused by the inability to sustain a productive preprotein import function. The translocation defect in vitro was not overcome by artificial unfolding of the preprotein prior to the import reaction. Despite a reduced substrate affinity, the presence of a minimal lid segment in Ssc1 was sufficient to support preprotein import. However, at low reaction temperatures or low matrix ATP levels, protein import rates were significantly reduced due to an unproductive interaction with the preprotein in transit. We conclude that the carboxy-terminal domain performs a crucial role in the import process by enhancing the import motor function of Ssc1 during polypeptide translocation.

The Hsp70 peptide-binding domain determines the interaction of the ATPase domain with Tim44 in mitochondria.

Ssc1, a molecular chaperone of the Hsp70 family, drives preprotein import into the mitochondrial matrix by a specific interaction with the translocase component Tim44. Two other mitochondrial Hsp70s, Ssc3 (Ecm10) and Ssq1, show high sequence homology to Ssc1 but fail to replace Ssc1 in vivo, possibly due to their inability to interact with Tim44. We analyzed the structural basis of the Tim44 interaction by the construction of chimeric Hsp70 proteins. The ATPase domains of all three mitochondrial Hsp70s were shown to bind to Tim44, supporting the active motor model for the Hsp70 mechanism during preprotein translocation. The peptide-binding domain of Ssc1 sustained binding of Tim44, while the peptide-binding domains of Ssc3 and Ssq1 exerted a negative effect on the interaction of the ATPase domains with Tim44. A mutation in the peptide-binding domain of Ssc1 resulted in a similar negative effect not only on the ATPase domain of Ssc1, but also of Ssq1 and Ssc3. Hence, the determination of a crucial Hsp70 function via the peptide-binding domain suggests a new regulatory principle for Hsp70 domain cooperation.