Inhibition of neuronal nitric oxide synthase by 4-amino pteridine derivatives: structure-activity relationship of antagonists of (6R)-5,6,7,8-tetrahydrobiopterin cofactor

Comparative routes to 7-carboxymethyl-pterin: A useful medicinal chemistry building block

Pterins, such as folate and biopterin, and their derivatives hold significant importance given their biological relevance, as well as the numerous pterin-based inhibitors developed for targeting various biological targets. For this reason, pterins can be viewed as a privileged scaffold, as the discovery of new pterin analogs gives rise to a vast array of potential drug candidates. 7-carboxymethyl-pterin (7-CMP) represents a useful scaffold for the rapid generation of structurally diverse pterin amides and has been a key building block in medicinal chemistry. In an effort to facilitate rapid generation of this pterin scaffold, we have explored multiple routes towards 7-CMP to assess the most efficient method of generation. Methods were evaluated based on yield, regioselectivity, reaction time, and hazardous reaction conditions. This work can aide in the synthesis and discovery of new pterin-based drug candidates.

Nitric Oxide Synthase Inhibitors into the Clinic at Last

The 1998 Nobel Prize in Medicine and Physiology for the discovery of nitric oxide, a nitrogen containing reactive oxygen species (also termed reactive nitrogen or reactive nitrogen/oxygen species) stirred great hopes. Clinical applications, however, have so far pertained exclusively to the downstream signaling of cGMP enhancing drugs such as phosphodiesterase inhibitors and soluble guanylate cyclase stimulators. All clinical attempts, so far, to inhibit NOS have failed even though preclinical models were strikingly positive and clinical biomarkers correlated perfectly. This rather casts doubt on our current way of target identification in drug discovery in general and our way of patient stratification based on correlating but not causal biomarkers or symptoms. The opposite, NO donors, nitrite and enhancing NO synthesis by eNOS/NOS3 recoupling in situations of NO deficiency, are rapidly declining in clinical relevance or hold promise but need yet to enter formal therapeutic guidelines, respectively. Nevertheless, NOS inhibition in situations of NO overproduction often jointly with enhanced superoxide (or hydrogen peroxide production) still holds promise, but most likely only in acute conditions such as neurotrauma (Stover et al., J Neurotrauma 31(19):1599-1606, 2014) and stroke (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016; Casas et al., Proc Natl Acad Sci U S A 116(14):7129-7136, 2019). Conversely, in chronic conditions, long-term inhibition of NOS might be too risky because of off-target effects on eNOS/NOS3 in particular for patients with cardiovascular risks or metabolic and renal diseases. Nitric oxide synthases (NOS) and their role in health (green) and disease (red). Only neuronal/type 1 NOS (NOS1) has a high degree of clinical validation and is in late stage development for traumatic brain injury, followed by a phase II safety/efficacy trial in ischemic stroke. The pathophysiology of NOS1 (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016) is likely to be related to parallel superoxide or hydrogen peroxide formation (Kleinschnitz et al., J Cereb Blood Flow Metab 1508-1512, 2016; Casas et al., Proc Natl Acad Sci U S A 114(46):12315-12320, 2017; Casas et al., Proc Natl Acad Sci U S A 116(14):7129-7136, 2019) leading to peroxynitrite and protein nitration, etc. Endothelial/type 3 NOS (NOS3) is considered protective only and its inhibition should be avoided. The preclinical evidence for a role of high-output inducible/type 2 NOS (NOS2) isoform in sepsis, asthma, rheumatic arthritis, etc. was high, but all clinical development trials in these indications were neutral despite target engagement being validated. This casts doubt on the role of NOS2 in humans in health and disease (hence the neutral, black coloring).

One-Pot Synthesis of 7-(Benzimidazol-2-yl)thioxolumazine and -lumazine Derivatives via H2SO4-Catalyzed Rearrangement of Quinoxalinones When Exposed to 5,6-Diamino-2-mercapto- and 2,5,6-Triaminopyrimidin-4-ols

A facile approach to a range of substituted 7-(benzimidazol-2-yl)thioxolumazines [7-(benzimidazol-2-yl)-2-thioxo-2,3-dihydropteridin-4(1 H)-ones] and 7-(benzimidazol-2-yl)lumazines [7-(benzimidazol-2-yl)pteridine-2,4(1 H,3 H)-diones] is described. These new biheterocyclic systems are obtained via H₂SO₄-catalyzed rearrangement of quinoxalin-2-ones in the presence of 5,6-diamino-2-mercapto- and 2,5,6-triaminopyrimidin-4-ols. Thus, benzimidazole and pteridine rings are constructed in one synthetic step. A plausible ANRORC ( addition of nucleophile, ring opening and ring closure)-type reaction mechanism is proposed. Applying the rearrangement to the aza-analogue of 3-benzoylquinoxalin-2(1 H)-one-i.e., 3-benzoylpyrido[2,3- b]pyrazin-2(1 H)-one-with 5,6-diamino-2-mercaptopyrimidin-4-ol makes it possible to synthesize inaccessible 7-(1 H-imidazo[4,5- b]pyridin-2-yl)-6-phenyl-2-thioxo-2,3-dihydropteridin-4(1 H)-one. 7-(Benzimidazol-2-yl)-6-(2-fluorophenyl)-2-thioxo-2,3-dihydropteridin-4(1 H)-ones undergoes intramolecular nucleophilic substitution of fluorine by a nitrogen of the benzimidazole fragment with the formation of benzo[4',5']imidazo[1',2':1,2]quinolino[4,3- g]pteridine-2,4(1 H,3 H)-diones as new heterocyclic systems.

Therapeutic potential of pteridine derivatives: A comprehensive review

Pteridines are aromatic compounds formed by fused pyrazine and pyrimidine rings. Many living organisms synthesize pteridines, where they act as pigments, enzymatic cofactors, or immune system activation molecules. This variety of biological functions has motivated the synthesis of a huge number of pteridine derivatives with the aim of studying their therapeutic potential. This review gathers the state-of-the-art of pteridine derivatives, describing their biological activities and molecular targets. The antitumor activity of pteridine-based compounds is one of the most studied and advanced therapeutic potentials, for which several molecular targets have been identified. Nevertheless, pteridines are also considered as very promising therapeutics for the treatment of chronic inflammation-related diseases. On the other hand, many pteridine derivatives have been tested for antimicrobial activities but, although some of them resulted to be active in preliminary assays, a deeper research is needed in this area. Moreover, pteridines may be of use in the treatment of many other diseases, such as diabetes, osteoporosis, ischemia, or neurodegeneration, among others. Thus, the diversity of the biological activities shown by these compounds highlights the promising therapeutic use of pteridine derivatives. Indeed, methotrexate, pralatrexate, and triamterene are Food and Drug Administration approved pteridines, while many others are currently under study in clinical trials.

Pterin-sulfa conjugates as dihydropteroate synthase inhibitors and antibacterial agents

The sulfonamide class of antibiotics has been in continuous use for over 70years. They are thought to act by directly inhibiting dihydropteroate synthase (DHPS), and also acting as prodrugs that sequester pterin pools by forming dead end pterin-sulfonamide conjugates. In this study, eight pterin-sulfonamide conjugates were synthesized using a novel synthetic strategy and their biochemical and microbiological properties were investigated. The conjugates were shown to competitively inhibit DHPS, and inhibition was enhanced by the presence of pyrophosphate that is crucial to catalysis and is known to promote an ordering of the DHPS active site. The co-crystal structure of Yersinia pestis DHPS bound to one of the more potent conjugates revealed a mode of binding that is similar to that of the enzymatic product analog pteroic acid. The antimicrobial activities of the pterin-sulfonamide conjugates were measured against Escherichia coli in the presence and absence of folate precursors and dependent metabolites. These results show that the conjugates have appreciable antibacterial activity and act by an on target, anti-folate pathway mechanism rather than as simple dead end products.

Pharmacology and Clinical Drug Candidates in Redox Medicine

SIGNIFICANCE

Oxidative stress is suggested to be a disease mechanism common to a wide range of disorders affecting human health. However, so far, the pharmacotherapeutic exploitation of this, for example, based on chemical scavenging of pro-oxidant molecules, has been unsuccessful.

RECENT ADVANCES

An alternative emerging approach is to target the enzymatic sources of disease-relevant oxidative stress. Several such enzymes and isoforms have been identified and linked to different pathologies. For some targets, the respective pharmacology is quite advanced, that is, up to late-stage clinical development or even on the market; for others, drugs are already in clinical use, although not for indications based on oxidative stress, and repurposing seems to be a viable option.

CRITICAL ISSUES

For all other targets, reliable preclinical validation and drug ability are key factors for any translation into the clinic. In this study, specific pharmacological agents with optimal pharmacokinetic profiles are still lacking. Moreover, these enzymes also serve largely unknown physiological functions and their inhibition may lead to unwanted side effects.

FUTURE DIRECTIONS

The current promising data based on new targets, drugs, and drug repurposing are mainly a result of academic efforts. With the availability of optimized compounds and coordinated efforts from academia and industry scientists, unambiguous validation and translation into proof-of-principle studies seem achievable in the very near future, possibly leading towards a new era of redox medicine.

Synthesis and Biological Evaluation of Novel Gigantol Derivatives as Potential Agents in Prevention of Diabetic Cataract

As a continuation of our efforts directed towards the development of natural anti-diabetic cataract agents, gigantol was isolated from Herba dendrobii and was found to inhibit both aldose reductase (AR) and inducible nitric oxide synthase (iNOS) activity, which play a significant role in the development and progression of diabetic cataracts. To improve its bioefficacy and facilitate use as a therapeutic agent, gigantol (compound 14f) and a series of novel analogs were designed and synthesized. Analogs were formulated to have different substituents on the phenyl ring (compounds 4, 5, 8, 14a-e), substitute the phenyl ring with a larger steric hindrance ring (compounds 10, 17c) or modify the carbon chain (compounds 17a, 17b, 21, 23, 25). All of the analogs were tested for their effect on AR and iNOS activities and on D-galactose-induced apoptosis in cultured human lens epithelial cells. Compounds 5, 10, 14a, 14b, 14d, 14e, 14f, 17b, 17c, 23, and 25 inhibited AR activity, with IC50 values ranging from 5.02 to 288.8 μM. Compounds 5, 10, 14b, and 14f inhibited iNOS activity with IC50 ranging from 432.6 to 1188.7 μM. Compounds 5, 8, 10, 14b, 14f, and 17c protected the cells from D-galactose induced apoptosis with viability ranging from 55.2 to 76.26%. Of gigantol and its analogs, compound 10 showed the greatest bioefficacy and is warranted to be developed as a therapeutic agent for diabetic cataracts.

Anti-osmotic and antioxidant activities of gigantol from Dendrobium aurantiacum var. denneanum against cataractogenesis in galactosemic rats

ETHNOPHARMACOLOGICAL RELEVANCE

Dendrobium aurantiacum var. denneanumis widespread in southern China, locally known as "Shihu", "Huangcao" or "Fengdou", has long been used in traditional Chinese medicine for antipyretic, immunomodulatory, anti-aging effects and eye benefiting.

AIM OF THIS STUDY

To investigate the effects of gigantol extracted from the stem of D. aurantiacum var. denneanum on the formation of galactose-induced cataractogenesis and the potential mechanisms underlying these effects.

MATERIALS AND METHODS

Cataract lens models were induced by d-galactose both in vitro and in vivo. The transparency of the rat lenses in vitro and in vivo was observed with an anatomical microscope and a slit lamp microscope. The differential protein and action targets of gigantol were determined and compared among the control group, model group, and gigantol group using two-dimensional electrophoresis and mass spectrometry (MS). Enzyme kinetics was used to show the ability of gigantol to respress aldose reductase (AR) and inducible nitric oxide synthase (iNOS). Quantitative real-time PCR (RT-qPCR). was used to detect repression of the expression of AR and iNOS genes. Molecular docking and dynamic simulation were used to predict the interaction points and combination patterns between gigantol, AR, and iNOS.

RESULTS

Gigantol was found to prevent galactose-induced damage to the rat lenses both in vitro and in vivo, to delay lens turbidity, and to keep the lenses transparent. Differential proteomes, MS, and RT-qPCR showed AR and iNOS to be the target proteins of gigantol. Gigantol reduced the galactose-induced AR and iNOS mRNA expression by 51.2% and 60.9%, respectively. The IC50 of gigantol for inhibition of AR and iNOS activities were 65.67 μg/mL and 8.768 μg/mL, respectively. Gigantol-AR binding sites were Trp111, His110, Tyr48, and Trp20, and gigantol-iNOS binding sites were Ile195 and Gln257. The main forms of interaction were hydrophobic forces, hydrogen bonds, and van der Waals forces.

CONCLUSION

Gigantol extracted from D. aurantiacum var. denneanum was found to inhibit galactose-induced formation of cataracts through repression of the gene expression and activity of AR and iNOS.

Development of nitric oxide synthase inhibitors for neurodegeneration and neuropathic pain

Nitric oxide (NO) is an important signaling molecule in the human body, playing a crucial role in cell and neuronal communication, regulation of blood pressure, and in immune activation. However, overproduction of NO by the neuronal isoform of nitric oxide synthase (nNOS) is one of the fundamental causes underlying neurodegenerative disorders and neuropathic pain. Therefore, developing small molecules for selective inhibition of nNOS over related isoforms (eNOS and iNOS) is therapeutically desirable. The aims of this review focus on the regulation and dysregulation of NO signaling, the role of NO in neurodegeneration and pain, the structure and mechanism of nNOS, and the use of this information to design selective inhibitors of this enzyme. Structure-based drug design, the bioavailability and pharmacokinetics of these inhibitors, and extensive target validation through animal studies are addressed.

Exploration of the in vitro Antiviral Activity of a Series of New Pyrimidine Analogues on the Replication of HIV and HCV

Background:

In continuation of our search for new anti-HIV and anti-HCV agents, the suggestion, synthesis and structure elucidation of a new series of 2,6-diamino-4-alkylthio- or (2-benzylhydrazinyl)-5- p-chlorophenylazopyrimidines), 2,6-diamino-4-(2-benzylhydrazinyl)-5-(aryl-[1,1′-biphenyl]-4-yl)pyrimidines, 2,6-diamino-4-(aryl)-5-(aryl[1,1′-biphenyl]-4-yl) pyrimidines), 6-(aryl)-1,3-dimethyl-5-nitro pyrimidine-2,4-dione and 6-amino-4-methoxy- N,N-dimethyl-6-arylpyrimidines were described.

Methods:

The anti-HIV-1 (strain IIIB) and HIV-2 (strain ROD) activity of the newly synthesized pyrimidine analogues was evaluated in vitro in human MT-4 cells using the MT-4/MTT assay. Similarly, the same compounds were evaluated in vitro for their selective antiviral activity against HCV in the Huh 5–2 replicon system (type 1b, Con1 strain).

Results:

None of the tested compounds exhibited inhibition of HIV-1 and HIV-2 replication in cell culture. Even though many compounds yielded a 50% effective concentration in the HCV replicon system with selectivity indexes up to 6.9, none of the compounds matched the selection criteria of a selective inhibitor of virus replication in this assay (that is, >70% inhibition at concentrations that do not elicit an anti-metabolic effect on the host cells).

Conclusions:

Structural modification of these compounds might optimize their anti-HCV activity by introducing diverse and potent functional groups at the pyrimidine backbone, like nitrile residue. Because of the nature of the molecules, these new derivatives will also be evaluated for their potential anti-HIV activity.

Arginine-based inhibitors of nitric oxide synthase: therapeutic potential and challenges

In the past three decades, nitric oxide has been well established as an important bioactive molecule implicated in regulation of cardiovascular, nervous, and immune systems. Therefore, it is not surprising that much effort has been made to find specific inhibitors of nitric oxide synthases (NOS), the enzymes responsible for production of nitric oxide. Among the many NOS inhibitors developed to date, inhibitors based on derivatives and analogues of arginine are of special interest, as this category includes a relatively high number of compounds with good potential for experimental as well as clinical application. Though this group of inhibitors covers early nonspecific compounds, modern drug design strategies such as biochemical screening and computer-aided drug design have provided NOS-isoform-specific inhibitors. With an emphasis on major advances in this field, a comprehensive list of inhibitors based on their structural characteristics is discussed in this paper. We provide a summary of their biochemical properties as well as their observed effects both in vitro and in vivo. Furthermore, we focus in particular on their pharmacology and use in recent clinical studies. The potential of newly designed specific NOS inhibitors developed by means of modern drug development strategies is highlighted.

Tetrahydrobiopterin analogues with NO-dependent pulmonary vasodilator properties

Reduced NO levels due to the deficiency of tetrahydrobiopterin (BH(4)) contribute to impaired vasodilation in pulmonary hypertension. Due to the chemically unstable nature of BH(4), it was hypothesised that oxidatively stable analogues of BH(4) would be able to support NO synthesis to improve endothelial dysfunction in pulmonary hypertension. Two analogues of BH(4), namely 6-hydroxymethyl pterin (HMP) and 6-acetyl-7,7-dimethyl-7,8-dihydropterin (ADDP), were evaluated for vasodilator activity on precontracted rat pulmonary artery rings. ADDP was administered to pulmonary hypertensive rats, followed by measurement of pulmonary vascular resistance in perfused lungs and eNOS expression by immunohistochemistry. ADDP and HMP caused significant relaxation in vitro in rat pulmonary arteries depleted of BH(4) with a maximum relaxation at 0.3μM (both P<0.05). Vasodilator activity of ADDP and HMP was completely abolished following preincubation with the NO synthase inhibitor, L-NAME. ADDP and HMP did not alter relaxation induced by carbachol or spermine NONOate. BH(4) itself did not produce relaxation. In rats receiving ADDP 14.1mg/kg/day, pulmonary vasodilation induced by calcium ionophore A23187 was augmented and eNOS immunoreactivity was increased. In conclusion, ADDP and HMP are two analogues of BH(4), which can act as oxidatively stable alternatives to BH(4) in causing NO-mediated vasorelaxation. Chronic treatment with ADDP resulted in improvement of NO-mediated pulmonary artery dilation and enhanced expression of eNOS in the pulmonary vascular endothelium. Chemically stable analogues of BH(4) may be able to limit endothelial dysfunction in the pulmonary vasculature.

The novel nitric oxide synthase inhibitor 4-amino-tetrahydro-L-biopterine prevents brain edema formation and intracranial hypertension following traumatic brain injury in mice

Brain edema formation, resulting in increased intracranial pressure (ICP), is one of the most deleterious consequences of traumatic brain injury (TBI). Nitric oxide (NO) has previously been shown to be involved in the damage of the blood-brain barrier (BBB) and, thus, in the formation of post-traumatic brain edema; however, this knowledge never resulted in a clinically relevant therapeutic option because available NO synthase inhibitors have serious side effects in man. The aim of the current study was to investigate the therapeutic efficacy of VAS203, a novel tetrahydrobiopterine (BH3)-based NOS inhibitor, in experimental TBI. When added to isolated vessels rings obtained from rat basilar and middle cerebral arteries (n = 32-35) VAS203 showed the same vasoconstrictive effect as the classical NO synthase inhibitor L-(G)-nitro-arginine-methylester (L-NAME). VAS203 passed the BBB both in healthy and traumatized mouse brain (C57/BL6, n = 5 per group) and did not show any systemic side effects at therapeutic concentrations. When administered 30 min after experimental TBI (controlled cortical impact, 2.2 mg/kg/min i.v., n = 7 per group), VAS203 prevented any further increase in ICP or deterioration of cerebral blood flow. This effect was dose-dependent and long-lasting (i.e., 24 h after trauma, brain edema formation was still significantly reduced [-40%, p < 0.008; n = 7 per group] and functional improvements were present up to 7 days after TBI [p < 0.02 on post-trauma day 6; n = 8 per group]). Therefore, VAS203 may represent a promising candidate for the treatment of acute intracranial hypertension following TBI.

Discovery of 3,3'-(2,4-diaminopteridine-6,7-diyl)diphenol as an isozyme-selective inhibitor of PI3K for the treatment of ischemia reperfusion injury associated with myocardial infarction

In studies aimed toward identifying effective and safe inhibitors of kinase signaling cascades that underlie ischemia/reperfusion (I/R) injury, we synthesized a series of pteridines and pyridopyrazines. The design strategy was inspired by the examination of naturally occurring PI3K inhibitors such as wortmannin and quercetin, and building a pharmacophore-based model used for optimization. Structural modifications led to hybrid molecules which incorporated aminopyrimidine and aminopyridine moieties with ATP mimetic characteristics into the pharmacophore motifs to modulate kinase affinity and selectivity. Elaborations involving substitutions of the 2 and 4 positions of the pyrimidine or pyridine ring and the 6 and 7 positions of the central pyrazine ring resulted in in vivo activity profiles which identified potent inhibitors of vascular endothelial growth factor (VEGF) induced vascular leakage. Pathway analysis identified a diaminopteridine-diphenol as a potent and selective phosphatidylinositol-3-kinase (PI3K) inhibitor. The structure-activity relationship studies of various analogues of diaminopteridine-diphenol-based on biochemical assays resulted in potent inhibitors of PI3K.

A Cascade Reaction Consisting of Pictet−Spengler-Type Cyclization and Smiles Rearrangement: Application to the Synthesis of Novel Pyrrole-Fused Dihydropteridines

[reaction: see text] Tandem Pictet-Spengler-type cyclization and Smiles rearrangement have been discovered in the synthesis of pyrimidine-fused heterocycles. The reaction of 4-chloro-5-pyrrol-1-ylpyrimidine amino aldehyde with an amine under an acidic condition yielded the Pictet-Spengler-type cyclization product diazepine, which readily underwent Smiles rearrangement to give a novel pyrrolo[1,2-f]pteridine derivative.

Synthesis of New 2-Phenyladenines and 2-Phenylpteridines and Biological Evaluation as Adenosine Receptor Ligands

Synthesis and biological assays of a series of 2-phenylpteridine derivatives are described to compare their affinities to adenosine receptors with those of the corresponding adenines, purposely prepared, and 8-azaadenines previously described. This study demonstrates that the enlargement of the five-membered ring of the adenine nucleus to a six-membered one is a modification that does not allow the molecules to maintain high activity towards adenosine receptors; in fact, pteridine derivatives did not show themselves to be good adenosine receptor ligands. On the contrary, N(6)-cycloalkyl- or N(6)-alkyl-2-phenyladenines showed a very high affinity and selectivity for A(1) adenosine receptors. We demonstrate also that the 9-benzyl substituent is crucial for conferring high affinity for A(3) receptors to molecules having a 2-phenyladenine-like nucleus.

Structural analysis of isoform-specific inhibitors targeting the tetrahydrobiopterin binding site of human nitric oxide synthases

Nitric oxide synthesized from l-arginine by nitric oxide synthase isoforms (NOS-I-III) is physiologically important but also can be deleterious when overproduced. Selective NOS inhibitors are of clinical interest, given their differing pathophysiological roles. Here we describe our approach to target the unique NOS (6R,1'R,2'S)-5,6,7,8-tetrahydrobiopterin (H(4)Bip) binding site. By a combination of ligand- and structure-based design, the structure-activity relationship (SAR) for a focused set of 41 pteridine analogues on four scaffolds was developed, revealing selective NOS-I inhibitors. The X-ray crystal structure of rat NOS-I dimeric-oxygenase domain with H(4)Bip and l-arginine was determined and used for human isoform homology modeling. All available NOS structural information was subjected to comparative analysis of favorable protein-ligand interactions using the GRID/concensus principal component analysis (CPCA) approach to identify the isoform-specific interaction site. Our interpretation, based on protein structures, is in good agreement with the ligand SAR and thus permits the rational design of next-generation inhibitors targeting the H(4)Bip binding site with enhanced isoform selectivity for therapeutics in pathology with NO overproduction.

Biology and chemistry of the inhibition of nitric oxide synthases by pteridine-derivatives as therapeutic agents

Inhibitors of the family of nitric oxide synthases (NOS-I-III; EC 1.14.13.39) are of interest as pharmacological agents to modulate pathologically high nitric oxide (NO) levels in inflammation, sepsis, and stroke. In this article, we discuss the approach for targeting the unique (6R)-5,6,7,8-tetrahydro-L-biopterin (H4Bip) binding site of NOS by appropriate inhibitors. This binding site maximally increases enzyme activity and NO production from the substrate L-arginine upon cofactor binding. The first generation of H4Bip-based NOS inhibitors was based on 4-amino H4Bip derivatives in analogy to anti-folates such as methotrexate. In addition, we discuss the structure-activity relationship of a related series of 4-oxo-pteridine derivatives. Furthermore, molecular modeling studies provide an understanding of pterin antagonism on a structural level based on favorable and unfavorable interactions between protein binding site and ligands. These techniques include 3D-QSAR (CoMFA, CoMSIA) to understand ligand affinity and GRID/consensus principal component analysis (CPCA) to learn about selectivity requirements. Collectively these approaches, in combination with the presented SAR and structural data, provide useful information for the design of novel NOS inhibitors with increased isoform selectivity.

Novel inhibitors of neuronal nitric oxide synthase

Selective inhibitors of neuronal nitric oxide synthase (nNOS), which are devoid of any effect on the endothelial isoform (eNOS), may be required for the treatment of some neurological disorders. In our search for novel nNOS inhibitors, we recently described some 1-[(Aryloxy)ethyl]-1H-imidazoles as interesting molecules for their selectivity for nNOS against eNOS. This work reports a new series of 1-[(Aryloxy)alkyl]-1H-imidazoles in which a longer methylene chain is present between the imidazole and the phenol part of molecule. Some of these molecules were found to be more potent nNOS inhibitors than the parent ethylenic compounds, although this increase in potency resulted in a partial loss of selectivity. The most interesting compound was investigated to establish its mechanism of action and was found to interact with the tetrahydrobiopterin (BH(4)) binding site of nNOS, without interference with any other cofactors or substrate binding sites.

Structural requirements for inhibition of the neuronal nitric oxide synthase (NOS-I): 3D-QSAR analysis of 4-oxo- and 4-amino-pteridine-based inhibitors

The family of homodimeric nitric oxide synthases (NOS I-III) catalyzes the generation of the cellular messenger nitric oxide (NO) by oxidation of the substrate L-arginine. The rational design of specific NOS inhibitors is of therapeutic interest in regulating pathological NO levels associated with sepsis, inflammatory, and neurodegenerative diseases. The cofactor (6R)-5,6,7,8-tetrahydrobiopterin (H(4)Bip) maximally activates all NOSs and stabilizes enzyme quaternary structure by promoting and stabilizing dimerization. Here, we describe the synthesis and three-dimensional (3D) quantitative structure-activity relationship (QSAR) analysis of 65 novel 4-amino- and 4-oxo-pteridines (antipterins) as inhibitors targeting the H(4)Bip binding site of the neuronal NOS isoform (NOS-I). The experimental binding modes for two inhibitors complexed with the related endothelial NO synthase (NOS-III) reveal requirements of biological affinity and form the basis for ligand alignment. Different alignment rules were derived by building other compounds accordingly using manual superposition or a genetic algorithm for flexible superposition. Those alignments led to 3D-QSAR models (comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA)), which were validated using leave-one-out cross-validation, multiple analyses with two and five randomly chosen cross-validation groups, perturbation of biological activities by randomization or progressive scrambling, and external prediction. An iterative realignment procedure based on rigid field fit was used to improve the consistency of the resulting partial least squares models. This led to consistent and highly predictive 3D-QSAR models with good correlation coefficients for both CoMFA and CoMSIA, which correspond to experimentally determined NOS-II and -III H(4)Bip binding site topologies as well as to the NOS-I homology model binding site in terms of steric, electrostatic, and hydrophobic complementarity. These models provide clear guidelines and accurate activity predictions for novel NOS-I inhibitors.

Current trends in QSAR on NO donors and inhibitors of nitric oxide synthase (NOS)*

This article evaluates the quantitative structure-activity relationships (QSAR) of nitric oxide (NO) radical donors and nitric oxide synthases (NOS) inhibitors, using the C-QSAR program of Biobyte. Furoxans, triazines, amidoximes, tetrazoles, imidazoles and N(omega)-2-nitroarylamino acid analogues were included in this survey. In nine out of seventeen cases, the clog P plays a significant part in the QSAR of the NO radical donors and of the NOS inhibition. Many of the compounds must be interacting with a hydrophobic space in a non-specific way. In some cases molecular refractivity CMR/MR as well as sterimol parameters (B(1) and L) are important. Electronic effects, with the exception of the Hammett's constant sigma and the Swain-Lupton parameter F, are not found to govern the biological activity. Stereochemical and electronic features are also found to be important. Indicator variables were used after the best model was found to account for the usual structural features.

Structural basis for pterin antagonism in nitric-oxide synthase. Development of novel 4-oxo-pteridine antagonists of (6R)-5,6,7,8-tetrahydrobiopterin

Pathological nitric oxide (NO) generation in sepsis, inflammation, and stroke may be therapeutically controlled by inhibiting NO synthases (NOS). Here we targeted the (6R)-5,6,7,8-tetrahydro-l-biopterin (H(4)Bip)-binding site of NOS, which, upon cofactor binding, maximally increases enzyme activity and NO production from substrate l-arginine. The first generation of H(4)Bip-based NOS inhibitors employed a 4-amino pharmacophore of H(4)Bip analogous to antifolates such as methotrexate. We developed a novel series of 4-oxo-pteridine derivatives that were screened for inhibition against neuronal NOS (NOS-I) and a structure-activity relationship was determined. To understand the structural basis for pterin antagonism, selected derivatives were docked into the NOS pterin binding cavity. Using a reduced 4-oxo-pteridine scaffold, derivatives with certain modifications such as electron-rich aromatic phenyl or benzoyl groups at the 5- and 6-positions, were discovered to markedly inhibit NOS-I, possibly due to hydrophobic and electrostatic interactions with Phe(462) and Ser(104), respectively, within the pterin binding pocket. One of the most effective 4-oxo compounds and, for comparisons an active 4-amino derivative, were then co-crystallized with the endothelial NOS (NOS-III) oxygenase domain and this structure solved to confirm the hypothetical binding modes. Collectively, these findings suggest (i) that, unlike the antifolate principle, the 4-amino substituent is not essential for developing pterin-based NOS inhibitors and (ii), provide a steric and electrostatic basis for their rational design.

Inhibitory effects of methanol extract of Cyperus rotundus rhizomes on nitric oxide and superoxide productions by murine macrophage cell line, RAW 264.7 cells

The rhizomes of Cyperus rotundus (C. rotundus) have been used in oriental traditional medicines for the treatment of stomach and bowel disorders, and inflammatory diseases. Nitric oxide (NO) and superoxide (O2-) are important mediators in the pathogenesis of inflammatory diseases. This study was undertaken to address whether the metanol (MeOH) extract of rhizomes of C. rotundus could modulate NO and O2- productions by murine macrophage cell line, RAW 264.7 cells. The MeOH extract of rhizomes of C. rotundus showed the inhibition of NO production in a dose-dependent manner by RAW 264.7 cells stimulated with interferon-gamma plus lipopolysaccharide. The inhibition of NO production by the extract was due to the suppression of iNOS protein, as well as iNOS mRNA expression, determined by Western and Northern blotting analyses, respectively. In addition, the MeOH extract suppressed the production of O2- by phorbol ester-stimulated RAW 264.7 cells in dose- and time-dependent manners. Collectively, these results suggest that the MeOH extract of rhizomes of C. rotundus could be developed as anti-inflammatory candidate for the treatment of inflammatory diseases mediated by overproduction of NO and O2-.

Pterin interactions with distinct reductase activities of NO synthase

Besides oxidizing L-arginine, neuronal NO synthase (NOS) NADPH-dependently reduces various electron acceptors, including cytochrome c and tetrazolium salts. The latter NADPH diaphorase reaction is used as a NOS-specific histochemical stain. Both reductase activities have been utilized to analyse electron transfer mechanisms within NOS. Basal L-arginine turnover by homodimeric NOS is enhanced by exogenous tetrahydrobiopterin, and the intra-subunit electron flow may include intermediate trihydrobiopterin. In the present work we have investigated the possible role of the tetrahydrobiopterin binding site of NOS in its reductase activities by examining the effects of anti-pterin type (PHS) NOS inhibitors. Although the type I anti-pterin, PHS-32, which does not affect basal dimeric NOS activity, also had no effect on either reductase activity, the type II anti-pterin, PHS-72, which inhibits basal NOS activity, inhibited both reductase activities and the NADPH diaphorase histochemical stain. Pterin-free NOS monomers catalysed both cytochrome c and tetrazolium salt reduction. Our data suggest that both NOS reductase activities are independent of tetrahydrobiopterin. However, occupation of an exosite near the pterin site in NOS by type II anti-pterins may interfere with the electron flow within the active centre, suggesting that steric perturbation of the pterin binding pocket or reductase interaction contribute to the mechanism of inhibition by this class of NOS inhibitors.

Inhibition of endotoxin-induced vascular hyporeactivity by 4-amino-tetrahydrobiopterin

The 4-amino analogue of tetrahydrobiopterin (4-ABH(4)) is a potent pterin-site inhibitor of nitric oxide synthases (NOS). Although 4-ABH(4) does not exhibit selectivity between purified NOS isoforms, a pronounced selectivity of the drug towards inducible NOS (iNOS) is apparent in intact cells. This work was carried out to investigate the potential iNOS selectivity of 4-ABH(4) in isolated pig pulmonary and coronary arteries. Endothelium-dependent relaxations of pig pulmonary and coronary artery strips to bradykinin or calcium ionophore A23187 were inhibited by 4-ABH(4) in a concentration-dependent manner. Half-maximal inhibition was observed at 60 - 65 microM (pulmonary artery) and 200 - 250 microM 4-ABH(4) (coronary artery). Pig coronary artery strips precontracted with 0.1 microM 9, 11-dideoxy-9, 11-methanoepoxy-prosta-glandin F(2alpha) (U46619) showed a time-dependent relaxation (monitored for up to 18 h) upon incubation with 1 microg ml(-1) lipopolysaccharide (LPS). Addition of 10 microM 4-ABH(4) 1 h after LPS led to a pronounced inhibition of the LPS-triggered relaxation, whereas the pterin antagonist had no effect when given> or =4 h after LPS. Incubation of pulmonary and coronary artery strips with 1 microg ml(-1) LPS attenuated contractile responses to norepinephrine (1 microM) and U46619 (0.1 microM). This hyporeactivity of the blood vessels to vasoconstrictor agents was inhibited by 4-ABH(4) in a concentration-dependent manner [IC(50)=17.5+/-5.9 microM (pulmonary artery) and 20.7+/-3 microM (coronary artery)]. The effect of 0.1 mM 4-ABH(4) was antagonized by coincubation with 0.1 mM sepiapterin, which is known to supply intracellular BH(4) via a salvage pathway. These results demonstrate that 4-ABH(4) is a fairly selective inhibitor of iNOS in an in vitro model of endotoxaemia, suggesting that this drug and/or related pterin-site NOS inhibitors may be useful to increase blood pressure in severe infections associated with a loss of vascular responsiveness to constrictor agents caused by endotoxin-triggered iNOS induction in the vasculature.

2-thiouracil is a selective inhibitor of neuronal nitric oxide synthase antagonising tetrahydrobiopterin-dependent enzyme activation and dimerisation

2-thiouracil (TU), an established antithyroid drug and melanoma-seeker, was found to selectively inhibit neuronal nitric oxide synthase (nNOS) in a competitive manner (K(i)=20 microM), being inactive on the other NOS isoforms. The drug apparently interfered with the substrate- and tetrahydrobiopterin (BH(4))-binding to the enzyme. It caused a 60% inhibition of H(2)O(2) production in the absence of L-arginine and BH(4), and antagonised BH(4)-induced dimerisation of nNOS, but did not affect cytochrome c reduction. These results open new perspectives in the understanding of the antithyroid action of TU and provide a new lead structure for the development of selective nNOS inhibitors to elucidate the interdependence of the substrate and pteridine sites and to modulate pathologically aberrant NO formation.