Targeting the CSF-1/CSF-1R Axis: Exploring the Potential of CSF1R Inhibitors in Neurodegenerative Diseases

We report herein the potential of colony-stimulating factor-1 receptor (CSF1R) inhibitors as therapeutic agents in neuroinflammatory diseases, with a focus on Alzheimer's disease (AD). Employing a carefully modified scaffold, N-(4-heterocycloalkyl-2-cycloalkylphenyl)-5-methylisoxazole-3-carboxamide, we identify highly selective and potent CSF1R inhibitors─7dri and 7dsi. Molecular docking studies shed light on the binding modes of these key compounds within the CSF1R binding site. Remarkably, kinome-wide selectivity assessment underscores the impressive specificity of 7dri for CSF-1R. Notably, 7dri emerges as a potent CSF-1R inhibitor with favorable cellular activity and minimal cytotoxicity among the synthesized compounds. Demonstrating efficacy in inhibiting CSF1R phosphorylation in microglial cells and successfully mitigating neuroinflammation in an in vivo LPS-induced model, 7dri establishes itself as a promising antineuroinflammatory agent.

Discovery of N-(5-amido-2-methylphenyl)-5-methylisoxazole-3-carboxamide as dual CSF-1R/c-Kit Inhibitors with improved stability and BBB permeability

This study explores the potential of CSF-1R inhibitors as therapeutic agents for neurodegenerative diseases. CSF-1R, a receptor tyrosine kinase primarily expressed in macrophage lineages, plays a pivotal role in regulating various cellular processes. Recent research highlights the significance of CSF-1R inhibition in mitigating neuroinflammation, particularly in Alzheimer's disease, where microglial overactivation contributes to neurodegeneration. The research reveals a series of N-(5-amido-2-methylphenyl)-5-methylisoxazole-3-carboxamide CSF-1R inhibitors, where compounds 7d, 7e, and 9a exhibit outstanding inhibitory activities and selectivity, with IC50 values of 33, 31, and 64 nM, respectively. These most promising compounds in this series were profiled for cellular potency and subjected to in vitro pharmacokinetic profiling. These inhibitors exhibit minimal cytotoxicity, even at higher concentrations, and possess promising blood-brain barrier permeability, making them potential candidates for central nervous system diseases. The investigation into the in vitro ADME properties, including plasma and microsomal stability, reveals that these CSF-1R inhibitors maintain their structural integrity and plasma concentration. This resilience positions them for further development as therapeutic agents for neurodegenerative diseases.

An Innovative Approach to Address Neurodegenerative Diseases through Kinase-Targeted Therapies: Potential for Designing Covalent Inhibitors

Owing to the dysregulation of protein kinase activity in various diseases such as cancer and autoimmune, cardiovascular, neurodegenerative, and inflammatory conditions, the protein kinase family has emerged as a crucial drug target in the 21st century. Notably, many kinases have been targeted to address cancer and neurodegenerative diseases using conventional ATP-mimicking kinase inhibitors. Likewise, irreversible covalent inhibitors have also been developed for different types of cancer. The application of covalent modification to target proteins has led to significant advancements in the treatment of cancer. However, while covalent drugs have significantly impacted medical treatment, their potential for neurodegenerative diseases remains largely unexplored. Neurodegenerative diseases present significant risks to brain function, leading to progressive deterioration in sensory, motor, and cognitive abilities. Alzheimer's disease (AD), Huntington's disease (HD), Parkinson's disease (PD), and multiple sclerosis (MS) are among the various examples of such disorders. Numerous research groups have already reported insights through reviews and research articles on FDA-approved covalent inhibitors, revealing their mechanisms and the specific covalent warheads that preferentially interact with particular amino acid residues in intricate detail. Hence, in this review, we aim to provide a concise summary of these critical topics. This summary endeavors to guide medicinal chemists in their quest to design covalent inhibitors for protein kinases, specifically targeting neurodegenerative diseases.

An Intriguing Purview on the Design of Macrocyclic Inhibitors for Unexplored Protein Kinases through Their Binding Site Comparison

Kinases play an important role in regulating various intracellular signaling pathways that control cell proliferation, differentiation, survival, and other cellular processes, and their deregulation causes more than 400 diseases. Consequently, macrocyclization can be considered a noteworthy approach to developing new therapeutic agents for human diseases. Macrocyclization has emerged as an effective drug discovery strategy over the past decade to improve target selectivity and potency of small molecules. Small compounds with linear structures upon macrocyclization can lead to changes in their physicochemical and biological properties by firmly reducing conformational flexibility. A number of distinct protein kinases exhibit similar binding sites. Comparison of protein binding sites provides crucial insights for drug discovery and development. Binding site similarities are helpful in understanding polypharmacology, identifying potential off-targets, and repurposing known drugs. In this review, we focused on comparing the binding sites of those kinases for which macrocyclic inhibitors are available/studied so far. Furthermore, we calculated the volume of the binding site pocket for each targeted kinase and then compared it with the binding site pocket of the kinase for which only acyclic inhibitors were designed to date. Our review and analysis of several explored kinases might be useful in targeting new protein kinases for macrocyclic drug discovery.

Carbamate JNK3 Inhibitors Show Promise as Effective Treatments for Alzheimer's Disease: In Vivo Studies on Mouse Models

We have been developing new inhibitors for c-Jun N-terminal kinase 3 (JNK3) as a potential treatment for Alzheimer's disease (AD). We identified potential JNK3 inhibitors through pharmacodynamic optimization studies, including benzimidazole compounds 2 and 3, but their unreliable pharmacokinetic properties led us to develop carbamate inhibitors 2h and 3h. In vitro studies validated carbamate inhibitors 2h and 3h as potent and highly selective JNK3 inhibitors with favorable pharmacokinetic profiles. Oral administration of 2h and 3h to both APP/PS1 and 3xTg AD mouse models improved cognitive function, indicating their potential as effective treatments for Alzheimer's disease. Carbamate JNK3 inhibitor 3h, in particular, restored cognitive function to near-normal levels in the 3xTg mice model of AD and led to pTau reduction in the hippocampal tissues of 3xTg-AD mice during in vivo behavioral evaluations. We intend to further develop these carbamate JNK3 inhibitors in preclinical studies as a potential first-in-class treatment for AD.

Design of Novel IRAK4 Inhibitors Using Molecular Docking, Dynamics Simulation and 3D-QSAR Studies

Treatment of several autoimmune diseases and types of cancer has been an intense area of research over the past two decades. Many signaling pathways that regulate innate and/or adaptive immunity, as well as those that induce overexpression or mutation of protein kinases, have been targeted for drug discovery. One of the serine/threonine kinases, Interleukin-1 Receptor Associated Kinase 4 (IRAK4) regulates signaling through various Toll-like receptors (TLRs) and interleukin-1 receptor (IL1R). It controls diverse cellular processes including inflammation, apoptosis, and cellular differentiation. MyD88 gain-of-function mutations or overexpression of IRAK4 has been implicated in various types of malignancies such as Waldenström macroglobulinemia, B cell lymphoma, colorectal cancer, pancreatic ductal adenocarcinoma, breast cancer, etc. Moreover, over activation of IRAK4 is also associated with several autoimmune diseases. The significant role of IRAK4 makes it an interesting target for the discovery and development of potent small molecule inhibitors. A few potent IRAK4 inhibitors such as PF-06650833, RA9 and BAY1834845 have recently entered phase I/II clinical trial studies. Nevertheless, there is still a need of selective inhibitors for the treatment of cancer and various autoimmune diseases. A great need for the same intrigued us to perform molecular modeling studies on 4,6-diaminonicotinamide derivatives as IRAK4 inhibitors. We performed molecular docking and dynamics simulation of 50 ns for one of the most active compounds of the dataset. We also carried out MM-PBSA binding free energy calculation to identify the active site residues, interactions of which are contributing to the total binding energy. The final 50 ns conformation of the most active compound was selected to perform dataset alignment in a 3D-QSAR study. Generated RF-CoMFA (q² = 0.751, ONC = 4, r² = 0.911) model revealed reasonable statistical results. Overall results of molecular dynamics simulation, MM-PBSA binding free energy calculation and RF-CoMFA model revealed important active site residues of IRAK4 and necessary structural properties of ligand to design more potent IRAK4 inhibitors. We designed few IRAK4 inhibitors based on these results, which possessed higher activity (predicted pIC₅₀) than the most active compounds of the dataset selected for this study. Moreover, ADMET properties of these inhibitors revealed promising results and need to be validated using experimental studies.

Rational design and synthesis of 2-(1H-indazol-6-yl)-1H-benzo[d]imidazole derivatives as inhibitors targeting FMS-like tyrosine kinase 3 (FLT3) and its mutants

Fms-like tyrosine kinase 3 (FLT3) has been verified as a therapeutic target for acute myeloid leukaemia (AML). In this study, we report a series of 2-(1H-indazol-6-yl)-1H-benzo[d]imidazol-5-yl benzamide and phenyl urea derivatives as potent FLT3 inhibitors based on the structural optimisation of previous FLT3 inhibitors. Derivatives were synthesised as benzamide 8a–k, 8n–z, and phenyl urea 8l–m, with various substituents. The most potent inhibitor, 8r, demonstrated strong inhibitory activity against FLT3 and FLT3 mutants with a nanomolar IC₅₀ and high selectivity profiles over 42 protein kinases. In addition, these type II FLT3 inhibitors were more potent against FLT3 mutants correlated with drug resistance. Overall, we provide a theoretical basis for the structural optimisation of novel benzimidazole analogues to develop strong inhibitors against FLT3 mutants for AML therapeutics.

Generation of Non-Nucleotide CD73 Inhibitors Using a Molecular Docking and 3D-QSAR Approach

Radiotherapy and chemotherapy are conventional cancer treatments. Around 60% of all patients who are diagnosed with cancer receive radio- or chemotherapy in combination with surgery during their disease. Only a few patients respond to the blockage of immune checkpoints alone, or in combination therapy, because their tumours might not be immunogenic. Under these circumstances, an increasing level of extracellular adenosine via the activation of ecto-5’-nucleotidase (CD73) and consequent adenosine receptor signalling is a typical mechanism that tumours use to evade immune surveillance. CD73 is responsible for the conversion of adenosine monophosphate to adenosine. CD73 is overexpressed in various tumour types. Hence, targetting CD73’s signalling is important for the reversal of adenosine-facilitated immune suppression. In this study, we selected a potent series of the non-nucleotide small molecule inhibitors of CD73. Molecular docking studies were performed in order to examine the binding mode of the inhibitors inside the active site of CD73 and 3D-QSAR was used to study the structure–activity relationship. The obtained CoMFA (q² = 0.844, ONC = 5, r² = 0.947) and CoMSIA (q² = 0.804, ONC = 4, r² = 0.954) models showed reasonable statistical values. The 3D-QSAR contour map analysis revealed useful structural characteristics that were needed to modify non-nucleotide small molecule inhibitors. We used the structural information from the overall docking and 3D-QSAR results to design new, potent CD73 non-nucleotide inhibitors. The newly designed CD73 inhibitors exhibited higher activity (predicted pIC₅₀) than the most active compound of all of the derivatives that were selected for this study. Further experimental studies are needed in order to validate the new CD73 inhibitors.

A Perspective on the Development of c-Jun N-terminal Kinase Inhibitors as Therapeutics for Alzheimer's Disease: Investigating Structure through Docking Studies

c-Jun N-terminal kinase (JNK) plays an important role in cell death caused by various stimuli. Because the isoform JNK3 is mainly expressed in the brain, it is believed to play a pivotal role in various neurodegenerative diseases, including Alzheimer’s disease (AD) and Parkinson’s disease (PD), which still lack plausible therapeutics. To develop a novel and selective JNK3 inhibitor, we conducted a decadal review (2011 to 2021) of published articles on JNK inhibitors, particularly those focusing on a structural perspective and docking insights. We observed the structures of three isoforms of JNK, namely holo-proteins and co-crystal structures, with JNK3 inhibitors and summarized the significant structural aspects of selective JNK3 inhibitors as AD therapeutics.

Design and Synthesis of a Novel PLK1 Inhibitor Scaffold Using a Hybridized 3D-QSAR Model

Polo-like kinase 1 (PLK1) plays an important role in cell cycle progression and proliferation in cancer cells. PLK1 also contributes to anticancer drug resistance and is a valuable target in anticancer therapeutics. To identify additional effective PLK1 inhibitors, we performed QSAR studies of two series of known PLK1 inhibitors and proposed a new structure based on a hybridized 3D-QSAR model. Given the hybridized 3D-QSAR models, we designed and synthesized 4-benzyloxy-1-(2-arylaminopyridin-4-yl)-1H-pyrazole-3-carboxamides, and we inspected its inhibitory activities to identify novel PLK1 inhibitors with decent potency and selectivity.

Discovery of 5-methyl-N-(2-arylquinazolin-7-yl)isoxazole-4-carboxamide analogues as highly selective FLT3 inhibitors

A series of 4-arylamido 5-methylisoxazole derivatives with quinazoline core was designed and synthesised based on conformational rigidification of a previous type II FMS inhibitor. Most of quinazoline analogues displayed activity against FLT3 and FLT3-ITD. Compound 7d, 5-methyl-N-(2-(3-(4-methylpiperazin-1-yl)-5-(trifluoromethyl)phenyl)quinazolin-7-yl)isoxazole-4-carboxamide, exhibited the most potent inhibitory activity against FLT3 (IC₅₀= 106 nM) with excellent selectivity profiles over 36 other protein kinases including cKit and FMS kinase. Compound 7d was also active in FLT-ITD, with an IC₅₀ value of 301 nM, and other FLT3 mutants showing potential as an AML therapeutics.

Discovery of 3-alkyl-5-aryl-1-pyrimidyl-1H-pyrazole derivatives as a novel selective inhibitor scaffold of JNK3

3-alkyl-5-aryl-1-pyrimidyl-1H-pyrazole derivatives were designed and synthesised as selective inhibitors of JNK3, a target for the treatment of neurodegenerative diseases. Following previous studies, we have designed JNK3 inhibitors to reduce the molecular weight and successfully identified a lead compound that exhibits equipotent activity towards JNK3. Kinase profiling results also showed high selectivity for JNK3 among 38 kinases. Among the derivatives, the IC₅₀ value of 8a, (R)-2-(1-(2-((1-(cyclopropanecarbonyl)pyrrolidin-3-yl)amino)pyrimidin-4-yl)-5-(3,4-dichlorophenyl)-1H-pyrazol-3-yl)acetonitrile exhibited 227 nM, showing the highest inhibitory activity against JNK3.

Computer-aided design and synthesis of 3-carbonyl-5-phenyl-1H-pyrazole as highly selective and potent BRAFV600E and CRAF inhibitor

BRAF belongs to the upstream portion of the MAPK pathway, which is involved in cell proliferation and survival. When mutations occur in BRAF, downstream MEK and ERK are phosphorylated irrespective of RAS, resulting in melanoma-like cancer. Over the years, small molecules targeting BRAFV600E have been discovered to be very effective melanoma drugs, but they are known to cause the BRAF paradox. Recently, it was shown that this paradox is caused by the heterodimer phenomenon of BRAF/CRAF. Here, we suggest one method by which paradoxical activation can be avoided by selectively inhibiting BRAFV600E and CRAF but not wild-type BRAF. From previous report of N-(3-(3-alkyl-1H-pyrazol-5-yl) phenyl) aryl amide as a selective inhibitor of BRAFV600E and CRAF, we present compounds that offer enhanced selectivity and efficacy with the aid of molecular modelling.

Conformational restriction of a type II FMS inhibitor leading to discovery of 5-methyl-N-(2-aryl-1H-benzo[d]imidazo-5-yl)isoxazole-4-carboxamide analogues as selective FLT3 inhibitors

A series of 4-arylamido 5-methylisoxazole derivatives incorporating benzimidazole was designed and synthesised by conformational restriction of an in-house type II FMS inhibitor. Kinase profiling of one compound revealed interesting features, with increased inhibitory potency towards FLT3 and concomitant loss of potency towards FMS. Several benzimidazole derivatives 5a–5g and 6a–6c containing various hydrophobic moieties were synthesised, and their inhibitory activity against FLT3 was evaluated. Specifically, 5a, 5-methyl-N-(2-(3-(4-methylpiperazin-1-yl)-5-(trifluoromethyl)phenyl)-1H-benzo[d]imidazole-5-yl) isoxazole-4-carboxamide, exhibited the most potent inhibitory activity against FLT3 (IC₅₀ = 495 nM), with excellent selectivity profiles.

Discovery of novel 4-aryl-thieno[1,4]diazepin-2-one derivatives targeting multiple protein kinases as anticancer agents

A series of 4-aryl-thieno[1,4]diazepin-2-one were synthesized and evaluated for their antiproliferative activities against the A375P melanoma and U937 hematopoietic cell lines. Several compounds showed very potent antiproliferative activities toward both cell lines and the activities were better than that of sorafenib, the reference standard. Derivatives were made as amide (8a-8i, 9a-9m) and urea (10a-10d, 11a-11d) with diverse hydrophobic moieties. One of the most potent inhibitor 10d, 1-(4-((4-ethylpiperazin-1-yl)methyl)-3-(trifluoromethyl)phenyl)-3-(4-(2-oxo-2,3-dihydro-1H-thieno [3,4-b][1,4]diazepin-4-yl)phenyl)urea was found to be very potent inhibitor of multi-protein kinases including FMS kinase (IC₅₀ = 3.73 nM) and is a promising candidate for further development in therapeutics for cancer.

Nec-1 alleviates cognitive impairment with reduction of Aβ and tau abnormalities in APP/PS1 mice

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive symptoms of learning and memory deficits. Such cognitive impairments are attributed to brain atrophy resulting from progressive neuronal and synaptic loss; therefore, alleviation of neural cell death is as an important target of treatment as other classical hallmarks of AD, such as aggregation of amyloid‐β (Aβ) and hyperphosphorylation of tau. Here, we found that an anti‐necroptotic molecule necrostatin‐1 (Nec‐1) directly targets Aβ and tau proteins, alleviates brain cell death and ameliorates cognitive impairment in AD models. In the cortex and hippocampus of APP/PS1 double‐transgenic mice, Nec‐1 treatment reduced the levels of Aβ oligomers, plaques and hyperphosphorylated tau without affecting production of Aβ, while it altered the levels of apoptotic marker proteins. Our results showing multiple beneficial modes of action of Nec‐1 against AD provide evidence that Nec‐1 may serve an important role in the development of preventive approach for AD.

Novel scaffold evolution through combinatorial 3D-QSAR model studies of two types of JNK3 inhibitors

JNK3 is an emerging target for neurodegenerative diseases including AD and PD, with histological selectivity. Specifically, in AD, JNK3 is the main protein kinase for APP phosphorylation, which is an important mechanism for Aβ processing, and a biomarker of Alzheimer's disease. Therefore, targeting JNK3 is a reasonable strategy for drug discovery in neurodegenerative diseases. In order to find a novel scaffold for JNK3 inhibitors, we performed 3D-QSAR modeling studies with two different JNK3 inhibitor series. The CoMFA model was obtained with a q² value of 0.806 and an r² value of 0.850. Based on CoMFA and CoMSIA models, rational design was conducted and led to a novel scaffold, N-(thiophen-2-yl)-8H-pyrazolo[1,5-a]pyrido[1,2-c]pyrimidine-10-carboxamide.

New diarylureas and diarylamides possessing acet(benz)amidophenyl scaffold: design, synthesis, and antiproliferative activity against melanoma cell line

A series of new diarylurea and diarylamide derivatives possessing acet(benz)amidophenyl scaffold was synthesized. Their in vitro antiproliferative activity was tested against A375P human melanoma cell line. Compounds 1c,d and 2c,d showed the highest potencies with IC(50) values in sub-micromolar scale. In addition, compounds 1b,e,l and 2e,l were more potent than Sorafenib but with IC(50) values in micromolar range. Moreover, compound 2c was equipotent to Vemurafenib, and 2d showed higher potency than Vemurafenib against A375P. Molar refractometry calculation and ADME profiling of the highest potent four derivatives 1c,d and 2c,d are also reported.

Correlation between performance of QM/MM docking and simple classification of binding sites

Use of SiteMap for binding site classification and its connection to QM/MM (quantum mechanics/ molecular mechanics) docking performance were investigated. Using the hydrophilic/hydrophobic character values along with balance between them which SiteMap calculates, we sorted 455 cocrystal complexes available from protein data bank into three groups and tested how Glide, a conventional docking program, and QPLD, a QM/MM docking program, perform on them. QPLD showed improvements on all three groups over Glide but with varying degrees. Analysis of the results was carried out, and establishment of correlations between the classification of binding sites and QM/MM docking performance was attempted. It was found that QM/MM docking delivers the most improvements for primarily hydrophobic binding sites with substantial hydrophilic interactions. Based on our findings, we make suggestions for use of QM/MM docking and directions for further developments.

Light-mediated liberation of enzymatic activity: "small molecule" caged protein equivalents

Light-activatable ("caged") proteins have been used to correlate, with exquisite temporal and spatial control, intracellular biochemical action with global cellular behavior. However, the chemical or genetic construction of caged proteins is nontrivial, with subsequent laborious introduction into living cells, potentially problematic competition with natural endogenous counterparts, and challenging intracellular incorporation at levels equivalent to the natural enzymes. We describe the design, synthesis, and characterization of small molecular equivalents of a caged Src kinase. These compounds are easy to prepare and function by inhibiting the action of the natural unmodified enzyme.

Acquisition of a "Group A"-selective Src kinase inhibitor via a global targeting strategy

A "global" strategy for the acquisition of selective high affinity inhibitors for the Src kinase subfamily of tyrosine kinases is described. Members of the Src family exhibit a strong amino acid sequence homology. However, recent studies have revealed differences in the relative spatial relationships of the three distinct protein-binding domains present in these enzymes. We have constructed an inhibitor, using an amalgamation of combinatorial methods and directed design, which simultaneously associates with the active site and an ancillary protein-binding region (SH2 domain). The inhibitor exhibits high inhibitory potency and selectivity for the Group A versus Group B subset of Src kinases.

Structural basis for dipeptide amide isoform-selective inhibition of neuronal nitric oxide synthase

Three nitric oxide synthase (NOS) isoforms, eNOS, nNOS and iNOS, generate nitric oxide (NO) crucial to the cardiovascular, nervous and host defense systems, respectively. Development of isoform-selective NOS inhibitors is of considerable therapeutic importance. Crystal structures of nNOS-selective dipeptide inhibitors in complex with both nNOS and eNOS were solved and the inhibitors were found to adopt a curled conformation in nNOS but an extended conformation in eNOS. We hypothesized that a single-residue difference in the active site, Asp597 (nNOS) versus Asn368 (eNOS), is responsible for the favored binding in nNOS. In the D597N nNOS mutant crystal structure, a bound inhibitor switches to the extended conformation and its inhibition of nNOS decreases >200-fold. Therefore, a single-residue difference is responsible for more than two orders of magnitude selectivity in inhibition of nNOS over eNOS by L-N(omega)-nitroarginine-containing dipeptide inhibitors.

Aromatic reduced amide bond peptidomimetics as selective inhibitors of neuronal nitric oxide synthase

Nitric oxide synthase inhibitors could act as important therapies for disorders arising from overstimulation or overexpression of individual nitric oxide synthase (NOS) isoforms. But preservation of physiologically important nitric oxide functions require the use of isoform-selective inhibitors. Recently we reported reduced amide bond pseudodipeptide analogues as potent and selective neuronal nitric oxide synthase (nNOS) inhibitors (Hah, J.-M.; Roman, L. J.; Martasek, P.; Silverman, R. B. J. Med. Chem. 2001, 44, 2667-2670). To increase the lipophilicity a series of aromatic, reduced amide bond analogues (6-25) were designed and synthesized as potential selective nNOS inhibitors. The hypothesized large increase in isoform selectivity of nNOS over inducible NOS was not obtained in this series. However, the high potency with nNOS as well as high selectivity of nNOS over endothelial NOS was retained in some of these compounds (15, 17, 21), as well as good selectivity over inducible NOS. The most potent nNOS inhibitor among these compounds is N-(4S)-[4-amino-5-[2-(2-aminoethyl)phenylamino]-pentyl]-N'-nitroguanidine (17) (K(i) = 50 nM), which also shows the highest selectivity over eNOS (greater than 2100-fold) and 70-fold selectivity over iNOS. Further modification of compound 17 should lead to even more potent and selective nNOS inhibitors.

Reduced amide bond peptidomimetics. (4S)-N-(4-amino-5-[aminoakyl]aminopentyl)-N'-nitroguanidines, potent and highly selective inhibitors of neuronal nitric oxide synthase

Selective inhibition of the isoforms of nitric oxide synthase (NOS) could be therapeutically useful in the treatment of certain disease states arising from the overproduction of nitric oxide. Recently, we reported nitroarginine-containing dipeptide amides (Huang, H; Martasek, P.; Roman, L. J.; Masters, B. S. S.; Silverman, R. B. J. Med. Chem. 1999, 42, 3147.) and some peptidomimetic analogues (Huang, H; Martasek, P.; Roman, L. J.; Silverman, R.B. J. Med Chem. 2000, 43, 2938.) as potent and selective inhibitors of neuronal NOS (nNOS). Here, reduced amide bond pseudodipeptide analogues are synthesized and evaluated for their activity. The deletion of the carbonyl group from the amide bond either preserves or improves the potency for nNOS. Significantly, the selectivities for nNOS over eNOS (endothelial NOS), and iNOS (inducible NOS) are greatly increased in these series. The most potent nNOS inhibitor among these compounds is (4S)-N-(4-amino-5-[aminoethyl]aminopentyl)-N'-nitroguanidine (7) (K(i) = 120 nM), which also shows the highest selectivity over eNOS (greater than 2500-fold) and 320-fold selectivity over iNOS. The reduced amide bond is an excellent surrogate of the amide bond, and it will facilitate the design of new potent and selective inhibitors of nNOS.

Deuterium isotope effects and product studies for the oxidation of N(omega)-allyl-L-arginine and N(omega)-allyl-N(omega)-hydroxy-L-arginine by neuronal nitric oxide synthase

The nitric oxide synthases (NOS), which require heme, tetrahydrobiopterin, FMN, FAD, and NADPH, catalyze the O2-dependent conversion of L-arginine to L-citrulline and nitric oxide. N(omega)-Allyl-L-arginine, a mechanism-based inactivator of neuronal NOS, also is a substrate, producing L-arginine, acrolein, and H2O (Zhang, H. Q.; Dixon, R. P., Marletta, M. A.; Nikolic, D.; Van Breemen, R.; Silverman, R. B. J. Am. Chem. Soc. 1997, 119, 10888). Two possible mechanisms for this turnover are proposed, one initiated by allyl C-H bond cleavage and the other by guanidino N H cleavage, and these mechanisms are investigated with the use of N(omega)-allyl-L-arginine (1), N(omega)-[1,1-(2)H2]allyl-L-arginine (7), N(omega)-allyl-N(omega)-hydroxy-L-arginine (2) and N(omega)-[1,1-(2)H2]allyl-N(omega)-hydroxy-L-arginine (8) as substrates. Significant isotope effects on the two kinetic parameters, kcat and kcat/Km, are observed in case of 1 and 7 during turnover, but not with 2 and 8. No kinetic isotope effects are observed for either compound in their role as inactivators. These results support a mechanism involving initial C-H bond cleavage of N(omega)-allyl-L-arginine followed by hydroxylation and breakdown to products.