Design and synthesis of new orally active nonpeptidic inhibitors of human neutrophil elastase

Thiol-promoted intermolecular cyclization to synthesize 1,2,4-oxadiazoles including tioxazafen under transition metal-free conditions

A simple and efficient one-pot intermolecular annulation reaction for the synthesis of 1,2,4-oxadiazoles from amidoximes and benzyl thiols has been developed, in which benzyl thiols act as not only reactants but also organo-catalysts. The control experiments proved that thiol substrates could facilitate the dehydroaromatization step. High yield, functional group diversity and transition metal-free, extra oxidant-free, and mild conditions are the important practical features. Moreover, this protocol provides an effective alternative method for the synthesis of a commercially available broad-spectrum nematicide, tioxazafen.

Synthesis and Receptor Binding Studies of α5 GABAAR Selective Novel Imidazodiazepines Targeted for Psychiatric and Cognitive Disorders

GABA mediates inhibitory actions through various GABA_A receptor subtypes, including 19 subunits in human GABAAR. Dysregulation of GABAergic neurotransmission is associated with several psychiatric disorders, including depression, anxiety, and schizophrenia. Selective targeting of α2/3 GABAARs can treat mood and anxiety, while α5 GABAA-Rs can treat anxiety, depression, and cognitive performance. GL-II-73 and MP-III-022, α5-positive allosteric modulators have shown promising results in animal models of chronic stress, aging, and cognitive disorders, including MDD, schizophrenia, autism, and Alzheimer's disease. Described in this article is how small changes in the structure of imidazodiazepine substituents can greatly impact the subtype selectivity of benzodiazepine GABAAR. To investigate alternate and potentially more effective therapeutic compounds, modifications were made to the structure of imidazodiazepine 1 to synthesize different amide analogs. The novel ligands were screened at the NIMH PDSP against a panel of 47 receptors, ion channels, including hERG, and transporters to identify on- and off-target interactions. Any ligands with significant inhibition in primary binding were subjected to secondary binding assays to determine their K_i values. The newly synthesized imidazodiazepines were found to have variable affinities for the benzodiazepine site and negligible or no binding to any off-target profile receptors that could cause other physiological problems.

Design and synthesis of thiadiazole-oxadiazole-acetamide derivatives: Elastase inhibition, cytotoxicity, kinetic mechanism, and computational studies

Considering the biological significance of 1,3,4-thiadiazole/oxadiazole heterocyclic scaffolds, a novel series of 1,3,4-thiadiazole-1,3,4-oxadiazole-acetamide derivatives (7a-j) was designed and synthesized using molecular hybridization. The inhibitory effects of the target compounds on elastase were evaluated, and all of these molecules were found to be potent inhibitors compared to the standard reference oleanolic acid. Compound 7f exhibited the excellent inhibitory activity (IC₅₀ = 0.06 ± 0.02 μM), which is 214-fold more active than oleanolic acid (IC₅₀ = 12.84 ± 0.45 μM). Kinetic analysis was also performed on the most potent compound (7f) to determine the mode of binding with the target enzyme, and it was discovered that 7f inhibits the enzyme in a competitive manner. Furthermore, the MTT assay method was used to assess their toxicity on the viability of B16F10 melanoma cell lines, and all compounds did not display any toxic effect on the cells even at high concentrations. The molecular docking studies of all compounds also justified with their good docking score and among them, compound 7f had a good conformational state with hydrogen bond interactions within the receptor binding pocket, which is consistent with the experimental inhibition studies.

Synthesis, X-ray diffraction analysis, quantum chemical studies and α-amylase inhibition of probenecid derived S-alkylphthalimide-oxadiazole-benzenesulfonamide hybrids

Sulphonamide and 1,3,4-oxadiazole moieties are present as integral structural parts of many drugs and pharmaceuticals. Taking into account the significance of these moieties, we herein present the synthesis, single-crystal X-ray analysis, DFT studies, and α-amylase inhibition of probenecid derived two S-alkylphthalimide-oxadiazole-benzenesulfonamide hybrids. The synthesis has been accomplished in high yields. The final structures of both hybrids have been established completely with the help of different spectro-analytical techniques, including NMR, FTIR, HR-MS, and single-crystal X-ray diffraction analyses. In an effort to confirm the experimental findings, versatile quantum mechanical calculations and Hirshfeld Surface analysis have been performed. α-Amylase inhibition assay has been executed to investigate the enzyme inhibitory potential of both hybrids. The low IC50 value (76.92 ± 0.19 μg/mL) of hybrid 2 shows the good α-amylase inhibition potential of the respective compound. Ultimately, the binding affinities and features of the two hybrids are elucidated utilising a molecular docking technique against the α-amylase enzyme.

Synthesis, Antibacterial study and ADME Evaluation of Novel Isonicotinoyl Hydrazide Derivative Containing 1,3,4-Oxadiazole Moiety

Novel derivative of isoniazid containing 1,3,4-oxadiazole pharmacophore has been synthesized. The chemical structure of the compound was characterized and confirmed by using FT-IR and 1H-NMR spectroscopy.   The desired compound was tested against gram-positive and gram-negative bacteria using agar well-diffusion technique for their ability as antibacterial agent and showed good antibacterial activity against gram-negative bacteria and gram-positive bacteria. In addition, ADME evaluations were performed using Swiss ADME to predict if the synthesized compound can be given orally, the bioavailability, topological polar surface area, and drug-likeness. The result showed that all tested compounds absorbed orally and fulfilled the Lipinski rule.

Bis-1,3,4-Oxadiazole Derivatives as Novel and Potential Urease Inhibitors; Synthesis, In Vitro, and In Silico Studies

AIMS

Synthesis of bis-1,3,4-oxadiazole derivatives as novel and potential urease inhibitors.

BACKGROUND

Despite many important biological activities associated with oxadiazoles, they are still neglected by medicinal chemists for their possible urease inhibitory activity. Keeping in view the countless importance of urease inhibitors, we have synthesized a new library of substituted bis-oxadiazole derivatives (1-21) to evaluate their urease inhibitory potential.

OBJECTIVE

Synthesis of substituted bis-oxadiazole derivatives (1-21) to evaluate their urease inhibitory potential.

METHOD

Bis-1,3,4-oxadiazole derivatives 1-21 were synthesized through sequential reactions using starting material isophthalic acid. Esterification reaction was done by refluxing in methanol for 2 h in the presence of the catalytic amount of concentrated H2SO4 till dissolution. In the second step, dimethyl isophthalate and hydrazine hydrate in excess (1:5) were refluxed in methanol to afford isophthalic dihydrazide. Then, isophthalic dihydrazide was treated with different substituted benzaldehydes in a 1:2 ratio under acidic conditions Result: In vitro urease, the inhibitory activity of the synthesized compounds were evaluated and results demonstrated good activities with IC50 values in the range of 13.46 ± 0.34 to 74.45 ± 3.81 µM as compared to the standard thiourea (IC50 = 21.13 ± 0.415 µM). Most of the compounds were found to be more potent than the standard. The structure-activity relationship (SAR) suggested that the variations in the inhibitory activities of the compounds were due to different substitutions. Furthermore; in silico study was also performed.

CONCLUSION

Current study identified a new class of urease inhibitors. All synthetic compounds 1-21 showed potent as well as good to moderate urease inhibitory activities except 3. SAR suggested that hydroxy-bearing analogs were identified exceptionally good. Molecular docking revealed many important interactions made by compounds with the active site of the urease enzyme.

Protease-Specific Biomarkers to Analyse Protease Inhibitors for Emphysema Associated with Alpha 1-Antitrypsin Deficiency. An Overview of Current Approaches

As a known genetic cause of chronic obstructive pulmonary disease (COPD), alpha1-antitrypsin deficiency (AATD) can cause severe respiratory problems at a relatively young age. These problems are caused by decreased or absent levels of alpha1-antitrypsin (AAT), an antiprotease which is primarily functional in the respiratory system. If the levels of AAT fall below the protective threshold of 11 µM, the neutrophil-derived serine proteases neutrophil elastase (NE) and proteinase 3 (PR3), which are targets of AAT, are not sufficiently inhibited, resulting in excessive degradation of the lung parenchyma, increased inflammation, and increased susceptibility to infections. Because other therapies are still in the early phases of development, the only therapy currently available for AATD is AAT augmentation therapy. The controversy surrounding AAT augmentation therapy concerns its efficiency, as protection of lung function decline is not demonstrated, despite the treatment's proven significant effect on lung density change in the long term. In this review article, novel biomarkers of NE and PR3 activity and their use to assess the efficacy of AAT augmentation therapy are discussed. Furthermore, a series of seven synthetic NE and PR3 inhibitors that can be used to evaluate the specificity of the novel biomarkers, and with potential as new drugs, are discussed.

Synthesis and biological evaluation of dehydroabietic acid-pyrimidine hybrids as antitumor agents

A series of novel dehydroabietic acid derivatives containing pyrimidine moieties were designed and synthesized. Some of them displayed more potent inhibitory activities compared with 5-FU.

Synthesis and in Vitro Antimicrobial Activity Screening of New 3-Acetyl-2,5-disubstituted-1,3,4-oxadiazoline Derivatives

Thirteen new 3-acetyl-2,5-disubstituted-1,3,4-oxadiazoline derivatives were synthesized from corresponding hydrazide-hydrazones of isonicotinic acid in the reaction with acetic anhydride. The obtained compounds were identified with the use of spectral methods (IR, ¹ H-NMR, ¹³ C-NMR, MS). In vitro antimicrobial activity screening of synthesized compounds against a panel of bacteria and fungi revealed interesting antibacterial and antifungal activity of tested 1,3,4-oxadiazoline derivatives, which is comparable to that of commonly used antimicrobial agents.

Toward Second-Generation Cardiomyogenic and Anti-cardiofibrotic 1,4-Dihydropyridine-Class TGFβ Inhibitors

Innovative therapeutic modalities for pharmacological intervention of transforming growth factor β (TGFβ)-dependent diseases are of great value. b-Annelated 1,4-dihydropyridines (DHPs) might be such a class, as they induce TGFβ receptor type II degradation. However, intrinsic drawbacks are associated with this compound class and were systematically addressed in the presented study. It was possible to install polar functionalities and bioisosteric moieties at distinct sites of the molecules while maintaining TGFβ-inhibitory activities. The introduction of a 2-amino group or 7-N-alkyl modification proved to be successful strategies. Aqueous solubility was improved by up to seven-fold at pH 7.4 and 200-fold at pH 3 relative to the parent ethyl 4-(biphenyl-4-yl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate. The therapeutic potential of the presented DHPs was further underscored in view of a potential dual mode of action: The differentiation of committed human iPSC-derived cardiac progenitor cells (CPCs) was potently stimulated, and the rescue of cardiac fibrosis phenotypes was observed in engineered heart tissue (EHT) constructs.

Some Novel Mannich Bases of 5-(3,4-Dichlorophenyl)-1,3,4-oxadiazole-2(3H)-one and Their Anti-Inflammatory Activity

Non-steroidal anti-inflammatory drugs (NSAIDs), which are widely used for the treatment of rheumatic arthritis, pain, and many different types of inflammatory disorders, cause serious gastrointestinal (GI) side effects. The free carboxylic acid group existing on their chemical structure is correlated with GI toxicity related with all routine NSAIDs. Replacing this functional group with the 1,3,4-oxadiazole bioisostere is a generally used strategy to obtain an anti-inflammatory agent devoid of GI side effects. In the present work, a novel group of 5-(3,4-dichlorophenyl)-1,3,4-oxadiazole-2(3H)-one Mannich bases were synthesized and characterized on the basis of IR, ¹ H NMR, and elemental analysis results. The target compounds were first tested for cytotoxicity to determine a non-toxic concentration for anti-inflammatory screening. Anti-inflammatory effects of the compounds were evaluated by in vitro lipopolysaccharide (LPS)-induced NO production and in vivo carrageenan footpad edema with ulcerogenic profile. In LPS-induced RAW 264.7 macrophages, most of the compounds showed inhibitory activity on nitrite production while compounds 5a, 5h, and 5j exhibited the best profiles by suppressing the NO production. To evaluate the in vivo anti-inflammatory potency of the compounds, the inflammatory response was quantified by increment in paw size in the carrageenan footpad edema assay. The anti-inflammatory data scoring showed that compounds 5a-d, 5g, and 5j, at the dose of 100 mg/kg, exhibited anti-inflammatory activity, which for compound 5g was comparable to that of the reference drug indomethacin with 53.9% and 55.5% inhibition in 60 and 120 min, respectively.

Novel 2-benzylthio-5-(1,3,4-oxadiazol-2-yl)benzenesulfonamides with anticancer activity: Synthesis, QSAR study, and metabolic stability

A series of novel 2-benzylthio-4-chloro-5-(5-substituted 1,3,4-oxadiazol-2-yl)benzenesulfonamides (4-27) have been synthesized as potential anticancer agents. MTT assay was carried out to determine the cytotoxic activity against three human cancer cell lines: colon cancer HCT-116, breast cancer MCF-7 and cervical cancer HeLa as well as to determine the influence on human keratinocyte cell line HaCaT. Relatively high (IC50: 7-17 μM) cytostatic activity and selectivity against HeLa cell line was found for compounds 6, 7, 9-11 and 16. While compounds 23-27 bearing styryl moieties attached to a 1,3,4-oxadiazole ring at position 5, exhibited significant activity against two and/or three cancer cell lines with IC50: 11-29 μM. Further quantitative structure-activity relationships based on molecular descriptors calculated by DRAGON software, were investigated by Orthogonal Projections to Latent Structures (OPLS) technique and Variable Influence on Projection (VIP) analysis. Considering molecular descriptors with the highest influence on projection (highest VIP values) lipophilicity of tested compounds was pointed as main factor affecting activity towards HCT-116 cell line, while structural parameters associated with presence of styryl substituent in position 5 of 1,3,4-oxadiazole ring were identified as essential for activity towards MCF-7 breast cancer. In vitro tests for metabolic stability in the presences of pooled human liver microsomes and NADPH showed that some of the most active compounds 26 and 27 presented favorable metabolic stability with t1/2 in the range of 28.1-36.0 min.

Emerging challenges in the design of selective substrates, inhibitors and activity-based probes for indistinguishable proteases

Proteases are enzymes that hydrolyze the peptide bond of peptide substrates and proteins. Despite significant progress in recent years, one of the greatest challenges in the design and testing of substrates, inhibitors and activity‐based probes for proteolytic enzymes is achieving specificity toward only one enzyme. This specificity is particularly important if the enzyme is present with other enzymes with a similar catalytic mechanism and substrate specificity but completely different functionality. The cross‐reactivity of substrates, inhibitors and activity‐based probes with other enzymes can significantly impair or even prevent investigations of a target protease. In this review, we describe important concepts and the latest challenges, focusing mainly on peptide‐based substrate specificity techniques used to distinguish individual enzymes within major protease families.

Synthesis, molecular docking and α-glucosidase inhibition of 5-aryl-2-(6'-nitrobenzofuran-2'-yl)-1,3,4-oxadiazoles

Twenty derivatives of 5-aryl-2-(6'-nitrobenzofuran-2'-yl)-1,3,4-oxadiazoles (1-20) were synthesized and evaluated for their α-glucosidase inhibitory activities. Compounds containing hydroxyl and halogens (1-6, and 8-18) were found to be five to seventy folds more active with IC50 values in the range of 12.75±0.10-162.05±1.65μM, in comparison with the standard drug, acarbose (IC50=856.45±5.60μM). Current study explores the α-glucosidase inhibition of a hybrid class of compounds of oxadiazole and benzofurans. These findings may invite researchers to work in the area of treatment of hyperglycemia. Docking studies showed that most compounds are interacting with important amino acids Glu 276, Asp 214 and Phe 177 through hydrogen bonds and arene-arene interaction.

Synthesis and biological evaluation of novel fluconazole analogues bearing 1,3,4-oxadiazole moiety as potent antifungal agents

A novel series of fluconazole based mimics incorporating 1,3,4-oxadiazole moiety were designed and synthesized. All the title compounds were characterized by (1)H-NMR, (13)C-NMR, and Q-TOF-MS. Preliminary results revealed that most of analogues exhibited significant antifungal activity against seven pathogenic fungi. Compounds 9g and 9k (MIC80 ≤ 0.125 μg/mL, respectively) were found more potent than the positive controls itraconazole and fluconazole as broad-spectrum antifungal agents. The observed docking results showed that the 1,3,4-oxadiazole moiety enhanced the affinity binding to the cytochrome P450 14α-demethylase (CYP51).

The Influence of Bioisosteres in Drug Design: Tactical Applications to Address Developability Problems

The application of bioisosteres in drug discovery is a well-established design concept that has demonstrated utility as an approach to solving a range of problems that affect candidate optimization, progression, and durability. In this chapter, the application of isosteric substitution is explored in a fashion that focuses on the development of practical solutions to problems that are encountered in typical optimization campaigns. The role of bioisosteres to affect intrinsic potency and selectivity, influence conformation, solve problems associated with drug developability, including P-glycoprotein recognition, modulating basicity, solubility, and lipophilicity, and to address issues associated with metabolism and toxicity is used as the underlying theme to capture a spectrum of creative applications of structural emulation in the design of drug candidates.

Oxadiazoles as privileged motifs for promising anticancer leads: recent advances and future prospects

Taking into account the rising trend of the incidence of cancers of various organs, effective therapies are urgently needed to control human malignancies. The rapid emergence of hundreds of new agents that modulate an ever-growing list of cancer-specific molecular targets offers tremendous hope for cancer patients. However, almost all of the chemotherapy drugs currently on the market cause serious side effects. Based on these facts, the design of new chemical entities as anticancer agents requires the simulation of a suitable bioactive pharmacophore. The pharmacophore not only should have the required potency but must also be safer on normal cell lines than on tumor cells. In this perspective, oxadiazole scaffolds with well-defined anticancer activity profile have fueled intense academic and industrial research in recent years. This paper is intended to highlight the recent advances along with current developments as well as future outlooks for the design of novel and efficacious anticancer agents based on oxadiazole motifs.

Synthesis, biological evaluation and molecular docking studies of some pyrimidine derivatives

Some novel pyrimidine-5-carbonitrile derivatives bearing various substituent have been synthesized. The structures of target compounds were confirmed by elemental analysis and spectral data. Some selected members of the newly synthesized compounds were investigated for their cytotoxic potency against certain human tumor cell lines. Five representative active anticancer compounds 6a, 6c, 6d, 17a and 18a were subjected to docking using MOE program on the 3D structure of two enzymes, namely; thymidylate synthase and dihydrofolate reductase. The antimicrobial activities of the synthesized compounds were tested against Staphylococcus aureus, Pseudomonas aeruginosa, Shigella flexneri and Candida albicans. Compounds 2c, 7a and 9c showed broad spectrum antimicrobial activity.

Synthetic approaches and pharmacological activity of 1,3,4-oxadiazoles: a review of the literature from 2000-2012

This review provides readers with an overview of the main synthetic methodologies for 1,3,4-oxadiazole derivatives, and of their broad spectrum of pharmacological activities as reported over the past twelve years.

Neutrophil elastase inhibitors: recent advances in the development of mechanism-based and nonelectrophilic inhibitors

Due to its implication in pathologies of prevalent diseases such as chronic obstructive pulmonary disease, fibrosis, bronchiectasis and ARDS, the serine protease, human neutrophil elastase, has been in focus for drug-development efforts over the last two decades. In recent years, continued efforts to identify and optimize novel mechanism-based inhibitors have led to a number of new inhibitors being reported. These compounds show promising potency and selectivity profiles, although their use is still limited by their inherent stability. Recently, two novel classes of potent and selective, synthetic, nonelectrophilic human neutrophil elastase inhibitors that display improved stability and overall drug-like properties have been reported. The most advanced compound from these classes, AZD9668, has been reported to show significant effects on relevant biomarkers in bronchiectasis and cystic fibrosis patient populations.

Oxadiazoles in medicinal chemistry

Oxadiazoles are five-membered heteroaromatic rings containing two carbons, two nitrogens, and one oxygen atom, and they exist in different regioisomeric forms. Oxadiazoles are frequently occurring motifs in druglike molecules, and they are often used with the intention of being bioisosteric replacements for ester and amide functionalities. The current study presents a systematic comparison of 1,2,4- and 1,3,4-oxadiazole matched pairs in the AstraZeneca compound collection. In virtually all cases, the 1,3,4-oxadiazole isomer shows an order of magnitude lower lipophilicity (log D), as compared to its isomeric partner. Significant differences are also observed with respect to metabolic stability, hERG inhibition, and aqueous solubility, favoring the 1,3,4-oxadiazole isomers. The difference in profile between the 1,2,4 and 1,3,4 regioisomers can be rationalized by their intrinsically different charge distributions (e.g., dipole moments). To facilitate the use of these heteroaromatic rings, novel synthetic routes for ready access of a broad spectrum of 1,3,4-oxadiazoles, under mild conditions, are described.

Targeting COPD: advances on low-molecular-weight inhibitors of human neutrophil elastase

Chronic obstructive pulmonary disease (COPD) is a major increasing health problem and the World Health Organization (WHO) reports COPD as the fifth leading cause of death worldwide. COPD refers to a condition of inflammation and progressive weakening of the structure of the lung as well as irreversible narrowing of the airways. Current treatment is only palliative and no available drug halts the progression of the disease. Human neutrophil elastase (HNE) is a serine protease, which plays a major role in the COPD inflammatory process. The protease/anti-protease imbalance leads to an excess of extracellular HNE hydrolyzing elastin, the structural protein that confers elasticity to the lung tissue. Although HNE was identified as a therapeutic target for COPD more than 30 years ago, only Sivelestat (ONO-5046), an HNE inhibitor from Ono Pharmaceutical, has been approved for clinical use. Nevertheless, Sivelestat is only approved in Japan and its development in the USA was terminated in 2003. Other inhibitors in pre-clinical or phase I trials were discontinued for various reasons. Hence, there is an urgent need for low-molecular-weight synthetic elastase inhibitors and the present review discusses the recent advances on this field covering acylating agents, transition-state inhibitors, mechanism-based inhibitors, relevant natural products, and major patent disclosures.

Design of novel and potent cPLA2α inhibitors containing an α-methyl-2-ketothiazole as a metabolically stable serine trap

We report the design of novel, potent cPLA(2)α inhibitors that possess an α-methyl-2-ketothiazole that acts as a serine-reactive moiety. We describe the optimization of the series for potency and metabolic stability towards ketone reduction. This was achieved by attenuating the reactivity of the ketone using a combination of electronic and steric effects.

Effects of the Neutrophil Elastase Inhibitor (ONO-6818) on Acetic Acid Induced Colitis in Syrian Hamsters

Neutrophil elastase (NE) released from neutrophils during inflammation is related to tissue disturbance and organ failure. We investigated the effects of an orally active NE inhibitor, ONO-6818, on acetic acid induced colitis in Syrian hamsters. The ulcer area, hemoglobin level in the colonic lumen, NE activity, and tissue myeloperoxidase (MPO) activity in the colitis control animals were significantly increased compared to the normal control ones. Either oral or subcutaneous treatment with ONO-6818 had significant inhibitory effects on the ulcer area, hemoglobin level and NE activity in the colonic lumen, but ONO-6818 did not have a significant inhibitory effect on tissue MPO activity. We conclude that NE is closely related to the development of inflammation in acetic acid-induced colitis in Syrian hamsters and that the condition is improved by the inhibition of NE.

Synthesis of various 3-substituted 1,2,4-oxadiazole-containing chiral beta 3- and alpha-amino acids from Fmoc-protected aspartic acid

Various 3-substituted chiral 1,2,4-oxadiazole-containing Fmoc-beta(3)- and -alpha-amino acids were synthesized from Fmoc-(l or d)-Asp(OtBu)-OH and Fmoc-l-Asp-OtBu, respectively, in three steps (i.e., condensation of an aspartyl derivative with differentially substituted amidoximes, formation of the 1,2,4-oxadiazole, and cleavage of the tert-butyl ester). These compounds represent new series of nonnatural amino acids, which could be used in combinatorial synthesis. A simple protocol has been developed to generate the 1,2,4-oxadiazole ring. Indeed, common methods resulted in cleavage of the Fmoc group or required long reaction times. We found that sodium acetate in refluxing ethanol/water (86 degrees C) was a convenient and efficient catalyst to promote conversion of Fmoc-amino acyl amidoximes to 1,2,4-oxadiazoles, and this procedure proved to be compatible with Fmoc protection. It is shown that these compounds can be prepared without significant loss of enantiomerical purity. Furthermore, the alkaline conditions used to cleave the Fmoc protecting group from these compounds did not induce epimerization of their chiral center.

Neutrophil elastase inhibitors as treatment for COPD

Chronic obstructive pulmonary disease, characterised by a slowly progressive, irreversible airways limitation, is a major worldwide cause of chronic morbidity and mortality. The imbalance between human neutrophil elastase and endogenous antiproteases may cause excess human neutrophil elastase in pulmonary tissues, which may be considered a major pathogenic factor in chronic obstructive pulmonary disease. Great effort has been devoted to finding a method to restore the balance, resulting in the discovery of potent two-typed small-molecular-weight human neutrophil elastase inhibitors. In the application of chronic obstructive pulmonary disease therapy, the human neutrophil elastase inhibitors mainly focused upon include ONO-5046, MR-889, L-694,458, CE-1037, GW-311616 and TEI-8362 as the acyl-enzyme inhibitors; and ONO-6818, AE-3763, FK-706, ICI-200,880, ZD-0892 and ZD-8321 as the transition-state inhibitors. In this review, various problems that remain to be solved in the clinical use of human neutrophil elastase inhibitors are discussed.