Oxadiazoles in medicinal chemistry

Improvement in aqueous solubility achieved via small molecular changes

Overcoming poor solubility is a significant issue in drug discovery. The most common solution is to replace carbon atoms with polar heteroatoms such as N and O or by attaching a solubilizing appendage. This approach can lead to other issues such as poor activity and PK or the increased risk for toxicity. However, there are more subtle structural changes which can be employed that lead to an increase in solubility. These include, excising hydrophobic groups which do not efficiently contribute to binding, modifying stereo- and regiochemistry, increasing or decreasing the degree of unsaturation or adding small hydrophobic groups such as fluorine or methyl.

Research progress in modern structure of platinum complexes

Since the antitumor activity of cisplatin was discovered in 1967 by Rosenberg, platinum-based anticancer drugs have played an important role in chemotherapy in clinic. Nevertheless, platinum anticancer drugs also have caused severe side effects and cross drug resistance which limited their applications. Therefore, a significant amount of efforts have been devoted to developing new platinum-based anticancer agents with equal or higher antitumor activity but lower toxicity. Until now, a large number of platinum-based complexes have been prepared and extensively investigated in vitro and in vivo. Among them, some platinum-based complexes revealing excellent anticancer activity showed the potential to be developed as novel type of anticancer agents. In this account, we present such platinum-based anticancer complexes which owning various types of ligands, such as, amine carrier ligands, leaving groups, reactive molecule, steric hindrance groups, non-covalently binding platinum (II) complexes, Platinum(IV) complexes and polynuclear platinum complexes. Overall, platinum-based anticancer complexes reported recently years upon modern structure are emphasized.

Ribosome Rescue Inhibitors Kill Actively Growing and Nonreplicating Persister Mycobacterium tuberculosis Cells

The emergence of Mycobacterium tuberculosis (MTB) strains that are resistant to most or all available antibiotics has created a severe problem for treating tuberculosis and has spurred a quest for new antibiotic targets. Here, we demonstrate that trans-translation is essential for growth of MTB and is a viable target for development of antituberculosis drugs. We also show that an inhibitor of trans-translation, KKL-35, is bactericidal against MTB under both aerobic and anoxic conditions. Biochemical experiments show that this compound targets helix 89 of the 23S rRNA. In silico molecular docking predicts a binding pocket for KKL-35 adjacent to the peptidyl-transfer center in a region not targeted by conventional antibiotics. Computational solvent mapping suggests that this pocket is a druggable hot spot for small molecule binding. Collectively, our findings reveal a new target for antituberculosis drug development and provide critical insight on the mechanism of antibacterial action for KKL-35 and related 1,3,4-oxadiazole benzamides.

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.

Bioisosteric replacement of central 1,2,4-oxadiazole ring of high affinity CB2 ligands by regioisomeric 1,3,4-oxadiazole ring

It has been reported that bioisosteric replacement of an 1,2,4-oxadiazole ring by an 1,3,4-oxadiazole ring leads to higher polarity, reduced metabolic degradation by human liver microsomes and reduced interaction with hERG channels. In a seven to eight step synthesis 1,3,4-oxadiazles 9a-c were synthesized as bioisosteric analogs of high-affinity but rather lipophilic CB₂ ligands 1a-c containing an 1,2,4-oxadiazole ring. The 1,3,4-oxadiazole derivatives 9a and 9b show 10- and 50-fold reduced CB₂ affinity compared to the 1,2,4-oxadiazole derivatives 1a and 1b, respectively. However, the 1,3,4-oxadiazole 9a has high CB₂ affinity (K_i = 25 nM) and high selectivity over the CB₁ receptor.

Oxetane Substrates of Human Microsomal Epoxide Hydrolase

Oxetanyl building blocks are increasingly used in drug discovery because of the improved drug-like properties they confer on drug candidates, yet little is currently known about their biotransformation. A series of oxetane-containing analogs was studied and we provide the first direct evidence of oxetane hydrolysis by human recombinant microsomal epoxide hydrolase (mEH). Incubations with human liver fractions and hepatocytes were performed with and without inhibitors of cytochrome P450 (P450), mEH and soluble epoxide hydrolase (sEH). Reaction dependence on NADPH was investigated in subcellular fractions. A full kinetic characterization of oxetane hydrolysis is presented, in both human liver microsomes and human recombinant mEH. In human liver fractions and hepatocytes, hydrolysis by mEH was the only oxetane ring-opening metabolic route, with no contribution from sEH or from cytochrome P450-catalyzed oxidation. Minimally altering the structural elements in the immediate vicinity of the oxetane can greatly modulate the efficiency of hydrolytic ring cleavage. In particular, higher pK_a in the vicinity of the oxetane and an increased distance between the oxetane ring and the benzylic nitrogen improve reaction rate, which is further enhanced by the presence of methyl groups near or on the oxetane. This work defines oxetanes as the first nonepoxide class of substrates for human mEH, which was previously known to catalyze the hydrolytic ring opening of electrophilic and potentially toxic epoxide-containing drugs, drug metabolites, and exogenous organochemicals. These findings will be of value for the development of biologically active oxetanes and may be exploited for the biocatalytic generation of enantiomerically pure oxetanes and diols.

Metal-free hydroarylation of the side chain carbon-carbon double bond of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles in triflic acid

The metal-free reaction of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles with arenes in neat triflic acid (TfOH, CF3SO3H), both under thermal and microwave conditions, leads to 5-(2,2-diarylethyl)-3-aryl-1,2,4-oxadiazoles. The products are formed through the regioselective hydroarylation of the side chain carbon-carbon double bond of the starting oxadiazoles in yields up to 97%. According to NMR data and DFT calculations, N4,C-diprotonated forms of oxadiazoles are the electrophilic intermediates in this reaction.

A convergent synthesis of 1,3,4-oxadiazoles from acyl hydrazides under semiaqueous conditions

The 1,3,4-oxadiazole is an aromatic heterocycle valued for its low-lipophilicity in drug development. Substituents at the 2- and/or 5-positions can modulate the heterocycle's electronic and hydrogen bond-accepting capability, while exploiting its use as a carbonyl bioisostere. A new approach to 1,3,4-oxadiazoles is described wherein α-bromo nitroalkanes are coupled to acyl hydrazides to deliver the 2,5-disubstituted oxadiazole directly, avoiding a 1,2-diacyl hydrazide intermediate. Access to new building blocks of oxadiazole-substituted secondary amines is improved by leveraging chiral α-bromo nitroalkane or amino acid hydrazide substrates. The non-dehydrative conditions for oxadiazole synthesis are particularly notable, in contrast to alternatives reliant on highly oxophilic reagents to effect cyclization of unsymmetrical 1,2-diacyl hydrazides. The mild conditions are punctuated by the straightforward removal of co-products by a standard aqueous wash.

Discovery of a series of 1,3,4-oxadiazole-2(3H)-thione derivatives containing piperazine skeleton as potential FAK inhibitors

Focal adhesion kinase (FAK) is an important drug target that plays a fundamental role in mediating signal transduction system. We report herein the discovery of a novel class of 1,3,4-oxadiazole-2(3H)-thione derivatives containing piperazine skeleton with improved potency toward FAK. All of the 17 new synthesized compounds were assayed for the anticancer activities against four cancer cells, HepG2, Hela, SW116 and BGC823. Because of the combination of 1,4-benzodioxan, 1,3,4-oxadiazole and piperazine ring, most of them exhibited remarkable antitumor activities. Notably, compound 5m showed the most potent biological activities (IC₅₀=5.78μM for HepG2, and IC₅₀=47.15μM for SW1116), and its anti-FAK inhibitory activity (IC₅₀=0.78μM) was also the best. Computational docking studies also showed that compound 5m has interaction with FAK key residues in the active site.

Enabling the facile conversion of acyl hydrazides into N-acyl carbamates via metal-free ionic-based rupture of the N–N linkage

A facile, one-pot procedure for the conversion of readily accessible acyl hydrazides into N-acyl carbamates.

Facile one-pot synthesis of 2-amino-1,3,4-oxadiazole tethered peptidomimetics by molecular-iodine-mediated cyclodeselenization

Mild and highly efficient I₂ and Et₃N aided cyclodeselenization of in situ generated selenosemicarbazide is described to obtain 2-amino-1,3,4-oxadiazole peptidomimetics.

Matched Molecular Pair Analysis in Short: Algorithms, Applications and Limitations

Molecular matched pair (MMP) analysis has been used for more than 40 years within molecular design and is still an important tool to analyse potency data and other compound properties. The methods used to find matched pairs range from manual inspection, through supervised methods to unsupervised methods, which are able to find previously unknown molecular pairs. Recent publications demonstrate the value of automatic MMP analysis of publicly available bioactivity databases. The MMP concept has its limitations, but because of its easy to use and intuitive nature, it will remain one of the most important tools in the toolbox of many drug designers.

Open Source Drug Discovery: Highly Potent Antimalarial Compounds Derived from the Tres Cantos Arylpyrroles

The development of new antimalarial compounds remains a pivotal part of the strategy for malaria elimination. Recent large-scale phenotypic screens have provided a wealth of potential starting points for hit-to-lead campaigns. One such public set is explored, employing an open source research mechanism in which all data and ideas were shared in real time, anyone was able to participate, and patents were not sought. One chemical subseries was found to exhibit oral activity but contained a labile ester that could not be replaced without loss of activity, and the original hit exhibited remarkable sensitivity to minor structural change. A second subseries displayed high potency, including activity within gametocyte and liver stage assays, but at the cost of low solubility. As an open source research project, unexplored avenues are clearly identified and may be explored further by the community; new findings may be cumulatively added to the present work.

Design, synthesis, evaluation and molecular modelling studies of some novel 5,6-diphenyl-1,2,4-triazin-3(2H)-ones bearing five-member heterocyclic moieties as potential COX-2 inhibitors: A hybrid pharmacophore approach

A series of novel hybrids comprising of 1,3,4-oxadiazole/thiadiazole and 1,2,4-triazole tethered to 5,6-diphenyl-1,2,4-triazin-3(2H)-one were designed, synthesised and evaluated as COX-2 inhibitors for the treatment of inflammation. The synthesised hybrids were characterised using FT-IR, 1H NMR, 13C NMR, elemental (C,H,N) analyses and assessed for their anti-inflammatory potential by in vitro albumin denaturation assay. Compounds exhibiting activity comparable to indomethacin and celecoxib were further evaluated for in vivo anti-inflammatory activity. Oral administration of promising compounds 3c-3e and 4c-4e did not evoke significant gastric, hepatic and renal toxicity in rats. These potential compounds exhibited reduced malondialdehyde (MDA) content on the gastric mucosa suggesting their protective effects by inhibition of lipid peroxidation. Based on the outcome of in vitro COX assay, compounds 3c-3e and 4c-4e (IC₅₀ 0.60-1.11μM) elicited an interesting profile as competitive selective COX-2 inhibitors. Further, selected compounds 3e and 4c were found devoid of cardiotoxicity post evaluation on myocardial infarcted rats. The in silico binding mode of the potential compounds into the COX-2 active site through docking and molecular dynamics exemplified their consensual interaction and subsequent COX-2 inhibition with significant implications for structure-based drug design.

Expanding Synthesizable Space of Disubstituted 1,2,4-Oxadiazoles

One-pot synthesis of 3,5-disubstituted 1,2,4-oxadiazoles from carboxylic acids and nitriles was optimized to parallel chemistry. The method was validated on a 141 member library; the desired products were recovered with a high success rate and in moderate yields. Practical application of the approach was demonstrated in the synthesis of bioactive compound pifexole and agonists of free fatty acid receptor 1. A library of 4 948 100 synthesizable drug-like 3,5-disubstituted 1,2,4-oxadiazoles was enumerated based on the method and available validated reagents.

Evolution of physicochemical properties of melanin concentrating hormone receptor 1 (MCHr1) antagonists

One pharmacological principle for the treatment of obesity is blockade of the melanin concentrating hormone receptor 1 (MCHr1), which in rodents has been shown to be strongly associated with food intake and energy expenditure. However, discovery of safe and efficacious MCHr1 antagonists has proved to be complex. So far, six compounds have been progressed into clinical trials, but clinical validation of the concept is still lacking. An account of discovery of the three most recent clinical candidates targeting the MCHr1 receptor is given, with an emphasis on their physicochemical properties.

Palladium-catalyzed one-pot synthesis of diazoles via tert-butyl isocyanide insertion

An efficient one-pot palladium-catalyzed reaction for the synthesis of diazoles from readily available hydrazides and aryl halide via isocyanide insertion/cyclization sequence has been developed. This methodology efficiently constructs diazoles in good to excellent yields with the advantages of wide functional group tolerance and operational simplicity.

Development of Simplified Heterocyclic Acetogenin Analogues as Potent and Selective Trypanosoma brucei Inhibitors

Neglected tropical diseases caused by parasitic infections are an ongoing and increasing concern. They are a burden to human and animal health, having the most devastating effect on the world's poorest countries. Building upon our previously reported triazole analogues, in this study we describe the synthesis and biological testing of other novel heterocyclic acetogenin-inspired derivatives, namely 3,5-isoxazoles, furoxans, and furazans. Several of these compounds maintain low-micromolar levels of inhibition against Trypanosoma brucei, whilst having no observable inhibitory effect on mammalian cells, leading to the possibility of novel lead compounds for selective treatment.

Design and evaluation of novel oxadiazole derivatives as potential prostate cancer agents

Various 1,3,4-oxadiazole derivatives have been synthesized and their antiproliferative properties have been studied. The in vitro screening was performed against androgen dependent (LNCaP) and androgen independent (PC-3) prostate cancer cell lines. Most of the compounds showed promising activity. Among them, compounds 2d (IC50=0.22 and 1.3μM) and 2a (IC50=8.34 and 2,5μM) have shown significant activities on PC-3 and LNCaP cell lines respectively. To investigate the mechanism of cell death we performed cell apoptosis staining and cell cycle arrest assay on more sensitive PC-3 cell lines on 2d. The results demonstrated that 2d induced apoptosis and shifted the cells to the sub G0/G1 and S phase. Our study evidently identified the potency of compound 2d as potential anti-prostate cancer agent.

Structural Requirements and Docking Analysis of Amidine-Based Sphingosine Kinase 1 Inhibitors Containing Oxadiazoles

Sphingosine 1-phosphate (S1P) is a potent growth-signaling lipid that has been implicated in cancer progression, inflammation, sickle cell disease, and fibrosis. Two sphingosine kinases (SphK1 and 2) are the source of S1P; thus, inhibitors of the SphKs have potential as targeted cancer therapies and will help to clarify the roles of S1P and the SphKs in other hyperproliferative diseases. Recently, we reported a series of amidine-based inhibitors with high selectivity for SphK1 and potency in the nanomolar range. However, these inhibitors display a short half-life. With the goal of increasing metabolic stability and maintaining efficacy, we designed an analogous series of molecules containing oxadiazole moieties. Generation of a library of molecules resulted in the identification of the most selective inhibitor of SphK1 reported to date (705-fold selectivity over SphK2), and we found that potency and selectivity vary significantly depending on the particular oxadiazole isomer employed. The best inhibitors were subjected to in silico molecular dynamics docking analysis, which revealed key insights into the binding of amidine-based inhibitors by SphK1. Herein, the design, synthesis, biological evaluation, and docking analysis of these molecules are described.

Structural requirements of acylated Gly-l-Ala-d-Glu analogs for activation of the innate immune receptor NOD2

The fragment of bacterial peptidoglycan muramyl dipeptide (MDP) has long been known for its adjuvant activity, however the underlying mechanism of this action has only recently been elucidated. It is ascribed to its agonist action on the nucleotide-binding oligomerization domain-containing protein 2 (NOD2). In spite of the pressing need for novel adjuvants for human use, this discovery is hampered, by not knowing the structural requirements underlying the immunostimulatory activity. We have investigated how minor modifications of hit compound acyl Gly-L-Ala-D-Glu derivative I modulate the molecular recognition by NOD2. A series of novel desmuramyldipeptides has been designed and synthesized leading to the identification of compound 16, in which the sugar moiety is replaced by a 6-phenylindole moiety, that exhibits the strongest NOD2 activation to date sans the carbohydrate moiety. The results have enabled a deeper understanding of the structural requirements of desmuramylpeptides for NOD2 activation.