Heme oxygenase inhibition by 2-oxy-substituted 1-azolyl-4-phenylbutanes: effect of variation of the azole moiety. X-ray crystal structure of human heme oxygenase-1 in complex with 4-phenyl-1-(1H-1,2,4-triazol-1-yl)-2-butanone

Stereoselective Asymmetric Syntheses of Molecules with a 4,5-Dihydro-1H-[1,2,4]-Triazoline Core Possessing an Acetylated Carbohydrate Appendage: Crystal Structure, Spectroscopy, and Pharmacology

A new series of chiral 4,5-dihydro-1H-[1,2,4]-triazoline molecules, featuring a β-ᴅ-glucopyranoside appendage, were synthesized via a 1,3-dipolar cycloaddition reaction between various hydrazonyl chlorides and carbohydrate Schiff bases. The isolated enantiopure triazolines (8a-j) were identified through high-resolution mass spectrometry (HRMS) and vibrational spectroscopy. Subsequently, their solution structures were elucidated through NMR spectroscopic techniques. Single-crystal X-ray analysis of derivative 8b provided definitive evidence for the 3-D structure of this compound and revealed important intermolecular forces in the crystal lattice. Moreover, it confirmed the (S)-configuration at the newly generated stereo-center. Selected target compounds were investigated for anti-tumor activity in 60 cancer cell lines, with derivative 8c showing the highest potency, particularly against leukemia. Additionally, substituent-dependent anti-fungal and anti-bacterial behavior was observed.

Novel quinolinepiperazinyl-aryltetrazoles targeting the blood stage of Plasmodium falciparum

The emergence of drug resistance against the frontline antimalarials is a major challenge in the treatment of malaria. In view of emerging reports on drug-resistant strains of Plasmodium against artemisinin combination therapy, a dire need is felt for the discovery of novel compounds acting against novel targets in the parasite. In this study, we identified a novel series of quinolinepiperazinyl-aryltetrazoles (QPTs) targeting the blood stage of Plasmodium. In vitro anti-plasmodial activity screening revealed that most of the compounds showed IC50 < 10 μM against chloroquine-resistant PfINDO strain, with the most promising lead compounds 66 and 75 showing IC50 values of 2.25 and 1.79 μM, respectively. Further, compounds 64-66, 68, 75-77 and 84 were found to be selective (selectivity index >50) in their action against Pf over a mammalian cell line, with compounds 66 and 75 offering the highest selectivity indexes of 178 and 223, respectively. Explorations into the action of lead compounds 66 and 75 revealed their selective cidal activity towards trophozoites and schizonts. In a ring-stage survival assay, 75 showed cidal activity against the early rings of artemisinin-resistant PfCam3.1R539T. Further, 66 and 75 in combination with artemisinin and pyrimethamine showed additive to weak synergistic interactions. Of these two in vitro lead molecules, only 66 restricted rise in the percentage of parasitemia to about 10% in P. berghei-infected mice with a median survival time of 28 days as compared to the untreated control, which showed the percentage of parasitemia >30%, and a median survival of 20 days. Promising antimalarial activity, high selectivity, and additive interaction with artemisinin and pyrimethamine indicate the potential of these compounds to be further optimized chemically as future drug candidates against malaria.

Preliminary studies of an imidazole-based alcohol derivative for imaging of Heme oxygenase 1

Heme oxygenase-1 (HO-1) has been involved in the pathogenesis of Alzheimer's disease (AD), thus constituting a promising target for AD drug development. Positron emission tomography (PET) is a fully translational imaging technology, which will help us understand the role of HO-1 in the progression of AD, facilitating to validate promising HO-1 inhibitors in clinical trials. To our knowledge, there is no report on PET imaging probe targeting HO-1 in animals and humans. We report herein the synthesis and characterization of a ¹¹C-labeled imidazole-based alcohol derivative ([¹¹C]QC-33) for imaging of HO-1 in the brain. The desired product [¹¹C]QC-33 was afforded with a radiochemical yield of 16 ± 9% (n = 3, decay corrected). The radiochemical purity was greater than 99%, and the molar radioactivity was greater than 185 GBq/μmol. In vitro autoradiography studies indicated specific binding of [¹¹C]QC-33 in the HO-1 rich regions, showing 75%, 75%, and 69% radioactivity binding reductions in cerebellum, brain stem, and midbrain, respectively. PET/CT scanning in C57BL/6 mice showed low brain uptake and poor blood-brain barrier (BBB) penetration of [¹¹C]QC-33. These results suggested that [¹¹C]QC-33 can serve as a lead compound to advance the development of next generation PET tracer with the potential to monitor HO-1 in AD progression.

Discovery of Novel Acetamide-Based Heme Oxygenase-1 Inhibitors with Potent In Vitro Antiproliferative Activity

Heme oxygenase-1 (HO-1) promotes heme catabolism exercising cytoprotective roles in normal and cancer cells. Herein, we report the design, synthesis, molecular modeling, and biological evaluation of novel HO-1 inhibitors. Specifically, an amide linker in the central spacer and an imidazole were fixed, and the hydrophobic moiety required by the pharmacophore was largely modified. In many tumors, overexpression of HO-1 correlates with poor prognosis and chemoresistance, suggesting the inhibition of HO-1 as a possible antitumor strategy. Accordingly, compounds 7i and 7l-p emerged for their potency against HO-1 and were investigated for their anticancer activity against prostate (DU145), lung (A549), and glioblastoma (U87MG, A172) cancer cells. The selected compounds showed the best activity toward U87MG cells. Compound 7l was further investigated for its in-cell enzymatic HO-1 activity, expression levels, and effects on cell invasion and vascular endothelial growth factor (VEGF) extracellular release. The obtained data suggest that 7l can reduce cell invasivity acting through modulation of HO-1 expression.

Fourfold Filtered Statistical/Computational Approach for the Identification of Imidazole Compounds as HO-1 Inhibitors from Natural Products

Over-regulation of Heme oxygenase 1 (HO-1) has been recently identified in many types of human cancer, and in these cases, poor clinical outcomes are normally reported. Indeed, the inhibition of HO-1 is being considered as an anticancer approach. Imidazole scaffold is normally present in most of the classical HO-1 inhibitors and seems indispensable to the inhibitory activity due to its strong interaction with the Fe(II) of the heme group. In this paper, we searched for new potentially HO-1 inhibitors among three different databases: Marine Natural Products (MNP), ZINC Natural Products (ZNP) and Super Natural II (SN2). 484,527 compounds were retrieved from the databases and filtered through four statistical/computational filters (2D descriptors, 2D-QSAR pharmacophoric model, 3D-QSAR pharmacophoric model, and docking). Different imidazole-based compounds were suggested by our methodology to be potentially active in inhibiting the HO-1, and the results have been rationalized by the bioactivity of the filtered molecules reported in the literature.

Progress in the development of selective heme oxygenase-1 inhibitors and their potential therapeutic application

Heme oxygenases (HOs) are a family of enzymes involved in the selective catabolism of free circulating heme. While HO-2 is constitutively expressed, HO-1 is strongly overexpressed under stressful stimuli (e.g., oxidative stress). Under these conditions, HO-1 exerts its strong cytoprotective activities and plays a crucial role in stimulating cell survival by removing the pro-oxidant heme and by producing carbon monoxide and biliverdin (promptly reduced to bilirubin). Unfortunately, the broad spectrum of HO-1 cytoprotective effects has been well experimentally documented both in normal and tumor cells, where the enzyme can be overexpressed, making it an exciting target in the management of some type of tumors. Development of non-competitive HO-1 inhibitors dates back in 2002 with the discovery of Azalanstat. Since then, many efforts have been devoted to the identification of selective HO-1 and HO-2 inhibitors and to unravel the molecular determinants responsible for selectivity. Molecular modeling studies supported the identification of chemical features involved in the recognition and inhibition of these enzymes. Herein, medicinal chemistry aspects and in silico studies related to the development of HO inhibitors will be discussed. The purpose of this review is to highlight recent advances in the development of new selective HO-1 and HO-2 inhibitors and covers the last six years (2013-2018).

Novel Structural Insight into Inhibitors of Heme Oxygenase-1 (HO-1) by New Imidazole-Based Compounds: Biochemical and In Vitro Anticancer Activity Evaluation

In this paper, the design, synthesis, and molecular modeling of a new azole-based HO-1 inhibitors was reported, using compound 1 as a lead compound, in which an imidazole moiety is linked to a hydrophobic group by means of an ethanolic spacer. The tested compounds showed a good inhibitor activity and possessed IC₅₀ values in the micromolar range. These results were obtained by targeting the hydrophobic western region. Molecular modeling studies confirmed a consolidated binding mode in which the nitrogen of the imidazolyl moiety coordinated the heme ferrous iron, meanwhile the hydrophobic groups were located in the western region of HO-1 binding pocket. Moreover, the new compounds were screened for in silico ADME-Tox properties to predict drug-like behavior with convincing results. Finally, the in vitro antitumor activity profile of compound 1 was investigated in different cancer cell lines and nanomicellar formulation was synthesized with the aim of improving compound's 1 water solubility. Finally, compound 1 was tested in melanoma cells in combination with doxorubicin showing interesting synergic activity.

Potholing of the hydrophobic heme oxygenase-1 western region for the search of potent and selective imidazole-based inhibitors

Here we report the design, synthesis, and molecular modeling of new potent and selective imidazole-based HO-1 inhibitors in which the imidazole nucleus and the hydrophobic groups are linked by a phenylethanolic spacer. Most of the tested compounds showed a good inhibitor activity with IC₅₀ values in the low micromolar range, with two of them (1b and 1j) exhibiting also high selectivity toward HO-2. These results were obtained by the idea of potholing the entire volume of the principal hydrophobic western region with an appropriate ligand volume. Molecular modeling studies showed that these molecules bind to the HO-1 in the consolidated fashion where the imidazolyl moiety coordinates the heme iron while the aromatic groups are stabilized by hydrophobic interaction in the western region of the binding pocket. Finally, the synthesized compounds were analyzed for in silico ADME-Tox properties to establish oral drug-like behavior and showed satisfactory results.

A Rh(iii)-catalyzed redox-neutral C–H alkylation reaction with allylic alcohols by using a traceless oxidizing directing group

A Rh(iii)-catalyzed C–H alkylation of N-phenoxyacetamides with allylic alcohols has been developed to provide valuable β-aryl ketones under mild and redox-neutral conditions by using a traceless oxidizing directing group.

Differential inhibition of rat and mouse microsome heme oxygenase by derivatives of imidazole and benzimidazole

Metalloporphyrin heme oxygenase (HO) inhibitors have made an important contribution to elucidating the role of HO in physiological processes. Nevertheless, their off-target effects have drawn substantial criticism, which prompted us to develop non-porphyrin, azole-based inhibitors of HO. These second-generation HO inhibitors were evaluated using spleen and brain microsomes from rats as native sources of HO-1 and HO-2, respectively. Recently, the use of azole-based inhibitors of HO has been extended to other mammalian species and, as a consequence, it will be important to characterize the inhibitors in these species. The goal of this study was to compare the inhibitory profile of imidazole- and benzimidazole-based inhibitors of HO in a breast-cancer-implanted mouse to that of an untreated rat. For spleen and brain microsomes from both species, HO protein expression was determined by Western blotting and concentration-response curves for imidazole- and benzimidazole-derivative inhibition of HO activity were determined using a headspace gas-chromatographic assay. It was found that the effects on HO activity by imidazole and benzimidazole derivatives were different between the 2 species and were not explained by differences in HO expression. Thus, the HO inhibitory profile should be determined for azole derivatives before they are used in mammalian species other than rats.

How does binding of imidazole-based inhibitors to heme oxygenase-1 influence their conformation? Insights combining crystal structures and molecular modelling

Heme oxygenase-1 (HO-1) inhibition is associated with antitumor activity. Imidazole-based analogues show effective and selective inhibitory potency of HO-1. In this work, five single-crystal structures of four imidazole-based compounds are presented, with an in-depth structural analysis. In order to study the influence of the conformation of the ligands on binding to protein, conformational data from crystallography are compared with quantum mechanics analysis and molecular docking studies. Molecular docking of imidazole-based analogues in the active site of HO-1 is in good agreement with the experimental structures. Inhibitors interact with the heme cofactor and a hydrophobic pocket (Met34, Phe37, Val50, Leu147 and Phe214) in the HO-1 binding site. An alternate binding mode can be hypothesized for some inhibitors in the series.

Tetrazole regioisomers in the development of nitro group-containing antitubercular agents

Tetrazole derivatives containing nitro substituents have been identified as promising antitubercular agents.

Synthesis and bioevaluation of novel 4-aminoquinoline-tetrazole derivatives as potent antimalarial agents

A series of novel tetrazole derivatives of 4-aminoquinoline were synthesized and screened for their antimalarial activities against both chloroquine-senstive (3D7) and chloroquine-resistant (K1) strains of Plasmodium falciparum as well as for cytotoxicity against VERO cell lines. Most of the synthesized compounds exhibited potent antimalarial activity as compared to chloroquine against K1-strain. Compounds with significant in vitro antimalarial activity were then evaluated for their in vivo efficacy in Swiss mice against Plasmodium yoelii following both intraperitoneal (ip) and oral administration, wherein compounds 20 and 23 each showed in vivo suppression of 99.99% parasitaemia on day 4.

Structural insights into human heme oxygenase-1 inhibition by potent and selective azole-based compounds

The development of heme oxygenase (HO) inhibitors, especially those that are isozyme-selective, promises powerful pharmacological tools to elucidate the regulatory characteristics of the HO system. It is already known that HO has cytoprotective properties and may play a role in several disease states, making it an enticing therapeutic target. Traditionally, the metalloporphyrins have been used as competitive HO inhibitors owing to their structural similarity with the substrate, heme. However, given heme's important role in several other proteins (e.g. cytochromes P450, nitric oxide synthase), non-selectivity is an unfortunate side-effect. Reports that azalanstat and other non-porphyrin molecules inhibited HO led to a multi-faceted effort to develop novel compounds as potent, selective inhibitors of HO. This resulted in the creation of non-competitive inhibitors with selectivity for HO, including a subset with isozyme selectivity for HO-1. Using X-ray crystallography, the structures of several complexes of HO-1 with novel inhibitors have been elucidated, which provided insightful information regarding the salient features required for inhibitor binding. This included the structural basis for non-competitive inhibition, flexibility and adaptability of the inhibitor binding pocket, and multiple, potential interaction subsites, all of which can be exploited in future drug-design strategies.

Heme oxygenase inhibition by α-(1H-imidazol-1-yl)-ω-phenylalkanes: effect of introduction of heteroatoms in the alkyl linker

Several α-(1H-imidazol-1-yl)-ω-phenylalkanes were synthesized and evaluated as novel inhibitors of heme oxygenase (HO). These compounds were found to be potent and selective for the stress-induced isozyme HO-1, showing mostly weak activity toward the constitutive isozyme HO-2. The introduction of an oxygen atom in the alkyl linker produced analogues with decreased potency toward HO-1, whereas the presence of a sulfur atom in the linker gave rise to analogues with greater potency toward HO-1 than the carbon-containing analogues. The most potent compounds studied contained a five-atom linker between the imidazolyl and phenyl moieties, whereas the most HO-1-selective compounds contained a four-atom linker between these groups. The compounds with a five-atom linker containing a heteroatom (O or S) were found to be the most potent inhibitors of HO-2; 1-(N-benzylamino)-3-(1H-imidazol-1-yl)propane dihydrochloride, with a nitrogen atom in the linker, was found to be inactive.

A novel, "double-clamp" binding mode for human heme oxygenase-1 inhibition

The development of heme oxygenase (HO) inhibitors is critical in dissecting and understanding the HO system and for potential therapeutic applications. We have established a program to design and optimize HO inhibitors using structure-activity relationships in conjunction with X-ray crystallographic analyses. One of our previous complex crystal structures revealed a putative secondary hydrophobic binding pocket which could be exploited for a new design strategy by introducing a functional group that would fit into this potential site. To test this hypothesis and gain further insights into the structural basis of inhibitor binding, we have synthesized and characterized 1-(1H-imidazol-1-yl)-4,4-diphenyl-2-butanone (QC-308). Using a carbon monoxide (CO) formation assay on rat spleen microsomes, the compound was found to be ∼15 times more potent (IC₅₀ = 0.27±0.07 µM) than its monophenyl analogue, which is already a potent compound in its own right (QC-65; IC₅₀ = 4.0±1.8 µM). The crystal structure of hHO-1 with QC-308 revealed that the second phenyl group in the western region of the compound is indeed accommodated by a definitive secondary proximal hydrophobic pocket. Thus, the two phenyl moieties are each stabilized by distinct hydrophobic pockets. This “double-clamp” binding offers additional inhibitor stabilization and provides a new route for improvement of human heme oxygenase inhibitors.

Heme oxygenase inhibition by 1-aryl-2-(1h-imidazol-1-yl/1h-1,2,4-triazol-1-yl)ethanones and their derivatives

Previous studies by our research group have been concerned with the design of selective inhibitors of heme oxygenases (HO-1 and HO-2). The majority of these were based on a four-carbon linkage of an azole, usually an imidazole, and an aromatic moiety. In the present study, we designed and synthesized a series of inhibition candidates containing a shorter linkage between these groups, specifically, a series of 1-aryl-2-(1H-imidazol-1-yl/1H-1,2,4-triazol-1-yl)ethanones and their derivatives. As regards HO-1 inhibition, the aromatic moieties yielding best results were found to be halogen-substituted residues such as 3-bromophenyl, 4-bromophenyl, and 3,4-dichlorophenyl, or hydrocarbon residues such as 2-naphthyl, 4-biphenyl, 4-benzylphenyl, and 4-(2-phenethyl)phenyl. Among the imidazole-ketones, five (36-39, and 44) were found to be very potent (IC(50)<5 muM) toward both isozymes. Relative to the imidazole-ketones, the series of corresponding triazole-ketones showed four compounds (54, 55, 61, and 62) having a selectivity index >50 in favor of HO-1. In the case of the azole-dioxolanes, two of them (80 and 85), each possessing a 2-naphthyl moiety, were found to be particularly potent and selective HO-1 inhibitors. Three non-carbonyl analogues (87, 89, and 91) of 1-(4-chlorophenyl)-2-(1H-imidazol-1-yl)ethanone were found to be good inhibitors of HO-1. For the first time in our studies, two azole-based inhibitors (37 and 39) were found to exhibit a modest selectivity index in favor of HO-2. The present study has revealed additional candidates based on inhibition of heme oxygenases for potentially useful pharmacological and therapeutic applications.

Recombinant truncated and microsomal heme oxygenase-1 and -2: differential sensitivity to inhibitors

Recombinant truncated forms of heme oxygenase-1 and -2 (HO-1 and HO-2) were compared with their crude microsomal counterparts from brain and spleen tissue of adult male rats with respect to their inhibition by azole-based, nonporphyrin HO inhibitors. The drugs tested were an imidazole-alcohol, an imidazole-dioxolane, and a triazole-ketone. Both the recombinant and crude forms of HO-2 were similarly inhibited by the 3 drugs. The crude microsomal spleen form of HO-1 was more susceptible to inhibition than was the truncated recombinant form. This difference is attributed to the extra amino acids in the full-length enzyme. These observations may be relevant in the design of drugs as inhibitors of HO and other membrane proteins.

The effects of azole-based heme oxygenase inhibitors on rat cytochromes P450 2E1 and 3A1/2 and human cytochromes P450 3A4 and 2D6

Heme oxygenases (HOs) catalyze the degradation of heme to biliverdin, carbon monoxide (CO), and free iron. The two major isoforms, HO-1 (inducible) and HO-2 (constitutive), are involved in a variety of physiological functions, including inflammation, apoptosis, neuromodulation, and vascular regulation. Major tools used in exploring these actions have been metalloporphyrin analogs of heme that inhibit the HOs. However, these tools are limited by their lack of selectivity; they affect other heme-dependent enzymes, such as cytochromes P450 (P450s), soluble guanylyl cyclase (sGC), and nitric-oxide synthase (NOS). Our laboratory has successfully synthesized a number of nonporphyrin azole-based HO inhibitors (QC-xx) that had little or no effect on sGC and NOS activity. However, their effects on various P450 isoforms have yet to be fully elucidated. To determine the effects of the QC-xx inhibitors on P450 enzyme activity, microsomal preparations of two rat P450 isoforms (2E1 and 3A1/3A2) and two human P450 supersome isoforms (3A4 and 2D6) were incubated with varying concentrations of HO inhibitor, and the activity was determined by spectrophotometric or fluorometric analysis. Results indicated that some QC compounds demonstrated little to no inhibition of the P450s, whereas others did inhibit these P450 isoforms. Four structural regions of QC-xx were analyzed, leading to the identification of structures that confer a decreased effect on both rat and human P450 isoforms studied while maintaining an inhibitory effect on the HOs.