Possible role of thioformamide as a proximate toxicant in the nephrotoxicity of thiabendazole and related thiazoles in glutathione-depleted mice: structure-toxicity and metabolic studies

Peroxidases-based enticing biotechnological platforms for biodegradation and biotransformation of emerging contaminants

Rampant industrial boom, urbanization, and exponential population growth resulted in widespread environmental pollution, with water being one of the leading affected resources. All kinds of pollutants, including phenols, industrial dyes, antibiotics, pharmaceutically active residues, and persistent/volatile organic compounds, have a paramount effect, either directly or indirectly, on human health and aquatic entities. Strategies for affordable and efficient decontamination of these emerging pollutants have become the prime focus of academic researchers, industry, and government to constitute a sustainable human society. Classical treatment techniques for environmental contaminants are associated with several limitations, such as inefficiency, complex pretreatments, overall high process cost, high sludge generation, and highly toxic side-products formation. Enzymatic remediation is considered a green and ecologically friendlier method that holds considerable potential to mitigate any kinds of contaminating agents. Exploiting the potential of various peroxidases for pollution abatement is an emerging research area and has considerable advantages, such as efficiency and ease of handling, over other methods. This work is designed to provide recent progress in deploying peroxidases as green and versatile biocatalytic tools for the degradation and transformation of a spectrum of potentially hazardous environmental pollutants to broaden their scope for biotechnological and environmental purposes. More studies are required to explicate the degradation mechanisms, assess the toxicology levels of bio-transformed metabolites, and standardize the treatment strategies for economic viability.

Reactive Metabolites from Thiazole-Containing Drugs: Quantum Chemical Insights into Biotransformation and Toxicity

Drugs containing thiazole and aminothiazole groups are known to generate reactive metabolites (RMs) catalyzed by cytochrome P450s (CYPs). These RMs can covalently modify essential cellular macromolecules and lead to toxicity and induce idiosyncratic adverse drug reactions. Molecular docking and quantum chemical hybrid DFT study were carried out to explore the molecular mechanisms involved in the biotransformation of thiazole (TZ) and aminothiazole (ATZ) groups leading to RM epoxide, S-oxide, N-oxide, and oxaziridine. The energy barrier required for the epoxidation is 13.63 kcal/mol, that is lower than that of S-oxidation, N-oxidation, and oxaziridine formation (14.56, 17.90, and 20.20, kcal/mol respectively). The presence of the amino group in ATZ further facilitates all the metabolic pathways, for example, the barrier for the epoxidation reaction is reduced by ∼2.5 kcal/mol. Some of the RMs/their isomers are highly electrophilic and tend to form covalent bonds with nucleophilic amino acids, finally leading to the formation of metabolic intermediate complexes (MICs). The energy profiles of these competitive pathways have also been explored.

Meloxicam methyl group determines enzyme specificity for thiazole bioactivation compared to sudoxicam

Meloxicam is a thiazole-containing NSAID that was approved for marketing with favorable clinical outcomes despite being structurally similar to the hepatotoxic sudoxicam. Introduction of a single methyl group on the thiazole results in an overall lower toxic risk, yet the group's impact on P450 isozyme bioactivation is unclear. Through analytical methods, we used inhibitor phenotyping and recombinant P450s to identify contributing P450s, and then measured steady-state kinetics for bioactivation of sudoxicam and meloxicam by the recombinant P450s to determine relative efficiencies. Experiments showed that CYP2C8, 2C19, and 3A4 catalyze sudoxicam bioactivation, and CYP1A2 catalyzes meloxicam bioactivation, indicating that the methyl group not only impacts enzyme affinity for the drugs, but also alters which isozymes catalyze the metabolic pathways. Scaling of relative P450 efficiencies based on average liver concentration revealed that CYP2C8 dominates the sudoxicam bioactivation pathway and CYP2C9 dominates meloxicam detoxification. Dominant P450s were applied for an informatics assessment of electronic health records to identify potential correlations between meloxicam drug-drug interactions and drug-induced liver injury. Overall, our findings provide a cautionary tale on assumed impacts of even simple structural modifications on drug bioactivation while also revealing specific targets for clinical investigations of predictive factors that determine meloxicam-induced idiosyncratic liver injury.

2-aminothiazoles in drug discovery: Privileged structures or toxicophores?

The 2-aminothiazole functionality has long been established as a privileged structural feature and therefore frequently exploited in the process of drug discovery and development. It has been introduced into numerous compounds due to its capacity for targeting a wide range of therapeutic target proteins. On the other hand, the aminothiazole group has also been classified as a toxicophore susceptible to metabolic activation and the ensuing reactive metabolite formation, hence caution is warranted when used in drug design. This review is divided into three parts entailing: (i) the general characteristics of the aminothiazole group, (ii) the advantages of the aminothiazole group in medicinal chemistry, and (iii) the impact of the integrated aminothiazole group on compound safety profile.

Design, synthesis, and biological evaluation of novel oxadiazole- and thiazole-based histamine H3R ligands

Histamine H₃ receptor (H₃R) is largely expressed in the CNS and modulation of the H₃R function can affect histamine synthesis and liberation, and modulate the release of many other neurotransmitters. Targeting H₃R with antagonists/inverse agonists may have therapeutic applications in neurodegenerative disorders, gastrointestinal and inflammatory diseases. This prompted us to design and synthesize azole-based H₃R ligands, i.e. having oxadiazole- or thiazole-based core structures. While ligands of oxadiazole scaffold were almost inactive, thiazole-based ligands were very potent and several exhibited binding affinities in a nanomolar concentration range. Ligands combining 4-cyanophenyl moiety as arbitrary region, para-xylene or piperidine carbamoyl linkers, and/or pyrrolidine or piperidine basic heads were found to be the most active within this series of thiazole-based H₃R ligands. The most active ligands were in silico screened for ADMET properties and drug-likeness. They fulfilled Lipinski's and Veber's rules and exhibited potential activities for oral administration, blood-brain barrier penetration, low hepatotoxicity, combined with an overall good toxicity profile.

Species Differences in the Oxidative Desulfurization of a Thiouracil-Based Irreversible Myeloperoxidase Inactivator by Flavin-Containing Monooxygenase Enzymes

N1-Substituted-6-arylthiouracils, represented by compound 1 [6-(2,4-dimethoxyphenyl)-1-(2-hydroxyethyl)-2-thioxo-2,3-dihydropyrimidin-4(1H)-one], are a novel class of selective irreversible inhibitors of human myeloperoxidase. The present account is a summary of our in vitro studies on the facile oxidative desulfurization in compound 1 to a cyclic ether metabolite M1 [5-(2,4-dimethoxyphenyl)-2,3-dihydro-7H-oxazolo[3,2-a]pyrimidin-7-one] in NADPH-supplemented rats (t1/2 [half-life = mean ± S.D.] = 8.6 ± 0.4 minutes) and dog liver microsomes (t1/2 = 11.2 ± 0.4 minutes), but not in human liver microsomes (t1/2 > 120 minutes). The in vitro metabolic instability also manifested in moderate-to-high plasma clearances of the parent compound in rats and dogs with significant concentrations of M1 detected in circulation. Mild heat deactivation of liver microsomes or coincubation with the flavin-containing monooxygenase (FMO) inhibitor imipramine significantly diminished M1 formation. In contrast, oxidative metabolism of compound 1 to M1 was not inhibited by the pan cytochrome P450 inactivator 1-aminobenzotriazole. Incubations with recombinant FMO isoforms (FMO1, FMO3, and FMO5) revealed that FMO1 principally catalyzed the conversion of compound 1 to M1. FMO1 is not expressed in adult human liver, which rationalizes the species difference in oxidative desulfurization. Oxidation by FMO1 followed Michaelis-Menten kinetics with Michaelis-Menten constant, maximum rate of oxidative desulfurization, and intrinsic clearance values of 209 μM, 20.4 nmol/min/mg protein, and 82.7 μl/min/mg protein, respectively. Addition of excess glutathione essentially eliminated the conversion of compound 1 to M1 in NADPH-supplemented rat and dog liver microsomes, which suggests that the initial FMO1-mediated S-oxygenation of compound 1 yields a sulfenic acid intermediate capable of redox cycling to the parent compound in a glutathione-dependent fashion or undergoing further oxidation to a more electrophilic sulfinic acid species that is trapped intramolecularly by the pendant alcohol motif in compound 1.

Degradation and monitoring of acetamiprid, thiabendazole and their transformation products in an agro-food industry effluent during solar photo-Fenton treatment in a raceway pond reactor

In this study, pesticides acetamiprid and thiabendazole and their transformation products (TPs), seven from each pesticide, were successfully monitored during solar photo-Fenton treatment in a real secondary effluent from an agro-food industry spiked with 100μgL(-1) of each pesticide. To this end, a highly sensitive procedure was developed, based on liquid chromatography (LC) coupled to hybrid quadrupole-linear ion trap mass spectrometry (QqLIT-MS). In addition, finding low-cost and operational technology for the application of AOPs would then facilitate their use on a commercial level. Simple and extensive photoreactors such as raceway pond reactors (RPRs) are therefore proposed as an alternative for the application of solar photo-Fenton. Results showed that high degradation could be achieved in a complex water matrix (>99% TBZ and 91% ACTM in 240min) using a 120-L RPR pilot plant as novel technology. The analyses indicated that after the treatment only three TPs from ACTM were still present in the effluent, while the others had been removed. The study showed that the goal of either just removing the parent compounds, or going one step further and removing all the TPs, can significantly change the treatment time, which would affect process costs.

Metabolism of the phytoalexins camalexins, their bioisosteres and analogues in the plant pathogenic fungus Alternaria brassicicola

The metabolism of the phytoalexins camalexin (1), 1-methylcamalexin (10) and 6-methoxycamalexin (11) by Alternaria brassicicola and their antifungal activity is reported. This work establishes that camalexins are slowly biotransformed (ca. six days) to the corresponding indole-3-thiocarboxamides, which are further transformed to the indole-3-carboxylic acids. These metabolites are substantially less inhibitory to A. brassicicola than the parent camalexins, indicating that these enzyme-mediated transformations are detoxifications. In addition, analyses of the metabolism of synthetic isomers and bioisosteres of camalexin (1) indicate that isomers of camalexin in the thiazole ring are not metabolized. Based on these results, the potential intermediates that lead to formation of indole-3-thiocarboxamides are proposed.

Metabolomic screening and identification of the bioactivation pathways of ritonavir

Ritonavir-boosted protease inhibitor regimens are widely used for HIV chemotherapy. However, ritonavir causes multiple side effects, and the mechanisms are not fully understood. The current study was designed to explore the metabolic pathways of ritonavir that may be related to its toxicity. Metabolomic analysis screened out 26 ritonavir metabolites in mice, and half of them are novel. These novel ritonavir metabolites include two glycine conjugated, two N-acetylcysteine conjugated, and three ring-open products. Accompanied with the generation of ritonavir ring-open metabolites, the formation of methanethioamide and 2-methylpropanethioamide were expected. Upon the basis of the structures of these novel metabolites, five bioactivation pathways are proposed, which may be associated with sulfation and epoxidation. By using Cyp3a-null mice, we confirmed that CYP3A is involved in four pathways of RTV bioactivation. In addition, all these five bioactivation pathways were recapitulated in the incubation of ritonavir in human liver microsomes. Further studies are suggested to determine the role of CYP3A and these bioactivation pathways in ritonavir toxicity.

In vitro and in silico identification and characterization of thiabendazole as a mechanism-based inhibitor of CYP1A2 and simulation of possible pharmacokinetic drug-drug interactions

Thiabendazole (TBZ) and its major metabolite 5-hydroxythiabendazole (5OH-TBZ) were screened for potential time-dependent inhibition (TDI) against CYP1A2. Screen assays were carried out in the absence and presence of NADPH. TDI was observed with both compounds, with k(inact) and K(I) values of 0.08 and 0.02 min(-1) and 1.4 and 63.3 microM for TBZ and 5OH-TBZ, respectively. Enzyme inactivation was time-, concentration-, and NADPH-dependent. Inactivation by TBZ was irreversible by dialysis and oxidation by potassium ferricyanide, and there was no protection by glutathione. 5OH-TBZ was a weak TDI of CYP1A2, and enzyme activity was recovered by dialysis. IC(50) determination of TBZ and 5OH-TBZ showed both compounds to be potent inhibitors, with IC(50) values of 0.83 and 13.05 microM, respectively. IC(50) shift studies also demonstrated that TBZ was a TDI of CYP1A2. In silico methods identified the thiazole group as a TDI fragment and predicted it as the site of metabolism. The observation pointed to epoxidation of the thiazole and the benzyl rings of TBZ as possible routes of metabolism and mechanisms of TDI. Drug-drug interaction (DDI) simulation studies using SimCyp showed good predictions for competitive inhibition. However, predictions for mechanism-based inhibition (MBI)-based DDI were not in agreement with clinical observations. There was no TBZ accumulation upon chronic administration of the drug. The in vitro MBI findings might therefore not be capturing the in vivo situation in which the proposed bioactivation route is minor. This might be the case for TBZ in which, in vivo, UDP glucuronosyltransferases and sulfanotransferase metabolize and eliminate the 5OH-TBZ.

In vitro microsomal metabolic studies on a selective mGluR5 antagonist MTEP: characterization of in vitro metabolites and identification of a novel thiazole ring opening aldehyde metabolite

In vitro liver microsomal studies revealed that [14C] MTEP (3-[2-methyl-1,3-thiazol-4-yl)ethynyl] pyridine) was metabolized into three major oxidative metabolites. Metabolite 1 (M1) was shown to be a hydroxymethyl metabolite; M2 was shown to be a pyridine oxide. Moreover, a novel aldehyde metabolite (M3) was identified from mouse liver microsomes. The structure of the aldehyde M3 was elucidated by LC/MS/MS. In addition, methoxyamine, an aldehyde-trapping agent, and accurate mass measurement using a high-resolution quadrupole-time of flight (Q-TOF) instrument, were used to confirm the proposed thiazole ring-opening structure of M3. A mechanism for aldehyde M3 formation was postulated based on MTEP incubation studies with 18O2 and H2 18O using mouse liver microsomes. MTEP was initially oxidized at sulfur, followed by subsequent C4-C5 of thiazole epoxidation, thiozole ring opening and further oxidative desulfation. This proposed thiazole ring-opening mechanism might represent a novel metabolism pathway for xenobiotics containing a thiazole moiety. Species differences in the metabolism of MTEP were observed in mouse, rat, dog, monkey and human liver microsomes. Mouse appears to generate all three oxidative metabolites to a greater extent than other species examined.

Evidence for cytochrome P4501A2-mediated protein covalent binding of thiabendazole and for its passive intestinal transport: use of human and rabbit derived cells

Thiabendazole (TBZ), an anthelmintic and fungicide benzimidazole, was recently demonstrated to be extensively metabolized by cytochrome P450 (CYP) 1A2 in man and rabbit, yielding 5-hydroxythiabendazole (5OH-TBZ), the major metabolite furtherly conjugated, and two minor unidentified metabolites (M1 and M2). In this study, exposure of rabbit and human cells to 14C-TBZ was also shown to be associated with the appearance of radioactivity irreversibly bound to proteins. The nature of CYP isoforms involved in this covalent binding was investigated by using cultured rabbit hepatocytes treated or not with various CYP inducers (CYP1A1/2 by beta-naphthoflavone, CYP2B4 by phenobarbital, CYP3A6 by rifampicine, CYP4A by clofibrate) and human liver and bronchial CYP-expressing cells. The covalent binding to proteins was particularly increased in beta-naphthoflavone-treated rabbit cells (2- to 4-fold over control) and human cells expressing CYP1A2 (22- to 42-fold over control). Thus, CYP1A2 is a major isoenzyme involved in the formation of TBZ-derived residues bound to protein. Furthermore, according to the good correlation between covalent binding and M1 or 5OH-TBZ production, TBZ would be firstly metabolized to 5OH-TBZ and subsequently converted to a chemically reactive metabolic intermediate binding to proteins. This metabolic activation could take place preferentially in liver and lung, the main biotransformation organs, rather than in intestines where TBZ was shown to be not metabolized. Moreover, TBZ was rapidly transported by passive diffusion through the human intestinal cells by comparison with the protein-bound residues which were not able to cross the intestinal barrier. Consequently, the absence of toxicity measured in intestines could be related to the low degree of TBZ metabolism and the lack of absorption of protein adducts. Nevertheless, caution is necessary in the use of TBZ concurrently with other drugs able to regulate CYP1A2, particularly in respect to liver and lung tissues, recognised as sites of covalent-binding.

Induction of class alpha glutathione S-transferases by 4-methylthiazole in the rat liver: role of oxidative stress

The expression of glutathione S-transferase (GST) is a crucial factor in determining the sensitivity of cells and organs in response to a variety of toxicants. Expression of class alpha GST genes by methyl-substituted thiazoles was assessed in the rat liver. Northern blot analysis revealed that 4-methylthiazole (4-MT) elevated rGSTA2, A3, A5 and M1 mRNAs in the liver by 19-, 4-, 6- and 9-fold at 24 h after treatment, respectively, as compared to control. Consecutive 3-day treatment with 4-MT resulted in 4- to 7-fold increases in rGSTA and M1 mRNAs. Multiple treatments with 5-methylthiazole (5-MT) caused marginal increases in GST mRNAs in spite of the large increases in certain GST mRNAs at 24 h. Either 4, 5-dimethylthiazole (DT) or 2,4,5-trimethylthiazole (TT) minimally affected the rGSTA and rGSTM mRNA expression at 1-3 day(s). Western blot analysis showed that 4-MT induced rGSTA1/2, rGSTA3/5 and rGSTM1 proteins by 2.6-, 2.1- and 2.1-fold at 3 days, respectively, while other methylthiazoles failed to induce the GST subunits. Starving rats were treated with a lower dose of methylthiazoles to study the role of oxidative stress in the mRNA expression. The levels in rGSTA2/3/5 mRNAs were significantly enhanced by 4-MT in starving rats, whereas rGSTM1/2 mRNAs were not further increased. Other methylthiazoles were inactive in enhancing the mRNAs in starving animals. Pretreatment of starving rats with either cysteine or methionine completely prevented the increases in class alpha GST mRNAs by 4-MT. Data showed that 4-MT induces class alpha GSTs with the increases in the mRNAs, whereas 5-methyl-, dimethyl- and trimethyl-substituted thiazoles were minimally active. Increases in the class alpha GST mRNAs by 4-MT may be associated with the oxidative stress in hepatocytes, as supported by starvation and sulfur amino acid experiments.

Enhanced expression of rat hepatic microsomal epoxide hydrolase by methylthiazole in conjunction with liver injury

Microsomal epoxide hydrolase (mEH) is inducible by a number of xenobiotics. Induction of mEH by certain chemopreventive agents may implicate the protective effect. In contrast, many of carcinogenic agents also induce the enzyme. The hepatotoxicity and mEH expression by methylthiazoles, which are incorporated as functional groups in a number of therapeutic agents, were assessed in the rat liver to study the structural basis for the enzyme induction and the correlative enzyme expression with hepatotoxicity. Among the methylthiazoles examined, 4-methylthiazole (MT) at the daily dose of 1.17 mmol/kg body weight caused hepatic necrosis and degeneration after 1-3 consecutive daily treatment(s), whereas 4, 5-dimethylthiazole (DT) and 2,4,5-trimethylthiazole (TT) elicited no toxicity. Treatment of rats with MT at the daily dose of 1.17 mmol/kg increased the mEH mRNA by 17- and 7-fold at day 1 and day 3, respectively, relative to control. Whereas DT caused 5- and 2-fold increases in mEH mRNA at day 1 and day 3, respectively, TT minimally affected mEH expression. The mRNA increase was consistent with the protein induction. Hence, the methylthiazole causing hepatotoxicity was more active in inducing the enzyme. Whereas treatment with MT at the dose of 0.35 mmol/kg caused no hepatotoxicity, MT caused hepatic necrosis in starving rats. Northern blot analysis showed that the mEH mRNA level was increased to a greater extent by MT in starving rats than in control animals. Conversely, treatment of starving rats with either cysteine or methionine prior to MT prevented the hepatic necrosis. Elevation of the mEH mRNA by MT in starving animals was also inhibited by either cysteine or methionine pretreatment. These results demonstrated that the methylthiazole which caused hepatotoxicity also up-regulated mEH expression, whereas other methylthiazoles showing no toxicity minimally increased the gene expression. The observation that the extent of mEH expression by MT was highly associated with that of liver injury raised the notion that mEH expression by xenobiotics may not necessarily represent the beneficial and protective effects.

Evidence for the involvement of N-methylthiourea, a ring cleavage metabolite, in the hepatotoxicity of methimazole in glutathione-depleted mice: structure-toxicity and metabolic studies

In mice depleted of GSH by treatment with buthionine sulfoximine (BSO), methimazole (2-mercapto-1-methylimidazole, MMI) causes liver injury characterized by centrilobular necrosis of hepatocytes and an increase in serum alanine transaminase (SALT) activity. MMI requires metabolic activation by both P450 monooxygenase and flavin-containing monooxygenase (FMO) before it produces the hepatotoxicity. MMI and its analogues were examined for the ability to increase SALT activity in GSH-depleted mice. Saturation of the C-4,5 double bond in MMI resulted in a complete loss of hepatotoxicity. Similarly, ring fusion of a benzene nucleus to the C-4,5 double bond, forming 2-mercapto-1-methylbenzimidazole, abolished the toxic potency. As for MMI, 2-mercapto-1,4,5-trimethylimidazole, and 2-mercapto-1-methyl-4, 5-di-n-propylimidazole, the toxic potency decreased with the increasing bulk of the 4- and 5-alkyl substituents. Furthermore, methylation of the thiol group of MMI totally reduced its toxicity. These structural requirements and the known toxicity of thiono-sulfur compounds led us to the hypothesis that MMI would undergo epoxidation of the C-4,5 double bond by P450 enzymes and, after being hydrolyzed, the resulting epoxide would be then decomposed to form N-methylthiourea, a proximate toxicant. Before N-methylthiourea would produce toxicity, it would be further biotransformed to its S-oxidized metabolites mainly by FMO. Evidence for this hypothesis was provided by the facts that N-methylthiourea and glyoxal as the accompanying fragment were identified as urinary metabolites in mice treated with MMI and that N-methylthiourea caused a marked increase in SALT activity when administered to mice in combination with BSO.

Relative hepatotoxicity of 2-(substituted phenyl)thiazoles and substituted thiobenzamides in mice: evidence for the involvement of thiobenzamides as ring cleavage metabolites in the hepatotoxicity of 2-phenylthiazoles

The hepatotoxicity of the 3 isomers of para-substituted thiobenzamides and the 3 isomers of 2-(para-substituted phenyl)-4-methylthiazoles was evaluated in mice depleted of glutathione (GSH) by pretreatment with buthionine sulfoximine (BSO). In accordance with previous studies with the rat, p-methoxythiobenzamide was more toxic than thiobenzamide, and conversely p-chlorothiobenzamide was markedly less toxic as assessed by serum alanine aminotransferase (ALT) activity. The hepatotoxicity of 2-phenyl-4-methylthiazole was also altered by the addition of para-substituents to the phenyl ring in the same way as observed for thiobenzamide derivatives: the rank order of toxicity was 4-methylthiazoles having p-methoxyphenyl > phenyl >> p-chlorophenyl at the 2-position. This good correlation of the rank order of hepatotoxicity between series of 2-(para-substituted phenyl)-4-methylthiazoles and para-substituted thiobenzamides supports the concept that thiobenzamides as ring cleavage metabolites play a role in the hepatotoxicity of 2-phenylthiazole derivatives.