Bioactivation of 2-(Alkylthio)-1,3,4-Thiadiazoles and 2-(Alkylthio)-1,3-Benzothiazoles

Abstract

Certain functional groups/structural motifs are known to generate chemically reactive metabolites that can covalently modify essential cellular macromolecules and, therefore, have the potential to disrupt biological function and elicit idiosyncratic adverse drug reactions. In this report, we describe the bioactivation of 5-substituted 2-(alkylthio)-1,3,4-thiadiazoles and 2-(alkylthio)-1,3-benzothiazoles, which can be added to the growing list of structural alerts. When 5-substituted 2-(methylthio)-1,3,4-thiadiazoles and 2-(methylthio)-1,3-benzothiazole were incubated with pooled human liver microsomes in the presence of NADPH and GSH, unusual GSH adducts were formed. Characterization of these GSH adducts by high-resolution mass spectrometry indicated the replacement of the methylthio- group by GSH, and NMR experiments ascertained the proposed structures. On the basis of the metabolic profile change in incubation samples with/without GSH, we proposed that the GSH adduct formation involved two steps: (1) enzymatic oxidation of the alkylthio- group to form sulfoxide and sulfone and (2) nucleophilic displacement of the formed sulfoxide and sulfone by GSH. The proposed mechanism was confirmed by the formation of the same GSH adduct from the incubation of synthetically prepared sulfoxide and sulfone compounds in buffer. We found the sulfur oxidation step was significantly inhibited (80-100%) by preincubation with 1-aminobenzotriazole but was much less affected by thermoinactivation (0-45%), suggesting that the sulfoxidation step is primarily catalyzed by cytochrome P450s and not by flavin monooxygenases. We also investigated the presence of this bioactivation pathway in more than a dozen compounds containing 2-(alkylthio)-1,3,4-thiadiazole and 2-(alkylthio)-1,3-benzothiazoles. The common GSH adduct formation pathway demonstrated by current studies raises a new structural alert and potential liability in drug safety when 2-alkylthio derivatives of 1,3-benzothiazoles and 1,3,4-thiadiazoles are incorporated in drug design.

MymA Bioactivated Thioalkylbenzoxazole Prodrug Family Active against Mycobacterium tuberculosis

“…Examples of metabolically labile structures featuring a alkylthio­(hetero)­aryl moiety from the literature indicated that upon S-oxidation, this structural motif could behave as a potential source of chemically reactive metabolites. − Reduced glutathione (GSH) is the most common trapping agent used for detection of chemically reactive metabolites in vitro . Compound 1 was incubated with human liver microsomes in the presence of the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) and GSH.…”

Demonstration of the Innate Electrophilicity of 4-(3-(Benzyloxy)phenyl)-2-(ethylsulfinyl)-6-(trifluoromethyl)pyrimidine (BETP), a Small-Molecule Positive Allosteric Modulator of the Glucagon-Like Peptide-1 Receptor

“…(Clapp, 1956;Colucci and Buyske, 1965;Conroy et al, 1984;Graham et al, 1989;Woltersdorf et al, 1989;Graham et al, 1990;Kishida et al, 1990;Huwe et al, 1991;Zhao et al, 1999;Teffera et al, 2008;Inoue et al, 2009;Litchfield et al, 2010). More recently, Yang et al (2012) have also demonstrated the susceptibility of 2-(alkylthio)-1,3,4-thiadiazoles and 2-(alkylthio)-1,3-benzothiazoles to undergo nucleophilic displacement with GSH in human liver microsomes. The requirement of NADPH cofactor in the GSH displacement reactions suggested that the rate-limiting step involved oxidation of the alkylthio functionality to the corresponding electrophilic sulfoxide and sulfone metabolites, followed by nucleophilic displacement of the formed sulfoxide and/or sulfone by GSH.…”

Reactive Metabolite Trapping Studies on Imidazo- and 2-Methylimidazo[2,1-b]thiazole-Based Inverse Agonists of the Ghrelin Receptor

“…The outlined mechanism is certainly not beyond the realm of possibilities, considering the well-known chemical (and glutathione transferase-mediated) displacement reaction of GSH with electron-deficient heteroaromatic rings (e.g., pyridine, pyrimidine, etc.) that contain alkylsulfoxide and/or alkylsulfone as leaving groups (Clapp, 1956;Colucci and Buyske, 1965;Conroy et al, 1984;Graham et al, 1989;Yang et al, 2012). A literature example that bears much commonality to the present situation is evident with the proton pump inhibitor pantoprazole wherein the benzimidazole-2-sulfoxide motif undergoes nucleophilic attack by GSH at the electron-deficient C2 position on the benzimidazole ring (Zhong et al, 2005).…”