N-Methylation of BI 187004 by Thiol S-Methyltransferase

Detailed resume of S-methyltransferases: Categories, structures, biological functions and research advancements in related pathophysiology and pharmacotherapy

Methylation is a vital chemical reaction in the metabolism of many drugs, neurotransmitters, hormones, and exogenous compounds. Among them, S-methylation plays a significant role in the biotransformation of sulfur-containing compounds, particularly chemicals with sulfhydryl groups. Currently, only three S-methyltransferases have been reported: thiopurine methyltransferase (TPMT), thiol methyltransferase (TMT), and thioether methyltransferase (TEMT). These enzymes are involved in various biological processes such as gene regulation, signal transduction, protein repair, tumor progression, and biosynthesis and degradation reactions in animals, plants, and microorganisms. Furthermore, they play pivotal roles in the metabolic pathways of essential drugs and contribute to the advancement of diseases such as tumors. This paper reviews the research progress on relevant structural features, metabolic mechanisms, inhibitor development, and influencing factors (gene polymorphism, S-adenosylmethionine level, race, sex, age, and disease) of S-methyltransferases. We hope that a better comprehension of S-methyltransferases will help to provide a reference for the development of novel strategies for related disorders and improve long-term efficacy.

METTL7A (TMT1A) and METTL7B (TMT1B) Are Responsible for Alkyl S-Thiol Methyl Transferase Activity in Liver

S-methylation of drugs containing thiol-moieties often alters their activity and results in detoxification. Historically, scientists attributed methylation of exogenous aliphatic and phenolic thiols to a putative S-adenosyl-L-methionine (SAM)-dependent membrane-associated enzyme referred to as thiol methyltransferase (TMT). This putative TMT appeared to have a broad substrate specificity and methylated the thiol metabolite of spironolactone, mertansine, ziprasidone, captopril, and the active metabolites of the thienopyridine prodrugs, clopidogrel, and prasugrel. Despite TMT's role in the S-methylation of clinically relevant drugs, the enzyme(s) responsible for this activity remained unknown. We recently identified methyltransferase-like protein 7B (METTL7B) as an alkyl thiol methyltransferase. METTL7B is an endoplasmic reticulum-associated protein with similar biochemical properties and substrate specificity to the putative TMT. Yet, the historic TMT inhibitor 2,3-dichloro-α-methylbenzylamine (DCMB) did not inhibit METTL7B, indicating that multiple enzymes contribute to TMT activity. Here we report that methyltransferase-like protein 7A (METTL7A), an uncharacterized member of the METTL7 family, is also a SAM-dependent thiol methyltransferase. METTL7A exhibits similar biochemical properties to METTL7B and putative TMT, including inhibition by DCMB (IC50 = 1.17 μM). Applying quantitative proteomics to human liver microsomes and gene modulation experiments in HepG2 and HeLa cells, we determined that TMT activity correlates closely with METTL7A and METTL7B protein levels. Furthermore, purification of a novel His-GST-tagged recombinant protein and subsequent activity experiments prove that METTL7A can selectively methylate exogenous thiol-containing substrates, including 7α-thiospironolactone, dithiothreitol, 4-chlorothiophenol, and mertansine. We conclude that the METTL7 family encodes for two enzymes, METTL7A and METTL7B, which are now renamed thiol methyltransferase 1A (TMT1A) and thiol methyltransferase 1B (TMT1B), respectively, that are responsible for thiol methylation activity in human liver microsomes. SIGNIFICANCE STATEMENT: We identified methyltransferase-like protein 7A (thiol methyltransferase 1A) and methyltransferase-like protein 7B (thiol methyltransferase 1B) as the enzymes responsible for the microsomal alkyl thiol methyltransferase (TMT) activity. These are the first two enzymes directly associated with microsomal TMT activity. S-methylation of commonly prescribed thiol-containing drugs alters their pharmacological activity and/or toxicity, and identifying the enzymes responsible for this activity will improve our understanding of the drug metabolism and pharmacokinetic (DMPK) properties of alkyl- or phenolic thiol-containing therapeutics.

Absorption, Metabolism, and Excretion of Taselisib (GDC-0032), a Potent β-Sparing PI3K Inhibitor in Rats, Dogs, and Humans

Taselisib (also known as GDC-0032) is a potent and selective phosphoinositide 3-kinase (PI3K) inhibitor that displays greater selectivity for mutant PI3Kα than wild-type PI3Kα To better understand the absorption, distribution, metabolism, and excretion properties of taselisib, mass balance studies were conducted following single oral doses of [¹⁴C]taselisib in rats, dogs, and humans. Absolute bioavailability (ABA) of taselisib in humans was determined by oral administration of taselisib at the therapeutic dose followed by intravenous dosing of [¹⁴C]taselisib as a microtracer. The ABA in humans was 57.4%. Absorption of taselisib was rapid in rats and dogs and moderately slow in humans. The recovery of radioactivity in excreta was high (>96%) in the three species where feces was the major route of excretion. Taselisib was the major circulating component in the three species with no metabolite accounting for >10% of the total drug-derived material. The fraction absorbed of taselisib was 35.9% in rats and 71.4% in dogs. In rats, absorbed drug underwent moderate to extensive metabolism and biliary excretion of taselisib was minor. In dog, biliary excretion and metabolism were major clearance pathways. In humans, 84.2% of the dose was recovered as the parent drug in excreta indicating that metabolism played a minor role in the drug's clearance. Major metabolism pathways were oxidation and amide hydrolysis in the three species while methylation was another prominent metabolism pathway in dogs. The site of methylation was identified on the triazole moiety. In vitro experiments characterized that the N-methylation was dog-specific and likely mediated by a thiol methyltransferase. SIGNIFICANCE STATEMENT: This study provides a comprehensive description of the absorption, distribution, and metabolism and pharmacokinetic properties of taselisib in preclinical species and humans. This study demonstrated the importance of oral bioavailability results for understanding taselisib's clearance pathways. The study also describes the identification and characterization of a unique dog-specific N-methylation metabolite of taselisib and the enzyme mediating N-methylation in vitro.

Biotransformation and bioactivation reactions - 2018 literature highlights

In the past three decades, ADME sciences have become an integral component of the drug discovery and development process. At the same time, the field has continued to evolve, thus, requiring ADME scientists to be knowledgeable of and engage with diverse aspects of drug assessment: from pharmacology to toxicology, and from in silico modeling to in vitro models and finally in vivo models. Progress in this field requires deliberate exposure to different aspects of ADME; however, this task can seem daunting in the current age of mass information. We hope this review provides a focused and brief summary of a wide array of critical advances over the past year and explains the relevance of this research ( Table 1 ). We divided the articles into categories of (1) drug optimization, (2) metabolites and drug metabolizing enzymes, and (3) bioactivation. This annual review is the fourth of its kind (Baillie et al. 2016 ; Khojasteh et al. 2017 , 2018 ). We have followed the same format we used in previous years in terms of the selection of articles and the authoring of each section. This effort in itself also continues to evolve. I am pleased that Rietjens, Miller, and Mitra have again contributed to this annual review. We would like to welcome Namandjé N. Bumpus, James P. Driscoll, and Donglu Zhang as authors for this year's issue. We strive to maintain a balance of authors from academic and industry settings. We would be pleased to hear your opinions of our commentary, and we extend an invitation to anyone who would like to contribute to a future edition of this review. Cyrus Khojasteh, on behalf of the authors.