Reduction and methylation of ziprasidone by glutathione, aldehyde oxidase, and thiol s-methyltransferase in humans: anin vitrostudy

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.

Challenges and Opportunities for In Vitro-In Vivo Extrapolation of Aldehyde Oxidase-Mediated Clearance: Toward a Roadmap for Quantitative Translation

Underestimation of aldehyde oxidase (AO)-mediated clearance by current in vitro assays leads to uncertainty in human dose projections, thereby reducing the likelihood of success in drug development. In the present study we first evaluated the current drug development practices for AO substrates. Next, the overall predictive performance of in vitro-in vivo extrapolation of unbound hepatic intrinsic clearance (CLint,u) and unbound hepatic intrinsic clearance by AO (CLint,u,AO) was assessed using a comprehensive literature database of in vitro (human cytosol/S9/hepatocytes) and in vivo (intravenous/oral) data collated for 22 AO substrates (total of 100 datapoints from multiple studies). Correction for unbound fraction in the incubation was done by experimental data or in silico predictions. The fraction metabolized by AO (fmAO) determined via in vitro/in vivo approaches was found to be highly variable. The geometric mean fold errors (gmfe) for scaled CLint,u (mL/min/kg) were 10.4 for human hepatocytes, 5.6 for human liver cytosols, and 5.0 for human liver S9, respectively. Application of these gmfe's as empirical scaling factors improved predictions (45%-57% within twofold of observed) compared with no correction (11%-27% within twofold), with the scaling factors qualified by leave-one-out cross-validation. A road map for quantitative translation was then proposed following a critical evaluation on the in vitro and clinical methodology to estimate in vivo fmAO In conclusion, the study provides the most robust system-specific empirical scaling factors to date as a pragmatic approach for the prediction of in vivo CLint,u,AO in the early stages of drug development. SIGNIFICANCE STATEMENT: Confidence remains low when predicting in vivo clearance of AO substrates using in vitro systems, leading to de-prioritization of AO substrates from the drug development pipeline to mitigate risk of unexpected and costly in vivo impact. The current study establishes a set of empirical scaling factors as a pragmatic tool to improve predictability of in vivo AO clearance. Developing clinical pharmacology strategies for AO substrates by utilizing mass balance/clinical drug-drug interaction data will help build confidence in fmAO.

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.

Investigating the Metabolic Mechanisms of Butaselen, An Ebselen Analog

BACKGROUND

Butaselen is an ebselen analog that is under clinical trials for treating hepatic and pulmonary fibrosis. Our previous studies showed that butaselen is mainly present in human plasma in the form of M2, a free Se-methylated metabolite.

OBJECTIVE

This study aimed to investigate the metabolic mechanisms of butaselen.

METHODS AND RESULTS

Butaselen was incubated with human plasma. Butaselen immediately disappeared, and the butaselen-HSA (human serum albumin) adduct was detected by HPLC-HRMS, showing that butaselen covalently binds to HSA. The butaselen-HSA adduct was precipitated using acetonitrile and then incubated with PBS, Cys, and GSH for 1 hour. The product was M1, a reduced form of butaselen. The results indicated that HSA, Cys, and GSH can reduce the butaselen-HSA covalent bond. The binding site for butaselen could be the cysteine-34 residue of HSA through pronase and trypsin hydrolysis. Incubating butaselen with cysteine, butaselen-Cys, butaselen-2Cys, and M1 were generated, indicating the covalent binding and reduction of butaselen by cysteine. We incubated liver microsomes and cytosol with butaselen, 6.22 and 246 nM M2 were generated, respectively. The results demonstrated that cytosolic enzymes are mainly involved in M2 production. The amount of M2 in the liver cytosol decreased from 246 nM to 2.21 nM when 10 mM m-anisic acid (a specific TPMT enzyme inhibitor) was added, showing that TPMT is responsible for M2 formation.

CONCLUSION

Butaselen was covalently bound to HSA, and the binding site was the cysteine-34 residue of HSA. The butaselen-HSA adduct was reduced by free thiol compounds to generate M1. M1 was further metabolized to M2 by cytosolic TPMT. This study provides a basis for studying the pharmacokinetics of selenium-containing drugs.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, "general chemicals," natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10-15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Non-cytochrome P450 enzymes involved in the oxidative metabolism of xenobiotics: Focus on the regulation of gene expression and enzyme activity

Oxidative metabolism is one of the major biotransformation reactions that regulates the exposure of xenobiotics and their metabolites in the circulatory system and local tissues and organs, and influences their efficacy and toxicity. Although cytochrome (CY)P450s play critical roles in the oxidative reaction, extensive CYP450-independent oxidative metabolism also occurs in some xenobiotics, such as aldehyde oxidase, xanthine oxidoreductase, flavin-containing monooxygenase, monoamine oxidase, alcohol dehydrogenase, or aldehyde dehydrogenase-dependent oxidative metabolism. Drugs form a large portion of xenobiotics and are the primary target of this review. The common reaction mechanisms and roles of non-CYP450 enzymes in metabolism, factors affecting the expression and activity of non-CYP450 enzymes in terms of inhibition, induction, regulation, and species differences in pharmaceutical research and development have been summarized. These non-CYP450 enzymes are detoxifying enzymes, although sometimes they mediate severe toxicity. Synthetic or natural chemicals serve as inhibitors for these non-CYP450 enzymes. However, pharmacokinetic-based drug interactions through these inhibitors have rarely been reported in vivo. Although multiple mechanisms participate in the basal expression and regulation of non-CYP450 enzymes, only a limited number of inducers upregulate their expression. Therefore, these enzymes are considered non-inducible or less inducible. Overall, this review focuses on the potential xenobiotic factors that contribute to variations in gene expression levels and the activities of non-CYP450 enzymes.

Human METTL7B is an alkyl thiol methyltransferase that metabolizes hydrogen sulfide and captopril

Methylation of alkyl thiols is a biotransformation pathway designed to reduce thiol reactivity and potential toxicity, yet the gene and protein responsible for human alkyl thiol methyltransferase (TMT) activity remain unknown. Here we demonstrate with a range of experimental approaches using cell lines, in vitro systems, and recombinantly expressed enzyme, that human methyltransferase-like protein 7B (METTL7B) catalyzes the transfer of a methyl group from S-adenosyl-L-methionine (AdoMet) to hydrogen sulfide (H2S) and other exogenous thiol small molecules. METTL7B gene modulation experiments, including knockdown in HepG2 cells and overexpression in HeLa cells, directly alter the methylation of the drug captopril, a historic probe substrate for TMT activity. Furthermore, recombinantly expressed and purified wild-type METTL7B methylates several thiol compounds, including H2S, 7α-thiospironolactone, L-penicillamine, and captopril, in a time- and concentration-dependent manner. Typical for AdoMet-dependent small molecule methyltransferases, S-adenosyl-L-homocysteine (AdoHcy) inhibited METTL7B activity in a competitive fashion. Similarly, mutating a conserved aspartate residue, proposed to anchor AdoMet into the active site, to an alanine (D98A) abolished methylation activity. Endogenous thiols such as glutathione and cysteine, or classic substrates for other known small molecule S-, N-, and O-methyltransferases, were not substrates for METTL7B. Our results confirm, for the first time, that METTL7B, a gene implicated in multiple disease states including rheumatoid arthritis and breast cancer, encodes a protein that methylates small molecule alkyl thiols. Identifying the catalytic function of METTL7B will enable future pharmacological research in disease pathophysiology where altered METTL7B expression and, potentially H2S levels, can disrupt cell growth and redox state.

Metabolism by Aldehyde Oxidase: Drug Design and Complementary Approaches to Challenges in Drug Discovery

Aldehyde oxidase (AO) catalyzes oxidations of azaheterocycles and aldehydes, amide hydrolysis, and diverse reductions. AO substrates are rare among marketed drugs, and many candidates failed due to poor pharmacokinetics, interspecies differences, and adverse effects. As most issues arise from complex and poorly understood AO biology, an effective solution is to stop or decrease AO metabolism. This perspective focuses on rational drug design approaches to modulate AO-mediated metabolism in drug discovery. AO biological aspects are also covered, as they are complementary to chemical design and important when selecting the experimental system for risk assessment. The authors' recommendation is an early consideration of AO-mediated metabolism supported by computational and in vitro experimental methods but not an automatic avoidance of AO structural flags, many of which are versatile and valuable building blocks. Preferably, consideration of AO-mediated metabolism should be part of the multiparametric drug optimization process, with the goal to improve overall drug-like properties.

N-Methylation of BI 187004 by Thiol S-Methyltransferase

BI 187004, an 11β-hydroxysteroid dehydrogenase 1 inhibitor, was administered once daily for 14 days to eight patients with type 2 diabetes mellitus. N-methylation was identified as a major biotransformation pathway. In four patients treated with BI 187004, the plasma exposure of an N-methylbenzimidazole metabolite [N-methylbenzimidazole regioisomer 1 (M1)] was 7-fold higher than the remaining four patients, indicating a substantial degree of metabolic variation. To identify the methyltransferase enzymes responsible for N-methylation, BI 187004 was incubated with human liver microsomes (HLM), human kidney microsomes (HKM), and their respective cytosolic preparations in the presence and absence of isoform-selective chemical inhibitors. Additionally, BI 187004 was incubated with several human recombinant methyltransferases: catechol O-methyltransferase (rhCOMT), histamine N-methyltransferase (rhHNMT), nicotinamide N-methyltransferase (rhNNMT), glycine N-methyltransferase (rhGNMT), and thiopurine S-methyltransferase (rhTPMT). M1 was principally observed in HLM and HKM incubations, minimally formed in liver and kidney cytosol, and not formed during incubations with recombinant methyltransferase enzymes. In all microsomal and cytosolic incubations, the formation of M1 was inhibited only by 2,3-dichloro-α-methylbenzylamine (DCMB), an inhibitor of thiol S-methyltransferase (TMT), providing evidence that TMT catalyzed the formation of M1. Interestingly, the N-methylbenzimidazole regioisomer (M14) was only observed in vitro, predominantly during incubations with human kidney cytosol and rhHNMT. The formation of M14 was inhibited by amodiaquine (an HNMT inhibitor) and DCMB, providing additional evidence that both HNMT and TMT catalyzed M14 formation. Overall, using BI 187004 as a substrate, this study demonstrates a novel TMT-mediated N-methylation biotransformation and an HNMT-mediated regioselective N-methylation.

An Inter-Ethnic Comparison Study of Ziprasidone Plasma Levels, Dosage and Clinical Response in Patients with Schizophrenia

OBJECTIVE

The aim of this study was to investigate ziprasidone plasma concentration, daily dose and clinical efficacy and safety in Han Chinese and Mongolian patients with first-episode schizophrenia.

METHODS

A total of 123 inpatients affected by schizophrenia were recruited from the Mental Health Center of Inner Mongolia in China. Ziprasidone plasma concentration, clinical efficacy and side effects were systematically evaluated at baseline, and at 1, 2, 4, and 6 weeks. Metabolic measures such as changes in weight, body mass index (BMI), fasting blood glucose (FBG), triglycerides, and cholesterol, were also recorded.

RESULTS

90 patients completed the study. Compared with Han patients, on average, Mongolian patients received a significantly higher ziprasidone dosage for adequate symptom control during the 6-week period and had a lower plasma concentration-to-dose ratio. The Mongolian patients also experienced greater increases in weight and BMI. No significant differences between the two ethnic groups were found in the rate of reduction in the Positive and Negative Syndrome Scale (PANSS) score, Treatment Emergent Symptom Scale (TESS) total score, FBG, triglycerides, cholesterol or Q-Tc interval.

CONCLUSION

Compared to Han Chinese patients, Mongolian patients appeared to have increased ziprasidone clearance and require higher doses to achieve effective treatment for schizophrenia.

Challenges and Opportunities with Non-CYP Enzymes Aldehyde Oxidase, Carboxylesterase, and UDP-Glucuronosyltransferase: Focus on Reaction Phenotyping and Prediction of Human Clearance

Over the years, significant progress has been made in reducing metabolic instability due to cytochrome P450-mediated oxidation. High-throughput metabolic stability screening has enabled the advancement of compounds with little to no oxidative metabolism. Furthermore, high lipophilicity and low aqueous solubility of presently pursued chemotypes reduces the probability of renal excretion. As such, these low microsomal turnover compounds are often substrates for non-CYP-mediated metabolism. UGTs, esterases, and aldehyde oxidase are major enzymes involved in catalyzing such metabolism. Hepatocytes provide an excellent tool to identify such pathways including elucidation of major metabolites. To predict human PK parameters for P450-mediated metabolism, in vitro-in vivo extrapolation using hepatic microsomes, hepatocytes, and intestinal microsomes has been actively investigated. However, such methods have not been sufficiently evaluated for non-P450 enzymes. In addition to the involvement of the liver, extrahepatic enzymes (intestine, kidney, lung) are also likely to contribute to these pathways. While there has been considerable progress in predicting metabolic pathways and clearance primarily mediated by the liver, progress in characterizing extrahepatic metabolism and prediction of clearance has been slow. Well-characterized in vitro systems or in vivo animal models to assess drug-drug interaction potential and intersubject variability due to polymorphism are not available. Here we focus on the utility of appropriate in vitro studies to characterize non-CYP-mediated metabolism and to understand the enzymes involved followed by pharmacokinetic studies in the appropriately characterized surrogate species. The review will highlight progress made in establishing in vitro-in vivo correlation, predicting human clearance and avoiding costly clinical failures when non-CYP-mediated metabolic pathways are predominant.

Evaluation of the pharmacokinetics, safety and clinical efficacy of ziprasidone for the treatment of schizophrenia and bipolar disorder

INTRODUCTION

Multiple strategies exist for the pharmacological treatment of schizophrenia and related disorders. In the last 20 years, several 'new' compounds have been introduced, called 'atypical antipsychotics', which have higher efficacy and better tolerability than first-generation neuroleptics. Among them, ziprasidone (ZPR) is currently finding widespread use, and it has also been shown to be active as an augmenter in bipolar disorder therapy.

AREAS COVERED

This review aims to provide the latest information on ZPR, an 'atypical' agent for the pharmacological therapy of schizophrenia and bipolar disorder. A literature search has been carried out with the keywords 'ziprasidone', 'schizophrenia', 'psychosis', 'bipolar', 'pharmacokinetics' and 'clinical trials'. In this process, particular attention has been paid to the drug pharmacokinetic characteristics and its safety in clinical use.

EXPERT OPINION

ZPR shares most advantages and disadvantages with other atypical antipsychotics. However, it can be useful for its low tendency to cause metabolic syndrome and hyperprolactinaemia, especially in patients suffering from excess weight, hyperlipidaemia, diabetes or who have suffered from hyperprolactinaemia when using other antipsychotics. However, there are serious doubts as to whether ZPR should be administered to patients suffering from arrhythmias or QTc prolongation, and even more for administration to bipolar patients undergoing polypharmacy with antidepressants.

Hepatic microsomal thiol methyltransferase is involved in stereoselective methylation of pharmacologically active metabolite of prasugrel

Prasugrel, a thienopyridine antiplatelet drug, is converted in animals and humans to the pharmacologically active metabolite R-138727 [(2Z)-{1-[(1RS)-2-cyclopropyl-1-(2-fluorophenyl)-2-oxoethyl]-4-sulfanylpiperidin-3-ylidene}ethanoic acid], which has two chiral centers, occurring as a mixture of four isomers. The RS and RR isomers are more active than the SS and SR isomers (RS > RR > > SR = SS). The pharmacologically active metabolite is further metabolized to an S-methylated metabolite that is the major identified inactive metabolite in humans. In rat, dog, and human liver microsomes supplemented with S-adenosyl methione, the SS and SR isomers of the active metabolite were extensively S-methylated while the RS and RR isomers were not. Addition of 2,3-dichloromethyl benzylamine (50 µM) completely inhibited the S-methylation reaction, indicating that the microsomal and cytosolic thiol methyltransferase but not the cytosolic thiopurine S-methyltransferase is involved in the methylation. The hepatic intrinsic clearance values for methylation of the RS, RR, SS, and SR isomers (ml/min/kg) were 0, 0, 40.4, and 37.6, respectively, in rat liver microsomes, 0, 0, 11.6, and 2.5, respectively, in dog liver microsomes, and 0, 0, 17.3, and 17.7, respectively, in human liver microsomes, indicating that the RS and RR isomers are not methylated in vitro and that the methylation of SS and SR isomers is high with rat > human > dog. This finding in vitro agreed well with the in vivo observation in rats and dogs, where the S-methylated SS and SR isomers were the major metabolites in the plasma whereas negligible amounts of S-methylated RS and RR isomers were detected after intravenous administration of the pharmacologically active metabolites.