Discovery of a Novel Microsomal Epoxide Hydrolase-Catalyzed Hydration of a Spiro Oxetane

Applications of oxetanes in drug discovery and medicinal chemistry

The compact and versatile oxetane motifs have gained significant attention in drug discovery and medicinal chemistry campaigns. This review presents an overview of the diverse applications of oxetanes in clinical and preclinical drug candidates targeting various human diseases, including cancer, viral infections, autoimmune disorders, neurodegenerative conditions, metabolic disorders, and others. Special attention is given to biologically active oxetane-containing compounds and their disease-related targets, such as kinases, epigenetic and non-epigenetic enzymes, and receptors. The review also details the effect of the oxetane motif on important properties, including aqueous solubility, lipophilicity, pKa, P-glycoprotein (P-gp) efflux, metabolic stability, conformational preferences, toxicity profiles (e.g., cytochrome P450 (CYP) suppression and human ether-a-go-go related gene (hERG) inhibition), pharmacokinetic (PK) properties, potency, and target selectivity. We anticipate that this work will provide valuable insights that can drive future discoveries of novel bioactive oxetane-containing small molecules, enabling their effective application in combating a wide range of human diseases.

Oxetanes in Drug Discovery Campaigns

The oxetane ring is an emergent, underexplored motif in drug discovery that shows attractive properties such as low molecular weight, high polarity, and marked three-dimensionality. Oxetanes have garnered further interest as isosteres of carbonyl groups and as molecular tools to fine-tune physicochemical properties of drug compounds such as pKa, LogD, aqueous solubility, and metabolic clearance. This perspective highlights recent applications of oxetane motifs in drug discovery campaigns (2017-2022), with emphasis on the effect of the oxetane on medicinally relevant properties and on the building blocks used to incorporate the oxetane ring. Based on this analysis, we provide an overview of the potential benefits of appending an oxetane to a drug compound, as well as potential pitfalls, challenges, and future directions.

Unusual Biotransformation Reactions of Drugs and Drug Candidates

Detailed assessment of the fate of drugs in nonclinical test species and humans is essential to ensure the safety and efficacy of medicines in patients. In this context, biotransformation of drugs and drug candidates has been an area of keen interest over many decades in the pharmaceutical industry as well as academia. Although many of the enzymes and biotransformation pathways involved in the metabolism of xenobiotics and more specifically drugs have been well characterized, each drug molecule is unique and constitutes specific challenges for the biotransformation scientist. In this mini-review written for the special issue on the occasion of the 50th Anniversary celebration of Drug Metabolism and Disposition and to celebrate contributions of F. Peter Guengerich, one of the pioneers of the drug metabolism field, recently reported "unusual" biotransformation reactions are presented. Scientific and technological advances in the "toolbox" of the biotransformation scientists are summarized. As the pharmaceutical industry continues to explore therapeutic modalities different from the traditional small molecule drugs, the new challenges confronting the biotransformation scientist as well as future opportunities are discussed. SIGNIFICANCE STATEMENT: For the biotransformation scientists, it is essential to share and be aware of unexpected biotransformation reactions so that they can increase their confidence in predicting metabolites of drugs in humans to ensure the safety and efficacy of these metabolites before the medicines reach large numbers of patients. The purpose of this review is to highlight recent observations of "unusual" metabolites so that the scientists working in the area of drug metabolism can strengthen their readiness in expecting the unexpected.

Rational enzyme design for enabling biocatalytic Baldwin cyclization and asymmetric synthesis of chiral heterocycles

Chiral heterocyclic compounds are needed for important medicinal applications. We report an in silico strategy for the biocatalytic synthesis of chiral N- and O-heterocycles via Baldwin cyclization modes of hydroxy- and amino-substituted epoxides and oxetanes using the limonene epoxide hydrolase from Rhodococcus erythropolis. This enzyme normally catalyzes hydrolysis with formation of vicinal diols. Firstly, the required shutdown of the undesired natural water-mediated ring-opening is achieved by rational mutagenesis of the active site. In silico enzyme design is then continued with generation of the improved mutants. These variants prove to be versatile catalysts for preparing chiral N- and O-heterocycles with up to 99% conversion, and enantiomeric ratios up to 99:1. Crystal structural data and computational modeling reveal that Baldwin-type cyclizations, catalyzed by the reprogrammed enzyme, are enabled by reshaping the active-site environment that directs the distal RHN and HO-substituents to be intramolecular nucleophiles.

Meeting report of the second European Biotransformation Workshop

Challenges and opportunities in the field of biotransformation were presented and discussed at the 2nd European Biotransformation workshop which was conducted virtually in collaboration with the DMDG on November 24/25, 2021. Here we summarise the presentations and discussions from this workshop.The following topics were covered:Regulatory requirements and biotransformation studies for antibody drug conjugates (ADCs) and antisense oligonucleotides (ASOs)Solutions for mass spectral data processing of peptides and oligonucleotidesFuture outsourcing needs in biotransformation for new modalitiesEstablished quantitative and qualitative workflows for metabolite identificationNew in vitro systems to study new chemical entities (NCEs) with low metabolic turnoverNew strategies on the timing of the human ADME (absorption, distribution, metabolism, excretion) study and to investigate the impact of human microbiome on drug development.

Put a ring on it: application of small aliphatic rings in medicinal chemistry

Aliphatic three- and four-membered rings including cyclopropanes, cyclobutanes, oxetanes, azetidines and bicyclo[1.1.1]pentanes have been increasingly exploited in medicinal chemistry for their beneficial physicochemical properties and applications as functional group bioisosteres. This review provides a historical perspective and comparative up to date overview of commonly applied small rings, exemplifying key principles with recent literature examples. In addition to describing the merits and advantages of each ring system, potential hazards and liabilities are also illustrated and explained, including any significant chemical or metabolic stability and toxicity risks.

Spirocyclic Scaffolds in Medicinal Chemistry

Spirocyclic scaffolds are incorporated in various approved drugs and drug candidates. The increasing interest in less planar bioactive compounds has given rise to the development of synthetic methodologies for the preparation of spirocyclic scaffolds. In this Perspective, we summarize the diverse synthetic routes to obtain spirocyclic systems. The impact of spirocycles on potency and selectivity, including the aspect of stereochemistry, is discussed. Furthermore, we examine the changes in physicochemical properties as well as in in vitro and in vivo ADME using selected studies that compare spirocyclic compounds to their nonspirocyclic counterparts. In conclusion, the value of spirocyclic scaffolds in medicinal chemistry is discussed.

Novel advances in biotransformation and bioactivation research-2019 year in review

Biotransformation is one of the main mechanisms used by the body to eliminate drugs. As drug molecules become more complicated, the involvement of drug metabolizing enzymes increases beyond those that are typically studied, such as the cytochrome P450 enzymes. In this review, we try to capture the many outstanding articles that were published in the past year in the field of biotransformation (see Table 1). We have divided the articles into two categories of (1) metabolites and drug metabolizing enzymes, and (2) bioactivation and safety.  This annual review is the fifth of its kind since 2016 (Baillie et al. 2016; Khojasteh et al. 2017, 2018, 2019). This effort in itself also continues to evolve. We have followed the same format we used in previous years in terms of the selection of articles and the authoring of each section. I am pleased of the continued support of Rietjens, Miller, Zhang, Driscoll and Mitra to this review. We would like to welcome Klarissa D. Jackson as a new author 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.

Metabolism of Strained Rings: Glutathione S-transferase-Catalyzed Formation of a Glutathione-Conjugated Spiro-azetidine without Prior Bioactivation

AZD1979 [(3-(4-(2-oxa-6-azaspiro[3.3]heptan-6-ylmethyl)phenoxy)azetidin-1-yl)(5-(4-methoxyphenyl)-1,3,4-oxadiazol-2-yl)methanone] is a melanin-concentrating hormone receptor 1 antagonist designed for the treatment of obesity. In this study, metabolite profiles of AZD1979 in human hepatocytes revealed a series of glutathione-related metabolites, including the glutathionyl, cysteinyl, cysteinylglycinyl, and mercapturic acid conjugates. The formation of these metabolites was not inhibited by coincubation with the cytochrome P450 (P450) inhibitor 1-aminobenzotriazole. In efforts to identify the mechanistic features of this pathway, investigations were performed to characterize the structure of the glutathionyl conjugate M12 of AZD1979 and to identify the enzyme system catalyzing its formation. Studies with various human liver subcellular fractions established that the formation of M12 was NAD(P)H-independent and proceeded in cytosol and S9 fractions but not in microsomal or mitochondrial fractions. The formation of M12 was inhibited by ethacrynic acid, an inhibitor of glutathione S-transferases (GSTs). Several human recombinant GSTs, including GSTA1, A2-2, M1a, M2-2, T1-1, and GST from human placenta, were incubated with AZD1979. All GSTs tested catalyzed the formation of M12, with GSTA2-2 being the most efficient. Metabolite M12 was purified from rat liver S9 incubations and its structure elucidated by NMR. These results establish that M12 is the product of the GST-catalyzed glutathione attack on the carbon atom α to the nitrogen atom of the strained spiro-azetidinyl moiety to give, after ring opening, the corresponding amino-thioether conjugate product, a direct conjugation pathway that occurs without the prior substrate bioactivation by P450. SIGNIFICANCE STATEMENT: The investigated compound, AZD1979, contains a 6-substituted-2-oxa-6-azaspiro[3.3]heptanyl derivative that is an example of strained heterocycles, including spiro-fused ring systems, that are widely used in synthetic organic chemistry. An unusual azetidinyl ring-opening reaction involving a nucleophilic attack by glutathione, which does not involve prior cytochrome P450-catalyzed bioactivation of the substrate and which is catalyzed by glutathione transferases, is reported. We propose a mechanism involving the protonated cyclic aminyl intermediate that undergoes nucleophilic attack by glutathione thiolate anion in this reaction, catalyzed by glutathione transferases.

Biotransformation and bioactivation reactions - 2017 literature highlights *

This annual review is the third one to highlight recent advances in the study and assessment of biotransformations and bioactivations ( Table 1 ). We followed the same format as the previous years with selection and authoring each section (see Baillie et al. 2016 ; Khojasteh et al. 2017 ). We acknowledge that many universities no longer train students in mechanistic biotransformation studies reflecting a decline in the investment for those efforts by public funded granting institutions. We hope this work serves as a resource to appreciate the knowledge gained each year to understand and hopefully anticipate toxicological outcomes dependent on biotransformations and bioactivations. This effort itself also continues to evolve. I am pleased that Drs. Rietjens and Miller have again contributed to this annual review. We would like to welcome Kaushik Mitra as an author for this year's issue, and we thank Deepak Dalvie for his contributions to last year's edition. We have intentionally maintained a balance of authors such that two come from an academic setting and two come from industry. As always, please drop us a note if you find this review helpful. 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.

Oxetane Substrates of Human Microsomal Epoxide Hydrolase

Oxetanyl building blocks are increasingly used in drug discovery because of the improved drug-like properties they confer on drug candidates, yet little is currently known about their biotransformation. A series of oxetane-containing analogs was studied and we provide the first direct evidence of oxetane hydrolysis by human recombinant microsomal epoxide hydrolase (mEH). Incubations with human liver fractions and hepatocytes were performed with and without inhibitors of cytochrome P450 (P450), mEH and soluble epoxide hydrolase (sEH). Reaction dependence on NADPH was investigated in subcellular fractions. A full kinetic characterization of oxetane hydrolysis is presented, in both human liver microsomes and human recombinant mEH. In human liver fractions and hepatocytes, hydrolysis by mEH was the only oxetane ring-opening metabolic route, with no contribution from sEH or from cytochrome P450-catalyzed oxidation. Minimally altering the structural elements in the immediate vicinity of the oxetane can greatly modulate the efficiency of hydrolytic ring cleavage. In particular, higher pK_a in the vicinity of the oxetane and an increased distance between the oxetane ring and the benzylic nitrogen improve reaction rate, which is further enhanced by the presence of methyl groups near or on the oxetane. This work defines oxetanes as the first nonepoxide class of substrates for human mEH, which was previously known to catalyze the hydrolytic ring opening of electrophilic and potentially toxic epoxide-containing drugs, drug metabolites, and exogenous organochemicals. These findings will be of value for the development of biologically active oxetanes and may be exploited for the biocatalytic generation of enantiomerically pure oxetanes and diols.