The letrozole phase 1 metabolite carbinol as a novel probe drug for UGT2B7

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Chemical Probes for Human UDP-Glucuronosyltransferases: A Comprehensive Review

UGTs play crucial roles in the metabolism and detoxification of both endogenous and xenobiotic compounds. The key roles of UGTs in human health have garnered great interest in the design and development of specific probes for human UGTs. However, in contrast to other human enzymes, the probe substrates for human UGTs are rarely reported, owing to the highly overlapping substrate specificities of UGTs and the lack of the integrated crystal structures of UGTs. Over the past decades, many efforts are made to develop specific probe substrates for UGTs and use them in both basic research and drug discovery. This review focuses on recent progress in the development of probe substrates for UGTs and their biomedical applications. A long list of chemical probes for UGTs, including non-fluorescent and fluorescent probes along with their structural information and kinetic parameters, are prepared and analyzed. Additionally, challenges and future directions in this field are highlighted in the final section. All information and knowledge presented in this review provide practical tools/methods for measuring UGT activities in complex biological samples, which will be very helpful for rapid screening and characterization of UGT modulators, and for exploring the relevance of UGT enzymes to human diseases.

The use of aromatase inhibitors in boys with short stature: what to know before prescribing?

Aromatase is a cytochrome P450 enzyme (CYP19A1 isoform) able to catalyze the conversion of androgens to estrogens. The aromatase gene mutations highlighted the action of estrogen as one of the main regulators of bone maturation and closure of bone plate. The use of aromatase inhibitors (AI) in boys with short stature has showed its capability to improve the predicted final height. Anastrozole (ANZ) and letrozole (LTZ) are nonsteroidal inhibitors able to bind reversibly to the heme group of cytochrome P450. In this review, we describe the pharmacokinetic profile of both drugs, discussing possible drug interactions between ANZ and LTZ with other drugs. AIs are triazolic compounds that can induce or suppress cytochrome P450 enzymes, interfering with metabolism of other compounds. Hydroxilation, N-dealkylation and glucoronidation are involved in the metabolism of AIs. Drug interactions can occur with azole antifungals, such as ketoconazole, by inhibiting CYP3A4 and by reducing the clearance of AIs. Antiepileptic drugs (lamotrigine, phenobarbital, and phenytoin) also inhibit aromatase. Concomitant use of phenobarbital or valproate has a synergistic effect on aromatase inhibition. Therefore, it is important to understand the pharmacokinetics of AIs, recognizing and avoiding possible drug interactions and offering a safer prescription profile of this class of aromatase inhibitors. Arch Endocrinol Metab. 2017;61(3):391-7.

Letrozole Determination by Capillary Zone Electrophoresis and UV Spectrophotometry Methods

Objective: Letrozole is a highly potent oral nonsteroidal aromatase inhibitor triazole derivative. The aim of this study was to quantify letrozole from bulk, pharmaceutical formulation, and spiked urine samples by developing a simple, rapid and cost effective capillary electrophoresis method. Methods: A capillary zone electrophoresis method was optimized and validated. Additionally, an UV spectrophotometry method was used for comparing results. Results:The capillary zone electrophoresis method using a 90 mM sodium tetraborate background electrolyte proved to be an efficient method for determination of letrozole in a very short time, less than 2 minutes, using 20 kV voltage, 50 mbar/2 seconds pressure and 50°C temperature as optimum parameters. Additionally, the UV spectrophotometry method proved to be simple and efficient to quantify letrozole from bulk material and pharmaceutical formulation with linearity of response between 5 to 20 μg·mL-1 concentrations. For both methods, validation parameters, including linearity, detection and quantification limits were determined. Also we proved that our electrophoretic method has potential in analyzing letrozole from biological samples, obtaining encouraging results on estimation of letrozole from spiked urine samples without any special treatment. Conclusions: To quantify letrozole from bulk material, pharmaceutical preparations, and spiked urine samples the capillary zone electrophoresis method using a tetraborate sodium background electrolyte has proven to be simple and appropriate. Also a simple UV spectrophotometric method has been developed and validated for the same purposes.

AM-2201 Inhibits Multiple Cytochrome P450 and Uridine 5'-Diphospho-Glucuronosyltransferase Enzyme Activities in Human Liver Microsomes

AM-2201 is a synthetic cannabinoid that acts as a potent agonist at cannabinoid receptors and its abuse has increased. However, there are no reports of the inhibitory effect of AM-2201 on human cytochrome P450 (CYP) or uridine 5′-diphospho-glucuronosyltransferase (UGT) enzymes. We evaluated the inhibitory effect of AM-2201 on the activities of eight major human CYPs (1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4) and six major human UGTs (1A1, 1A3, 1A4, 1A6, 1A9, and 2B7) enzymes in pooled human liver microsomes using liquid chromatography–tandem mass spectrometry to investigate drug interaction potentials of AM-2201. AM-2201 potently inhibited CYP2C9-catalyzed diclofenac 4′-hydroxylation, CYP3A4-catalyzed midazolam 1′-hydroxylation, UGT1A3-catalyzed chenodeoxycholic acid 24-acyl-glucuronidation, and UGT2B7-catalyzed naloxone 3-glucuronidation with IC₅₀ values of 3.9, 4.0, 4.3, and 10.0 µM, respectively, and showed mechanism-based inhibition of CYP2C8-catalyzed amodiaquine N-deethylation with a K_i value of 2.1 µM. It negligibly inhibited CYP1A2, CYP2A6, CYP2B6, CYP2C19, CYP2D6, UGT1A1, UGT1A4, UGT1A6, and UGT1A9 activities at 50 μM in human liver microsomes. These in vitro results indicate that AM-2201 needs to be examined for potential pharmacokinetic drug interactions in vivo due to its potent inhibition of CYP2C8, CYP2C9, CYP3A4, UGT1A3, and UGT2B7 enzyme activities.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.