Deeper Insight into the Reducing Biotransformation of Bupropion in the Human Liver

Stereoselective Metabolism of Bupropion to Active Metabolites in Cellular Fractions of Human Liver and Intestine

Striking stereoselective disposition of the antidepressant and smoking cessation aid bupropion (BUP) and its active metabolites observed clinically influence patients' response to BUP therapy and its clinically important drug-drug interactions (DDI) with CYP2D6 substrates. However, understanding of the biochemical mechanisms responsible is incomplete. This study comprehensively examined hepatic and extrahepatic stereoselective metabolism of BUP in vitro Racemic-, R-, and S-BUP were incubated separately with pooled cellular fractions of human liver [microsomes (HLMs), S9 fractions (HLS9s), and cytosols (HLCs)] and intestinal [microsomes (HIMs), S9 fractions (HIS9s), and cytosols (HICs)] and cofactors. Formations of diastereomers of 4-hydroxyBUP (OHBUP), threohydroBUP (THBUP), and erythrohydroBUP (EHBUP) were quantified using a novel chiral ultra-high performance liquid chromatography/tandem mass spectrometry method. Racemic BUP (but not R- or S-BUP) was found suitable to determine stereoselective metabolism of BUP; both enantiomers showed complete racemization. Compared with that of RR-THBUP, the in vitro intrinsic clearance (Clint) for the formation of SS-THBUP was 42-, 19-, and 8.3-fold higher in HLMs, HLS9 fractions, and HLCs, respectively; Clint for the formation of SS-OHBUP and RS-EHBUP was also higher (2.7- to 3.9-fold) than their R-derived counterparts. In cellular fractions of human intestine, ≥ 95% of total reduction was accounted by the formation of RR-THBUP. Ours is the first to demonstrate marked stereoselective reduction of BUP in HLCs, HIMs, HIS9 fractions, and HICs, providing the first evidence for tissue- and cellular fraction-dependent stereoselective metabolism of BUP. These data may serve as the first critical step toward understanding factors dictating BUP's stereoselective disposition, effects, and DDI risks. SIGNIFICANCE STATEMENT: This work provides a deeper insight into bupropion (BUP) stereoselective oxidation and reduction to active metabolites in cellular fractions of human liver and intestine tissues. The results demonstrate tissue- and cellular fraction-dependent stereospecific metabolism of BUP. These data may improve prediction of BUP stereoselective disposition and understanding of BUP's effects and CYP2D6-dependent drug-drug interaction in vivo.

Bruton’s Tyrosine Kinase Inhibitor Zanubrutinib Effectively Modulates Cancer Resistance by Inhibiting Anthracycline Metabolism and Efflux

Zanubrutinib (ZAN) is a Bruton’s tyrosine kinase inhibitor recently approved for the treatment of some non-Hodgkin lymphomas. In clinical trials, ZAN is often combined with standard anthracycline (ANT) chemotherapy. Although ANTs are generally effective, drug resistance is a crucial obstacle that leads to treatment discontinuation. This study showed that ZAN counteracts ANT resistance by targeting aldo-keto reductase 1C3 (AKR1C3) and ATP-binding cassette (ABC) transporters. AKR1C3 catalyses the transformation of ANTs to less potent hydroxy-metabolites, whereas transporters decrease the ANT-effective concentrations by pumping them out of the cancer cells. In our experiments, ZAN inhibited the AKR1C3-mediated inactivation of daunorubicin (DAUN) at both the recombinant and cellular levels. In the drug combination experiments, ZAN synergistically sensitised AKR1C3-expressing HCT116 and A549 cells to DAUN treatment. Gene induction studies further confirmed that ZAN did not increase the intracellular level of AKR1C3 mRNA; thus, the drug combination effect is not abolished by enzyme induction. Finally, in accumulation assays, ZAN was found to interfere with the DAUN efflux mediated by the ABCB1, ABCG2, and ABCC1 transporters, which might further contribute to the reversal of ANT resistance. In summary, our data provide the rationale for ZAN inclusion in ANT-based therapy and suggest its potential for the treatment of tumours expressing AKR1C3 and/or the above-mentioned ABC transporters.

Pregnancy Has No Clinically Significant Effect on the Pharmacokinetics of Bupropion or Its Metabolites

BACKGROUND

Bupropion (BUP) is a chiral antidepressant and smoking cessation aide with benefits and side effects correlated with parent and active metabolite concentrations. BUP is metabolized by CYP2B6, CYP2C19, and CYP3A4 to hydroxy-BUP (OH-BUP) as well as by 11β-hydroxysteroid dehydrogenase-1 and aldo-keto reductases to threohydrobupropion (Threo) and erythrohydrobupropion (Erythro), respectively. As pregnancy alters the activity of drug-metabolizing enzymes, the authors hypothesized that BUP metabolism and BUP metabolite concentrations would be altered during pregnancy, potentially affecting the efficacy and safety of BUP in pregnant women.

METHODS

Pregnant women (n = 8) taking BUP chronically were enrolled, and steady-state plasma samples and dosing interval urine samples were collected during pregnancy and postpartum. Maternal and umbilical cord venous blood samples were collected at delivery from 3 subjects, and cord blood/maternal plasma concentration ratios were calculated. The concentrations of BUP stereoisomers and their metabolites were measured. Paired t tests were used to compare pharmacokinetic parameters during pregnancy and postpartum.

RESULTS

No significant changes were observed in the steady-state plasma concentrations, metabolite to parent ratios, formation clearances, or renal clearance of any of the compounds during pregnancy when compared with postpartum. The umbilical cord venous plasma concentrations of BUP and its metabolites were 30%-60% lower than maternal plasma concentrations.

CONCLUSIONS

This study showed that there are no clinically meaningful differences in the stereoselective disposition of BUP or its metabolites during pregnancy, indicating that dose adjustment during pregnancy may not be necessary. The results also showed that the placenta provides a partial barrier for bupropion and its metabolite distribution to the fetus, with possible placental efflux transport of bupropion and its metabolites.

Overview of human 20 alpha-hydroxysteroid dehydrogenase (AKR1C1): Functions, regulation, and structural insights of inhibitors

Human aldo-keto reductase family 1C1 (AKR1C1) is an important enzyme involved in human hormone metabolism, which is mainly responsible for the metabolism of progesterone in the human body. AKR1C1 is highly expressed and has an important relationship with the occurrence and development of various diseases, especially some cancers related to hormone metabolism. Nowadays, many inhibitors against AKR1C1 have been discovered, including some synthetic compounds and natural products, which have certain inhibitory activity against AKR1C1 at the target level. Here we briefly reviewed the physiological and pathological functions of AKR1C1 and the relationship with the disease, and then summarized the development of AKR1C1 inhibitors, elucidated the interaction between inhibitors and AKR1C1 through molecular docking results and existing co-crystal structures. Finally, we discussed the design ideals of selective AKR1C1 inhibitors from the perspective of AKR1C1 structure, discussed the prospects of AKR1C1 in the treatment of human diseases in terms of biomarkers, pre-receptor regulation and single nucleotide polymorphisms, aiming to provide new ideas for drug research targeting AKR1C1.

Inhibition of AKR1B10-mediated metabolism of daunorubicin as a novel off-target effect for the Bcr-Abl tyrosine kinase inhibitor dasatinib

Bcr-Abl tyrosine kinase inhibitors significantly improved Philadelphia chromosome-positive leukaemia therapy. Apart from Bcr-Abl kinase, imatinib, dasatinib, nilotinib, bosutinib and ponatinib are known to have additional off-target effects that might contribute to their antitumoural activities. In our study, we identified aldo-keto reductase 1B10 (AKR1B10) as a novel target for dasatinib. The enzyme AKR1B10 is upregulated in several cancers and influences the metabolism of chemotherapy drugs, including anthracyclines. AKR1B10 reduces anthracyclines to alcohol metabolites that show less antineoplastic properties and tend to accumulate in cardiac tissue. In our experiments, clinically achievable concentrations of dasatinib selectively inhibited AKR1B10 both in experiments with recombinant enzyme (Ki = 0.6 µM) and in a cellular model (IC₅₀ = 0.5 µM). Subsequently, the ability of dasatinib to attenuate AKR1B10-mediated daunorubicin (Daun) resistance was determined in AKR1B10-overexpressing cells. We have demonstrated that dasatinib can synergize with Daun in human cancer cells and enhance its therapeutic effectiveness. Taken together, our results provide new information on how dasatinib may act beyond targeting Bcr-Abl kinase, which may help to design new chemotherapy regimens, including those with anthracyclines.

Metabolic-Hydroxy and Carboxy Functionalization of Alkyl Moieties in Drug Molecules: Prediction of Structure Influence and Pharmacologic Activity

Alkyl moieties-open chain or cyclic, linear, or branched-are common in drug molecules. The hydrophobicity of alkyl moieties in drug molecules is modified by metabolic hydroxy functionalization via free-radical intermediates to give primary, secondary, or tertiary alcohols depending on the class of the substrate carbon. The hydroxymethyl groups resulting from the functionalization of methyl groups are mostly oxidized further to carboxyl groups to give carboxy metabolites. As observed from the surveyed cases in this review, hydroxy functionalization leads to loss, attenuation, or retention of pharmacologic activity with respect to the parent drug. On the other hand, carboxy functionalization leads to a loss of activity with the exception of only a few cases in which activity is retained. The exceptions are those groups in which the carboxy functionalization occurs at a position distant from a well-defined primary pharmacophore. Some hydroxy metabolites, which are equiactive with their parent drugs, have been developed into ester prodrugs while carboxy metabolites, which are equiactive to their parent drugs, have been developed into drugs as per se. In this review, we present and discuss the above state of affairs for a variety of drug classes, using selected drug members to show the effect on pharmacologic activity as well as dependence of the metabolic change on drug molecular structure. The review provides a basis for informed predictions of (i) structural features required for metabolic hydroxy and carboxy functionalization of alkyl moieties in existing or planned small drug molecules, and (ii) pharmacologic activity of the metabolites resulting from hydroxy and/or carboxy functionalization of alkyl moieties.

Transport of Bupropion and its Metabolites by the Model CHO and HEK293 Cell Lines

BACKGROUND

Bupropion (BUP) is widely used as an antidepressant and smoking cessation aid. There are three major pharmacologically active metabolites of BUP, Erythrohydrobupropion (EB), Hydroxybupropion (OHB) and Threohydrobupropion (TB). At present, the mechanisms underlying the overall disposition and systemic clearance of BUP and its metabolites have not been well understood, and the role of transporters has not been studied.

OBJECTIVE

The goal of this study was to investigate whether BUP and its active metabolites are substrates of the major hepatic uptake and efflux transporters.

METHOD

CHO or HEK293 cell lines or plasma membrane vesicles that overexpress OATP1B1, OATP1B3, OATP2B1, OATP4A1, OCT1, BCRP, MRP2 or P-gp were used in cellular or vesicle uptake and inhibition assays. Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) was used to quantify transport activity.

RESULTS

BUP and its major active metabolites were actively transported into the CHO or HEK293 cells overexpressing OATP1B1, OATP1B3 or OATP2B1; however, such cellular active uptake could not be inhibited at all by prototypical inhibitors of any of the OATP transporters. These compounds were not transported by OCT1, BCRP, MRP2 or P-gp either. These results suggest that the major known hepatic transporters likely play a minor role in the overall disposition and systemic clearance of BUP and its active metabolites in humans. We also demonstrated that BUP and its metabolites were not transported by OATP4A1, an uptake transporter on the apical membrane of placental syncytiotrophoblasts, suggesting that OATP4A1 is not responsible for the transfer of BUP and its metabolites from the maternal blood to the fetal compartment across the placental barrier in pregnant women.

CONCLUSION

BUP and metabolites are not substrates of the major hepatic transporters tested and thus these hepatic transporters likely do not play a role in the overall disposition of the drug. Our results also suggest that caution should be taken when using the model CHO and HEK293 cell lines to evaluate potential roles of transporters in drug disposition.

Comparison of In Vitro Stereoselective Metabolism of Bupropion in Human, Monkey, Rat, and Mouse Liver Microsomes

BACKGROUND AND OBJECTIVES

Bupropion is an atypical antidepressant and smoking cessation aid associated with wide intersubject variability. This study compared the formation kinetics of three phase I metabolites (hydroxybupropion, threohydrobupropion, and erythrohydrobupropion) in human, marmoset, rat, and mouse liver microsomes. The objective was to establish suitability and limitations  for subsequent use of nonclinical species to model bupropion central nervous system (CNS) disposition in humans.

METHODS

Hepatic microsomal incubations were conducted separately for the R- and S-bupropion enantiomers, and the formation of enantiomer-specific metabolites was determined using LC-MS/MS. Intrinsic formation clearance (CL_int) of metabolites across the four species was determined from the formation rate versus substrate concentration relationship.

RESULTS

The total clearance of S-bupropion was higher than that of R-bupropion in monkey and human liver microsomes. The contribution of hydroxybupropion to the total racemic bupropion clearance was 38%, 62%, 17%, and 96% in human, monkey, rat, and mouse, respectively.  In the same species order, threohydrobupropion contributed 53%, 23%, 17%, and 3%, and erythrohydrobupropion contributed 9%, 14%, 66%, and 1.3%, respectively, to racemic bupropion clearance.

CONCLUSION

The results demonstrate that phase I metabolism in monkeys best approximates that observed in humans, and support the preferred use of this species to investigate possible pharmacokinetic factors that influence the CNS disposition of bupropion and contribute to its high intersubject variability.

Cyclin-dependent kinase inhibitors AZD5438 and R547 show potential for enhancing efficacy of daunorubicin-based anticancer therapy: Interaction with carbonyl-reducing enzymes and ABC transporters

Daunorubicin (DAUN) has served as an anticancer drug in chemotherapy regimens for decades and is still irreplaceable in treatment of acute leukemias. The therapeutic outcome of DAUN-based therapy is compromised by its cardiotoxicity and emergence of drug resistance. This phenomenon is often caused by pharmacokinetic mechanisms such as efflux of DAUN from cancer cells through ATP-binding cassette (ABC) transporters and its conversion to less cytostatic but more cardiotoxic daunorubicinol (DAUN-OL) by carbonyl reducing enzymes (CREs). Here we aimed to investigate, whether two cyclin-dependent kinase inhibitors, AZD5438 and R547, can interact with these pharmacokinetic mechanisms and reverse DAUN resistance. Using accumulation assays, we revealed AZD5438 as potent inhibitor of ABCC1 showing also weaker inhibitory effect to ABCB1 and ABCG2. Combination index analysis, however, shown that inhibition of ABCC1 does not significantly contribute to synergism between AZD5438 and DAUN in MDCKII-ABCC1 cells, suggesting predominant role of other mechanism. Using pure recombinant enzymes, we found both tested drugs to inhibit CREs with aldo-keto reductase 1C3 (AKR1C3). This interaction was further confirmed in transfected HCT-116 cells. Moreover, these cells were sensitized to DAUN by both compounds as Chou-Talalay combination index analysis showed synergism in AKR1C3 transfected HCT-116, but not in empty vector transfected control cell line. In conclusion, we propose AZD5438 and R547 as modulators of DAUN resistance that can prevent AKR1C3-mediated DAUN biotransformation to DAUN-OL. This interaction could be beneficially exploited to prevent failure of DAUN-based therapy as well as the undesirable cardiotoxic effect of DAUN-OL.

The hop-derived compounds xanthohumol, isoxanthohumol and 8-prenylnaringenin are tight-binding inhibitors of human aldo-keto reductases 1B1 and 1B10

Xanthohumol (XN), a prenylated chalcone unique to hops (Humulus lupulus) and two derived prenylflavanones, isoxanthohumol (IX) and 8-prenylnaringenin (8-PN) gained increasing attention as potential anti-diabetic and cancer preventive compounds. Two enzymes of the aldo-keto reductase (AKR) superfamily are notable pharmacological targets in cancer therapy (AKR1B10) and in the treatment of diabetic complications (AKR1B1). Our results show that XN, IX and 8-PN are potent uncompetitive, tight-binding inhibitors of human aldose reductase AKR1B1 (Ki = 15.08 μM, 0.34 μM, 0.71 μM) and of human AKR1B10 (Ki = 20.11 μM, 2.25 μM, 1.95 μM). The activity of the related enzyme AKR1A1 was left unaffected by all three compounds. This is the first time these three substances have been tested on AKRs. The results of this study may provide a basis for further quantitative structure?activity relationship models and promising scaffolds for future anti-diabetic or carcinopreventive drugs.

Selective Inhibition of Human AKR1B10 by n-Humulone, Adhumulone and Cohumulone Isolated from Humulus lupulus Extract

Hop-derived compounds have been subjected to numerous biomedical studies investigating their impact on a wide range of pathologies. Isomerised bitter acids (isoadhumulone, isocohumulone and isohumulone) from hops, used in the brewing process of beer, are known to inhibit members of the aldo-keto-reductase superfamily. Aldo-keto-reductase 1B10 (AKR1B10) is upregulated in various types of cancer and has been reported to promote carcinogenesis. Inhibition of AKR1B10 appears to be an attractive means to specifically treat RAS-dependent malignancies. However, the closely related reductases AKR1A1 and AKR1B1, which fulfil important roles in the detoxification of endogenous and xenobiotic carbonyl compounds oftentimes crossreact with inhibitors designed to target AKR1B10. Accordingly, there is an ongoing search for selective AKR1B10 inhibitors that do not interact with endogeneous AKR1A1 and AKR1B1-driven detoxification systems. In this study, unisomerised α-acids (adhumulone, cohumulone and n-humulone) were separated and tested for their inhibitory potential on AKR1A1, AKR1B1 and AKR1B10. Also AKR1B10-mediated farnesal reduction was effectively inhibited by α-acid congeners with Ki-values ranging from 16.79 ± 1.33 µM (adhumulone) to 3.94 ± 0.33 µM (n-humulone). Overall, α-acids showed a strong inhibition with selectivity (115⁻137 fold) for AKR1B10. The results presented herein characterise hop-derived α-acids as a promising basis for the development of novel and selective AKR1B10-inhibitors.

The identification of new substrates of human DHRS7 by molecular modeling and in vitro testing

Human DHRS7 (SDR34C1) is one of insufficiently described enzymes of the short-chain dehydrogenase/reductase superfamily. The members of this superfamily often play an important pato/physiological role in the human body, participating in the metabolism of diverse substrates (e.g. retinoids, steroids, xenobiotics). A systematic approach to the identification of novel, physiological substrates of DHRS7 based on a combination of homology modeling, structure-based virtual screening and experimental evaluation has been used. Three novel substrates of DHRS7 (dihydrotestosterone, benzil and 4,4'-dimetylbenzil) have been described.

Pharmacokinetics and Pharmacogenomics of Bupropion in Three Different Formulations with Different Release Kinetics in Healthy Human Volunteers

The purpose of this pharmacokinetics (PK) study was to investigate whether different release kinetics from bupropion hydrochloride (HCl) immediate release (IR), sustained release (SR), and extended release (ER) formulations alter its metabolism and to test the hypothesis that the unsuccessful bioequivalence (BE) study of the higher strength (300 mg) of bupropion HCl ER tablets based on the successful BE study of the lower strength (150 mg) was due to metabolic saturation in the gastrointestinal (GI) lumen. A randomized six-way crossover study was conducted in healthy volunteers. During each period, subjects took a single dose of IR (75/100 mg), SR (100/150 mg), or ER (150/300 mg) formulations of bupropion HCl; plasma samples for PK analysis were collected from 0-96 h for all formulations. In addition, each subject's whole blood was collected for the genotyping of various single-nucleotide polymorphisms (SNPs) of bupropion's major metabolic enzymes. The data indicates that the relative bioavailability of the ER formulations was 72.3-78.8% compared with IR 75 mg. No differences were observed for ratio of the area under the curve (AUC) of metabolite to AUC of parent for the three major metabolites. The pharmacogenomics analysis suggested no statistically significant correlation between polymorphisms and PK parameters of the various formulations. Altogether, these data suggested that the different release kinetics of the formulations did not change metabolites-to-parent ratio. Therefore, the differing BE result between the 150 and 300 mg bupropion HCl ER tablets was unlikely due to the metabolic saturation in the GI lumen caused by different release patterns.

Reductive metabolism of tiaprofenic acid by the human liver and recombinant carbonyl reducing enzymes

Tiaprofenic acid is a widely used anti-inflammatory drug; however, the reductive metabolism of tiaprofenic acid is not yet well understood. Here, we compared the reduction of tiaprofenic acid in microsomes and cytosol from the human liver. The microsomes exhibited lower K_m value toward tiaprofenic acid than the cytosol (K_m = 164 ± 18 μM vs. 569 ± 74 μM, respectively), whereas the cytosol showed higher specific activity during reduction than the microsomes (V_max = 728 ± 52 pmol mg of protein^-1 min^-1 vs. 285 ± 11 pmol mg of protein^-1 min^-1, respectively). Next, a panel of recombinant carbonyl reducing enzymes from AKR and SDR superfamilies has been studied to find the enzymes responsible for the cytosolic reduction of tiaprofenic acid. CBR1 was identified as the reductase of tiaprofenic acid with high specific activity (56,965 ± 6741 pmol mg of protein^-1 min^-1). Three other enzymes, AKR1A1, AKR1B10, and AKR1C4, were also able to reduce tiaprofenic acid, but with very low activity. Thus, CBR1 was shown to be a tiaprofenic acid reductase in vitro and was also suggested to be the principal tiaprofenic acid reductase in vivo.

Identification of non-reported bupropion metabolites in human plasma

Bupropion and its three active metabolites exhibit clinical efficacy in the treatment of major depression, seasonal depression and smoking cessation. The pharmacokinetics of bupropion in humans is highly variable. It is not known if there are any non‐reported metabolites formed in humans in addition to the three known active metabolites. This paper reports newly identified and non‐reported metabolites of bupropion in human plasma samples. Human subjects were dosed with a single oral dose of 75 mg of an immediate release bupropion HCl tablet. Plasma samples were collected and analysed by LC–MS/MS at 0, 6 and 24 h. Two non‐reported metabolites (M1 and M3) were identified with mass‐to‐charge (m/z) ratios of 276 (M1, hydration of bupropion) and 258 (M3, hydroxylation of threo/erythrohydrobupropion) from human plasma in addition to the known hydroxybupropion, threo/erythrohydrobupropion and the glucuronidation products of the major metabolites (M2 and M4–M7). These new metabolites may provide new insight and broaden the understanding of bupropion's variability in clinical pharmacokinetics. © 2016 The Authors Biopharmaceutics & Drug Disposition Published by John Wiley & Sons Ltd.

Stereoselective Metabolism of Bupropion to OH-bupropion, Threohydrobupropion, Erythrohydrobupropion, and 4'-OH-bupropion in vitro

Bupropion is a widely used antidepressant, smoking cessation aid, and weight-loss therapy. It is administered as a racemic mixture, but the pharmacokinetics and activity of bupropion are stereoselective. The activity and side effects of bupropion are attributed to bupropion and its metabolites S,S- and R,R-OH-bupropion, threohydrobupropion, and erythrohydrobupropion. Yet the stereoselective metabolism in vitro and the enzymes contributing to the stereoselective disposition of bupropion have not been characterized. In humans, the fraction of bupropion metabolized (fm) to the CYP2B6 probe metabolite OH-bupropion is 5-16%, but ticlopidine increases bupropion exposure by 61%, suggesting a 40% CYP2B6 and/or CYP2C19 fm for bupropion. Yet, the CYP2C19 contribution to bupropion clearance has not been defined, and the enzymes contributing to overall bupropion metabolite formation have not been fully characterized. The aim of this study was to characterize the stereoselective metabolism of bupropion in vitro to explain the stereoselective pharmacokinetics and the effect of drug-drug interactions (DDIs) and CYP2C19 pharmacogenetics on bupropion exposure. The data predict that threohydrobupropion accounts for 50 and 82%, OH-bupropion for 34 and 12%, erythrohydrobupropion for 8 and 4%, and 4'-OH-bupropion for 8 and 2% of overall R- and S-bupropion clearance, respectively. The fm,CYP2B6 was predicted to be 21%, and the fm,CYP2C19, 6% for racemic bupropion. Importantly, ticlopidine was found to inhibit all metabolic pathways of bupropion in vitro, including threohydrobupropion, erythrohydrobupropion, and 4'OH-bupropion formation, explaining the in vivo DDI. The stereoselective pharmacokinetics of bupropion were quantitatively explained by the in vitro metabolic clearances and in vivo interconversion between bupropion stereoisomers.

Stereoselective method to quantify bupropion and its three major metabolites, hydroxybupropion, erythro-dihydrobupropion, and threo-dihydrobupropion using HPLC-MS/MS

Bupropion metabolites formed via oxidation and reduction exhibit pharmacological activity, but little is known regarding their stereoselective disposition. A novel stereoselective liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to separate and quantify enantiomers of bupropion, 4-hydroxybupropion, and erythro- and threo-dihydrobupropion. Liquid-liquid extraction was implemented to extract all analytes from 50 μL human plasma. Acetaminophen (APAP) was used as an internal standard. The analytes were separated on a Lux 3 μ Cellulose-3 250×4.6 mm column by methanol: acetonitrile: ammonium bicarbonate: ammonium hydroxide gradient elution and monitored using an ABSciex 5500 QTRAP triple-quadrupole mass spectrometer equipped with electrospray ionization probe in positive mode. Extraction efficiency for all analytes was ≥70%. The stability at a single non-extracted concentration for over 48 h at ambient temperature resulted in less than 9.8% variability for all analytes. The limit of quantification (LOQ) for enantiomers of bupropion and 4-hydroxybupropion was 0.3 ng/mL, while the LOQ for enantiomers of erythro- and threo-hydrobupropion was 0.15 ng/mL. The intra-day precision and accuracy estimates for enantiomers of bupropion and its metabolites ranged from 3.4% to 15.4% and from 80.6% to 97.8%, respectively, while the inter-day precision and accuracy ranged from 6.1% to 19.9% and from 88.5% to 99.9%, respectively. The current method was successfully implemented to determine the stereoselective pharmacokinetics of bupropion and its metabolites in 3 healthy volunteers administered a single 100mg oral dose of racemic bupropion. This novel, accurate, and precise HPLC-MS/MS method should enhance further research into bupropion stereoselective metabolism and drug interactions.

Stereoselective Glucuronidation of Bupropion Metabolites In Vitro and In Vivo

Bupropion is a widely used antidepressant and smoking cessation aid in addition to being one of two US Food and Drug Administration-recommended probe substrates for evaluation of cytochrome P450 2B6 activity. Racemic bupropion undergoes oxidative and reductive metabolism, producing a complex profile of pharmacologically active metabolites with relatively little known about the mechanisms underlying their elimination. A liquid chromatography-tandem mass spectrometry assay was developed to simultaneously separate and detect glucuronide metabolites of (R,R)- and (S,S)-hydroxybupropion, (R,R)- and (S,S)-hydrobupropion (threo) and (S,R)- and (R,S)-hydrobupropion (erythro), in human urine and liver subcellular fractions to begin exploring mechanisms underlying enantioselective metabolism and elimination of bupropion metabolites. Human liver microsomal data revealed marked glucuronidation stereoselectivity [Cl(int), 11.4 versus 4.3 µl/min per milligram for the formation of (R,R)- and (S,S)-hydroxybupropion glucuronide; and Cl(max), 7.7 versus 1.1 µl/min per milligram for the formation of (R,R)- and (S,S)-hydrobupropion glucuronide], in concurrence with observed enantioselective urinary elimination of bupropion glucuronide conjugates. Approximately 10% of the administered bupropion dose was recovered in the urine as metabolites with glucuronide metabolites, accounting for approximately 40%, 15%, and 7% of the total excreted hydroxybupropion, erythro-hydrobupropion, and threo-hydrobupropion, respectively. Elimination pathways were further characterized using an expressed UDP-glucuronosyl transferase (UGT) panel with bupropion enantiomers (both individual and racemic) as substrates. UGT2B7 catalyzed the stereoselective formation of glucuronides of hydroxybupropion, (S,S)-hydrobupropion, (S,R)- and (R,S)-hydrobupropion; UGT1A9 catalyzed the formation of (R,R)-hydrobupropion glucuronide. These data systematically describe the metabolic pathways underlying bupropion metabolite disposition and significantly expand our knowledge of potential contributors to the interindividual and intraindividual variability in therapeutic and toxic effects of bupropion in humans.

Inhibition of human anthracycline reductases by emodin - A possible remedy for anthracycline resistance

The clinical application of anthracyclines, like daunorubicin and doxorubicin, is limited by two factors: dose-related cardiotoxicity and drug resistance. Both have been linked to reductive metabolism of the parent drug to their metabolites daunorubicinol and doxorubicinol, respectively. These metabolites show significantly less anti-neoplastic properties as their parent drugs and accumulate in cardiac tissue leading to chronic cardiotoxicity. Therefore, we aimed to identify novel and potent natural inhibitors for anthracycline reductases, which enhance the anticancer effect of anthracyclines by preventing the development of anthracycline resistance. Human enzymes responsible for the reductive metabolism of daunorubicin were tested for their sensitivity towards anthrachinones, in particular emodin and anthraflavic acid. Intense inhibition kinetic data for the most effective daunorubicin reductases, including IC50- and Ki-values, the mode of inhibition, as well as molecular docking, were compiled. Subsequently, a cytotoxicity profile and the ability of emodin to reverse daunorubicin resistance were determined using multiresistant A549 lung cancer and HepG2 liver cancer cells. Emodin potently inhibited the four main human daunorubicin reductases in vitro. Further, we could demonstrate that emodin is able to synergistically sensitize human cancer cells towards daunorubicin at clinically relevant concentrations. Therefore, emodin may yield the potential to enhance the therapeutic effectiveness of anthracyclines by preventing anthracycline resistance via inhibition of the anthracycline reductases. In symphony with its known pharmacological properties, emodin might be a compound of particular interest in the management of anthracycline chemotherapy efficacy and their adverse effects.

Flavones Inhibit the Activity of AKR1B10, a Promising Therapeutic Target for Cancer Treatment

AKR1B10 is an NADPH-dependent reductase that plays an important function in several physiological reactions such as the conversion of retinal to retinol, reduction of isoprenyl aldehydes, and biotransformation of procarcinogens and drugs. A growing body of evidence points to the important role of the enzyme in the development of several types of cancer (e.g., breast, hepatocellular), in which it is highly overexpressed. AKR1B10 is regarded as a therapeutic target for the treatment of these diseases, and potent and specific inhibitors may be promising therapeutic agents. Several inhibitors of AKR1B10 have been described, but the area of natural plant products has been investigated sparingly. In the present study almost 40 diverse phenolic compounds and alkaloids were examined for their ability to inhibit the recombinant AKR1B10 enzyme. The most potent inhibitors-apigenin, luteolin, and 7-hydroxyflavone-were further characterized in terms of IC50, selectivity, and mode of action. Molecular docking studies were also conducted, which identified putative binding residues important for the interaction. In addition, cellular studies demonstrated a significant inhibition of the AKR1B10-mediated reduction of daunorubicin in intact cells by these inhibitors without a considerable cytotoxic effect. Although these compounds are moderately potent and selective inhibitors of AKR1B10, they constitute a new structural type of AKR1B10 inhibitor and may serve as a template for the development of better inhibitors.

Rifampin enhances cytochrome P450 (CYP) 2B6-mediated efavirenz 8-hydroxylation in healthy volunteers

The effect of rifampin on the in vivo metabolism of the antiretroviral drug efavirenz was evaluated in healthy volunteers. In a cross-over placebo control trial, healthy subjects (n = 20) were administered a single 600 mg oral dose of efavirenz after pretreatment with placebo or rifampin (600 mg/day for 10 days). Plasma and urine concentrations of efavirenz, 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz were measured by LC-MS/MS. Compared to placebo treatment, rifampin increased the oral clearance (by ∼2.5-fold) and decreased maximum plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC0-∞) of efavirenz (by ∼1.6- and ∼2.5-fold respectively) (p < 0.001). Rifampin treatment substantially increased the Cmax and AUC0-12h of 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz, metabolic ratio (AUC0-72h of metabolites to AUC0-72h efavirenz) and the amount of metabolites excreted in urine (Ae0-12hr) (all, p < 0.01). Female subjects had longer elimination half-life (1.6-2.2-fold) and larger weight-adjusted distribution volume (1.6-1.9-fold) of efavirenz than male subjects (p < 0.05) in placebo and rifampin treated groups respectively. In conclusion, rifampin enhances CYP2B6-mediated efavirenz 8-hydroxylation in vivo. The metabolism of a single oral dose of efavirenz may be a suitable in vivo marker of CYP2B6 activity to evaluate induction drug interactions involving this enzyme.

11β-Hydroxysteroid dehydrogenase 1: Regeneration of active glucocorticoids is only part of the story

11β-Hydroxysteroid dehydrogenase 1 (11β-HSD1) is an endoplasmic reticulum membrane enzyme with its catalytic site facing the luminal space. It functions primarily as a reductase, driven by the supply of its cosubstrate NADPH by hexose-6-phosphate dehydrogenase (H6PDH). Extensive research has been performed on the role of 11β-HSD1 in the regeneration of active glucocorticoids and its role in inflammation and metabolic disease. Besides its important role in the fine-tuning of glucocorticoid action, 11β-HSD1 is a multi-functional carbonyl reductase converting several 11- and 7-oxosterols into the respective 7-hydroxylated forms. Moreover, 11β-HSD1 has a role in phase I biotransformation reactions and catalyzes the carbonyl reduction of several non-steroidal xenobiotics. Recent observations from experiments using selective inhibitors and studies with transgenic mice indicated a role for 11β-HSD1 in oxysterol metabolism and in bile acid homeostasis, with evidence for glucocorticoid-independent effects on gene expression. This review focuses on the promiscuity of 11β-HSD1 to accept structurally distinct substrates and discusses recent progress mainly on non-glucocorticoid substrates. This article is part of a Special Issue entitled 'Enzyme Promiscuity and Diversity'.

Pharmacokinetic interactions of breast cancer chemotherapeutics with human doxorubicin reductases

Paclitaxel (PTX), docetaxel (DTX), 5-fluorouracil (5-FU), cyclophosphamide (CYC) or tamoxifen (TMX) are combined with doxorubicin (DOX) in first-line chemotherapy regimens that are indicated for breast cancer patients. Although the efficacies of these drugs in combination treatments have been demonstrated in clinical practice, their possible interference with DOX metabolism has not been described in detail to date. In the present study, we investigated the possible interactions of human carbonyl reducing enzymes with 5-FU, PTX, DTX, CYC and TMX. First, the reducing activities of carbonyl reducing enzymes toward DOX were tested using incubations with purified recombinant enzymes. In the subsequent studies, we investigated the possible effects of the tested anticancer agents on the DOX-reducing activities of the most potent enzymes (AKR1C3, CBR1 and AKR1A1) and on the DOX metabolism driven by MCF7, HepG2 and human liver cytosols. In both of these assays, we observed that CYC and its active metabolites inhibited DOX metabolism. In the final study, we tracked the changes in AKR1C3, CBR1 and AKR1A1 expression levels following exposure to the tested cytostatics in MCF7 and HepG2 cells. Consequently, no significant changes in the expression levels of tested enzymes were detected in either cell line. Based on these findings, it is feasible to presume that inhibition rather than induction plays a role in the interactions of the tested anticancer agents with DOX-reducing enzymes. In conclusion, our results describe important molecular events that occur during combination breast cancer therapies and might modulate pharmacokinetic DOX resistance and/or behaviour.

Metabolism of bupropion by carbonyl reductases in liver and intestine

Bupropion's metabolism and the formation of hydroxybupropion in the liver by cytochrome P450 2B6 (CYP2B6) has been extensively studied; however, the metabolism and formation of erythro/threohydrobupropion in the liver and intestine by carbonyl reductases (CR) has not been well characterized. The purpose of this investigation was to compare the relative contribution of the two metabolism pathways of bupropion (by CYP2B6 and CR) in the subcellular fractions of liver and intestine and to identify the CRs responsible for erythro/threohydrobupropion formation in the liver and the intestine. The results showed that the liver microsome generated the highest amount of hydroxybupropion (Vmax = 131 pmol/min per milligram, Km = 87 μM). In addition, liver microsome and S9 fractions formed similar levels of threohydrobupropion by CR (Vmax = 98-99 pmol/min per milligram and Km = 186-265 μM). Interestingly, the liver has similar capability to form hydroxybupropion (by CYP2B6) and threohydrobupropion (by CR). In contrast, none of the intestinal fractions generate hydroxybupropion, suggesting that the intestine does not have CYP2B6 available for metabolism of bupropion. However, intestinal S9 fraction formed threohydrobupropion to the extent of 25% of the amount of threohydrobupropion formed by liver S9 fraction. Enzyme inhibition and Western blots identified that 11β-dehydrogenase isozyme 1 in the liver microsome fraction is mainly responsible for the formation of threohydrobupropion, and in the intestine AKR7 may be responsible for the same metabolite formation. These quantitative comparisons of bupropion metabolism by CR in the liver and intestine may provide new insight into its efficacy and side effects with respect to these metabolites.

Carbonyl-reducing enzymes as targets of a drug-immobilised affinity carrier

Proteins, peptides and nucleic acids are commonly isolated and purified in almost all bioscience laboratories. Methods based on molecular recognition are currently the most powerful tool in separation processes due to their selectivity and recovery. The aim of this study was to prove the versatility and the ability of an affinity carrier containing the immobilised ligand oracin (previously developed by our workgroup) to selectively bind carbonyl-reducing enzymes. These enzymes play an important role in metabolic pathways of various endogenic compounds and xenobiotics. Many important drugs, such as doxorubicin, daunorubicin, haloperidol and the model anticancer drug oracin, are metabolised by carbonyl-reducing enzymes. The functionality of the presented carrier was demonstrated with pure recombinant enzymes (AKR1A1, AKR1B1, AKR1B10, AKR1C1, AKR1C2, AKR1C3, AKR1C4, CBR1 and CBR3) as well as with two model biological samples (cell extract from genetically modified Escherichia coli and pre-purified human liver cytosol). Enzymes that show an affinity toward oracin were efficiently captured, gently eluted using 150 mM ammonium hydroxide and subsequently identified by MS. The method is highly selective and robust and may be applied to the purification and identification of various carbonyl-reducing enzymes from any biological sample.

Molecular and biochemical characterisation of human short-chain dehydrogenase/reductase member 3 (DHRS3)

Dehydrogenase/reductase (SDR family) member 3 (DHRS3), also known as retinal short-chain dehydrogenase/reductase (retSDR1) is a member of SDR16C family. This family is thought to be NADP(H) dependent and to have multiple substrates; however, to date, only all-trans-retinal has been identified as a DHRS3 substrate. The reductive reaction catalysed by DHRS3 seems to be physiological, and recent studies proved the importance of DHRS3 for maintaining suitable retinoic acid levels during embryonic development in vivo. Although it seems that DHRS3 is an important protein, knowledge of the protein and its properties is quite limited, with the majority of information being more than 15 years old. This study aimed to generate a more comprehensive characterisation of the DHRS3 protein. Recombinant enzyme was prepared and demonstrated to be a microsomal, integral-membrane protein with the C-terminus oriented towards the cytosol, consistent with its preference of NADPH as a cofactor. It was determined that DHRS3 also participates in the metabolism of other endogenous compounds, such as androstenedione, estrone, and DL-glyceraldehyde, and in the biotransformation of xenobiotics (e.g., NNK and acetohexamide) in addition to all-trans-retinal. Purified and reconstituted enzyme was prepared for the first time and will be used for further studies. Expression of DHRS3 was shown at the level of both mRNA and protein in the human liver, testis and small intestine. This new information could open other areas of DHRS3 protein research.

Isoquinoline alkaloids as a novel type of AKR1C3 inhibitors

AKR1C3 is an important human enzyme that participates in the reduction of steroids and prostaglandins, which leads to proliferative signalling. In addition, this enzyme also participates in the biotransformation of xenobiotics, such as drugs and procarcinogens. AKR1C3 is involved in the development of both hormone-dependent and hormone-independent cancers and was recently demonstrated to confer cell resistance to anthracyclines. Because AKR1C3 is frequently upregulated in various cancers, this enzyme has been suggested as a therapeutic target for the treatment of these pathological conditions. In this study, nineteen isoquinoline alkaloids were examined for their ability to inhibit a recombinant AKR1C3 enzyme. As a result, stylopine was demonstrated to be the most potent inhibitor among the tested compounds and exhibited moderate selectivity towards AKR1C3. In the follow-up cellular studies, stylopine significantly inhibited the AKR1C3-mediated reduction of daunorubicin in intact cells without considerable cytotoxic effects. This inhibitor could therefore be used as a model AKR1C3 inhibitor in research or evaluated as a possible therapeutic anticancer drug. Furthermore, based on our results, stylopine can serve as a model compound for the design and future development of structurally related AKR1C3 inhibitors.

The Activity of SN33638, an Inhibitor of AKR1C3, on Testosterone and 17β-Estradiol Production and Function in Castration-Resistant Prostate Cancer and ER-Positive Breast Cancer

AKR1C3 is a novel therapeutic target in castration-resistant prostate cancer (CRPC) and estrogen receptor (ER)-positive breast cancer because of its ability to produce testosterone and 17β-estradiol intratumorally, thus promoting nuclear receptor signaling and tumor progression. A panel of CRPC, ER-positive breast cancer and high/low AKR1C3-expressing cell lines were treated with SN33638, a selective inhibitor of AKR1C3, in the presence of hormone or prostaglandin (PG) precursors, prior to evaluation of cell proliferation and levels of 11β-PG F2α (11β-PGF2α), testosterone, 17β-estradiol, and prostate-specific antigen (PSA). A meta-analysis of AKR1C3 mRNA expression in patient samples was also conducted, which revealed that AKR1C3 mRNA was upregulated in CRPC, but downregulated in ER-positive breast cancer. 11β-PGF2α and testosterone levels in the cell line panel correlated with AKR1C3 protein expression. SN33638 prevented 11β-PGF2α formation in cell lines that expressed AKR1C3, but partially inhibited testosterone formation and subsequently cell proliferation and/or PSA expression only in high (LAPC4 AKR1C3-overexpressing cells) or moderate (22RV1) AKR1C3-expressing cell lines. SN33638 had little effect on 17β-estradiol production or estrone-stimulated cell proliferation in ER-positive breast cancer cell lines. Although SN33638 could prevent 11β-PGF2α formation, its ability to prevent testosterone and 17β-estradiol production and their roles in CRPC and ER-positive breast cancer progression was limited due to AKR1C3-independent steroid hormone production, except in LAPC4 AKR1C3 cells where the majority of testosterone was AKR1C3-dependent. These results suggest that inhibition of AKR1C3 is unlikely to produce therapeutic benefit in CRPC and ER-positive breast cancer patients, except possibly in the small subpopulation of CRPC patients with tumors that have upregulated AKR1C3 expression and are dependent on AKR1C3 to produce the testosterone required for their growth.