Propranolol oxidation by human liver microsomes--the use of cumene hydroperoxide to probe isoenzyme specificity and regio- and stereoselectivity

Glucuronidation Pathways of 5- and 7-Hydroxypropranolol: Determination of Glucuronide Structures and Enzyme Selectivity

Propranolol, a non-selective beta-blocker medication, has been utilized in the treatment of cardiovascular diseases for several decades. Its hydroxynaphthyl metabolites have been recognized to possess varying degrees of beta-blocker activity due to the unaltered side-chain. This study achieved the successful separation and identification of diastereomeric glucuronic metabolites derived from 4-, 5-, and 7-hydroxypropranolol (4-OHP, 5-OHP, and 7-OHP) in human urine. Subsequently, reaction phenotyping of 5- and 7-hydroxypropranolol by different uridine 5'-diphospho-glucuronosyltransferases (UGTs) was carried out, with a comparison to the glucuronidation of 4-hydroxypropranolol (4-OHP). Among the 19 UGT enzymes examined, UGT1A1, UGT1A3, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2A1, and UGT2A2 were found to be involved in the glucuronidation of 5-OHP. Furthermore, UGT1A6 exhibited glucuronidation activity towards 7-OHP, along with the aforementioned eight UGTs. Results obtained by glucuronidation of corresponding methoxypropranolols and MS/MS analysis of 1,2-dimethylimidazole-4-sulfonyl (DMIS) derivatives of hydroxypropranolol glucuronides suggest that both the aromatic and aliphatic hydroxy groups of the hydroxypropranolols may be glucuronidated in vitro. However, the analysis of human urine samples collected after the administration of propranolol leads us to conclude that aromatic-linked glucuronidation is the preferred pathway under physiological conditions.

Complete Reaction Phenotyping of Propranolol and 4-Hydroxypropranolol with the 19 Enzymes of the Human UGT1 and UGT2 Families

Propranolol is a competitive non-selective beta-receptor antagonist that is available on the market as a racemic mixture. In the present study, glucuronidation of propranolol and its equipotent phase I metabolite 4-hydroxypropranolol by all 19 members of the human UGT1 and UGT2 families was monitored. UGT1A7, UGT1A9, UGT1A10 and UGT2A1 were found to glucuronidate propranolol, with UGT1A7, UGT1A9 and UGT2A1 mainly acting on (S)-propranolol, while UGT1A10 displays the opposite stereoselectivity. UGT1A7, UGT1A9 and UGT2A1 were also found to glucuronidate 4-hydroxypropranolol. In contrast to propranolol, 4-hydroxypropranolol was found to be glucuronidated by UGT1A8 but not by UGT1A10. Additional biotransformations with 4-methoxypropanolol demonstrated different regioselectivities of these UGTs with respect to the aliphatic and aromatic hydroxy groups of the substrate. Modeling and molecular docking studies were performed to explain the stereoselective glucuronidation of the substrates under study.

How Can Drug Metabolism and Transporter Genetics Inform Psychotropic Prescribing?

Many genetic variants in drug metabolizing enzymes and transporters have been shown to be relevant for treating psychiatric disorders. Associations are strong enough to feature on drug labels and for prescribing guidelines based on such data. A range of commercial tests are available; however, there is variability in included genetic variants, methodology, and interpretation. We herein provide relevant background for understanding clinical associations with specific variants, other factors that are relevant to consider when interpreting such data (such as age, gender, drug-drug interactions), and summarize the data relevant to clinical utility of pharmacogenetic testing in psychiatry and the available prescribing guidelines. We also highlight areas for future research focus in this field.

Monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 enzymes

This review examines the monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 (CYP) enzymes in bacterial, archaeal and mammalian systems. CYP enzymes catalyze monooxygenation reactions by inserting one oxygen atom from O2 into an enormous number and variety of substrates. The catalytic versatility of CYP stems from its ability to functionalize unactivated carbon-hydrogen (C-H) bonds of substrates through monooxygenation. The oxidative prowess of CYP in catalyzing monooxygenation reactions is attributed primarily to a porphyrin π radical ferryl intermediate known as Compound I (CpdI) (Por•+FeIV=O), or its ferryl radical resonance form (FeIV-O•). CYP-mediated hydroxylations occur via a consensus H atom abstraction/oxygen rebound mechanism involving an initial abstraction by CpdI of a H atom from the substrate, generating a highly-reactive protonated Compound II (CpdII) intermediate (FeIV-OH) and a carbon-centered alkyl radical that rebounds onto the ferryl hydroxyl moiety to yield the hydroxylated substrate. CYP enzymes utilize hydroperoxides, peracids, perborate, percarbonate, periodate, chlorite, iodosobenzene and N-oxides as surrogate oxygen atom donors to oxygenate substrates via the shunt pathway in the absence of NAD(P)H/O2 and reduction-oxidation (redox) auxiliary proteins. It has been difficult to isolate the historically elusive CpdI intermediate in the native NAD(P)H/O2-supported monooxygenase pathway and to determine its precise electronic structure and kinetic and physicochemical properties because of its high reactivity, unstable nature (t½~2 ms) and short life cycle, prompting suggestions for participation in monooxygenation reactions of alternative CYP iron-oxygen intermediates such as the ferric-peroxo anion species (FeIII-OO-), ferric-hydroperoxo species (FeIII-OOH) and FeIII-(H2O2) complex.

Induction of propranolol metabolism in the Hep G2 human hepatoma cell line

Metabolism of propranolol by the human hepatoma cell line Hep G2 was studied. Although metabolism qualitatively was similar to that in-vivo, the P450-mediated N-desisopropylation clearly predominated. Pretreatment of cells with 3-methylcholanth-rene increased the activity of this pathway 14-fold, whereas phenobarbitone had no effect. This is similar to the pathway-selective inductive response observed for cigarette smoking in-vivo. As in-vivo, secondary metabolism of N-desisopropylpropranolol was extensive. This could, however, be completely blocked by 0·1 μm clorgyline, a potent MAO type A inhibitor. As in human liver microsomes, the stereochemistry of propranolol metabolism demonstrated a preference for the R(+)-enantiomer. These observations emphasize the usefulness of the Hep G2 cell line as a model of man.

Microbial production of phase I and phase II metabolites of propranolol

1. The production in multimilligram amounts of 4- and 5-hydroxylated metabolites of (R)- or (S)-propranolol by biotransformation with two fungal strains, an Absidia sp. M50002 and a Cunninghamella sp. M50036, was carried out, starting from either the racemic drug or pure enantiomers. 2. While both enantiomers of propranolol were hydroxylated in the 5-position by incubation with strain M50002, the strain M50036 operated a chiral discrimination, resulting in the exclusive formation of the 4-hydroxy-(R)-enantiomer. 3. In addition, a Streptomyces sp. strain M52104, isolated from a soil sample, was selected for the high-yield regioselective β-glucuronidation of propranolol and its 4- and 5-hydroxylated derivatives. 4. NMR and mass spectroscopic data have been extensively used for the unambiguous characterization of 4- and 5-hydroxylated and glucuronidated derivatives, all of them corresponding to the major animal and human metabolites of propranolol, a typical substrate of CYP2D6.

Prediction of Human Drug Clearance from in Vitro and Preclinical Data Using Physiologically Based and Empirical Approaches

PURPOSE

The aim of this study is to compare the accuracy of five methods for predicting in vivo intrinsic clearance (CL(int)) and seven for predicting hepatic clearance (CL(h)) in humans using in vitro microsomal data and/or preclinical animal data.

METHODS

The human CL(int) was predicted for 33 drugs by five methods that used either in vitro data with a physiologic scaling factor (SF), with an empirical SF, with the physiologic and drug-specific (the ratio of in vivo and in vitro CL(int) in rats) SFs, or rat CL(int) directly and with allometric scaling. Using the estimated CL(int), the CL(h) in humans was calculated according to the well-stirred liver model. The CL(h) was also predicted using additional two methods: using direct allometric scaling or drug-specific SF and allometry.

RESULTS

Using in vitro human microsomal data with a physiologic SF resulted in consistent underestimation of both CL(int) and CL(h). This bias was reduced by using either an empirical SF, a drug-specific SF, or allometry. However, for allometry, there was a substantial decrease in precision. For drug-specific SF, bias was less reduced, precision was similar to an empirical SF. Both CL(int) and CL(h) were best predicted using in vitro human microsomal data with empirical SF. Use of larger data set of 52 drugs with the well-stirred liver model resulted in a best-fit empirical SF that is 9-fold increase on the physiologic SF.

CONCLUSIONS

Overall, the empirical SF method and the drug-specific SF method appear to be the best methods; they show lower bias than the physiologic SF and better precision than allometric approaches. The use of in vitro human microsomal data with an empirical SF may be preferable, as it does not require extra information from a preclinical study.

Inactivation of rat cytochrome P450 2D enzyme by a further metabolite of 4-hydroxypropranolol, the major and active metabolite of propranolol

Repetitive administration of propranolol (PL) in rats decreases the activities of cytochrome P450 (CYP) 2D enzyme(s) in hepatic microsomes. We examined the properties of 4-hydroxypropranolol (4-OH-PL) as an inactivator of rat liver microsomal CYP2D enzyme(s) using bunitrolol (BTL) 4-hydroxylation and PL 5- and 7-hydroxylations as indices of CYP2D enzyme activity. Rat microsomal BTL 4-hydroxylase activity was inhibited by the addition of 4-OH-PL to the incubation medium. The inhibition was greater after preincubation of microsomes with 4-OH-PL in the presence of NADPH than in its absence. The type of inhibition kinetics of BTL 4-hydroxylase by 4-OH-PL was changed from a competitive type to a noncompetitive type by the preincubation. The inhibition of rat liver microsomal PL 5- and 7-hydroxylases by 4-OH-PL was blocked efficiently by co-incubation with quinine, a typical inhibitor of rat CYP2D enzyme(s), or to a lesser extent by BTL. However, quinidine, a diastereomer of quinine, did not significantly protect against the enzyme inactivation. The protective capacities of the substrate and inhibitors reflected their affinities for rat CYP2D enzyme(s). BTL hydroxylase was not affected by either 1,4-naphthoquinone or 1,4-dihydroxynaphthalene which are possible metabolites of 4-OH-PL. These results provide further evidence to support the notion that PL is biotransformed by rat CYP2D enzyme(s) to 4-OH-PL, which is further oxidized to a chemically reactive metabolite in the active site. The inactivation of CYP is likely the result of covalent binding of the reactive species to an amino acid residue of the active site.

Stereoselective propranolol metabolism in two drug induced rat hepatic microsomes

AIM: To study the influence of inducers BNF and PB on the stere oselective metabolism of propranolol in rat hepatic microsomes.

METHODS: Phase I metabolism of propranolol was studied by using the microsomes induced by BNF and PB and the non-induced microsome as the control. The enzymatic kinetic parameters of propranolol enantiomers were calculated by regression analysis of Lineweaver-Burk plots. Propranolol concentrations we re assayed by HPLC.

RESULTS: A RP-HPLC method was developed to determine propranol ol concentration in rat hepatic microsomes. The linearity equations for R (+)pr opranolol and S (-)propranolol were A = 705.7C + 311.2C (R = 0.9987) and A = 697.2C+311.4C (R = 0.9970) respectively. Recoveries of each enant iomer were 98.9%, 99.5%, 101.0% at 60 μmol/L, 120 μmol/L, 240 μmol/L respectively. At the concentration level of 120 μmol/L, propranolol enantiomers were metabolized at different rates in different microsomes. The concentration ratio R (+)/S (-) of control and PB induced microsomes increased with time, whereas that of microsome induced by BNF decreased. The assayed enzyme parameters were: 1. Km. Control group: R (+)30 ± 8, S (-) 18 ± 5; BNF group: R (+) 34 ± 3, S (-)39 ± 7; PB group: R (+)38 ± 17, S (-) 36 ± 10. 2. Vmax. Control group: R (+)1.5 ± 0.2, S (-)2.9 ± 0.3; BNF group: R (+)3.8 ± 0.3, S (-)3. 3 ± 0.5 ; PB group: R (+)0.07 ± 0.03, S (-)1.94 ± 0. 07. 3. Clint. Control group: R (+)60 ± 3, S (-) 170 ± 30; BNF group: R (+)111.0 ± 1, S (-) 84 ± 5; PB group: R (+)2.0 ± 2, S (-)56.0 ± 1. The enzyme parameters compared with unpaired t tests showed that no stereoselectivity was observed in enzymatic affinity of three microsomes to enantiomers and their catalytic abilities were quite different and had stereoselectivities. Compared with the control, microsome induced by BNF enhanced enzyme activity to propranolol R (+)enantiomer, and microsome induced by PB showed less enzyme activity to propranolol S (-)-enan tiomer which remains the same stereoselectivities as that of the control.

CONCLUSION: Enzyme activity centers of the microsome were changed in composition and regioselectivity after the induction of BNF and PB, and the stereoselectivities of propranolol cytochrome P450 metabolism in rat hepatic microsomes were likely due to the stereoselectivities of the catalyzing function in enzyme. CYP-1A subfamily induced by BNF exhibited pronounced contribution to propranolol metabolism with stereoselectivity to R (+)-enantiomer. CYP-2B subfamily induced by PB exhibited moderate contribution to propranolol metabolism, but still had the stereoselectivity of S (-)-enantiomer.

Covalent binding of a reactive metabolite derived from propranolol and its active metabolite 4-hydroxypropranolol to hepatic microsomal proteins of the rat

Repeated administration of propranolol (PL) to rats causes the inhibition of cytochrome P450-2D (P450-2D) enzyme. We recently found that 4-hydroxypropranolol (4-OH-PL) was biotransformed to 1,4-naphthoquinone (1,4-NQ) by superoxide (SO) anions in medium containing rat liver microsomes and NADPH and proposed that the binding of the quinone to P450-2D apoproteins might be one of mechanisms for the enzyme inhibition [Narimatsu et al. (1995) Chem. Res. Toxicol. 8, 721-728]. In this study, we have searched for possible sources of SO for the conversion of 4-OH-PL to 1,4-NQ in rat liver microsomes and determined the radioactivity covalently bound to microsomal proteins after incubation of radioactive PL and 4-OH-PL with rat liver microsomes. Elimination of 4-OH-PL from a mixture containing microsomes and NADPH was suppressed by carbon monoxide. Antibodies raised to P450-2B1 and -3A2 partially, and antibody against NADPH-cytochrome P450 reductase (fp2) markedly suppressed the reaction. 1,4-NQ was formed concomitantly with 4-OH-PL elimination by a reconstituted preparation of fp2. Binding studies using naphthalene ring (NR)- and side chain (SC)-radiolabeled PL and 4-OH-PL showed that radioactivity covalently bound to microsomal proteins was much higher from 4-OH-PL than from PL for the NR-labeled compounds, but higher from PL than from 4-OH-PL for the SC-labeled compounds. These results suggest that the 4-OH-PL formed from PL by P450-2D enzyme is converted to 1,4-NQ with loss of the side chain, and the 1,4-NQ accounts for most of the radioactivity covalently bound to microsomal proteins, including the P450-2D enzymes. The SO for conversion of 4-OH-PL to 1,4-NQ is supplied mainly by fp2 with some contribution by P450 enzymes.

Identification of human CYP isoforms involved in the metabolism of propranolol enantiomers--N-desisopropylation is mediated mainly by CYP1A2

1. Studies using human liver microsomes and six recombinant human CYP isoforms (i.e. CYP1A2, 2A6, 2B6, 2D6, 2E1 and 3A4) were performed to identify the cytochrome P450 (CYP) isoform(s) involved in the ring 4-hydroxylation and side-chain N-desisopropylation of propranolol enantiomers in humans. 2. alpha-Naphthoflavone and 7-ethoxyresorufin (selective inhibitors of CYP1A1/2) inhibited the N-desisopropylation of R- and S-propranolol by human liver microsomes by 20 and 40%, respectively, while quinidine (a selective inhibitor of CYP2D6) abolished the 4-hydroxylation of both propranolol enantiomers almost completely. In contrast, sulphaphenazole (CYP2C8/9 inhibitor), S-mephenytoin (CYP2C19 inhibitor), troleandomycin (CYP3A3/4 inhibitor) and diethyldithiocarbamate (CYP2E1 inhibitor) elicited only weak inhibitory effects on propranolol metabolism via the two measured metabolic pathways. 3. Significant (P < 0.01) correlations were observed between the microsomal N-desisopropylation of both propranolol enantiomers and that for the O-deethylation of phenacetin among the 11 different human liver microsome samples (r = 0.98 and 0.77 for R- and S-propranolol, respectively). A marginally significant (r = 0.60, P congruent to 0.05) correlation was also observed between N-desisopropylation of S-, but not of R-propranolol and the 4'-hydroxylation of S-mephenytoin. No significant correlations were observed between the N-desisopropylation of propranolol enantiomers and the 2-hydroxylation of desipramine, the hydroxylation of tolbutamide or the 6 beta-hydroxylation of testosterone. 4. Significant (P < 0.01) correlations were observed between the microsomal 4-hydroxylation of R- and S-propranolol and the 2-hydroxylation of desipramine (r = 0.85 and 0.98, respectively). A weak (r = 0.66), albeit significant (P < 0.05) correlation was observed between the 4-hydroxylation of R-, but not of S-propranolol and the hydroxylation of tolbutamide. No significant correlations were observed between the 4-hydroxylation of propranolol enantiomers and the oxidation of other substrates for CYP1A2, 2C19, and 3A3/4. 5. Recombinant human CYP1A2 and CYP2D6 exhibited comparable catalytic activity with respect to the N-desisopropylation of both propranolol enantiomers; only expressed CYP2D6 exhibited a marked catalytic activity with respect to the 4-hydroxylation of both propranolol enantiomers.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the CYP2D subfamily in metabolism-dependent covalent binding of propranolol to liver microsomal protein in rats

In vitro covalent binding of a chemically reactive metabolite of propranolol to microsomal macromolecules, which is presumed to cause inhibition of its own metabolism in rats, was diminished in liver microsomes from rats pretreated with propranolol. Covalent binding was suppressed by the addition of an antibody against P450BTL, which is a cytochrome P450 (P450) isozyme belonging to the CYP2D subfamily. SDS-PAGE of microsomal proteins after incubation with [3H]propranolol and NADPH indicated that the binding was non-selective but prominent at the molecular mass of approx. 50 kDa, corresponding to those of the P450 protein. The radioactivity peak was markedly but not completely diminished by the addition of reduced glutathione. In a reconstituted system containing P450BTL, NADPH-cytochrome P450 reductase (fp2) and dilauroylphosphatidylcholine, propranolol 4-, 5- and 7-hydroxylase activities decreased time dependently following preincubation with propranolol in the presence of NADPH, indicating time-dependent inactivation of P450BTL. The covalent binding of a reactive metabolite of [3H]propranolol to the proteins was also observed in this system. SDS-PAGE showed that among the three proteins in the reconstituted system, fp2 and P450BTL consisting of two polypeptides with molecular masses of 49 and 32 kDa, the binding was specific for a polypeptide corresponding to the P450 isozyme with a molecular mass of 49 kDa. In addition, the ratio of the amount of covalently bound radiolabelled materials to that of P450BTL which was estimated from each impaired propranolol hydroxylase activity under the same reconstitutional conditions was calculated to be approx. 1.0. These findings indicate that propranolol is a mechanism-based inactivator of a cytochrome P450 isozyme(s) belonging to the CYP2D subfamily.

Inhibition of CYP2D6 activity by treatment with propranolol and the role of 4-hydroxy propranolol

1. The 4-hydroxylation of propranolol by rat and human liver microsomes is associated with formation of a chemically reactive species which binds irreversibly to cytochrome P4502D6 (CYP2D6) destroying its catalytic function. Therefore, the effect of propranolol treatment (80 mg twice daily) on debrisoquine phenotype was examined, to see if it resulted in phenocopying in vivo. The role of 4-hydroxypropranolol (4OHP) in the inhibition of CYP2D6 activity was also studied using microsomes from yeast expressing CYP2D6 and from human livers; metoprolol was used as the CYP2D6 substrate. 2. Although a significant effect on apparent oxidation phenotype was demonstrated, the absolute change in the urinary debrisoquine/4-hydroxydebrisoquine ratio (D/4HD) was small, such that no extensive metaboliser who received propranolol treatment was reclassified as a poor metaboliser. The in vitro studies indicated that 4OHP is a potent inhibitor of metoprolol metabolism (Ki approximately 1 microM). This inhibitory effect was enhanced when 4OHP was pre-incubated in the presence of a NADPH generating system and human liver microsomes. The effect was decreased significantly when reduced glutathione was added to the pre-incubation mixture. Metabolism of 4OHP occurred when incubated with human liver microsomes in the presence of a NADPH generating system and irrespective of CYP2D6 phenotype; yeast expressing CYP2D6 did not metabolise 4OHP. 3. We conclude that, although treatment with propranolol 80 mg twice daily significantly decreases the catalytic function of CYP2D6, the inhibition is insufficient to result in phenocopying. The reactive intermediate produced by further metabolism of 4OHP is probably scavenged effectively in vivo by glutathione and other nucleophiles.

Substrate stereoselectivity and enantiomer/enantiomer interaction in propranolol metabolism in rat liver microsomes

The substrate stereoselectivity and enantiomer/enantiomer interaction of (S)- and (R)- propranolol for the formation of their metabolites were investigated in rat liver microsomal fractions. The enantiomers of primary metabolites of propranolol, 4-, 5-, 7-hydroxy- and N-desisopropyl-propranolol were separated and assayed by an HPLC method employing a chiral ovomucoid column. Regioselective substrate stereoselectivity (R < S for 4- and 5-hydroxylations; R > S for 7-hydroxylation; R = S for N-desisopropylation) was observed in the formation of propranolol metabolites when the individual enantiomers or a racemic mixture of propranolol were used as substrates. Concentration-dependent metabolic inhibition of propranolol enantiomers by their optical isomers was also observed. In addition, the inhibition of propranolol 4-, 5- and 7-hydroxylations between the enantiomers showed a typical competitive nature. These findings suggested that the propranolol enantiomers competed for the same enzyme, probably a cytochrome P450 isozyme in the CYP2D subfamily.

The hepatic first-pass metabolism of problematic drugs

The first-pass hepatic metabolism of a number of important therapeutic agents is inconsistent with traditional models that assume that the hepatic extraction ratio of a drug is constant in each individual (independent of the concentration of drug in the hepatic sinusoidal blood and also independent of the history of exposure to the drug). In this review, the authors examine the first-pass metabolism of five "problematic drugs" (propranolol, lidocaine, propafenone, verapamil, and nitroglycerin). Each of these compounds has unique facets to its hepatic clearance and pharmacokinetics as well as striking similarities. Selected aspects of first-pass metabolism are reviewed, and a theory that may explain some of the unusual behavior of the four lipophilic bases (propranolol, lidocaine, propafenone, and verapamil) is presented. Finally, the unusual and variable clearance of nitroglycerin is discussed.