Variable contribution of CYP2D6 to the N-dealkylation of S-(-)-propranolol by human liver microsomes

Combining Antidepressants with β-Blockers: Evidence of a Clinically Significant CYP2D6 Drug Interaction

BACKGROUND

The β-blockers and antidepressants are two of the most commonly prescribed drug classes in the United States. Several antidepressants are potent inhibitors of cytochrome P450 2D6 liver enzymes (CYP2D6) and can increase the plasma concentrations of certain β-blockers when administered concomitantly, potentially leading to serious medical consequences such as hypotension, bradycardia, and falls.

OBJECTIVE

The primary objective of this investigation was to determine whether initiating an antidepressant in patients receiving β-blockers increased the risk of hemodynamic adverse events. Our primary outcome was time to hospital admissions or emergency department (ED) visits for an International Classification of Diseases-9 diagnosis suggestive of excessive β-blockade.

METHODS

We conducted a survival analysis for adults continuously enrolled in the California Medicaid system (Medi-Cal) between 2004 and 2012. Eligible patients were required to be receiving β-blocker medications that are primarily CYP2D6 substrates (e.g., metoprolol, propranolol, or carvedilol). Univariate and multivariable analyses were performed for patients who concurrently received antidepressants with β-blockers. An additional multivariable analysis analyzed the association of this combination upon hospitalizations or ED visits for all causes.

RESULTS

A total of 21,292 beneficiaries met the inclusion criteria, and 4.3% of patients required hospitalization or ED visits within 30 days of co-medication. In multivariable analysis, patients receiving antidepressants with moderate to strong CYP2D6 inhibitory potential (fluoxetine, paroxetine, duloxetine, or bupropion) had a greater risk for hospitalization or ED visits for hemodynamic events than those initiated on antidepressants with weak CYP2D6 inhibition for 30 days or less when each was compared with patients receiving no antidepressants (hazard ratio [HR] 1.53, 95% confidence interval [CI] 1.03-2.81; p=0.04 vs HR 1.24; 95% CI 0.82-1.88; p=0.30). Other demographic variables associated with increased morbidity included advanced age, male sex, higher β-blocker doses, and African American race or Hispanic ethnicity.

CONCLUSIONS

Results of this analysis suggest that initiation of certain antidepressants was associated with an increased risk for serious medical sequelae among patients concurrently receiving β-blockers. Greater risk was observed with antidepressants that potently inhibit the CYP2D6 enzyme, implying that increased morbidity may be mediated by a metabolic drug interaction.

Development and validation of an HPLC-UV method for the simultaneous determination of the antipsychotics clozapine, olanzapine and quetiapine, several beta-blockers and their metabolites

A simple, accurate and selective column-switching high-performance liquid chromatography (HPLC) method was developed and validated for simultaneous quantification of six beta-blockers (metoprolol, timolol, bisoprolol, propranolol, carvedilol and nebivolol), three of their metabolites (α-hydroxy metoprolol, N-desisopropyl propranolol and 4'-hydroxy carvedilol 4-HCAR), three antipsychotics (olanzapine, clozapine and quetiapine) and three of their metabolites (N-desmethyl olanzapine, N-desmethyl clozapine and N-desalkyl quetiapine) in human serum. After pretreatment on a Merck LiChrospher RP-4 ADS column (25 μm), drugs were separated on a Phenomenex Gemini Phenyl Hexyl 110 A column (250 × 4.6 mm, 5 μm) using a gradient mixture of acetonitrile and potassium dihydrogen phosphate buffer pH 3.1 (containing 10% methanol) as a mobile phase at a flow rate of 1 mL/min. The total analysis time was 40 min. For detection of the analytes, four different UV wavelengths were used: 215, 226, 242 and 299 nm. The method was validated according to the guidelines of the Society of Toxicology and Forensic Chemistry in terms of selectivity, linearity, accuracy, precision and stability and successfully applied for the analysis of the 15 described analytes in human serum.

Pharmacogenetics in reproductive and perinatal medicine

The clinical application of pharmacogenetics has been well accepted by some medical specialties, but not all. The aim of this review is to discuss the current use of pharmacogenetics in reproductive and perinatal medicine and to highlight areas where pharmacogenetics may be able to help in the future to predict response to medicines in terms of efficacy and safety. This applies to drugs that are specific to pregnancy and reproduction, as well as drugs prescribed for the treatment of medical disorders in pregnancy. Our review points out the need for well-designed clinical studies on the efficacy and safety of medicines used in women of childbearing age in order to define the additional utility provided by pharmacogenetic testing.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Thein vitro metabolism of desglymidodrine, an active metabolite of prodrug midodrine by human liver microsomes

The human cytochrome P450 (CYP) isoforms catalyzing the oxidation metabolism of desglymidodrine (DMAE), an active metabolite of midodrine, were studied. Recombinant human CYP2D6, 1A2 and 2C19 exhibited appreciable catalytic activity with respect to the 5'-O-demethylation of DMAE. The O-demethylase activity by the recombinant CYP2D6 was much higher than that of other CYP isoforms. Quinidine (a selective inhibitor of CYP2D6) inhibited the O-demethylation of DMAE in pooled human microsomes by 86%, while selective inhibitors for other forms of CYP did not show any appreciable effect. Although the activity of CYP2D6 was almost negligible in the PM microsomes, the O-demethylase activity of DMAE was found to be maintained by about 25% of the pooled microsomes. Furafylline (a selective inhibitor of CYP1A2) inhibited the M-2 formation in the PM microsomes by 57%. The treatment of pooled microsomes with an antibody against CYP2D6 inhibited the formation of M-2 by about 75%, whereas that of the PM microsomes did not show drastic inhibition. In contrast, the antibody against CYP1A2 suppressed the activity by 40 to 50% in the PM microsomes. These findings suggest that CYP2D6 have the highest catalytic activity of DMAE 5'-O-demethylation in human liver microsomes, followed by CYP1A2 to a small extent.

Comparative molecular field analysis and QSAR on substrates binding to cytochrome P450 2D6

In this study, we utilized comparative molecular field analysis (CoMFA) to gain a better understanding of the steric and electrostatic features of the cytochrome p450 2D6 (CYP2D6) active site. The training set consists of 24 substrates with reported K(M) values from liver microsomal CYP2D6 spanning an activity range of almost three log units. The low energy conformers were fit by root mean square (RMS) to minaprine at the site of metabolism and to the protonated nitrogen. In this manner, we constructed two CoMFA models, one model with a distance constraint and another without. The model with the distance parameter (non-cross-validated R(2)=0.99) was approximately equal to the CoMFA without a distance parameter (non-cross-validated R(2)=0.98). Validation of our CoMFA was accomplished by predicting the K(M) values of 15 diverse CYP2D6 substrates not in the original training set resulting in a predictive R(2)=0.62. Finally, we also pursued correlations of pK(a) and log P with CYP2D6 substrate K(M) in an effort to investigate other physicochemical properties.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Cytochrome P450 2D6.1 and cytochrome P450 2D6.10 differ in catalytic activity for multiple substrates

CYP2D6 is involved in the metabolism of several classes of drugs, including tricyclic antidepressants, selective serotonin reuptake inhibitors and various amphetamines. CYP2D6*10 is an allelic variant, producing an enzyme with Pro34Ser and Ser486Thr amino acid substitutions. Approximately 75% of Asians possess the *10 allele. We sought to further characterize CYP2D6.10 catalytically in vitro in a baculovirus expression system using various substrates and inhibitors, in comparison to CYP2D6.1 (wild-type). Using dextromethorphan (DEX), P-methoxyamphetamine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and (+/-)3,4-methylenedioxymethamphetamine (MDMA), the ratios of intrinsic clearance (Vmax/Km) of *1 to *10 were 50, 34, 22 and 123, respectively. The CYP2D6 substrates amitriptyline, and (+) and (-) methamphetamine (MAMP) are both p-hydroxylated and N-demethylated (NDM). The intrinsic clearance *1/*10 ratios were 42, 30 and 67 for the p-hydroxylation; and 60, 120 and 157 for the NDM, respectively, illustrating chemical pathway and enantiomeric selectivity for MAMP. It was apparent that (+) and (-) MAMP NDM and MDMA demethylenation were most significantly different in CYP2D6.10. Using DEX as the substrate, the ratios of Ki(*10)/Ki(*1) for inhibitors were: budipine (1.3), sparteine (1.6), debrisoquine (8.1), fluoxetine (16), norfluoxetine (30), paroxetine (14), MDMA (21) and MMDA-2 (7.1), indicating that CYP2D6.10 shows drug-specific altered susceptibility to inhibition. Taken together, these data suggest that CYP2D6*10/*10 individuals may be expected to require different drug doses; and show altered susceptibility to toxicity, interaction risk and, in the case of the amphetamines, drug dependence and toxicity compared to CYP2D6*1/*1 individuals.

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.

Evidence that serine 304 is not a key ligand-binding residue in the active site of cytochrome P450 2D6

Homology models of cytochrome P450 2D6 (CYP2D6) have identified serine 304 as an active-site residue and implicated a putative role for this residue in substrate enantioselectivity and the differential inhibition of enzyme activity by the diastereoisomers quinine and quinidine. The role of serine 304 in selectivity is thought to be achieved through a preferential hydrogen-bond interaction between the hydroxyl group of the residue and one of the stereoisomers of each ligand. We have tested this hypothesis by substituting serine 304 with alanine, a non-hydrogen-bonding residue, and compared the properties of the wild-type and mutant enzymes in microsomes prepared from yeast cells expressing the appropriate cDNA-derived enzyme. The Ser(304)Ala substitution did not alter the enantioselective oxidation of metoprolol; the O-demethylation reaction remained R-(+)-enantioselective (wild-type, R/S, 1.7; mutant, R/S, 1.6), whereas alpha-hydroxylation remained S-(-)-enantioselective (wild-type and mutant, R/S, 0.7). Similarly, the selective oxidation of the R-(+) and S-(-) enantiomers of propranolol to the major 4-hydroxy metabolite was identical with both wild-type and mutant forms of the enzyme (R/S 0.9), although the formation of minor metabolites (5-hydroxy and deisopropylpropranolol) did show some slight alteration in enantioselectivity. The differential inhibition of enzyme activity by quinine and quinidine was also identical with both forms of CYP2D6, the IC(50) values for each enzyme being approx. 10 microM and 0.1 microM for quinine and quinidine, respectively. The kinetics of formation of alpha-hydroxymetoprolol and 4-hydroxydebrisoquine by wild-type and the Ser(304)Ala mutant was also very similar. However, modest changes in the regioselective oxidation of metoprolol and debrisoquine were observed with the Ser(304)Ala mutant. The regio- and enantioselective oxidation of an analogue of metoprolol, in which the hydroxyl group attached to the chiral carbon was replaced by a methyl moiety, was again identical with both wild-type and Ser(304)Ala mutant. However, the observed selectivity was the reverse of that observed with metoprolol. Collectively, these data indicate that Ser(304) is unlikely to be a key ligand-binding residue, although the residue may indeed be located in the active-site cavity. The reversal of selectivity with the methyl analogue of metoprolol indicates that the hydroxyl group attached to the chiral centre of ligands, such as metoprolol, is important in defining the enzyme's selective properties, and that a hydrogen-bonding residue, other than Ser(304), may be involved in this interaction. Current homology models of the active site of CYP2D6 that predict a hydrogen-bond interaction between Ser(304) and specific ligands will need to be re-evaluated, and other candidate residues capable of such an interaction nominated and tested by site-directed mutagenesis studies.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.

A novel approach to predicting P450 mediated drug metabolism. CYP2D6 catalyzed N-dealkylation reactions and qualitative metabolite predictions using a combined protein and pharmacophore model for CYP2D6

A combined protein and pharmacophore model for cytochrome P450 2D6 (CYP2D6) has been extended with a second pharmacophore in order to explain CYP2D6 catalyzed N-dealkylation reactions. A group of 14 experimentally verified N-dealkylation reactions form the basis of this second pharmacophore. The combined model can now accommodate both the usual hydroxylation and O-demethylation reactions catalyzed by CYP2D6, as well as the less common N-dealkylation reactions. The combined model now contains 72 metabolic pathways catalyzed by CYP2D6 in 51 substrates. The model was then used to predict the involvement of CYP2D6 in the metabolism of a "test set" of seven compounds. Molecular orbital calculations were used to suggest energetically favorable sites of metabolism, which were then examined using modeling techniques. The combined model correctly predicted 6 of the 8 observed metabolites. For the well-established CYP2D6 metabolic routes, the predictive value of the current combined protein and pharmacophore model is good. Except for the highly unusual metabolism of procainamide and ritonavir, the known metabolites not included in the development of the model were all predicted by the current model. Two possible metabolites have been predicted by the current model, which have not been detected experimentally. In these cases, the model may be able to guide experiments. P450 models, like the one presented here, have wide applications in the drug design process which will contribute to the prediction and elimination of polymorphic metabolism and drug-drug interactions.

Novel approach to predicting P450-mediated drug metabolism: development of a combined protein and pharmacophore model for CYP2D6

A combined protein and pharmacophore model for cytochrome P450 2D6 (CYP2D6) has been derived using various computational chemistry techniques. A combination of pharmacophore modeling (using 40 substrates), protein modeling, and molecular orbital calculations was necessary to derive a model which incorporated steric, electronic, and chemical stability properties. The initial pharmacophore and protein models used to construct the combined model were derived independently and showed a high level of complementarity. The combined model is in agreement with experimental results concerning the substrates used to derive the model, with site-directed mutagenesis data available for the CYP2D6 protein, and takes into account the site-directed mutagenesis results for a variety of other 2-family P450s.

Variable contribution of cytochromes P450 2D6, 2C9 and 3A4 to the 4-hydroxylation of tamoxifen by human liver microsomes

4-Hydroxylation is an important pathway of tamoxifen metabolism because the product of this reaction is intrinsically 100 times more potent as an oestrogen receptor antagonist than is the parent drug. Although tamoxifen 4-hydroxylation is catalysed by human cytochrome P450 (CYP), data conflict on the specific isoforms responsible. The aim of this study was to define unequivocally the role of individual CYPs in the 4-hydroxylation of tamoxifen by human liver microsomes. Microsomes from each of 10 human livers catalysed the reaction [range = 0.6-2.9 pmol/mg protein/min (1 microM substrate concentration) and 6-25 pmol/mg protein/min (18 microM)]. Three of the livers with the lowest tamoxifen 4-hydroxylation activity were from genetically poor metabolisers with respect to CYP2D6. Inhibition of activity by quinidine (1 microM), sulphaphenazole (20 microM) and ketoconazole (2 microM), selective inhibitors of CYPs 2D6, 2C9 and 3A4, respectively, was 0-80%, 0-80% and 12-57%. The proportion of activity inhibited by quinidine correlated positively with total microsomal tamoxifen 4-hydroxylation activity (rs = 0.89, P < 0.01), indicating a major involvement of CYP2D6 in this reaction. Recombinant human CYPs 2D6, 2C9 and 3A4 but not CYPs 1A1, 1A2, 2C19 and 2E1 displayed significant 4-hydroxylation activity. Similar inhibition and correlation experiments confirmed that tamoxifen N-demethylation is catalysed predominantly by CYP3A4. These findings indicate that the 4-hydroxylation of tamoxifen is catalysed almost exclusively by CYPs 2D6, 2C9 and 3A4 in human liver microsomes. However, the marked between-subject variation in the contribution of these isoforms underlines the need to study metabolic reactions in a sufficient number of livers that are characterised with respect to a range of cytochrome P450 activities.