Phenobarbital induces the 2-hydroxylation of desipramine

Physiologically Based Pharmacokinetic Model of the CYP2D6 Probe Atomoxetine: Extrapolation to Special Populations and Drug-Drug Interactions

Physiologically based pharmacokinetic (PBPK) modeling of drug disposition and drug-drug interactions (DDIs) has become a key component of drug development. PBPK modeling has also been considered as an approach to predict drug disposition in special populations. However, whether models developed and validated in healthy populations can be extrapolated to special populations is not well established. The goal of this study was to determine whether a drug-specific PBPK model validated using healthy populations could be used to predict drug disposition in specific populations and in organ impairment patients. A full PBPK model of atomoxetine was developed using a training set of pharmacokinetic (PK) data from CYP2D6 genotyped individuals. The model was validated using drug-specific acceptance criteria and a test set of 14 healthy subject PK studies. Population PBPK models were then challenged by simulating the effects of ethnicity, DDIs, pediatrics, and renal and hepatic impairment on atomoxetine PK. Atomoxetine disposition was successfully predicted in 100% of healthy subject studies, 88% of studies in Asians, 79% of DDI studies, and 100% of pediatric studies. However, the atomoxetine area under the plasma concentration versus time curve (AUC) was overpredicted by 3- to 4-fold in end stage renal disease and hepatic impairment. The results show that validated PBPK models can be extrapolated to different ethnicities, DDIs, and pediatrics but not to renal and hepatic impairment patients, likely due to incomplete understanding of the physiologic changes in these conditions. These results show that systematic modeling efforts can be used to further refine population models to improve the predictive value in this area.

CYP2D6 Is Inducible by Endogenous and Exogenous Corticosteroids

Although cytochrome P450 (CYP) 2D6 has been widely considered to be noninducible on the basis of human hepatocyte studies, in vivo data suggests that it is inducible by endo- and xenobiotics. Therefore, we investigated if the experimental conditions routinely used in human hepatocyte studies may be a confounding factor in the lack of in vitro induction of CYP2D6. Sandwich cultured human hepatocytes (SCHH) were preincubated with or without dexamethasone (100 nM) for 72 hours before incubation with 1μM endogenous (cortisol or corticosterone) or exogenous (dexamethasone or prednisolone) corticosteroids. At 72 hours, CYP2D6 mRNA, protein, and activity were quantified by real-time quantitative polymerase chain reaction, quantitative proteomics, and formation of dextrorphan from dextromethorphan, respectively. In the absence of supplemental dexamethasone, CYP2D6 activity, mRNA, and protein were significantly and robustly (>10-fold) induced by all four corticosteroids. However, this CYP2D6 induction was abolished in cells preincubated with supplemental dexamethasone. These data show, for the first time, that CYP2D6 is inducible in vitro but the routine presence of 100 nM dexamethasone in the culture medium masks this induction. Our cortisol data are in agreement with the clinical observation that CYP2D6 is inducible during the third trimester of pregnancy when the plasma concentrations of cortisol increase to ∼1μM. These findings, if confirmed in vivo, have implications for predicting CYP2D6-mediated drug-drug interactions and call for re-evaluation of regulatory guidelines on screening for CYP2D6 induction by xenobiotics. Our findings also suggest that cortisol may be a causative factor in the in vivo induction of CYP2D6 during pregnancy.

Pharmacokinetic and pharmacodynamic interactions between antiepileptics and antidepressants

INTRODUCTION

Antiepileptic-antidepressant combinations are frequently used by clinicians; their pharmacokinetic (PK) and pharmacodynamic (PD) drug interactions (DIs) have not been well studied but are frequently likely to be clinically relevant.

AREAS COVERED

This article provides a comprehensive review of PK DIs between antiepileptics and antidepressants. In the absence of PD DI studies, PD information on pharmacological mechanisms and studies on efficacy and safety of individual drugs are reviewed.

EXPERT OPINION

The clinical relevance of the inductive properties of carbamazepine, phenytoin, phenobarbital and primidone and the inhibitory properties of valproic acid and some antidepressants are well understood; correction factors are provided if appropriate DI studies have been completed. More PK studies are needed for: i) antiepileptics with potent inductive effects for all recently approved antidepressants; ii) high doses of mild CYP3A4 inducers, such as clobazam, eslicarbazepine, oxcarbazepine, rufinamide and topiramate for reboxetine and vilazodone; iii) valproate as a possible inhibitor, mild inducer or both a mild inducer and competitive inhibitor of some antidepressants; and iv) inhibitory effects of long-term fluoxetine use on clobazam, lacosamide, phenobarbital, primidone, carbamazepine, felbamate, tiagabine and zonisamide. Possible synergistic or additive beneficial PD DIs in generalized anxiety disorder, chronic pain, migraine prophylaxis, weight control and menopausal symptoms need study.

The effect of mirabegron, a potent and selective β3-adrenoceptor agonist, on the pharmacokinetics of CYP2D6 substrates desipramine and metoprolol

Mirabegron is a potent and selective β3-adrenoceptor agonist developed for the treatment of overactive bladder. In vitro studies demonstrated that mirabegron partly acts as a (quasi-) irreversible, metabolism-dependent inhibitor of CYP2D6. The effect of steady-state mirabegron on single doses of the sensitive CYP2D6 substrates metoprolol (100 mg) and desipramine (50 mg) was assessed in two open-label, one-sequence crossover studies in healthy subjects (CYP2D6 extensive metabolizers). Mirabegron 160 mg/day increased metoprolol maximum plasma concentration (C max) 1.90-fold (90 % confidence interval [CI] 1.54; 2.33) and total exposure (AUC0-∞) 3.29-fold (90 % CI 2.70; 4.00) in 12 males (study 1). Mean metoprolol half-life increased from 2.96 to 4.11 h. α-Hydroxymetoprolol C max and AUC to last measurable concentration decreased 2.6-fold and 2.2-fold, respectively. In study 2, mirabegron 100 mg/day increased desipramine C max 1.79-fold (90 % CI 1.69; 1.90) and AUC0-∞ 3.41-fold (90 % CI 3.07; 3.80) in 14 males and 14 females. Mean desipramine half-life increased from 19.5 to 35.8 h. C max of 2-hydroxydesipramine decreased ~twofold, while AUC increased ~1.3-fold. Desipramine was administered again 2 weeks after the last mirabegron dose. Desipramine C max and AUC0-∞ were still ~1.13-fold increased; the 90 % CIs fell within the 0.80-1.25 interval. All treatments were well tolerated. In conclusion, mirabegron is a moderate CYP2D6 inhibitor (ratio and 90 % CI <5.0).

An assessment of drug-drug interactions: the effect of desvenlafaxine and duloxetine on the pharmacokinetics of the CYP2D6 probe desipramine in healthy subjects

A number of antidepressants inhibit the activity of the cytochrome P450 2D6 enzyme system, which can lead to drug-drug interactions. Based on its metabolic profile, desvenlafaxine, administered as desvenlafaxine succinate, a new serotonin-norepinephrine reuptake inhibitor, is not expected to have an impact on activity of CYP2D6. This single-center, randomized, open-label, four-period, crossover study was undertaken to evaluate the effect of multiple doses of desvenlafaxine (100 mg/day, twice the recommended therapeutic dose for major depressive disorder in the United States) and duloxetine (30 mg b.i.d.) on the pharmacokinetics (PK) of a single dose of desipramine (50 mg). A single dose of desipramine was given first to assess its PK. Desvenlafaxine or duloxetine was then administered, in a crossover design, so that steady-state levels were achieved; a single dose of desipramine was then coadministered. The geometric least-square mean ratios (coadministration versus desipramine alone) for area under the plasma concentration versus time curve (AUC) and peak plasma concentrations (C(max)) of desipramine and 2-hydroxydesipramine were compared using analysis of variance. Relative to desipramine alone, increases in AUC and C(max) of desipramine associated with duloxetine administration (122 and 63%, respectively) were significantly greater than those associated with desvenlafaxine (22 and 19%, respectively; P < 0.001). Duloxetine coadministered with desipramine was also associated with a decrease in 2-hydroxydesipramine C(max) that was significant compared with the small increase seen with desvenlafaxine and desipramine (-24 versus 9%; P < 0.001); the difference between changes in 2-hydroxydesipramine AUC did not reach statistical significance (P = 0.054). Overall, desvenlafaxine had a minimal impact on the PK of desipramine compared with duloxetine, suggesting a lower risk for CYP2D6-mediated drug interactions.

Clinical pharmacokinetics of drugs used to treat urge incontinence

Urge incontinence (also known as overactive bladder) is a common form of urinary incontinence, occurring alone or as a component of mixed urinary incontinence, frequently together with stress incontinence. Because of the pathophysiology of urge incontinence, anticholinergic/antispasmodic agents form the cornerstone of therapy. Unfortunately, the pharmacological activity of these agents is not limited to the urinary tract, leading to systemic adverse effects that often promote nonadherence. Although the pharmacokinetics of flavoxate, propantheline, scopolamine, imipramine/desipramine, trospium chloride and propiverine are also reviewed here, only for oxybutynin and tolterodine are there adequate efficacy/tolerability data to support their use in urge incontinence. Oxybutynin is poorly absorbed orally (2-11% for the immediate-release tablet formulation). Controlled-release oral formulations significantly prolong the time to peak plasma concentration and reduce the degree of fluctuation around the average concentration. Significant absorption occurs after intravesical (bladder) and transdermal administration, although concentrations of the active N-desethyl metabolite are lower after transdermal compared with oral administration, possibly improving tolerability. Food has been found to significantly affect the absorption of one of the controlled-release formulations of oxybutynin, enhancing the rate of drug release. Oxybutynin is extensively metabolised, principally via N-demethylation mediated by the cytochrome P450 (CYP) 3A isozyme. The pharmacokinetics of tolterodine are dependent in large part on the pharmacogenomics of the CYP2D6 and 3A4 isozymes. In an unselected population, oral bioavailability of tolterodine ranges from 10% to 74% (mean 33%) whereas in CYP2D6 extensive metabolisers and poor metabolisers mean bioavailabilities are 26% and 91%, respectively. Tolterodine is metabolised via CYP2D6 to the active metabolite 5-hydroxymethyl-tolterodine and via CYP3A to N-dealkylated metabolites. Urinary excretion of parent compound plays a minor role in drug disposition. Drug effect is based upon the unbound concentration of the so-called 'active moiety' (sum of tolterodine + 5-hydroxymethyl-tolterodine). Terminal disposition half-lives of tolterodine and 5-hydroxymethyl-tolterodine (in CYP2D6 extensive metabolisers) are 2-3 and 3-4 hours, respectively. Coadministration of antacid essentially converts the extended-release formulation into an immediate-release formulation. Knowledge of the pharmacokinetics of these agents may improve the treatment of urge incontinence by allowing the identification of individuals at high risk for toxicity with 'usual' dosages. In addition, the use of alternative formulations (controlled-release oral, transdermal) may also facilitate adherence, not only by reducing the frequency of drug administration but also by enhancing tolerability by altering the proportions of parent compound and active metabolite in the blood.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve) and primarily to acute exposure (peak concentration). Similar analyses were performed with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

The impact of CYP2C19 and CYP2D6 genotypes on metabolism of amitriptyline in Japanese psychiatric patients

We investigated the effect of the CYP2C19 and CYP2D6 genotypes on the metabolism of amitriptyline (AT) in Japanese psychiatric patients. Steady-state concentrations of AT and its metabolites (nortriptyline [NT], trans-10-hydroxy-nortriptyline [EHNT], cis-10-hydroxy-nortriptyline [ZHNT], trans-10-hydroxy-amitriptyline [EHAT], and cis-10-hydroxy-amitriptyline [ZHAT]) in 50 patients were determined by high-performance liquid chromatography. Significantly higher plasma concentrations of AT corrected for dose and body weight in the subjects with two mutated alleles of CYP2C19 than in those with no mutated alleles of CYP2C19 were observed (no mutated alleles vs. two mutated alleles: 36.0 +/- 18.2 vs. 64.0 +/- 25.2 ng/mL/mg/kg, p = 0.025). A significantly higher AT/NT ratio was seen in the subjects with two mutated alleles of CYP2C19 than in those with no mutated alleles of CYP2C19 (no mutated alleles vs. two mutated alleles: 1.27 +/- 0.59 vs. 3.40 +/- 1.02, p = 0.001). A trend for higher NT/EHNT ratio in the subjects with two mutated alleles of CYP2D6 than in those with no mutated alleles of CYP2D6 was observed (no mutated alleles vs. two mutated alleles: 0.73 +/- 0.39 vs. 1.31 +/- 0.81, p = 0.068). A trend for higher plasma concentrations of total hydroxylated metabolites of AT (EHAT + ZHAT) corrected for dose and body weight in the subjects with two mutated alleles of CYP2C19 than in those with no mutated alleles of CYP2C19 was found (no mutated alleles vs. two mutated alleles: 9.5 +/- 5.8 vs. 17.8 +/- 8.9, p = 0.051). Therefore, the genotype of CYP2C19 is one of the important determinants of the plasma concentrations of AT and the capacity to desmethylate AT. Mother compound AT is shunted via hydroxylation pathways from AT to EHAT and ZHAT in the subjects with homozygotes of mutated alleles of CYP2C19 in order to compensate for the decreased capacity to desmethylate AT.

Clinically significant drug interactions with antidepressants in the elderly

Pharmacological treatment of depression in old age is associated with an increased risk of adverse pharmacokinetic and pharmacodynamic drug interactions. Elderly patients may have multiple disease states and, therefore, may require a variety of other drugs. In addition to polypharmacy, other factors such as age-related physiological changes, diseases, genetic constitution and diet may alter drug response and, therefore, predispose elderly patients to adverse effects and drug interactions. Antidepressant drugs currently available differ in their potential for drug interactions. In general, older compounds, such as tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs), have a higher potential for interactions than newer compounds, such as selective serotonin reuptake inhibitors (SSRIs) and other relatively novel agents with a more specific mechanism of action. In particular, TCAs and MAOIs are associated with clinically significant pharmacodynamic interactions with many medications frequently prescribed to elderly patients. Moreover, TCAs may be susceptible to pharmacokinetic interactions when given in combination with inhibitors or inducers of the cytochrome P450 (CYP) isoenzymes involved in their metabolism. Because of a more selective mechanism of action, newer antidepressants have a low potential for pharmacodynamic drug interactions. However, the possibility of the serotonin syndrome should be taken into account when drugs affecting serotonergic transmission, such as SSRIs, venlafaxine or nefazodone, are coadministered with other serotonergic agents. Newer agents have a differential potential for pharmacokinetic interactions because of their selective effects on CYP isoenzymes. Within the group of SSRIs, fluoxetine and paroxetine are potent inhibitors of CYP2D6, while fluvoxamine predominantly affects CYP1A2 and CYP2C19 activity. Therefore, these agents should be closely monitored or avoided in elderly patients treated with substrates of these isoforms, especially those with a narrow therapeutic index. On the other hand, citalopram and sertraline have a low inhibitory activity on different drug metabolising enzymes and appear particularly suitable in an elderly population. Among other newer antidepressants, nefazodone is a potent inhibitor of CYP3A4 and its combination with substrates of this isoform should be avoided.

Clinical significance of pharmacokinetic interactions between antiepileptic and psychotropic drugs

As antiepileptic drugs (AEDs) and psychotropic agents are increasingly used in combination, the possibility of pharmacokinetic interactions between these compounds is relatively common. Most pharmacokinetic interactions between AEDs and psychoactive drugs occur at a metabolic level, and usually involve changes in the activity of the cytochrome P450 mixed-function oxidases (CYP) involved in their biotransformation. As a consequence of CYP inhibition or induction, plasma concentrations of a given drug may reach toxic or subtherapeutic levels, and dosage adjustments may be required to avoid adverse effects or clinical failure. Enzyme-inducing AEDs, such as carbamazepine (CBZ), phenytoin (PHT), and barbiturates, stimulate the oxidative biotransformation of many concurrently prescribed psychotropics. In particular, these AEDs may decrease the plasma concentrations of tricyclic antidepressants, many antipsychotics, including traditional compounds, i.e., haloperidol and chlorpromazine, and newer agents, i.e., clozapine, risperidone, olanzapine, quetiapine, and ziprasidone, and some benzodiazepines. Conversely, new AEDs appear to have a lower potential for interactions with all psychotropic drugs. While antipsychotics and anxiolytics do not significantly influence the pharmacokinetics of most AEDs, some newer antidepressants, such as viloxazine, fluoxetine, and fluvoxamine, may lead to higher serum levels of some AEDs, namely CBZ and PHT, through inhibition of CYP enzymes. No significant pharmacokinetic interactions have been documented between AEDs and lithium. Information about CYP enzymes responsible for the biotransformation of individual agents and about the effects of these compounds on the activity of specific CYP enzymes may help in predicting and avoiding clinically significant interactions. Apart from careful clinical observation, serum level monitoring of AEDs and psychotropic drugs can be useful in determining the need for dosage adjustments, especially if there is any change in seizure control, or possible toxicity.

The effect of CYP2C19 and CYP2D6 genotypes on the metabolism of clomipramine in Japanese psychiatric patients

In this study, the authors investigated the relationship between the metabolism of clomipramine (C) and the genotypes of cytochrome P450 (CYP) CYP2C19 and CYP2D6. Fifty-one Japanese patients (18 men and 33 women) were administered 10 to 250 mg/day of C by mouth and maintained on the same daily dose of C for at least 2 weeks to obtain steady-state concentrations. Plasma levels of C and its metabolites N-desmethylclomipramine (DC), 8-hydroxyclomipramine, and 8-hydroxy-N-desmethylclomipramine (HDC) were determined by high-performance liquid chromatography. The allele frequencies of CYP2C19*2, CYP2C19*3, CYP2D6*5, and CYP2D6*10 were 27.5%, 12.8%, 2.9%, and 43.1%, respectively. Subjects who were homozygous for mutated alleles of CYP2C19 showed approximately 75% higher concentrations of C corrected by dose and body weight compared with those who were homozygous for wild-type alleles. Also, subjects who were homozygous for mutated alleles of CYP2C19 showed an approximately 68% higher value of C/DC compared with those who were homozygous for wild-type alleles. No significant difference in the ratio of DC/HDC was observed between subjects who were homozygous for mutated alleles of CYP2D6 and those who were homozygous for wild-type alleles. These results suggest that genotyping CYP2C19 is useful for grossly predicting the risk of getting high plasma concentrations of C and the low individual capacity to demethylate C because there is marked interindividual variability within each genotype. However, the genotyping of CYP2D6 is not useful for predicting the individual capacity to hydroxylate DC.

P-hydroxylation of phenobarbital: relationship to (S)-mephenytoin hydroxylation (CYP2C19) polymorphism

The aim of the current study was to compare the pharmacokinetics of phenobarbital (PB) in extensive metabolizers (EMs) and poor metabolizers (PMs) of S-mephenytoin. Ten healthy volunteers (5 EMs and 5 PMs) were given 30 mg PB daily for 14 days. PB and p-hydroxyphenobarbital (p-OHPB) in serum and urine were measured by high-performance liquid chromatography (HPLC). Urinary excretion (12.5% versus 7.7%) and formation clearance (29.8 versus 21.1 mL/h) of p-OHPB, one of the main metabolites of PB, were significantly lower (p < .05) in PMs than in EMs. However, area under the serum concentration-time curve (153.3 in the EMs versus 122.9 microg x h/mL in the PMs), total (210.8 versus 254.9 mL/h) and renal clearance (53.1 versus 66.1 mL/h) of PB were identical between the two groups. To compare the inducibility of CYP2C19, mephenytoin was also given prior to and on the last day of PB treatment. The urinary level of 4'-hydroxymephenytoin was analyzed by a validated gas chromatograpy/mass spectrometry (GC/MS) method. The mephenytoin hydroxylation index did not change in either EMs (1.42 versus 1.42) or PMs (341.4 versus 403.5), showing that CYP2C19 was not induced by treatment with PB. These results indicated that the p-hydroxylation pathway of PB co-segregates with the CYP2C19 metabolic polymorphism. However, the overall disposition kinetics of PB were not different between EMs and PMs, and therefore polymorphic CYP2C19 seems have no major clinical implications.

Steady-state plasma levels of nortriptyline and its hydroxylated metabolites in Japanese patients: impact of CYP2D6 genotype on the hydroxylation of nortriptyline

The authors investigated the impact of the CYP2D6 genotype on steady-state concentrations of nortriptyline (NT) and its metabolites, trans-10-hydroxynortriptyline (EHNT) and cis-10-hydroxynortriptyline in a Japanese population of psychiatric patients. Forty-one patients (20 men and 21 women) were orally administered nortriptyline hydrochloride. The allele frequencies of the CYP2D6*5 and CYP2D6*10 were 4.9% and 34.1%, respectively. Significant differences in NT concentrations corrected for dose and weight were observed between the subjects with no mutated alleles and those with one mutated allele (mean +/- SD for no mutated alleles vs. one mutated allele: 70.3 +/- 25.4 vs. 98.4 +/- 36.6 ng/mL x mg(-1) x kg(-1); t = 2.54, dcf = 33, p < 0.05) and between the subjects with no mutated alleles and two mutated alleles (no mutated alleles vs. two mutated alleles: 70.3 +/- 25.4 vs. 147 +/- 31.1 ng/mL x mg(-1) x kg(-1); t = 5.87, df = 19, p < 0.0001). Also, a significant difference in the NT/EHNT ratio, which is representative of the hydroxylation ratio of NT, was observed between the subjects with no mutated alleles and those with two mutated alleles (no mutated alleles vs. two mutated alleles: 0.82 +/- 0.30 vs. 2.71 +/- 0.84; t = 7.86, df = 19, p < 0.0001). Multiple regression analysis showed that the number of mutated alleles of CYP2D6, which was the only significant factor, accounted for 41% and 48% of the variability in log(NT corrected for dose and weight) and log(NT/EHNT), respectively.

Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs

The human cytochrome P450 (CYP) isoforms mediating nortriptyline 10-hydroxylation have been identified using kinetic studies on heterologously expressed human CYPs and chemical inhibition studies on human liver microsomes. Nortriptyline was metabolized to E-10-hydroxynortriptyline by human lymphoblast-expressed CYPs 2D6 (Km 2.1 microM) and 3A4 (Km 37.4 microM) with high and low affinity, respectively, whereas CYPs 1A2, 2A6, 2B6, 2C9, 2C19, and 2E1 had no detectable activity. Human liver microsomal nortriptyline E-10-hydroxylation displayed biphasic kinetics. The high-affinity component (Km 1.3 +/- 0.4 microM, n = 11 livers) was selectively inhibited by the CYP 2D6 inhibitor quinidine, whereas the CYP3A4 inhibitor ketoconazole selectively inhibited the low-affinity component (K(m) 24.4 +/- 7 microM, n = 11 livers). Inhibition by ketoconazole increased with increasing substrate concentration, whereas the reverse was true for quinidine. The Vmax of the low-affinity component in human liver microsomes was significantly correlated (r2 = 0.84) with the relative activity factor for CYP3A4, a measure of the amount of catalytically active enzyme. A simulation of the relative contribution of CYPs 2D6 and 3A4 to net nortriptyline hydroxylation rate suggested that the relative contribution of CYP3A4 is only 20% even at the higher end of the therapeutic range. Induction of CYP3A4 will increase its importance and increase the net metabolic rate, whereas inhibition of CYP3A4 will be of little importance due to its minimal relative contribution under uninduced conditions. The identification of CYP3A4 as a low-affinity nortriptyline E-10-hydroxylase explains the ability of poor metabolizers of debrisoquin to hydroxylate nortriptyline, as well as the increased in vivo clearance via this pathway caused by CYP3A4-inducing drugs such as pentobarbital, carbamazepine, and rifampin.

Interindividual variation of plasma haloperidol concentrations and the impact of concomitant medications: the analysis of therapeutic drug monitoring data

We analyzed therapeutic drug monitoring (TDM) data from 231 schizophrenic inpatients (137 men, 94 women) and investigated interindividual differences of plasma haloperidol (HAL) concentrations and drug/drug interactions between HAL and various concomitant drugs. Plasma HAL concentrations were determined by an enzyme immunoassay (EIA) method. Plasma HAL concentrations per daily dose of HAL per body weight (HAL C/D ratio) demonstrated an approximately 11-fold interindividual variation. The patient subjects who received carbamazepine (CBZ) concomitantly had a mean HAL C/D ratio that was 37% lower than that of the patient subjects without CBZ. The patient subjects treated with concomitant phenobarbital (PB) also showed a mean HAL C/D ratio that was 22% lower than those without PB. We concluded that careful evaluation of HAL TDM data and consideration of the impact of concomitant medication such as CBZ or PB that might influence the metabolism of HAL is necessary in daily clinical settings to avoid insufficient clinical response because of lowered concentrations of HAL or adverse effects because of high concentrations of HAL.

Time course of enzyme induction in humans: effect of pentobarbital on nortriptyline metabolism

To study the effect of induction we gave six male volunteers 10 mg nortriptyline three times a day for 4 weeks and 0.2 gm pentobarbital on days 8 to 21. Plasma and urinary levels of nortriptyline and metabolites were measured. The rate and extent of induction of the enzyme(s) were estimated by a model with use of nortriptyline concentrations. There was a marked decrease of nortriptyline levels after 2 days of pentobarbital treatment. Total clearance of nortriptyline increased more than twofold (range, 1.6-fold to 4.1-fold). Apparent metabolic clearance by 10-hydroxylation increased markedly. The decrease in nortriptyline levels was more rapid than the increase after pentobarbital cessation, fitting with the theory of the model. The induction of nortriptyline metabolism is probably mainly the result of an increase in a non-CYP 2D6 P450 isozyme, possibly CYP 3A4 or a CYP 2C form. More knowledge of induction characteristics of drugs should lead to better predictions of decreased effects and appearance of adverse effects. The kinetic model used for analysis of our data could then be useful.