Pomegranate juice and pomegranate extract do not impair oral clearance of flurbiprofen in human volunteers: divergence from in vitro results

Nutrient interactions with prescription drugs are a topic of ongoing basic and clinical research. Pomegranate juice and a 1-g capsule containing pomegranate extract were evaluated in vitro and in vivo as inhibitors of cytochrome P450 2C9 (CYP2C9), with flurbiprofen serving as the index substrate. Fluconazole was the positive control inhibitor. The in vitro 50% inhibitory concentration (IC(50)) values for pomegranate juice and extract were below 1% (vol/vol), with no evidence of mechanism-based (irreversible) inhibition. In clinical studies, flurbiprofen pharmacokinetics were unchanged by pomegranate juice or extract as compared to a low-polyphenol placebo control beverage. However, fluconazole significantly reduced the oral clearance of flurbiprofen. Despite inhibition of CYP2C9 in vitro, pomegranate juice and extract had no effect on CYP2C9 activity in human subjects, and can be consumed by patients taking CYP2C9 substrate drugs with negligible risk of a pharmacokinetic interaction.

Short-term clarithromycin administration impairs clearance and enhances pharmacodynamic effects of trazodone but not of zolpidem

The kinetic and dynamic interactions of 5 mg zolpidem and 50 mg trazodone with 500 mg clarithromycin (4 doses given over 32 h) were investigated in a 5-way double crossover study with 10 healthy volunteers. The five treatment conditions were: placebo + placebo; zolpidem + placebo; zolpidem + clarithromycin; trazodone + placebo; and trazodone + clarithromycin. Coadministration of clarithromycin increased trazodone area under the curve, prolonged elimination half-life, increased peak plasma concentration (C(max)), and reduced oral clearance. In contrast, clarithromycin had no significant effect on any kinetic parameter for zolpidem. Clarithromycin did not potentiate sedation caused by zolpidem. However, clarithromycin coadministered with trazodone significantly increased self- and observer-rated sedation and ratings of feeling "spacey." Thus, short-term clarithromycin coadministration significantly impairs trazodone clearance, elevates plasma concentrations, and enhances sedative effects. However, clarithromycin has no significant kinetic or dynamic interaction with zolpidem.

Pharmacokinetic and Pharmacodynamic Interactions Between Zolpidem and Caffeine

The kinetic and dynamic interaction of caffeine and zolpidem was evaluated in a double-blind, single-dose, six-way crossover study of 7.5 mg zolpidem (Z) or placebo (P) combined with low-dose caffeine (250 mg), high-dose caffeine (500 mg), or placebo. Caffeine coadministration modestly increased maximum plasma concentration (C(max)) and area under the plasma concentration-time curve of zolpidem by 30-40%, whereas zolpidem did not significantly affect the pharmacokinetics of caffeine or its metabolites. Compared to P+P, Z+P significantly increased sedation, impaired digit-symbol substitution test performance, slowed tapping speed and reaction time, increased EEG relative beta amplitude, and impaired delayed recall. Caffeine partially, but not completely, reversed most pharmacodynamic effects of zolpidem. Thus, caffeine only incompletely reverses zolpidem's sedative and performance-impairing effects, and cannot be considered as an antidote to benzodiazepine agonists.

Molecular and pharmacokinetic evaluation of rat hepatic and gastrointestinal cytochrome p450 induction by tamoxifen

Tamoxifen (TAM) is a first-line endocrine treatment for all stages of postmenopausal breast cancer. The cytochrome P450 (CYP) enzymes catalyze the majority of TAM's primary metabolism, producing N-desmethyltamoxifen (DMT) and 4-hydroxytamoxifen (4-OH-TAM) in both humans and rats. CYP 3A isoforms are the predominant subfamily involved in the formation of DMT and recent studies have shown that TAM induces hepatic forms of these enzymes. TAM's inductive effect on gastrointestinal CYP 3A has not been previously reported. The current studies investigated TAM's induction of CYP isoforms (3A and 2B) in female rat gastrointestinal and hepatic tissue at the mRNA, protein, and catalytic level. Since previous studies have not addressed whether TAM induction causes changes to the overall pharmacokinetics (PKs), a rat PK model was used to determine if TAM induced its own metabolism, and/or the metabolism of a CYP 3A substrate, midazolam (MDZ). Phenobarbital (PB) and/or dexamethasone (DEX) were used as positive controls for all studies. TAM significantly induced, or caused a trend towards induction of all studied parameters for hepatic CYP 3A and 2B, whereas intestinal CYP 3A and 2B analysis did not show significant induction by TAM at any level. A study evaluating time-dependent alterations in the PK profile of TAM showed no change in apparent oral clearance (Cl(app)) during two weeks of chronic dosing with TAM. However, the Cl(app) for MDZ was shown to trend towards an increase after two weeks of dosing with TAM, in a second PK study. These combined investigations suggest that TAM is an inducer of rat hepatic CYP 3A and 2B isoforms, and this agent has the potential of influencing the PK of coadministered 3A substrates.

Relative contribution of CYP3A to amitriptyline clearance in humans: in vitro and in vivo studies

The relative contribution of cytochrome P450 3A (CYP3A) to the oral clearance of amitriptyline in humans has been assessed using a combination of in vitro approaches together with a clinical pharmacokinetic interaction study using the CYP3A-selective inhibitor ketoconazole. Lymphoblast-expressed CYPs were used to study amitriptyline N-demethylation and E-10 hydroxylation in vitro. The relative activity factor (RAF) approach was used to predict the relative contribution of each CYP isoform to the net hepatic intrinsic clearance (sum of N-demethylation and E-10 hydroxylation). Assuming no extrahepatic metabolism, the model-predicted contribution of CYP3A to net intrinsic clearance should equal the fractional decrement in apparent oral clearance of amitriptyline upon complete inhibition of the enzyme. This hypothesis was tested in a clinical study of amitriptyline (50 mg, p.o.) with ketoconazole (three 200 mg doses spaced 12 hours apart) in 8 healthy volunteers. The RAF approach predicted CYP2C19 to be the dominant contributor (34%), with a mean 21% contribution of CYP3A (range: 8%-42% in a panel of 12 human livers). The mean apparent oral clearance of amitriptyline in 8 human volunteers was decreased from 2791 ml/min in the control condition to 2069 ml/min with ketoconazole. The average 21% decrement (range: 2%-40%) was identical to the mean value predicted in vitro using the RAF approach. The central nervous system (CNS) sedative effects of amitriptyline were slightly greater when ketoconazole was coadministered, but the differences were not statistically significant. In conclusion, CYP3A plays a relatively minor role in amitriptyline clearance in vivo, which is consistent with in vitro predictions using the RAF approach.

Escitalopram (S-citalopram) and its metabolites in vitro: cytochromes mediating biotransformation, inhibitory effects, and comparison to R-citalopram

Transformation of escitalopram (S-CT), the pharmacologically active S-enantiometer of citalopram, to S-desmethyl-CT (S-DCT), and of S-DCT to S-didesmethyl-CT (S-DDCT), was studied in human liver microsomes and in expressed cytochromes (CYPs). Biotransformation of the R-enantiomer (R-CT) was studied in parallel. S-CT was transformed to S-DCT by CYP2C19 (K(m) = 69 microM), CYP2D6 (K(m) = 29 microM), and CYP3A4 (K(m) = 588 microM). After normalization for hepatic abundance, relative contributions to net intrinsic clearance were 37% for CYP2C19, 28% for CYP2D6, and 35% for CYP3A4. At 10 microM S-CT in liver microsomes, S-DCT formation was reduced to 60% of control by 1 microM ketoconazole, and to 80 to 85% of control by 5 microM quinidine or 25 microM omeprazole. S-DDCT was formed from S-DCT only by CYP2D6; incomplete inhibition by quinidine in liver microsomes indicated participation of a non-CYP pathway. Based on established index reactions, S-CT and S-DCT were negligible inhibitors (IC(50) > 100 microM) of CYP1A2, -2C9, -2C19, -2E1, and -3A, and weakly inhibited CYP2D6 (IC(50) = 70-80 microM). R-CT and its metabolites, studied using the same procedures, had properties very similar to those of the corresponding S-enantiomers. Thus S-CT, biotransformed by three CYP isoforms in parallel, is unlikely to be affected by drug interactions or genetic polymorphisms. S-CT and S-DCT are also unlikely to cause clinically important drug interactions via CYP inhibition.

Inhibition of human cytochrome P450 isoforms by nonnucleoside reverse transcriptase inhibitors

The capacity of three clinically available nonnucleoside reverse transcriptase inhibitors (NNRTIs) to inhibit the activity of human cytochromes P450 (CYPs) was studied in vitro using human liver microsomes. Delavirdine, nevirapine, and efavirenz produced negligible inhibition of phenacetin O-deethylation (CYP1A2) or dextromethorphan O-demethylation (CYP2D6). Nevirapine did not inhibit hydroxylation of tolbutamide (CYP2C9) or S-mephenytoin (CYP2C19), but these CYP isoforms were importantly inhibited by delavirdine and efavirenz. This indicates the likelihood of significantly impaired clearance of CYP2C substrate drugs (such as phenytoin, tolbutamide, and warfarin) upon initial exposure to these two NNRTIs. Delavirdine and efavirenz (but not nevirapine) also were strong inhibitors of CYP3A, consistent with clinical hazards of initial cotreatment with either of these drugs and substrates of CYP3A. The in vitro microsomal model provides relevant predictive data on probable drug interactions with NNRTIs when the mechanism is inhibition of CYP-mediated drug biotransformation. However, the model does not incorporate interactions attributable to enzyme induction.

Comparison between cytochrome P450 (CYP) content and relative activity approaches to scaling from cDNA-expressed CYPs to human liver microsomes: ratios of accessory proteins as sources of discrepancies between the approaches

Relative activity factors (RAFs) and immunoquantified levels of cytochrome P450 (CYP) isoforms both have been proposed as scaling factors for the prediction of hepatic drug metabolism from studies using cDNA-expressed CYPs. However, a systematic comparison of the two approaches, including possible mechanisms underlying differences, is not available. In this study, RAFs determined for CYPs 1A2, 2B6, 2C19, 2D6, and 3A4 in 12 human livers using lymphoblast-expressed enzymes were compared to immunoquantified protein levels. 2C19, 2D6, and 3A4 RAFs were similar to immunoquantified enzyme levels. In contrast, 1A2 RAFs were 5- to 20-fold higher than CYP1A2 content, and the RAF:content ratio was positively correlated with the molar ratio of NADPH:CYP oxidoreductase (OR) to CYP1A2. The OR:CYP1A2 ratio in lymphoblast microsomes was 92-fold lower than in human liver microsomes. Reconstitution experiments demonstrated a 10- to 20-fold lower activity at OR:CYP1A2 ratios similar to those in lymphoblasts, compared with those in human livers. CYP2B6-containing lymphoblast microsomes had 29- and 13-fold lower OR:CYP and cytochrome b(5):CYP ratios, respectively, than did liver microsomes and yielded RAFs that were 6-fold higher than CYP2B6 content. Use of metabolic rates from cDNA-expressed CYPs containing nonphysiologic concentrations of electron-transfer proteins (relative to human liver microsomes) in conjunction with hepatic CYP contents may lead to incorrect predictions of liver microsomal rates and relative contributions of individual isoforms. Scaling factors used in bridging the gap between expression systems and liver microsomes should not only incorporate relative hepatic abundance of individual CYPs but also account for differences in activity per unit enzyme in the two systems.

Kinetics and dynamics of lorazepam during and after continuous intravenous infusion

OBJECTIVE

To evaluate the kinetics and dynamics of lorazepam during administration as a bolus plus an infusion, using electroencephalography as a pharmacodynamic end point.

METHODS

Nine volunteers received a 2-mg bolus loading dose of lorazepam, coincident with the start of a 2 microg/kg/hr zero-order infusion. The infusion was stopped after 4 hrs. Plasma lorazepam concentrations and electroencephalographic activity in the 13- to 30-Hz range were monitored for 24 hrs.

RESULTS

The bolus-plus-infusion scheme rapidly produced plasma lorazepam concentrations that were close to those predicted to be achieved at true steady state. Mean kinetic values for lorazepam were as follows: volume of distribution, 126 L; elimination half-life, 13.8 hrs; and clearance, 109 mL/min. Electroencephalographic effects were maximal 0.5 hr after the loading dose, were maintained essentially constant during infusion, and then declined in parallel with plasma concentrations after the infusion was terminated. There was no evidence of tolerance. Plots of pharmacodynamic electroencephalographic effect vs. plasma lorazepam concentration demonstrated counterclockwise hysteresis, consistent with an effect-site equilibration delay. This was incorporated into a kinetic-dynamic model in which hypothetical effect-site concentration was related to pharmacodynamic electroencephalographic effect via the sigmoid Emax model. The analysis yielded the following mean estimates: maximum electroencephalographic effect, 12.7% over baseline; 50% effective concentration, 13.1 ng/mL; and effect-site equilibration half-life, 8.8 mins.

CONCLUSION

Despite the delay in effect onset, continuous infusion of lorazepam, preceded by a bolus loading dose, produces a relatively constant sedative effect on the central nervous system, which can be utilized in the context of critical care medicine.

Differences in pharmacodynamics but not pharmacokinetics between subjects with panic disorder and healthy subjects after treatment with a single dose of alprazolam

The pharmacokinetics and pharmacodynamics of the benzodiazepine alprazolam (1 mg, administered orally) were compared between eight patients with panic disorder and eight age- and sex-matched healthy volunteers. Subjects received orally administered placebo and alprazolam in a randomized, double-blind, single-dose crossover study. The elimination half-life, time of maximum plasma concentration, maximum concentration, volume of distribution, and clearance of alprazolam were similar for both groups. For each cohort, alprazolam treatment (vs. placebo) produced significant changes in typical benzodiazepine agonist effects, such as increased sedation and impaired cognitive performance on the digit-symbol substitution test. For the panic disorder group only, there was a significant increase in the subjective rating of"contented" and a reduction in the rating of "easily irritated." For the healthy volunteer group, alprazolam produced increases in ratings of "fatigued" and "slowed thinking," but also increases in ratings of "relaxed." In each group, alprazolam significantly increased the electroencephalographic (EEG) measure of relative beta amplitude (range, 13-30 Hz) compared with placebo. Concentration-EEG response curves fit a sigmoid E(max) model, and there was greater sensitivity to EEG effects, as measured by a 28% reduction in the EC50 value, in the panic disorder group compared with healthy control subjects. After alprazolam treatment, there was increased sensitivity to EEG and mood effects and fewer aversive effects in the panic disorder group compared with healthy subjects. There were no differences in the pharmacodynamic measures of sedation and cognition or differences in pharmacokinetics between the two groups.

Differential impairment of triazolam and zolpidem clearance by ritonavir

BACKGROUND

The viral protease inhibitor ritonavir has the capacity to inhibit and induce the activity of cytochrome P450-3A (CYP3A) isoforms, leading to drug interactions that may influence the efficacy and toxicity of other antiretroviral therapies, as well as pharmacologic treatments of coincident or complicating diseases.

METHODS

The inhibitory effect of ritonavir on the biotransformation of the hypnotic agents triazolam and zolpidem was tested in vitro using human liver microsomes. In a double-blind clinical study, volunteer study subjects received 0.125 mg triazolam or 5.0 mg zolpidem concurrent with low-dose ritonavir (four doses of 200 mg), or with placebo.

RESULTS

Ritonavir was a potent in vitro inhibitor of triazolam hydroxylation but was less potent as an inhibitor of zolpidem hydroxylation. In the clinical study, ritonavir reduced triazolam clearance to < 4% of control values (p < .005), prolonged elimination half-life (41 versus 3 hours; p < .005), and magnified benzodiazepine agonist effects such as sedation and performance impairment. In contrast, ritonavir reduced zolpidem clearance to 78% of control values (p < .08), and slightly prolonged elimination half-life (2.4 versus 2.0 hours; NS). Benzodiazepine agonist effects of zolpidem were not altered by ritonavir.

CONCLUSION

Short-term low-dose administration of ritonavir produces a large and significant impairment of triazolam clearance and enhancement of clinical effects. In contrast, ritonavir produced small and clinically unimportant reductions in zolpidem clearance. The findings are consistent with the complete dependence of triazolam clearance on CYP3A activity, compared with the partial dependence of zolpidem clearance on CYP3A.

Comparative kinetics and response to the benzodiazepine agonists triazolam and zolpidem: evaluation of sex-dependent differences

Eighteen healthy volunteers (10 men and 8 women) participated in a single-dose, double-blind, three-way crossover pharmacokinetic and pharmacodynamic study. Treatment conditions were 0.25 mg of triazolam, a full-agonist benzodiazepine ligand; 10 mg of zolpidem, an imidazopyridine having relative selectivity for the type 1 benzodiazepine receptor subtype; and placebo. Weight-normalized clearance of triazolam was higher in women than in men (8.7 versus 5. 5 ml/min/kg), but the difference was not significant. In contrast, zolpidem clearance was lower in women than in men (3.5 versus 6.7 ml/min/kg, P <.06). Compared to placebo, both active medications produced significant benzodiazepine agonist-like pharmacodynamic effects: sedation, impaired psychomotor performance, impaired information recall, and increased electroencephalographic beta-amplitude. Effects of triazolam and zolpidem in general were comparable and less than 8 h in duration. There was no evidence of a substantial or consistent sex difference in pharmacodynamic effects or in the kinetic-dynamic relationship, although subtle differences could not be ruled out due to low statistical power. The complete dependence of triazolam clearance on CYP3A activity, as opposed to the mixed CYP participation in zolpidem clearance, may explain the differing sex effects on clearance of the two compounds.

Alprazolam-ritonavir interaction: implications for product labeling

BACKGROUND

Pharmacokinetic interactions involving antiretroviral therapies may critically influence the efficacy and toxicity of these drugs, as well as pharmacologic treatments of coincident or complicating diseases. The viral protease inhibitor ritonavir is of particular concern since it both inhibits and induces the activity of cytochrome P450 3A (CYP3A) isoforms.

METHODS

The inhibitory effect of ritonavir on the metabolism of alprazolam, a CYP3A-mediated reaction in humans, was tested in vitro using human liver microsomes. In a double-blind clinical study, volunteer subjects received 1.0 mg of alprazolam concurrent with low-dose ritonavir (four doses of 200 mg) or with placebo.

RESULTS

Ritonavir was a potent in vitro inhibitor of alprazolam hydroxylation. The 50% inhibitory concentration was 0.11 micromol/L (0.08 microg/mL); this is below the usual therapeutic plasma concentration range (generally exceeding 2 microg/mL). In the clinical study, ritonavir reduced alprazolam clearance to 41% of control values (P < .001), prolonged elimination half-life (mean values, 30 versus 13 hours; P < .005), and magnified benzodiazepine agonist effects such as sedation and performance impairment.

CONCLUSION

Consistent with in vitro results, administration of low doses of ritonavir for a short duration of time resulted in large impairment of alprazolam clearance and enhancement of clinical effects. Removal from product labeling of a warning against coadministration of ritonavir and alprazolam was based on a previous study only of extended exposure to ritonavir, in which CYP3A induction offset inhibition. Kinetic interactions involving antiretroviral therapies may be complex and time dependent. Product labeling should reflect this complexity.

In vitro metabolism of trazodone by CYP3A: inhibition by ketoconazole and human immunodeficiency viral protease inhibitors

BACKGROUND

Pharmacologic treatment of emotional disorders in HIV-infected patients can be more easily optimized by understanding of potential interactions of psychotropic drugs with medications used to treat HIV infection and its sequelae.

METHODS

Biotransformation of the antidepressant trazodone to its principal metabolite, meta-chlorophenylpiperazine (mCPP), was studied in vitro using human liver microsomes and heterologously expressed individual human cytochromes. Interactions of trazodone with the azole antifungal agent, ketoconazole, and with human immunodeficiency virus protease inhibitors (HIVPIs) were studied in the same system.

RESULTS

Formation of mCPP from trazodone in liver microsomes had a mean (+/- SE) K(m) value of 163 (+/- 21) micromol/L. Ketoconazole, a relatively specific CYP3A inhibitor, impaired mCPP formation consistent with a competitive mechanism, having an inhibition constant (K(i)) of 0.12 (+/- 0.01) micromol/L. Among heterologously expressed human cytochromes, only CYP3A4 mediated formation of mCPP from trazodone; the K(m) was 180 micromol/L, consistent with the value in microsomes. The HIVPI ritonavir was a potent inhibitor of mCPP formation in liver microsomes (K(i) = 0.14 +/- 0.04 micromol/L). The HIVPI indinavir was also a strong inhibitor, whereas saquinavir and nelfinavir were weaker inhibitors.

CONCLUSIONS

CYP3A-mediated clearance of trazodone is inhibited by ketoconazole, ritonavir and indinavir, and indicates the likelihood of pharmacokinetic interactions in vivo.

Effects of age on in vitro midazolam biotransformation in male CD-1 mouse liver microsomes

To study age-related changes in drug metabolism, we examined the in vitro biotransformation of midazolam (MDZ), a human cytochrome P-450 (CYP) 3A substrate, using liver microsomes from three age groups of male CD-1 mice ranging from 6 weeks to 2 years old. MDZ was metabolized to two major products, alpha-OH- and 4-OH-MDZ, which were quantified by HPLC. For both metabolites, V(max) values were reduced in old livers (P <.05), while K(m) values did not change with age. The net intrinsic clearance (the sum of V(max)/K(m) for both pathways) also was reduced in the old animals (P <.05). The capacity of ketoconazole, a CYP3A inhibitor in humans, to inhibit the biotransformation of MDZ and of alprazolam, another human CYP3A substrate, did not differ significantly with age. At 100 microM alprazolam, 0.5 microM ketoconazole inhibited metabolite formation by >80%. At 30 microM MDZ, 2.5 microM ketoconazole impaired 4-OH-MDZ formation by 88%, whereas it reduced alpha-OH-MDZ formation by only 46%. Immunoinhibition studies with polyclonal anti-rat CYP3A1/2 and CYP2C11 antibodies confirmed that 4-OH-MDZ formation was largely CYP3A-dependent, while alpha-OH-MDZ formation was mediated by CYP3A and -2C isoforms. Western blot analysis revealed decreased microsomal content of CYP3A in old livers. Net intrinsic clearance of MDZ was correlated with total CYP3A content (P <.001). These results demonstrate a reduction in MDZ biotransformation in old male mice, which may be attributable, in part, to decreased CYP3A content in old livers. Changes in expression and activity of CYP2C isoforms also may contribute to age-related changes in MDZ biotransformation, but this requires more investigation.

Human cytochromes and some newer antidepressants: kinetics, metabolism, and drug interactions

The appearance of selective serotonin reuptake inhibitor antidepressants in the mid-1980s caused the discipline of clinical psychopharmacology to refocus attention to the topics of drug metabolism and drug interactions. This article reviews the metabolic profiles of some newer antidepressants, the clinical implications of metabolic properties, and research methodology that can be applied in determining which specific human cytochromes P450 (CYP) mediate metabolic pathways. Also reviewed are the relative activities of various new antidepressants as inhibitors of CYPs, and the benefits and drawbacks of in vivo and in vitro methodologies for identification and quantitation of drug interactions.

Citalopram and desmethylcitalopram in vitro: human cytochromes mediating transformation, and cytochrome inhibitory effects

BACKGROUND

Biotransformation of citalopram (CT), a newly available selective serotonin reuptake inhibitor antidepressant, to its principal metabolite, desmethycitalopram (DCT), and the capacity of CT and DCT to inhibit human cytochromes P450, were studied in vitro.

METHODS

Formation of DCT from CT was evaluated using human liver microsomes and microsomes from cDNA-transfected human lymphoblastoid cells. Cytochrome inhibition by CT and DCT in liver microsomes was studied using isoform-specific index reactions.

RESULTS

Formation of DCT from CT in liver microsomes had a mean apparent K(m) of 174 mumol/L. Coincubation with 1 mumol/L ketoconazole reduced reaction velocity to 46 to 58% of control values, while omeprazole, 10 mumol/L, reduced velocity to 80% of control. Quinidine produced minimal inhibition. DCT was formed from CT by heterologously expressed human P450-2D6, -2C19, -3A4. After accounting for the relative abundance of individual cytochromes, 3A4 and 2C19 were estimated to make major contributions to net reaction velocity, with a possible contribution of 2D6 at therapeutic CT concentrations. CT and DCT themselves produced negligible inhibition of 2C9, 2E1, and 3A, and only weak inhibition of 1A2, 2C19, and 2D6.

CONCLUSIONS

Formation of DCT from CT is mediated mainly by P450-3A4 and 2C19, with an additional contribution of 2D6. CT at therapeutic doses in humans may produce a small degree of inhibition of P450-1A2, -2C19, and -2D6, but negligible inhibition of P450-2C9, -2E1, and -3A.

Population pharmacokinetics of methylphenidate in children with attention-deficit hyperactivity disorder

Sources of individual variation in plasma methylphenidate (MP) concentrations during usual clinical use are not established. This was evaluated in a series of patients receiving clinical treatment with MP. A single plasma MP concentration was determined in each of 273 children and adolescents ages 5 to 18 years (mean: 11.1 years) who were clinically good responders to MP for the treatment of attention-deficit hyperactivity disorder. MP was given on a twice-daily schedule (mean dose: 25 mg/day) in 40% of patients and three times daily (mean dose: 39.3 mg/day) in 60%. A nonlinear regression model was applied to estimate overall population values of MP clearance and elimination half-life (t1/2), assuming a one-component model with first-order absorption and elimination, and further assuming that clearance is linearly related to body weight. The model incorporated each patient's dosage size and schedule, body weight, and time of the plasma sample. Iterated solutions of best fit were: t1/2, 4.5 hours (95% confidence interval [CI]: 3.1-8.1 hours), and apparent clearance, 90.7 ml/min/kg (95% CI: 74.6-106.7 ml/min/kg). The model explained 43% of the overall variance in MP concentrations (r2 = 0.43, p < .001). In a small subsample (N = 16), a second plasma sample was drawn at the same time of day and at the same dose; the correlation between the two concentration values was 0.83. The relatively noninvasive approach used in this study allows the assessment of pharmacokinetic properties of medications under conditions of appropriate clinical use in special populations such as children, adolescents, and the elderly.

Zolpidem metabolism in vitro: responsible cytochromes, chemical inhibitors, and in vivo correlations

AIMS

To determine the human cytochromes mediating biotransformation of the imidazopyridine hypnotic, zolpidem, and the clinical correlates of the findings.

METHODS

Kinetic properties of zolpidem biotransformation to its three hydroxylated metabolites were studied in vitro using human liver microsomes and heterologously expressed individual human cytochromes.

RESULTS

The metabolic product termed M-3 accounted for more than 80% of net intrinsic clearance by liver microsomes in vitro. Microsomes containing human cytochromes CYP1A2, 2C9, 2C19, 2D6, and 3 A4 expressed by cDNA-transfected human lymphoblastoid cells mediated zolpidem metabolism in vitro. The kinetic profile for zolpidem metabolite formation by each individual cytochrome was combined with estimated relative abundances based on immunological quantification, yielding projected contributions to net intrinsic clearance of: 61% for 3 A4, 22% for 2C9, 14% for 1A2, and less than 3% for 2D6 and 2C19. These values were consistent with inhibitory effects of ketoconazole and sulfaphenazole on zolpidem biotransformation by liver microsomes. Ketoconazole had a 50% inhibitory concentration (IC50 ) of 0.61 microm vs formation of the M-3 metabolite of zolpidem in vitro; in a clinical study, ketoconazole coadministration reduced zolpidem oral clearance by approximately 40%, somewhat less than anticipated based on the IC50 value and total plasma ketoconazole levels, but much more than predicted based on unbound plasma ketoconazole levels.

CONCLUSIONS

The incomplete dependence of zolpidem clearance on CYP3A activity has clinical implications for susceptibility to metabolic inhibition.

Nefazodone, meta-chlorophenylpiperazine, and their metabolites in vitro: cytochromes mediating transformation, and P450-3A4 inhibitory actions

RATIONALE

Understanding of the mechanisms of biotransformation of antidepressant drugs, and of their capacity to interact with other medications, is of direct relevance to rational clinical psychopharmacology.

OBJECTIVES

To determine the human cytochromes P450 mediating the metabolism of nefazodone, and the inhibitory activity of nefazodone and metabolites versus human P450-3A.

METHODS

Biotransformation of nefazodone to its metabolic products, and of meta-chlorophenylpiperazine (mCPP) to para-hydroxy-mCPP, was studied in vitro using human liver microsomes and heterologously expressed human cytochromes. Nefazodone and metabolites were also tested as inhibitors of alprazolam hydroxylation, reflecting activity of cytochrome P450-3A isoforms.

RESULTS

mCPP and two hydroxylated derivatives were the principal metabolites formed from nefazodone by liver microsomes. Metabolite production was strongly inhibited by ketoconazole or troleandomycin (relatively specific P450-3A inhibitors), and by an anti-P450-3A antibody. Only heterologously expressed human P450-3A4 mediated formation of nefazodone metabolites from the parent compound. Nefazodone, hydroxy-nefazodone, and para-hydroxy-nefazodone were strong 3A inhibitors, being more potent than norfluoxetine and fluvoxamine, but less potent than ketoconazole. The triazoledione metabolite and mCPP had weak or negligible 3A-inhibiting activity. Formation of parahydroxy-mCPP from mCPP was mediated by heterologously expressed P450-2D6; in liver microsomes, the reaction was strongly inhibitable by quinidine, a relatively specific 2D6 inhibitor.

CONCLUSION

The complex parallel biotransformation pathways of nefazodone are mediated mainly by human cytochrome P450-3A, whereas clearance of mCPP is mediated by P450-2D6. Nefazodone and two of its hydroxylated metabolites are potent 3A inhibitors, accounting for pharmacokinetic drug interactions of nefazodone with 3A substrate drugs such as triazolam and alprazolam.

O- and N-demethylation of venlafaxine in vitro by human liver microsomes and by microsomes from cDNA-transfected cells: effect of metabolic inhibitors and SSRI antidepressants

The biotransformation of venlafaxine (VF) into its two major metabolites, O-desmethylvenlafaxine (ODV) and N-desmethylvenlafaxine (NDV) was studied in vitro with human liver microsomes and with microsomes containing individual human cytochromes from cDNA-transfected human lymphoblastoid cells. VF was coincubated with selective cytochrome P450 (CYP) inhibitors and several selective serotonin reuptake inhibitors (SSRIs) to assess their inhibitory effect on VF metabolism. Formation rates for ODV incubated with human microsomes were consistent with Michaelis-Menten kinetics for a single-enzyme mediated reaction with substrate inhibition. Mean parameters determined by non-linear regression were: Vmax = 0.36 nmol/min/mg protein, K(m) = 41 microM, and Ks 22901 microM (Ks represents a constant which reflects the degree of substrate inhibition). Quinidine (QUI) was a potent inhibitor of ODV formation with a Ki of 0.04 microM, and paroxetine (PX) was the most potent SSRI at inhibiting ODV formation with a mean Ki value of 0.17 microM. Studies using expressed cytochromes showed that ODV was formed by CYP2C9, -2C19, and -2D6. CYP2D6 was dominant with the lowest K(m), 23.2 microM, and highest intrinsic clearance (Vmax/K(m) ratio). No unique model was applicable to the formation of NDV for all four livers tested. Parameters determined by applying a single-enzyme model were Vmax = 2.14 nmol/min/mg protein, and K(m) = 2504 microM. Ketoconazole was a potent inhibitor of NDV production, although its inhibitory activity was not as great as observed with pure 3A substrates. NDV formation was also reduced by 42% by a polyclonal rabbit antibody against rat liver CYP3A1. Studies using expressed cytochromes showed that NDV was formed by CYP2C9, -2C19, and -3A4. The highest intrinsic clearance was attributable to CYP2C19 and the lowest to CYP3A4. However the high in vivo abundance of 3A isoforms will magnify the importance of this cytochrome. Fluvoxamine (FX), at a concentration of 20 microM, decreased NDV production by 46% consistent with the capacity of FX to inhibit CYP3A, 2C9, and 2C19. These results are consistent with previous studies that show CYP2D6 and -3A4 play important roles in the formation of ODV and NDV, respectively. In addition we have shown that several other CYPs have important roles in the biotransformation of VF.

Extrapolating in vitro data on drug metabolism to in vivo pharmacokinetics: evaluation of the pharmacokinetic interaction between amitriptyline and fluoxetine

Recently, models have been proposed to extrapolate in vitro data on the influence of inhibitors on drug metabolism to in vivo decrement in drug clearance. Many factors influence drug clearance such as age, gender, habits, diet, environment, liver disease, heredity, and other drugs. In vitro investigation of hepatic cytochrome P450 activity has generally centered on genetic influences and interactions with other drugs. This group of enzymes is involved in many, although not all, drug interactions. The interaction of amitriptyline and fluoxetine is an example. Of the different in vitro paradigms, interaction studies utilizing human liver microsomal preparations have proved to be the most generally applicable for in vitro scaling models. Assuming Michaelis-Menten conditions and applying nonlinear regression, a hybrid inhibition constant (Ki) can be generated that allows classification of the inhibitory potency of an inhibitor toward a specific reaction. This constant is largely independent of the substrate concentration, but in vivo relevance is critically dependent on the inhibitor concentration in the site of metabolic activity, the liver cell cytosol. Many lipophilic drugs are extensively bound to plasma protein but, nonetheless, demonstrate extensive partitioning into liver tissue. This is not compatible with diffusion only of the unbound drug fraction into liver cells. The introduction of a partition factor, based on data from a number of possible sources, provided a reasonable basis for the scaling of in vitro data to in vivo conditions. Many interactions could be reconstructed or predicted with greater accuracy and clinical relevance for interactions such as terfenadine or midazolam and ketoconazole. Even for less marked interactions such as amitriptyline and fluoxetine, this model provides a forecast consistent with the clinically observed range of 22-45% reduction in oral clearance, although this interaction is complicated by the presence of two inhibitors, fluoxetine and norfluoxetine. The concept of in vitro-in vivo scaling is promising and might ultimately yield a fast and more cost-effective screening for drug interactions with reduced human drug exposure and risk.

Gepirone and 1-(2-pyrimidinyl)-piperazine in vitro: human cytochromes mediating transformation and cytochrome inhibitory effects

Biotransformation of gepirone to its principal metabolite, 1-(2-pyrimidinyl)-piperazine (1-PP), was studied in human liver microsomes and in microsomes from cDNA-transfected human lymphoblastoid cells. Formation of 1-PP from gepirone in liver microsomes proceeded with a mean apparent Km ranging from 335 to 677 microM. Coincubation with 1 microM ketoconazole reduced reaction velocity to less than 5% of control values at a gepirone concentration of 250 microM. Three other metabolites, presumed to be hydroxylated products, were also formed from gepirone. Formation of all three products was reduced to approximately 20% of control values by 1 microM ketoconazole; quinidine at 1 microM produced a small reduction in formation (91-94% of control) of two of the metabolites. 1-PP was formed from gepirone exclusively by pure P450-3A4 with a Km of 849 microM; Km values for the other metabolites were 245, 240, and 415 microM. Two of the products were also formed by P450-2D6. The results indicate that 3A4 is the principal cytochrome mediating 1-PP formation, as well as formation of the other metabolites. The properties of gepirone and 1-PP themselves as cytochrome inhibitors were tested in human liver microsomes using index reactions representing activity of P450-1A2, -2C9, -2C19, -2D6, -2E1 and -3A. Gepirone and 1-PP produced negligible inhibition of all these reactions. Thus gepirone at therapeutic doses in humans has a low likelihood of inhibiting P450-mediated drug metabolism involving these cytochromes.

Kinetic and dynamic interaction study of zolpidem with ketoconazole, itraconazole, and fluconazole

BACKGROUND

Azole antifungal agents may impair hepatic clearance of drugs metabolized by cytochrome P450-3A isoforms. The imidazopyridine hypnotic agent zolpidem is metabolized in humans in part by P450-3A, as well as by a number of other cytochromes. Potential interactions of zolpidem with 3 commonly prescribed azole derivatives were evaluated in a controlled clinical study.

METHODS

In a randomized, double-blind, 5-way, crossover, clinical pharmacokinetic-pharmacodynamic study, 12 volunteers received (A) zolpidem placebo plus azole placebo, (B) 5 mg zolpidem plus azole placebo (C) zolpidem plus ketoconazole, (D) zolpidem plus itraconazole, and (E) zolpidem plus fluconazole.

RESULTS

Mean apparent oral clearance of zolpidem when given with placebo was 422 mL/min, and elimination half-life was 1.9 hours. Clearance was significantly reduced to 250 mL/min when zolpidem was given with ketoconazole, and half-life was prolonged to 2.4 hours. Coadministration of zolpidem with itraconazole or fluconazole also reduced clearance (320 and 338 mL/min), but differences compared to the zolpidem plus placebo treatment did not reach significance. Zolpidem-induced benzodiazepine agonist effects (increased electrocardiographic beta activity, digit-symbol substitution test impairment, and delayed recall) during the first 4 hours after dosage were enhanced by ketoconazole but not by itraconazole or fluconazole.

CONCLUSION

Coadministration of zolpidem with ketoconazole impairs zolpidem clearance and enhances its benzodiazepine-like agonist pharmacodynamic effects. Itraconazole and fluconazole had a small influence on zolpidem kinetics and dynamics. The findings are consistent with in vitro studies of differentially impaired zolpidem metabolism by azole derivatives.

Comparative kinetics and dynamics of zaleplon, zolpidem, and placebo

PURPOSE

This study evaluated the relationship of dose, plasma concentration, and time to the pharmacodynamics of zaleplon and zolpidem, 2 structurally distinct benzodiazepine receptor agonists.

METHOD

Ten healthy male volunteers received single oral doses of placebo, 10 mg zaleplon, 20 mg zaleplon, 10 mg zolpidem, and 20 mg zolpidem in a double-blind, 5-condition crossover study, with 48 hours elapsing between trials. Plasma drug concentrations and pharmacodynamic effects were measured during the 8 to 24 hours after administration.

RESULTS

Kinetics of zaleplon and zolpidem were not significantly related to dose. However, zaleplon had more rapid elimination (apparent elimination half-life [t1/2] of 1 hour) and higher apparent oral clearance (approximately 4300 mL/min) than zolpidem (t1/2, 2.0 to 2.2 hours; apparent oral clearance, 340 to 380 mL/min). Active treatments produced pharmacodynamic effects consistent with benzodiazepine agonist activity: self- and observer-rated sedation, impairment of digit symbol substitution test (DSST) performance, impaired memory, and increased electroencephalographic activity in the beta frequency range. The overall order of agonist potency was as follows: placebo < 10 mg zaleplon < 20 mg zaleplon < 10 mg zolpidem < 20 mg zolpidem; on a number of measures, 20 mg zaleplon was comparable to 10 mg zolpidem. Quantitative effects of zolpidem 20 mg far exceeded those of other treatments. Dynamic effects of both drugs were significantly related to plasma concentration.

CONCLUSIONS

Benzodiazepine agonist effects of zaleplon and zolpidem were dose and concentration dependent. At the usual clinically effective hypnotic dose (10 mg of either drug), agonist effects of zolpidem exceeded those of zaleplon.

Drug interactions with newer antidepressants: role of human cytochromes P450

Selective serotonin reuptake inhibitors and related antidepressant compounds have the secondary pharmacologic property of inhibiting the activity of human cytochrome P450 enzymes responsible for the oxidative metabolism of many drugs. A number of clinically important pharmacokinetic drug interactions are a consequence of these cytochrome inhibiting effects. This review evaluates the clinical implications of the metabolic profiles of the newer antidepressants, the relative activities of various new antidepressants as inhibitors of human cytochrome P450, and the various in vivo and in vitro methodologies that can be used for identification and quantification of drug interactions.

Inhibition of desipramine hydroxylation (Cytochrome P450-2D6) in vitro by quinidine and by viral protease inhibitors: relation to drug interactions in vivo

Pharmacokinetic drug interactions with viral protease inhibitors are of potential clinical importance. An in vitro model was applied to the quantitative identification of possible interactions of protease inhibitors with substrates of cytochrome P450-2D6. Biotransformation of desipramine (DMI) to hydroxydesipramine (OH-DMI), an index reaction used to profile activity of human cytochrome P450-2D6, was studied in vitro using human liver microsomes. Quinidine and four viral protease inhibitors currently used to treat human immunodeficiency virus infection were tested as chemical inhibitors in this system. Formation of OH-DMI from DMI was consistent with Michaelis-Menten kinetics, having a mean Km value of 11.7 microM (range: 9.9-15.3 microM). Quinidine, a highly potent and relatively selective inhibitor of P450-2D6, strongly inhibited OH-DMI formation with an apparent competitive mechanism, having a mean inhibition constant of 0.16 microM (range: 0.13-0.18 microM). All four protease inhibitors impaired OH-DMI formation; the pattern was consistent with a mixed competitive-noncompetitive mechanism. Mean inhibition constants (small numbers indicating greater inhibiting potency) were as follows: ritonavir, 4.8 microM; indinavir, 15.6 microM; saquinavir, 24.0 microM; nelfinavir, 51.9 microM. In a clinical pharmacokinetic study, coadministration of ritonavir with DMI inhibited DMI clearance by an average of 59%. The in vitro findings, together with observed plasma ritonavir concentrations, provided a reasonable quantitative forecast of this interaction, whereas estimated unbound plasma or intrahepatic ritonavir concentrations yielded poor quantitative forecasts. Thus the in vitro model correctly identifies ritonavir as a potent and clinically important inhibitor of human P450-2D6. Other protease inhibitors may also inhibit 2D6 activity in humans, but with lower potency than ritonavir.

Inhibition of triazolam clearance by macrolide antimicrobial agents: in vitro correlates and dynamic consequences

BACKGROUND

Macrolide antimicrobial agents may impair hepatic clearance of drugs metabolized by cytochrome P4503A isoforms. Potential interactions of triazolam, a substrate metabolized almost entirely by cytochrome P4503A in humans, with 3 commonly prescribed macrolides were identified using an in vitro metabolic model. The actual interactions, and their pharmacodynamic consequences, were verified in a controlled clinical study.

METHODS

In an in vitro model using human liver microsomes, 250 mumol/L triazolam was incubated with ascending concentrations (0 to 250 mumol/L of troleandomycin, azithromycin, erythromycin, and clarithromycin. In a randomized, double-blind, 5-trial clinical pharmacokinetic-pharmacodynamic study, 12 volunteers received 0.125 mg triazolam orally, together with placebo, azithromycin, erythromycin, or clarithromycin. In a fifth trial they received placebo plus placebo.

RESULTS

Mean 50% inhibitory concentrations versus 4-hydroxytriazolam formation in vitro were as follows: 3.3 mumol/L troleandomycin, 27.3 mumol/L erythromycin, 25.2 mumol/L clarithromycin, and greater than 250 mumol/L azithromycin. Apparent oral clearance of triazolam when given with placebo or azithromycin was nearly identical (413 and 416 mL/min), as were peak plasma concentrations (1.25 and 1.32 ng/mL) and elimination half-life (2.7 and 2.6 hours). Apparent oral clearance was significantly reduced (P < .05) during erythromycin and clarithromycin trials (146 and 95 mL/min). Peak plasma concentration was correspondingly increased, and elimination half-life was prolonged. The effects of triazolam on dynamic measures were nearly identical when triazolam was given with placebo or azithromycin, but benzodiazepine agonist effects were enhanced during erythromycin and clarithromycin trials.

CONCLUSION

The in vitro model identifies macrolides that may impair triazolam clearance. Anticipated interactions, and their pharmacodynamic consequences in volunteer subjects, were verified in vivo.

Multiple human cytochromes contribute to biotransformation of dextromethorphan in-vitro: role of CYP2C9, CYP2C19, CYP2D6, and CYP3A

Cytochromes mediating the biotransformation of dextromethorphan to dextrorphan and 3-methoxymorphinan, its principal metabolites in man, have been studied by use of liver microsomes and microsomes containing individual cytochromes expressed by cDNA-transfected human lymphoblastoid cells. In-vitro formation of dextrorphan from dextromethorphan by liver microsomes was mediated principally by a high-affinity enzyme (Km (substrate concentration producing maximum reaction velocity) 3-13 microM). Formation of dextrorphan from 25 microM dextromethorphan was strongly inhibited by quinidine (IC50 (concentration resulting in 50% inhibition) = 0.37 microM); inhibition by sulphaphenazole was approximately 18% and omeprazole and ketoconazole had minimal effect. Dextrorphan was formed from dextromethorphan by microsomes from cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, and -2D6 but not by those expressing CYP1A2, -2E1 or -3A4. Despite the low in-vivo abundance of CYP2D6, this cytochrome was identified as the dominant enzyme mediating dextrorphan formation at substrate concentrations below 10 microM. Formation of 3-methoxy-morphinan from dextromethorphan in liver microsomes proceeded with a mean Km of 259 microM. For formation of 3-methoxymorphinan from 25 microM dextromethorphan the IC50 for ketoconazole was 1.15 microM; sulphaphenazole, omeprazole and quinidine had little effect. 3-Methoxymorphinan was formed by microsomes from cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, -2D6, and -3A4, but not by those expressing CYP1A2 or -2E1. CYP2C19 had the highest affinity (Km = 49 microM) whereas CYP3A4 had the lowest (Km = 1155 microM). Relative abundances of the four cytochromes were determined in liver microsomes by use of the relative activity factor approach. After adjustment for relative abundance, CYP3A4 was identified as the dominant enzyme mediating 3-methoxymorphinan formation from dextromethorphan, although CYP2C9 and -2C19 were estimated to contribute to 3-methoxymorphinan formation, particularly at low substrate concentrations. Although formation of dextrorphan from dextromethorphan appears to be sufficiently specific to be used as an in-vitro or in-vivo index reaction for profiling of CYP2D6 activity, the findings raise questions about the specificity of 3-methoxymorphinan formation as an index of CYP3A activity.

Ketoconazole inhibition of triazolam and alprazolam clearance: differential kinetic and dynamic consequences

BACKGROUND

Kinetic and dynamic consequences of metabolic inhibition were evaluated in a study of the interaction of ketoconazole, a P4503A inhibitor, with alprazolam and triazolam, two 3A substrate drugs with different kinetic profiles.

METHODS

In a double-blind, 5-way crossover study, healthy volunteers received (A) ketoconazole placebo plus 1.0 mg alprazolam orally, (B) 200 mg ketoconazole twice a day plus 1.0 mg alprazolam, (C) ketoconazole placebo plus 0.25 mg triazolam orally, (D) 200 mg ketoconazole twice a day plus 0.25 mg triazolam, and (E) 200 mg ketoconazole twice a day plus benzodiazepine placebo. Plasma concentrations and pharmacodynamic parameters were measured after each dose.

RESULTS

For trial B versus trial A, alprazolam clearance was reduced (27 versus 86 mL/min; P < .002) and apparent elimination half-life (t1/2) prolonged (59 versus 15 hours; P < .03), whereas peak plasma concentration (Cmax) was only slightly increased (16.1 versus 14.7 ng/mL). The 8-hour pharmacodynamic effect areas for electroencephalographic (EEG) beta activity were increased by a factor of 1.35, and those for digit-symbol substitution test (DSST) decrement were increased by 2.29 for trial B versus trial A. For trial D versus trial C, triazolam clearance was reduced (40 versus 444 mL/min; P < .002), t1/2 was prolonged (18.3 versus 3.0 hours; P < .01), and Cmax was increased (2.6 versus 5.4 ng/mL; P < .001). The 8-hour effect area for EEG was increased by a factor of 2.51, and that for DSST decrement was increased by 4.33. Observed in vivo clearance decrements due to ketoconazole were consistent with those anticipated on the basis of an in vitro model, together with in vivo plasma concentrations of ketoconazole.

CONCLUSION

For triazolam, an intermediate-extraction compound, impaired clearance by ketoconazole has more profound clinical consequences than those for alprazolam, a low extraction compound.

Appetite suppressant drugs as inhibitors of human cytochromes P450: in vitro inhibition of P450-2D6 by D- and L-fenfluramine, but not phentermine

The activity of D-fenfluramine, L-fenfluramine, and phentermine as inhibitors of five human cytochromes P450 was evaluated using human liver microsomes in vitro. All three compounds produced negligible inhibition of P450-1A2, -2C9, -2E1, and -3A. Phentermine also did not inhibit P450-2D6. However, D- and L-fenfluramine significantly inhibited P450-2D6 activity as measured by dextromethorphan O-demethylation, with mean 50% inhibitory concentrations (15.1 microM) within one order of magnitude of that for fluoxetine (2.7 microM). Findings from the in vitro assay are consistent with clinical studies showing significant inhibition of desipramine clearance by coadministration of fenfluramine.

Pharmacokinetics and pharmacodynamics of diphenhydramine 25 mg in young and elderly volunteers

Thirty-seven young and elderly male and female volunteers 21 to 76 years of age received a single 25-mg oral dose of diphenhydramine or matching placebo in a double-blind, randomized, two-way crossover study. Plasma diphenhydramine concentrations, self-ratings of sedation, mood, and autonomic effects, performance on the digit-symbol substitution test (DSST), and heart rate were determined for 24 hours after administration. Information acquisition and recall were tested at 2.5 and 24 hours after administration. Age and gender did not significantly influence diphenhydramine peak plasma concentration, time of peak concentration, elimination half-life, area under the plasma concentration curve, or apparent oral clearance. Effects on psychomotor performance, sedation, mood, and memory did not differ between diphenhydramine and placebo in either group. Thus, the pharmacokinetics of single 25-mg oral doses of diphenhydramine are not influenced by age or gender. This dose of diphenhydramine produces essentially undetectable pharmacodynamic effects in both the young and elderly.

Protease inhibitors as inhibitors of human cytochromes P450: high risk associated with ritonavir

Four protease inhibitor antiviral agents (ritonavir, indinavir, nelfinavir, saquinavir) were evaluated as in vitro inhibitors of the activity of six human cytochromes using an in vitro model based on human liver microsomes. Ritonavir was a highly potent inhibitor of P450-3A activity (triazolam hydroxylation), having inhibitory potency slightly less than ketoconazole. Indinavir was also a potent 3A inhibitor, while nelfinavir and saquinavir were less potent. Ritonavir had high inhibition potency against cytochrome P450-2C9 (tolbutamide hydroxylation), -2C19 (S-mephenytoin hydroxylation), and -2D6 (dextromethorphan O-demethylation and desipramine hydroxylation), while the other protease inhibitors had one or more orders of magnitude lower inhibitory activity against these reactions. None of the protease inhibitors had important inhibitory potency against P450-1A2 (phenacetin O-deethylation) or -2E1 (chlorzoxazone hydroxylation). Thus, among available protease inhibitors, ritonavir carries the highest risk of incurring drug interactions due to inhibition of cytochrome P450 activity.

Five distinct human cytochromes mediate amitriptyline N-demethylation in vitro: dominance of CYP 2C19 and 3A4

The human cytochromes P450 (CYPs) mediating amitriptyline N-demethylation have been identified using a combination of enzyme kinetic and chemical inhibition studies. Amitriptyline was N-demethylated to nortriptyline by microsomes from cDNA transfected human lymphoblastoid cells expressing human CYPs 1A2, 2C9, 2C19, 2D6, and 3A4. CYP 2E1 showed no detectable activity. While CYP 2C19 and CYP 2D6 showed high affinity, CYP 3A4 showed low affinity; CYP 2C9 and 1A2 showed intermediate affinities. Based on these kinetic parameters and estimated relative abundance of the different CYPs in human liver, CYP 2C19 was identified as the major amitriptyline N-demethylase at low (therapeutically relevant) amitriptyline concentrations, whereas CYP 3A4 may be more important at higher amitriptyline concentrations. Chemical inhibition studies with ketoconazole and omeprazole indicate that CYP 3A4 is the major amitriptyline N-demethylase at 100 mumol/L amitriptyline, while CYP 2C19 is equally important at a substrate concentration of 5 mumol/L. The CYP 1A2 inhibitor alpha-naphthoflavone and the CYP 2C9 inhibitor sulfaphenazole produced much less inhibition of amitriptyline N-demethylation at both substrate concentrations. Quinidine produced no detectable inhibition. The kinetics of amitriptyline N-demethylation by human liver microsomes were consistent with a two enzyme model, with the high affinity component exhibiting Michaelis Menten kinetics and the low affinity component exhibiting Hill enzyme kinetics. No difference was apparent in the kinetics of amitriptyline N-demethylation in two liver samples with low levels of CYP 2C19 activity compared with two other samples with relatively normal 2C19 activity. This may reflect the importance of higher substrate concentration values in estimation of kinetic parameters in vitro.

In vitro approaches to predicting drug interactions in vivo

In vitro metabolic models using human liver microsomes can be applied to quantitative prediction of in vivo drug interactions caused by reversible inhibition of metabolism. One approach utilizes in vitro Ki, values together with in vivo values of inhibitor concentration to forecast in vivo decrements of clearance caused by coadministration of inhibitor. A critical limitation is the lack of a general scheme for assigning intrahepatic exposure of enzyme to inhibitor or substrate based only on plasma concentration; however, the assumption that plasma protein binding necessarily restricts hepatic uptake is not tenable. Other potential limitations include: flow-dependent hepatic clearance, "mechanism-based" chemical inhibition, concurrent induction, or a major contribution of gastrointestinal P450-3A isoforms to presystemic extraction. Nonetheless, the model to date has provided reasonably accurate forecasts of in vivo inhibition of clearance of several substrates (desipramine, terfenadine, triazolam, alprazolam, midazolam) by coadministration of selective serotonin reuptake-inhibitor antidepressants and azole antifungal agents. Such predictive models deserve further evaluation, since they may ultimately yield more cost-effective and expeditious screening for drug interactions, with reduced human drug exposure and risk.

Single-dose pharmacokinetics and pharmacodynamics of alprazolam in elderly and young subjects

The pharmacokinetics and pharmacodynamics of the benzodiazepine anxiolytic alprazolam (1 mg orally) were compared between young and elderly healthy volunteers. Eight young subjects (mean age 29.8 years) and eight elderly volunteers (mean age 68.4 years) received oral placebo and alprazolam (1.0 mg) in a randomized, double-blind, single-dose crossover study. In the elderly subjects, plasma concentrations were higher, although not significantly so, than in young volunteers 0.25, 0.5, and 0.75 hours after dosage. Apparent elimination half-life, time of maximum concentration, maximum concentration, volume of distribution, and apparent clearance were similar for the two groups. In both groups, alprazolam treatment (versus placebo) produced significant changes in typical benzodiazepine agonist effects, such as increased sedation and fatigue, reduced excitement, increased feelings of spaciness, and perception of thinking slowed. For some measures, the alprazolam-placebo difference was greater in young than in elderly subjects. In both groups, alprazolam significantly impaired performance on the digit-symbol substitution test (DSST). EEG studies indicated significant increases in relative beta amplitude (13-30 Hz range) after alprazolam compared to placebo. Percent DSST decrement and percent EEG change were highly correlated with plasma alprazolam concentrations for both groups. There were modest increases in alprazolam plasma concentration in the elderly compared to the younger group shortly after drug administration, but there was no evidence of increased sensitivity to the pharmacodynamic effects of alprazolam in the elderly.

Human cytochromes mediating N-demethylation of fluoxetine in vitro

Biotransformation of the selective serotonin reuptake inhibitor antidepressant, fluoxetine, to its principal metabolite, norfluoxetine, was evaluated in human liver microsomes and in microsomes from transfected cell lines expressing pure human cytochromes. In human liver microsomes, formation of norfluoxetine from R,S-fluoxetine was consistent with Michaelis-Menten kinetics (mean K(m) = 33 microM), with evidence of substrate inhibition at high substrate concentrations in a number of cases. The reaction was minimally inhibited by coincubation with chemical probes inhibitory for P450-2D6 (quinidine), -1A2 (furafylline, alpha-naphthoflavone), and -2E1 (diethyldithiocarbamate). Substantial inhibition was produced by coincubation with sulfaphenazole (Ki = 2.8 microM), an inhibitory probe for P450-2C9, and by ketoconazole (Ki = 2.5 microM) and fluvoxamine (Ki = 5.2 microM). However, ketoconazole, relatively specific for P450-3A isoforms only at low concentrations, reduced norfluoxetine formation by only 20% at 1 microM, and triacetyloleandomycin (> or = 5 microM) reduced the velocity by only 20-25%. Microsomes from cDNA-transfected human lymphoblastoid cells containing human P450-2C9 produced substantial quantities of norfluoxetine when incubated with 100 microM fluoxetine. Smaller amounts of product were produced by P450-2C19 and -2D6, but no product was produced by P450-1A2, -2E1, or 3A4. Cytochrome P450-2C9 appears to be the principal human cytochrome mediating fluoxetine N-demethylation. P450-2C19 and -3A may make a further small contribution, but P450-2D6 is unlikely to make an important contribution.

Dose-dependent pharmacokinetics and psychomotor effects of caffeine in humans

Twelve healthy volunteers received oral placebo, 250 mg of caffeine, and 500 mg of caffeine in a randomized, double-blind, single-dose crossover study. Caffeine kinetics were nonlinear, with clearance significantly reduced and elimination half-life prolonged at the 500-mg compared to the 250-mg dose. The lower dose of caffeine produced more favorable subjective effects than the higher dose (elation, peacefulness, pleasantness), whereas unpleasant effects (tension, nervousness, anxiety, excitement, irritability, nausea, palpitations, restlessness) following the 500-mg dose exceeded those of the 250-mg dose. The lower dose of caffeine enhanced performance on the digit symbol substitution test and a tapping speed test compared to placebo; high-dose caffeine produced less performance enhancement than the lower dose. The plasma concentration versus response relationship revealed concentration-dependent increases in anxiety and improvements in cognitive and motor performance at low to intermediate concentrations. Both caffeine doses reduced electroencephalographic amplitude over the 4 Hz to 30 Hz spectrum, as well as in the alpha (8-11 Hz) and beta (12-30 Hz) ranges; however, effects were not dose-dependent. While favorable subjective and performance-enhancing stimulant effects occur at low to intermediate caffeine doses, the unfavorable subjective and somatic effects, as well as performance disruption, from high doses of caffeine may intrinsically limit the doses of caffeine used in the general population.

Influence of oral contraceptive use and cigarette smoking, alone and together, on antipyrine pharmacokinetics

The pharmacokinetics of antipyrine following a single 1-g intravenous dose was determined in 63 healthy women. Subjects were divided into 4 groups as follows: 1) cigarette smokers using low-dose oral contraceptives (n = 15); 2) nonsmokers using low-dose oral contraceptives (n = 12); 3) cigarette smokers not using oral contraceptives (n = 10); and 4) controls, neither cigarette smokers nor oral contraceptive users. Plasma antipyrine concentrations during 24 to 48 hours after dosage were measured by high-performance liquid chromatography. Mean kinetic variables in the nonsmoking, non-oral contraceptive using control group were: volume of distribution, 37.7 L; elimination half-life, 13.2 hours; and clearance, 34.4 mL/min. In cigarette smoking, non-oral contraceptive users versus controls, elimination half-life was reduced (8.0 vs. 13.2 hours, P < 0.05) and clearance increased (56.0 vs. 34.4 mL/min, P < 0.05). In nonsmoker oral contraceptive users, the reverse was true (elimination half-life was significantly increased: 16.6 vs. 13.2 hours, P < 0.05; and clearance was significantly decreased: 24.8 vs. 34.4 mL/min, P < 0.05). In smokers who were using oral contraceptives, values were not significantly different from controls (elimination half-life, 11.2 hours; clearance, 39.5 mL/min). Volume of distribution did not differ among the four groups. Thus the opposing effects on antipyrine clearance of the induction of metabolism by cigarette smoking and the inhibition due to low dose oral contraceptive use in effect negate each other when combined in humans.

Alprazolam hydroxylation by mouse liver microsomes in vitro: the effect of age and phenobarbital induction

The effects of age on hepatic microsomal enzyme induction were studied in male CD-1 mice. Six week old and 1 year old animals were treated with either phenobarbital (80 mg kg-1) or saline once daily for 3d. Twenty-four hours after the last treatment, animals were sacrificed and livers were harvested. Hepatic microsomal fractions were isolated and incubated with alprazolam, a triazolobenzodiazepine metabolized by cytochrome P-450-3A isoforms in humans. Metabolites were identified and quantitated by HPLC. All microsomal preparations produced two principal metabolites (alpha-OH- and 4-OH-alprazolam) while microsomes from phenobarbital-treated animals also produced a third metabolite (alpha, 4-dihydroxyalprazolam). Vmax, K(m), and intrinsic clearance (Vmax/K(m) ratio) for both alpha-OH- and 4-OH-alprazolam in the saline-treated control animals were not significantly different between age groups. Vmax and intrinsic clearance for both metabolites were more than three times greater in phenobarbital-treated animals than in the control mice (p < 0.001). Age did not influence the extent of induction, and both pathways were induced to approximately an equal extent. Thus the present in vitro study of liver microsomal preparations from male CD-1 mice does not delineate a mechanism for impaired alprazolam clearance in aging organisms in vivo. There is no evidence that age alters susceptibility to induction by phenobarbital.

Phenacetin O-deethylation by human liver microsomes in vitro: inhibition by chemical probes, SSRI antidepressants, nefazodone and venlafaxine

Biotransformation of phenacetin via O-deethylation to acetaminophen, an index reaction reflecting activity of Cytochrome P450-1A2, was studied in microsomal preparations from a series of human livers. Acetaminophen formation was consistent with a double Michaelis-Menten system, with low-Km (mean Km1 = 68 microM) and high-Km (mean Km2 = 7691 microM) components. The low-K(m) enzyme accounted for an average of 96% of estimated intrinsic clearance, and was predicted to contribute more than 50% of net reaction velocity at phenacetin concentrations less than 2000 microM. Among index inhibitor probes, alpha-naphthoflavone was a highly potent inhibitor of the low-Km enzyme (Ki1 = 0.013 microM); furafylline also was a moderately active inhibitor (Ki1 = 4.4 microM), but its inhibiting potency was increased by preincubation with microsomes. Ketoconazole was a relatively weak inhibitor (Ki1 = 32 microM); quinidine and cimetidine showed minimal inhibiting activity. Among six selective serotonin reuptake inhibitor (SSRI) antidepressants, fluvoxamine was a potent inhibitor of 1A2 (mean Ki1 = 0.24 microM). The other SSRIs were more than tenfold less potent. Mean Ki1 values were: fluoxetine, 4.4 microM; norfluoxetine, 15.9 microM; sertraline, 8.8 microM; desmethylsertraline, 9.5 microM; paroxetine, 5.5 microM. The antidepressant nefazodone and four of its metabolites (meta-chloro-phenylpiperazine, two hydroxylated derivatives, and a triazoledione) were very weak inhibitors of P450-1A2. Venlafaxine and its O- and N-desmethyl metabolites showed minimal inhibitory activity.

Midazolam hydroxylation by human liver microsomes in vitro: inhibition by fluoxetine, norfluoxetine, and by azole antifungal agents

Biotransformation of the imidazobenzodiazepine midazolam to its alpha-hydroxy and 4-hydroxy metabolites was studied in vitro using human liver microsomal preparations. Formation of alpha-hydroxy-midazolam was a high-affinity (Km = 3.3 mumol/L) Michaelis-Menten process coupled with substrate inhibition at high concentrations of midazolam. Formation of 4-hydroxy-midazolam had much lower apparent affinity (57 mumol/L), with minimal evidence of substrate inhibition. Based on comparison of Vmax/Km ratios for the two pathways, alpha-hydroxy-midazolam formation was estimated to account for 95% of net intrinsic clearance. Three azole antifungal agents were inhibitors of midazolam metabolism in vitro, with inhibition being largely consistent with a competitive mechanism. Mean competitive inhibition constants (Ki) versus alpha-hydroxy-midazolam formation were 0.0037 mumol/L for ketoconazole, 0.27 mumol/L for itraconazole, and 1.27 mumol/L for fluconazole. An in vitro-in vivo scaling model predicted inhibition of oral midazolam clearance due to coadministration of ketoconazole or itraconazole; the predicted inhibition was consistent with observed interactions in clinical pharmacokinetic studies. The selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine and its principal metabolite, norfluoxetine, also were inhibitors of both pathways of midazolam biotransformation, with norfluoxetine being a much more potent inhibitor than was fluoxetine itself. This finding is consistent with results of other in vitro studies and of clinical studies, indicating that fluoxetine, largely via its metabolite norfluoxetine, may impair clearance of P450-3A substrates.

Enzyme kinetic modelling as a tool to analyse the behaviour of cytochrome P450 catalysed reactions: application to amitriptyline N-demethylation

1. To determine kinetic parameters (Vmax, K(m)) for cytochrome P450 (CYP) mediated metabolic pathways, nonlinear least squares regression is commonly used to fit a model equation (e.g., Michaelis Menten [MM]) to sets of data points (reaction velocity vs substrate concentration). This method can also be utilized to determine the parameters for more complex mechanisms involving allosteric or multi-enzyme systems. Akaike's Information Criterion (AIC), or an estimation of improvement of fit as successive parameters are introduced in the model (F-test), can be used to determine whether application of more complex models is helpful. To evaluate these approaches, we have examined the complex enzyme kinetics of amitriptyline (AMI) N-demethylation in vitro by human liver microsomes. 2. For a 15-point nortriptyline (NT) formation rate vs substrate (AMI) concentration curve, a two enzyme model, consisting of one enzyme with MM kinetics (Vmax = 1.2 nmol min-1 mg-1, K(m) = 24 microM) together with a sigmoidal component (described by an equation equivalent to the Hill equation for cooperative substrate binding; Vmax = 2.1 nmol min-1 mg-1, K' = 70 microM; Hill exponent n = 2.34), was favoured according to AIC and the F-test. 3. Data generated by incubating AMI under the same conditions but in the presence of 10 microM ketoconazole (KET), a CYP3A3/4 inhibitor, were consistent with a single enzyme model with substrate inhibition (Vmax = 0.74 nmol min-1 mg-1, K(m) = 186 microM, K1 = 0.0028 microM-1). 4. Sulphaphenazole (SPA), a CYP2C9 inhibitor, decreased the rate of NT formation in a concentration dependent manner, whereas a polyclonal rat liver CYP2C11 antibody, inhibitory for S-mephenytoin 4'-hydroxylation in humans, had no important effect on this reaction. 5. Incubation of AMI with 50 microM SPA resulted in a curve consistent with a two enzyme model, one with MM kinetics (Vmax = 0.72 nmol min-1 mg-1, K(m) = 54 microM) the other with 'Hill-kinetics' (Vmax = 2.1 nmol min-1 mg-1, K' = 195 microM; n = 2.38). 6. A fourth data-set was generated by incubating AMI with 10 microM KET and 50 microM SPA. The proposed model of best fit describes two activities, one obeying MM-kinetics (Vmax = 0.048 nmol min-1 mg-1, K(m) = 7 microM) and the other obeying MM kinetics but with substrate inhibition (Vmax = 0.8 nmol min-1 mg-1, K(m) = 443 microM, K1 = 0.0041 microM-1). 7. The combination of kinetic modelling tools and biological data has permitted the discrimination of at least three CYP enzymes involved in AMI N-demethylation. Two are identified as CYP3A3/4 and CYP2C9, although further work in several more livers is required to confirm the participation of the latter.

Inhibition of terfenadine metabolism in vitro by azole antifungal agents and by selective serotonin reuptake inhibitor antidepressants: relation to pharmacokinetic interactions in vivo

Biotransformation of the H-1 antagonist terfenadine to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations of human liver. These metabolic reactions are presumed to be mediated by Cytochrome P450-3A isoforms. The azole antifungal agent ketoconazole was a highly potent inhibitor of both reactions, having mean inhibition constants (Ki) of 0.037 and 0.34 microM for desalkyl- and hydroxy-terfenadine formation, respectively. Itraconazole also was a potent inhibitor, with Ki values of 0.28 and 2.05 microM, respectively. Fluconazole, on the other hand, was a weak inhibitor. Six selective serotonin reuptake inhibitor antidepressants tested in this system were at least 20 times less potent inhibitors of terfenadine metabolism than was ketoconazole. An in vitro-in vivo scaling model used in vitro Ki values, typical clinically relevant plasma concentrations of inhibitors, and presumed liver:plasma partition ratios to predict the degree of terfenadine clearance impairment during coadministration of terfenadine with these inhibitors in humans. The model predicted a large and potentially hazardous impairment of terfenadine clearance by ketoconazole and, to a slightly lesser extent, by itraconazole. However, fluconazole and the six selective serotonin reuptake inhibitors (SSRIs) at usual clinical doses were not predicted to impair terfenadine clearance to a degree that would be of clinical importance. Caution is nonetheless warranted with the coadministration of SSRIs and terfenadine when high doses of SSRIs (particularly fluoxetine) are administered. Also, some individuals may be unusually susceptible to metabolic inhibition for a variety of reasons.