In vitro prediction of clinical drug interactions with CYP3A substrates: we are not there yet

In 1973, Malcolm Rowland and associates described an approach to predicting clinical pharmacokinetic drug-drug interactions (DDIs) using an inhibition constant determined in vitro (Ki) together with anticipated inhibitor exposure in vivo ([I]). Despite numerous modifications and refinements of the core model over the following 40 years, we still have not achieved a predictive paradigm having accuracy sufficient to justify bypassing all, or even most, clinical DDI studies in the course of drug development.

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.

Identification of polymorphisms in the 3'-untranslated region of the human pregnane X receptor (PXR) gene associated with variability in cytochrome P450 3A (CYP3A) metabolism

Single nucleotide polymorphisms in the 3'-untranslated region (3'UTR) of the human pregnane X receptor (PXR) gene might contribute to interindividual variability in cytochrome P450 3A (CYP3A) activity. Genotype-phenotype associations involving PXR-3'UTR single nucleotide polymorphisms were investigated through in vitro (53 human livers from primarily White donors) and in vivo (26 mainly White or African-American volunteers) studies using midazolam 1'-hydroxylation and midazolam apparent oral clearance (CL/F), respectively, as CYP3A-specific probes. PXR-3'UTR resequencing identified twelve single nucleotide polymorphisms, including two that were novel. Although none of the single nucleotide polymorphisms evaluated were associated with altered midazolam 1'-hydroxylation in the liver bank, both rs3732359 homozygotes and rs3732360 carriers showed 80% higher (p < 0.05) CL/F compared with homozygous reference individuals. These differences in CL/F were even larger (100% and 120% higher, respectively; p < 0.01) when only African-American subjects (n = 14) were considered. Five major haplotypes were identified containing the PXR-3'UTR single nucleotide polymorphisms and previously identified intron single nucleotide polymorphisms. Although CL/F differences were not statistically significant within the entire study cohort, African-American carriers of Haplotype-1 (which includes both rs3732359 and rs3732360 variants) exhibited 70% higher median CL/F compared with African-American non-carriers (p = 0.036). The results identify rs3732359 and rs3732360 as PXR-3'UTR single nucleotide polymorphisms associated with higher CYP3A activity in vivo in African-Americans.

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.

Ritonavir and dexamethasone induce expression of CYP3A and P-glycoprotein in rats

1. The consequences of extended exposure to the human immunodeficiency viral protease inhibitor ritonavir (RIT) on the expression and function of CYP3A isoforms in the liver and in enteric mucosal cells, and on the expression of the efflux transport protein P-glycoprotein (P-gp) in enteric mucosa and in brain microvessel endothelial cells, were evaluated in rat. Dexamethasone (DEX), a known inducer of CYP3A and P-gp in rodents, served as a positive control. 2. Male CD-1 rats received RIT (20 mg kg(-1)), DEX (80 mg kg(-1)) or vehicle by oral/duodenal gavage once daily for 3 days. 3. Compared with vehicle control, CYP3A activity in liver microsomes (intrinsic clearance for triazolam hydroxylation in vitro) was increased by a factor of 2-4 by RIT, and by 10-14-fold by DEX. Similar increases were observed in expression of immunoactive CYP3A protein. Overall, maximum reaction velocity and immunoactive protein were highly intercorrelated (r2 = 0.89). Both RIT and DEX also increased function and expression of enteric CYP3A, although to a more modest extent (about 1.7-fold for RIT, about 3.3-fold for DEX). 4. Enteric P-gp expression was equally induced (by 2.8-fold) by both RIT and DEX. P-gp expressed in brain microvessel endothelial cells was increased by a factor of 1.3 by both compounds. 5. Thus, increased expression of CYP3A isoforms and of P-gp occurs with 3 days of exposure to RIT in rats. Qualitatively similar changes occur in human cell culture models and in clinical studies, and might contribute to drug interactions involving RIT (and other antiretroviral agents) in humans.

Benzodiazepines: some aspects of their clinical pharmacology

Benzodiazepine derivatives are the most commonly prescribed anti-anxiety agents in clinical practice. Six benzodiazepine anxiolytics are now available in the United States. Additional drugs are used in other parts of the world, and many others are in various stages of clinical testing. All these benzodiazepine derivatives have similar neuropharmacological properties--they reduce anxiety, produce sedation and sleep, have anticonvulsant effects, and can produce muscle relaxation. Faced with this bewildering array of drugs from the same class which are very similar in intrinsic effects upon the brain, the clinician may well ask how best to make a rational choice among the available derivatives. Despite neuropharmacological similarities, there are differences among benzodiazepines in patterns of absorption, distribution, and elimination by the human body. These pharmacokinetic differences may in turn lead to apparent differences in clinical action. This review summarizes pertinent pharmacokinetic characteristics of benzodiazepine anti-anxiety agents.

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.

Lorazepam concentrations, pharmacokinetics and pharmacodynamics in a cohort of mechanically ventilated ICU patients

OBJECTIVES

To evaluate plasma concentrations, pharmacokinetics and pharmacodynamics of lorazepam in a cohort of mechanically ventilated patients.

INTERVENTIONS

Patients underwent simultaneous measurement of lorazepam concentration and sedation assessments using the Sedation-Agitation Scale (SAS) and Bispectral Index (BIS). Lorazepam administration was classified as either continuous intravenous infusion (CIVS) or bolus.

MAIN RESULTS

A total of 124 observations were made in 13 patients. The median concentration was 59 ng/ml, interquartile range 23 - 93 ng/ml, range 0 - 1,072 ng/ml. Clearance was preserved at 92 +/- 71 ml/min. Higher concentrations were associated with deeper sedation determined by both SAS and BIS. Two patients were managed with CIVS and received more lorazepam than those managed without (288 +/- 53.5 versus 55 +/- 25.2 mg, p-value < 0.005). CIVS administration was associated with higher concentrations (629 +/- 36 versus 49 +/- 15 ng/ml, p-value < 0.001) and deeper sedation by both SAS and BIS.

CONCLUSIONS

Lorazepam clearance was preserved with a wide range of concentrations. Higher concentrations were associated with deeper sedation and use of CIVS. Elevated concentrations during CIVS were attributable to administration of larger doses.

Role of CYP3A enzymes in the biotransformation of triazolam in rat liver

1. Triazolam (TRZ) has been used extensively in rat to evaluate its benzodiazepine agonist central nervous system effects. However, the pharmacokinetics of TRZ in the male rat are not well understood. 2. To characterize further TRZ biotransformation across species, the NADPH-dependent biotransformation of TRZ was examined in rat and human liver microsomes. The role of specific cytochrome P450s (CYPs) in the biotransformation of TRZ in the rat were also determined using both rat cDNA-expressed CYPs and chemical and antibody inhibition techniques. 3. The formation of TRZ's primary hydroxylated products, alpha-OH- and 4-OH-TRZ, was consistent with a single-enzyme Michaelis-Menten model in humans. 4. Although 4-OH-TRZ formation in the male rat liver was also approximated by a single-enzyme model, a second low-affinity component was identified as contributing to alpha-OH-TRZ formation in the rat. 5. The K(m) values for the primary metabolic pathway differed between the two species. However, the net intrinsic clearances were similar for the rat and human. 6. As observed previously for humans, chemical and antibody inhibition studies suggested that CYP3A enzymes contribute significantly to TRZ hydroxylation in the rat. This finding was further supported by the use of rat cDNA-expressed CYPs. 7. The male rat might serve as a useful model for evaluating mechanisms regulating TRZ metabolism in vivo.

Serotonin (5-hydroxytryptamine) glucuronidation in vitro: assay development, human liver microsome activities and species differences

1. The main purpose was to develop a high-performance liquid chromatography (HPLC)-based method to assay serotonin glucuronidation activity using liver microsomal fractions. Application of this method was then demonstrated by determining serotonin UDP-glucuronosyltransferase (UGT) enzyme kinetics using human liver microsomes and recombinant human UGT1A6. Interspecies differences were also evaluated using liver microsomes from 10 different mammalian species. 2. Incubation of liver microsomes with serotonin, UDP-glucuronic acid and magnesium resulted in the formation of a single product peak using HPLC with fluorescence and ultraviolet absorbance detection. This peak was confirmed as serotonin glucuronide based on sensitivity to beta-glucuronidase and by obtaining the expected mass of 352 with positive-ion mass spectrometry. 3. Following a preparative HPLC isolation, the structure of this metabolite was established as serotonin-5-O-glucuronide by (1)H-NMR spectroscopy. 4. Enzyme kinetic studies showed apparent K(m) and V(max) of 8.8 +/- 0.3 mM and 43.4 +/- 0.4 nmoles min(-1) mg(-1) protein, respectively, for human liver microsomes, and 5.9 +/- 0.2 mM and 15.8 +/- 0.2 nmoles min(-1) mg(-1), respectively, for recombinant UGT1A6. 5. The order of serotonin-UGT activities in animal liver microsomes was rat > mouse > human > cow > pig > horse > dog > rabbit > monkey > ferret. Cat livers showed no serotonin-UGT activity. Heterozygous and homozygous mutant Gunn rat livers had 40 and 13%, respectively, of the activity of the normal Wistar rat, indicating a significant contribution by a rat UGT1A isoform to serotonin glucuronidation. 6. This assay provides a novel sensitive and specific technique for the measurement of serotonin-UGT activity in vitro.

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.

Interindividual variability in acetaminophen glucuronidation by human liver microsomes: identification of relevant acetaminophen UDP-glucuronosyltransferase isoforms

Interindividual variability in acetaminophen (APAP) glucuronidation may contribute to differences in susceptibility to APAP intoxication in humans. The purpose of this study was to identify the relevant UDP-glucuronosyltransferase (UGT) isoforms mediating APAP-UGT activity in human liver microsomes (HLMs). APAP-UGT activities and enzyme kinetics were determined using HLMs from 56 donors and nine recombinant human UGTs. Activities mediated by UGT1A1, UGT1A4, UGT1A9, and UGT2B7, and relative UGT1A6 protein content were quantified using 20 livers. More than 15-fold variation in liver microsomal APAP-UGT activities was observed with a distribution skewed toward lower activities. Although most UGTs could glucuronidate APAP, UGT1A1, UGT1A6, and UGT1A9 were most active. UGT1A6 was a relatively high-affinity (K(m) = 2.2 mM), low-capacity enzyme; UGT1A1 was intermediate in affinity (K(m) = 9.4 mM) and capacity; and UGT1A9 was a low-affinity (K(m) = 21 mM), high-capacity enzyme. K(m) values were similar to UGT1A1 in high- and intermediate-activity HLMs (6-10 mM) and UGT1A9 in low-activity HLMs (10-55 mM). APAP-UGT activities correlated best with propofol-UGT (r = 0.85; UGT1A9) and bilirubin-UGT (r = 0.66; UGT1A1) activities, but poorly with UGT1A6 protein (r = 0.30). A kinetic model was constructed from these data that identified UGT1A9 as the predominant APAP-UGT (>55% total activity) in HLMs over a clinically relevant APAP concentration range (50 microM-5 mM). UGT1A1 was also predicted to contribute substantially at toxic concentrations (>1 mM; >28% activity), whereas UGT1A6 was most active at relatively low concentrations (<50 microM; >29% activity).

Ritonavir induces P-glycoprotein expression, multidrug resistance-associated protein (MRP1) expression, and drug transporter-mediated activity in a human intestinal cell line

The present study characterized the response of P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP1) to chronic ritonavir (RIT) exposure by assessing increases in P-gp and MRP1 protein expression and activity. LS-180V intestinal carcinoma cells were exposed for 3 days to 1-100 microM RIT concurrently with controls. P-gp and MRP1 protein was quantified by Western blot analysis. Cell accumulation assays, using the P-gp substrate rhodamine 123 (RH123), the P-gp/MRP1 substrate doxorubicin (DOX), and the MRP substrate carboxyfluorescein (CBF), were performed as a measure of transporter activity. RIT strongly induced P-gp and MRP1 expression (maximum 6-fold and 3-fold increases, respectively) in a concentration-dependent fashion. Following extended exposure to RIT (> 10 microM), cells accumulated < 50% of the RH123 and DOX compared with controls, whereas accumulation of CBF was decreased by 30% at 30 microM. Differences in cell accumulation of RH123 could be eliminated with verapamil (100 microM; a P-gp inhibitor), whereas decreased DOX cell accumulation was only partially reversed by verapamil. Indomethacin (100 microM; an MRP1 inhibitor) had no significant effect on RH123 or DOX accumulation, suggesting limited MRP1-mediated activity. Thus, RIT induced protein expression of P-gp and MRP1 and increased cellular drug exclusion of RH123, DOX, and CBF. Similar in vivo phenomena may occur during anti-HIV drug therapy, explaining potential decrements in therapeutic efficacy due to decreases in bioavailability or alterations in drug distribution.

Human drug metabolism and the cytochromes P450: application and relevance of in vitro models

The cytochromes P450 (CYPs) constitute a superfamily of hemoprotein enzymes that are responsible for the biotransformation of numerous xenobiotics, including therapeutic agents. Studies of the biochemical and enzymatic properties of these enzymes and their molecular genetics and regulation of gene expression and activity have greatly enhanced our understanding of several aspects of clinical pharmacology such as pharmacokinetic variability, drug toxicity, and drug interactions. This review evaluates the major human hepatic drug-metabolizing CYP enzymes and their clinically relevant substrates, inhibitors, and inducers. Also discussed are the molecular bases and clinical implications of genetic polymorphisms that affect the CYPs. Much of the information on the specificity of substrates and inhibitors of the CYP enzymes is derived from in vitro studies using human liver microsomes and heterologously expressed CYP enzymes. These methods are discussed, and guidelines are provided for designing enzyme kinetic and reaction phenotyping studies using multiple approaches. The strengths, weaknesses, and discrepancies among the different approaches are considered using representative examples. The mathematical models used in predicting the pharmacokinetic clearance of a drug from in vitro estimates of intrinsic clearance and the principles of quantitative in vitro-in vivo scaling of metabolic drug interactions are also discussed.

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.

Methadone inhibits rhodamine123 transport in Caco-2 cells

This study investigated the effects of racemic methadone (MET) on P-glycoprotein (P-gp) activity in cell culture. MET showed no differential rates of passage between the basolateral to apical (B to A) and apical to basolateral (A to B) direction across Caco-2 cell monolayers in a transwell system. MET transport in either direction was not importantly influenced by the P-gp inhibitor verapamil. However, MET was a potent inhibitor (IC(50) = 7.5 microM) of rhodamine123 B to A transport across Caco-2 cell monolayers, causing a reduction to 25% of control at 100 microM MET. In this model of Caco-2 monolayers, rates of MET passage between B to A and A to B directions could not be distinguished. However, MET can inhibit P-gp activity at intraluminal concentrations that might be achieved clinically. This may lead to increased bioavailability of coadministered compounds.

P-glycoprotein interactions of nefazodone and trazodone in cell culture

This study investigated the effects of nefazodone (NFZ) and trazodone (TZD) on P-glycoprotein (P-gp) activity and expression in cell culture. NFZ and TZD showed no differential transport between the basolateral to apical and apical to basolateral direction across Caco-2 cell monolayers. Transport in either direction was not affected by verapamil. NFZ was a potent inhibitor (IC50 = 4.7 microM) of rhodamine123 (Rh123) B to A transport across Caco-2 cell monolayers, while TZD had minimal effect. Following 72-hour exposure of LS180V cells to NFZ and TZD (10 microM), a twofold increase in immunoreactive P-gp was observed. Rh123 accumulation into these cells was reduced to 65% and 74% of control by NFZ and TZD (10 microM), respectively. It was concluded that differential rates of transport of NFZ and TZD in Caco-2 cells were not evident. However, NFZ is an inhibitor of P-gp activity at clinically relevant in vivo concentrations and may have the potential to increase bioavailability of coadministered compounds that are substrates for transport. Concentrations of NFZ and TZD achieved in the intestine after chronic oral dosing may induce P-gp expression and reduce absorption of coadministered drugs.

Pharmacodynamic and receptor binding changes during chronic lorazepam administration

To assess pharmacodynamic and neurochemical aspects of tolerance, lorazepam (2 mg/kg/day), or vehicle was administered chronically to male Crl: CD-1(ICR)BR mice via implantable osmotic pump. Open-field behavior, benzodiazepine receptor binding in vitro, receptor autoradiography, and muscimol-stimulated chloride uptake were examined at both 1 and 14 days. Open-field activity was depressed in lorazepam-treated animals on Day 1. On Day 14, open-field parameters were indistinguishable from those of vehicle-treated animals, indicating behavioral tolerance. Benzodiazepine binding, as determined by the specific binding of [125I]diazepam, was also decreased in cortex on Day 14. Hippocampal binding was unchanged following chronic lorazepam exposure. Apparent affinity in cortical membrane preparations was unchanged, indicating that altered ligand uptake was due to decreased receptor number. Muscimol-stimulated chloride uptake into cortical synaptoneurosomes from lorazepam-treated animals was not significantly different on Day 1 or Day 14 compared to vehicle-treated animals. These results confirm that down-regulation of benzodiazepine receptor binding is closely associated with behavioral tolerance to benzodiazepines. These observed changes in binding are not necessarily associated with robust changes in receptor function.

A benzodiazepine mood effect scale: reliability and validity determined for alcohol-dependent subjects and adults with a parental history of alcoholism

The Tufts Addiction Research Center Inventory--Morphine Benzedrine Group (ARCI-MBG) scale was designed to measure benzodiazepine-induced mood elevation. The reliability and validity of the Tufts ARCI-MBG scale were determined in 64 subjects with a history of alcoholism (HA), a positive history of parental alcoholism, defined as one or both parents meeting DSM-III-R criteria for alcohol dependence (PHP), and matched control subjects. Significant correlations were found for within-day Tufts ARCI-MBG scale scores for all groups and for between-day scores for PHP and matched control subjects. Interitem reliability was significant for pooled baseline scores. For HA subjects, correlations between mean Tufts ARCI-MBG scale and Drug Liking scores that were obtained after either alprazolam or diazepam administration were significant. These results suggest that the Tufts ARCI-MBG scale is a reliable test that is a valid measure of benzodiazepine-induced mood elevation.

Application of the relative activity factor approach in scaling from heterologously expressed cytochromes p450 to human liver microsomes: studies on amitriptyline as a model substrate

The relative activity factor (RAF) approach is being increasingly used in the quantitative phenotyping of multienzyme drug biotransformations. Using lymphoblast-expressed cytochromes P450 (CYPs) and the tricyclic antidepressant amitriptyline as a model substrate, we have tested the hypothesis that the human liver microsomal rates of a biotransformation mediated by multiple CYP isoforms can be mathematically reconstructed from the rates of the biotransformation catalyzed by individual recombinant CYPs using the RAF approach, and that the RAF approach can be used for the in vitro-in vivo scaling of pharmacokinetic clearance from in vitro intrinsic clearance measurements in heterologous expression systems. In addition, we have compared the results of two widely used methods of quantitative reaction phenotyping, namely, chemical inhibition studies and the prediction of relative contributions of individual CYP isoforms using the RAF approach. For the pathways of N-demethylation (mediated by CYPs 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4) and E-10 hydroxylation (mediated by CYPs 2B6, 2D6, and 3A4), the model-predicted biotransformation rates in microsomes from a panel of 12 human livers determined from enzyme kinetic parameters of the recombinant CYPs were similar to, and correlated with the observed rates. The model-predicted clearance via N-demethylation was 53% lower than the previously reported in vivo pharmacokinetic estimates. Model-predicted relative contributions of individual CYP isoforms to the net biotransformation rate were similar to, and correlated with the fractional decrement in human liver microsomal reaction rates by chemical inhibitors of the respective CYPs, provided the chemical inhibitors used were specific to their target CYP isoforms.

Comparative pharmacokinetics of alprazolam and lorazepam in humans and in African Green Monkeys

The comparative pharmacokinetics of alprazolam and lorazepam were evaluated in African Green Monkeys and in healthy male human volunteers. Six monkeys received a single 250 micrograms/kg oral dose of alprazolam and of lorazepam on two separate occasions. Mean kinetic values for the two drugs, respectively, were: elimination half-life, 5.7 and 1.7 h; oral clearance, 5.5 and 40.2 ml/min/kg. Healthy male volunteers (n = 22) received a single 1 mg oral dose of alprazolam; another group (n = 24) received a single 2 mg oral dose of lorazepam. Mean values of elimination half-life in humans (11.5 and 12.4 h, for alprazolam and lorazepam, respectively) were substantially longer than corresponding values in the primate animal model, and human values of clearance (0.85 and 1.40 ml/min/kg) likewise were much lower. However, in humans, kinetic differences between the two drugs were much smaller than in the primate animals. Thus comparative studies of the behavioral effects of these two drugs in African Green Monkeys should utilize relative dosages that reflect the pharmacokinetic properties of the drugs in that species. Use of dosage ratios analogous to those used in humans may lead to results that cannot be extrapolated to humans.

Relative contributions of CYP2C9 and 2C19 to phenytoin 4-hydroxylation in vitro: inhibition by sulfaphenazole, omeprazole, and ticlopidine

OBJECTIVES

To determine the relative contribution of cytochromes P450 (CYP) 2C9 and 2C19 to the formation of 5-(-4-hydroxyphenyl)-5-phenylhydantion (HPPH) from phenytoin (PPH).

DESIGN

Hydroxylation of PPH to form HPPH was studied in vitro using human liver microsomes and microsomes from cDNA-transfected human lymphoblastoid cells.

RESULTS

Formation of HPPH from PPH in liver microsomes had a mean (+/- SEM) apparent Km [substrate concentration corresponding to 50% of maximal reaction velocity (Vmax)] of 23.6 +/- 1.8 mumol/l. Coincubation with the CYP2C9 inhibitor, sulfaphenazole (SPA), at 5 mumol/l reduced reaction velocity to less than 15% of control values. The mean inhibitor concentration at which 50% inhibition is achieved (IC50 value) for SPA versus PPH hydroxylation (0.49 microM) was similar to the SPA IC50 versus flurbiprofen hydroxylation (0.46 microM) and tolbutamide hydroxylation (0.7-1.5 microM). In contrast, the CYP2C19 inhibitor omeprazole (OME) at 10 mumol/l produced only a small degree of inhibition. Incubation of PPH with microsomes from cDNA-transfected human lymphoblastoid cells containing CYP1A2, 2A6, 2B6, 2C8, 2D6, 2E1, or 3A4 yielded no detectable formation of HPPH. Only CYP2C9 and 2C19 had PPH hydroxylation activity, with apparent Km values for the high-affinity component of 14.6 mumol/l and 24.1 mumol/l, respectively. Based on Vmax values in liver microsomes, the Vmax and Km values in expressed CYPs and the relative abundance of the two isoforms in human liver, CYP2C9, and 2C19 were estimated to have relative contributions of 90% and 10%, respectively, to net intrinsic clearance.

CONCLUSIONS

Formation of HPPH from PPH is mediated exclusively by CYP2C9 and 2C19, with CYP2C9 playing the major role.

CYP3A4 is the major CYP isoform mediating the in vitro hydroxylation and demethylation of flunitrazepam

The kinetics of flunitrazepam (FNTZ) N-demethylation to desmethylflunitrazepam (DM FNTZ), and 3-hydroxylation to 3-hydroxyflunitrazepam (3-OH FNTZ), were studied in human liver microsomes and in microsomes containing heterologously expressed individual human CYPs. FNTZ was N-demethylated by cDNA-expressed CYP2A6 (K(m) = 1921 microM), CYP2B6 (K(m) = 101 microM), CYP2C9 (K(m) = 50 microM), CYP2C19 (K(m) = 60 microM), and CYP3A4 (K(m) = 155 microM), and 3-hydroxylated by cDNA-expressed CYP2A6 (K(m) = 298 microM) and CYP3A4 (K(m) = 286 microM). The 3-hydroxylation pathway was predominant in liver microsomes, accounting for more than 80% of intrinsic clearance compared with the N-demethylation pathway. After adjusting for estimated relative abundance, CYP3A accounted for the majority of intrinsic clearance via both pathways. This finding was supported by chemical inhibition studies in human liver microsomes. Formation of 3-OH FNTZ was reduced to 10% or less of control values by ketoconazole (IC(50) = 0.11 microM) and ritonavir (IC(50) = 0.041 microM). Formation of DM FNTZ was inhibited to 40% of control velocity by 2.5 microM ketoconazole and to 30% of control by 2.5 microM ritonavir. Neither 3-OH FNTZ nor DM FNTZ formation was inhibited to less than 85% of control activity by alpha-naphthoflavone (CYP1A2), sulfaphenazole (CYP2C9), omeprazole (CYP2C19), or quinidine (CYP2D6). Thus, CYP-dependent FNTZ biotransformation, like that of many benzodiazepine derivatives, is mediated mainly by CYP3A. Clinical interactions of FNTZ with CYP3A inhibitors can be anticipated.

Ritonavir, efavirenz, and nelfinavir inhibit CYP2B6 activity in vitro: potential drug interactions with bupropion

Since antiretroviral drugs are known to inhibit many cytochrome P450 isoforms, the inhibition of CYP2B6 by non-nucleoside reverse transcriptase inhibitors and viral protease inhibitors was studied in vitro in human liver microsomes using bupropion hydroxylation as the CYP2B6 index reaction. Mean IC(50) values (microM) for inhibition of bupropion hydroxylation were: nelfinavir (2.5), ritonavir (2.2), and efavirenz (5.5). The reaction was only weakly inhibited by indinavir, saquinavir, amprenavir, delavirdine, and nevirapine. The inhibition of bupropion hydroxylation in vitro by nelfinavir, ritonavir, and efavirenz indicates inhibitory potency versus CYP2B6 and suggests the potential for clinical drug interactions.

Pharmacokinetics and pharmacodynamics of vecuronium in children receiving phenytoin or carbamazepine for chronic anticonvulsant therapy

The pharmacokinetics and time course of action of vecuronium in normal children and children receiving anticonvulsant drugs for prolonged periods were characterized. A bolus dose of vecuronium 0.15 mg kg(-1) was administered i.v. to 10 non-epileptic children and to 10 children on phenytoin and 10 children on carbamazepine, who were matched for age and weight. Plasma concentrations of vecuronium, 3-OH desacetylvecuronium (the primary metabolite of vecuronium) and alpha1-acid glycoprotein (AAG) were determined. Pharmacokinetic variables were derived from plasma samples collected before and after administration of vecuronium. Neuromuscular transmission was monitored by evoked compound electromyography. Recovery of the first twitch of the train-of-four (T1/T0) and the recovery index (RI), the time for 25-75% recovery of T1/T0, were determined. The elimination half-life of vecuronium was significantly reduced in both anticonvulsant groups compared with control [control 48.2 (SD 40.3), phenytoin 23.5 (13.1), carbamazepine 18.4 (16.6) min, P<0.05]. Vecuronium clearance was increased in both anticonvulsant groups [control 9.0 (3.6), phenytoin 15.1 (8.9), carbamazepine 18.8 (13.1) ml kg(-1) min(-1), 0.05<P<0.1]. Children on chronic anticonvulsant therapy had a significantly shorter RI than control [control 21.8 (11), phenytoin 12.5 (8.3), carbamazepine 10.6 (5.9) min, P<0.05]. Concentrations of vecuronium at different degrees of recovery of T1, volumes of distribution and AAG concentrations were not different between groups. Our data confirm anticonvulsant-induced resistance to vecuronium in children and support a pharmacokinetic component contributing to the resistance.

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.

Cytochrome P-450 2B6 is responsible for interindividual variability of propofol hydroxylation by human liver microsomes

BACKGROUND

Oxidation of propofol to 4-hydroxypropofol represents a significant pathway in the metabolism of this anesthetic agent in humans. The aim of this study was to identify the principal cytochrome P-450 (CYP) isoforms mediating this biotransformation.

METHODS

Propofol hydroxylation activities and enzyme kinetics were determined using human liver microsomes and cDNA-expressed CYPs. CYP-specific marker activities and CYP2B6 protein content were also quantified in hepatic microsomes for correlational analyses. Finally, inhibitory antibodies were used to ascertain the relative contribution of CYPs to propofol hydroxylation by hepatic microsomes.

RESULTS

Propofol hydroxylation by hepatic microsomes showed more than 19-fold variability and was most closely correlated to CYP2B6 protein content (r = 0.904), and the CYP2B6 marker activities, S-mephenytoin N-demethylation (r = 0.919) and bupropion hydroxylation (r = 0.854). High- and intermediate-activity livers demonstrated high-affinity enzyme kinetics (K(m) < 8 microm), whereas low-activity livers displayed low-affinity kinetics (K(m) > 80 microm). All of the CYPs evaluated were capable of hydroxylating propofol; however, CYP2B6 and CYP2C9 were most active. Kinetic analysis indicated that CYP2B6 is a high-affinity (K(m) = 10 +/- 2 microm; mean +/- SE of the estimate), high-capacity enzyme, whereas CYP2C9 is a low-affinity (K(m) = 41 +/- 8 microm), high-capacity enzyme. Furthermore, immunoinhibition showed a greater contribution of CYP2B6 (56 +/- 22% inhibition; mean +/- SD) compared with CYP2C isoforms (16 +/- 7% inhibition) to hepatic microsomal activity.

CONCLUSIONS

Cytochrome P-450 2B6, and to a lesser extent CYP2C9, contribute to the oxidative metabolism of propofol. However, CYP2B6 is the principal determinant of interindividual variability in the hydroxylation of this drug by human liver microsomes.

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.

A comparative pharmacokinetic and dynamic evaluation of alprazolam sustained-release, bromazepam, and lorazepam

Sustained-release (SR) alprazolam may facilitate compliance with oral benzodiazepine treatment of panic disorders that currently requires doses administered three or four times daily. To compare the pharmacokinetic, psychomotor performance, and subjective effects of alprazolam SR (1.5 mg), bromazepam (3 mg taken three times daily), and lorazepam (1 mg taken three times daily), 13 male volunteers (aged 20-45 years) randomly received on four separate occasions one of these medications or placebo. Once before and 11 times after drug administration, the subjects were tested using psychomotor performance tests (manual tracking and digit-symbol substitution test [DSST]) and computerized questionnaires (such as the Tufts University Benzodiazepine Scale [TUBS], the Addiction Research Center Inventory, and the visual analog scales) to determine the subjective effects of the drugs. Blood samples for the determination of the plasma levels of the drugs were collected before and 17 times after the drug was administered. A peak plateau of plasma alprazolam began approximately 6 hours after the dose, which was later than the initial peaks for lorazepam and bromazepam (1-2 hours after the dose). Once this plateau had begun, alprazolam SR sustained that concentration better than did the other two formulations. Of the 10 measures on which the response averaged for the first 14 hours differed among drugs (p < 0.05), bromazepam differed from placebo on two measures, lorazepam on four (including DSST Performance and TUBS Sedation), and alprazolam SR on nine (including all four affected by lorazepam). Lorazepam and alprazolam, but not bromazepam, produced significantly more sedation than placebo. The doses of the three drugs were not equipotent in sedation and mood effects. None of the drugs tested differed from placebo on measures relevant to abuse liability.

Imipramine and buspirone in treatment of patients with generalized anxiety disorder who are discontinuing long-term benzodiazepine therapy

OBJECTIVE

Patients with generalized anxiety disorder (N=107) who had been long-term benzodiazepine users (average duration of use=8.5 years) were enrolled in a benzodiazepine discontinuation program that assessed the effectiveness of concomitant imipramine (180 mg/day) and buspirone (38 mg/day) compared to placebo in facilitating benzodiazepine discontinuation.

METHOD

After a benzodiazepine stabilization period taking either diazepam, lorazepam, or alprazolam, patients were treated for 4 weeks with imipramine, buspirone, or placebo under double-blind conditions while benzodiazepine intake was kept stable (treatment phase). Patients then entered a 4-6 week benzodiazepine taper and a 5-week posttaper phase with imipramine, buspirone, and placebo treatment being continued until 3 weeks into the posttaper phase, at which time all patients were switched to placebo for 2 weeks. Benzodiazepine plasma levels were assayed weekly. Benzodiazepine-free status was assessed 3 and 12 months posttaper.

RESULTS

Study subjects were long-term benzodiazepine users with an average of three unsuccessful prior taper attempts. The success rate of the taper in this study was significantly higher for patients who received imipramine (82.6%), and nonsignificantly higher for patients who received buspirone (67.9%), than for patients who received placebo (37.5%). The imipramine effect remained highly significant even after the analysis adjusted for three other independent predictors of taper success: benzodiazepine dose, level of anxious symptoms at baseline, and duration of benzodiazepine therapy.

CONCLUSIONS

Management of benzodiazepine discontinuation can be facilitated significantly by co-prescribing imipramine before and during the benzodiazepine taper. Daily benzodiazepine dose, severity of baseline symptoms of anxiety and depression, and duration of benzodiazepine use were additional significant predictors of successful taper outcome.

Scaling drug biotransformation data from cDNA-expressed cytochrome P-450 to human liver: a comparison of relative activity factors and human liver abundance in studies of mirtazapine metabolism

The present study represents a comparison of three approaches to transform recombinant cytochrome P-450 (rCYP) enzyme kinetic data to human liver activity using mirtazapine (MIR) biotransformation as a model. MIR metabolite rCYP formation rates were corrected using I) relative activity factors (RAFs) determined on site, II) RAFs based on activity data provided by the rCYP manufacturer, and III) immunologically determined human liver abundance of CYP isoforms reported in the literature. For 2.5, 25, and 250 microM MIR, predictions of 1) the relative contribution of CYP isoforms to a particular reaction, 2) absolute metabolite formation rates, 3) the relative contribution of each pathway to net MIR biotransformation, and 4) the relative contribution of CYP isoforms to net MIR biotransformation were generated, and the results were compared with data obtained with human liver microsomes (HLM). We found that RAFs determined on site most accurately predict the results observed in HLM. Estimations based on liver abundance systematically underestimated CYP1A2 and overestimated CYP3A and CYP2C9 contributions to MIR metabolism and, therefore, seem less suitable to predict CYP isoform involvement in a particular reaction. Normalized RAFs calculated from the manufacturer activity data fell within the range of RAFs determined on site and lead to similar results for CYP isoform contribution to metabolic reactions and to net MIR biotransformation. Considering the time and resource-intensive step of RAF determination, manufacturer RAFs are an alternative to RAFs determined on site for the transformation of rCYP enzyme kinetic data; both of them provide more accurate estimations than immunologically determined human liver CYP isoform content.

Metabolism of the antidepressant mirtazapine in vitro: contribution of cytochromes P-450 1A2, 2D6, and 3A4

The metabolism of the antidepressant mirtazapine (MIR) was investigated in vitro using human liver microsomes (HLM) and recombinant enzymes. Mean K(m) values (+/-S.D., n = 4) were 136 (+/-44) microM for MIR-hydroxylation, 242 (+/-34) microM for N-demethylation, and 570 (+/-281) microM for N-oxidation in HLM. Based on the K(m) and V(max) values, MIR-8-hydroxylation, N-demethylation, and N-oxidation contributed 55, 35, and 10%, respectively, to MIR biotransformation in HLM at an anticipated in vivo liver MIR concentration of 2 microM. Recombinant CYP predicted a 65% contribution of CYP2D6 to MIR-hydroxylation at 2 microM MIR, decreasing to 20% at 250 microM. CYP1A2 contribution increased correspondingly from 30 to 50%. In HLM, quinidine and alpha-naphthoflavone reduced MIR-hydroxylation to 75 and 45% of control, respectively, at 250 microM MIR. A >50% contribution of CYP3A4 to MIR-N-demethylation at <1 microM MIR was indicated by recombinant enzymes. In HLM, ketoconazole (1 microM) reduced N-desmethylmirtazapine formation rates to 60% of control at 250 microM. Twenty percent of MIR-N-oxidation was accounted for by CYP3A4 at 2 microM MIR, increasing to 85% at 250 microM, while CYP1A2 contribution decreased from 80 to 15%. Ketoconazole reduced MIR-N-oxidation to 50% of control at 250 microM. MIR did not substantially inhibit CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP1E2, and CYP3A4 activity in vitro. Induction/inhibition or genetic polymorphisms of CYP2D6, CYP1A2, and CYP3A4 may affect MIR metabolism, but involvement of several enzymes in different metabolic pathways may prevent large alterations in in vivo drug clearance.

CYP2B6 mediates the in vitro hydroxylation of bupropion: potential drug interactions with other antidepressants

The in vitro biotransformation of bupropion to hydroxybupropion was studied in human liver microsomes and microsomes containing heterologously expressed human cytochromes P450 (CYP). The mean (+/-S.E.) K(m) in four human liver microsomes was 89 (+/-14) microM. In microsomes containing cDNA-expressed CYPs, hydroxybupropion formation was mediated only by CYP2B6 at 50 microM bupropion (K(m) 85 microM). A CYP2B6 inhibitory antibody produced more than 95% inhibition of bupropion hydroxylation in four human livers. Bupropion hydroxylation activity at 250 microM was highly correlated with S-mephenytoin N-demethylation activity (yielding nirvanol), another CYP2B6-mediated reaction, in a panel of 32 human livers (r = 0.94). The CYP2B6 content of 12 human livers highly correlated with bupropion hydroxylation activity (r = 0.96). Thus bupropion hydroxylation is mediated almost exclusively by CYP2B6 and can serve as an index reaction reflecting activity of this isoform. IC(50) values for inhibition of a CYP2D6 index reaction (dextromethorphan O-demethylation) by bupropion and hydroxybupropion were 58 and 74 microM, respectively. This suggests a low inhibitory potency versus CYP2D6, the clinical importance of which is not established. Since bupropion is frequently coadministered with other antidepressants, IC(50) values (microM) for inhibition of bupropion hydroxylation were determined as follows: paroxetine (1.6), fluvoxamine (6.1), sertraline (3.2), desmethylsertraline (19.9), fluoxetine (59.5), norfluoxetine (4.2), and nefazodone (25.4). Bupropion hydroxylation was only weakly inhibited by venlafaxine, O-desmethylvenlafaxine, citalopram, and desmethylcitalopram. The inhibition of bupropion hydroxylation in vitro by a number of newer antidepressants suggests the potential for clinical drug interactions.

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.

The influence of the menstrual cycle on triazolam and indocyanine green pharmacokinetics

The aim of this study was to investigate the effects of the menstrual cycle phase on the pharmacokinetics of two high-clearance agents, triazolam and indocyanine green (ICG). Eleven nonsmoking, healthy, eumenorrheic women were enrolled in this study. Triazolam (0.25 mg) was administered orally, and indocyanine green was administered as an i.v. bolus (0.5 mg/kg) during the follicular, ovulatory, and luteal phases of a single menstrual cycle. Blood samples were collected over 10 hours for triazolam and over 30 minutes for ICG. Triazolam and indocyanine green concentrations were quantitated by electron capture gas chromatography and spectrophotometry, respectively. Noncompartmental analysis was used to determine relevant pharmacokinetics parameters, which were statistically assessed using two-way ANOVA (p < 0.05). No statistical differences for triazolam were observed. Vd/F was lower in the luteal phase (107 L) as compared to the follicular (138 L) and ovulatory (133 L) phases. Clearance of triazolam was comparable in the follicular (583 ml/min), ovulatory (565 ml/min), and luteal (538 ml/min) phases. ICG also revealed no significant differences across the phases. These results suggest that the phases of the menstrual cycle do not influence triazolam or ICG pharmacokinetics.

Evidence for propofol hydroxylation by cytochrome P4502B11 in canine liver microsomes: breed and gender differences

1. The study aimed to ascertain the enzyme kinetic basis for breed differences in the biotransformation of propofol in dog and to identify the responsible canine cytochrome P450 (CYP) isoenzymes. 2. The NADPH-dependent formation of 4-hydroxypropofol (the rate-limiting biotransformation in dog) was assayed using hepatic microsomes from the male greyhound and beagle, and from both sexes in mixed-breed dogs (five of each). 3. Enzyme kinetic analysis revealed that whereas there were no significant differences in Km, Vmax averaged > 3-fold lower in greyhound compared with beagle (p = 0.032). Although average Vmax was > 3-fold higher in the male compared with female mixed-breed dogs, this difference did not achieve statistical significance (p = 0.095), probably because of the high variability of data from mixed-breed dogs. 4. Chloramphenicol (a specific CYP2B11 inhibitor) and diethyldithiocarbamate (a non-specific CYP2 inhibitor) inhibited propofol hydroxylation in all microsomes. Quinine (a CYP2D15 inhibitor) was also inhibitory, but only in one-half of the microsomes examined. Immuno-inhibition by anti-CYP2B1 sera resulted in > 50% reduction in metabolite formation in all dogs except mixed-breed females, which showed a 30% reduction. Differences in propofol hydroxylase activity between microsomal preparations were primarily attributed to a component that was sensitive to inhibition by chloramphenicol and anti-CYP2B1 sera. 5. The results indicate that propofol hydroxylation in dog is primarily mediated by CYP2B11 and that breed (and possibly gender) differences in propofol metabolism may result from differences in the liver content of this CYP.

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.