The generation, detection, and effects of reactive drug metabolites.

The decline in approval of new drugs during the past decade has led to a close analysis of the drug discovery process. One of the main reasons for attrition is preclinical toxicity, frequently attributed to the generation of protein-reactive drug metabolites. In this review, we present a critique of such reactive metabolites and evaluate the evidence linking them to observed toxic effects.

Pharmacokinetic-pharmacodynamic modeling of mood and withdrawal symptoms in relation to plasma concentrations of methadone in patients undergoing methadone maintenance treatment.

The aims of the present study were to characterize the relationship between plasma racemic methadone and its enantiomers' concentrations with respect to their pharmacodynamic effects and to investigate the influence of potential covariates on the pharmacodynamic parameters in patients on methadone maintenance treatment (MMT). Eighty-eight regular subjects at the Sheffield Care Trust Substance Misuse Services were studied. Samples of blood and urine were collected before the daily dose of methadone.

Contribution of the activities of CYP3A, CYP2D6, CYP1A2 and other potential covariates to the disposition of methadone in patients undergoing methadone maintenance treatment.

Aims: To investigate the influence of different cytochrome P450 (CYP) activities and other potential covariates on the disposition of methadone in patients on methadone maintenance therapy (MMT).

Methods: Eighty-eight patients (58 male; 21-55 years; 84 White) on MMT were studied. CYP2D6 activity [3 h plasma metabolic ratio of dextromethorphan (DEX) to dextrorphan (DOR)] was determined in 44 patients (29 male; 24-55 years), CYP1A2 activity (salivary caffeine elimination half-life) in 44 patients (21 male; 24-55 years) and CYP3A activity (oral clearance of midazolam) in 49 patients (33 male; 23-55 years).

Single-dose pharmacokinetic study of clomiphene citrate isomers in anovular patients with polycystic ovary disease.

The pharmacokinetics of the zuclomiphene (Zu) and enclomiphene (En) isomers of clomiphene citrate following a single oral dose (50 mg) were characterized for the first time in patients receiving the drug (ie, infertile women with polycystic ovary syndrome). Plasma concentrations of Zu and En were measured in 9 patients from the second day of their menstrual cycle (day 1 of dosing) up to 21 days. The mean (+/- coefficient of variation) of C(max), t(max), and AUC of Zu was 15 +/- 41 ng/mL, 7 +/- 87 h, and 1289 +/- 34 ng/mL.

CYP2D6 is primarily responsible for the metabolism of clomiphene.

Clomiphene is a first line treatment for anovulation, a common cause of infertility. Response to clomiphene is variable and unpredictable. Tamoxifen is structurally related to clomiphene, and also shows considerable variation in response.

Evaluation of the relationship between plasma concentrations of en- and zuclomiphene and induction of ovulation in anovulatory women being treated with clomiphene citrate.

Objective: To investigate the relationship between the plasma concentrations of clomiphene citrate (CC) isomers zu- (Zu) and enclomiphene (En), and ovulation outcome.

Design: Prospective, cohort study.

Setting: Reproductive medicine and fertility center in a university teaching hospital, United Kingdom.

Inactivation of CYP2D6 by methylenedioxymethamphetamine in different recombinant expression systems.

Recombinantly expressed CYP450 systems (rCYPs) are often used to screen for irreversible/quasi-irreversible enzyme inhibitors during drug development. The concentration- and time-dependent inactivation of CYP2D6 by methylenedioxymethamphetamine (MDMA) was compared in three different rCYP2D6 systems (yeast microsomes, Supersomestrade mark and Bactosomestrade mark) under the conditions of the most commonly used protocols in assessing mechanism-based inactivation (MBI). MDMA (2-20microM) was pre-incubated with enzyme for 0, 2.

PRN prescribing in psychiatric inpatients: potential for pharmacokinetic drug interactions.

Medications are commonly prescribed to psychiatric inpatients on a PRN (pro re nata/as required) basis, allowing drugs to be administered on patient request or at nurses' discretion for psychiatric symptoms, treatment side effects or physical complaints. However, there has been no formal study of the pharmacokinetic implications of PRN prescribing. The objective of the study was to determine the prevalence of PRN drug prescription and administration, and to assess the potential for interactions involving CYP2D6 and CYP3A4 between drugs prescribed and administered to inpatients on psychiatry wards.

The use of mechanistic DM-PK-PD modelling to assess the power of pharmacogenetic studies -CYP2C9 and warfarin as an example.

Aim: To assess the power of in vivo studies needed to discern the effect of genotype on pharmacokinetics (PK) and pharmacodynamics (PD) using CYP2C9 and (S)-warfarin as an example.

Methods: Information on the in vitro metabolism of (S)-warfarin and genetic variation in CYP2C9 was incorporated into a mechanistic population-based PK-PD model. The influence of study design on the ability to detect significant differences in PK (AUC(0-12 h)) and PD (AUEC(0-12 h) INR) between CYP2C9 genotypes was investigated.

Incorporating in vitro information on drug metabolism into clinical trial simulations to assess the effect of CYP2D6 polymorphism on pharmacokinetics and pharmacodynamics: dextromethorphan as a model application.

In vitro-in vivo extrapolation of clearance, embedded in a clinical trial simulation, was used to investigate differences in the pharmacokinetics and pharmacodynamics of dextromethorphan between CYP2D6 poor and extensive metabolizer phenotypes. Information on the genetic variation of CYP2D6, as well as the in vitro metabolism and pharmacodynamics of dextromethorphan and its active metabolite dextrorphan, was integrated to assess the power of studies to detect differences between phenotypes. Whereas 6 subjects of each phenotype were adequate to achieve 80% power in showing pharmacokinetic differences, the power required to detect a difference in antitussive response was less than 80% with 500 subjects in each study arm.

Evaluation of an existing nomogram for predicting the response to clomiphene citrate.

Objective: To evaluate the ability of an existing nomogram to predict response to clomiphene citrate (CC) in infertile couples with World Health Organization group II ovulatory disorders, using the free androgen index, body mass index, and menstrual-cycle history.

Design: Retrospective case-notes study.

Setting: Reproductive medicine and fertility center at a university teaching hospital in the United Kingdom.

Does previous response to clomifene citrate influence the selection of gonadotropin dosage given in subsequent superovulation treatment cycles?

Purpose: To determine whether ovarian response to previous clomifene treatment could influence the selection of the starting dose of gonadotropins in subsequent in vitro fertilization (IVF) or intra uterine insemination (IUI).

Methods: Forty three anovular women who had received clomifene for ovulation induction followed by gonadotropins for IUI or IVF superovulation were reviewed retrospectively. Data on gonadotropin dose were compared between clomifene-resistant patients and clomifene responders.

Solid-phase microextraction: investigation of the metabolism of substances that may be abused by inhalation.

Purified liquefied petroleum gas (LPG), a mixture of butane, isobutane, and propane, is commonly abused by inhalation. Little is known about the mammalian metabolism of these substances. Metabolism of other hydrocarbons, including n-hexane and cyclohexane, has been studied in vitro using a range of liver preparations, with metabolites analyzed by static headspace techniques.

The impact of experimental design on assessing mechanism-based inactivation of CYP2D6 by MDMA (Ecstasy).

MDMA (3-4-methylenedioxymethamphetamine, commonly known as Ecstasy) is a potent mechanism-based inhibitor (MBI) of cytochrome P450 2D6 (CYP2D6), causing quasi-irreversible inhibition of the enzyme in vitro. An evaluation of the in vivo implications of this phenomenon depends on the accuracy of the estimates of the parameters that define the inhibition in vitro, namely k(inact) (the maximal inhibition rate) and KI (the inactivation constant). These values are determined in two steps, pre-incubation of the enzyme with the inhibitor (enzyme inactivation), followed by dilution and further incubation to measure residual enzyme activity with a probe substrate.

Can saliva replace plasma for the monitoring of methadone?

The aims of this study were to determine the relationship between saliva and plasma methadone concentrations and the influence of variability in saliva pH. Saliva and plasma samples were taken before the daily dose of methadone in 60 patients undergoing methadone maintenance treatment (MMT). Saliva pH was measured immediately after sampling, and concentrations of (RS)-, (R)-, and (S)-methadone in saliva and plasma were assayed by LC/MS.

Monitoring plasma concentrations to individualize treatment with clomiphene citrate.

Objective: Recent reports have indicated that a number of individual patient characteristics are responsible for the success or failure of clomiphene citrate treatment. However, a priori individualization of doses in different patients has not been investigated. We examined the thesis that wide variability in the metabolism of the active component (zuclomiphene) contributes to variability in response.

Metabolism of ethylbenzene by human liver microsomes and recombinant human cytochrome P450s (CYP).

The enzyme kinetics of the initial hydroxylation of ethylbenzene to form 1-phenylethanol were determined in human liver microsomes. The individual cytochrome P450 (CYP) forms catalysing this reaction were identified using selective inhibitors and recombinant preparations of hepatic CYPs. Production of 1-phenylethanol in hepatic microsomes exhibited biphasic kinetics with a high affinity, low Km, component (mean Km = 8 microM; V(max) = 689 pmol/min/mg protein; n = 6 livers) and a low affinity, high Km, component (Km = 391 microM; V(max) = 3039 pmol/min/mg protein; n = 6).

Influence of dose, cigarette smoking, age, sex, and metabolic activity on plasma clozapine concentrations: a predictive model and nomograms to aid clozapine dose adjustment and to assess compliance in individual patients.

The measurement of plasma clozapine concentrations is useful in assessing compliance, optimizing therapy, and minimizing toxicity. We measured plasma clozapine and norclozapine (N-desmethylclozapine) concentrations in samples from 3782 patients (2648 male, 1127 female). No clozapine was detected in 291 samples (227 patients, median prescribed dose 300 mg/d).

Metabolism of tamoxifen by recombinant human cytochrome P450 enzymes: formation of the 4-hydroxy, 4'-hydroxy and N-desmethyl metabolites and isomerization of trans-4-hydroxytamoxifen.

The cytochrome P450 (P450)-mediated biotransformation of tamoxifen is important in determining both the clearance of the drug and its conversion to the active metabolite, trans-4-hydroxytamoxifen. Biotransformation by P450 forms expressed extrahepatically, such as in the breast and endometrium, may be particularly important in determining tissue-specific effects of tamoxifen. Moreover, tamoxifen may serve as a useful probe drug to examine the regioselectivity of different forms.