Early stage autoinduction of carbamazepine metabolism in humans

Animal-free assessment of developmental toxicity: Combining PBPK modeling with the ReproTracker assay

in vitro screening platforms to assess teratogenic potential of compounds are emerging rapidly. ReproTracker is a human induced pluripotent stem cells (hiPSCs)-based biomarker assay that is shown to identify the teratogenicity potential of new pharmaceuticals and chemicals reliably. In its current state, the assay is limited to identifying the potential teratogenic effects and does not immediately quantify a clinical dose relevant to the exposure of chemicals or drugs observable in mothers or fetuses. The goal of this study was to evaluate whether the ReproTracker assay can be extrapolated in vivo and quantitatively predict developmental toxicity exposure levels of two known human teratogens, thalidomide, and carbamazepine. Here, we utilized Physiologically Based Pharmacokinetic (PBPK) modeling to describe the pharmacokinetic behavior of these compounds and conducted an in vitro to in vivo extrapolation (IVIVE) approach to predict human equivalent effect doses (HEDs) that correspond with in vitro concentrations potentially associated with adverse outcomes in ReproTracker. The HEDs derived from the ReproTracker concentration predicted to cause developmental toxicity were close to the reported teratogenic human clinical doses and the HED derived from the rat or rabbit developmental toxicity study. The ReproTracker derived-HED revealed to be sensitive and protective of humans. Overall, this pilot study demonstrated the importance of integrating PBPK model in extrapolating and assessing developmental toxicity in vitro. The combination of these tools demonstrated that they could improve the safety assessment of drugs and chemicals without animal testing.

Source-specific ecological risks and critical source identification of PPCPs in surface water: Comparing urban and rural areas

To control the concentrations of pharmaceutical and personal care products (PPCPs) in the surface water of urban and rural areas, it is important to explore the spatial variation in source-specific ecological risks and identify critical sources. Here, we focused on 22 PPCPs found in the effluent from wastewater treatment plants and surface water in Tianjin, and source-specific risk was quantitatively apportioned combining positive matrix factorization with ecological risk assessment. Results showed that rural areas exhibited a more severe contamination level than urban areas. Medical wastewater (30.1 %) accounted for the highest proportion, while domestic sewage posed the greatest threat to aquatic ecosystems. The incidence of potential risks (RQ > 0.01) caused by domestic sewage in urban areas (88.9 %) was higher than that in rural areas (75.9 %). However, PPCP risks caused by farmland drainage, aquaculture, and livestock discharge were mainly distributed in rural areas. The critical source identified in the entire region was domestic sewage (weight, 0.36), and its weight (0.51) in urban areas was greater than that in rural areas (0.32). The impact of aquaculture (weight, 0.16) in rural areas was noteworthy. These findings may contribute to developing environmental management strategies in key areas to help alleviate PPCP contamination worldwide.

Using Prescription and Wastewater Data to Estimate the Correction Factors of Atenolol, Carbamazepine, and Naproxen for Wastewater-Based Epidemiology Applications

The correction factor (CF) is a critical parameter in wastewater-based epidemiology (WBE) that significantly influences the accuracy of the final consumption estimates. However, most CFs have been derived from a few old pharmacokinetic studies and should be re-evaluated and refined to improve the accuracy of the WBE approach. This study aimed to review and estimate the CFs for atenolol, carbamazepine, and naproxen for WBE using the daily mass loads of those pharmaceuticals in wastewater and their corresponding dispensed prescription data in Australia. Influent wastewater samples were collected from wastewater treatment plants serving approximately 24% of the Australian population and annual national dispensed prescription data. The estimated CFs for atenolol and carbamazepine are 1.37 (95% CI: 1.17-1.66) and 8.69 (95% CI: 7.66-10.03), respectively. Due to significant over-the-counter sales of naproxen, a reliable CF could not be estimated based on prescription statistics. Using an independent dataset of 186 and 149 wastewater samples collected in an urban catchment in 2011 and 2012, WBE results calculated using the new CFs matched well with the dispensed data for atenolol and carbamazepine in the catchment area.

Pharmacokinetics of the CYP3A4 and CYP2B6 Inducer Carbamazepine and Its Drug-Drug Interaction Potential: A Physiologically Based Pharmacokinetic Modeling Approach

The anticonvulsant carbamazepine is frequently used in the long-term therapy of epilepsy and is a known substrate and inducer of cytochrome P450 (CYP) 3A4 and CYP2B6. Carbamazepine induces the metabolism of various drugs (including its own); on the other hand, its metabolism can be affected by various CYP inhibitors and inducers. The aim of this work was to develop a physiologically based pharmacokinetic (PBPK) parent-metabolite model of carbamazepine and its metabolite carbamazepine-10,11-epoxide, including carbamazepine autoinduction, to be applied for drug-drug interaction (DDI) prediction. The model was developed in PK-Sim, using a total of 92 plasma concentration-time profiles (dosing range 50-800 mg), as well as fractions excreted unchanged in urine measurements. The carbamazepine model applies metabolism by CYP3A4 and CYP2C8 to produce carbamazepine-10,11-epoxide, metabolism by CYP2B6 and UDP-glucuronosyltransferase (UGT) 2B7 and glomerular filtration. The carbamazepine-10,11-epoxide model applies metabolism by epoxide hydroxylase 1 (EPHX1) and glomerular filtration. Good DDI performance was demonstrated by the prediction of carbamazepine DDIs with alprazolam, bupropion, erythromycin, efavirenz and simvastatin, where 14/15 DDI AUClast ratios and 11/15 DDI Cmax ratios were within the prediction success limits proposed by Guest et al. The thoroughly evaluated model will be freely available in the Open Systems Pharmacology model repository.

Environmental exposure to pharmaceuticals: A new technique for trace analysis of carbamazepine and its metabolites in human urine

Pharmaceutically active compounds are taken up and accumulate in crops irrigated with treated wastewater. This raises the concern of chronic human exposure to pharmaceuticals via food consumption. Thus, there is a need to develop a reliable technique to detect and quantify pharmaceuticals at environmentally relevant concentrations in human biological matrices, particularly urine. In this study, we focus on carbamazepine, an antiepileptic drug and recalcitrant compound that is taken up by crops-making it an excellent model compound for this study. This paper presents a new analytical technique enabling quantification of trace concentrations of carbamazepine and its metabolites in the urine of individuals who have been environmentally exposed. Sample preparation included extraction with acetonitrile followed by clean-up through mixed-mode ion-exchange cartridges and analysis using LC/MS/MS. This technique, which was validated for a wide range of concentrations (5-2000 ng L(-1)), exhibits low limits of quantification (3.0-7.2 ng L(-1)), acceptable recovery levels (70-120%), and low relative standard deviation (<20%). Unlike currently available methods for the analysis of water or treated wastewater that require large volumes (up to 1 L), the new method uses only 10 mL of urine. Moreover, relative to available methods for carbamazepine detection in the urine of individuals who are chronically treated with this drug, the limit of quantification values with our method are six orders of magnitude lower. The newly developed method has been successfully applied for the quantification of carbamazepine and its metabolites in the urine of healthy people exposed to this pharmaceutical through their diet. Our analytical protocol can provide the scientific community and stakeholders with real data for risk assessments and the design of policies ensuring safe use of wastewater for crop irrigation.

Human Exposure to Wastewater-Derived Pharmaceuticals in Fresh Produce: A Randomized Controlled Trial Focusing on Carbamazepine

Fresh water scarcity has led to increased use of reclaimed wastewater as an alternative and reliable source for crop irrigation. Beyond microbiological safety, concerns have been raised regarding contamination of reclaimed wastewater by xenobiotics including pharmaceuticals. This study focuses on carbamazepine, an anticonvulsant drug which is ubiquitously detected in reclaimed wastewater, highly persistent in soil, and taken up by crops. In a randomized controlled trial we demonstrate that healthy individuals consuming reclaimed wastewater-irrigated produce excreted carbamazepine and its metabolites in their urine, while subjects consuming fresh water-irrigated produce excreted undetectable or significantly lower levels of carbamazepine. We also report that the carbamazepine metabolite pattern at this low exposure level differed from that observed at therapeutic doses. This "proof of concept" study demonstrates that human exposure to xenobiotics occurs through ingestion of reclaimed wastewater-irrigated produce, providing real world data which could guide risk assessments and policy designed to ensure the safe use of wastewater for crop irrigation.

Reducing PEC uncertainty in coastal zones: a case study on carbamazepine, oxcarbazepine and their metabolites

Concentrations of the antiepileptic drugs carbamazepine (Cbz), oxcarbazepine (OxCz) and their main metabolites were predicted in a wastewater treatment plant (WTP) and in the vicinity of its submarine outfall located in a Mediterranean coastal zone. Refined predicted environmental concentrations (PECs) were calculated in effluents based on consumption data and human excretion rates. PECs were estimated in the sea using the hydrodynamic MARS 3D model integrating meteorological data, oceanic conditions (wind, tide, atmospheric pressure), freshwater and sewage inputs. Measured environmental concentrations (MECs) were compared to PECs to assess the estimation relevance. In the coastal zone, PEC and MEC were in the same magnitude range. Modeling of Cbz diffusion and advection just above the submarine outfall showed the influence of the thermocline during summer, with low diffusion of Cbz from the bottom to the surface. This work allowed understanding the dispersion of target compounds and deserved further development for a better acknowledgement of vulnerability at local scales.

Characterization of the time course of carbamazepine deinduction by an enzyme turnover model

BACKGROUND AND OBJECTIVE

Carbamazepine is a potent inducer of drug metabolizing enzymes, which results in a number of clinically significant drug-drug interactions. Deinduction occurs when long-term carbamazepine therapy is discontinued. The goal of this study was to develop a population pharmacokinetic model to describe the time course of carbamazepine deinduction.

PATIENTS AND METHODS

Stable-labelled carbamazepine was administered intravenously on three occasions during the deinduction period to 15 patients with epilepsy for whom carbamazepine therapy was being discontinued. Data were analysed using a nonlinear mixed-effects model (NONMEM). An enzyme turnover model consisting of a one-compartment model linked with a hypothetical enzyme compartment was applied to characterize the time course of carbamazepine deinduction. Model evaluation was performed using the bootstrap approach and a visual predictive check.

RESULTS

In the final model, the deinduction process was accomplished by decreasing the rate of enzyme synthesis, resulting in a decrease in the relative amount of enzymes. The estimated rate constant for enzyme degradation was 0.00805 h-1, corresponding to a half-life of the combined enzymes of 86.1 hours (3.6 days).

CONCLUSION

An enzyme turnover model adequately characterized the experimental data. Based on the predicted enzyme half-life from the final model, the deinduction process should be completed within 2 weeks after carbamazepine therapy is terminated.

No pharmacokinetic interaction between lacosamide and carbamazepine in healthy volunteers

Lacosamide is a new antiepileptic drug for adjunctive treatment of adult partial-onset seizures. Two open-label, multiple-dose clinical trials were conducted to evaluate the potential for pharmacokinetic interaction between lacosamide and carbamazepine. The influence of carbamazepine on lacosamide pharmacokinetics (trial A) and lacosamide on carbamazepine pharmacokinetics (trial B) was investigated in 19 (trial A) and 18 (trial B) healthy male participants. Trial A participants received lacosamide 200 mg bid alone and with carbamazepine 200 mg bid. Trial B participants received carbamazepine 200 mg bid alone and with lacosamide 200 mg bid. Pharmacokinetic parameters, area under the concentration-time curve during a dosage interval at steady state (AUC(tau,ss)), and maximum steady-state plasma drug concentration during a dosage interval (C(max,ss)) of lacosamide, carbamazepine, and carbamazepine-10,11-epoxide were measured and compared for each drug alone and together. The AUC(tau,ss) and C(max,ss) point estimates (combined vs sole treatment) showed relative bioavailability of approximately 100% for both drugs. All 90% confidence intervals of AUC(tau,ss) and C(max,ss) were within the generally accepted bioequivalence ranges of 80% to 125%. No changes in rate or extent of absorption or terminal half-life were observed. These results suggest that lacosamide and carbamazepine have a low potential for pharmacokinetic drug-drug interaction in clinical use.

Carbamazepine and diclofenac: removal in wastewater treatment plants and occurrence in water bodies

In the aquatic environment, pharmaceuticals have been widely found. Among them, carbamazepine and diclofenac were detected at the highest frequency. To evaluate the worldwide environmental impacts of both drugs, their global consumption volumes are estimated, based on the dose per capita. The metabolites of these pharmaceuticals are also of environmental concerns, especially trans-10,11-dihydro-10,11- dihydroxycarbamazepine (CBZ-diol) which probably has a similar concentration in water bodies to that of its parent drug. The removal efficiencies and mechanisms of both drugs in the wastewater treatment plants (WWTPs) are discussed with the actual state of knowledge. The occurrences of both drugs are examined in various water bodies including WWTP effluents, surface waters, groundwater and drinking water. Their chemical, physical and pharmacological properties are also addressed in context, which can largely influence their environmental behaviors. The ecotoxicological studies of both drugs imply that they do not easily cause acute toxic effects at their environmental concentrations. However their chronic effects need cautious attention.

Antiepileptic drugs--best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies

Although no randomized studies have demonstrated a positive impact of therapeutic drug monitoring (TDM) on clinical outcome in epilepsy, evidence from nonrandomized studies and everyday clinical experience does indicate that measuring serum concentrations of old and new generation antiepileptic drugs (AEDs) can have a valuable role in guiding patient management provided that concentrations are measured with a clear indication and are interpreted critically, taking into account the whole clinical context. Situations in which AED measurements are most likely to be of benefit include (1) when a person has attained the desired clinical outcome, to establish an individual therapeutic concentration which can be used at subsequent times to assess potential causes for a change in drug response; (2) as an aid in the diagnosis of clinical toxicity; (3) to assess compliance, particularly in patients with uncontrolled seizures or breakthrough seizures; (4) to guide dosage adjustment in situations associated with increased pharmacokinetic variability (e.g., children, the elderly, patients with associated diseases, drug formulation changes); (5) when a potentially important pharmacokinetic change is anticipated (e.g., in pregnancy, or when an interacting drug is added or removed); (6) to guide dose adjustments for AEDs with dose-dependent pharmacokinetics, particularly phenytoin.

Carbamazepine-provoked hepatotoxicity and possible aetiopathological role of glutathione in the events. Retrospective review of old data and call for new investigation

The antiepileptic drug (AED) carbamazepine is widely used in the treatment of different kinds of seizures as well as affective and behavioural disorders. This paper presents an epidemiological study of carbamazepine-induced hepatic injuries and death, and describes the possible mechanisms of its toxicity. A retrospective analysis of clinical data revealed that the likelihood of hepatic death was comparatively higher in children, particularly when they were receiving medication with multiple AEDs, whereas reversible hepatic injuries were more likely to be seen in elderly patients. As suggested in this paper, the development of carbamazepine hepatotoxicity is rare, and unpredictable with the present state of knowledge, but it is somehow related to disturbance of glutathione metabolism, although data in this regard are imperfect. There appear to be two types of carbamazepine-initiated idiosyncratic liver injury, hypersensitivity and toxin-induced. It is feasible that both are due to the accumulation of toxic metabolite(s), and arene oxides may probably be considered as damaging derivatives of carbamazepine metabolism. Despite the lack of clear-cut underlying clinical and experimental findings in those patients in whom an inherited weakness of drug eliminating capacity is present, those conditions that may deteriorate glutathione balance, may increase the possibility of the emergence of toxic events during carbamazepine therapy. Finally, some recommendations for carbamazepine therapy are presented.

The use of gabapentin for the treatment of postherpetic neuralgia

BACKGROUND

Varicella-zoster virus causes chickenpox and can reemerge later in life to cause herpes zoster or shingles. One of the most common and disabling complications of herpes zoster is postherpetic neuralgia (PHN).

OBJECTIVES

This article reviews the current primary literature about the efficacy and tolerability of gabapentin for the treatment of PHN. Gabapentin pharmacokinetics and drug interactions are also reviewed.

METHODS

A literature search in the English language was conducted using OVID Web, which contained the following databases: MEDLINE (1966-present), EMBASE (1980-2002), Current Contents/Clinical Medicine (1999-2002), Cochrane Controlled Trials Register (1898-present), Cochrane Database of Systemic Reviews (fourth quarter, 2002), and International Pharmaceutical Abstracts (1970-2002). Search terms used were postherpetic neuralgia; zoster; gabapentin; neuropathic pain; pain; pharmacoeconomic; cost; controlled clinical trial; randomized, controlled trial; postherpetic neuralgia and gabapentin; gabapentin and pain; treatment and postherpetic neuralgia; gabapentin and age; gabapentin and gender; gabapentin and ethnicity; and gabapentin and pharmacokinetics.

RESULTS

Gabapentin displays nonlinear absorption kinetics, is minimally protein bound (< 3%), has a high mean (SD) volume of distribution (50.4 [8.0] L), and is excreted via the kidneys as unchanged drug. Two randomized, placebo-controlled, parallel-group, multicenter clinical trials demonstrated the effectiveness of gabapentin at doses of up to 3600 mg/d to significantly reduce pain (P < 0.01 and P < 0.001), improve sleep (P < 0.01), and improve some parameters on the Short Form-McGill Pain Questionnaire (P < 0.05). Dizziness and somnolence were the most common side effects leading to withdrawal from the trials. The recommended dosage in adults is 300 mg at bedtime on day 1,300 mg BID on day 2, and 300 mg TID on day 3, titrating up as needed to 2400 to 3600 mg/d. To reduce adverse events in patients with renal impairment, the dose should be adjusted based on the patient's creatinine clearance.

CONCLUSIONS

Gabapentin appears to be effective and well tolerated for the short-term treatment of PHN. However, future controlled studies are needed to determine whether the effectiveness of gabapentin for PHN is maintained for > 2 months, to establish the optimal dose of gabapentin for PHN, and to compare the efficacy of gabapentin with that of other pharmacologic agents used for the treatment of PHN.

Population pharmacokinetic modelling of carbamazepine by using the iterative Bayesian (IT2B) and the nonparametric EM (NPEM) algorithms: implications for dosage

OBJECTIVE

To estimate individual and population postinduction pharmacokinetics of carbamazepine (CBZ) in epileptic adult and paediatric patients who received chronic CBZ monotherapy.

METHODS

We have used the USC*PACK collection of PC programs for the estimations. The preinduction CBZ metabolism was also estimated in 16 volunteers after a single dose of CBZ (200 mg). We used a linear one-compartmental model with oral absorption and found the pharmacokinetic parameter values of CBZ behaviour to be in good agreement with those reported earlier.

RESULTS

Serum CBZ concentrations correlated poorly with daily doses in both the adult and child populations. Because of the diversity within the population, use of the mean population model without knowledge of an individual patient's pharmacokinetic characteristics gives poor prediction. In contrast, the individual Bayesian posterior models gave good prediction for all subjects in the population, due to the removal of the interindividual variability.

CONCLUSION

This approach permits one to individualize drug therapy for patients even when only sparse therapeutic drug monitoring (TDM) data are available. Future individual CBZ serum level predictions were acceptable from a clinical point of view (mean absolute error = 13.2 +/- 9.7%). The optimal sampling strategy approach helped to design an optimal cost-effective TDM protocol for CBZ therapy management.

Dose-response evaluation of the interaction between sertraline and alprazolam in vivo

In vitro data show the inhibition of alprazolam metabolism by sertraline via CYP3A4; therefore, using a randomized, double-blind, placebo-controlled design, the authors conducted this study to assess the potential for similar in vivo inhibition in humans. Ten healthy volunteers participated in two test sessions (placebo/alprazolam 1 mg orally) before the initiation of sertraline treatment. Blood samples were obtained over a 32-hour period and pharmacodynamic measures (sedation, psychomotor performance, memory function) were obtained over an 8-hour period. After a minimum of 2 weeks of daily sertraline self-administration (50, 100, or 150 mg/day), test sessions were repeated. Alprazolam concentrations (N = 6, 4, and 6 at sertraline doses of 50, 100, and 150 mg/day, respectively) showed no significant changes based on peak concentration (Cmax), time to maximum concentration (Tmax), elimination half-life (t1/2[beta]), and area under the concentration-time curve (AUC(0-8)), with the exception of a reduced Cmax in the 50 mg/day group. Similarly, dynamic data showed no significant variations based on peak effect, Tmax, and AUC(0-infinity), with the exception of increased peak impairment in one measure of psychomotor performance. No differences were detected between placebo alone and placebo plus sertraline. These findings suggest that sertraline (50-150 mg/day) does not alter the single-dose kinetics or dynamics of alprazolam; therefore, the combination may be prescribed without an increased risk of alprazolam toxicity.

Therapeutic drug monitoring--antiepileptic drugs

AIMS

To provide a brief critical review of the basis and contemporary practice of monitoring the concentrations of antiepileptic drugs in biological fluids.

METHODS

The review is based on literature data and observations from clinical practice.

RESULTS

As experience has accumulated, monitoring of antiepileptic drug concentrations has come to be applied mainly to certain of the drugs when present in whole plasma. For these drugs there is a reasonably established relationship between drug concentrations and biological effects, but attention still needs to be paid to issues such as the timing of the measurements in relation to drug intake, the presence or absence of steady-state conditions, the presence in plasma of active metabolites and possible nonlinear pharmacokinetics of particular agents e.g. phenytoin.

CONCLUSIONS

Plasma antiepileptic drug concentration monitoring is coming to be used in a more thoughtful and critical manner. Lack of adequate knowledge of matters such as the relationship between the plasma concentrations and antiepileptic and toxic effects of the drugs, not only the newer, but also the longer established ones, in particular clinical situations, remains more important than deficiencies in analytical methodology in limiting the clinical usefulness of antiepileptic drug concentration monitoring.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Pharmacokinetics of oxcarbazepine and carbamazepine in human placenta

PURPOSE

To study the transfer and metabolism of oxcarbazepine (OCBZ) and 10-hydroxy-10,11-dihydrocarbamazepine (10-OH-CBZ) and carbamazepine (CBZ) metabolism and its possible induction in human placenta.

METHODS

A dual recirculating human placental perfusion system, blood sampling, high performance liquid chromatography (HPLC), reverse transcriptase-polymerase chain reaction (RT-PCR), and enzyme assays.

RESULTS

OCBZ was metabolized into 10-OH-CBZ in five human placental cotyledons perfused for 2 h in a dual recirculating perfusion system. The same metabolite was found by HPLC in three sample pairs of maternal and cord blood taken during delivery from patients on OCBZ therapy. In all of the clinical samples, 10,11-trans-dihydroxy-10,11-dihydrocarbamazepine (10,11-D) was also found, but not in the perfusions. In addition, 10-OH-CBZ was not metabolized in the placental perfusions. The transfer of OCBZ through the perfused placentas was quicker than the transfer of antipyrine, while the transfer of 10-OH-CBZ was slower. Both OCBZ and 10-OH-CBZ also accumulated in placental tissue. CBZ metabolism was studied in three perfusions using placentas from mothers on CBZ therapy. No metabolism could be detected in the perfused placentas, while metabolites were found in both maternal and cord blood of the same mothers. Another series of placentas of mothers on CBZ therapy did not differ significantly from the placenta of a healthy mother as to CYP activities or the level of CYP3A4 mRNA.

CONCLUSIONS

OCBZ is metabolized into 10-OH-CBZ to some extent in human placenta in vitro, suggesting that the placenta also participates in the metabolism of OCBZ in vivo. On the contrary, the placenta does not participate in the metabolism of CBZ. No induction of placental CBZ metabolism in vitro can be detected after maternal CBZ treatment during pregnancy.

Dose-dependent metabolism of carbamazepine in humans

48-h steady-state metabolic balance studies were carried out in 17 adults receiving long-term anticonvulsant monotherapy. With increasing carbamazepine dosage (1) carbamazepine overall plasma apparent clearance (CL/F), (2) plasma clearance of carbamazepine to urinary carbamazepine-10,11-epoxide, (3) plasma clearance of carbamazepine-10,11-epoxide to urinary unconjugated carbamazepine-10,11-trans-diol and (4) plasma clearances of carbamazepine to urinary 2- and 3-hydroxy carbamazepine all increased. However, with increasing carbamazepine dose there was no increase in the clearance of carbamazepine to (5) its acridan derivative in urine or of (6) the diol, phenolic or acridan metabolites to their metabolically subsequent conjugates excreted in urine. These findings are consistent with ongoing dose-dependent autoinduction of carbamazepine metabolism along the first two stages, but not the final stage, of the epoxide-diol pathway and, to a lesser extent, along pathways yielding phenolic metabolites. However, conjugations of the various plasma phase I metabolites of carbamazepine are not dose-dependent. Plasma concentration ratios of substances involved in consecutive stages of the epoxide-diol pathway, as in previous published studies, suggested apparent dose dependence of the epoxide-->unconjugated diol stage only. Presumably, increased flux along the first two stages of the full epoxide-diol pathway reduces plasma carbamazepine and carbamazepine-10,11-epoxide concentrations largely in parallel, concealing the dose dependence of the conversion of carbamazepine to its epoxide.