A pharmacodynamic approach to the estimate of carbamazepine autoinduction

Pharmacokinetic Interaction Between Fingolimod and Carbamazepine in Healthy Subjects

This open‐label, single‐sequence study in healthy subjects investigated the effects of steady‐state carbamazepine on the pharmacokinetic (PK) profile of a single 2‐mg dose of fingolimod. In period 1, a single oral dose of fingolimod 2 mg (day 1) was followed by PK and safety assessments up to 36 days. In period 2, carbamazepine was administered in flexible, up‐titrated doses (600 mg twice daily maximum) for 49 days. Fingolimod was administered on day 35, followed by a study completion evaluation (day 71). The PK analysis included 23 of 26 of the enrolled subjects (88.5%). Coadministration of fingolimod at steady‐state carbamazepine concentrations resulted in increased fingolimod CL/F by 67% through the induction of CYP3A4, a cytochrome with negligible involvement in fingolimod clearance in an uninduced state. Fingolimod C_max was reduced by 18% and AUC_inf by 40%, as was T_1/2 (106 vs 163 hours). A similar trend was observed for fingolimod‐P. Models linking fingolimod‐P blood concentrations to lymphocyte count or annual relapse rate suggest that such a decrease would have a low impact on the treatment effect. However, in the absence of efficacy data of fingolimod at doses lower than the therapeutic dose, their coadministration should be used with caution.

Time-dependent pharmacokinetics of dexamethasone and its efficacy in human breast cancer xenograft mice: a semi-mechanism-based pharmacokinetic/pharmacodynamic model

Dexamethasone (DEX) is the substrate of CYP3A. However, the activity of CYP3A could be induced by DEX when DEX was persistently administered, resulting in auto-induction and time-dependent pharmacokinetics (pharmacokinetics with time-dependent clearance) of DEX. In this study we investigated the pharmacokinetic profiles of DEX after single or multiple doses in human breast cancer xenograft nude mice and established a semi-mechanism-based pharmacokinetic/pharmacodynamic (PK/PD) model for characterizing the time-dependent PK of DEX as well as its anti-cancer effect. The mice were orally given a single or multiple doses (8 mg/kg) of DEX, and the plasma concentrations of DEX were assessed using LC-MS/MS. Tumor volumes were recorded daily. Based on the experimental data, a two-compartment model with first order absorption and time-dependent clearance was established, and the time-dependence of clearance was modeled by a sigmoid E_max equation. Moreover, a semi-mechanism-based PK/PD model was developed, in which the auto-induction effect of DEX on its metabolizing enzyme CYP3A was integrated and drug potency was described using an E_max equation. The PK/PD model was further used to predict the drug efficacy when the auto-induction effect was or was not considered, which further revealed the necessity of adding the auto-induction effect into the final PK/PD model. This study established a semi-mechanism-based PK/PD model for characterizing the time-dependent pharmacokinetics of DEX and its anti-cancer effect in breast cancer xenograft mice. The model may serve as a reference for DEX dose adjustments or optimization in future preclinical or clinical studies.

Intravenous carbamazepine as short-term replacement therapy for oral carbamazepine in adults with epilepsy: Pooled tolerability results from two open-label trials

OBJECTIVE

To report tolerability findings and maintenance of seizure control from a pooled analysis of phase I open-label trial OV-1015 (NCT01079351) and phase III study 13181A (NCT01128959).

METHODS

Patients receiving a stable oral dosage of carbamazepine were switched to an intravenous (IV) carbamazepine formulation solubilized in a cyclodextrin matrix (at a 70% dosage conversion) for either a 15- or a 30-min infusion every 6 h for up to 7 days and then switched back. A subset of patients who tolerated 15-min infusions also received 2- to 5-min (rapid) infusions. Assessments included physical and laboratory evaluations, electrocardiography (ECG) studies, as well as adverse event (AE) monitoring for tolerability. Convulsion/seizure AE terms and data from seizure diaries were used as proxies for the assessment of consistency of seizure control between formulations.

RESULTS

Of the 203 patients exposed to IV carbamazepine (30 min, n = 43; 15 min, n = 160), 113 received 149 rapid infusions. During infusion, the most commonly reported AEs (≥ 5%) were dizziness (19%), somnolence (6%), headache (6%), and blurred vision (5%). IV carbamazepine was not associated with clinically relevant cardiac AEs. The tolerability profile appeared similar between patients who received <1,600 mg/day (n = 174) and ≥ 1,600 mg/day (n = 29) carbamazepine. Cyclodextrin exposure was not associated with clinically relevant changes in AEs or renal biomarkers. Seizure control was maintained as patients transitioned between oral and IV carbamazepine.

SIGNIFICANCE

IV carbamazepine administered as multiple 30- or 15-min infusions every 6 h, and as a single rapid infusion, was well tolerated as a short-term replacement in adults with epilepsy receiving stable dosages of oral carbamazepine. Infusion site reactions, which were generally mild, were the only unique AEs identified; seizure control was generally unchanged when patients were switching between formulations.

Hepatoprotective and antioxidant activity of N-acetyl cysteine in carbamazepine-administered rats

Objectives:

The present study evaluates the hepatoprotective activity of N-acetyl cysteine (NAC) against carbamazepine (CBZ)-induced hepatotoxicity.

Materials and Methods:

Rats were treated with CBZ (50 mg/kg p.o.) and CBZ supplemented with NAC 50, 100 and 200 mg/kg for 45 days, after which blood samples were collected and subjected to liver function tests. Animals were killed, liver was separated, weighed and the levels of antioxidants and liver enzymes were estimated. In addition, histopathological investigation was also performed.

Results:

Serum glutamate pyruvate transaminase (SGPT), serum glutamate oxaloacetate (SGOT) transaminase, alkaline phosphatase (ALP), bilirubin, lipid peroxidation, absolute and relative liver weights were significantly (P < 0.05) elevated, whereas serum levels of albumin, total protein and body weight were decreased in the CBZ-treated animals. CBZ also produced vacuolar degeneration, centrilobular congestion and hepatic necrosis as evidenced from histopathological report. NAC significantly reduced the levels of serum transaminase, ALP, bilirubin and liver weight and increased the levels of total protein, albumin and body weight.

Conclusion:

It was observed that NAC increased the glutathione (GSH) content, reduced lipid peroxidation and reversed the CBZ-induced histopathological abnormalities. CBZ-induced hepatotoxicity may be due its toxic epoxide metabolite-induced oxidative stress.

Pregnane xenobiotic receptors and their effect on drug elimination from the organism

Nuclear receptors are intracellular proteins which, having been activated by their more or less specific ligands, regulate (usually increase) the transcription of target genes. They thus participate in a regulation of a number of physiologic functions. Some of them - especially pregnane xenobiotic receptors - serve primarily as protection of the organism from the xenobiotic intoxication. This is because many xenobiotics activate their function which consists in increasing the gene expression of enzymes involved in the metabolism of xenobiotics and detoxication drug transporters. Clarification of these mechanisms enabled the understanding of the substance of many drug-drug interactions observed in the clinical practice. Polymorphism of the nuclear receptors appears to be one of the causes of the interindividual variability in response to drug administration.

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.

Antiepileptic drugs and liver disease

Antiepileptic drugs (AEDs) are no longer restricted to the treatment of epilepsy. These are widely used in a broad spectrum of psychiatric and neurological disorders. Liver plays a major role in the metabolism of a majority of these drugs. Hepatotoxicity is rare, but a real concern when initiating therapy. Likewise, liver disease can adversely affect the biotransformation of some of these drugs. This manuscript addresses the significance of elevated liver enzymes associated with AED use, the role of therapeutic drug monitoring, pharmacokinetics during liver disease and potential risk of hepatotoxicity.

A mechanism-based pharmacokinetic model for the cytochrome P450 drug-drug interaction between cyclophosphamide and thioTEPA and the autoinduction of cyclophosphamide

Cyclophosphamide (CP) is widely used in high-dose chemotherapy regimens in combination with thioTEPA. CP is a prodrug and is activated by cytochrome P450 to 4-hydroxycyclophosphamide (HCP) which yields the final cytotoxic metabolite phosphoramide mustard (PM). The metabolism of CP into HCP exhibits autoinduction but is inhibited by thioTEPA. The aim of this study was to develop a population pharmacokinetic model for the bioactivation route of CP incorporating the phenomena of both autoinduction and the drug-drug interaction between CP and thioTEPA. Plasma samples were collected from 34 patients who received high-dose CP, thioTEPA and carboplatin in short infusions during 4 consecutive days. Elimination of CP was described by a noninducible route and an inducible route leading to HCP. The latter route was mediated by a hypothetical amount of enzyme. Autoinduction leads to a zero-order increase in amount of this enzyme during treatment. Inhibition by thioTEPA was modeled as a reversible, competitive, concentration-dependent inhibition. PM pharmacokinetics were described by first-order formation from HCP and first-order elimination. The final models for CP, HCP, and PM provided an adequate fit of the experimental data. The volume of distribution, noninducible and initial inducible clearances of CP were 31.0 L, 1.58 L/hr and 4.76 L/hr, respectively. The enzyme amount increased with a zero-order rate constant of 0.041 amount * hr-1. After each thioTEPA infusion, however, approximately 80% of the enzyme was inhibited. This inhibition was reversible with a half-life of 6.5 hr. The formation and elimination rate constants of PM were 1.58 and 0.338 hr-1, respectively. The developed model enabled the assessment of the complex pharmacokinetics of CP in combination with thio TEPA. This model provided an adequate description of enzyme induction and inhibition and can be used for treatment optimization in this combination.

A mechanism-based pharmacokinetic-enzyme model for cyclophosphamide autoinduction in breast cancer patients

AIMS

This study investigated the pharmacokinetics of cyclophosphamide (CP) and its main metabolite 4-hydroxycyclophosphamide (4-OH-CP) in patients with breast cancer undergoing high dose chemotherapy prior to autologous stem cell transplantation. An enzyme turn-over model was also developed to study the time course of cyclophosphamide induction.

METHODS

Fourteen patients received a combination of CP (6 g m-2 ), thiotepum (500 mg m-2 ) and carboplatin (800 mg m-2 ) as a 96 h infusion. Plasma concentrations of CP and 4-OH-CP were determined with h.p.l.c. and a pharmacokinetic and enzyme turn-over model applied to data using NONMEM.

RESULTS

CP plasma concentrations were described by a two-compartment model with a noninducible and an inducible pathway, the latter forming 4-OH-CP. In the final enzyme model, CP affects the amount of enzymes by increasing the enzyme production rate. CP concentrations decreased during the infusion with no subsequent change in 4-OH-CP concentrations. CP inducible and noninducible clearance were estimated to 1.76 l h-1 (90% C.I. 0.92-2.58) and 1.14 l h-1 (0.31-1.85), respectively. The induction resulted in an approximately doubled CP clearance through the inducible pathway at the end of treatment. The model predicted the enzyme turn-over half-life to be 24 h.

CONCLUSIONS

The presented mechanism-based enzyme induction model where the pharmacokinetics of the inducer and the enzyme pool counterbalance each other successfully described CP autoinduction. It is reasonable to believe that CP affects its own elimination by increasing the enzyme production rate and thereby increasing the amount of enzyme by which CP is eliminated.

Multiple dose pharmacokinetics of artemether in Chinese patients with uncomplicated falciparum malaria

Multiple dose pharmacokinetics of artemether and dihydroartemisinin were investigated in chinese patients treated for malaria. They received over 2 days either 4 x 80 mg artemether orally (n = 48) or 4 x 80-480 mg co-artemether (n = 40), a combination of artemether and lumefantrine (benflumetol). Lag time = 0.48 h (mean), Cmax after first dose = 157 ng/ml, t(max) = 1.73 h and elimination half-life = 1.16 h. The lag and absorption times were 0.5 h longer for co-artemether compared with artemether. Dihydroartemisinin paralleled artemether pharmacokinetics. Artemether Cmax after the last dose was one-third of the Cmax after the first dose while, inversely, dihydroartemisinin Cmax increased over time. We suggest that auto-induction of gut mucosa enzymes and/or liver enzymes causes a time-dependent increase in first-pass metabolisation of artemether.

Predictive capacity of carbamazepine pharmacokinetic parameters in a Portuguese outpatient population

The individualization of anticonvulsant therapy regimens can contribute to the implementation of appropriate carbamazepine (CBZ) maintenance doses in epileptic patients. An accurate method for the prediction of concentrations based on a determination of parameters and serum concentrations could be of clinical relevance in the management of epilepsy. In this study, we retrospectively evaluated the predictive performance in an adult outpatient population of six different methods, representing six sets of CBZ pharmacokinetic parameters selected according to the literature using a Bayesian computer program (PKS System; Abbott Laboratories, Abbott Park, IL, USA). The study involved 50 patients with two or more available concentrations selected under several inclusion criteria. The patients were taking CBZ (between 200 and 1600 mg/d) in monotherapy or polytherapy regimens and had no hepatic or renal disease. Steady state concentrations were predicted according to the use of prior information and using one and two feedback patient concentrations. Accuracy and precision were assessed by mean prediction error (ME), mean squared prediction error (MSE) and root mean squared prediction error (RMSE). The analysis showed CL = 0.067 L/hour/kg and Vd = 1.19 L/kg as the most accurate and precise set of pharmacokinetic parameters, presenting the highest percentage of clinically acceptable estimates (error < 2 microg/mL). Additionally, predictions based on one measured feedback concentration were found to be more accurate and precise than prior population-based predictions; the use of two previous patient concentrations further improved predictive capacity but failed to show a significant difference when compared with predictions based on one measured feedback concentration. In conclusion, the adoption of the previously mentioned set of parameters as population estimates and the use of at least one feedback concentration through the Bayesian approach seems to be essential for a better CBZ use in clinical practice. Finally, despite the obtained results, we believe that the Portuguese pharmacokinetic parameter determination of antiepileptics should be carried out to improve the rationale and cost-effectiveness of anticonvulsant therapy.

Autoregulation of 5-fluorouracil metabolism

5-Fluorouracil (5-FU) is a commonly used anticancer agent for the treatment of gastrointestinal, head and neck, and breast tumours. This study determined the influence of 5-FU on dihydropyrimidine dehydrogenase (DPD) activity, the enzyme responsible for its in vivo degradation. DPD activity was measured in mononuclear cells obtained prior to and after the administration of 5-FU in 20 patients with colorectal cancer. Following the results from the human studies, DPD activity was measured in Sprague-Dawley rat liver up to 72 h after administration of 5-FU 200 mg/kg as a single injection. Total liver P450 content and the production of testosterone metabolites (indicative of CYP3A activity) were also analysed to determine the specificity of 5-FU-associated alteration in rat liver metabolism. Human mononuclear cell DPD activity decreased by a median of 38.7% following the administration of 5-FU (P = 0.001). 5-FU-induced alterations in rat liver DPD were also observed, with the lowest activity occurring 48 h after injection (50% of control activity; P = 0.009). Rat liver DPD activity returned to near control values by 72 h postinjection. Rat liver total P450 content and CYP3A activity were not significantly different in 5-FU treated or control tissues. Thus, 5-FU demonstrates autoregulation of its metabolism through inhibition of DPD activity. Although this inhibition appears to be specific for DPD, the mechanism for enzyme inhibition is not clear. These findings may aid in the design of 5-FU treatment regimens and provide the basis for further studies into the regulation of DPD.

Kinetic profile of carbamazepine in an adult Portuguese outpatient population

OBJECTIVE

The aim of our work was to define the kinetic profile of carbamazepine (CBZ), in order to improve on dosing schedules through a Bayesian approach.

METHOD

Carbamazepine dose/steady-state trough concentrations data pairs and associated information were collected retrospectively on a population of adult epileptic patients.

RESULTS

Fifty patients (index population) with two or more available concentrations (total of 174 determinations) met our inclusion criteria. Patients were taking CBZ (200-1800 mg/day) in mono- or polytherapy regimens. The analysis assumed a one-compartmental model with first-order absorption and elimination. Due to the data source (only trough concentrations were measured as part of hospital routine), the volume of distribution was fixed at 1.19 l/kg. The final estimates for CL were: 0.075 +/- 0.027 (mono- and polytherapy), 0.069 +/- 0.020 (monotherapy), and 0.106 +/- 0.037 l/h/kg (polytherapy). In order to validate these results, we assessed their predictive capacity using 18 new patients (validation population), submitted to the same inclusion criteria and using Prediction-Error analysis. The results suggested a different CL value for our population compared to earlier published clearance values. The results also pointed to an increased metabolic rate associated with polytherapy. The prediction capacity of the optimization method derived from a Portuguese population made in an a priori evaluation indicated a low error (-0.04 microg/ml), close to the theoretical zero value.

CONCLUSION

Our results provide specific data on CBZ disposition in a Portuguese population and given the wide variability in the literature values, our data may help improve dosing of CBZ in Portuguese patients.

Intravenous and oral pharmacokinetic evaluation of a 2-hydroxypropyl-beta-cyclodextrin-based formulation of carbamazepine in the dog: comparison with commercially available tablets and suspensions

Complexation of carbamazepine with 2-hydroxypropyl-beta-cyclodextrin was performed to provide improved formulations of this widely used antiepileptic drug. Based on this approach, liquid dosage forms were configured for both parenteral and oral use. Intravenous administration of an aqueous carbamazepine x 2-hydroxypropyl-beta-cyclodextrin (CBZ x HPbetaCD) complex at a CBZ dose of 20 mg/kg was well tolerated and generated high initial drug levels that fell monoexponentially as a function of time, yielding a plasma elimination half-life of 38 min. Oral studies were completed with three preparations: a commercially available tablet and suspension, as well as a CBZ x HPbetaCD oral solution. Oral administration of tablets gave erratic and slow absorption, leading to maximum CBZ concentrations (C(max)) of <2 microg/mL, which were manifested only at 2.5 h after drug dosing. The absolute bioavailability of CBZ from the tablets was approximately 25%. Both the suspension and CBZ x HPbetaCD solution gave a significantly improved profile. Thus, the liquid oral dosage forms approximately doubled the oral bioavailability of CBZ compared with the tablets.