Intradose and circadian variation in circulating carbamazepine and its epoxide in epileptic patients: a consequence of autoinduction of metabolism

Sodium Channel Blockers in the Treatment of Epilepsy

Sodium channel blockers have been the mainstay of the pharmacological management of focal and generalised tonic-clonic seizures for more than 70 years. The focus of this paper will be on phenytoin, carbamazepine, lamotrigine, oxcarbazepine, rufinamide, lacosamide and eslicarbazepine acetate. All these antiepileptic drugs have similar efficacy and share similar dose-dependent, adverse effect profiles, although phenytoin, carbamazepine and oxcarbazepine are more likely to cause idiosyncratic reactions than the others. With the exception of lamotrigine, rufinamide and lacosamide, all are enzyme inducers and most are minor teratogens; although data on teratogenicity are sparse with lacosamide and eslicarbazepine acetate. There is increasing evidence that these drugs differ mechanistically, with the newer agents, lacosamide and eslicarbazepine acetate, having their major pharmacological effect on the slow inactivation state of the sodium channel, which may be associated with better tolerability at higher dosage, although hard evidence in support of this observation is currently not available. Rufinamide is licensed only for Lennox-Gastaut syndrome in children aged 4 years and above. There is a move away from using enzyme inducers, particularly phenytoin and carbamazepine, in everyday clinical practice. There seems little doubt, however, that some sodium channel blockers will have an enduring place in the management of epilepsy well into the 21st century.

Tolerability and Safety of Commonly Used Antiepileptic Drugs in Adolescents and Adults: A Clinician's Overview

This paper discusses the issues surrounding the tolerability and safety of the commonly used antiepileptic drugs (AEDs) in adolescents and adults. The content includes dose-related adverse effects, idiosyncratic reactions, behavioural and psychiatric comorbidities, chronic problems, enzyme induction and teratogenesis. Twenty-one AEDs are discussed in chronological order of their introduction into the UK, starting with phenobarbital and ending with brivaracetam. Wherever possible, advice is given on anticipating, recognising and managing these issues and thereby improving the lives of people with epilepsy, most of whom will need to take one or more of these agents for life. Avoidance of side effects will increase the possibility of achieving and maintaining long-term seizure freedom. Alternatively, adverse events from AEDs will substantially reduce quality of life and often result in higher healthcare costs.

Extended-release formulations in epilepsy

In the past years, the extended-release antiepileptic drug formulations have been developed and then approved for the treatment of many types of epilepsy. Among these extended-release formulations of antiepileptic drugs, the main drugs are valproic acid, carbamazepine, and phenytoin. This review analyzes the chemical and structural characteristics of the extended-release formulations of these 3 antiepileptic drugs, analyzing their bioequivalence and the studies about their clinical use. The results of these studies are encouraging and suggest a good tolerability and efficacy of these extended-release formulations, although larger studies are needed.

The effect of carbamazepine on injury-induced ectopic discharge in the lingual nerve

Previous studies have shown that the development of ectopic activity from damaged axons following nerve injury may contribute to the aetiology of sensory disturbances, including dysaesthesia. Pharmacological manipulation of this activity could provide a method of treatment for this intractable condition. In this study we have investigated the effect of carbamazepine, an anti-convulsant, as it is known to have membrane stabilising properties. In eight anaesthetised adult ferrets the left lingual nerve was sectioned and the animals allowed to recover for 3 days. Then, in terminal experiments under general anaesthesia, the nerve was re-exposed and electrophysiological recordings were made from spontaneously active units in fine filaments dissected from the nerve proximal to the injury site. Carbamazepine in a modified cyclodextrin (hydroxypropyl-beta-cyclodextrin) was administered intravenously in increments, in order to achieve a progressively increasing systemic concentration, and serum levels were determined at the point that activity ceased. Twenty-one spontaneously active units were studied, with conduction velocities of 2.1-28.9 m s(-1) and discharge frequencies of 0.25-15.3 Hz. Spontaneous activity ceased in 13 units with a serum concentration of carbamazepine ranging from 3.5 to 8.4 mg/l, which was within the normal therapeutic range (4-12 mg/l). Four units ceased activity with carbamazepine levels above the therapeutic range (15.4-17.2 mg/ml), but the remaining four continued to discharge throughout the recording period. These data suggest that systemic carbamazepine can reduce the level of spontaneous activity initiated in some axons following lingual nerve injury.

Bayesian estimation of six different sets of carbamazepine pharmacokinetic parameters in Egyptian adult epileptic patients

The individualization of carbamazepine (CBZ) dosage regimen based on estimation of pharmacokinetic (PK) parameters and measurement of serum drug concentration in epileptic patients can help to control epilepsy. In a retrospective study, the predictive performance of six different sets of CBZ PK parameters selected according to the literature was evaluated in 60 adult epileptic patients. Patients were administered controlled release CBZ (dose range: 200-1200 mg day(-1)) as monotherapy and one steady state serum concentration of the drug was available for each patient. The Bayesian Program of Abbott (PKS system; Abbott Laboratories, Wiesbaden, Germany) was used in the prediction process. Predictive measures included estimation of mean prediction error (mpe) for bias, mean squared prediction error (mspe) and root mean squared prediction error (rmspe) for precision. The analysis showed that three of the investigated six sets achieved the best predictive performance in Egyptian patients and consequently, the PK parameters of any of these three sets can be used by the Bayesian approach as prior information for CBZ dose optimization among the Egyptian adult population.

Circadian rhythms: interactions with seizures and epilepsy

Circadian rhythms are endogenously-mediated 24 h cycles of behavioral or physiological activity. The interactions among the mammalian circadian clock, acute seizures, and chronic epilepsy are not well-characterized. Evidence suggests that seizures are susceptible to circadian modulation, and that this modulation varies with epilepsy syndrome and location of seizure foci. The circadian timing system and secondary circadian cycles of hormone secretion, sleep and wakefulness, and recurrent environmental factors are discussed as potential systems that effect spontaneous seizure recurrence. Experimental designs should take into account time-of-day effects on seizure threshold and occurrence. Further work is required to determine what mechanisms account for daily variation in seizure susceptibility.

Established antiepileptic drugs

Despite the recent entry into the market-place of a range of new pharmacological treatments for epilepsy, most patients still receive the standard antiepileptic drugs. This review considers the clinical place and practical use of these agents. Detailed consideration is given to carbamazepine, phenytoin, sodium valproate, phenobarbital and ethosuximide, with lesser emphasis on primidone, clobazam and clonazepam. Individualization of therapy, polypharmacy, refractory epilepsy, therapeutic drug monitoring, pregnancy, withdrawing treatment, epilepsy prophylaxis and referral to an epilepsy centre are also discussed. The paper concludes with a statement of 12 basic rules in prescribing established antiepileptic drugs.

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.

Mutual interaction between remacemide hydrochloride and carbamazepine: two drugs with active metabolites

PURPOSE

We wished to determine mutual interaction of two drugs with active metabolism: remacemide, hydrochloride and carbamazepine (CBZ).

METHODS

A randomized, double-blind, placebo-controlled cross-over study of add-on remacemide hydrochloride was performed in 10 of 14 recruited patients being treated with CBZ monotherapy. Forty-eight-hour concentration profiles of CBZ, its active epoxide metabolite (CBZ-E), remacemide, and its desglycinyl metabolite (ARL12495XX) were assayed after single and multiple dosing.

RESULTS

After patients were treated with 300 mg remacemide hydrochloride twice daily for 14 days, the mean area under the concentration-time curve (AUC) of CBZ was increased by 22% (p = 0.12), Cmax was increased by 27% (p = 0.07), and Cmin was increased by 22% (p = 0.29). Trough concentrations of CBZ were higher (p = 0.0037) during active treatment as compared with placebo treatment. CBZ-E levels were unaffected. No symptoms of CBZ toxicity were reported. There was no evidence of autoinduction of remacemide metabolism. However, in CBZ-treated patients, the AUC of remacemide and its active metabolite was 60 and 30%, respectively, of values observed in healthy volunteers treated previously with the same dose.

CONCLUSIONS

Remacemide hydrochloride inhibits CBZ metabolism, which itself induces that of remacemide hydrochloride and its active metabolite. This mutual interaction between remacemide hydrochloride and CBZ is predictable and modest and should not present a barrier to their clinical use in combination.

A comprehensive study of the relation between serum concentrations, concentration ratios, and level/dose ratios of carbamazepine and its metabolites with age, weight, dose, and clearances in epileptic children

We made a comprehensive study of the relation between age, weight, carbamazepine (CBZ) dose, total clearance (TC), and intrinsic clearance (IC) and concentrations, concentration ratios, and level/dose ratios of CBZ, carbamazepine-10,11-epoxide (CBZ-E) and trans-10,11-dihydroxy-10,11- dihydro-carbamazepine (CBZ-H) in a group of epileptic children receiving CBZ monotherapy. Body weight and age showed negative correlations with TC, IC, CBZ dose, and CBZ-E/CBZ and CBZ-H/CBZ concentration ratios, and had positive relation with CBZ, CBZ-E, and CBZ-H level/dose ratios. These results indicate decreased CBZ metabolism with patient maturity. Correlations between CBZ dose with TC, IC, and the concentration ratios of CBZ-E/CBZ, CBZ-H/CBZ-E, and CBZ-H/CBZ were positive. CBZ dose also had negative associations with CBZ and CBZ-E level/dose ratios, indicating dose-dependent autoinduction of CBZ metabolism. Our data suggest that weight, age, and CBZ dose have less influence on epoxide-hydrolase activities than on epoxidase activities. The CBZ-E/CBZ concentration ratio can be used as an indicator of the degree of autoinduction of CBZ metabolism, even in patients receiving CBZ monotherapy.

Clinical pharmacology and pharmacokinetics of oxcarbazepine

Oxcarbazepine (OCBZ) is a new antiepileptic drug (AED) structurally related to carbamazepine (CBZ) but differing in several important aspects, notably metabolism and induction of metabolic pathways. Consequently, OXCB has fewer drug-drug interactions compared with CBZ. Absorption of OCBZ is rapid and complete. In animals it is responsible for the pharmacological effect. In humans, however, the parent compound is rapidly and extensively metabolized to a monohydroxy derivative (MHD), which is responsible for the therapeutic effect. Exposure to the MHD increases dose proportionally, and steady state is achieved after only three or four doses in a twice-daily regimen. When given with food, systemic exposure to MHD increases by about 17%. MHD is eliminated with a half-life of about 8-10 h. About 27% of the dose is recovered in the urine as unchanged MHD and a further 49% as a glucuronide conjugate of MHD. Results suggest that the kinetics of OCBZ should not be affected by impaired liver function. Impaired kidney function does not affect the kinetics of MHD; the glucuronide conjugate will, however, accumulate in these patients. The conversion of OCBZ to MHD is catalyzed by reductase enzymes, which are not subject to induction. Furthermore, OCBZ itself does not appear to induce the cytochrome P-450 family in general, although it does induce the P-450 IIIA subfamily, which is responsible for the metabolism of estrogens and the dihydropyridine calcium-channel blockers (e.g., nifedipine, felodipine). In patients, linear and dose-proportional kinetics with no autoinduction of metabolism simplify OCBZ dosage adjustment.

A double-blind, placebo-controlled interaction study between oxcarbazepine and carbamazepine, sodium valproate and phenytoin in epileptic patients

1. The effect of carbamazepine (CBZ), sodium valproate (VPA) and phenytoin (PHT) on the pharmacokinetics of oxcarbazepine (OXC) was explored in three groups of 12 epileptic patients taking one of these drug as monotherapy. 2. Each patient took a single 600 mg dose of OXC followed 7 days later by 3 weeks' treatment with OXC 300 mg thrice daily and matched placebo in random order. 3. Seven untreated patients, acting as controls, were prescribed the single OXC dose and 3 weeks' active treatment only. 4. In those patients completing the study, the area under the concentration-time curve (AUC) at steady-state for hydroxycarbazepine (OHCZ), the active metabolite of OXC, was significantly lower in the CBZ-treated group than in controls (P < 0.05). 5. No other differences in AUCs or elimination half-lives for OHCZ were found between treated and untreated patients following single or multiple OXC dosing. 6. Median AUCs of CBZ, VPA and PHT during a dosage interval did not differ significantly after treatment with OXC and placebo. 7. Ten patients completing the study complained of side-effects during treatment with OXC compared with one taking placebo (P < 0.01). 8. There were no important changes in cognitive function testing during administration of OXC compared with placebo. 9. Standard doses of OXC can be given as add-on therapy in epileptic patients receiving CBZ, VPA or PHT without producing a clinically relevant pharmacokinetic interaction.

Double dummy comparison between once and twice daily dosing with modified-release carbamazepine in epileptic patients

1. Fourteen patients with refractory epilepsy on a twice daily regimen of modified-release carbamazepine (CBZ-MR. Tegretol Retard. Ciba Pharmaceuticals) completed a balanced, double-blind, double dummy, random order, crossover comparison of 8 weeks treatment with once (o.d.) and twice daily (b.d.) dosing. In order to obtain a profile of serum CBZ concentrations over 24 h on once daily dosing, patients were randomised to taking it in the morning (o.d. a.m.) or evening (o.d. p.m.) for 4 weeks. Each treatment was taken with a placebo of the other and total tablet numbers were matched. Blood sampling was undertaken 0, 2, 4, 6, 8, 10, 12 and 24 h after the morning tablets at the end of each 4 week treatment period. 2. Overall, trough serum drug concentrations (Cmin) were lower with once daily dosing (Cmin: b.d. 7.5 mg l-1, o.d. 6.5 mg l-1, P < 0.05, 95% CI of the difference -1.3 to -0.1), but no significant differences were found in average (Cav) or peak (Cmax) concentrations, AUC values or fluctuations in CBZ concentrations. 3. Pharmacokinetic parameters for CBZ 10.11 epoxide, the active metabolite of CBZ did not differ significantly between the dosage schedules. 4. Seizure control was similar during once and twice daily dosing with CBZ-MR (median seizures/month (range): b.d. 1 (0-14.5), o.d. 0.5 (0-11), NS, 95% CI of the difference -1.8 to + 0.25). 5. There were no differences in psychomotor performance between the treatment periods. 6. More patients (n = 11) preferred treatment (P < 0.05) with once daily than twice daily dosing (n = 3) with CBZ-MR. 7. Once daily dosing with CBZ-MR should be possible in the majority of patients receiving the drug as monotherapy.

Steady state pharmacokinetics of conventional versus controlled-release carbamazepine in patients with epilepsy

The purpose of the present study was to compare the pharmacokinetic parameters of two carbamazepine (CBZ) tablet formulations (conventional (CBZ-CO) and controlled-release (CBZ-CR)) in patients with epilepsy receiving the drug as monotherapy or polytherapy. The absorption rate constant (Ka), steady-state volume of distribution (Vdss) and total clearance (CL) were computed with the APIS software using 31 blood level profiles from 23 patients who were divided into four groups: patients receiving CBZ-CO in polytherapy, the same patients switched to CBZ-CR in the same polytherapy conditions, patients receiving CBZ-CO in monotherapy and patients receiving CBZ-CR in monotherapy. The four groups were compared in order to assess the significance of differences in Ka, Vdss, CL and diurnal fluctuations of plasma CBZ concentration. The results show a significant decrease of the Ka in the CBZ-CR groups compared to the CBZ-CO groups, both on monotherapy and on polytherapy. The comparison between the monotherapy and polytherapy groups shows increases of Vdss and CL, both in CBZ-CO and CBZ-CR polytherapy groups. Dispersion of pharmacokinetic data was higher in patients on CBZ-CO; among patients on CBZ-CR, dispersion was lowest in the monotherapy group. Clinical improvement was found in four of eight patients switched from CBZ-CO to CBZ-CR. CBZ-CR is therefore a valuable alternative to CBZ-CO.

Oxcarbazepine. A review of its pharmacology and therapeutic potential in epilepsy, trigeminal neuralgia and affective disorders

Oxcarbazepine is the 10-keto analogue of carbamazepine but has a distinct pharmacokinetic profile. In contrast to the oxidative metabolism of carbamazepine, oxcarbazepine is rapidly reduced to its active metabolite, 10,11-dihydro-10-hydroxy-carbamazepine. With the possible exception of the P450IIIA isozyme of the cytochrome P450 family, neither oxcarbazepine nor its monohydroxy derivative induce hepatic oxidative metabolism. Direct comparison of oxcarbazepine and carbamazepine has shown no difference in efficacy between these 2 agents in terms of reducing seizure frequency in patients with partial epilepsy with or without secondary generalisation, or with tonic-clonic seizures. Substitution of oxcarbazepine for carbamazepine in multiple antiepileptic drug regimens improved seizure control in some patients with refractory epilepsy; however, the rise in serum concentrations of concurrent antiepileptic agents secondary to elimination of carbamazepine-associated hepatic enzyme induction may have also played a role. Substitution of oxcarbazepine for carbamazepine was associated with improved cognition and alertness in some patients with epilepsy. Limited data indicate that oxcarbazepine may be a useful alternative to carbamazepine in the management of trigeminal neuralgia. Experience in patients with acute mania is promising, but the value of oxcarbazepine in managing affective disorders, particularly as a prophylactic agent, is not established. Oxcarbazepine may be better tolerated than carbamazepine; however, the current published database is small and the potential for oxcarbazepine to induce the type of serious idiosyncratic reactions occasionally associated with carbamazepine is unknown. Hyponatraemia has been reported in patients treated with oxcarbazepine. Although apparently asymptomatic, fluid restriction may be deemed necessary in some patients to reduce the risk of precipitating seizures secondary to low serum sodium. Thus, oxcarbazepine appears to be an effective substitute for carbamazepine in those patients intolerant of this agent, or experiencing significant drug interactions. Wider clinical experience should help clarify the long term efficacy and tolerability of oxcarbazepine. Pharmacokinetic advantages over current antiepileptic drugs, carbamazepine in particular, may then favour oxcarbazepine for consideration as a first-line agent in the management of partial and tonic-clonic epilepsy.

Rapid tolerance to acute psychomotor impairment with carbamazepine in epileptic patients

Thirteen patients with newly diagnosed epilepsy performed a battery of psychomotor function tests before and during the first 12 weeks of carbamazepine (CBZ) therapy. Movement time (P less than 0.025), total choice reaction time (P less than 0.025), finger tapping rate (P less than 0.005) and number cancellation (P less than 0.05) were all significantly impaired after 1 week's treatment, but had returned to baseline by 4 weeks. Mean (+/- s.d.) serum CBZ concentrations and those of its active metabolite CBZ 10,11 epoxide (CBZ-E) were comparable at 1 (CBZ: 8.5 +/- 2.1 mg l-1, CBZ-E: 1.1 +/- 0.68 mg l-1) and 4 weeks (CBZ: 8.1 +/- 4.4 mg l-1, CBZ-E: 0.93 +/- 0.27 mg l-1). These results suggest the rapid development of tolerance to the acute deleterious psychomotor effects of CBZ.

Monotherapy with conventional and controlled-release carbamazepine: a double-blind, double-dummy comparison in epileptic patients

1. Twenty-one epileptic patients completed a double-blind, double-dummy, random order, crossover comparison of conventional carbamazepine (CBZ, Tegretol, Ciba-Geigy) with a new controlled-release formulation (CBZ-CR, Tegretol Retard). All participants were stabilised on maximally tolerated doses of CBZ as monotherapy (one twice daily, twelve three times daily, eight four times daily). Each preparation was taken with a matched placebo of the other for 4 weeks. 2. Peak serum CBZ concentrations (mean +/- s.e. mean) were lower (CBZ 11.4 +/- 0.4 mg l-1; CBZ-CR 10.4 +/- 0.5 mg l-1; P less than 0.01) and times to peak longer (CBZ 3.6 +/- 0.5 h, CBZ-CR 5.2 +/- 0.7 h, P less than 0.01) during CBZ-CR treatment. Mean CBZ concentrations, however, were also slightly reduced with the new formulation (CBZ 9.9 +/- 0.3 mg l-1; CBZ-CR 9.1 +/- 0.5 mg l-1, P less than 0.05) and this was associated with greater seizure frequency (CBZ 2.8 +/- 1.2, CBZ-CR 3.8 +/- 0.9; P less than 0.05) during the CBZ-CR treatment phase. 3. Diurnal fluctuations (CBZ 41 +/- 3%, CBZ-CR 28 +/- 2%, P less than 0.01) and variations (CBZ 53 +/- 5%, CBZ-CR 33 +/- 3%; P less than 0.01) in serum CBZ concentration were substantially less with CBZ-CR and were similar to those calculated during a 6 or 8 hourly dosage interval with conventional CBZ (fluctuation 33 +/- 3%, variation 42 +/- 5%).(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the efficacy and tolerability of controlled-release carbamazepine with conventional carbamazepine

We compared the efficacy and tolerability of controlled-release carbamazepine (CBZ-CR) with conventional carbamazepine (CBZ) in 131 epileptic patients (both men and women, ages 6-65 years) in an open, multicentre, cross-over trial. Patients entered into the trial were previously on CBZ monotherapy or polytherapy. During the first 4 weeks, patients were treated with equivalent daily doses of CBZ and then switched to CBZ-CR for the subsequent 4 weeks. The majority of patients were switched to the more convenient b.i.d. dosing schedule of the controlled-release (CR) preparation without a detrimental effect on seizure frequency or adverse effects. In 44/131 (34%) of patients, the switch to CBZ-CR was accompanied by an improvement in tolerability, primarily due to a reduction in peak-dependent CNS side-effects such as tiredness, double or blurred vision, dizziness and ataxia. At the end of the study, investigators preferred CBZ-CR for 76% of their patients and 70% of the patients preferred CBZ-CR.

Lack of enzyme induction with oxcarbazepine (600 mg daily) in healthy subjects

1. Oxcarbazepine (OXC), the 10-keto analogue of carbamazepine (CBZ), has similar anticonvulsant efficacy and possibly improved patient tolerability. Unlike CBZ, it is metabolised by reduction and may not induce hepatic monooxygenase enzymes. 2. Serum concentrations of OXC and its active metabolite 10-OH-carbazepine (10-OH-CZ) were followed after a single 300 mg dose and during and after 300 mg OXC twice daily for 29 doses in eight healthy male volunteers. 3. Antipyrine metabolism, urinary 6-beta-hydroxycortisol excretion, sex hormone binding globulin (SHBG) levels and circulating androgens were measured as indices of hepatic enzyme induction before, during and after treatment with OXC. 4. Elimination half-lives (mean +/- s.e. mean) of 10-OH-CZ were unaltered by 2 weeks' therapy with OXC (before 11.3 +/- 1.1 h; after 13.9 +/- 3 h). Trough plasma concentrations of 10-OH-CZ at steady-state (31 +/- 2.2 mumol l-1) were higher than predicted (16.5 +/- 4 mumol l-1). 5. Antipyrine metabolism, urinary 6-beta-hydroxycortisol excretion, SHBG levels and circulating androgens were unaltered by treatment with OXC. 6. OXC (600 mg daily) does not induce hepatic monooxygenase enzymes and so is likely to have more predictable dose-concentration relationships and to produce fewer physiological and pharmacological interactions than CBZ.

Pharmacokinetic comparison of two carbamazepine slow-release formulations

In a single-blind pharmacokinetic study patients being treated with conventional carbamazepine (CBZ) in a t.i.d. regimen were randomly allocated to identical doses of two CBZ slow-release formulations (Trimonil Retard and Tegretol Retard) administered once daily. Statistical analysis involved the following parameters: AUC, Cmin, Cmax and Fluctuation Index (FI). With regard to Cmin and FI statistically significant differences in favour of Tegretol Retard were observed.

A double-blind comparison of conventional and controlled-release carbamazepine in healthy subjects

1. Eight healthy subjects took part in a balanced, double-blind, crossover comparison of conventional carbamazepine (Tegretol, Ciba-Geigy Ltd, CBZ-C) and a novel controlled-release formulation (Tegretol CR Divitabs, Ciba-Geigy Ltd; CBZ-CR). An initial single dose of either preparation was followed 1 week later by a 2 week course of 200 mg twice daily. 2. Following the single dose, CBZ-CR produced a concentration plateau from 6-56 h at 50-60% of the CBZ-CR peak. 3. After 2 weeks' treatment, CBZ daytime levels measured as area under the concentration-time curve over a dosage interval were 7% lower with CBZ-CR, but this difference was not statistically significant. 4. CBZ-CR showed less diurnal fluctuation (12%) of CBZ than CBZ-C (24%; P less than 0.025) with less rapid changes in concentration (P less than 0.02). 5. Diurnal fluctuation of free CBZ and of CBZ 10,11 epoxide, the active metabolite, did not differ significantly between the two preparations. 6. Auto-induction of CBZ metabolism resulted from the administration of both formulations. The mean elimination half-life was 23 h (CBZ-C) and 25 h (CBZ-CR) after dose 29 compared with a base-line value of 37 h (both P less than 0.02). Antipyrine metabolism was also induced to a similar extent in both legs of the study (P less than 0.01). 7. No significant alteration in psychomotor function was demonstrated with either preparation. 8. CBZ-CR fulfils the criteria for a controlled-release preparation with comparable apparent bioavailability to CBZ-C. Further pharmacokinetic and, more importantly, pharmacodynamic studies are required in epileptic patients to confirm a clinical advantage over the currently available formulation.

Carbamazepine and its epoxide: relation of plasma levels to toxicity and seizure control

We studied the relation of plasma levels of carbamazepine (CBZ) and carbamazepine 10,11 epoxide (CBZ-E), and their ratio to drug toxicity and seizure control in 7 patients with complex partial seizures. CBZ-E/CBZ increased with increasing CBZ levels and was higher when patients were taking phenytoin or valproic acid. There were weak correlations between CBZ, CBZ-E levels, toxicity scores, and seizure control when patients were taking CBZ alone, but not when other drugs were given as well. There were no significant differences in the correlation with toxicity score or seizure frequency between CBZ, CBZ-E, or their sum. Measurement of CBZ-E levels did not provide additional information useful for monitoring clinical response to CBZ therapy.

Effect of sodium valproate on carbamazepine disposition and psychomotor profile in man

1 The effect of sodium valproate (VPA; 500 mg thrice daily for 7 days) and matched placebo on the disposition and psychomotor profile of a single dose of carbamazepine (CBZ; 10 mg kg-1) was studied in eight healthy male subjects using a randomised balanced crossover design. 2 VPA alone had no effect on antipyrine clearance, urinary 6 beta-hydroxycortisol excretion and a battery of psychomotor function tests after 3 days' treatment despite achieving a mean steady-state concentration (90 +/- 6 mg 1(-1)) well within the target range (50-100 mg 1(-1)) for the drug. 3 VPA pre-treatment did not alter total CBZ area under the concentration-time curve (AUC 0-59 h) but did prolong CBZ elimination half life by 12% (P less than 0.01). AUC 0-59 h for free plasma CBZ was 13% higher (P less than 0.02) and half-life of unbound CBZ 16% longer (P less than 0.02) during VPA treatment. CBZ-10,11 epoxide (CBZ-E) levels (52%) and CBZ-E/CBZ ratios (45%) were both elevated by concurrent VPA (P less than 0.05) and free CBZ fraction was increased by 7% (P less than 0.02). 4 The sole effect of VPA on the psychomotor profile of CBZ was prolongation of card sorting time (P less than 0.05), although CBZ-related side effects were reported as more severe when VPA was also taken (P less than 0.01). 5 These data suggest that VPA displaces CBZ from plasma protein binding sites and inhibits the metabolism of both the parent drug and its epoxide.(ABSTRACT TRUNCATED AT 250 WORDS)