Pharmacokinetic interactions of the new antiepileptic drugs

Modified-Release Formulations of Second-Generation Antiepileptic Drugs: Pharmacokinetic and Clinical Aspects

Modified-release or extended-release (XR) formulations are used to decrease the frequency of dosing for drugs with rapid elimination, to improve convenience and adherence. Use of a modified-release formulation can decrease the peak to trough fluctuations in serum concentrations and theoretically improve the therapeutic benefit of the drug, by decreasing adverse events associated with the higher peak concentrations. Once-daily formulations of lamotrigine (Lamictal XR(®)), levetiracetam (Keppra XR(®)), oxcarbazepine (Oxtellar XR(®), Apydan(®) extent) and topiramate (Qudexy XR™, Trokendi XR™) are approved for the treatment of focal and/or generalized onset seizures. Other seizure medications have been approved for non-epileptic symptoms. Gabapentin XR (Gralise(®)) is approved for the treatment of post-herpetic neuralgias. Gabapentin enacarbil XR (Horizant(®)) is a prodrug of gabapentin and is indicated for treatment of post-herpetic neuralgia and restless leg syndrome, a novel indication. For all but Qudexy XR™, the tablets/capsules must be swallowed whole, without cutting, crushing or chewing, in order to maintain the XR properties of the formulation. Qudexy XR™ can be swallowed intact or the capsules can be opened and sprinkled onto soft food for those with swallowing difficulties, for example, children and the elderly. The bioavailability of Gralise(®) and Horizant(®) is significantly affected by food, specifically fat content, and should be taken with a meal to maximize absorption. Overall, the primary advantage of the newly released XR formulations is the once-daily dosing to improve convenience and adherence, with very limited data suggesting improved tolerability.

Oxcarbazepine induced maculopapular rash - a case report

Unlike carbamazepine, newer anti epileptic drug like oxcarbazepine, reports fewer side effects. In this report we describe a case of oxcarbazepine induced maculopapular rash probably happened because of a drug interaction with isoniazid, and a brief review of the existing literature is presented herewith. A 40-year-old male patient received oxcarbazepine 300mg twice daily along with other anti-tubercular drugs including isoniazid (300mg) once daily since two days. Extensive cutaneous rash with intense itching developed which subsided on discontinuation of oxcarbazepine. This case highlights the fact that there is a potential possibility of drug-drug interaction between oxcarbazepine and isoniazid and concomitant use of these two drugs should better be avoided during clinical practice.

Pharmacokinetic aspects of the anti-epileptic drug substance vigabatrin: focus on transporter interactions

Drug transporters in various tissues, such as intestine, kidney, liver and brain, are recognized as important mediators of absorption, distribution, metabolism and excretion of drug substances. This review gives a current status on the transporter(s) mediating the absorption, distribution, metabolism and excretion properties of the anti-epileptic drug substance vigabatrin. For orally administered drugs, like vigabatrin, the absorption from the intestine is a prerequisite for the bioavailability. Therefore, transporter(s) involved in the intestinal absorption of vigabatrin in vitro and in vivo are discussed in detail. Special focus is on the contribution of the proton-coupled amino acid transporter 1 (PAT1) for intestinal vigabatrin absorption. Furthermore, the review gives an overview of the pharmacokinetic parameters of vigabatrin across different species and drug–food and drug–drug interactions involving vigabatrin.

The association between oxcarbazepine-induced maculopapular eruption and HLA-B alleles in a northern Han Chinese population

Background

We investigated the association between oxcarbazepine (OXC)-induced maculopapular eruption (MPE) and HLA-B alleles in a northern Han Chinese population, and conducted an analysis of clinical risk factors for OXC-MPE.

Methods

Forty-two northern Han Chinese patients who had been treated with OXC in Changchun, China were genotyped. Among them were 14 cases with OXC-induced MPE; the remaining 28 were OXC-tolerant. The HLA-B allele frequencies of the normal control group were found in the Allele Frequency Net Database. Polymerase chain reaction-sequence specific primer( PCR-SSP )was used for HLA-B*1502 testing and direct sequencing for four-digit genotype determination.

Results

Four-digit allele sequencing showed that there was no statistically significant difference in the frequency of the HLA-B*1502 allele between the OXC-MPE and OXC-tolerant controls (3.6% versus 7.5%, OR = 0.38, 95% CI = 0.04–3.40, P = 0.65), as well as between OXC-MPE and normal controls (3.6% versus 2.4%, OR = 1.54, 95% CI = 0.20–11.73, P = 0.49). However, a significant difference in the frequency of HLA-B*3802 alleles was found between the MPE group and normal controls (10.7% versus 1.9%, OR = 6.329, 95% CI = 1.783-22.460, P = 0.018). There was no significant difference in terms of age, gender, or final OXC dose between the OXC-MPE and OXC-tolerant groups.

Conclusions

There was no significant association between OXC-MPE and HLA-B*1502 in the northern Han Chinese population in our study. Instead, HLA-B*3802 was found to be a potential risk factor for OXC-MPE.

Population pharmacokinetics of lamotrigine in Chinese children with epilepsy

AIM

To establish a population pharmacokinetics (PPK) model for lamotrigine (LTG) in Chinese children with epilepsy in order to formulate an individualized dosage guideline.

METHODS

LTG steady-state plasma concentration data from therapeutic drug monitoring (TDM) were collected retrospectively from 284 patients, with a total of 404 plasma drug concentrations. LTG concentrations were determined using a HPLC method. The patients were divided into 2 groups: PPK model group (n=116) and PPK valid group (n=168). A PPK model of LTG was established with NONMEM based on the data from PPK model group according to a one-compartment model with first order absorption and elimination. To validate the basic and final model, the plasma drug concentrations of the patients in PPK model group and PPK valid group were predicted by the two models.

RESULTS

The final regression model for LTG was as follows: CL (L/h)=1.01*(TBW/27.87)(0.635)*e(-0.753*VPA)*e(0.868*CBZ)*e(0.633*PB), Vd (L)= 16.7*(TBW/27.87). The final PPK model was demonstrated to be stable and effective in the prediction of serum LTG concentrations by an internal and external approach validation.

CONCLUSION

A PPK model of LTG in Chinese children with epilepsy was successfully established with NONMEM. LTG concentrations can be predicted accurately by this model. The model may be very useful for establishing initial LTG dosage guidelines.

Population pharmacokinetics of lamotrigine with data from therapeutic drug monitoring in German and Spanish patients with epilepsy

This study develops a population pharmacokinetic model for lamotrigine (LTG) in Spanish and German patients diagnosed with epilepsy. LTG steady-state plasma concentration data from therapeutic drug monitoring were collected retrospectively from 600 patients, with a total of 1699 plasma drug concentrations. The data were analyzed according to a one-compartment model using the nonlinear mixed effect modelling program. The influences of origin (Germany or Spain), sex, age, total body weight, and comedication with valproic acid (VPA), levetiracetam, and enzyme-inducing antiepileptic drugs (phenobarbital [PB], phenytoin [PHT], primidone [PRM], and carbamazepine [CBZ]) were investigated using step-wise generalized additive modelling. The final regression model for LTG clearance (CL) was as follows: CL(L/h) = 0.028*total body weight*e(-0.713*VPA)*e0.663*PHT*e0.588*(PB or PRM)*e0.467*CBZ*e0.864*IND, where IND refers to two or more inducers added to LTG treatment; this factor as well as VPA, PHT, PB, PRM, and CBZ take a value of zero or one according to their absence or presence, respectively. The administration of inducers led to a significant increase in mean LTG CL (values of 0.045-0.070 L/h/kg vs. 0.028 L/h/kg being reached in monotherapy), whereas VPA led to a significant decrease in CL (0.014 L/h/kg). Thus, comedication with these analyzed drugs can partly explain the interindividual variability in population LTG CL, which decreased from the basic model by more than 40%. The proposed model may be very useful for clinicians in establishing initial LTG dosage guidelines. However, the interindividual variability remaining in the final model (clearance coefficient of variation close to 30%) make these a priori dosage predictions imprecise and justifies the need for LTG plasma level monitoring to optimize dosage regimens. Thus, this final model allows easy implementation in clinical pharmacokinetic software and its application in dosage individualization using the Bayesian approach.

Role of cytochrome P450 in drug interactions

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues. Many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Isobolographic and behavioral characterizations of interactions between vigabatrin and gabapentin in two experimental models of epilepsy

The aim of this study was to characterize the pharmacodynamic, pharmacokinetic and adverse-effect profiles of vigabatrin and gabapentin. Isobolographic analysis was used in two mouse experimental models of epilepsy: the maximal electroshock seizure threshold test and pentylenetetrazole-induced seizures. In the maximal electroshock seizure threshold test, electroconvulsions were produced by a current with various intensities whilst in the pentylenetetrazole test a CD(97) dose (100 mg/kg) was used. Potential adverse-effect profiles of interactions of vigabatrin with gabapentin at three fixed-ratios of 1:3, 1:1 and 3:1 from both seizure tests were evaluated in the chimney (motor performance) and grip-strength (skeletal muscular strength) tests. Vigabatrin and gabapentin total brain concentrations were determined with high performance liquid chromatography. Vigabatrin and gabapentin administered singly increased the electroconvulsive threshold (TID(20) - 226.2 and 70.0 mg/kg, respectively). With isobolography, the combination of vigabatrin with gabapentin at the fixed-ratio of 1:3 exerted supra-additive (synergistic) interactions whilst at 1:1 and 3:1 additivity occurred. Similarly, vigabatrin and gabapentin administered singly suppressed the pentylenetetrazole-induced seizures (ED(50) values - 622.5 and 201.1 mg/kg, respectively). Isobolography revealed that vigabatrin with gabapentin in combination at the fixed-ratio of 1:1 produced supra-additive (synergistic) interaction whilst at 1:3 and 3:1 additivity occurred. In combination neither motor coordination nor skeletal muscular strength was affected. Total vigabatrin and gabapentin brain concentrations revealed that neither drug affected the pharmacokinetics of the other. Vigabatrin and gabapentin have a favorable pharmacodynamic interaction in animal seizure models in the absence of acute adverse effects or concurrent pharmacokinetic changes.

Carbamazepine overdose after exposure to simethicone: a case report

INTRODUCTION

Carbamazepine is an anticonvulsant drug and is also used as a treatment for patients with manic-depressive illness, post-herpetic neuralgia or phantom limb pain. The drug itself has many drug interactions. Simethicone is an antifoaming agent and is reported to be an inert material with no known drug interaction with carbamazepine.

CASE PRESENTATION

We present a case of a patient who was routinely using carbamazepine 400 mg three times per day and levetiracetam 500 mg twice daily, and experienced carbamazepine overdose after exposure to simethicone. After cessation of simethicone therapy normal drug levels of carbamazepine were obtained again with the standard dose of the drug. The mechanism of interaction is unknown but the risk of overdose should be considered when prescribing simethicone to a patient who is using carbamazepine.

CONCLUSION

Simethicone and carbamazepine, when taken together, may be a cause of carbamazepine toxicity. The risk of carbamazepine overdose should be considered when prescribing simethicone to a patient who is using carbamazepine.

Isobolographic characterization of interactions between vigabatrin and tiagabine in two experimental models of epilepsy

To characterize the type of interactions between vigabatrin (VGB) and tiagabine (TGB) -- two newer antiepileptic drugs influencing GABA-ergic neurotransmitter system, the isobolographic analysis was used in two experimental models of epilepsy: the maximal electroshock seizure threshold (MEST) test and pentylenetetrazole (PTZ)-induced seizures in mice. Results indicated that VGB and TGB administered separately (i.p.) increased the electroconvulsive threshold in mice, which allowed the calculation of their TID(20) values (threshold increasing doses by 20% over the threshold of control animals) in the MEST test. The TID(20) for VGB was 226.2 mg/kg and that for TGB was 4.4 mg/kg. With isobolography, the combinations of VGB with TGB (at fixed-ratios of 1:3, 1:1 and 3:1) exerted additive interactions in the MEST test in mice. Similarly, VGB and TGB injected separately (i.p.) suppressed the PTZ-induced seizures, and their ED(50) values (median effective doses, protecting 50% of the animals tested against clonic convulsions) for VGB and TGB were 622.5 mg/kg and 0.8 mg/kg, respectively. Isobolographic analysis of interactions revealed that the combinations of VGB with TGB at the fixed-ratios of 1:3 and 1:1 produced supra-additive (synergistic) interactions against PTZ-induced seizures. Only the combination of VGB with TGB at the fixed-ratio of 3:1 was additive in the PTZ test. The evaluation of acute adverse-effect potential for all fixed-ratio combinations of VGB with TGB (administered at their TID(20) and ED(50) values from the MEST and PTZ tests) revealed that none of the examined combinations affected motor coordination in the chimney test and altered neuromuscular tone in the grip-strength test in mice. In contrast, VGB in combinations with TGB produced the antinociceptive effects with respect to suppression of acute thermal pain in animals subjected to the hot-plate test. Based on this preclinical study, one can ascertain that the combination of VGB with TGB would provide an adequate seizure control in epileptic patients.

Anticonvulsants in the treatment of bipolar mania

A series of antiepileptic drugs have been investigated in terms of their ability to treat mania (with later applications for the treatment of bipolar depression and prevention of relapses). These include divalproex, carbamazepine, oxcarbazepine, gabapentin, lamotrigine, levetiracetam, tiagabine, topiramate and zonisamide. Although these drugs are all antiepileptic in action, they bring about these effects by different mechanisms; in particular, their impact on GABA differs significantly. Perhaps for this reason, their impact on mania varies greatly, with double-blind significant results evident only for valproate, carbamazepine and oxcarbazepine. Only valproate and carbamazepine are approved by the US FDA for use in mania; oxcarbazepine has never been found significantly effective in large-scale studies. Of the other options, both gabapentin and topiramate failed in large-scale investigations; tiagabine failed in small sample reports. Although lamotrigine has been successful in the prevention of depression relapse in bipolar disorder, it has not been effective in treating mania. Finally, there are no findings of large scale double-blind studies on the use of levetiracetam and zonisamide. A review of the kinetics, side effects and complications of the antiepileptic drugs indicates that carbamazepine is useful, and has adverse event benefit over all other options. The potential of zonisamide awaits further testing.

Synergistic interaction of gabapentin and oxcarbazepine in the mouse maximal electroshock seizure model--an isobolographic analysis

The anticonvulsant effects produced by mixtures of oxcarbazepine and gabapentin (two second-generation antiepileptic drugs) in numerous fixed-ratio combinations of 1:1, 1:2, 1:5, 1:10, 1:15, and 1:20 were examined isobolographically in the mouse maximal electroshock seizure model. Results displayed that mixtures of both drugs at the fixed-ratios of 1:2, 1:5, 1:10, 1:15, and 1:20 exerted supra-additive (synergistic) interactions against electroconvulsions. Only a fixed-ratio of 1:1 was indifferent with isobolography, although the combination displayed the trend towards supra-additivity. Furthermore, the combinations of oxcarbazepine with gabapentin, administered at their median effective doses (ED(50 mix)s), did not alter motor performance of animals challenged with the chimney test. Additionally, neither gabapentin nor oxcarbazepine affected total brain concentrations of co-administered drug, indicating a pharmacodynamic nature of interaction between these antiepileptics. Finally, based on preclinical data presented here the combination of oxcarbazepine and gabapentin is of particular importance for further therapy in patients with refractory partial seizures.

Zonisamide: a review of its use in the management of partial seizures in epilepsy

Zonisamide (Zonegran, Excegran) is a new-generation, broad-spectrum antiepileptic drug (AED) currently approved as adjunctive therapy for the treatment of medically refractory partial seizures in adults in the US and as adjunctive therapy or monotherapy in the control of partial and generalised seizures in adults and children in Japan and Korea. Either as adjunctive therapy or monotherapy, zonisamide effectively reduces the frequency of partial seizures, with or without secondary generalisation to tonic-clonic seizures, in adults and children with epilepsy. The drug is generally well tolerated and, additionally, has a favourable pharmacokinetic profile permitting once- or twice-daily administration. Direct head-to-head comparisons with other AEDs would be beneficial in fully defining the place of zonisamide in therapy. In the meantime, adjunctive therapy or monotherapy with zonisamide is a convenient, useful option for the management of partial seizures, including those refractory to other AEDs.

Effect of probenecid on the pharmacokinetics of carbamazepine in healthy subjects

OBJECTIVES

Carbamazepine (CBZ) undergoes biotransformation by CYP3A4 and CYP2C8, and glucuronide conjugation. There has been no clear demonstration to reveal the role of glucuronidation in the disposition of CBZ. We evaluated the effect of probenecid, a UDP-glucuronosyltransferase inhibitor, on the pharmacokinetics of CBZ in humans.

METHODS

In a randomized, open-label, two-way crossover study, ten healthy male subjects were treated twice daily for 10 days with 500 mg probenecid or with a matched placebo. On day 6, a single dose of 200 mg CBZ was administered orally. Concentrations of CBZ and CBZ 10,11-epoxide (CBZ-E) in plasma and urine were measured.

RESULTS

Probenecid decreased the area under the plasma concentration-time curve (AUC) of CBZ from 1253.9 micromol h/l to 1020.7 micromol h/l (P < 0.001) while increasing that of CBZ-E from 137.6 micromol h/l to 183.5 micromol h/l (P = 0.033). The oral clearance of CBZ was increased by probenecid by 26% (90% confidence interval, 17-34%; P < 0.001). Probenecid increased the AUC ratio of CBZ-E/CBZ from 0.11 to 0.16 (P < 0.001). However, probenecid had minimal effect on the recovery of the conjugated and free forms of CBZ and CBZ-E in urine.

CONCLUSION

Although probenecid showed a minimal effect on the glucuronidation of CBZ and CBZ-E, it increased CBZ biotransformation to CBZ-E, most likely reflecting the induction of CYP3A4 and CYP2C8 activities, in humans. These results demonstrate that glucuronide conjugation plays a minor role in the metabolism of CBZ and CBZ-E in humans, and that probenecid has an inducing effect on the disposition of CBZ.

Effect of felbamate and its combinations with conventional antiepileptics in amygdala-kindled rats

We investigated the effect of felbamate, administered singly and in combination with carbamazepine, phenobarbital, phenytoin or clonazepam, on various behavioral and electrographic correlates of seizures in amygdala-kindled rats. Felbamate (5 or 10 mg/kg) significantly increased afterdischarge threshold, shortened seizure and afterdischarge durations but remained without effect on seizure severity. Furthermore, the combination of felbamate (2.5 mg/kg) with carbamazepine (7.5 mg/kg; both drugs at their subeffective doses), was associated with the reduction in seizure severity and afterdischarge duration. In relation to the afterdischarge duration, the antiseizure potency of felbamate and carbamazepine, in combination, was comparable with that of carbamazepine (10 mg/kg) administered alone. Neither carbamazepine (7.5 and 10 mg/kg) nor felbamate (2.5-10 mg/kg) affected seizure severity, whereas the combined administration of felbamate (2.5 mg/kg) with carbamazepine (7.5 mg/kg) led to significant reduction in seizure severity from the fifth to the third stage of Racine's scale. Among the conventional antiepileptic drugs evaluated in this study, only valproate (100 mg/kg) and clonazepam (0.1 mg/kg) exerted similar action on seizure severity. However, the combinations of felbamate (2.5 mg/kg), with subeffective doses of valproate, phenobarbital, phenytoin or clonazepam, were not associated with any protective action. As blood and brain felbamate and carbamazepine concentrations were unaffected, a pharmacokinetic interaction can be excluded and a pharmacodynamic interaction concluded. These data suggest that felbamate and carbamazepine, administered in combination, may be useful in patients with drug-resistant partial epilepsy.

Lamotrigine: a review of its use in bipolar disorder

Lamotrigine (Lamictal), a phenyltriazine derivative, is a well established anticonvulsant agent that has shown efficacy in the prevention of mood episodes in adult patients with bipolar I disorder. The mechanism of action of the drug in patients with bipolar disorder may be related to the inhibition of sodium and calcium channels in presynaptic neurons and subsequent stabilisation of the neuronal membrane. Lamotrigine monotherapy significantly delayed time to intervention with additional pharmacotherapy or electroconvulsive therapy for any new mood episode (mania, hypomania, depression and mixed episodes), compared with placebo, in two large, randomised, double-blind trials of 18 months' duration. Additionally, lamotrigine was significantly superior to placebo at prolonging time to intervention for depression. These effects of lamotrigine were demonstrated in both recently manic/hypomanic and recently depressed patients. Lamotrigine showed efficacy in delaying manic/hypomanic episodes in pooled data only, although lithium was superior to lamotrigine on this measure. Two of four double-blind, short-term studies have shown lamotrigine to be more effective than placebo in the treatment of patients with treatment-refractory bipolar disorder or those with bipolar depression. Lamotrigine has not demonstrated efficacy in the treatment of acute mania. Lamotrigine was generally well tolerated in maintenance studies with the most common adverse events being headache, nausea, infection and insomnia. Incidences of diarrhoea and tremor were significantly lower in lamotrigine- than in lithium-treated patients. The incidence of serious rash with lamotrigine treatment was 0.1% in all studies of bipolar disorder and included one case of mild Stevens-Johnson syndrome. Lamotrigine did not appear to cause bodyweight gain. The dosage of lamotrigine is titrated over a 6-week period to 200 mg/day to minimise the incidence of serious rash. Adjustments to the initial and target dosages are required if coadministered with valproate semisodium or carbamazepine.

CONCLUSION

Lamotrigine has been shown to be an effective maintenance therapy for patients with bipolar I disorder, significantly delaying time to intervention for any mood episode. Additionally, lamotrigine significantly delayed time to intervention for a depressive episode and showed limited efficacy in delaying time to intervention for a manic/hypomanic episode, compared with placebo. Although not approved for the short-term treatment of mood episodes, lamotrigine has shown efficacy in the acute treatment of patients with bipolar depression but has not demonstrated efficacy in the treatment of acute mania. Lamotrigine is generally well tolerated, does not appear to cause bodyweight gain and, unlike lithium, generally does not require monitoring of serum levels.

Oxcarbazepine: a review of its use in children with epilepsy

Oxcarbazepine (Trileptal, Timox) is structurally related to carbamazepine and has anticonvulsant activity. Studies suggest that the anticonvulsant activity of oxcarbazepine is mediated via the blocking of neuronal ion channels. In patients aged <18 years, the efficacy of oxcarbazepine monotherapy was similar to that of phenytoin in children with partial onset or generalized tonic-clonic seizures in a 48-week trial. Additional supporting findings demonstrated that 43-71% of patients with partial onset, generalized or undetermined epilepsy were seizure free after oxcarbazepine monotherapy (mean dosage 27.7-50 mg/kg/day; duration 1-5 years). In contrast, one small nonblind trial showed more patients treated with oxcarbazepine monotherapy than with carbamazepine monotherapy had recurrent seizures during 16 months of therapy (although the conclusions that can be drawn from this trial are limited). As adjunctive therapy, oxcarbazepine was significantly better than placebo at reducing seizure frequency in children and adolescents with refractory partial onset seizures with or without secondary generalization: the median percentage change in partial onset seizure frequency was 35% vs 9%, respectively, during 16 weeks of therapy. In noncomparative trials of adjunctive oxcarbazepine (mean dosage of 34.5-56.7 mg/kg/day), 7-11% of patients with partial onset or generalized seizures were seizure free during treatment, and 20-54% had seizure reductions of > or=50%. Oxcarbazepine was generally well tolerated during monotherapy and adjunctive therapy; 2.5% and 10% of patients withdrew from well controlled trials of oxcarbazepine monotherapy and adjunctive therapy. Oxcarbazepine monotherapy was better tolerated than phenytoin and events observed in oxcarbazepine-treated patients were transient. Oxcarbazepine metabolism is largely unaffected by induction of the cytochrome (CYP) P450 system. However, oxcarbazepine can inhibit CYP2C19 and induce CYP3A4 and CYP3A5, thereby interfering with the metabolism of other drugs (e.g. phenytoin). In addition, oxcarbazepine decreases plasma levels of oral contraceptives and alternative contraceptive methods should be used. In conclusion, oxcarbazepine (as both monotherapy and adjunctive therapy) has shown efficacy in the treatment of partial onset seizures in children with epilepsy. Nevertheless, the generally favorable tolerability profile and relatively low potential for drug interactions of oxcarbazepine make it a valuable option in the treatment of childhood epilepsy.

Tiagabine synergistically interacts with gabapentin in the electroconvulsive threshold test in mice

Polytherapy, based on the rational combining of antiepileptic drugs (AEDs), is required for patients with drug-resistant epilepsy. In such cases, the combinations of AEDs usually offer a significant enhancement of their protective effects against seizures. There has appeared a hypothesis that combining two AEDs, influencing the same neurotransmitter system, results in the potentialization of their anticonvulsant effects. For corroborating this hypothesis, a pharmacological character of interaction between tiagabine (TGB) and gabapentin (GBP)-two novel AEDs affecting the GABA-ergic system, in the maximal electroshock seizure threshold (MEST)-test in mice was evaluated. TGB at the dose of 4 mg/kg and GBP at 75 mg/kg significantly raised the electroconvulsive threshold. Further, using the isobolographic calculations, TGB was coadministered with GBP at three fixed-ratios (1 : 3, 1 : 1, and 3 : 1) of their respective protective drug doses. All examined combinations of TGB with GBP exerted supra-additive (synergistic) interactions against MEST-induced seizures in mice. The interaction index, describing the strength and magnitude of interaction, ranged between 0.25 and 0.50 indicating supra-additivity. Adverse (neurotoxic) effects were evaluated in the chimney (motor performance) and the step-through, light-dark passive avoidance (long-term memory) tests in mice. The examined combinations of TGB with GBP did not affect the motor coordination, except for the fixed-ratio of 1 : 1, at which significant impairment of motor performance was observed. Moreover, all combinations selectively impaired the acquisition of the task in the passive avoidance test, having no impact on consolidation and retrieval in the long-term memory test. The pain threshold test revealed that the observed disturbances in the passive avoidance testing resulted presumably from the antinociceptive activity of these AEDs in combinations. After lengthening the exposing time to the direct current stimulus in the passive avoidance test from 2 to 6 s, the acquisition of the task, in animals receiving the combinations of TGB and GBP was not impaired. Neither the plasma, nor brain concentrations of GBP were affected by TGB application, so pharmacokinetic events that might negatively influence the observed effects are not probable. Results of this study clearly indicate that the activation of the same neurotransmitter system (GABA-ergic) leads to a synergistic interaction. The pain threshold test is a very good paradigm for screening the antinociceptive properties of AEDs, which may disturb the long-term memory testing in animals. Combinations of TGB with GBP (very promising from a preclinical point of view) should be clinically verified for elaborating the most effective treatment regimen in patients with intractable seizures.

Lack of pharmacokinetic interaction between retigabine and phenobarbitone at steady-state in healthy subjects

AIMS

To evaluate potential pharmacokinetic interactions between phenobarbitone and retigabine, a new antiepileptic drug.

METHODS

Fifteen healthy men received 200 mg of retigabine on day 1. On days 4-32, phenobarbitone 90 mg was administered at 22.00 h. On days 26-32, increasing doses of retigabine were given to achieve a final dose of 200 mg every 8 h on day 32. The pharmacokinetics of retigabine were determined on days 1 and 32, and those for phenobarbitone on days 25 and 31.

RESULTS

After administration of a single 200 mg dose, retigabine was rapidly absorbed and eliminated with a mean terminal half-life of 6.7 h, a mean AUC of 3936 ng x ml(-1) x h and a mean apparent clearance of 0.76 l x h(-1) x kg(-1). Similar exposure to the partially active acetylated metabolite (AWD21-360) of retigabine was observed. After administration of phenobarbitone dosed to steady-state, the pharmacokinetics of retigabine at steady-state were similar (AUC of 4433 ng x ml(-1) x h and t1/2 of 8.5 h) to those of retigabine alone. The AUC of phenobarbitone was 298 mg x l(-1) x h when administered alone and 311 mg x ml(-1) x h after retigabine administration. The geometric mean ratios and 90% confidence intervals of the AUC were 1.11 (0.97, 1.28) for retigabine, 1.01 (0.88, 1.06) for AWD21-360 and 1.04 (0.96, 1.11) for phenobarbitone. Individual and combined treatments were generally well tolerated. One subject was withdrawn from the study on day 10 due to severe abdominal pain. Headache was the most commonly reported adverse event. No clinically relevant changes were observed in the electrocardiograms, vital signs or laboratory measurements.

CONCLUSIONS

There was no pharmacokinetic interaction between retigabine and phenobarbitone in healthy subjects. No dosage adjustment is likely to be necessary when retigabine and phenobarbitone are coadministered to patients.

Does lamotrigine influence valproate concentrations?

The aim of this study is to investigate the effect of lamotrigine (LTG) on valproate (VPA) concentrations dependent on LTG dose, LTG concentration, and additional enzyme-inducing antiepileptic drugs (AED) as well. For this purpose the following patient groups were compared: VPA monotherapy, VPA + one enzyme-inducing AED, VPA + LTG, and VPA + LTG + one enzyme-inducing AED. A total of 400 serum concentrations from 372 patients were evaluated. Two or more serum samples from the same patient were considered only if the comedication had been changed. For statistical evaluation, regression analytical methods and an analysis of variance were performed. For the analysis of variance, the VPA serum concentration in relation to VPA dose:body weight (level:dose ratio, LDR) was calculated and compared for different drug combinations. The analysis of variance revealed a significant effect of enzyme-inducing comedication (as expected) and age on the VPA LDR. Patients on LTG had a slightly lower VPA LDR, but this effect was not statistically significant. In addition, nonlinear regression analysis confirmed that patients on enzyme-inducing AED (carbamazepine, phenytoin, phenobarbital, methsuximide) had significantly lower VPA concentrations. Patients on ethosuximide had slightly but not significantly lower VPA concentrations. Patients on LTG also had significantly lower VPA levels, but this effect was only minor (-7%) and most probably not of any clinical relevance. Furthermore, the regression analysis showed that the relationship between the VPA dose per body weight and the serum concentration deviates significantly from linearity. Children less than 6 years old had lower VPA levels than older children and adults on a comparable VPA dose per body weight. Gender had no significant influence on VPA serum concentration. In addition, a subgroup of 40 patients was analyzed to see whether changing the LTG dose influences VPA serum concentrations. It did not. Thus, the authors conclude that the effect of LTG on VPA concentrations is not of clinical relevance.

The interaction of medications used in palliative care

Palliative care uses several classes of drugs, which are handled by the CYP P450 system. Interaction of drugs in this setting requires ongoing vigilance by the physician. Phenocopying may be more common than previously realized.

Drugs in pregnancy. Anticonvulsants and drugs for neurological disease

The use of anticonvulsant drugs in pregnancy presents unique challenges to clinicians and their patients. The need for control of maternal epilepsy must be balanced with the fetal and neonatal risks associated with anticonvulsant drugs. Anticonvulsant drugs may have potential effects on embryogenesis, neurological development, growth and subsequent paediatric progress. Drug selection and dose adjustment must be appropriate and based on a combination of known maternal and fetal risks as well as the clinical status of the patient. Overall, no one drug can be specifically recommended but monotherapy with most of the recognized first-line drugs will result in a satisfactory outcome. Polytherapy is associated with an increase in congenital malformations and should be avoided if possible. It is possible that newer second-line agents, for example, gabapentin, may be safer as add-on therapy. Neurological disorders such as migraine, and the less common conditions of myasthenia gravis and multiple sclerosis, may require the use of drugs which have not been well studied in pregnancy. Information is provided about the use of drugs to control symptoms and prevent disease progression in these disorders during pregnancy.

Oxcarbazepine, an antiepileptic agent

BACKGROUND

Epilepsy is a common neurologic condition. Many of the currently approved pharmacologic agents for its treatment are associated with numerous adverse drug reactions and drug interactions.

OBJECTIVE

This review describes the pharmacology and therapeutic use of oxcarbazepine, an analogue of the well-known antiepileptic agent carbamazepine.

METHODS

Articles for review were identified through a search of MEDLINE, International Pharmaceutical Abstracts, and EMBASE for the years 1980 through 2000. The terms used individually and in combination were oxcarbazepine, carbamazepine, epilepsy, and seizures.

RESULTS

Oxcarbazepine and its primary metabolite have been effective in animal models of epilepsy that generally predict efficacy in generalized tonic-clonic seizures and partial seizures in humans. The exact mechanism of action of oxcarbazepine is unknown, although as with carbamazepine, it is believed to involve blockade of voltage-gated sodium channels. The pharmacokinetic profile of oxcarbazepine is less complicated than that of carbamazepine, with less metabolism by the cytochrome P450 system, no production of an epoxide metabolite, and lower plasma protein binding. The clinical efficacy and tolerability of oxcarbazepine have been demonstrated in trials in adults, children, and the elderly. In a double-blind, randomized, crossover trial in adults, oxcarbazepine 300 mg was associated with a decrease in the mean frequency of tonic seizures (21.4 vs 30.5 seizures during steady-state periods) and tonic-clonic seizures (8.2 vs 10.4) compared with carbamazepine 200 mg (P = 0.05). A multinational, multicenter, double-blind, placebo-controlled, randomized, 28-week trial assessed the efficacy and tolerability of oxcarbazepine at doses of 600, 1200, and 2400 mg as adjunctive therapy in patients with uncontrolled partial seizures. All 3 oxcarbazepine groups demonstrated a reduction in seizure frequency per 28-day period compared with placebo (600 mg, 26% reduction; 1200 mg, 40% reduction; 2400 mg, 50% reduction; placebo, 7.6% reduction; all, P < 0.001). A trial in children assessed the efficacy and toxicity of oxcarbazepine (median dose, 31.4 mg/kg/d) as adjunctive therapy for partial seizures. Patients receiving oxcarbazepine experienced a 35% reduction in seizure frequency, compared with a 9% reduction in the placebo group (P < 0.001). The most common adverse effects associated with oxcarbazepine are related to the central nervous system (eg, dizziness, headache, diplopia, and ataxia) and the gastrointestinal system (eg, nausea and vomiting). Compared with carbamazepine, there is an increased risk of hyponatremia with oxcarbazepine. The frequency and severity of drug interactions are less with oxcarbazepine than with carbamazepine or other antiepileptic agents.

CONCLUSIONS

Oxcarbazepine may be considered an appropriate alternative to carbamazepine for the treatment of partial seizures in patients who are unable to tolerate carbamazepine. Its use in nonseizure disorders remains to be examined in large-scale clinical trials, and pharmacoeconomic comparisons of oxcarbazepine with other antiepileptic agents, particularly carbamazepine, are needed.

Clinical study of lamotrigine and valproic acid in patients with epilepsy: using a drug interaction to advantage?

Lamotrigine (LTG) is one of the newer antiepileptic drugs which has been shown to have a spectrum of drug interactions (including with other epilepsy drugs) that can have a pronounced effect on LTG kinetics. The present study examined the LTG metabolic inhibition dose-response relationship with valproic acid (VPA) in eight patients with epilepsy with a view to using this to benefit the patient. This could benefit the patient not only by attaining higher plasma LTG concentrations with "standard" dosages of LTG, but also possibly by achieving better seizure control through providing a less variable peak-to-trough fluctuation in LTG concentrations as a result of extending the half-life of LTG. The dosages of VPA trialed were 0, 200, 500, and 1,000 mg/d which resulted in a mean increase in LTG area under the curve of 83.7 +/- 14.7% at 200 mg VPA/d, to and 160 +/- 37.9% at 1,000 mg VPA/d. The presence of concomitant enzyme inducers in some patients did not influence the percentage increase from baseline in half-life observed, although clearly those on inducers started from a lower absolute half-life as a result of the induction. The effect was shown to be quite variable, particularly at the highest dosage of VPA tested (1,000 mg/d), suggesting that this effect could be best applied with the support of the therapeutic drug monitoring laboratory determining plasma LTG concentrations to allow individualization of the LTG dosage.

Enzyme induction and inhibition by new antiepileptic drugs: a review of human studies

The aim of this paper is to review a number of new antiepileptic agents (i.e. felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide) for their inducing and/or inhibitory properties in humans, mainly considering the interactions where they are involved as the cause rather than the object of such interactions. Two aspects have been particularly taken into account: the changes or absence of changes in plasma/serum concentrations of concomitant drugs and the direct or indirect evidence of induction, inhibition or lack of effect on the six major human hepatic CYP isozymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), as well as on other CYP isozymes or enzyme systems. Felbamate clearly affects the pharmacokinetics of a number of drugs, generally increasing but also decreasing their concentrations. It induces enzymes such as CYP3A4 and inhibits enzymes such as CYP2C19 and those of the beta-oxidation pathway. Topiramate is not devoid of potential interaction properties: it decreases the plasma concentrations of ethinylestradiol, induces CYP3A4 and inhibits CYP2C19. For oxcarbazepine, no inhibitory, only inductive effects have been observed thus far. Felbamate. topiramate and oxcarbazepine may induce the metabolism of steroidal oral contraceptives. In this respect, tiagabine has been studied at a rather low dose. Pharmacodynamic or pharmacokinetic interaction seems to exist between lamotrigine and carbamazepine. Lamotrigine appears to be a weak inducer of UGTs, whereas induction of CYP3A4 seems improbable as the compound does not change the concentrations of oral contraceptives or the urinary excretion of 6beta-hydroxycortisol. Zonisamide has very peculiar pharmacokinetics and an extensive metabolism. Additional information on its enzyme inducing or inhibiting properties would be necessary, as data so far collected on its effect on the pharmacokinetics of other drugs are conflicting. Gabapentin, vigabatrin and in particular levetiracetam appear to be devoid of significant enzyme inducing or inhibiting properties.

Adjunctive therapy with oxcarbazepine in children with partial seizures. The Oxcarbazepine Pediatric Study Group

OBJECTIVE

To evaluate the safety and efficacy of oxcarbazepine (OXC) as adjunctive therapy in children with inadequately controlled partial seizures on one or two concomitant antiepileptic drugs (AEDs).

BACKGROUND

OXC has shown antiepileptic activity in several comparative monotherapy trials in newly diagnosed patients with epilepsy, and in a placebo-controlled monotherapy trial in hospitalized patients evaluated for epilepsy surgery.

DESIGN

A total of 267 patients were evaluated in a multicenter, randomized, placebo-controlled trial consisting of three phases: 1) a 56-day baseline phase (patients maintained on their current AEDs); 2) a 112-day double-blind treatment phase (patients received either OXC 30-46 mg/kg/day orally or placebo); and 3) an open-label extension phase. Data are reported only from the double-blind treatment phase; the open-label extension phase is ongoing.

METHODS

Children (3 to 17 years old) with inadequately controlled partial seizures (simple, complex, and partial seizures evolving to secondarily generalized seizures) were enrolled.

RESULTS

Patients treated with OXC experienced a significantly greater median percent reduction from baseline in partial seizure frequency than patients treated with placebo (p = 0.0001; 35% versus 9%, respectively). Forty-one percent of patients treated with OXC experienced a > or =50% reduction from baseline in partial seizure frequency per 28 days compared with 22% of patients treated with placebo (p = 0.0005). Ninety-one percent of the group treated with OXC and 82% of the group treated with placebo reported > or =1 adverse event; vomiting, somnolence, dizziness, and nausea occurred more frequently (twofold or greater) in the group treated with OXC.

CONCLUSION

OXC adjunctive therapy administered in a dose range of 6 to 51 mg/kg/day (median 31.4 mg/kg/day) is safe, effective, and well tolerated in children with partial seizures.

Drug interactions in palliative care

PURPOSE

This review of drug interactions in palliative care examines the relevant literature in this area and summarizes the information on interactions of drugs, nutrients, and natural products that are used in the palliative care setting. Particular emphasis is placed on describing the newer information on the cytochrome P450 (CYP) system and the interactions of opioids, antidepressants, and the antitussive, dextromethorphan.

METHODS

We performed a search of the MEDLINE database of the time period from 1966 until April 1998, using medical subject headings such as the names of selective serotonin reuptake inhibitors and other relevant medications in palliative care. Literature reviewed included both human and animal articles as well as non-English literature. Bibliographies of these articles and the personal libraries of several palliative care specialists were reviewed. Software developed by The Medical Letter-The Drug Interaction Program was also used.

RESULTS

Drug interactions can be categorized in several ways. Drug-drug interactions are the most well known and can be kinetic, dynamic, or pharmaceutical. Pharmacokinetic interactions can involve CYP 2D6, which acts on drugs such as codeine and is responsible for its conversion to morphine. Poor metabolizers, either genotypic or due to phenocopying, are at risk for undertreatment if not recognized. Pharmacodynamic interactions with dextromethorphan may produce serotonin syndrome.

CONCLUSION

Drug interactions are important in palliative care as in other aspects of medicine. These interactions are similar to those seen in other areas of medical care but have significant consequences in pain management. Failure to recognize these interactions can lead to either overdosing or undertreatment.

The clinical pharmacokinetics of the new antiepileptic drugs

Because pharmacokinetics is a major determinant of the magnitude and duration of pharmacologic response, understanding the kinetic properties of the new antiepileptic drugs (AEDs) is essential for the correct use of these compounds in clinical practice. After oral administration, absorption is rapid and relatively efficient for the new AEDs, the most notable exception being gabapentin, whose bioavailability decreases with increasing dosage. None of the new AEDs is extensively bound to plasma proteins except for tiagabine, which is over 95% protein-bound. The route of elimination differs to an important extent from one compound to another, and elimination half-lives range from over 30 h for zonisamide to 5-7 h for gabapentin. For all drugs that are metabolized, half-life is shortened and clearance is increased when patients receive concomitant enzyme-inducing agents such as barbiturates, phenytoin, and carbamazepine. Lamotrigine metabolism is markedly inhibited by valproic acid, and felbamate may increase the serum levels of most other AEDs. Felbamate, topiramate, and oxcarbazepine may also reduce the efficacy of the contraceptive pill by stimulating its metabolism.

Vigabatrin: a novel therapy for seizure disorders

OBJECTIVE

To review the pharmacology, pharmacokinetics, efficacy, and adverse effects of vigabatrin and its role in the management of seizure disorders.

METHODS

A MEDLINE search of English-language literature from January 1993 through January 1999 was conducted using vigabatrin as a search term to identify pertinent studies and review articles. Additional studies were identified from the bibliographies of reviewed literature. The manufacturer provided postmarketing surveillance data. Priority was given to randomized, double-blind, placebo-controlled studies.

FINDINGS

Vigabatrin is a selective and irreversible inhibitor of gamma-aminobutyric acid transaminase. In controlled clinical trials of vigabatrin add-on therapy in patients with uncontrolled partial seizures, 24-67% of patients achieved a < or =50% reduction in seizure frequency. Data from two comparative trials with carbamazepine monotherapy indicate that vigabatrin monotherapy reduces the frequency of partial seizures in patients with newly diagnosed epilepsy. Vigabatrin also controls infantile spasms, particularly those associated with tuberous sclerosis. Vigabatrin is more effective in patients with partial seizures than in those with generalized seizures. The drug is generally well tolerated. Headache and drowsiness were the most common adverse effects observed in controlled clinical trials; visual field defects, psychiatric reactions, and hyperactivity also have been reported. There are no known clinically significant drug interactions.

CONCLUSIONS

Vigabatrin improves seizure control as add-on therapy for refractory partial seizures and may produce therapeutic benefits in the treatment of infantile spasms. Vigabatrin is generally well tolerated, with a convenient administration schedule, a lack of known significant drug interactions, and no need for routine monitoring of plasma concentrations.

Treatment of epilepsy in the elderly

Management of epilepsy in the elderly involves many challenges, including the presence of concomitant diseases, polypharmacy and changes in body physiology. Age-related changes in pharmacokinetics and pharmacodynamics have to be taken into account in order to avoid potentially severe adverse drug reactions in elderly people. The present study reviews the most commonly used antiepileptic drugs (AEDs) in the elderly. Because some AEDs may induce the metabolism of other agents and reduce the effectiveness of several drugs, the physicians have to carefully evaluate concomitant drugs being administered. Moreover, the main problems appear to be when beginning therapy, the first choice drug, the appropriate dosage and pharmacologic compliance. Elderly patients must be screened for hepatic and renal functions before beginning a treatment with an AED, carefully interviewed to reduce complaints for drug side-effects which may negatively influence compliance and monitored for total and free blood levels. Besides the 'classic' AEDs, such as phenytoin, phenobarbital, carbamazepine, valproic acid, primidone and benzodiazepines, the review shows the possible advantages of new AEDs, such as felbamate, gabapentin, lamotrigine, oxcarbazepine and gamma-vinyl-GABA, which may be used in the elderly too for their good tolerability. A careful control of drug assumption is requested in the elderly, especially when it is difficult to achieve seizure control.

Assessing the needs of pharmacists and physicians in caring for patients with epilepsy

OBJECTIVE

To obtain primary care physicians' and community pharmacists' opinions of the Pharmacist Note, a model epilepsy patient profile maintained by the pharmacist and transmitted to the physician as needed, and the information it contains.

DESIGN

A cross-sectional descriptive study design was used for collecting and analyzing data. Separate surveys were developed and mailed to physicians and pharmacists.

PARTICIPANTS

554 primary care physicians and 114 community pharmacists in Ohio who interact regularly with epilepsy patients.

MAIN OUTCOME MEASURES

Pharmacist and physician opinions on the Pharmacist Note program.

RESULTS

Physicians ranked seizure frequency as their most useful piece of information, followed by medication compliance and drug interaction screening. For medication profile and drug interaction screening, most physicians currently use themselves as their primary source of information, although a significant number would prefer to use pharmacists as information sources in these areas (p < .05). A majority (62%) would like to have pharmacists more involved in the care of their patients. Pharmacists identified lack of time and lack of appointments with patients, inadequate pharmacy staff, and insufficient reimbursement as barriers to implementing the Pharmacist Note program.

CONCLUSION

Physicians desire pharmacist involvement in specific areas of care for patients with epilepsy, and the feasibility of implementing the Pharmacist Note and similar programs appears promising. However, pharmacists identified barriers to implementation, and these barriers need to be addressed if this type of program is to be successful.

Influence of oxcarbazepine and methsuximide on lamotrigine concentrations in epileptic patients with and without valproic acid comedication: results of a retrospective study

The aim of this retrospective study was to investigate the influence of oxcarbazepine (OCBZ) and methsuximide (MSM) on lamotrigine (LTG) serum concentrations. The effect of OCBZ compared to carbamazepine (CBZ) and the effect of MSM on LTG serum concentrations were examined in patients with and without valproic acid (VPA) comedication. Altogether, 376 samples from 222 patients were analyzed in routine drug monitoring. Two or more serum samples from the same patient were considered only if the comedication had been changed. For statistical evaluation, regression analytical methods and an analysis of variance were performed. For the analysis of variance, the LTG serum concentration in relation to LTG dose/ body weight--level-to-dose ratio (LDR), in (microg/mL)/(mg/kg)--was calculated and compared for different drug combinations. The nonlinear regression analysis including the LTG dose per body weight, age, gender, and the different kinds of comedication revealed that these variables have a significant influence on LTG serum concentration (r2 = 0.724). The relationship between LTG dose/body weight and serum concentration deviates only slightly from linearity, the LTG concentration was about 18% lower in women than in men, and age had a significant influence. The data indicate that children have significantly lower LTG concentrations than adults on a comparable LTG dose per body weight and that children may be more prone to enzyme induction by comedicated drugs. Methsuximide has a strong inducing effect on the LTG metabolism and decreases the LTG concentrations markedly (about 70% compared to LTG monotherapy). Carbamazepine also reduces the LTG concentrations considerably (by 54%). The inducing effect of OCBZ (29%) was less pronounced but also significant. The inducing effect of MSM, CBZ, and OCBZ was also seen in combination with VPA: VPA alone increases the LTG concentration approximately 211%, whereas in addition to MSM (8%), CBZ (21%), or OCBZ (111%), the increase of LTG was significantly smaller. The analysis of variance confirmed the results of the regression analysis. The effect of MSM on the LTG concentration should be considered if MSM is added or withdrawn in patients treated with LTG. Oxcarbazepine had a less pronounced inducing effect on LTG metabolism compared to CBZ. If CBZ is replaced by OCBZ as comedication, an increase in LTG serum concentrations should be expected.

Lamotrigine drug interactions in a TDM material

Using therapeutic drug monitoring (TDM) data from our laboratory, we have studied the concentration/dose relationship for lamotrigine and the influence of drug interactions in clinical practice. One hundred forty-nine lamotrigine samples from 104 adult patients were included in the study. The samples were collected as steady-state trough values, 9-16 hours after dose intake. Concomitant drug treatment was specified on the analysis request form. Lamotrigine serum concentrations were determined by high-performance liquid chromatography (HPLC). In 20 patients in monotherapy, the concentration/dose (C/D) ratio was 65 (range: 50-84) nmol/L/mg (mean and 95% confidence interval, antilog from lognormal distribution). In 37 patients with concomitant carbamazepine treatment, the C/D ratio was less than half that of the patients in monotherapy; 31 (2146) nmol/L/mg, and in 14 patients with phenytoin, it was even lower; 17 (13-23) nmol/L/mg. Valproic acid significantly increased the C/D to 251 (200-320) nmol/L/mg in 13 patients. Triple therapy with valproic acid and either carbamazepine or phenytoin (23 patients) yielded a C/D slightly above that of monotherapy, whereas a few patients on phenobarbital had a C/D slightly below that of monotherapy. The within-group C/D variation was comparatively small. The C/D ratio in a mixed lamotrigine TDM material shows a widespread intra- and interindividual variation, which can largely be explained by pharmacokinetic interactions with concomitantly used antiepileptic drugs. These results support the use of TDM in lamotrigine therapy.

Lamotrigine and therapeutic drug monitoring: retrospective survey following the introduction of a routine service

AIMS

To review (retrospectively) the relationships between lamotrigine (LTG) dosage and plasma concentrations based on data generated in a routine therapeutic drug monitoring laboratory from a heterogeneous sample of patients with epilepsy. To distinguish patients taking concomitant anti-epileptic therapy which induced or inhibited drug metabolising enzymes, or a combination of both, together with LTG. To survey medical staff who use a routine LTG assay service with a view to establishing the utility of higher plasma LTG concentrations than those used in early clinical trials.

METHODS

All patient assays for LTG received over a 12 month period (339 requests from 149 patients) were reviewed and relationships between dosage and concentration calculated and grouped according to concomitant antiepileptic drug therapy. The doctors requesting the tests were surveyed by questionnaire (n=40 of 67 responded). They were asked for details about the patient's seizure control, rationale used for LTG dosage adjustment and their acceptance of the proposed 'therapeutic range' adopted by the laboratory of 3-14 mg(-1).

RESULTS

Linear relationships were demonstrated between LTG dosage and concentration for the 3 treatment groups (LTG plus valproic acid (VPA), LTG plus enzyme inducing antiepileptic drugs, and LTG plus VPA and inducers), however, there were significant differences between groups (P<0.001) with a 4.4 fold difference in dosage: concentration ratios between the LTG plus VPA group and the LTG plus inducers group. The questionnaire showed that the therapeutic range was well accepted by 88% of responders, none of whom considered this higher range to be wrong.

CONCLUSIONS

Metabolic inhibition by VPA was shown to have a marked effect on LTG kinetics, suggesting either a significant LTG dosage reduction is required if plasma LTG concentrations are elevated, or alternatively, higher plasma LTG concentrations could be attained from lower dosages. The higher therapeutic range adopted by the laboratory (3-14 mg(-1)) was widely accepted and increasingly applied in clinical practice in the management of patients with epilepsy.

Modelling of the pharmacodynamic interaction between phenytoin and sodium valproate

Treatment of epilepsy with a combination of antiepileptic drugs remains the therapeutic choice when monotherapy fails. In this study, we apply pharmacokinetic-pharmacodynamic modelling to characterize the interaction between phenytoin (PHT) and sodium valproate (VPA). Male Wistar rats received a 40 mg kg(-1) intravenous dose of PHT over 5 min either alone or in combination with an infusion of VPA resulting in a steady-state concentration of 115.5+/-4.9 microg ml(-1). A control group received only the infusion of VPA. The increase in the threshold for generalized seizure activity (ATGS) was used as measure of the anticonvulsant effect. PHT pharmacokinetics was described by a pharmacokinetic model with Michaelis-Menten elimination. The concentration-time course and plasma protein binding of PHT were not altered by VPA. The pharmacokinetic parameters Vmax and Km were, respectively, 294+/-63 microg min(-1) and 7.8+/-2.4 microg ml(-1) in the absence of VPA and 562+/-40 microg min(-1) and 15.6+/-0.9 microg ml(-1) upon administration in combination with VPA. A delay of the onset of the effect relative to plasma concentrations of PHT was observed. The assessment of PHT concentrations at the effect site was based on the effect-compartment model, yielding mean ke0 values of 0.128 and 0.107 min(-1) in the presence and absence of VPA, respectively. A nonlinear relationship between effect-site concentration and the increase in the TGS was observed. The concentration that causes an increase of 50% in the baseline TGS (EC50%TGS) was used to compare drug potency. A shift of EC50%TGS from 13.27+3.55 to 4.32+/-0.52 microg ml(-1) was observed upon combination with VPA (P<0.01). It is concluded that there is a synergistic pharmacodynamic interaction between PHT and VPA in vivo.

Clinically important pharmacokinetic drug-drug interactions: role of cytochrome P450 enzymes

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues and many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In the future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

A mechanistic approach to antiepileptic drug interactions

OBJECTIVE

To describe the primary types of antiepileptic drug (AED) interactions by using a mechanistic approach.

DATA SOURCES

A literature search was performed using MEDLINE and bibliographies of recent review articles and published abstracts.

DISCUSSION

AEDs are associated with a wide range of drug interactions, including hepatic enzyme induction and inhibition and protein-binding displacement. Hepatic induction by AEDs affects the metabolism of a limited number of drugs with low therapeutic indices. Anticipation of induction interactions and careful clinical monitoring may alleviate potential problems. Most commonly used AEDs are eliminated through hepatic metabolism catalyzed by the cytochrome P450 (CYP) and uridine diphosphate glucuronosyltransferase (UGT) enzymes. Phenytoin, phenobarbital, and carbamazepine induce CYP and UGT enzymes. Lamotrigine is a weak inducer of UGT. Valproate is a broad-spectrum inhibitor of UGT enzymes, epoxide hydrolase, and CYP2C enzymes. Felbamate induces CYP3A4, but inhibits CYP2C19 substrates. Topiramate inhibits only CYP2C19 substrates. Ethosuximide, gabapentin, tiagabine, and vigabatrin are neither inducers nor inhibitors of drug metabolism. Hepatic enzyme inhibition usually occurs because of competition at the enzyme site. Knowledge of the specific metabolic enzymes involved in the metabolism of AEDs allows clinicians to predict potential interactions.

CONCLUSIONS

By understanding the mechanisms of drug interactions, the pharmacist can play a key role in patient care by anticipating and preventing AED drug interactions.