Preclinical profile of combinations of some second-generation antiepileptic drugs: an isobolographic analysis

Imperatorin interacts additively with novel antiseizure medications in the mouse maximal electroshock-induced seizure model: an isobolographic transformation

BACKGROUND

Anticonvulsant effects of imperatorin (IMP) have been experimentally confirmed earlier, but no information is available on the interaction profiles of this naturally occurring coumarin when combined with novel antiseizure medication (ASMs). This study aimed to determine the effects of IMP on the anticonvulsant effects of lacosamide (LCM), oxcarbazepine (OXC), pregabalin (PGB), and topiramate (TPM) in the maximal electroshock-induced seizure (MES) model in mice.

METHODS

The anticonvulsant effects exerted by novel ASMs (LCM, OXC, PGB, and TPM) when combined with constant doses of IMP (25 and 50 mg/kg) underwent isobolographic transformation to precisely classify the observed interactions in the mouse MES model. Total brain concentrations of ASMs were measured with high-pressure liquid chromatography to exclude the pharmacokinetic nature of interactions among IMP and the tested ASMs.

RESULTS

IMP (50 mg/kg) significantly enhanced (p < 0.01) the anticonvulsant potency of LCM, OXC, PGB, and TPM in the mouse MES model. IMP (25 mg/kg) mildly potentiated the anticonvulsant action of LCM, OXC, PGB, and TPM, but no statistical significance was reported for these combinations. The isobolographic transformation of data from the MES test revealed that the interactions of novel ASMs with IMP were additive. Moreover, IMP (50 mg/kg) did not affect the total brain content of any of the novel ASMs in experimental mice.

CONCLUSIONS

The additive interactions of IMP with LCM, OXC, PGB, and TPM in the mouse MES model accompanied by no pharmacokinetic changes in the total brain content of ASMs are worthy of recommendation for further studies.

Evaluation and management of drug resistant epilepsy in children

Epilepsy is one of the most common neurological conditions in children. Most children with epilepsy respond to anti- epileptic drugs (AEDs) but approximately 30% of children develop drug resistant epilepsy (DRE) defined as 'the failure of adequate trials of two tolerated, appropriately chosen and used anti-epileptic drugs (AEDs)'. DRE is associated with serious consequences including higher mortality and worse cognitive outcomes. DRE impacts several aspects of the child's and the caregiver's life.

Interactions among Lacosamide and Second-Generation Antiepileptic Drugs in the Tonic-Clonic Seizure Model in Mice

Combination therapy with two or three antiseizure medications (ASMs) is sometimes a preferred method of treatment in epilepsy patients. (1) Background: To detect the most beneficial combination among three ASMs, a screen test evaluating in vivo interactions with respect to their anticonvulsant properties, was conducted on albino Swiss mice; (2) Methods: Classification of interactions among lacosamide (LCM) and selected second-generation ASMs (lamotrigine (LTG), pregabalin (PGB), oxcarbazepine (OXC), and topiramate (TPM)) was based on the isobolographic analysis in the mouse maximal electroshock-induced seizure (MES) model. Interactions among LCM and second-generation ASMs were visualized using a polygonogram; (3) Results: In the mouse MES model, synergy was observed for the combinations of LCM + TPM + PGB and LCM + OXC + PGB. Additivity was reported for the other combinations tested i.e., LCM + LTG + TPM, LCM + LTG + PGB, LCM + LTG + OXC, and LCM + OXC + TPM in this seizure model. No adverse effects associated with triple ASM combinations, containing LCM and second-generation ASMs were observed in mice; (4) Conclusions: The combination of LCM + TPM + PGB was the most beneficial combination among the tested in this study, offering synergistic suppression of tonic-clonic seizures in mice subjected to the MES model. Both the isobolographic analysis and polygonogram method can be recommended for experimental epileptology when classifying interactions among the ASMs.

Developing precision treatments for epilepsy using patient and animal models

Introduction: Phenytoin was the first antiepileptic drug (AED) discovered in an animal model of seizures whose clinical efficacy was subsequently confirmed. This clearly indicated that a search for other AEDs had to consider animal studies.Areas covered: Main seizure tests used for the evaluation of possible anticonvulsive activity of potential anticonvulsants and their predictive values have been reviewed. Procedures used for the estimation of antiepileptogenic effects have been also included.Expert opinion: First-line seizure models comprise maximal electroshock (MES)-, pentylenetetrazol (PTZ)- and kindling-induced convulsions in rodents. The MES test may be considered as a convenient and easy model of generalized tonic-clonic seizures, PTZ test - as a model of generalized myoclonic seizures and to a certain degree - absence seizures. Kindled seizures (for example, from amygdala) may be regarded as a model of focal seizures. Some tests have been suggested for the search of AEDs effective in drug-resistant seizures - for instance, 6 Hz (44 mA) test or intrahippocampal kainate model of mesial temporal lobe epilepsy. There are also recommendations from experimental epileptology on synergistic AED combinations for patients with drug-resistant seizures. The clinical evidence on this issue is scarce and favors a combined treatment with valproate + lamotrigine.

Polygonogram with isobolographic synergy for three-drug combinations of phenobarbital with second-generation antiepileptic drugs in the tonic-clonic seizure model in mice

Background

Combination therapy consisting of two or more antiepileptic drugs (AEDs) is usually prescribed for patients with refractory epilepsy. The drug–drug interactions, which may occur among currently available AEDs, are the principal criterion taken by physicians when prescribing the AED combination to the patients. Unfortunately, the number of possible three-drug combinations tremendously increases along with the clinical approval of novel AEDs.

Aim

To isobolographically characterize three-drug interactions of phenobarbital (PB) with lamotrigine (LTG), oxcarbazepine (OXC), pregabalin (PGB) and topiramate (TPM), the maximal electroshock-induced (MES) seizure model was used in male albino Swiss mice.

Materials and method

The MES-induced seizures in mice were generated by alternating current delivered via auricular electrodes. To classify interactions for 6 various three-drug combinations of AEDs (i.e., PB + TPM + PGB, PB + OXC + TPM, PB + LTG + TPM, PB + OXC + PGB, PB + LTG + PGB and PB + LTG + OXC), the type I isobolographic analysis was used. Total brain concentrations of PB were measured by fluorescent polarization immunoassay technique.

Results

The three-drug mixtures of PB + TPM + PGB, PB + OXC + TPM, PB + LTG + TPM, PB + OXC + PGB, PB + LTG + PGB and PB + LTG + OXC protected the male albino Swiss mice from MES-induced seizures. All the observed interactions in this seizure model were supra-additive (synergistic) (p < 0.001), except for the combination of PB + LTG + OXC, which was additive. It was unable to show the impact of the studied second-generation AEDs on total brain content of PB in mice.

Conclusions

The synergistic interactions among PB and LTG, OXC, PGB and TPM in the mouse MES model are worthy of being transferred to clinical trials, especially for the patients with drug resistant epilepsy, who would benefit these treatment options.

Sub-additive (antagonistic) interaction of lacosamide with lamotrigine and valproate in the maximal electroshock-induced seizure model in mice: an isobolographic analysis

BACKGROUND

Launching polytherapy with two or three antiseizure drugs (ASDs) in patients with epilepsy is still problematic. The choice of ASDs to combine them together is usually based on clinicians' experience and it requires knowledge about mechanisms of action of the studied ASDs and their drug-drug interactions, whose nature may be favorable, neutral or unfavorable. To characterize three-drug interaction among lacosamide (LCM), lamotrigine (LTG) and valproate (VPA), the type I isobolographic analysis was used. The antiseizure effects of three-drug combination were analyzed in a model of maximal electroshock-induced seizures (MES) in albino Swiss mice.

MATERIALS AND METHODS

The seizure activity in mice was evoked by alternating current stimulation (25 mA, 500 V, 50 Hz, 0.2 s). Both, the type I isobolographic analysis and the test of parallelism of dose-response effects of the ASDs were used so as to properly classify interaction among three ASDs, administered in a fixed ratio combination of 1:1:1.

RESULTS

The three-drug mixture of LCM, LTG and VPA at the fixed ratio of 1:1:1 protected the experimental mice from MES-induced seizures; however, the reported interaction was sub-additive (antagonistic; p < 0.01) with isobolography.

CONCLUSION

The antagonistic pharmacodynamic interaction among LCM, LTG and VPA in the MES test in mice cannot be transferred to clinical settings and this unfavorable combination should not be recommended for patients with epilepsy.

Polygonogram and isobolographic analysis of interactions between various novel antiepileptic drugs in the 6-Hz corneal stimulation-induced seizure model in mice

Pharmacotherapy with two antiepileptic drugs in combination is usually prescribed to epilepsy patients with refractory seizures. The choice of antiepileptic drugs in combination should be based on synergistic cooperation of the drugs with respect to suppression of seizures. The selection of synergistic interactions between antiepileptic drugs is challenging issue for physicians, especially, if 25 antiepileptic drugs are currently available and approved to treat epilepsy patients. The aim of this study was to determine all possible interactions among 5 second-generation antiepileptic drugs (gabapentin (GBP), lacosamide (LCM), levetiracetam (LEV), pregabalin (PGB) and retigabine (RTG)) in the 6-Hz corneal stimulation-induced seizure model in adult male albino Swiss mice. The anticonvulsant effects of 10 various two-drug combinations of antiepileptic drugs were evaluated with type I isobolographic analysis associated with graphical presentation of polygonogram to visualize the types of interactions. Isobolographic analysis revealed that 7 two-drug combinations of LEV+RTG, LEV+LCM, GBP+RTG, PGB+LEV, GBP+LEV, PGB+RTG, PGB+LCM were synergistic in the 6-Hz corneal stimulation-induced seizure model in mice. The additive interaction was observed for the combinations of GBP+LCM, GBP+PGB, and RTG+LCM in this seizure model in mice. The most beneficial combination, offering the highest level of synergistic suppression of seizures in mice was that of LEV+RTG, whereas the most additive combination that protected the animals from seizures was that reporting additivity for RTG+LCM. The strength of interaction for two-drug combinations can be arranged from the synergistic to the additive, as follows: LEV+RTG > LEV+LCM > GBP+RTG > PGB+LEV > GBP+LEV > PGB+RTG > PGB+LCM > GBP+LCM > GBP+PGB > RTG+LCM.

Pharmacodynamic interactions of antiepileptic drugs: From bench to clinical practice

BACKGROUND

Approximately 50% of patients do not achieve seizure control with antiepileptic drug (AED) monotherapy, and polytherapy, with more than one AED, is often required. To date, no evidence-based criteria on how to combine AEDs exist.

OBJECTIVE

This narrative review aimed to provide critical findings of the available literature about the role of pharmacodynamic AEDs' interactions in patients whose epilepsies were treated with polytherapy.

METHODS

Electronic databases, Medical Literature Analysis and Retrieval System Online (MEDLINE) and Excerpta Medica dataBASE (EMBASE), were systematically searched to identify relevant studies on pharmacodynamic AEDs' interactions in patients with epilepsy.

RESULTS AND CONCLUSION

Most data on AED combinations are coming from animal models and preclinical studies. Combining AEDs with different mechanisms of actions seems to have greater effectiveness and lower risk of adverse event development. Conversely, the combination of AEDs may cause pharmacodynamic synergistic effects that may result in not only increased efficacy but also more adverse effects. Despite some AED associations that have been proven to be effective in specific epilepsy/seizure type (e.g., phenobarbital+/phenytoin for tonic seizures and ethosiximide + valproate for absences; lamotrigine + valproate for various epilepsy/seizure types), no clear and definitive evidence exists about AED combinations in humans. Examples of pharmacodynamic interactions that possibly explain the synergistic effects on efficacy or adverse effects include the combination between vigabatrin or pregabalin and sodium channel blockers (supra-additive antiseizure effect) and lacosamide combined with other sodium channel blockers (infra-additive antiseizure effect and neurotoxicity synergistic). The pharmacodynamic lamotrigine-valproate interaction is also supported by synergistic adverse events. Therefore, well-designed double-blind prospective studies recruiting a sufficient number of patients possibly with a crossover design and carefully ascertain the role of pharmacokinetic interactions and variations of AEDs' levels in the blood are needed.

Anticonvulsant and neurotoxic effects of a novel 1,2,4-triazole-3-thione derivative (TPF-34) and its isobolographic interaction profile with classical antiepileptic drugs in mice

BACKGROUND

Anticonvulsant and acute toxic effects of 5-[(3-fluorophenyl)ethyl]-4-(n-hexyl)-2,4-dihydro-3H-1,2,4-triazole-3-thione (TPF-34)-a candidate for novel antiepileptic drug-were examined in the maximal electroshock-induced seizure (MES) model and rotarod test in mice. The interaction profile of TPF-34 with four classical antiepileptic drugs (carbamazepine, phenobarbital, phenytoin and valproate) was also studied in the mouse MES model.

METHODS

Both ED₅₀ and TD₅₀ values for TPF-34 were determined at four treatment times (15, 30, 60 and 120 min after i.p. administration) in the MES model and rotarod test in adult male albino Swiss mice, respectively. The influence of TPF-34 on the protective anticonvulsant action of carbamazepine, phenobarbital, phenytoin and valproate in the mouse MES model was assessed with isobolographic analysis of interaction. Total brain antiepileptic drug concentrations were measured with fluorescence polarization immunoassay.

RESULTS

TPF-34, when administered alone at four pretreatment times (15, 30, 60 and 120 min before experiments), possessed a favorable preclinical profile with the protective index (a ratio of TD₅₀ and ED₅₀ values) ranging from 2.89 to 3.53. Moreover, TPF-34, when combined with carbamazepine, phenobarbital, phenytoin and valproate, exerted an additive interaction in the MES model in mice. TPF-34 had no impact on total brain antiepileptic drug concentrations in mice.

CONCLUSIONS

A protective index value higher than 3 allows recommending TPF-34 as a promising antiepileptic drug candidate for further preclinical testing using other experimental seizure models. The additive interaction of TPF-34 with carbamazepine, phenobarbital, phenytoin and valproate in the mouse MES model is worthy of recommendation to further clinical studies.

Clinical opinion: Earlier employment of polytherapy in sequential pharmacotherapy of epilepsy

Modern pharmacotherapy for epilepsy consists of orderly, sequential drug trials, in which antiepileptic drugs (AEDs) are chosen under the concept of individual patient-oriented (or - tailored) pharmacotherapy. Although monotherapy has been established as the preferred mode of AEDs therapy in both newly diagnosed and drug resistant epilepsies, there are still lack of evidence to favor either monotherapy or polytherapy in epilepsy, which has generated continuing controversies on the preferred mode of pharmacotherapy. However, each mode of pharmacotherapy may have both advantages and disadvantages, which are different and variable related to individual case scenario. We conducted a brief comparative overview between monotherapy and polytherapy to provide clues for earlier employment of polytherapy in each steps of sequential drug trials. Previous claims about the advantages of monotherapy over polytherapy are not supported but gradually losing its ground by the introduction of a large number of drugs carrying pharmacological advantages for combination therapy. Current evidence stresses the importance of combining drugs having synergistic interactions for better outcome of polytherapy, which has not been considered in previous clinical investigations comparing monotherapy and polytherapy. It is likely that a significant improvement in the outcome of current AEDs therapy is feasible by earlier employment of polytherapy as well as identification of combination drug regimens carrying synergistic interactions. At present, lamotrigine(LTG) and valproate(VPA) combination regimen is the only well documented synergistic regimen, but there are a long-list of candidate regimens requiring future trials in appropriate designs.

Effects of androsterone on the protective action of various antiepileptic drugs against maximal electroshock-induced seizures in mice

This study evaluated the effect of androsterone (AND), a metabolite of testosterone, on the ability of selected classical and novel antiepileptic drugs to prevent seizures caused by maximal electroshock (MES), which may serve as an experimental model of human generalized tonic-clonic seizures in mice. Single intraperitoneal (i.p.) administration of AND (80 mg kg^-1) significantly raised the threshold for convulsions in the MES seizure threshold test. Lower doses of AND (5, 10, 20, and 40 mg kg^-1) failed to change the threshold. AND at a subthreshold dose of 40 mg kg^-1 significantly enhanced the protective activity of carbamazepine, gabapentin, and phenobarbital against MES-induced seizures decreasing their median effective doses (ED₅₀) values ± SEM from 8.59 ± 0.76 to 6.05 ± 0.81 mg kg^-1 (p = 0.0308) for carbamazepine, from 419.9 ± 120.6 to 111.5 ± 41.1 mg kg^-1 (p = 0.0405) for gabapentin, and from 20.86 ± 1.64 to 10.0 ± 1.21 mg kg^-1 (p = 0.0007) for phenobarbital. There were no significant changes in total brain concentrations of carbamazepine, gabapentin, and phenobarbital following AND administration. This suggests that the enhancing effects of AND on the protective activity of these antiepileptic drugs are not related to pharmacokinetic factors. A lower dose of AND (20 mg kg^-1) had no effect on the protective activity of carbamazepine, gabapentin, and phenobarbital. AND administered at a dose of 40 mg kg^-1 failed to change the anticonvulsant activity of lamotrigine, oxcarbazepine, phenytoin, topiramate, and valproate in the MES test. In the chimney test, AND given at a dose enhancing the protective activity of carbamazepine, gabapentin, and phenobarbital (which alone was without effect on motor performance of mice) did not affect impairment of motor coordination produced by the antiepileptics. Our findings recommend further preclinical and clinical research on AND in respect of its use as adjuvant therapy in the management of epilepsy in men with deficiency of androgens.

Interactions of Mexiletine with Novel Antiepileptic Drugs in the Maximal Electroshock Test in Mice: An Isobolographic Analysis

The aim of the study was to evaluate precisely the type of interactions between mexiletine (an antiarrhythmic drug) and four new generation antiepileptic drugs: lamotrigine, oxcarbazepine, topiramate and pregabalin in the maximal electroshock test in mice (MES). The isobolographic analysis was used to assess the nature of interactions between the tested drugs. Total brain concentrations of antiepileptics were also measured to detect possible pharmacokinetic interactions. The results obtained indicated that the mixture of mexiletine and pregabalin at the fixed ratios of 1:1 and 3:1 led to supra-additive interaction in terms of seizure suppression, while the proportion of 1:3 occurred additive. Synergism was also demonstrated for the combination of mexiletine and topiramate in all three proportions. Combinations of mexiletine with lamotrigine and mexiletine with oxcarbazepine were found to be additive. Adverse-effect profiles of mexiletine, antiepileptics and drug combinations were evaluated in the chimney test (motor coordination) and step-through passive-avoidance task (long-term memory). Mexiletine and drug combinations did not impair long-term memory. Moreover, all combinations of mexiletine with lamotrigine, oxcarbazepine and topiramate had no significant effect on motor coordination. However, the results from the chimney test indicated that pregabalin, administered alone at its ED₅₀ dose from the MES-test, significantly impaired motor performance. Similar adverse effects were observed when mexiletine was co-administered with pregabalin at the fixed-dose ratio combinations of 1:1 and 1:3. However, reduction of pregabalin dose at the fixed ratio of 3:1 seems to prevent significant motor impairment. The results may indicate that mexiletine can be considered as an adjunctive drug in antiepileptic treatment, particularly in patients with concomitant cardiac arrhythmia.

A viewpoint on rational and irrational fixed-drug combinations

INTRODUCTION

Considering that there are around 30% of patients with epilepsy resistant to monotherapy, the use of synergistic combinations of antiepileptic drugs is of particular importance. This review shows most beneficial as well as irrational combined treatments both from an experimental and clinical point of view. Areas covered: Preferably, experimental data derived from studies evaluating synergy, additivity, or antagonism by relevant methods, in terms of anticonvulsant or neurotoxic effects and pharmacokinetic data have been considered. Although there have been no randomized clinical trials on this issue, the clinical data have been analyzed from studies on considerable numbers of patients. Case-report studies have been not considered. Expert commentary: The experimental data provide a strong support that co-administration of lamotrigine with carbamazepine is negative, considering the anticonvulsant and neurotoxic effects. Clinical reports do not entirely support this conclusion. Other experimentally documented negative combinations comprise lamotrigine+ oxcarbazepine and oxcarbazepine+ phenytoin. From the experimental and clinical point of view, a combination of lamotrigine+ valproate may deserve recommendation. Other most positive experimental and clinical combinations include carbamazepine+valproate, phenytoin+phenobarbital, carbamazepine+gabapentin, carbamazepine+topiramate, levetiracetam+valproate, levetiracetam+carbamazepine. Certainly, experimental data have some limitations (non-epileptic animals, acute administration of antiepileptic drugs) so all experimental recommendations need a careful clinical evaluation.

Beneficial Combination of Lacosamide with Retigabine in Experimental Animals: An Isobolographic Analysis

BACKGROUND/AIM

To isobolographically determine the types of interactions that occur between retigabine and lacosamide (LCM; two third-generation antiepileptic drugs) with respect to their anticonvulsant activity and acute adverse effects (sedation) in the maximal electroshock-induced seizures (MES) and chimney test (motor performance) in adult male Swiss mice.

METHODS

Type I isobolographic analysis for nonparallel dose-response effects for the combination of retigabine with LCM (at the fixed-ratio of 1:1) in both the MES and chimney test in mice was performed. Brain concentrations of retigabine and LCM were measured by high-pressure liquid chromatography (HPLC) to characterize any pharmacokinetic interactions occurring when combining these drugs.

RESULTS

Linear regression analysis revealed that retigabine had its dose-response effect line nonparallel to that of LCM in both the MES and chimney tests. The type I isobolographic analysis illustrated that retigabine combined with LCM (fixed-ratio of 1:1) exerted an additive interaction in the mouse MES model and sub-additivity (antagonism) in the chimney test. With HPLC, retigabine and LCM did not mutually change their total brain concentrations, thereby confirming the pharmacodynamic nature of the interaction.

CONCLUSION

LCM combined with retigabine possesses a beneficial preclinical profile (benefit index ranged from 2.07 to 2.50) and this 2-drug combination is worth recommending as treatment plan to patients with pharmacoresistant epilepsy.

Additive interactions between retigabine and oxcarbazepine in the chimney test and the model of generalized tonic-clonic seizures in mice

Introduction. Patients with pharmacoresistant epilepsy are usually treated with two or more antiepileptic drugs (AEDs). The search for therapeutically efficacious AED combinations is still a challenging issue for clinicians and epileptologists throughout the world.

Aim. To determine the interaction profile for the combination of retigabine (RTG) and oxcarbazepine (OXC) in both, the model of tonic-clonic seizures, the maximal electroshock (MES)-induced seizure model and chimney test (motor performance) in adult male albino Swiss mice.

Methods. Isobolographic analysis (type I) was applied to characterize interactions for the combination of RTG with OXC with respect to its anticonvulsant and acute side (neurotoxic) effects, as determined in the MES and chimney tests, respectively.

Results. The combination of RTG with OXC at the fixed-ratios of 1:3, 1:1 and 3:1 produced additive interactions in the MES test in mice. Similarly, the combination of RTG with OXC at the fixed-ratio of 1:1 produced additive interaction with a tendency towards sub-additivity in the chimney test in mice. Measurement of total brain concentrations of both AEDs revealed that RTG did not affect total brain concentrations of OXC and inversely, OXC had no impact on RTG’s total brain concentrations, confirming pharmacodynamic interaction between the drugs.

Conclusions. The additive pharmacodynamic interactions in both the MES and chimney tests in mice were observed for the combination of RTG with OXC.

Additive interaction of levetiracetam with lamotrigine in the mouse 6 Hz psychomotor seizure model – an isobolographic analysis

The aim of this study was to characterize the anticonvulsant effects of levetiracetam (LEV) in combination with lamotrigine (LTG – a second-generation antiepileptic drug), in the mouse 6 Hz psychomotor seizure model. Limbic (psychomotor) seizure activity was evoked in albino Swiss mice by a current (32 mA, 6 Hz, 3 s stimulus duration) delivered via ocular electrodes and isobolographic analysis for parallel dose-response relationship curves (DRRCs) was used to characterize the consequent anticonvulsant interactions between the drug combinations. Results indicated that LEV administered singly was associated with a DRRC that was parallel to that for LTG. With isobolography for parallel DRRCs, the combination of LEV with LTG at three fixed-ratios of 1:3, 1:1 and 3:1 exerted additive interaction. LEV combined with LTG exerted additive interaction in the mouse 6 Hz psychomotor seizure model.

Antiepileptic drugs: adverse effects and drug interactions

The goal of epilepsy treatment is no seizures and no side effects. Antiepileptic drugs (AEDs) are the mainstay of treatment. Chronic use, however, often leads to serial drug changes over time, exposing patients with epilepsy (PWE) to recurrent risks due to adverse effects (AEs) and drug interactions. Both unwanted acute and chronic AEs may occur that are usually dose-related, suggested by AED pharmacology, and addressed with appropriate use of serum drug concentrations. Idiosyncratic AEs can pose significant health issues for PWE and be lifesaving with early identification. AED pharmacokinetics and pharmacodynamic properties serve as the foundation for anumber of drug interactions. Hepatic enzyme systems may facilitate AED interaction with other coadministered medication through induction or inhibition of drug metabolism and result in AEs or seizures from drug interactions. A working knowledge of AED pharmacology is an essential component of good clinical practice to help clinicians predict potential AEs and DIs in the treatments of PWE.

Antiepileptic drug monotherapy versus polytherapy: pursuing seizure freedom and tolerability in adults

PURPOSE OF REVIEW

Despite the availability of many new antiepileptic drugs (AEDs), only around 50% of people with epilepsy will become seizure free on their first drug. This article explores treatment options and issues influencing whether AEDs should be substituted or combined in the remainder of the patient population.

RECENT FINDINGS

Prior to the introduction of novel AEDs, it was generally opined that combining traditional agents did not necessarily lead to an improvement in seizure control and might increase the propensity for side effects. Newer AEDs, many with different mechanisms of action, have increased the potential for polytherapy regimens, although robust data to support or refute this therapeutic strategy are sparse. It seems sensible to substitute rather than combine when the first AED produces an idiosyncratic reaction, is poorly tolerated at a low/moderate dose or shows no efficacy. Polytherapy may be preferred if the patient tolerates their first or second AED well, but with a suboptimal response, particularly when there is an identifiable anatomical substrate for the seizures. AED selection requires consideration of many factors some of which are discussed in this study.

SUMMARY

There are no definitive answers on whether to combine or substitute AEDs. Different strategies are required for different scenarios in different patients.

Involvement of GABAergic and glutamatergic systems in the anticonvulsant activity of 3-alkynyl selenophene in 21 day-old rats

In this study, we investigated the role of GABAergic and glutamatergic systems in the anticonvulsant action of 3-alkynyl selenophene (3-ASP) in a pilocarpine (PC) model of seizures. To this purpose, 21 day-old rats were administered with an anticonvulsant dose of 3-ASP (50 mg/kg, per oral, p.o.), and [(3)H]γ-aminobutyric acid (GABA) and [(3)H]glutamate uptakes were carried out in slices of cerebral cortex and hippocampus. [(3)H]GABA uptake was decreased in cerebral cortex (64%) and hippocampus (58%) slices of 21 day-old rats treated with 3-ASP. In contrast, no alteration was observed in [(3)H]glutamate uptake in cerebral cortex and hippocampus slices of 21 day-old rats that received 3-ASP. Considering the drugs that increase synaptic GABA levels, by inhibiting its uptake or catabolism, are effective anticonvulsants, we further investigated the possible interaction between sub-effective doses of 3-ASP and GABA uptake or GABA transaminase (GABA-T) inhibitors in PC-induced seizures in 21 day-old rats. For this end, sub-effective doses of 3-ASP (10 mg/kg, p.o.) and DL-2,4-diamino-n-butyric acid hydrochloride (DABA, an inhibitor of GABA uptake--2 mg/kg, intraperitoneally; i.p.) or aminooxyacetic acid hemihydrochloride (AOAA; a GABA-T inhibitor--10 mg/kg, i.p.) were co-administrated to 21 day-old rats before PC (400 mg/kg; i.p.) treatment, and the appearance of seizures was recorded. Results demonstrated that treatment with AOAA and 3-ASP or DABA and 3-ASP significantly abolished the number of convulsing animals induced by PC. The present study indicates that 3-ASP reduced [(3)H]GABA uptake, suggesting that its anticonvulsant action is related to an increase in inhibitory tonus.

Antiepileptic drug therapy: does mechanism of action matter?

This article represents a synthesis of presentations made by the authors during a scientific meeting held in London on 7 June 2010 and organized by GlaxoSmithKline. Each speaker produced a short précis of his lecture to answer a specific question, resulting in an overview of what we know about the relevance of the mechanisms of action of antiepileptic drugs in determining appropriate combination therapies for the treatment of drug-resistant epilepsy.

Combining antiepileptic drugs--rational polytherapy?

The global introduction of 14 new antiepileptic drugs (AEDs) over the past 20 years as adjunctive treatment in refractory epilepsy has triggered an increased interest in optimising combination therapy. With a widening range of available mechanisms of AED action, much activity has been focused on the defining and refining "rational polytherapy" with AEDs that have differing pharmacological properties. This paper reviews the available animal and human data exploring this issue. The experimental and clinical evidence in support of "rational polytherapy" is sparse, with only the combination of sodium valproate with lamotrigine demonstrating synergism. Robust evidence to guide clinicians on how and when to combine AEDs is lacking and current practice recommendations are largely empirical. Practical guidance for the clinician is summarised and discussed in this review. In particular, care should be taken to avoid excessive drug load, which can be associated with decreased tolerability and, therefore, reduced likelihood of seizure freedom. A palliative strategy should be defined early for the more than 30% of patients with refractory epilepsy. Nevertheless, the availability of an increasing number of pharmacologically distinct AEDs has produced a modest improvement in prognosis with combination therapy, which will encourage the clinician to persevere with continued pharmacological manipulation when other therapeutic options have been tried or are not appropriate.

Interactions of propofol and remifentanil on bispectral index under 66% N(2)O: analysis by dose-effect curve, isobologram, and combination index

Background

Propofol and remifentanil are usually co-administered and have shown synergistic effect for anesthesia. However, the synergistic effect of the two drugs on hypnosis measured by bispectral index (BIS) was controversial in previous studies. The aim of this study was to identify the interaction of propofol and remifentanil on BIS and the optimal dose combinations for hypnosis under 66% N₂O during surgery.

Methods

Patients (age 55-75 and American Society of Anesthesiologists [ASA] 1-2) undergoing gastrectomy were enrolled in this study. Propofol and remifentanil were co-administered incrementally at 1 : 1 potent ratio (the P1R1 group), at 1 : 2 potent ratio (the P1R2 group), or at 2 : 1 potent ratio (the P2R1 group) using effect site target-controlled infusion and BIS was measured. 66% N₂O was concomitantly administered to all groups. The dose-effect curves, the 90% effective dose (EC₉₀) for adequate hypnosis (BIS 40), isobolograms and combination index were obtained by Calcusyn program (Biosoft) to reveal the interaction of propofol and remifentanil.

Results

The P2R1 group showed synergistic action on BIS. However, the other groups needed larger amount of each drug than the doses of additive action. The EC₉₀ of the P2R1 group was propofol, 3.34 µg/ml and remifentanil, 2.41 ng/ml under 66% of N₂O.

Conclusions

Propofol dominant co-administration is needed for dose reduction in BIS guided hypnosis.

Dose-response or dose-effect curves in in vitro experiments and their use to study combined effects of neurotoxicants

Interactions among neurotoxicants in in vitro models, where the molecular mechanisms of toxicity are generally studied, represent today an emerging field in the experimental neurotoxicology. In this chapter, we define some general concepts about the optimization of in vitro experiments to assess the dose/concentration-effect/response relationships and to extrapolate the functions describing them. After describing the available models to study interactions (the Bliss independence criterion and the Loewe additivity model), we present a method to practically apply these models to experimental data. Finally, we provide some examples of the theory of interactions among neurotoxicants in in vitro models.

Rational Polytherapy with Antiepileptic Drugs

Approximately 30-40% of patients do not achieve seizure control with a single antiepileptic drug (AED). With the advent of multiple AEDs in the past 15 years, rational polytherapy, the goal of finding combinations of AEDs that have favorable characteristics, has become of greater importance. We review the theoretical considerations based on AED mechanism of action, animal models, human studies in this field, and the challenges in finding such optimal combinations. Several case scenarios are presented, illustrating examples of rational polytherapy.

Mechanisms of action of antiepileptic drugs: the search for synergy

PURPOSE OF REVIEW

The aim is to review rational polytherapy of antiepileptic drugs in terms of conventional and novel mechanisms of action, consider combinations that might be beneficial when used as polytherapy, and discuss whether animal models can predict clinical efficacy.

RECENT FINDINGS

Many patients with epilepsy require concurrent treatment with more than one antiepileptic drug (rational polytherapy), but there is little information available as to which drugs might work best in combination. Conventional antiepileptic drugs act by blocking sodium channels or enhancing gamma-aminobutyric acid function. Some newer antiepileptic drugs have novel mechanisms of action, including impairment of the slow inactivation of sodium channels, binding to the presynaptic vesicle protein SV2A, binding to the calcium channel alpha2delta subunit, and opening select potassium channels. Several antiepileptic drugs have multiple or uncertain mechanisms of action. Quantitative techniques such as isobolography can be used to compare the efficacy and side effects of antiepileptic drug combinations in animals. However, neither such methods nor antiepileptic drug mechanisms of action have yet proven useful in predicting clinical benefit in patients.

SUMMARY

Animal models can be used to help predict drug combinations that might be effective clinically, based on novel mechanisms of action. However, at this point, antiepileptic drug choice in patients with epilepsy remains empirical.

Pharmacodynamic interactions between antiepileptic drugs: preclinical data based on isobolography

BACKGROUND

At least 20 - 30% of epileptic patients do not sufficiently respond to monotherapy. Some of them can benefit from drug combinations; hence, animal data may provide some useful novel clues for rational polytherapy.

OBJECTIVE

To review combinations of antiepileptic drugs, evaluated with the help of isobolographic analysis, in terms of their efficacy and adverse effects.

METHODS

A literature search, on the basis of experimental studies, with no time limit was carried out.

RESULTS/CONCLUSION

Preclinical data indicate that a synergy occurred for the combinations of valproate + phenytoin, valproate + ethosuximide, lamotrigine + valproate, gabapentin + valproate, gabapentin + carbamazepine, topiramate + carbamazepine, topiramate + valproate, topiramate + oxcarbazepine, levetiracetam + topiramate, levetiracetam + oxcarbazepine, oxcarbazepine + gabapentin, tiagabine + gabapentin and lamotrigine + topiramate. On the other hand, lamotrigine combined with carbamazepine or oxcarbazepine resulted in a clear-cut antagonism. Interestingly, a combination of oxcarbazepine + clonazepam produced variable responses, including synergy, additivity or antagonism, depending on the dose ratio of these drugs. In no case did pharmacokinetic factors contribute to the final analysis of the effects of drug combinations. Pharmacokinetic factors can contribute to the final effect of drug combinations,such as when stiripentol is added to valproate, or clobazam is added to valproate. It may be concluded that the rational treatment of drug-resistant epilepsy needs to consider the results of preclinical studies.

Efficacy, safety, and potential of extended-release lamotrigine in the treatment of epileptic patients

Epilepsy is a frequent, chronic disease demanding long-term medication with antiepileptic drugs (AEDs). When slow release formulations of AEDs are used the chance of compliance and control of seizures is increased. Lamotrigine (LTG) is a broad spectrum antiepileptic drug (AED), effective against both generalized and partial seizures. Its immediate-release formulation (LTG-IR) requires twice-daily dosing. In contrast, an extended-release formulation (LTG-XR) may be given once daily, providing a flatter dose-concentration curve with apparently lower maximum serum levels. Simplified dosing positively affects compliance and LTG-XR has a similar profile of efficacy and tolerability to LTG-IR. Rashes, including Stevens-Johnson syndrome, are the most serious adverse effect impacting 0.8% of pediatric patients. Thus, LTG-XR should be discontinued upon the appearance of rash.

Indapamide enhances the protective action of carbamazepine, phenobarbital, and valproate against maximal electroshock-induced seizures in mice

PURPOSE

To determine the influence of indapamide on the protective action of numerous conventional and second-generation antiepileptic drugs (carbamazepine, lamotrigine, oxcarbazepine, phenobarbital, topiramate and valproate) in the mouse maximal electroshock seizure model.

MATERIAL AND METHODS

Electroconvulsions were evoked in Albino Swiss mice by a current (sine-wave, 0.2 s stimulus duration) delivered via auricular electrodes. Adverse-effect profiles with respect to motor performance, long-term memory and skeletal muscular strength were measured along with total brain antiepileptic drug concentrations.

RESULTS

Indapamide (up to 3 mg/kg, i.p., 120 min before the test) neither altered the threshold for maximal electroconvulsions, nor protected the animals against maximal electroshock-induced seizures in mice. Moreover, indapamide (3 mg/kg, i.p.) significantly enhanced the anticonvulsant action of carbamazepine, phenobarbital and valproate, but not that of lamotrigine, oxcarbazepine or topiramate in the maximal electroshock seizure test in mice. Indapamide (1.5 mg/kg) had no impact on the anticonvulsant action of all studied antiepileptic drugs in the maximal electroshock seizure test in mice. Estimation of total brain antiepileptic drug concentrations revealed that the observed interaction between indapamide and phenobarbital was complicated by a significant pharmacokinetic increase in total brain concentrations of phenobarbital. In contrast, indapamide had no impact on the total brain concentrations of carbamazepine and valproate in mice.

CONCLUSIONS

The selective potentiation of the anticonvulsant action of carbamazepine and valproate by indapamide and lack of any pharmacokinetic interactions between drugs, make the combinations of indapamide with carbamazepine or valproate of pivotal importance for epileptic patients taking these drugs together.

Isobolographic characterization of interactions of levetiracetam with the various antiepileptic drugs in the mouse 6 Hz psychomotor seizure model

The aim of this study was to characterize the anticonvulsant effects of levetiracetam (LEV) in combination with the various antiepileptic drugs (clonazepam [CZP], oxcarbazepine [OXC], phenobarbital [PB], tiagabine [TGB], and valproate [VPA]), in the mouse 6 Hz psychomotor seizure model. Limbic (psychomotor) seizure activity was evoked in albino Swiss mice by a current (32 mA, 6 Hz, 3s stimulus duration) delivered via ocular electrodes and isobolographic analysis for parallel and non-parallel dose-response effects was used to characterize the consequent anticonvulsant interactions between the various drug combinations. Potential concurrent adverse-effect profiles of interactions between LEV and CZP, OXC, PB, TGB, and VPA at the fixed-ratio of 1:1 were evaluated in the chimney (motor performance), passive avoidance (long-term memory), and grip-strength (muscular strength) tests. LEV administered singly was associated with a dose-response relationship curve (DRRC) that was parallel to that for CZP and non-parallel to that for OXC, PB, TGB and VPA. With isobolography for parallel DRRCs, the combination of LEV with CZP at three fixed-ratios of 1:3, 1:1 and 3:1 was additive in nature. With isobolography for non-parallel DRRCs the combinations of LEV with OXC, TGB and VPA at the fixed-ratio of 1:1 were also additive. In contrast, the isobolography for non-parallel DRRCs revealed that the interaction for the combination of LEV with PB at the fixed-ratio of 1:1 was supra-additive (synergistic). None of the combinations were associated with any concurrent adverse effects with regards to motor coordination, long-term memory or muscular strength. LEV is associated with favorable anticonvulsant synergism with PB and is additive with regards to CZP, OXC, TGB and VPA in the mouse 6 Hz psychomotor seizure model.

Effects of amlodipine, diltiazem, and verapamil on the anticonvulsant action of topiramate against maximal electroshock-induced seizures in micePresented in part at the 11th Congress of the European Federation of Neurological Societies, Brussels, Belgium, 25–28 August 2007

To assess the effect of 3 calcium channel antagonists (amlodipine, diltiazem, and verapamil) on the anticonvulsant action of topiramate (a new generation antiepileptic drug) in the mouse maximal electroshock seizure (MES) model. Amlodipine (20 mg/kg) significantly enhanced the anticonvulsant activity of topiramate in the MES test in mice, reducing its ED50 value from 54.83 to 33.10 mg/kg (p < 0.05). Similarly, diltiazem (5 and 10 mg/kg) markedly potentiated the antiseizure action of topiramate against MES, lowering its ED50 value from 54.83 to 32.48 mg/kg (p < 0.05) and 28.68 mg/kg (p < 0.01), respectively. In contrast, lower doses of amlodipine (5 and 10 mg/kg) and diltiazem (2.5 mg/kg) and all doses of verapamil (5, 10, and 20 mg/kg) had no significant impact on the antiseizure action of topiramate. Pharmacokinetic verification of the interaction of topiramate with amlodipine and diltiazem revealed that neither amlodipine nor diltiazem affected total brain topiramate concentration in experimental animals, and thus, the observed interactions were concluded to be pharmacodynamic in nature. The favorable combinations of topiramate with amlodipine or diltiazem deserve more attention from a clinical viewpoint because the enhanced antiseizure action of topiramate was not associated with any pharmacokinetic changes in total brain topiramate concentration.

Conversion from carbamazepine or oxcarbazepine to topiramate in adolescents and adults with epilepsy

OBJECTIVE

To explore effectiveness, tolerability and changes in quality of life in patients with epilepsy converting to topiramate (TPM) from carbamazepine (CBZ) or oxcarbazepine (OXC) due to insufficient effectiveness and/or tolerability.

METHODS

A multicenter, open-label, non-interventional trial was used to examine patients (> or = 12 years) with epilepsy, changing to TPM monotherapy from baseline mono- or combination therapy with CBZ or OXC. TPM was added to the existing antiepileptic drug (AED) treatment and started at a dose of 25 mg once daily. The dose was titrated up with 25 mg/day increments, once every 1-2 weeks, until a final dose between 50 and 200 mg/day was reached. On the basis of clinical judgment, the treating physician decided whether or not the existing AED treatment with CBZ or OXC could then be withdrawn. Type and number of seizures, preferred TPM dose, quality of life (QOLIE-10 questionnaire), subjective perception of improvement and adverse events (AE) were documented.

RESULTS

140 patients (53.5% women, mean age 47 years) decided to switch to TPM due to insufficient effectiveness (75% of patients) and/or poor tolerability (80%) of the CBZ/OXC treatment. Average duration of follow-up was 24 weeks with an overall discontinuation rate of 19.3%, mainly due to AEs (12.1%). At study endpoint, the intended shift to TPM monotherapy was achieved in 73% of patients at a median TPM dose of 100 mg/day. A seizure reduction of > or = 50% was achieved in 91% of patients in the last scheduled period (weeks 12-26); 62% of patients entering that period remained seizure free. Quality of life at endpoint improved significantly when compared with baseline for all domains of QOLIE-10 (P < 0.001). Most frequent AEs (reported by > or = 5% of patients) were paresthesia (9.3%), weight loss (7.9%), convulsions (5.7%) and memory disorders (5.0%).

CONCLUSION

In patients with epilepsy, previously not satisfactorily treated with CBZ or OXC, conversion to TPM may result in an improvement in seizure control as well as in quality of life.

Interactions of MRZ 2/576 with felbamate, lamotrigine, oxcarbazepine and topiramate in the mouse maximal electroshock-induced seizure model

This study focused on the evaluation of interactions between MRZ 2/576 (8-chloro-4-hydroxy-1-oxo-1,2-dihydropyridazino(4,5-b)quinoline-5-oxide choline salt), an N-methyl-D-aspartate (NMDA) receptor antagonist acting at the NMDA receptor/glycine(B) site and four newer antiepileptic drugs (felbamate, lamotrigine, oxcarbazepine, and topiramate) in the mouse maximal electroshock seizure model. Results indicate that MRZ 2/576 administered intraperitoneally, 5 min before the test, exerted a clear-cut anticonvulsant effect in the maximal electroshock seizure test in mice and its ED(50) value was 13.71 (11.95-15.73) mg/kg. In the subthreshold method, MRZ 2/576 (administered intraperitoneally, at a subthreshold dose of 5 mg/kg) significantly enhanced the anticonvulsant action of felbamate, oxcarbazepine and topiramate, by reducing their ED(50) values from 73.0 to 53.8 mg/kg (p < 0.05) for felbamate, from 10.77 to 7.48 mg/kg (p < 0.05) for oxcarbazepine, and from 49.3 to 28.7 mg/kg (p < 0.01) for topiramate. In contrast, MRZ 2/576 (5 mg/kg, i.p.) did not significantly affect the antiseizure effects of lamotrigine in the maximal electroshock seizure test in mice. Isobolographic transformation of data revealed that MRZ 2/576 (5 mg/kg, i.p.) exerted barely additive interactions with all investigated antiepileptic drugs in the maximal electroshock seizure test. In conclusion, the isobolographic analysis revealed that MRZ 2/576 additively cooperates with newer antiepileptic drugs in terms of suppression of maximal electroshock-induced seizures in mice.

Influence of aminophylline on the anticonvulsive action of gabapentin in the mouse maximal electroshock seizure threshold model

Accumulating evidence indicates that aminophylline [theophylline(2) x ethylenediamine] markedly attenuates the anticonvulsant action of conventional antiepileptic drugs in experimental animal models of epilepsy and evokes severe seizure activity in patients treated with this methylxanthine. The objective of this study was to determine the influence of acute (single) and chronic (twice daily for 14 consecutive days) treatments with aminophylline on the anticonvulsant potential of gabapentin (a second-generation antiepileptic drug) in the mouse maximal electroshock seizure threshold model. Additionally, the effects of acute and chronic administration of aminophylline on the adverse effect potential of gabapentin in terms of motor coordination impairment were assessed in the chimney test. To evaluate pharmacokinetic characteristics of interaction between drugs, total brain concentrations of gabapentin and theophylline were estimated with high-pressure liquid chromatography and fluorescence polarization immunoassay, respectively. Results indicated that gabapentin (at doses of 75 and 100 mg/kg, i.p.) increased the threshold for electroconvulsions in mice. Aminophylline in non-convulsive doses of 50 and 100 mg/kg (i.p.), both in acute and chronic experiments, did not attenuate the anticonvulsant potential of gabapentin in the maximal electroshock seizure threshold test in mice. Similarly, aminophylline at a dose of 100 mg/kg had no impact on the adverse effect potential of gabapentin in the chimney test. Pharmacokinetic evaluation of total brain concentrations of gabapentin and theophylline revealed no significant changes in total brain concentrations of the drugs after both, acute and chronic applications of aminophylline in combination with gabapentin. The data show that aminophylline did not alter the ability of gabapentin to protect mice against seizures induced by electroconvulsive shock. The observed interaction between gabapentin and aminophylline in both acute and chronic experiments was pharmacodynamic in nature.

Synergistic combinations of anticonvulsant agents: what is the evidence from animal experiments?

PURPOSE

Combination therapy is often used in the treatment of seizures refractory to monotherapy. At the same time, the pharmacodynamic mechanisms that determine the combined efficacy of antiepileptic drugs (AEDs) are unknown, and this prevents a rational use of these drug combinations. We critically evaluate the existing evidence for pharmacodynamic synergism between AEDs from preclinical studies in animal models of epilepsy to identify useful combinations of mechanisms and to determine whether study outcome depends on the various research methods that are in use.

METHODS

Published articles were included if the studies were placebo-controlled, in vivo, or ex vivo animal studies investigating marketed or experimental AEDs. The animal models that were used in these studies, the primary molecular targets of the tested drugs, and the methods of interpretation were recorded. The potential association of these factors with the study outcome (synergism: yes or no) was assessed through logistic regression analysis.

RESULTS

In total, 107 studies were identified, in which 536 interaction experiments were conducted. In 54% of these experiments, the possibility of a pharmacokinetic interaction was not investigated. The majority of studies were conducted in the maximal electroshock model, and other established models were the pentylenetetrazole model, amygdala kindling, and the DBA/2 model. By far the most widely used method for interpretation of the results was evaluation of the effect of a threshold dose of one agent on the median effective dose (ED50) of another agent. Experiments relying on this method found synergism significantly more often compared with experiments relying on other methods (p<0.001). Furthermore, experiments including antagonists of the AMPA receptor were more likely to find synergism in comparison with all other experiments (p<0.001).

CONCLUSIONS

Intensive preclinical research into the effects of AED combinations has not led to an understanding of the pharmacodynamic properties of AED combinations. Specifically, the majority of the preclinical studies are not adequately designed to distinguish between additive, synergistic, and antagonistic interactions. Quantitative pharmacokinetic-pharmacodynamic studies of selectively acting AEDs in a battery of animal models are necessary for the development of truly synergistic drug combinations.

Isobolographic analysis of interaction between drugs with nonparallel dose-response relationship curves: a practical application

The objective of this study was to characterize the anticonvulsant and acute adverse-effect potentials of topiramate (TPM) and gabapentin (GBP)-two second-generation antiepileptic drugs administered alone and in combination in the maximal electroshock (MES)-induced seizures and chimney test in mice. The anticonvulsant and acute adverse effects of the combination of TPM with GBP at the fixed ratio of 1:1 were determined using the type I isobolographic analysis for nonparallel dose-response relationship curves (DRRCs). To ascertain any pharmacokinetic contribution to the observed interaction between TPM and GBP, total brain concentrations of both drugs were determined. The isobolographic analysis of interaction for TPM and GBP, whose DRRCs were not parallel in both MES and chimney tests, was accompanied with a presentation of all required calculations allowing the determination of lower and upper lines of additivity. The isobolographic analysis revealed that TPM combined with GBP at the fixed-ratio combination of 1:1 interacted supraadditively (synergistically) in terms of suppression of MES-induced seizures, and simultaneously, the combination produced additive interaction with respect to motor coordination impairment (adverse effects) in the chimney test. The evaluation of pharmacokinetic characteristics of interaction for the combination of TPM with GBP revealed that neither TPM nor GBP affected their total brain concentrations in experimental animals, and thus, the observed interaction in the MES test was pharmacodynamic in nature. In conclusion, the combination of TPM with GBP, because of supraadditivity in the MES test and additivity in terms of motor coordination impairment in the chimney test as well as lack of pharmacokinetic interactions between drugs, fulfilled the criterion of a favorable combination, worthy of recommendation in further clinical practice.

Pharmacokinetic and pharmacodynamic interactions of aminophylline and topiramate in the mouse maximal electroshock-induced seizure model

The aim of this study was to determine the influence of acute (single) and chronic (twice daily for 14 consecutive days) treatments with aminophylline (theophylline(2).ethylenediamine) on the anticonvulsant potential of topiramate (a broad-spectrum antiepileptic drug) in the mouse maximal electroshock-induced seizure model. Additionally, the effects of acute and chronic administration of aminophylline on the adverse effect potential of topiramate were assessed in the chimney test (motor performance). To evaluate pharmacokinetic characteristics of interaction between topiramate and aminophylline, total brain concentrations of topiramate and theophylline were estimated with fluorescence polarization immunoassay technique. Results indicate that aminophylline in non-convulsive doses of 50 and 100 mg/kg (i.p.), both in acute and chronic experiments, markedly attenuated the anticonvulsant potential of topiramate by raising its ED(50) value against maximal electroconvulsions. Aminophylline at a lower dose of 25 mg/kg did not affect significantly the ED(50) value of topiramate in the acute experiment, but the drug markedly increased the ED(50) value of topiramate during the chronic treatment in mice. Only, aminophylline at 12.5 mg/kg, in both acute and chronic experiments, did not affect the antielectroshock action of topiramate in mice. Moreover, aminophylline at a dose of 100 mg/kg had no impact on the adverse effect potential of topiramate in the chimney test. Pharmacokinetic evaluation of total brain concentrations of topiramate and theophylline revealed that topiramate significantly increased total brain theophylline concentrations following both acute and chronic applications of aminophylline. Conversely, aminophylline did not alter total brain concentrations of topiramate in mice. Based on this preclinical study, one can conclude that aminophylline attenuated the antiseizure action of topiramate in the mouse maximal electroshock-induced seizure model and the observed interaction between drugs was both pharmacokinetic and pharmacodynamic in nature.

Refractory epilepsy: mechanisms and solutions

Pharmacoresistance remains a major challenge in epilepsy management. The availability of ten new antiepileptic drugs since the late 1980s has not dramatically improved the outcome of refractory epilepsy. This article provides an overview of the contemporary understanding of epilepsy and the limitation of current treatment modalities, discusses putative biological mechanisms of medical intractability and reviews some of the novel strategies under investigation to overcome the challenge of pharmacoresistance.