Michaelis-Menten kinetics of stiripentol in normal humans

Study of the metabolic stability profiles of perampanel, rufinamide and stiripentol and prediction of drug interactions using HepaRG cells as an in vitro human model

New-generation antiepileptic drugs as perampanel, rufinamide and stiripentol emerged as alternatives in chronic epilepsy polytherapy. Hence, their metabolic stability and potential involvement in relevant drug-drug interactions (DDI) are of great clinical interest, being HepaRG cells herein used as an in vitro human model. To characterize their metabolic stability profiles, HepaRG cells were incubated with perampanel (1 μM), rufinamide (100 μM) or stiripentol (5 μM) for 12-h. HepaRG cells, pretreated with known CYP450 isoenzymes inducers (rifampicin, phenytoin, phenobarbital, omeprazole and carbamazepine), were also incubated with perampanel, rufinamide or stiripentol to assess possible DDI mediated by CYP induction. Results suggest a considerable decrease in perampanel and stiripentol concentrations over 12-h; contrary, rufinamide concentrations did not variated. Cells pretreatment with all inducers significantly decreased stiripentol concentrations (between 20.3% and 31.9%), suggesting a considerable potential for DDI. Rufinamide concentrations only decreased when preincubated with rifampicin and with the highest tested concentrations of the remaining inducers. Perampanel levels decreased with rifampicin, carbamazepine and phenobarbital, supporting the involvement of CYP3A4-mediated metabolism. Besides relevant information concerning the metabolic stability profile and potential DDIs of the new antiepileptics here studied, it was also reinforced the HepaRG cells suitability as a reliable in vitro model to foresee in vivo metabolism in humans.

A Phase 1, Open-Label, Pharmacokinetic Trial to Investigate Possible Drug-Drug Interactions Between Clobazam, Stiripentol, or Valproate and Cannabidiol in Healthy Subjects

GW Pharmaceuticals' formulation of highly purified cannabidiol oral solution is approved in the United States for seizures associated with Lennox-Gastaut and Dravet syndromes in patients aged ≥2 years, for which clobazam, stiripentol, and valproate are commonly used antiepileptic drugs. This open-label, fixed-sequence, drug-drug interaction, healthy volunteer trial investigated the impact of cannabidiol on steady-state pharmacokinetics of clobazam (and N-desmethylclobazam), stiripentol, and valproate; the reciprocal effect of clobazam, stiripentol, and valproate on cannabidiol and its major metabolites (7-hydroxy-cannabidiol [7-OH-CBD] and 7-carboxy-cannabidiol [7-COOH-CBD]); and cannabidiol safety and tolerability when coadministered with each antiepileptic drug. Concomitant cannabidiol had little effect on clobazam exposure (maximum concentration [Cmax ] and area under the concentration-time curve [AUC], 1.2-fold), N-desmethylclobazam exposure increased (Cmax and AUC, 3.4-fold), stiripentol exposure increased slightly (Cmax , 1.3-fold; AUC, 1.6-fold), while no clinically relevant effect on valproate exposure was observed. Concomitant clobazam with cannabidiol increased 7-OH-CBD exposure (Cmax , 1.7-fold; AUC, 1.5-fold), without notable 7-COOH-CBD or cannabidiol increases. Stiripentol decreased 7-OH-CBD exposure by 29% and 7-COOH-CBD exposure by 13%. There was no effect of valproate on cannabidiol or its metabolites. Cannabidiol was moderately well tolerated, with similar incidences of adverse events reported when coadministered with clobazam, stiripentol, or valproate. There were no deaths, serious adverse events, pregnancies, or other clinically significant safety findings.

Audit of use of stiripentol in adults with Dravet syndrome

Objectives

There are very few data available in the literature on the use of stiripentol in adults with Dravet syndrome (DS). DS cases are increasingly recognized in adulthood, and more children with DS now survive to adulthood. The aim of the study was to document the effectiveness and tolerability of stiripentol in adults with DS.

Material and methods

We conducted an observational clinical audit in the epilepsy service of the National Hospital for Neurology and Neurosurgery, London (UK).

Results

We included 13 adult subjects with DS (eight females, five males). The responder (defined as more than 50% reduction in all seizure types) rate was 3/13 (23%) at 36 months. The following other outcomes were reported: seizure exacerbation (3/13, 23%), no change (3/13, 23%), less than 50% reduction in seizures (2/13, 15%), more than 50% reduction in generalized tonic‐clonic seizures but no other seizure types (1/13, 8%), undefined response (1/13, 8%). The retention rate was 62% after 1 year and 31% after 5 years. Adverse effects were reported in 7/13 (54%): the most frequent were anorexia, weight loss, unsteadiness and tiredness. Withdrawal due to adverse effects occurred in 3/13 (23%).

Conclusions

Compared with previous studies on children with DS, our results show a lower responder rate and a similar tolerability profile. Stiripentol can be effective with a good tolerability profile. Our audit is small, but supports the use of stiripentol in adults with DS when first‐line treatments are ineffective or not tolerated, in keeping with published guidelines.

Stiripentol and vigabatrin current roles in the treatment of epilepsy

INTRODUCTION

Stiripentol and vigabatrin are the two anticonvulsant drugs currently approved in severe infantile-onset epilepsies, respectively Dravet syndrome and infantile spasms.

AREAS COVERED

For both, the indication was discovered by chance thanks to an exploratory study. Both demonstrated indisputable efficacy through randomized-controlled trials. Stiripentol as adjunctive therapy to clobazam and valproate performed better than placebo, and vigabatrin as first-line monotherapy better than the reference steroid therapy in spasms due to tuberous sclerosis. At one-year treatment vigabatrin and steroids were equally efficient in the other etiologies of spasms. However, it took more than 20 years for both drugs to be approved world-wide.

EXPERT OPINION

Stiripentol suffered from pharmacokinetic potentiation of clobazam, thus raising the question whether it was efficient per se. Finally, animal models and pharmacogenetic data on CYP2C19 confirmed its specific anticonvulsant effect. Stiripentol (in comedication with clobazam and valproate) is therefore to be recommended for Dravet patients. Vigabatrin was found to have a frequent and irreversible retinal toxicity, which required an alternative visual testing to be detected in young children. Today the benefit/risk ratio of vigabatrin as first-line is considered to be positive in infantile spasms, given the severity of this epilepsy and the lack of a safer alternative therapy.

Pharmacological considerations in the use of stiripentol for the treatment of epilepsy

INTRODUCTION

Despite the fact that more than 20 antiepileptic drugs (AEDs) are currently available, about one-third of patients still present drug resistance. Further efforts are required to develop novel and more efficacious therapeutic strategies, especially for refractory epileptic syndromes showing few and anecdotic therapeutic options.

AREAS COVERED

Stiripentol (STP) is a second generation AED that shows GABAergic activity, with immature brain selectivity, and an indirect metabolic action on co-administered AEDs. Two pivotal studies demonstrated STP efficacy in patients with Dravet syndrome with refractory partial seizures, and marketing authorization in Europe, Canada and Japan was granted thereafter. Post-marketing surveys reported a good efficacy and tolerability profile. In addition, interesting data is currently emerging regarding off-label experimentation of STP in other forms of epilepsy.

EXPERT OPINION

STP is an important addition to the limited treatment options available for patients resistant to common AEDs. The possibility to inhibit seizures through the metabolic pathway of lactate dehydrogenase and the inhibitory effects on the entry of Na(+) and Ca(2+) are the most recent findings to emerge about STP and could be proof of its neuroprotective action. Moreover, its positive effects on cognitive function, its good safety and tolerability profile and the increasing data about STP efficacy on other refractory epileptic syndromes may prove to be fertile grounds for further investigation.

Pharmacokinetic of antiepileptic drugs in patients with hepatic or renal impairment

Many factors influence choice of antiepileptic drugs (AEDs), including efficacy of the drug for the indication (epilepsy, neuropathic pain, affective disorder, migraine), tolerability, and toxicity. The first-generation AEDs and some newer AEDs are predominately eliminated by hepatic metabolism. Other recent AEDs are eliminated by renal excretion of unchanged drug or a combination of hepatic metabolism and renal excretion. The effect of renal and hepatic disease on the dosing will depend on the fraction of the AED eliminated by hepatic and/or renal excretion, the metabolic isozymes involved, as well as the extent of protein binding, if therapeutic drug monitoring is used. For drugs that are eliminated by renal excretion, methods of estimating creatinine clearance can be used to determine dose adjustments. For drugs eliminated by hepatic metabolism, there are no specific markers of liver function that can be used to provide guidance in dosage adjustments. Based on studies with probe drugs, the hepatic metabolic enzymes are differentially affected depending on the cause and severity of hepatic disease, which can aid in predicting dose adjustment when clinical data are not available. Several AEDs are also associated with laboratory markers of mild hepatic dysfunction and, rarely, more severe hepatic injury. In contrast, the risk of renal injury from AEDs is generally low. In general, co-morbid hepatic or renal diseases influence the decision for the selection of an AED. For some patients dosing changes to their existing AEDs may be appropriate. For others, a change to another AED may be a better option.

Stiripentol : in severe myoclonic epilepsy of infancy (dravet syndrome)

Stiripentol is an anticonvulsant used as adjunctive therapy with valproate and clobazam in the management of patients with severe myoclonic epilepsy of infancy (SMEI; Dravet syndrome), a rare form of epilepsy that develops in the first year of life and is subsequently associated with significant morbidity and mortality. Results of a randomized, double-blind trial, in which patients (≥3 years of age) whose SMEI was inadequately controlled with valproate and clobazam received adjunctive therapy with stiripentol or placebo for 2 months, showed a significantly higher response rate in the stiripentol group compared with the placebo group (71 % vs. 5 %; p < 0.0001; primary endpoint). Responders were defined as those patients who experienced a ≥50 % reduction in clonic or tonic-clonic seizure frequency during the second month of the double-blind period compared with baseline. Almost half of the stiripentol recipients were seizure free during this period compared with none in the placebo group. Stiripentol was also statistically superior to placebo for secondary efficacy outcomes in the randomized controlled trial, which included the median number of seizures during the second month of the double-blind period and the mean percentage change from baseline in seizure frequency. These results are supported by efficacy data from other studies in patients with SMEI treated with stiripentol as adjunctive therapy, including a long-term retrospective analysis, prospectively conducted open-label studies and a meta-analysis. Drowsiness, loss of appetite and weight loss are the most frequently reported adverse events with stiripentol, and the drug inhibits various cytochrome P450 isoenzymes, potentially leading to clinically significant drug interactions. Stiripentol is an important addition to the limited treatment options available for the management of patients with SMEI.

[Therapeutic drug monitoring of stiripentol]

Stiripentol is a third generation antiepileptic, marketed since 2007 under the name of Diacomit(®). It is indicated, always in combination, in the treatment of severe myoclonic epilepsy in infancy or Dravet syndrome. Its pharmacokinetics is not linear. It is a potent inhibitor of CYP3A4, 1A2 and 2C19 and increases the plasma concentrations of many other antiepileptic drugs. Without this being considered as a validated therapeutic range, the trough plasma concentrations at steady-state, corresponding to the usual doses are between 10 and 15 mg/L. The concentration-efficacy relationship is not established, but there is some evidence for a concentration-related toxicity. However, because of its non-linear kinetics, stiripentol should be a good candidate for therapeutic drug monitoring (TDM). Nonetheless, the current level of evidence for the advantage of TDM is "remains to be estimated".

Biphasic characteristic of interactions between stiripentol and carbamazepine in the mouse maximal electroshock-induced seizure model: a three-dimensional isobolographic analysis

The anticonvulsant effects produced by stiripentol (STP), carbamazepine (CBZ), and their combination in the maximal electroshock (MES)-induced seizures in mice were investigated using three-dimensional (3D) isobolographic analysis. With 3D isobolography, the combinations of both drugs at the fixed-ratios of 1:3, 1:1, and 3:1 for 16%, 50% and 84% antiseizure effects, respectively, were examined in order to evaluate the preclinical characteristics of the interactions between STP and CBZ. Additionally, to characterize precisely the types of interactions observed in the MES test, free plasma and total brain CBZ concentrations were estimated for all fixed-ratios tested. The 3D isobolographic analysis showed that STP and CBZ combined at the fixed-ratio of 1:3 produced supra-additive (synergistic) interactions in the MES test for the anticonvulsant effects ranging between 16% and 84%. In contrast, the combination of STP with CBZ at the fixed-ratio of 3:1 exerted sub-additive (antagonistic) interactions in 3D isobolography for all antiseizure effects examined in the MES test. Only the combination of STP and CBZ at the fixed-ratio of 1:1 was additive for the investigated effects (16%, 50% and 84%) in 3D isobolography. Pharmacokinetic evaluation of CBZ concentrations revealed that STP increased both free plasma and total brain CBZ concentrations for all fixed-ratio combinations tested (1:3, 1:1 and 3:1). In conclusion, the 3D isobolographic findings suggest that the combination of STP with CBZ exerted biphasic characteristics of interactions in the MES test, despite the pharmacokinetic increase in CBZ content in plasma and brains of experimental animals.

Stiripentol

Stiripentol (STP) is a new antiepileptic compound produced by Biocodex. It is not structurally related to any of the other currently marketed antiepileptic products as it belongs to the group of aromatic allylic alcohols. It has recently been proved to increase GABAergic transmission in experimental models. It has been studied and used in France and Canada for > 10 years, but its clinical development was delayed due to the inhibitory effect of STP on hepatic cytochrome P450 (CYP). Clinical studies were based on the fact that STP also acts as an inhibitor of CYP3A4, CYP1A2 and CYP2C19 in vivo in epileptic patients. Although the studies in adult patients were disappointing, the trials conducted in paediatric populations demonstrated a specific efficacy of STP in a severe form of early childhood epilepsy, Dravet syndrome (severe myoclonic epilepsy in infancy), when combined with valproate and clobazam. Based on these results, STP was granted orphan drug status in the European Union for the treatment of Dravet syndrome. The French experience in compassionate use suggests that STP might also be of benefit when combined with carbamazepine in paediatric patients with pharmacoresistant partial epilepsy. Nevertheless, two controlled adjunctive-therapy trials were recently completed in paediatric populations with epilepsy. The interactions of STP with a large number of drugs need to be carefully taken into account by adjusting the doses of the combined antiepileptic drugs in order to improve the tolerability of the therapeutic association.

The clinical pharmacokinetics of the newer antiepileptic drugs. Focus on topiramate, zonisamide and tiagabine

Following the introduction of felbamate, gabapentin, lamotrigine, oxcarbazepine and vigabatrin in the early 1990s, other new antiepileptic drugs have been advancing in clinical development. Those most extensively evaluated to date include topiramate, zonisamide and tiagabine. Topiramate, licensed recently in the UK, acts multifactorially through the blockade of sodium channels and kainate/AMPA receptors, enhancement of gamma-aminobutyric acid (GABA)ergic transmission and inhibition of carbonic anhydrase. It is well absorbed from the gastrointestinal tract and negligibly bound to plasma proteins. When used as a monotherapy, topiramate is eliminated primarily in the urine in an unchanged form with a half-life of 20 to 30 hours; elimination is faster in patients receiving concurrent medication with enzyme-inducing anticonvulsants, in whom the extent of biotransformation becomes more prominent. Zonisamide, which has been commercially available in Japan for some years, also has a multifactorial mode of action, possibly involving the blockade of sodium channels, T-type calcium channels and inhibition of carbonic anhydrase. It is rapidly absorbed, 50% bound to plasma proteins and is eliminated predominantly by biotransformation; zonisamide has a half-life of 50 to 70 hours in monotherapy patients, or 25 to 35 hours in patients comedicated with enzyme-inducing anticonvulsants. Tiagabine, a nipecotic acid derivative which inhibits GABA reuptake, is rapidly and completely absorbed after oral intake. It is highly (96%) bound to plasma proteins and it is eliminated primarily by cytochrome P450 3A-mediated oxidation, with a half-life of about 7 hours in healthy volunteers. Tiagabine metabolism is also enhanced by concurrent medication with enzyme-inducing anticonvulsants, resulting in a need to use dosages larger than those required in monotherapy or valproic acid (sodium valproate)-treated patients. Additional investigational antiepileptic agents included in this article are rufinamide (CGP 33101), fosphenytoin, levetiracetam, losigamone, remacemide and stiripentol. All these drugs have undergone early characterisation with respect to pharmacokinetic features and interaction potential.

New anticonvulsant drugs. Focus on flunarizine, fosphenytoin, midazolam and stiripentol

In the past decade, several new antiepileptic drugs have been tested. Most recently, 5 new antiepileptic drugs have been launched onto European and US markets. These include vigabatrin, oxcarbazepine and lamotrigine in Europe, and felbamate and gabapentin in the US. In addition to these, 3 additional drugs are in the clinical investigational stage: flunarizine, fosphenytoin and stiripentol. A fourth agent is midazolam, which was originally introduced in 1986, but recently has shown effectiveness in the treatment of status epilepticus. Flunarizine is a selective calcium channel blocker that has shown anticonvulsant properties in both animal and human studies. It is a long-acting anticonvulsant that clinical studies have shown to have effects similar to those of phenytoin and carbamazepine in the treatment of partial, complex partial and generalised seizures. Fosphenytoin was developed to eliminate the poor aqueous solubility and irritant properties of intravenous phenytoin. It is rapidly converted to phenytoin after intravenous or intramuscular administration. In clinical studies, this prodrug showed minimal evidence of adverse events and no serious cardiovascular or respiratory adverse reactions. It may have a clear advantage over the present parenteral formulation of phenytoin. Midazolam is a benzodiazepine that is more potent than diazepam as a sedative, muscle relaxant and in its influence on electroencephalographic measures. It has been shown to be an effective treatment for refractory seizures in status epilepticus. Stiripentol has anticonvulsant properties as well as the ability to inhibit the cytochrome P450 system. There are significant metabolic drug interactions between stiripentol and phenytoin, carbamazepine and phenobarbital (phenobarbitone). Stiripentol has been studied in patients with partial seizures, refractory epilepsy and refractory absence seizures with some efficacious results.

Antiepileptic drugs in development: prospects for the near future

Among some 14 new antiepileptic drugs (AEDs), those most extensively tested in humans include felbamate (FBM), gabapentin (GBP), lamotrigine (LTG), oxcarbazepine (OCBZ), vigabatrin (VGB), and zonisamide (ZNS). All are currently marketed in some but not all countries. Although no large, comparative studies on efficacy have been conducted, all of these new AEDs are effective in adult localization-related epilepsies, and some have activity in specific syndromes. Although these drugs all have some CNS side effects, especially when administered in combination with other AEDs, they also all have low toxicity profiles. The availability of AEDs with different mechanisms of action may facilitate rational polytherapy. FBM is not teratogenic in animals. Half-life of FBM in humans is 11-28 h. Daily FBM dosages are 15-45 mg/kg in children and 2,400-4,800 mg in adults. Side effects include insomnia and anorexia, with weight loss. FBM increases phenytoin (PHT) and valproate (VPA) concentrations, and FBM concentration may be affected by other drugs. It is available in the United States for treatment of Lennox-Gastaut syndrome and partial seizures in adults. GBP is very water soluble. Half-life of GBP in humans is 5-7 h and daily dosages range from 900 to 2,400 mg in adults. Few side effects have been observed. GBP is not metabolized by the liver and has no drug interactions. It is available in the United Kingdom and the United States. LTG has no teratogenicity in animal models. Half-life of LTG in humans depends on co-medication: with enzyme inducers it is 15-24 h, and with VPA it is approximately 60 h. LTG dosages are 100-600 mg/day in adults. LTG is available in Europe. OCBZ is rapidly metabolized to 10,11-dihydro-10-hydroxy-carbazepine (MHD), the active compound. Animal studies have shown similar efficacy but superior toxicity to carbamazepine (CBZ) in animal models. For MHD, half-life ranges from 10 to 15 h in patients. OCBZ dosages range from 300 to 1,800 mg/day. VGB is a potent, irreversible inhibitor of GABA transaminase which elevates GABA levels in the CNS. Daily dosages of 2,000-4,000 mg of VGB are needed in adults. Although intramyelinic edema has developed in rats and dogs, it has not yet presented in other mammals or humans. ZNS is a sulfonamide effective in animal models of epilepsy. Half-life of ZNS is 27-36 h. ZNS daily dosage is 400-600 mg. ZNS has been effective in some cases of Baltic myoclonic epilepsy.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

Disposition of stiripentol in the pregnant and non-pregnant female rat

1. The disposition of stiripentol labelled with 14C and 3H on two positions has been studied in the pregnant and non-pregnant female rat after p.o. administration of a 200 mg/kg dose. 2. For both labelled species radioactivity was eliminated mainly in the faeces (69% within 72 h). Urinary excretion was rather low (22% within 72 h). No significant difference was found between the disposition of the two labelled species. 3. For both labelled species concentrations of radioactivity reached a plateau in the plasma and tissues between 1 and 6 h after administration. The liver, fat, mammary gland and adrenal gland were the most extensively-labelled organs. The affinity for the mammary gland was significantly greater in pregnant rats and for the adrenal gland was significantly greater in the non-pregnant rats. The fact that the concentration in the placenta was higher than in the foetus demonstrated that this membrane acts as a barrier for the penetration of the drug in the amniotic fluid. 4. Chromatographic analysis of the faeces and urine showed that an important portion of the dose remained unabsorbed through the gastrointestinal tract. The absorbed fraction undergoes an extensive first-pass metabolism involving mainly the oxidative cleavage of the methylenedioxy ring. Comparison with the results of other work conducted on the non-pregnant rat demonstrated that pregnancy did not affect the disposition and metabolic process.

Comparative pharmacokinetics of the newer antiepileptic drugs

During the past few years a major increase has taken place in the number of drugs which have become available in the antiepileptic arsenal. In fact, 3 new antiepileptic drugs, vigabatrin, oxcarbazepine and lamotrigine, were recently approved in several European countries. Two other drugs, felbamate and gabapentin, are expected to be approved in the US in the near future. This review comparatively evaluates the pharmacokinetics of the following 10 new antiepileptic drugs: felbamate, flunarizine, gabapentin, lamotrigine, oxcarbazepine, remacemide, stiripentol, tiagabine, topiramate and vigabatrin. Three of the new drugs, gabapentin, topiramate and vigabatrin, are more promising on the basis of their pharmacokinetic features. They are well absorbed, excreted mainly unchanged in the urine, and are not susceptible to enzyme induction or inhibition. Their drug interaction potential appears to be minimal. About 50% of felbamate is excreted unchanged, with the rest eliminated by metabolism. The remaining drugs are eliminated by metabolic processes such as glucuronidation (lamotrigine), deglycine formation (remacemide) or oxidative metabolism (flunarizine and stiripentol). Oxcarbazepine and remacemide have high hepatic clearance and are biotransformed to hydroxy and deglycine metabolites, respectively, with the activity of their metabolites contributing to the antiepileptic activity of the parent drug after oral administration, despite high first-pass effect metabolism. Gabapentin and oxcarbazepine do not behave pharmacokinetically as their original design intended. Gabapentin is not effective as a chemical drug delivery system for gamma-aminobutyric acid (GABA), and oxcarbazepine serves as a prodrug to its hydroxy metabolite, but does not act as a drug on its own. Nevertheless, these 2 agents demonstrate efficacy in extensive preclinical and clinical trials. Although the pharmacokinetics features of these drugs are important, these features are secondary to their pharmacodynamic properties--i.e. to the requirement that new antiepileptic drugs have to have proven clinical efficacy and safety in epileptic patients.

Stiripentol in atypical absence seizures in children: an open trial

Stiripentol (STP) was added to the antiepileptic drug (AED) regimen of 10 patients with uncontrolled atypical absence seizures (more than one seizure a day). Seven boys and three girls aged 6-16 years participated in the study. Concomitant AEDs included various combinations of phenobarbital (PB), phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA). Parents counted daily seizures over a 4-week baseline period before institution of STP, and in a 20-week period during STP therapy. To compensate for drug interactions, doses of other AEDs were adjusted during STP administration to keep serum levels close to levels of the baseline period. Maintenance doses of STP were 1,000-3,000 mg/day, giving serum levels of 4-22 micrograms/mL. All patients experienced a decrease in atypical absence seizures. Average decrease was 70% (range 5-95%). Side effects experienced by some patients were dose related and included anorexia, nausea, vomiting, and lethargy. In only 1 patient did an adverse effect (vomiting) require discontinuation of STP. We conclude that STP shows promise in treatment of atypical absence seizures in children, and further trials are warranted.

Stiripentol in acute/chronic efficacy tests in monkey model

Acute and chronic efficacy tests of stiripentol (4,4-dimethyl-1-[3,4-(methylenedioxy)-phenyl]-1-penten-3-ol) were conducted in alumina-gel rhesus monkeys. In the acute study (n = 6), discrete serial seizures precipitated by 150 mg/kg of 4-deoxypyridoxine hydrochloride were challenged by intravenous administration of stiripentol and the data compared with those obtained with valproate similarly tested in other monkeys (reported here) and with those from four other standard anticonvulsants (phenytoin, carbamazepine, phenobarbital, and diazepam--data published previously). In the acute challenge (Study 1), stiripentol performed comparably to valproate by delaying the onset of seizures but not eliminating them as did the other four drugs. In two separate chronic studies (at different doses, n = 6 each), stiripentol was given every 4 h by gastric catheter for 4 weeks, preceded and followed by 4 weeks of baseline. In these studies, stiripentol significantly reduced EEG interictal spike rates at mean plasma concentrations from 20 to 27 micrograms/ml in Study 2 and 11 to 14 micrograms/ml in Study 3. From these results, and those evinced in other studies, it appears that stiripentol should be evaluated for absence epilepsy and possible synergistic effects in polytherapy.