Stiripentol in acute/chronic efficacy tests in monkey model

An Update on Stiripentol Mechanisms of Action: A Narrative Review

Stiripentol (Diacomit®) (STP) is an orally active antiseizure medication (ASM) indicated as adjunctive therapy, for the treatment of seizures associated with Dravet syndrome (DS), a severe form of childhood epilepsy, in conjunction with clobazam and, in some regions valproic acid. Since the discovery of STP, several mechanisms of action (MoA) have been described that may explain its specific effect on seizures associated with DS. STP is mainly considered as a potentiator of gamma-aminobutyric acid (GABA) neurotransmission: (i) via uptake blockade, (ii) inhibition of degradation, but also (iii) as a positive allosteric modulator of GABAA receptors, especially those containing α3 and δ subunits. Blockade of voltage-gated sodium and T-type calcium channels, which is classically associated with anticonvulsant and neuroprotective properties, has also been demonstrated for STP. Finally, several studies indicate that STP could regulate glucose energy metabolism and inhibit lactate dehydrogenase. STP is also an inhibitor of several cytochrome P450 enzymes involved in the metabolism of other ASMs, contributing to boost their anticonvulsant efficacy as add-on therapy. These different MoAs involved in treatment of DS and recent data suggest a potential for STP to treat other neurological or non-neurological diseases.

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.

Stiripentol exhibits higher anticonvulsant properties in the immature than in the mature rat brain

PURPOSE

After the first positive experimental data in rodents in the early 1970s demonstrating the anticonvulsant effect of stiripentol (STP), in vitro studies showed that STP acts directly on γ-aminobutyric acid A (GABAA ) receptors. Chloride influx is higher when these receptors contain an α3 subunit, leading to the hypothesis that STP might exhibit higher efficacy in the immature brain.

METHODS

We explored this issue by studying the efficacy of STP in P21 and P75 rats using the pentylenetetrazol model of acute seizures or the lithium-pilocarpine status epilepticus model. P21 and adult rats received vehicle, 150, 250, or 350 mg/kg of STP, i.p., 1 h before evaluating the anticonvulsant. We also studied the blood and brain levels of STP as well as the expression and the messenger RNA (mRNA) levels of the α3 subunit of the GABAA receptors at both ages.

KEYS FINDINGS

STP exhibited anticonvulsant properties in both models at both ages, but STP was more effective in P21 than in P75 rats. This was shown by the significant suppression of seizure or status epilepticus occurrence in P21 with 350 mg/kg STP, whereas the same dose had no significant effect at P75. The blood level, brain level, and blood/brain ratio of STP did not explain these differences between the two age groups. Moreover, the higher anticonvulsant properties in the immature brain were not explained by the mRNA level or protein expression of the GABAA α3 subunit at either age.

SIGNIFICANCE

Stiripentol exhibits higher anticonvulsant properties in the immature than in the mature brain. These findings require further investigation because it might lead to new clinical developments.

Interactions of stiripentol with clobazam and valproate in the mouse maximal electroshock-induced seizure model

The aim of this study was to characterize the anticonvulsant effects of stiripentol (STP) in combination with clobazam [CLB], and valproate [VPA]) in the mouse maximal electroshock (MES)-induced seizure model using the type I isobolographic analysis for parallel and non-parallel dose-response relationship curves (DRRCs). Potential adverse-effect profiles of interactions of STP with CLB and VPA at the fixed-ratio of 1:1 in the MES test with respect to motor performance, long-term memory and skeletal muscular strength were measured along with total brain antiepileptic drug concentrations. In the mouse MES model, STP administered singly had its DRRC non-parallel to that for CLB and, simultaneously, parallel to that for VPA. With type I isobolography for parallel DRRCs, the combinations of STP with VPA at three fixed-ratios of 1:3, 1:1 and 3:1 exerted sub-additive (antagonistic) interaction. Isobolography for non-parallel DRRCs revealed that the combination of STP with CLB at the fixed-ratio of 1:1 produced additive interaction. For all combinations, neither motor coordination, long-term memory nor muscular strength was affected. Total brain antiepileptic drug concentrations revealed bi-direction changes with the most profound being an 18.6-fold increase in CLB by STP and a 2.3-fold increase in STP by VPA. In conclusion, the additive interaction between STP and CLB was associated with a concurrent pharmacokinetic interaction and these data may explain the clinical efficacy seen with this combination. In contrast, the antagonism between STP and VPA was surprising since synergism is observed clinically.

Characterization of the Anticonvulsant, Behavioral and Pharmacokinetic Interaction Profiles of Stiripentol in Combination with Clonazepam, Ethosuximide, Phenobarbital, and Valproate Using Isobolographic Analysis

PURPOSE

Isobolographic analysis was used to characterize the interactions between stiripentol (STP) and clonazepam (CZP), ethosuximide (ETS), phenobarbital (PB), and valproate (VPA) in suppressing pentylenetetrazole (PTZ)-induced clonic seizures in mice.

METHODS

The anticonvulsant and acute adverse (neurotoxic) effects of STP in combination with the various conventional antiepileptic drugs (AEDs), at fixed ratios of 1:3, 1:1, and 3:1, were evaluated in the PTZ and chimney tests in mice using the isobolographic analysis. Additionally, protective indices (PI) and benefit indices (BI) were calculated to identify their pharmacological profiles so that a ranking in relation to advantageous combination could be established. Moreover, adverse-effect paradigms were determined by use of the step-through passive avoidance task (long-term memory), threshold for the first pain reaction, grip-strength test (neuromuscular tone), and the hot plate test (acute thermal pain). Brain AED concentrations were also measured so as to ascertain any pharmacokinetic contribution to the pharmacodynamic interactions.

RESULTS

All AED combinations comprising of STP and CZP, ETS, PB, and VPA (at the fixed ratios of 1:3, 1:1 and 3:1) were additive in terms of clonic seizure suppression in the PTZ test. However, these interactions were complicated by changes in brain AED concentrations consequent to pharmacokinetic interactions. Thus STP significantly increased total brain ETS and PB concentrations, and decreased VPA concentrations, but was without effect on CZP concentrations. In contrast, PB significantly decreased and VPA increased total brain STP concentrations while CZP and ETS were without effect. Furthermore, while isobolographic analysis revealed that STP and CZP in combination, at the fixed ratios of 1:1 and 3:1, were supraadditive (synergistic; p < 0.05), the combinations of STP with CZP (1:3), ETS, PB, or VPA (at all fixed ratios of 1:3, 1:1, and 3:1) were barely additivity in terms of acute neurotoxic adverse effects in the chimney test. Additionally, none of the examined combinations of STP with conventional AEDs (CZP, ETS, PB, VPA--at their median effective doses from the PTZ-test) affected long-term memory, threshold for the first pain reaction, neuromuscular tone, and acute thermal pain.

CONCLUSIONS

Based on BI values, the combination of STP with PB at the fixed ratio of 1:3 appears to be a particularly favourable combination. In contrast, STP and CZP or ETS (at the fixed ratios of 1:1 and 3:1) were unfavorable combinations. However, these conclusions are confounded by the fact that STP is associated with significant pharmacokinetic interactions. The remaining combinations of STP with PB (1:1 and 3:1), CZP (1:3), ETS (1:3), and VPA (at all fixed ratios of 1:3, 1:1, and 3:1) do not appear to be potential favorable AED combinations.

Stiripentol, a Putative Antiepileptic Drug, Enhances the Duration of Opening of GABAA-Receptor Channels

PURPOSE

Stiripentol (STP) is currently an efficient drug for add-on therapy in infantile epilepsies because it improves the efficacy of antiepileptic drugs (AEDs) through its potent inhibition of liver cytochromes P450. In addition, STP directly reduces seizures in several animal models of epilepsy, suggesting that it might also have anticonvulsive effects of its own. However, its underlying mechanisms of action are unknown.

METHODS

We examined the interactions of STP with gamma-aminobutyric acid (GABA) transmission by using patch-clamp methods in CA3 pyramidal neurons in the neonatal rat.

RESULTS

STP markedly increased miniature inhibitory postsynaptic current (mIPSC) decay-time constant in a concentration-dependent manner. The prolongation of mIPSC duration does not result from an interaction with GABA transporters because it persisted in the presence of GAT-1 inhibitors (SKF-89976A and NO-711). An interaction with benzodiazepine or neurosteroid binding sites also was excluded because STP-mediated increase of decay time was still observed when these sites were initially saturated (by clobazam, zolpidem, or pregnanolone) or blocked (by flumazenil or dehydroepiandrosterone sulfate), respectively. In contrast, saturating barbiturate sites with pentobarbital clearly occluded this effect of STP, suggesting that STP and barbiturates interact at the same locus. This was directly confirmed by using outside-out patches, because STP increased the duration and not the frequency of opening of GABAA channels.

CONCLUSIONS

At clinically relevant concentrations, STP enhances central GABA transmission through a barbiturate-like effect, suggesting that STP should possess an antiepileptic effect by itself.

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.

Stiripentol: efficacy and tolerability in children with epilepsy

PURPOSE

Stiripentol (STP) is a new antiepileptic drug (AED) that inhibits cytochrome P450, resulting in increased plasma concentrations of concomitant AEDs. The efficacy and tolerability of STP as an add-on therapy in children were assessed.

METHODS

Two hundred twelve patients with refractory epilepsy, aged from 1 month to 20.5 years, received STP either in a single-blind, placebo-controlled trial (108 patients) or in a further open trial (104 other patients selected by epilepsy syndrome for possible efficacy based on the results of the previous trial).

RESULTS

Among the 97 patients who could be analyzed for efficacy in the placebo-controlled study, the median seizure frequency was lower at 3 months with STP than with the placebo (p<0.0001); 49% responded to the drug, including 10% who became seizure free. Patients with partial epilepsy had the highest response rate (57%). Results were confirmed in the open study where 68% of the 91 patients receiving STP responded at 3 months. These patients were mainly those with partial epilepsy (73%) who were receiving carbamazepine (CBZ) (75%) as comedication (p<0.001). Ten of the 20 children with severe myoclonic epilepsy in infancy also responded with clobazam (CLB) as comedication. Efficacy was sustained long term in 74% of the 94 patients still receiving STP at a mean 30-month follow-up. Adverse events were reported in 48% of the 212 patients, mainly anorexia and loss of weight, but these events required STP discontinuation in only nine cases. Side effects were minimized in the open trial by optimizing the dose of comedication.

CONCLUSIONS

STP seems to be a promising add-on drug, particularly when combined with CBZ in patients with partial childhood epilepsy refractory to vigabatrin (VGB) and with CLB in patients with severe myoclonic epilepsy in infancy.

New antiepileptic drugs--an explosion of activity

The low therapeutic index of established antiepileptic drugs coupled with a better understanding of the pathophysiology of seizure production has led to the development of a range of new therapeutic agents for the treatment of epilepsy. In this review, the three drugs recently licensed in the UK (vigabatrin, lamotrigine and gabapentin) are profiled, together with several of the more promising up-and-coming compounds (oxcarbazepine, felbamate, tiagabine, stiripentol, remacemide and topiramate). Future avenues for clinical research in the pharmacological management of the epilepsies involve their rational use both singly and in combination.

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.

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.

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.

Chemical systems for delivery of antiepileptic drugs to the central nervous system

The chemical delivery system (CDS) approach, a recently developed procedure conceived to enhance the specific central nervous system (CNS) uptake of drugs, has been applied to several antiepileptic agents. CDSs based on dihydropyridine<-->pyridinium salt type redox targetors, reversibly linked to the drug, were designed, synthesized and tested for some traditional (phenytoin, valproate) and potential (stiripentol) antiepileptic drugs, as well as some compounds (GABA, adenosine) with important roles in epileptogenesis. Physicochemical, in vitro stability, in vivo tissue distribution, activity and toxicity studies were performed for the new derivatives. The results of these investigations indicated that selected CDSs possessed properties required for delivering the drugs to the CNS. In vivo experiments indicated improved brain uptake and enhanced pharmacologic activity in some of the examined cases. On the other hand, no toxic side effects were registered during the studies. Properly developed CDSs could enhance the therapeutic indexes of the anticonvulsant drugs.

Efficacy of stiripentol in the intravenous pentylenetetrazol infusion seizure model in the rat

The potential effectiveness of stiripentol, a new allylic alcohol anticonvulsant, against generalized epilepsy of the absence type was evaluated in the intravenous pentylenetetrazol (PTZ) infusion seizure model in the rat. The ability of stiripentol to elevate the threshold dose of PTZ in eliciting clonic seizure (i.e., ratio of the post-drug threshold dose to the baseline threshold dose) was measured. Dose-response studies were performed after acute intraperitoneal injection and subacute oral drug treatment. Concentrations of stiripentol in plasma and whole brain were determined. Significant elevation in PTZ threshold dose was observed at a single 300 mg/kg intraperitoneal dose of stiripentol or at plasma levels exceeding 35 micrograms/ml. Maximal anticonvulsant response (i.e., a dose ratio of 3) was reached with doses at or above 450 mg/kg (or plasma concentration greater than or equal to 120 micrograms/ml), along with the appearance of neurotoxicity. Subacute treatment consisted of 9 consecutive oral doses of stiripentol over a 3 day period, until steady-state plasma stiripentol concentration was attained. Response data were obtained at dosage levels of 150, 400 and 800 mg/kg with respective mean steady-state levels of 33.2 +/- 7.8, 61.4 +/- 20.7, and 116 +/- 14 micrograms/ml. Maximal anticonvulsant effect was not reached even at the highest dose of 800 mg/kg. Correlation of threshold dose ratio with plasma and brain stiripentol concentrations showed an approximate 40% loss in anticonvulsant potency during subacute treatment. However, the animals also became more resistant to drug-induced neurotoxicity; about 40% higher plasma or brain stiripentol concentrations had to be reached for a given degree of neurotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbamazepine plus stiripentol: is polytherapy by design possible?

To test the idea that the combination of carbamazepine (CBZ) plus stiripentol (STP) is synergistic, an alumina-gel monkey model (N = 4) was used to compare polytherapy electroencephalographic (EEG) effects to those of CBZ monotherapy. The research design included five consecutive phases (2-3 weeks each): baseline, CBZ, CBZ + STP, CBZ, and postdrug baseline. Both drugs were administered in suspension through a chronic gastric catheter every 4 h (to minimize plasma level oscillations). Doses of CBZ were adjusted to maintain CBZ concentration at the same level in the drug periods (except during the initial polytherapy phase, where levels were allowed to increase prior to adjustment). Phased-reversed interictal spikes were manually counted (expressed as a rate per minute). Relative to baseline, CBZ (Cmin = 0.59; Cmax = 2.36 micrograms/ml) increased interictal EEG spikes by an average of 42%. Relative to CBZ monotherapy, the addition of STP (Cmin = 12.02; Cmax = 13.21 micrograms/ml) was associated with an average decrease in spike rate of 39%. This effect was reversible since removal of STP was associated with an increase in spike rate of 66%. The CBZ-epoxide/CBZ ratio decreased from 0.29 to 0.06 when STP was added and increased to 0.30 when STP was removed. The data fit a pharmacodynamic interpretation and suggest that in the case of CBZ + STP the benefits may outweigh the usual disadvantages of polytherapy.