[Pharmacokinetic study of 3H-stiripentol in rats (author's transl)]

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.

Influence of stiripentol on cytochrome P450-mediated metabolic pathways in humans: in vitro and in vivo comparison and calculation of in vivo inhibition constants

OBJECTIVE

The spectrum of cytochrome P450 inhibition of stiripentol, a new anticonvulsant, was characterized in vitro and in vivo.

METHODS

Stiripentol was incubated in vitro with (R)-warfarin, coumarin, (S)-warfarin, (S)-mephenytoin, bufuralol, p-nitrophenol, and carbamazepine as probes for CYPs 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, and 3A4, respectively. Caffeine demethylation and the 6 beta-hydroxycortisol/cortisol ratio were monitored in vivo before and after 14 days of treatment with stiripentol as measures of CYP1A2 and CYP3A4 activity, and dextromethorphan O- and N-demethylation were used to measure CYP2D6 and CYP3A4 activity, respectively. In vivo inhibition constants for CYP3A4 were calculated with use of data that previously documented the interaction between stripentol and carbamazepine.

RESULTS

In vitro, stiripentol inhibited CYPs 1A2, 2C9, 2C19, 2D6, and 3A4, with inhibition constant values at or slightly higher than therapeutic (total) concentrations of stiripentol, but it did not inhibit CYPs 2A6 and 2E1 even at tenfold therapeutic concentrations. In vivo inhibition of caffeine demethylation and dextromethorphan N-demethylation were consistent with inhibition of CYP1A2 and CYP3A4, respectively. The 6 beta-hydroxycortisol/cortisol ratio did not provide a reliable index of CYP3A4 inhibition. Inhibition of CYP2D6-mediated O-demethylation was not observed in vivo. With use of carbamazepine, in vivo inhibition constants for CYP3A4 ranged between 12 and 35 mumol/L, whereas the corresponding in vitro value was 80 mumol/L.

CONCLUSIONS

Stiripentol appears to inhibit several CYP450 enzymes in vitro and in vivo. In vivo inhibition constants show that stiripentol inhibition of CYP3A4 is linearly related to plasma concentration in patients with epilepsy.

Clinical pharmacokinetics of new antiepileptic drugs

We have reviewed the pharmacokinetics of six antiepileptic drugs that are marketed (felbamate, gabapentin, lamotrigine, oxcarbazepine, vigabatrin, and zonisamide) and six drugs that are undergoing evaluation (levetiracetam, ralitoline, remacemide, stiripentol, tiagabine, and topiramate). In addition, we have compared the prodrugs eterobarb and fosphenytoin and the controlled-release formulations of valproic acid and carbamazepine with their parent compounds. Finally, we have devised a scoring system to compare the pharmacokinetics of new antiepileptic drugs. Using this system, vigabatrin, levetiracetam, gabapentin, and topiramate appea to have the most favourable pharmacokinetic profiles, whilst ralitoline and stiripentol have the least favourable.

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.

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.

Pharmacokinetic profile of a new anticonvulsant, stiripentol, in the rhesus monkey

Stiripentol is a new anticonvulsant drug derived from phenyl-l-pentene-ol. One of its metabolites resulting from opening of the methylenedioxy ring also possesses anticonvulsant activity. This study undertook to define the overall pharmacokinetic profile of stiripentol in rhesus monkey prior to its efficacy evaluation. The experimental design included six treatments (three intravenous doses of 40, 80, and 120 mg; one oral dose of 80 mg; and two intraperitoneal doses of 80 and 120 mg) administered to five chaired rhesus monkeys in a randomized fashion. Sixteen plasma samples were obtained over 7 h and urine was collected for 24 h. A high-performance liquid chromatography assay was developed for the determination of stiripentol in plasma and urine (C8 reverse phase column and UV detection, lambda = 254 nm). The intravenous data revealed multiexponential behavior and therefore noncompartmental methods were used to describe the pharmacokinetics of stiripentol. Values for plasma clearances (L/h/kg) were 1.10 +/- 0.07 (40 mg), 0.92 +/- 0.08 (80 mg), and 0.86 +/- 0.15 (120 mg). The decrease in clearance with dose provided evidence of nonlinearity. The average mean residence time was 1.09 +/- 0.03 h. The average volume of distribution at steady state was 1.03 +/- 0.3 L/kg. The bioavailabilities obtained for the oral and intraperitoneal doses were consistent with first-pass effect predictions: 0.3 (oral), 0.32 (i.p. 80 mg), and 0.34 (i.p. 120 mg). The free fraction determined by equilibrium dialysis was less than 1%. The fraction of dose excreted unchanged in urine ranged between 0 and 3%. The metabolite with anticonvulsant activity could not be detected in plasma with any of the modes of administration. However, it was found in urine and accounted for 2% of the dose. The fraction metabolized by glucuronidation was 34.8 +/- 9.1%. The percentages of total amount of glucuronide excreted in the intervals 0-2, 2-4, 4-8, and 8-24 h were 70.6 +/- 6.2, 10.1 +/- 5.4, 13.6 +/- 5.1, and 5.7 +/- 3.1%. These findings suggested that the terminal phase was not associated with elimination but rather with drug distribution.

Pharmacokinetics of stiripentol in normal man: evidence of nonlinearity

The pharmacokinetics and metabolism of stiripentol, a new antiepileptic drug, were investigated in normal male subjects after single-dose and multiple-dose administration. Each of six subjects received single doses of 300, 600, and 1200 mg of stiripentol in powder form and another 600 mg in solution. In the multiple-dose study, each of six subjects received a 300-mg dose on day 1 and multiple doses (1200 mg/day) from day 2 to day 8. Five of these six subjects participated also in the single-dose study. Stiripentol and several of its metabolites, namely, stiripentol conjugate, DiOH, P-OH, and M-OH, were analyzed in plasma and urine. After single doses, the elimination curve of stiripentol appeared multiphasic. The oral clearance was 1.3 to 1.8 liter/hr/kg. The average mean residence time was 4 hours. There were no statistically significant differences in clearance or mean residence time among the three doses. However, dose dependence was found in all the four pathways when formation clearances were compared. Only trace amounts of the drug were excreted unchanged in urine. The active metabolite, P-OH, was not detectable in plasma. Stiripentol was very highly bound to plasma proteins in plasma from dosed subjects as well as spiked human plasma (free fraction of 1 per cent). In the multiple-dose study, there was a decrease (nearly eightfold) in oral clearance of stiripentol between day 1 and day 8. The fractions of dose metabolized through conjugation and methylenedioxy ring opening increased 183 and 49 per cent, respectively, but the formation clearances for all the pathways were decreased. These findings suggest that the steady-state plasma level/dose ratio of stiripentol will increase with the daily dose.