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

Effect of Hepatic Impairment on the Pharmacokinetics of Fenfluramine and Norfenfluramine

Fenfluramine (Fintepla®) is approved for the treatment of seizures associated with the rare epileptic encephalopathies Dravet syndrome and Lennox-Gastaut syndrome. Fenfluramine is extensively metabolized; thus, patients with hepatic impairment (HI) might experience changes in exposure to fenfluramine or its metabolites. In this phase 1 study, we investigated the pharmacokinetics (PK) and safety of a single oral dose of 0.35 mg/kg fenfluramine in subjects with mild (n = 8), moderate (n = 8), or severe (n = 7) HI (Child-Pugh A/B/C, respectively) and healthy control subjects (n = 22) matched for sex, age, and BMI. All subjects underwent serial sampling to determine total plasma concentrations of fenfluramine and its active metabolite, norfenfluramine. Hepatic impairment was associated with increases in fenfluramine exposures, mainly area-under-the-curve (AUC). Geometric least squares mean ratios (90% confidence intervals) for fenfluramine AUC0-∞ in mild, moderate, and severe HI versus healthy controls were 1.98 (1.36-2.90), 2.13 (1.43-3.17), and 2.77 (1.82-4.24), respectively. Changes in exposure to norfenfluramine in mild, moderate, and severe HI were minimal compared with normal hepatic function. Exposures to fenfluramine and norfenfluramine in all HI groups were within the ranges that have been characterized in the overall development program, including ranges examined in exposure-response relationships for efficacy and safety in patients, and determined to have an acceptable safety profile. Mild and moderate HI had a modest effect on fenfluramine exposure that was not clinically meaningful, whereas the higher fenfluramine exposure in severe HI may require dose reduction based on general caution in this population. The modest decrease in norfenfluramine exposure is not considered clinically relevant.

Interactions between antiepileptic drugs and direct oral anticoagulants for primary and secondary stroke prevention

INTRODUCTION

Direct oral anticoagulants (DOAC) are the guideline-recommended therapy for prevention of stroke in atrial fibrillation (AF) and venous thromboembolism. Since approximately 10% of patients using antiepileptic drugs (AED) also receive DOAC, aim of this review is to summarize data about drug-drug interactions (DDI) of DOAC with AED by using data from PubMed until December 2023.

AREAS COVERED

Of 49 AED, only 16 have been investigated regarding DDI with DOAC by case reports or observational studies. No increased risk for stroke was reported only for topiramate, zonisamide, pregabalin, and gabapentin, whereas for the remaining 12 AED conflicting results regarding the risk for stroke and bleeding were found. Further 16 AED have the potential for pharmacodynamic or pharmacokinetic DDI, but no data regarding DOAC are available. For the remaining 17 AED it is unknown if they have DDI with DOAC.

EXPERT OPINION

Knowledge about pharmacokinetic and pharmacodynamic DDI of AED and DOAC is limited and frequently restricted to in vitro and in vivo findings. Since no data about DDI with DOAC are available for 67% of AED and an increasing number of patients have a combined medication of DOAC and AED, there is an urgent need for research on this topic.

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 diversity amongst approved and emerging antiseizure medications for the treatment of developmental and epileptic encephalopathies

Developmental and epileptic encephalopathies (DEEs) are rare neurodevelopmental disorders characterised by early-onset and often intractable seizures and developmental delay/regression, and include Dravet syndrome and Lennox-Gastaut syndrome (LGS). Rufinamide, fenfluramine, stiripentol, cannabidiol and ganaxolone are antiseizure medications (ASMs) with diverse mechanisms of action that have been approved for treating specific DEEs. Rufinamide is thought to suppress neuronal hyperexcitability by preventing the functional recycling of voltage-gated sodium channels from the inactivated to resting state. It is licensed for adjunctive treatment of seizures associated with LGS. Fenfluramine increases extracellular serotonin levels and may reduce seizures via activation of specific serotonin receptors and positive modulation of the sigma-1 receptor. Fenfluramine is licensed for adjunctive treatment of seizures associated with Dravet syndrome and LGS. Stiripentol is a positive allosteric modulator of type-A gamma-aminobutyric acid (GABAA) receptors. As a broad-spectrum inhibitor of cytochrome P450 enzymes, its antiseizure effects may additionally arise through pharmacokinetic interactions with co-administered ASMs. Stiripentol is licensed for treating seizures associated with Dravet syndrome in patients taking clobazam and/or valproate. The mechanism(s) of action of cannabidiol remains largely unclear although multiple targets have been proposed, including transient receptor potential vanilloid 1, G protein-coupled receptor 55 and equilibrative nucleoside transporter 1. Cannabidiol is licensed as adjunctive treatment in conjunction with clobazam for seizures associated with Dravet syndrome and LGS, and as adjunctive treatment of seizures associated with tuberous sclerosis complex. Like stiripentol, ganaxolone is a positive allosteric modulator at GABAA receptors. It has recently been licensed in the USA for the treatment of seizures associated with cyclin-dependent kinase-like 5 deficiency disorder. Greater understanding of the causes of DEEs has driven research into the potential use of other novel and repurposed agents. Putative ASMs currently in clinical development for use in DEEs include soticlestat, carisbamate, verapamil, radiprodil, clemizole and lorcaserin.

Epilepsy in Dravet Syndrome—Current and Future Therapeutic Opportunities

Dravet Syndrome (DS) is a developmental epileptic encephalopathy characterized by drug-resistant seizures and other clinical features, including intellectual disability and behavioral, sleep, and gait problems. The pathogenesis is strongly connected to voltage-gated sodium channel dysfunction. The current consensus of seizure management in DS consists of a combination of conventional and recently approved drugs such as stiripentol, cannabidiol, and fenfluramine. Despite promising results in randomized clinical trials and extension studies, the prognosis of the developmental outcomes of patients with DS remains unfavorable. The article summarizes recent changes in the therapeutic approach to DS and discusses ongoing clinical research directions. Serotonergic agents under investigation show promising results and may replace less DS-specific medicines. The use of antisense nucleotides and gene therapy is focused not only on symptom relief but primarily addresses the underlying cause of the syndrome. Novel compounds, after expected safe and successful implementation in clinical practice, will open a new era for patients with DS. The main goal of causative treatment is to modify the natural course of the disease and provide the best neurodevelopmental outcome with minimum neurological deficit.

Long-term safety and effectiveness of stiripentol in patients with Dravet syndrome: Interim report of a post-marketing surveillance study in Japan

BACKGROUND

A post-marketing surveillance study is investigating the safety and effectiveness of stiripentol during real-world clinical use in Japanese patients with Dravet syndrome (DS).

METHODS

The safety and effectiveness of stiripentol were prospectively investigated over 104 weeks in all patients with DS who were administered the drug from November 2012 through July 2019 in Japan. Patients administered stiripentol for the first time after its approval were defined as "new patients," and those who continued to take the drug after participating in domestic clinical studies were defined as "continuous-use patients." The responder rate was defined as the proportion of patients with a ≥50 % decrease in seizure episodes at the time of assessment of stiripentol effectiveness compared with the 4 weeks before starting stiripentol. Overall improvement was evaluated by the physician in charge based on the comprehensive assessment of the patient's condition after stiripentol treatment.

RESULTS

Of 411 patients whose information was collected, 410 patients (376 new and 34 continuous-use) were included in the safety analysis set, and 409 (376 new and 33 continuous-use) were included in the effectiveness analysis set. The median age of new patients was 7 years (range: 0.5-50 years) at the time of stiripentol initiation; 99 % of patients were taking concomitant sodium valproate and 93 % clobazam. Adverse drug reactions occurred in 70 % of new patients; the most common were somnolence (39 %) and loss of appetite (25 %). No new safety concerns due to stiripentol were observed. The responder rate in new patients was 43 % (110/257 patients) for convulsive seizures (tonic-clonic and/or clonic convulsions), 55 % (58/105 patients) for focal impaired awareness seizures, and 62 % (56/90 patients) for generalized myoclonic seizures and/or generalized atypical absence seizures. Overall improvement (after 104 weeks or at the time of drug discontinuation) was rated as marked or moderate in 160/353 of new patients (45 %).

CONCLUSION

Stiripentol is safe and effective during long-term use in patients with DS in routine clinical practice.

Inhibition and induction of CYP enzymes in humans: an update

The cytochrome P450 (CYP) enzyme family is the most important enzyme system catalyzing the phase 1 metabolism of pharmaceuticals and other xenobiotics such as herbal remedies and toxic compounds in the environment. The inhibition and induction of CYPs are major mechanisms causing pharmacokinetic drug–drug interactions. This review presents a comprehensive update on the inhibitors and inducers of the specific CYP enzymes in humans. The focus is on the more recent human in vitro and in vivo findings since the publication of our previous review on this topic in 2008. In addition to the general presentation of inhibitory drugs and inducers of human CYP enzymes by drugs, herbal remedies, and toxic compounds, an in-depth view on tyrosine-kinase inhibitors and antiretroviral HIV medications as victims and perpetrators of drug–drug interactions is provided as examples of the current trends in the field. Also, a concise overview of the mechanisms of CYP induction is presented to aid the understanding of the induction phenomena.

Phenoconversion of Cytochrome P450 Metabolism: A Systematic Review

Phenoconversion is the mismatch between the individual’s genotype-based prediction of drug metabolism and the true capacity to metabolize drugs due to nongenetic factors. While the concept of phenoconversion has been described in narrative reviews, no systematic review is available. A systematic review was conducted to investigate factors contributing to phenoconversion and the impact on cytochrome P450 metabolism. Twenty-seven studies met the inclusion criteria and were incorporated in this review, of which 14 demonstrate phenoconversion for a specific genotype group. Phenoconversion into a lower metabolizer phenotype was reported for concomitant use of CYP450-inhibiting drugs, increasing age, cancer, and inflammation. Phenoconversion into a higher metabolizer phenotype was reported for concomitant use of CYP450 inducers and smoking. Moreover, alcohol, pregnancy, and vitamin D exposure are factors where study data suggested phenoconversion. The studies reported genotype–phenotype discrepancies, but the impact of phenoconversion on the effectiveness and toxicity in the clinical setting remains unclear. In conclusion, phenoconversion is caused by both extrinsic factors and patient- and disease-related factors. The mechanism(s) behind and the extent to which CYP450 metabolism is affected remain unexplored. If studied more comprehensively, accounting for phenoconversion may help to improve our ability to predict the individual CYP450 metabolism and personalize drug treatment.

Critical Aspects Affecting Cannabidiol Oral Bioavailability and Metabolic Elimination, and Related Clinical Implications

This article provides a critical appraisal of the available evidence concerning clinical exposure to orally administered cannabidiol (CBD), with special reference to factors affecting gastrointestinal absorption, presystemic elimination, and susceptibility to metabolic drug interactions. Although detailed studies have not been published, the available data suggest that the absolute bioavailability of CBD after oral dosing under fasting conditions is approximately 6%, and increases fourfold when the medication is co-administered with a high-fat meal. Based on measurements of CBD plasma exposure after oral dosing and a 6% absolute oral bioavailability estimate, the actual clearance of CBD in adults can be inferred to be in the order of 67 L/h, which is similar to the value of 74 ± 14 L/h (mean ± standard deviation) determined after intravenous injection of a 20-mg dose of deuterium-labeled CBD in five healthy subjects. Assuming that the CBD blood-to-plasma ratio is about 1, as in the case of tetrahydrocannabinol (THC), and that CBD metabolism takes place virtually entirely in the liver, it can be estimated that about 70 to 75% of an orally absorbed dose of CBD can be removed by hepatic metabolism before reaching the systemic circulation, and additionally CBD gastrointestinal absorption is incomplete. A formulation with improved biopharmaceutical properties could increase the extent of CBD absorption about fourfold (i.e., to the level achieved with the currently available formulations co-administered with a high-fat meal) and minimize the influence of food effects on CBD bioavailability. There is also potential for favoring the absorption of CBD through the enteric lymphatic system, thereby reducing the extent of presystemic hepatic elimination. Evidence that CBD can behave as a high hepatic clearance compound also has implications when predicting the magnitude of drug-drug interactions affecting CBD metabolism. These considerations have important clinical relevance, particularly with respect to the objective of minimizing pharmacokinetic variability and consequent intra- and interindividual differences in therapeutic response and susceptibility to adverse effects.

Stiripentol: A Review in Dravet Syndrome

Stiripentol (Diacomit^®) is an orally-active, structurally unique anti-epileptic drug (AED) with multiple potential mechanisms of action, including enhancement of central γ-aminobutyric acid transmission. In the EU, stiripentol is indicated for use in conjunction with clobazam and valproate as adjunctive therapy of refractory generalized tonic-clonic seizures in patients with Dravet syndrome (DS; previously known as severe myoclonic epilepsy of infancy), whose seizures are not adequately controlled with clobazam and valproate. This approval (and similar DS indications in the USA, Canada and Japan), reflect the results of the STICLO studies, two small, randomized controlled trials in which stiripentol as adjunctive therapy was associated with a markedly superior response rate after 2 months compared with placebo in patients aged between 3 and ≈ 21 years with DS that was inadequately controlled with clobazam and valproate. These short-term results have subsequently been supported and extended by findings from longer-term, open-label, observational studies, including a retrospective longitudinal cohort study, which showed that the efficacy of combining stiripentol with clobazam and valproate when started at paediatric age was maintained in mid-adulthood with up to 24 years of exposure, and up to 40 years of age. Drowsiness, appetite loss, weight loss, ataxia and tremor are the most common adverse events associated with the addition of stiripentol to clobazam and valproate. Based on the available evidence, stiripentol, as an adjunct to clobazam and valproate, is a demonstrably beneficial and generally well-tolerated second-line treatment for patients with DS.

The Potential for Pharmacokinetic Interactions Between Cannabis Products and Conventional Medications

PURPOSE

Increased cannabis use and recent drug approvals pose new challenges for avoiding drug interactions between cannabis products and conventional medications. This review aims to identify drug-metabolizing enzymes and drug transporters that are affected by concurrent cannabis use and, conversely, those co-prescribed medications that may alter the exposure to one or more cannabinoids.

METHODS

A systematic literature search was conducted utilizing the Google Scholar search engine and MEDLINE (PubMed) database through March 2019. All articles describing in vitro or clinical studies of cannabis drug interaction potential were retrieved for review. Additional articles of interest were obtained through cross-referencing of published bibliographies.

FINDINGS

After comparing the in vitro inhibition parameters to physiologically achievable cannabinoid concentrations, it was concluded that CYP2C9, CYP1A1/2, and CYP1B1 are likely to be inhibited by all 3 major cannabinoids Δ-tetrahydrocannabinol (THC), cannabidiol (CBD), and cannabinol (CBN). The isoforms CYP2D6, CYP2C19, CYP2B6, and CYP2J2 are inhibited by THC and CBD. CYP3A4/5/7 is potentially inhibited by CBD. Δ-Tetrahydrocannabinol also activates CYP2C9 and induces CYP1A1. For non-CYP drug-metabolizing enzymes, UGT1A9 is inhibited by CBD and CBN, whereas UGT2B7 is inhibited by CBD but activated by CBN. Carboxylesterase 1 (CES1) is potentially inhibited by THC and CBD. Clinical studies suggest inhibition of CYP2C19 by CBD, inhibition of CYP2C9 by various cannabis products, and induction of CYP1A2 through cannabis smoking. Evidence of CBD inhibition of UGTs and CES1 has been shown in some studies, but the data are limited at present. We did not identify any clinical studies suggesting an influence of cannabinoids on drug transporters, and in vitro results suggest that a clinical interaction is unlikely.

CONCLUSIONS

Medications that are prominent substrates for CYP2C19, CYP2C9, and CYP1A2 may be particularly at risk of altered disposition by concomitant use of cannabis or 1 or more of its constituents. Caution should also be given when coadministered drugs are metabolized by UGT or CES1, on which subject the information remains limited and further investigation is warranted. Conversely, conventional drugs with strong inhibitory or inductive effects on CYP3A4 are expected to affect CBD disposition.

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.

Recent Advances in the Drug Treatment of Dravet Syndrome

Dravet syndrome is a rare but severe epilepsy syndrome that begins in the first year of life with recurrent seizures triggered by fever that are typically prolonged and hemiclonic. The epilepsy is highly drug resistant. Although development is normal at onset, over time, most patients develop moderate-to-severe intellectual disability, behavior disorders, and a characteristic crouch gait. There is a significant mortality, predominantly owing to sudden unexpected death in epilepsy. Complete seizure control is rarely attainable. Initial therapy includes valproic acid and clobazam, but response is typically inadequate. The results of new drugs for Dravet syndrome, including stiripentol, cannabidiol, and fenfluramine, are very promising. Stiripentol was associated with a greater than 50% reduction in convulsive seizure frequency in 71% of cases, when added to valproic acid and clobazam, and also markedly reduced status epilepticus. Pharmaceutical-grade cannabidiol resulted in a median change in monthly motor seizures from baseline of - 36.5%. Fenfluramine was associated with a greater than 50% reduction in seizures of 70%, with one quarter of cases achieving near seizure freedom over the duration of the trial. These agents are generally well tolerated, with few patients discontinuing for adverse effects. There is limited evidence to date regarding improvement in cognition with these newer agents; however, a meaningful change is challenging to assess over short trial periods and requires longer follow-up studies. While current treatments focus predominantly on seizure control, newer therapies including genetic treatments and antisense oligonucleotides can target the SCN1A channelopathy, and thus, may also significantly impact the important co-morbidities associated with this syndrome.

A phase I, randomized, open-label, single-dose, 3-period crossover study to evaluate the drug-drug interaction between ZX008 (fenfluramine HCl oral solution) and a regimen of stiripentol, clobazam, and valproate in healthy subjects

OBJECTIVE

Phase I, open-label, randomized, single-dose, 3-period crossover study assessing pharmacokinetics (PK) and safety of ZX008, a liquid oral formulation of fenfluramine (FFA) under development for adjunctive treatment of Dravet syndrome and Lennox-Gastaut syndrome, administered with and without a combined antiepileptic drug (AED) regimen of stiripentol (STP), valproate (VPA), and clobazam (CLB) (STP regimen).

MATERIALS AND METHODS

26 healthy adults were administered the following treatments: ZX008 0.8 mg/kg; STP 3,500 mg, CLB 20 mg, VPA 25 mg/kg (max. 1,500 mg); and ZX008 0.8 mg/kg + STP regimen. Dose periods were 17 days apart. Blood samples were obtained for 72 hours after drug administration and used to calculate non-compartmental PK parameters.

RESULTS

Statistical bioequivalence-type analysis demonstrated ZX008 had no significant impact on the PK of any drug in the STP regimen, while the STP regimen moderately affected FFA PK. The 3-drug combination increased the geometric mean C_max, AUC_0-t, and AUC_0-inf of FFA while reducing the C_max and AUC_0-t of its major metabolite, norfenfluramine (norFFA). Adverse events (AEs) were mild to moderate and resolved spontaneously. ZX008 + STP regimen co-administration to healthy adult subjects modestly impacted the number but not severity of AEs.

CONCLUSION

Results show that the STP regimen had a moderate impact on FFA and norFFA PK and ZX008 had no significant impact on the 3 STP regimen drugs. ZX008 would not be expected to alter the clinical response of patients to this regimen by means of an effect on PK. When administering these drugs together, a downward dose adjustment of ZX008 may be warranted.
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Pharmacokinetic Interactions of Clinical Interest Between Direct Oral Anticoagulants and Antiepileptic Drugs

Direct oral anticoagulants (DOACs), namely apixaban, dabigatran, edoxaban, and rivaroxaban are being increasingly prescribed among the general population, as they are considered to be associated to lower bleeding risk than classical anticoagulants, and do not require coagulation monitoring. Likewise, DOACs are increasingly concomitantly prescribed in patients with epilepsy taking, therefore, antiepileptic drugs (AEDs), above all among the elderly. As a result, potential interactions may cause an increased risk of DOAC-related bleeding or a reduced antithrombotic efficacy. The objective of the present review is to describe the pharmacokinetic interactions between AEDs and DOACs of clinical relevance. We observed that there are only few clinical reports in which such interactions have been described in patients. More data are available on the pharmacokinetics of both drugs classes which allow speculating on their potential interactions. Older AEDs, acting on cytochrome P450 isoenzymes, and especially on CYP3A4, such as phenobarbital, phenytoin, and carbamazepine are more likely to significantly reduce the anticoagulant effect of DOACs (especially rivaroxaban, apixaban, and edoxaban). Newer AEDs not affecting significantly CYP or P-gp, such as lamotrigine, or pregabalin are not likely to affect DOACs efficacy. Zonisamide and lacosamide, which do not affect significantly CYP activity in vitro, might have a quite safe profile, even though their effects on P-gp are not well-known, yet. Levetiracetam exerts only a potential effect on P-gp activity, and thus it might be safe, as well. In conclusion, there are only few case reports and limited evidence on interactions between DOACs and AEDs in patients. However, the overall evidence suggests that the interaction between these drug classes might be of high clinical relevance and therefore further studies in larger patients' cohorts are warranted for the future in order to better clarify their pharmacokinetic and define the most appropriate clinical behavior.

Discovery of Beclabuvir: A Potent Allosteric Inhibitor of the Hepatitis C Virus Polymerase

The discovery of beclabuvir occurred through an iterative series of structure-activity relationship studies directed at the optimization of a novel class of indolobenzazepines. Within this research, a strategic decision to abandon a highly potent but physiochemically problematic series in favor of one of lower molecular weight and potency was key in the realization of the program’s objectives. Subsequent cycles of analog design incorporating progressive conformational constraints successfully addressed off-target liabilities and identified compounds with improved physiochemical profiles. Ultimately, a class of alkyl-bridged piperazine carboxamides was found to be of particular interest, and from this series, beclabuvir was identified as having superior antiviral, safety, and pharmacokinetic properties. The clinical evaluation of beclabuvir in combination with both the NS5A replication complex inhibitor daclatasvir and the NS3 protease inhibitor asunaprevir in a single, fixed-dose formulation (Ximency) resulted in the approval by the Japanese Pharmaceutical and Food Safety Bureau for its use in the treatment of patients infected with genotype 1 HCV.

Therapeutic Drug Monitoring of Clobazam and Its Metabolite-Impact of Age and Comedication on Pharmacokinetic Variability

BACKGROUND

Clobazam (CLB) has been used as an antiepileptic drug for several decades. There is still insufficient data regarding its pharmacokinetic variability in clinical practice. The purpose of this study was to investigate pharmacokinetic variability of CLB with emphasis on the impact of age and comedication in patients with epilepsy.

METHODS

Serum concentration measurements of CLB and its metabolite N-desmethylclobazam (NCLB), as well as demographic and clinical data were retrieved from the routine therapeutic drug monitoring service at the National Center for Epilepsy, Norway, 2009-2013. NCLB/CLB and total (CLB + NCLB), CLB and NCLB concentration/dose (C/D) ratios were calculated.

RESULTS

550 patients (296 women/254 men), average age 27 years (range 1-86), were included. The interindividual pharmacokinetic variability was extensive, as illustrated by a 100-fold variability in serum concentration compared with dose (total C/D ratio 0.03-3.29 µmol·L·mg). The CLB C/D ratio was 36% lower in young children (2-9 years) than in adults (18-64 years), reflecting a higher clearance. In patients receiving phenytoin, felbamate, stiripentol, oxcarbazepine or eslicarbazepine acetate, valproate, phenobarbital, zonisamide or carbamazepine one or more of the calculated ratios were significantly different from that in patients receiving no or neutral comedications. The mean values for the different groups were in the order of 20%-230% of C/D ratios in the neutral group and 200%-950% of the NCLB/CLB ratio.

CONCLUSIONS

The pharmacokinetic variability of CLB and its metabolite NCLB in clinical practice is extensive, and is influenced by drug-drug interactions, age, and pharmacogenetics. Therapeutic drug monitoring of CLB and NCLB is therefore valuable in patient management.

Pharmacotherapy for Dravet Syndrome

Dravet syndrome (DS) is an intractable pediatric epilepsy syndrome, starting in early childhood. This disorder typically manifests with febrile status epilepticus, and progresses to a multifocal epilepsy with febrile and non-febrile seizures with encephalopathy. Most cases are due to a mutation in the SCN1A gene. This article reviews treatments for DS, with an emphasis on pharmacotherapy. While many medications are used in treating the seizures associated with DS, these patients typically have medically refractory epilepsy, and polytherapy is often required. First-line agents include valproate and clobazam, although there are supportive data for topiramate, levetiracetam, stiripentol and the ketogenic diet. Other agents such as fenfluramine are promising therapies for Dravet syndrome. Sodium channel-blocking anticonvulsants such as carbamazepine and lamotrigine are generally contraindicated in this syndrome. Nonpharmacologic therapies (such as neurostimulation or surgery) are understudied in DS. Because DS is a global encephalopathy, pharmacologic treatment of non-epileptic manifestations of the disease is often necessary. Attention-deficit hyperactivity disorder is often encountered in patients with DS, and psychostimulants can be helpful for this indication. Other psychoactive drugs are less studied in this context. Extrapyramidal and gait disorders are often encountered in DS as well. While DS is a severe epileptic encephalopathy with a high (up to 15 %) mortality rate in childhood, careful pharmacologic management can improve these patients' clinical picture and quality of life.

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.

Advances in anti-epileptic drug testing

In the past twenty-one years, 17 new antiepileptic drugs have been approved for use in the United States and/or Europe. These drugs are clobazam, ezogabine (retigabine), eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, perampanel, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide. Therapeutic drug monitoring is often used in the clinical dosing of the newer anti-epileptic drugs. The drugs with the best justifications for drug monitoring are lamotrigine, levetiracetam, oxcarbazepine, stiripentol, and zonisamide. Perampanel, stiripentol and tiagabine are strongly bound to serum proteins and are candidates for monitoring of the free drug fractions. Alternative specimens for therapeutic drug monitoring are saliva and dried blood spots. Therapeutic drug monitoring of the new antiepileptic drugs is discussed here for managing patients with epilepsy.

Discovery and preclinical characterization of the cyclopropylindolobenzazepine BMS-791325, a potent allosteric inhibitor of the hepatitis C virus NS5B polymerase

Described herein are structure-activity relationship studies that resulted in the optimization of the activity of members of a class of cyclopropyl-fused indolobenzazepine HCV NS5B polymerase inhibitors. Subsequent iterations of analogue design and syntheses successfully addressed off-target activities, most notably human pregnane X receptor (hPXR) transactivation, and led to significant improvements in the physicochemical properties of lead compounds. Those analogues exhibiting improved solubility and membrane permeability were shown to have notably enhanced pharmacokinetic profiles. Additionally, a series of alkyl bridged piperazine carboxamides was identified as being of particular interest, and from which the compound BMS-791325 (2) was found to have distinguishing antiviral, safety, and pharmacokinetic properties that resulted in its selection for clinical evaluation.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

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.

Stiripentol: an example of antiepileptic drug development in childhood epilepsies

The efficacy of stiripentol (STP) in Dravet Syndrome (DS) was discovered first in an exploratory study in pediatric pharmacoresistant epilepsies. This efficacy signal, used as a proof of concept, led to - two independent multicenter randomized, double-blind, placebo-controlled trials in DS patients: STICLO-France and STICLO-Italy. In adjunction to valproate and clobazam, STP demonstrated marked efficacy and these trials became the basis for the registration of STP as an orphan drug for DS. Although STP had previously shown antiepileptic activity, since it inhibits cytochromes P450, the increased plasma levels of clobazam (CLB), norclobazam (NCLB), and NCLB/CLB ratio reported in STICLO studies brought into question the activity of STP per se. Recent pharmacological studies demonstrated that (i) STP is a direct allosteric modulator of the GABA receptors at a site distinct from benzodiazepines; (ii) STP and CLB/NCLB act independently at GABA(A) receptors; (iii) their combination increases the maximum response beyond that of either drug alone. All these effects are independent of considerations of changes in metabolism. Some responders in STICLO studies failed to display any increase of plasmatic concentrations of NCLB/CLB ratio as STP could not inhibit CYP2C19 because of its inhibition by progabide or due to an inactivating CYP polymorphism. The responder rate proved to be in the same range whether the NCLB/CLB ratio increased or not. These analyses confirmed that the effects of STP cannot result from a simple pharmacokinetic interaction. We propose that the success of STP should serve as a model for AED development in rare pediatric epileptic syndromes.

Efficacy of stiripentol in hyperthermia-induced seizures in a mouse model of Dravet syndrome

PURPOSE

We previously reported a mutant mouse carrying a severe myoclonic epilepsy in infancy (SMEI) mutation in Scn1a. In this study, we examined the susceptibility to hyperthermia-induced seizures of heterozygous Scn1a mutant mice (Scn1a(RX/+)) and wild-type (Scn1a(+/+) ) mice. Then we assessed the efficacy of stiripentol (STP) monotherapy versus STP and clobazam (CLB) combination therapy to prevent hyperthermia-induced seizures in Scn1a(RX/+) mice.

METHODS

The seizure-inducing body temperatures in Scn1a(RX/+) mice and age-matched Scn1a(+/+) mice were compared in three age groups (1 month, 3-5 months, > 6 months). Then STP, CLB, or STP + CLB was administered intraperitoneally to Scn1a(RX/+) mice of two age groups (p1M, aged 1 month; p5M, aged 5-10 months). The efficacy of medications was assessed by comparing the seizure-inducing body temperature and the duration of seizures.

KEY FINDINGS

The seizure-inducing body temperature was significantly lower in Scn1a(RX/+) than in Scn1a(+/+) mice for all age groups (p < 0.01). The seizure-inducing body temperature was significantly elevated after administration of STP in p1M (p < 0.05) but not in p5M (p > 0.05), and it was significantly elevated after administration of CLB in both age groups (p < 0.05). The seizure-inducing body temperature was significantly higher after administration of STP + CLB than after administration of CLB in p5M (p < 0.05).

SIGNIFICANCE

Scn1a (RX/+) mice have increased susceptibility to hyperthermia-induced seizure in all age groups. STP monotherapy is effective in preventing hyperthermia-induced seizures in Scn1a(RX/+) mice aged 1 month, but not in those aged 5 months and older. When used in combination therapy with CLB, STP inhibits the metabolism of CLB and probably synergistically enhances the anticonvulsant effect in mice aged 1 month.

The effects of stiripentol on GABA(A) receptors

The anticonvulsant stiripentol (Diacomittm) has been shown to have a positive impact on control of seizures for many patients with Dravet syndrome. As with most antiepileptic drugs, stiripentol has multiple mechanisms of action. Its direct anticonvulsant activity is likely due to enhancement of inhibitory, γ-aminobutyric acid (GABA)ergic neurotransmission. Stiripentol was shown to increase the activity of both neuronal and recombinant GABA(A) receptors at clinically relevant concentrations. At recombinant receptors, stiripentol was found to act through a unique site in a subunit-dependent manner. Positive modulation by stiripentol was most effective at GABA(A) receptors containing an α3 subunit. The expression of the α3 subunit is developmentally regulated, with highest levels in the immature brain. This subunit selectivity may explain the greater clinical efficacy of stiripentol in childhood-onset epilepsies, including Dravet syndrome.

Interactions between modulators of the GABA(A) receptor: Stiripentol and benzodiazepines

Many patients with refractory epilepsy are treated with polytherapy, and nearly 15% of epilepsy patients receive two or more anti-convulsant agents. The anti-convulsant stiripentol is used as an add-on treatment for the childhood epilepsy syndrome known as severe myoclonic epilepsy in infancy (Dravet syndrome). Stiripentol has multiple mechanisms of action, both enhancing GABA(A) receptors and reducing activity of metabolic enzymes that break down other drugs. Stiripentol is typically co-administered with other anti-convulsants such as benzodiazepines which also act through GABA(A) receptor modulation. Stiripentol slows the metabolism of some of these drugs through inhibition of a variety of cytochrome P450 enzymes, but could also influence their effects on GABAergic neurotransmission. Is it rational to co-administer drugs which can act through the same target? To examine the potential interaction between these modulators, we transiently transfected HEK-293T cells to produce α3β3γ2L or α3β3δ recombinant GABA(A) receptors. Using whole-cell patch clamp recordings, we measured the response to each benzodiazepine alone and in combination with a maximally effective concentration of stiripentol. We compared the responses to four different benzodiazepines: diazepam, clonazepam, clobazam and norclobazam. In all cases we found that these modulators were equally effective in the presence and absence of stiripentol. The δ-containing receptors were insensitive to modulation by the benzodiazepines, which did not affect potentiation by stiripentol. These data suggest that stiripentol and the benzodiazepines act independently at GABA(A) receptors and that polytherapy could be expected to increase the maximum effect beyond either drug alone, even without consideration of changes in metabolism.

Selected pharmacokinetic issues of the use of antiepileptic drugs and parenteral nutrition in critically ill patients

OBJECTIVES

To conduct a systematic review for the evidence supporting or disproving the reality of parenteral nutrition- antiepileptic drugs interaction, especially with respect to the plasma protein-binding of the drug.

METHODS

The articles related to the topic were identified through Medline and PubMed search (1968-Feburary 2010) for English language on the interaction between parenteral nutrition and antiepileptic drugs; the search terms used were anti-epileptic drugs, parenteral nutrition, and/or interaction, and/or in vitro. The search looked for prospective randomized and nonrandomized controlled studies; prospective nonrandomized uncontrolled studies; retrospective studies; case reports; and in vitro studies. Full text of the articles were then traced from the Universiti Sains Malaysia (USM) library subscribed databases, including Wiley-Blackwell Library, Cochrane Library, EBSCOHost, OVID, ScienceDirect, SAGE Premier, Scopus, SpringerLINK, and Wiley InterScience. The articles from journals not listed by USM library were traced through inter library loan.

RESULTS

There were interactions between parenteral nutrition and drugs, including antiepileptics. Several guidelines were designed for the management of illnesses such as traumatic brain injuries or cancer patients, involving the use of parenteral nutrition and antiepileptics. Moreover, many studies demonstrated the in vitro and in vivo parenteral nutrition -drugs interactions, especially with antiepileptics.

CONCLUSIONS

There was no evidence supporting the existence of parenteral nutrition-antiepileptic drugs interaction. The issue has not been studied in formal researches, but several case reports and anecdotes demonstrate this drug-nutrition interaction. However, alteration in the drug-free fraction result from parenteral nutrition-drug (i.e. antiepileptics) interactions may necessitate scrupulous reassessment of drug dosages in patients receiving these therapies. This reassessment may be particularly imperative in certain clinical situations characterized by hypoalbuminemia (e.g., burn patients).

Therapeutic Drug Monitoring of the Newer Anti-Epilepsy Medications

In the past twenty years, 14 new antiepileptic drugs have been approved for use in the United States and/or Europe. These drugs are eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide. In general, the clinical utility of therapeutic drug monitoring has not been established in clinical trials for these new anticonvulsants, and clear guidelines for drug monitoring have yet to be defined. The antiepileptic drugs with the strongest justifications for drug monitoring are lamotrigine, oxcarbazepine, stiripentol, and zonisamide. Stiripentol and tiagabine are strongly protein bound and are candidates for free drug monitoring. Therapeutic drug monitoring has lower utility for gabapentin, pregabalin, and vigabatrin. Measurement of salivary drug concentrations has potential utility for therapeutic drug monitoring of lamotrigine, levetiracetam, and topiramate. Therapeutic drug monitoring of the new antiepileptic drugs will be discussed in managing patients with epilepsy.

Therapeutic Drug Monitoring of the Newer Anti-Epilepsy Medications

In the past twenty years, 14 new antiepileptic drugs have been approved for use in the United States and/or Europe. These drugs are eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide. In general, the clinical utility of therapeutic drug monitoring has not been established in clinical trials for these new anticonvulsants, and clear guidelines for drug monitoring have yet to be defined. The antiepileptic drugs with the strongest justifications for drug monitoring are lamotrigine, oxcarbazepine, stiripentol, and zonisamide. Stiripentol and tiagabine are strongly protein bound and are candidates for free drug monitoring. Therapeutic drug monitoring has lower utility for gabapentin, pregabalin, and vigabatrin. Measurement of salivary drug concentrations has potential utility for therapeutic drug monitoring of lamotrigine, levetiracetam, and topiramate. Therapeutic drug monitoring of the new antiepileptic drugs will be discussed in managing patients with epilepsy.

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.

Too long or too short? New insights into abnormal cardiac repolarization in people with chronic epilepsy and its potential role in sudden unexpected death

SUMMARY

Sudden unexpected death in epilepsy (SUDEP) is probably caused by periictal cardiorespiratory alterations such as central apnea, bradyarrhythmia, and neurogenic pulmonary edema. These alterations may occur in people with epilepsy and vary in duration and severity. Seizure-related ventricular tachyarrhythmias have also been hypothesized to be involved in SUDEP, but compelling evidence of these, or of predisposition to these, is lacking. Ventricular tachyarrhythmias are facilitated by pathologic cardiac repolarization. Electrocardiography (ECG) indicators of pathologic cardiac repolarization, such as prolongation or shortening of QT intervals as well as increased QT dispersion, are established risk factors for life-threatening tachyarrhythmia and sudden cardiac death (SDC). Abnormalities in cardiac repolarization have recently been described in people with epilepsy. Importantly, periictal ventricular tachycardia and fibrillation have also been reported in the absence of any underlying cardiac disease. Therefore, pathologic cardiac repolarization could promote SCD in people with epilepsy and could be one plausible cause for SUDEP. Herein, we review abnormal cardiac repolarization in people with epilepsy, describe the putative contribution of antiepileptic drugs, and discuss the potential role of pathologic cardiac repolarization in SUDEP. Based on these, measures to reduce the risk of or prevent SUDEP may include antiarrhythmic medication and implantation of cardiac combined pacemaker-defibrillator devices.