Extended‐release antiepileptic drugs: A comparison of pharmacokinetic parameters relative to original immediate‐release formulations

Many antiepileptic drugs (AEDs) have short half‐lives with large fluctuations in peak‐to‐trough plasma concentrations. Consequences of these pharmacokinetic (PK) properties may include adverse events (AEs) and breakthrough seizures, potentially leading to poor adherence. To address these challenges, newer formulations of these AEDs have been developed using unique extended‐release (ER) technologies. These technologies extend the dosing interval such that dosing frequency can be minimized, which may improve patient adherence. Available ER formulations have the potential to minimize the spikes in maximum plasma concentrations (C_max) at steady‐state that often contribute to AEs during treatment with immediate‐release (IR) products. In so doing, tolerability advantages may lead to increased AED effectiveness by improving adherence and allowing higher doses if clinically indicated. Direct PK comparison studies of IR and ER formulations (e.g., carbamazepine, divalproate sodium, lamotrigine, oxcarbazepine, levetiracetam, and phenytoin) have found that dose‐normalized ER formulations may or may not be bioequivalent to their IR counterparts, but most ER formulations have a lower fluctuation index ([C_max–C_min]/C_avg) compared with the IR versions. This results in flatter concentration‐time plots. Not all ER preparations improve the various PK parameters to the same extent, and PK nuances may impact the effectiveness, tolerability, and adherence rates of various ER formulations.     

Effects of some antiepileptic drugs in pentetrazol-induced convulsions in mice lesioned with kainic acid

Mice were injected with intracerebroventricular (i.c.v.) kainic acid (KA; 0.1 micrograms per animal) and the pentetrazol test was carried out on the fifth day after the administration of the amino acid. The following antiepileptic drugs were tested for anticonvulsant activity in mice lesioned with KA: diazepam (0.4 mg/kg), phenobarbital (12.5 and 25 mg/kg), trimethadione (200 and 400 mg/kg), depakine (200 and 400 mg/kg), carbamazepine (10 and 20 mg/kg), lefadol (bromophenylsuccinimide; 20 mg/kg), and acetazolamide (320 mg/kg). All drugs were given intraperitoneally, except for carbamazepine, which was also given orally in doses of 100 and 200 mg/kg. Pentetrazol was administered subcutaneously in a dose of 110 mg/kg, and the animals were subsequently observed for the occurrence of clonic and tonic convulsions within 30 min. The protective effects of diazepam and phenobarbital were significantly reduced in the KA-lesioned animals, while the actions of the remaining anticonvulsants were unaltered. Moreover, a substantial loss of pyramidal cells in the CA 3 field of the hippocampus was noted after i.c.v. injection of KA. It may therefore be concluded that the mechanism of the action of diazepam and phenobarbital are partially dependent on the intact functions of the hippocampal formation.     

Problems with the use of conventional anticonvulsant drugs in mentally retarded individuals

Special problems in the use of anticonvulsant agents in the mentally retarded are reviewed. Effects of barbiturates on mental abilities and of phenytoin on facial appearance are especially undesirable side effects of these agents in mentally retarded individuals. Careful consideration should be given to selection of anticonvulsants that minimally interfere with the long-term rehabilitation of mentally retarded persons. The relationship of paroxysmal EEG abnormalities, episodic violence and hysteroepilepsy to epilepsy is discussed.     

The hippocampal slice: a system for studying the pharmacology of seizures and for screening anticonvulsant drugs

Interictal spikes with a configuration similar to that occurring in grand mal epilepsy were generated by the application of penicillin to a hippocampal slice preparation. This slice preparation has potential value for screening anticonvulsant drugs and for studying epileptic activity. The effect of anticonvulsant drugs on seizure activity was tested at concentrations comparable to reported clinical serum concentrations. Phenytoin and diazepam were maximally effective at concentrations of 20 microgram/ml and 3-4 microgram/ml, respectively, in good agreement with their effective concentrations in clinical practice. Phenobarbital was more potent (5 microgram/ml) and mesuximide (50% potent at 80 microgram/ml) was least effective.     

A comparison between microcrystalline and conventional phenytoin preparations: Relative bioavailability and steady-state plasma concentrations

Summary Plasma concentrations of two phenytoin products (a conventional phenytoin acid preparation and a microcrystalline form of phenytoin acid) were studied after single dose administration and during steady-state conditions in four healthy male volunteers.Relative bioavailability for the conventional tablet in comparison with the microcrystalline was in the range of 48–80% during single dose administration and in the range 54–95% at steady-state.The microcrystalline preparation gave, as expected, a higher rate of absorption. During steady-state conditions, however, this higher rate of absorption was associated with considerable fluctuations in plasma concentration during the dosage interval. The mean maximum plasma concentration was about 50% higher than the value at the beginning of the dose interval (2-dose concentration value) when the microcrystalline product was administered. The corresponding figure was only about 25% for the conventional tablet. Since upward fluctuations in plasma concentrations may be associated with side effects, the more even level obtained with the conventional product may be an advantage from the clinical point of view. An increased rate of bioavailability is not a clinical improvement if it occurs at the expense of greater fluctuations in plasma concentration during the dose interval.Supported by the Swedish Medical Research Council (Project No. 522), Stiftelsen Margarethahemmet, and Sällskapet Barnavård     

Correlation analysis between anticonvulsant ED50 values of antiepileptic drugs in mice and rats and their therapeutic doses and plasma levels

OBJECTIVE: Although there are several animal models of epilepsy, the extrapolation of antiepileptic drug (AEDs) performance to epileptic patients from anticonvulsant activity results in animals is not straightforward. Consequently, the aim of this work was to perform a correlation analysis between therapeutic daily doses (D) and average steady-state plasma concentrations (Css,av) of AEDs and their activity in common anticonvulsant animal models. METHODS: AED activity in anticonvulsant animal models was expressed as maximal electroshock seizure (MES) test ED50 values in mice and rats and ED50 values in audiogenic seizure-susceptible mice (AGS ED50). Data were examined, by use of linear and logarithmic approaches, for an association between Css,av (mg/L or micromol/L) and D (mg or mmol) for each AED in epileptic patients as the dependent variable (Y) and its MES ED50 in mice and rats and AGS ED50 in mice (mg/kg or micromol/kg) as the independent variable (X). RESULTS: Linear correlation analyses between Css,av (mg/L) and ED50 (mg/kg) for 11 AEDs gave the following correlation coefficients (R2): 0.68 (mice, MES); 0.73 (rat, MES); 0.64 (AGS). Switching the units from milligrams to micromoles improved the correlation significantly and gave the following R2 values: 0.88 (mice, MES); 0.90 (rat, MES); 0.76 (AGS). The linear correlation between Css,av and ED50 was better than that between D and ED50. CONCLUSIONS: The results of this analysis suggest that the relationship between Css,av and ED50 is useful in predicting target concentration ranges in humans. The Y intercepts of the Css,av-versus-ED50 and D-versus- ED50 plots were similar in all three animal models and ranged between 12 and 17 mg/L and between 570 and 890 mg, respectively, indicating that for all AEDs analyzed except valproic acid and ethosuximide, the therapeutic plasma concentration is in the range 10-20 mg/L.     

Evaluation of stereoselective anticonvulsant,teratogenic, and pharmacokinetic profile of valnoctylurea (capuride): A chiral stereoisomer of valproic acid urea derivative

Purpose: The purpose of this study was to evaluate the stereoselective anticonvulsant activity, neurotoxicity, pharmacokinetics, and teratogenic potential of two stereoisomers of valnoctylurea (VCU), a central nervous system (CNS)–active urea derivative of valnoctic acid, which is a constitutional isomer of valproic acid (VPA). Methods: VCU stereoisomers (2S,3S)‐VCU and (2R,3S)‐VCU were synthesized. Their anticonvulsant activity was determined and compared to VPA and racemic‐VCU in rats utilizing the maximal electroshock seizure (MES) and the subcutaneous pentylenetetrazole (scMet) tests. Neurotoxicity was determined in rats using the positional sense test, muscle tone test, and gait and stance test. The induction of neural tube defects (NTDs) by VCU stereoisomers was evaluated in a mouse strain highly susceptible to VPA‐induced teratogenicity. The pharmacokinetics of VCU was studied in a stereoselective manner following intraperitoneal (i.p.) administration to rats. Results: (2S,3S)‐VCU and (2R,3S)‐VCU median effective dose ED₅₀ values were 29 mg/kg [95% confidence interval (CI) = 8–60 mg/kg] and 42 mg/kg (95% CI = 36–51 mg/kg) (MES) and 22 mg/kg (95% CI = 13–51 mg/kg) and 12 mg/kg (95%CI = 7–21 mg/kg) (scMet), respectively. (2S,3S)‐VCU was more potent and had a wider safety margin (p < 0.05), defined as the protective index (PI = TD₅₀/ED₅₀), at both the MES (PI > 17) and scMet (PI > 23) tests than racemic‐VCU or (R,S)‐VCU (PI = 2.8 and 9.9, respectively). VCU stereoisomers caused NTDs at doses >4 times that of their anticonvulsant ED₅₀ values. At a dose of 112 mg/kg, (2R,3S)‐VCU was nonteratogenic and less embryotoxic than its stereoisomer (2S,3S)‐VCU. No stereoselective pharmacokinetics was observed following intraperitoneal dosing of racemic‐VCU to rats. VCU was mainly eliminated by metabolism with a half‐life of 2 h. Conclusions: VCU anticonvulsant activity and wide PI values make it a potential candidate for development as a new, potent antiepileptic drug (AED).     

Interaction of anticonvulsant drugs with adenosine receptors in the central nervous system

The anticonvulsant carbamazepine inhibits binding of 1 nM [3H]N6-cyclohexyladenosine to rat cerebral and cerebellar A1 adenosine receptors with an IC50 value of about 50 microM. This concentration is well within that expected for therapeutic regimens. Other anticonvulsants such as phenobarbital, phenytoin, primidone, valproate sodium, and ethosuximide had little or no effect on binding, while theophylline and caffeine caused marked inhibition. Carbamazepine had no marked effect on cyclic AMP levels in guinea pig cerebral cortical or hippocampal slices, but was a weak inhibitor (IC50 about 200 microM) of 2-chloroadenosine-elicited accumulations of cyclic AMP via an A2 adenosine receptor in cortical slices. Carbamazepine is thus a somewhat selective ligand for A1 adenosine receptors in brain. The nature of its activity at those receptors is unclear, but its lack of central stimulant effects contrasts to the stimulant properties of A1 adenosine receptor antagonist such as caffeine and theophylline.     

Clinical comparability of the new antiepileptic drugs in refractory partial epilepsy: a systematic review and meta-analysis

Purpose: Evaluate the clinical comparability of new antiepileptic drugs (AEDs) in partial refractory epilepsy. Methods: Systematic review of randomized trials (RCTs) comparing a new AED (add‐on treatment) with placebo or another AED. Primary outcomes: responder (≥50% seizure reduction) and withdrawal (tolerability) rates. Pooled estimates of odds ratios (ORs) and number needed treat/harm (NNT/NNH) taking into account baseline risk were derived by random‐effects meta‐analysis. Adjusted frequentist indirect comparisons between AEDs were estimated. Key Findings: Sixty‐two placebo‐controlled (12,902 patients) and eight head‐to‐head RCTs (1,370 patients) were included. Pooled ORs for responder and withdrawal rates (vs. placebo) were 3.00 [95% confidence interval (CI) 2.63–3.41] and 1.48 (1.30–1.68), respectively. Indirect comparisons of responder rate based on relative measurements of treatment effect (ORs) favored topiramate (1.52; 1.06–2.20) in comparison to all other AEDs, whereas gabapentin (0.67; 0.46–0.97) and lacosamide (0.66; 0.48–0.92) were less efficacious, without significant heterogeneity. When analyses were based on absolute estimates (NNTs), topiramate and levetiracetam were more efficacious, whereas gabapentin and tiagabine were less efficacious. Withdrawal rate was higher with oxcarbazepine (OR 1.60; 1.12–2.29) and topiramate (OR 1.68; 1.07–2.63), and lower with gabapentin (OR 0.65; 0.42–1.00) and levetiracetam (OR 0.62; 0.43–0.89). Significance: The differences found are of relatively small magnitude to allow a definitive conclusion about which new AED(s) has superior effectiveness. This uncertainty probably reflects the limitations of conclusions based on indirect evidence. The process of pharmacologic clinical decision making in partial refractory epilepsy probably depends more on other aspects, such as individual patient characteristics and pharmacoeconomics, than on available controlled randomized evidence.     

Effect of gabapentin on the anticonvulsant activity of antiepileptic drugs against electroconvulsions in mice: an isobolographic analysis

PURPOSE: The objective of this study was the isobolographic evaluation of the interactions between the novel antiepileptic drug (AED) gabapentin (GBP) and a number of other AEDs against electroconvulsion-induced convulsions in mice. METHODS: Electroconvulsions were produced by means of an alternating current (ear-clip electrodes, 0.2-s stimulus duration, tonic hindlimb extension taken as the end point). Adverse effects were evaluated with the chimney test (motor performance) and passive-avoidance task (long-term memory). Plasma levels of AEDs were measured by immunofluorescence or high-pressure liquid chromatography. RESULTS: GBP (< or =50 mg/kg) remained ineffective on the electroconvulsive threshold. According to the isobolographic analysis, GBP appears to act synergistically with carbamazepine, valproate, phenytoin, phenobarbital (PB), lamotrigine (LTG), and LY 300164. The pharmacokinetic events may be responsible for the interactions of GBP/PB and GBP/LTG, because only PB and LTG significantly elevated the plasma concentration of this AED. Conversely, GBP did not affect the plasma levels of other AEDs used in this study. No adverse effects were induced by combinations of GBP with these AEDs. CONCLUSIONS: The isobolographic analysis revealed that combinations of GBP with other AEDs generally results in synergistic (supraadditive) interactions.     

Anticonvulsant profile and teratogenicity of 3,3-dimethylbutanoylurea: a potential for a second generation drug to valproic acid

Purpose: The purpose of this study was to evaluate the anticonvulsant activity and teratogenic potential of branched aliphatic acylureas represented by isovaleroylurea (IVU), pivaloylurea (PVU) and 3,3‐dimethylbutanoylurea (DBU), as potential second‐generation drugs to valproic acid (VPA). Methods: The anticonvulsant activity of IVU, PVU, and DBU was determined in mice and rats utilizing the maximal electroshock seizure (MES) and the pentylenetetrazole (scMet) tests. The ability of DBU to block electrical‐, or chemical‐induced seizures was further examined in three acute seizure models: the psychomotor 6 Hz model, the bicuculline and picrotoxin models and one model of chronic epilepsy (i.e., the hippocampal kindled rat model). The induction of neural tube defects (NTDs) by IVU, PVU, and DBU was evaluated after i.p. administration at day 8.5 of gestation to a mouse strain highly susceptible to VPA‐induced teratogenicity. The pharmacokinetics of DBU was studied following i.v. administration to rats. Results: DBU emerged as the most potent compound having an MES‐ED ₅₀ of 186 mg/kg (mice) and 64 mg/kg (rats) and an scMet‐ED ₅₀ of 66 mg/kg (mice) and 26 mg/kg (rats). DBU underwent further evaluation in the hippocampal kindled rat (ED ₅₀ = 35 mg/kg), the psychomotor 6 Hz mouse model (ED ₅₀ = 80 mg/kg at 32 mA and ED ₅₀ = 133 mg/kg at 44 mA), the bicuculline‐ and picrotoxin‐induced seizure mouse model (ED ₅₀ = 205 mg/kg and 167 mg/kg, respectively). In contrast to VPA, DBU, IVU, and PVU did not induce a significant increase in NTDs as compared to control. DBU was eliminated by metabolism with a half‐life of 4.5 h. Conclusions: DBU's broad spectrum and potent anticonvulsant activity, along with its high safety margin and favorable pharmacokinetic profile, make it an attractive candidate to become a new, potent, and safe AED.     

The influence of antiepileptic drugs on cognition: a comparison of levetiracetam with topiramate

Levetiracetam (LEV) and topiramate (TPM) are considered highly effective novel antiepileptic drugs (AEDs) in the treatment of focal epilepsies. To explore potential side effects, this study investigated their influence on cognitive functions comparatively by means of a standardized neuropsychological test battery assessing several cognitive domains. In this observational study, cognitive changes were explored in 30 consecutively recruited patients with focal epilepsy treated with LEV and in 21 patients treated with TPM, comparing functions assessed prior to gradual initiation and after reaching steady state of the individual target dosage. Before titration, patient groups did not differ significantly with respect to cognitive performance. Whereas the LEV group manifested no change in cognitive performance after AED titration, the TPM group worsened in the cognitive domains of cognitive speed and verbal fluency, as well as short-term memory. These findings suggest that TPM, unlike LEV, may impair frontal lobe functions. The lack of cognitive side effects related to LEV treatment may be relevant for treatment decisions.     

Anticonvulsant drugs alter plasma tryptophan concentrations in epileptic patients: implications for antiepileptic action and mental function.

In epileptic patients carbamazepine and diphenylhydantoin have opposite effects on whole and free plasma tryptophan concentrations, the former elevating and the latter depressing them. It is unlikely that these observations relate to the similar anti-epileptic properties of these two drugs but they may relate to their different effects on mental function and mood.     

Differential effects of antiepileptic drugs on sexual function and reproductive hormones in men with epilepsy: interim analysis of a comparison between lamotrigine and enzyme-inducing antiepileptic drugs

PURPOSE: We compared sexual function and reproductive hormone levels among men with localization-related epilepsy (LRE) taking various antiepileptic drugs (AEDs) and normal controls (NC). METHODS: Subjects were 63 men with LRE [enzyme-inducing (EI) AEDs, 36; lamotrigine (LTG), 18; no AEDs, 9] and 18 NC. Sexual interest and function (S-score), hormone levels [bioactive testosterone (BAT) and estradiol (BAE)], hormone ratios [BAT/BAE], and gonadal efficiency [BAT/luteinizing hormone (LH)] were compared among the groups. RESULTS: S-scores, BAT levels, BAT/BAE, and BAT/LH were significantly lower in the EIAED group than in NC or LTG groups. Sex hormone-binding globulin (SHBG) was significantly higher in the EIAED group than in all other groups. Of men with LRE, 23.8% had abnormally low S-scores: 33.3% taking EIAEDs, 5.5% taking LTG, and 22.2% taking no AEDs (p < 0.01). BAT was low in 55.6% taking EIAEDs as compared with 33.3% taking LTG and 33.3% taking no AEDs (p < 0.05). Among men with low S-scores, 86.7% had low BAT as compared with 33.3% of men with normal scores (p < 0.01). BAT decline with age was greater among men with LRE than in controls (3.75 vs. 1.80 ng/dl/yr). The slope showed no significant difference among LRE groups. However, 89% of 40- to 50-year-old men taking EIAEDs had low BAT as compared with 33% taking LTG and 33% taking no AED (p < 0.01). CONCLUSIONS: Sexual function, BAT levels, BAT/BAE, and gonadal efficiency are greater with LTG than with EIAED. Abnormally low BAT levels are reached at an earlier age with EIAEDs than with LTG.     

P-glycoprotein-mediated efflux of antiepileptic drugs: preliminary studies in mdr1a knockout mice

Evidence suggests that the efflux transporter P-glycoprotein (P-gp) may play a facilitatory role in refractory epilepsy by limiting the brain access of antiepileptic drugs (AEDs). We have conducted a preliminary pharmacokinetic study of seven commonly used AEDs in mdr1a knockout mice, devoid of P-gp at the blood-brain barrier. A parallel group of matched wild-type mice served as controls. AEDs were administered by subcutaneous injection and serum and brain drug concentrations determined at 30, 60, and 240min post-dosing. The brain-serum concentration ratio for topiramate was higher in mdr1a(-/-) mice than in wild-type controls at all time points investigated. No consistent effects were observed with any other AED investigated. These findings suggest that topiramate may be a substrate for P-gp-mediated transport. Further studies employing a range of model systems are required to substantiate this observation and to address the potential role of drug transporters in refractory epilepsy.     

Preclinical profile of remacemide: a novel anticonvulsant effective against maximal electroshock seizures in mice

Anticonvulsant tests in mice revealed specific, potent actions of remacemide for protection of mice against maximal electroshock seizures (MES). Comparisons of oral efficacy to reference compounds yielded the following ED50 values (expressed as mg/kg): remacemide = 33, phenytoin = 11, phenobarbital = 20, carbamazepine = 13 and valproate = 631. The duration for protection by remacemide was longer than carbamazepine or valproate, but shorter than phenytoin or phenobarbital. In neural impairment tests (inverted screen or rotorod) to determine the oral toxic dose 50 (TD50) the following therapeutic indices (TD50/ED50) were obtained: (1) inverted screen--remacemide = 17.6, phenytoin = 57.4, phenobarbital = 5.1, carbamazepine = 10.2, and valproate = greater than 3; and (2) rotorod--remacemide = 5.6, phenytoin = 9.6, phenobarbital 4.8, and valproate = 1.9. Remacemide was devoid of sedative actions and possessed a favorable 28.1 margin of safety value (median estimated lethal dose/ED50 for MES). An intermediate potency against either audiogenic- or N-methyl-D-aspartate-induced seizures was exhibited by remacemide. Tolerance to MES was not apparent after 5 days of oral daily dosing of remacemide. Remacemide was inactive in vitro against gamma-aminobutyrate or benzodiazepine receptors and adenosine uptake mechanisms. Therapeutic utility for generalized tonic/clonic seizures is predicted for remacemide.     

Lecture: profile of risks and benefits of new antiepileptic drugs in brain tumor-related epilepsy

In patients with brain tumor, seizures are the onset symptom in 20-40% of the patients, while a further 20-45% of the patients will present them during the course of the disease. These data are important when considering the choice of antiepileptic drugs for this particular patient population, because brain tumor-related epilepsy (BTRE) is often drug resistant, has a strong impact on the quality of life and weighs heavily on public health expenditures. In brain tumor patients, the presence of epilepsy is considered as the most important risk factor for long-term disability. For this reason, the problem of the proper administration of medications and their potential side effects is of great importance, because good seizure control can significantly improve the patient's psychological and relational sphere. In these patients, new generation drugs such as gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, topiramate, and zonisamide are preferred, because they have fewer drug interactions and cause fewer side effects. Among the recently marketed drugs, lacosamide has demonstrated promising results and should be considered as a possible treatment option. Therefore, it is necessary to develop a customized treatment plan for each patient with BTRE, whose goals are complete seizure control, minimal or no side effects, and elimination of cognitive impairment and/or psychosocial problems.     

Pharmacodynamic and pharmacokinetic interactions between common antiepileptic drugs and acetone, the chief anticonvulsant ketone body elevated in the ketogenic diet in mice

Purpose:   Acetone is the principal ketone body elevated in the ketogenic diet (KD), with demonstrated robust anticonvulsant properties across a variety of seizure tests and models of epilepsy. Because the majority of patients continue to receive antiepileptic drugs (AEDs) during KD treatment, interactions between acetone and AEDs may have important clinical implications. Therefore, we investigated whether acetone could affect the anticonvulsant activity and pharmacokinetic properties of several AEDs against maximal electroshock (MES)–induced seizures in mice.
Methods:   Effects of acetone given in subthreshold doses were tested on the anticonvulsant effects of carbamazepine (CBZ), lamotrigine (LTG), oxcarbazepine (OXC), phenobarbital (PB), phenytoin (PHT), topiramate (TPM) and valproate (VPA) against MES-induced seizures in mice. In addition, acute adverse effects of acetone–AEDs combinations were assessed in the chimney test (motor performance) and passive avoidance task (long-term memory). Pharmacokinetic interactions between acetone and AEDs were also studied in the mouse brain tissue.
Results:   Acetone (5 or 7.5 mmol/kg, intraperitoneally [i.p.]) enhanced the anticonvulsant activity of CBZ, LTG, PB, and VPA against MES-induced seizures; effects of OXC, PHT, and TPM were not changed. Acetone (7.5 mmol/kg) did not enhance the acute adverse-effect profiles of the studied AEDs. Acetone (5 or 7.5 mmol/kg, i.p.) did not affect total brain concentrations of the studied AEDs. In contrast, VPA, CBZ, LTG, OXC, and TPM significantly decreased the concentration of free acetone in the brain; PB and PHT had no effect.
Conclusions:   Acetone enhances the anticonvulsant effects of several AEDs such as VPA, CBZ, LTG, and PB without affecting their pharmacokinetic and side-effect profiles.