Antiepileptic drug combinations and experimental background: The case of phenobarbital and phenytoin

Summary In view of the trend towards single drug therapy of epilepsy, the experimental background for thr combination of phenobarbital (PB) and phenytoin (PHT) was reassessed. Since potentiation of the anticonvulsant activity is in itself neither sufficient nor necessary to demonstrate the superiority of a drug combination, protection against maximal electroshock seizures as well as neurotoxicity were determined in mice. This allowed one to separate the neurotoxic from the anticonvulsant interaction and to base the analysis on changes in the therapeutic index (TI). Pharmacokinetic interactions were excluded from the analysis by expressing all results in terms of brain concentrations. The anticonvulsant interaction between PB and PHT was found to be additive, whereas the neurotoxic interaction was antagonistic. These results provide experimental documentation of one of the theories behind antiepileptic drug combinations. However, because PB had a markedly lower TI than PHT in this model, the TI of the drug combination was lower than the TI of PHT. Thus, the antagonism with regard to neurotoxicity was not sufficient to raise the TI of the combination above the TI of PHT.     

Drug interaction assessment following concomitant administration of posaconazole and phenytoin in healthy men

ABSTRACT

Objective: Posaconazole is an extended-spectrum triazole antifungal agent for the treatment and prophylaxis of invasive fungal infections. This randomized, open-label, parallel-group, multiple-dose study was conducted in healthy adult volunteers to assess the potential for a drug interaction between phenytoin and the posaconazole tablet formulation.

Methods: Subjects were randomly assigned for 10 days to one of the following treatments: posaconazole (200?mg once daily), phenytoin (200?mg once daily), or posacon­azole (200?mg once daily) and phenytoin (200?mg once daily). Blood samples were collected on days 1 and 10 for pharmacokinetic evaluation of posaconazole and pheny­toin concentrations.

Results: A total of 36 healthy men enrolled in the study. On day 1, the maximum plasma concentration (C_max) and area under the concentration–time curve calculated from time 0–24?h post-dose (AUC_(0–24)) were unchanged upon co-administration. At steady state (day 10), co-administra­tion of posaconazole with phenytoin resulted in 44% (?p = 0.012) and 52% (?p = 0.007) decreases in posaconazole C_max and AUC_(0–24), respectively. These decreases in exposure corresponded with a 90% increase in steady-state clearance of orally administered posaconazole. Phenytoin C_max and AUC_(0–24) were not significantly altered upon co-administration of the two agents, 24% increase in C_max (?p = 0.196) and 25% increase in AUC_(0–24) (?p = 0.212) values, although inter-subject variability was observed within this group.

Conclusion: Because co-administration of phenytoin and posaconazole significantly reduces posaconazole exposure and increases phenytoin levels in some subjects, concomitant use of these agents should be avoided unless the benefit outweighs the risk.     

Effects of ibuprofen on the disposition kinetics of phenytoin in pregnant rats

The effects of ibuprofen on maternal phenytoin pharmacokinetics and fetal phenytoin acquisition were investigated in 19-day gestation Sprague-Dawley rats. A 5 mg kg-1 bolus injection of 14C-phenytoin was given with and without (control) pretreatment with 12.5 mg kg-1 of ibuprofen. Maternal plasma and fetal whole body samples were obtained at various times after the phenytoin bolus and evaluated simultaneously using a three-compartment maternal-fetal model. Ibuprofen pretreatment increased the maternal plasma clearance of phenytoin about three-fold and the overall apparent volume of distribution almost four-fold. No changes in the volume of the maternal central compartment or terminal first-order disposition rate constant were observed. Additionally, the maternal-to-fetal clearance of phenytoin was not altered in the ibuprofen-treated rats. No differences in the apparent fetal volume of distribution or areas under the fetal phenytoin concentration-time curves were observed between the control and ibuprofen-treated rats. The results of this study were consistent with ibuprofen-induced alterations in organ and tissue blood perfusion and demonstrated that, while the maternal disposition kinetics of phenytoin were altered by sodium ibuprofen coadministration, the maternal changes did not affect the extent of fetal exposure to phenytoin.     

Subsensitivity to muscimol-induced catalepsy after long-term administration of phenytoin in rats

Male albino rats were injected with 25 mg/kg of phenytoin (PHT) every day for 20 consecutive days and were tested on days 21 and 28 for their response to 1 or 2 mg/kg of muscimol, a GABA receptor agonist. Rats treated with PHT showed a decreased responsiveness to muscimol-induced catalepsy (2 mg/kg) on day 21 but not on day 28. Acutely administered PHT, on the contrary, had a tendency to potentiate muscimol-induced catalepsy. Muscimol-induced catalepsy was not antagonized by acute treatment with bicuculline (0.5–2.0 mg/kg). It is proposed that withdrawal after long-term administration of PHT reduces the sensitivity of a GABA receptor site not sensitive to bicuculline.     

Effects of phenytoin on schedule-controlled performance of rats

The present study examined the effects of acute administrations of phenytoin on the lever pressing of rats maintained under fixed-ratio, fixed-interval, and interresponse-time-greater-than-T schedules of food delivery. The drug typically produced dose-dependent decreases in response rates under fixed-ratio and fixed-interval schedules, while response rates under the interresponse-time-greater-than-T schedule were affected little by the drug. These findings indicate that phenytoin effects on schedulecontrolled responding differ from those of other anticonvulsants.     

Effects of phenytoin, phenobarbital, and valproic acid, alone and in selected combinations, on schedule-controlled behavior of rats

The present study examined the effects of phenytoin (20, 30, 40, and 50 mg/kg), phenobarbital (10, 20, 30, and 40 mg/kg), and valproic acid (80, 120, 160, and 240 mg/kg), and those of phenobarbital (10 and 30 mg/kg) combined with phenytoin (20, 30, and 40 mg/kg) or valproic acid (80, 120, and 160 mg/kg), on the lever pressing of rats maintained under fixed-ratio and interresponse-time-greater-than-t schedules of food delivery. High doses of each individual drug significantly decreased mean group response (and reinforcement) rate under the fixed-ratio schedule. No dose of an individual agent significantly affected mean group response rate under the interresponse-time-greater-than-t schedule, although high doses of phenobarbital and valproic acid significantly reduced the mean group reinforcement rate under this schedule. When given in combination, phenobarbital and phenytoin and phenobarbital and valproic acid significantly reduced response (and reinforcement) rate under the fixed-ratio schedule and reinforcement rate under the interresponse-time-greater-than-t schedule. These reductions did not significantly differ in magnitude from those predicted by an additive model of drug interaction.     

应用高压液相色谱法研究苯妥英钠的临床药代动力学及生物利用度

应用高压液相色谱法测定六位正常人一次口服苯妥英钠胶囊剂200mg后血浆中药物浓度的动力学变化。根据药—时曲线半对数图确定以一房室开放模型,一级消除动力学求得苯妥英钠的药代动力学各项参数。苯妥英钠片剂和糖衣片剂的相对生物利用度分别为74.71%和89.76%。     

Comparative effects of carbamazepine,phenytoin, diazepam and clonazepam on inhibitory avoidance learning in mice

Four antiepileptic drugs were investigated in an inhibitory avoidance task in mice. Following IP administration 30 min prior to training, carbamazepine (32 mg/kg), phenytoin (30–60 mg/kg), diazepam (2–8 mg/kg) and clonazepam (0.125–0.5 mg/kg) impaired retention. When administered 30 min prior to the retention test none of the drugs under investigation affected retention. The drugs did not affect latencies in the hot plate test. This indicates that in the case of pretraining drug administration effects on retention cannot attributed to elevated pain thresholds. Carbamazepine and phenytoin impaired avoidance learning at doses above those which prevent electroshock induced tonic hindlimb convulsions. Diazepam and clonazepam were effective at lower than anticonvulsant doses. The results of the study are relevant to the evaluation of CNS side effects of antiepileptic drugs in mice.     

Effect of phenytoin on sodium and calcium currents in hippocampal CA1 neurons of phenytoin-resistant kindled rats.

About 20-30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms explaining why some patients' respond and others prove resistant to antiepileptic drugs are poorly understood. It has been proposed that pharmacoresistance is related to reduced sensitivity of sodium channels in hippocampal neurons to antiepileptic drugs such as carbamazepine or phenytoin. In line with this proposal, a reduced effect of carbamazepine on sodium currents in hippocampal CA1 neurons was found in the rat kindling model of temporal lobe epilepsy (TLE), i.e. a form of epilepsy with the poorest prognosis of all epilepsy types in adult patients. To address directly the possibility that neuronal sodium currents in the hippocampus play a crucial role in the pharmacoresistance of TLE, we selected amygdala-kindled rats with respect to their in vivo anticonvulsant response to phenytoin into responders and nonresponders and then compared phenytoin's effect on voltage-activated sodium currents in CA1 neurons. Furthermore, in view of the potential role of calcium current modulation in the anticonvulsant action of phenytoin, the effect of phenytoin on high-voltage-activated calcium currents was studied in CA1 neurons. Electrode-implanted but not kindled rats were used as sham controls for comparison with the kindled rats. In all experiments, the interval between last kindled seizure and ion channel measurements was at least 5 weeks. In kindled rats with in vivo resistance to the anticonvulsant effect of phenytoin (phenytoin nonresponders), in vitro modulation of sodium and calcium currents by phenytoin in hippocampal CA1 neurons did not significantly differ from respective data obtained in phenytoin responders, i.e. phenytoin resistance was not associated with a changed modulation of the sodium or calcium currents by this drug. Compared to sham controls, phenytoin's inhibitory effect on sodium currents was significantly reduced by kindling without difference between the responder and nonresponder subgroups. Further studies in phenytoin-resistant kindled rats may help to elucidate the mechanisms that can explain therapy resistance.     

The effect of administration of phenytoin on the pharmacokinetics of isoxicam

This study was designed to determine whether the disposition of isoxicam is influenced by the coadministration of another acidic drug, highly bound to plasma proteins and extensively metabolized, i.e., phenytoin. Ten healthy volunteers received an oral dose of 200 mg of isoxicam prior to and following the oral administration of phenytoin (100 mg) twice a day for 10 days. Eleven blood samples were drawn during the period following each dose of isoxicam. The area under the isoxicam plasma concentration-time curve (AUC infinity) increased from 389 +/- 66 to 464 +/- 62 micrograms h ml-1 (+/- SEM) (p less than 0.05) after treatment with phenytoin. This increase was due to an increase in isoxicam bioavailability; the absorption rate constant for isoxicam increased correspondingly from 0.34 +/- 0.06 to 1.16 +/- 0.38 h-1 (p less than 0.05). Distribution and clearance of isoxicam were probably not affected as its half-life was not changed, its plasma peak concentration increased, and the time to reach this peak decreased. It is concluded that phenytoin increases the rate and extent of absorption of isoxicam.     

Pharmacokinetics of verproside after intravenous and oral administration in rats

Verproside, a catalpol derivative iridoid glucoside isolated from Pseudolysimachion longifolium, is a candidate for anti-asthmatic drug. The dose-dependency of the pharmacokinetics of verproside was evaluated in rats after intravenous and oral administration. After intravenous administration of verproside (2, 5 and 10 mg/kg doses), the systemic clearance (Cl) was significantly reduced and AUC was significantly increased at 10 mg/kg dose compared to 2 and 5 mg/kg doses. The volume of distribution at steady state (V _ss) remained unchanged as the dose was increased. The extent of urinary excretion was low for both intravenous (3.3–6.2%) and oral (0.01–0.04%) doses. Isovanilloylcatalpol was identified as a metabolite after intravenous administration of verproside and showed the significant decreases in AUC and C _max at 10 mg/kg verproside dose. The reduced systemic clearance of verproside at high doses appears to be due to the saturable metabolism. Upon oral administration of verproside (20, 50 and 100 mg/kg doses), C _max was nonlinearly increased. The extent of verproside recovered from the gastrointestinal tract at 24 h after oral administration was 0.01–0.72% for all three doses studied. The absolute oral bioavailability (F) was 0.3 and 0.5% for 50 and 100 mg/kg doses, respectively. Low F appears to be due to first-pass metabolism.     

Chronic effects of ethosuximide,phenytoin, clonazepam,and valproic acid on the delayed-matching-to-sample performance of pigeons

Acute and chronic effects of ethosuximide (40, 80, and 120 mg/kg), phenytoin (2.5, 5, and 7.5 mg/kg), clonazepam (0.25, 0.5, and 0.75 mg/kg), and valproic acid (40, 80, and 120 mg/kg) were examined in pigeons performing under a delayed-matching-to-sample procedure. Acute administration of clonazepam or valproic acid produced generally dose-dependent decreases in accuracy; over 50 sessions of daily exposure, complete or nearly complete tolerance developed to the accuracy-reducing effects of these drugs. Whether administered acutely or chronically, ethosuximide and phenytoin failed to affect accuracy. When given acutely, all drugs typically reduced rate of responding to the sample stimulus. A degree of tolerance appeared to develop to the rate-decreasing effects of all of the drugs tested. Offprint requests to: A. Poling     

Prenatal exposure to sodium phenytoin in rats induces complex maze learning deficits comparable to those induced by exposure to phenytoin acid at half the dose

Gravid Sprague-Dawley CD (VAF) rats were administered sodium phenytoin suspended in corn oil by gavage once per day on embryonic days 7–18 at a dose of 100 mg/kg. Controls were administered corn oil alone by gavage on E7-18. Litters were randomly culled to 10. Offspring were regularly weighed, mortality noted, and males checked for preputial separation. At approximately 50 days of age offspring were evaluated in a straight water-filled channel for swimming proficiency and motivation to escape. Following this, rats were tested in the Cincinnati multiple T-water maze and scored for errors, latency to find the goal, and presence of phenytoin-induced abnormal circling behavior while swimming. Sodium phenytoin-exposed dams gained weight normally and delivered normally. Offspring mortality in the sodium phenytoin group was not increased above controls. No treatment effects on preputial separation or offspring growth were observed. No differences between groups in swimming proficiency in straight channel performance were obtained. In the Cincinnati maze, phenytoin offspring committed significantly more errors and had longer latencies to find the goal than controls. Among the phenytoin offspring, those exhibiting abnormal circling committed more errors than noncircling animals. When compared to previous data using the same maze and test protocol, it was found that 100 mg/kg of sodium phenytoin induced performance deficits similar to those induced by a dose of 200 mg/kg of phenytoin acid. Accordingly, the present data help explain why other investigators have reported sodium phenytoin to be more developmentally neurotoxic than phenytoin acid. Because the prenatal neurotoxic effects seen with the salt of phenytoin occur at lower doses, it suggests that phenytoin is more developmentally neurotoxic than previously believed.     

Estimation of Michaelis-Menten constant of efflux transporter considering asymmetric permeability

It was previously reported that the apparent K_m values of P-gp in apical to basal (A to B) and basal to apical (B to A) directions were different. The purpose of the present study was to derive a theoretical framework by which this asymmetric concentration-permeability profile can be explained using a single intrinsic K_m value. A three compartment model was used to represent the apical, cytosol and basal compartments. The difference of passive permeability and the surface areas between the apical and basolateral membrane were explicitly taken into account. Applying the steady state approximation and considering the mass balance in the cytosol compartment, an open analytical solution was obtained. By using this equation, the asymmetric concentration-permeability profile was appropriately reproduced. In addition, the expression level dependency of apparent K_m was also reproduced.     

Enhanced bioavailability of verapamil after oral administration with hesperidin in rats

The aim of this study was to investigate the effects of hesperidin on the pharmacokinetics of verapamil and its major metabolite, norverapamil, in rats. The pharmacokinetic parameters of verapamil and norverapamil in rats were measured after the oral administration of verapamil (9 mg/kg) in the presence or absence of hesperidin (3 or 10 mg/kg). Compared to the control group, the presence of hesperidin significantly (p<0.01) increased the area under the plasma concentration-time curve (AUC) of verapamil by 71.1–96.8% and the peak concentration (C_max) of verapamil by 98.3–105.2%. Hesperidin significantly (p<0.01) decreased the total plasma clearance (CL/F) of verapamil by 41.6–49.2% in rats. However there was no significant change in the time to reach the peak plasma concentration (T_max), the elimination rate constant (K_el) and the terminal half-life (T_1/2) of verapamil in the presence of hesperidin. The AUC and C_max of norverapamil were significantly (p<0.05) higher in rats coadministrated with hesperidin than those of the control. Consequently hesperidin significantly enhanced bioavailability of verapamil in rats. These results might be due to the decreased efflux and metabolism of verapamil in the intestine. Drug interactions should be concerned in the clinical setting when verapamil is used concomitantly with hesperidin or hesperidin-containing dietary.     

Chronic cyclosporine administration induces renal P-glycoprotein in rats

The effect of cyclosporine doses on renal P-glycoprotein expression was examined. Rats were given cyclosporine orally at 2, 10, 30 mg/kg/day or subcutaneously at 1, 5, 15 mg/kg/day for 28 days with or without 14 days of additional vehicle dosing. Following cyclosporine dosing, renal function and P-glycoprotein expression were measured. Renal function was reduced in rats receiving oral cyclosporine and the highest subcutaneous dose, 15 mg/kg/day. Western blot analysis showed that cyclosporine administered orally at 10 and 30 mg/kg/day and subcutaneously at 15 mg/kg/day induced significantly renal P-glycoprotein expression. After discontinuation of cyclosporine, renal P-glycoprotein returned to pre-dosing levels in oral groups, whereas the return was incomplete in subcutaneous groups. These results indicate that cyclosporine induces renal P-glycoprotein overexpression a dose-dependent manner.     

Differences in the transport of the antiepileptic drugs phenytoin, levetiracetam and carbamazepine by human and mouse P-glycoprotein

In view of the important role of P-glycoprotein (Pgp) and other drug efflux transporters for drug distribution and resistance, the identification of compounds as substrates of Pgp-mediated transport is one of the key issues in drug discovery and development, particularly for compounds acting on the central nervous system. In vitro transport assays with Pgp-transfected kidney cell lines are widely used to evaluate the potential of compounds to act as Pgp substrates or inhibitors. Furthermore, such cell lines are also frequently utilized as a substitute for more labor-intensive in vitro or in vivo models of the blood-brain barrier (BBB). Overexpression of Pgp or members of the multidrug resistance protein (MRP) family at the BBB has been implicated in the mechanisms underlying resistance to antiepileptic drugs (AEDs) in patients with epilepsy. Therefore, it is important to know which AEDs are substrates for Pgp or MRPs. In the present study, we used monolayers of polarized MDCKII dog kidney or LLC-PK1 pig kidney cells transfected with cDNA containing either human MDR1, MRP2 or mouse mdr1a and mdr1b sequences to measure the directional transport of AEDs. Cyclosporin A (CsA) and vinblastine were used as reference standards for Pgp and MRP2, respectively. The AEDs phenytoin and levetiracetam were directionally transported by mouse but not human Pgp, whereas CsA was transported by both types of Pgp. Carbamazepine was not transported by any type of Pgp and did not inhibit the transport of CsA. In contrast to vinblastine, none of the AEDs was transported by MRP2 in transfected kidney cells. The data indicate that substrate recognition or transport efficacy by Pgp differs between human and mouse for certain AEDs. Such species differences, which are certainly not restricted to human and mouse, may explain, at least in part, the controversial data which have been previously reported for AED transport by Pgp in preparations from different species. However, because transport efficacy of efflux transporters such as Pgp or MRP2 may not only differ between species but also between tissues, the present data do not exclude that the AEDs examined are weak substrates of Pgp or MRP2 at the human BBB.     

Acute and chronic effects of phenytoin on fixed-ratio performance of pigeons

The effects of phenytoin on the key-pecking of pigeons maintained under a fixed-ratio 50 schedule of food reinforcement were studied. When initially administered across a range of presession intervals, acute intramuscular injections of 10 and 20 mg/kg of the drug generally decreased response rates. The magnitude of this effect was greater at the larger dose. Comparison of the effects of 10 and 20 mg/kg doses before and after chronic exposure to phenytoin showed that a degree of tolerance to the drug's behavioral effects did develop.     

Dose and sex-dependent disposition of ketoconazole in rats

The disposition of the antifungal drug ketoconazole was studied in mature (60-day-old) male and female rats given single intravenous doses of 10, 20 or 40 mg/kg body weight. The plasma profiles of ketoconazole were characterized by an initial rapid decline, followed by an apparent zero-order decline and a subsequent first-order elimination phase. In male animals the zero-order phase was less pronounced, resulting in a 3–5 times higher overall rate of elimination. A consequence of the dose-dependent disposition was that a 4-fold increase in dose resulted in a 9- and 17-fold increase in the area under the plasma concentration-time curve (AUC) of females and males, respectively. Terminal half-lives were independent of dose in both sexes. The disproportionate increase in AUC with dose, together with the observation that no intact ketoconazole was excreted in urine and only very small amounts in bile (<1% of given dose), suggest that the dose-dependent disposition is caused by saturation of metabolizing enzymes. These enzymes are most likely under the influence of androgens, since the capacity of males to eliminate ketoconazole was reduced by castration and in females this capacity was increased by testosterone treatment.