Effect of sodium valproate on plasma protein binding of diphenylhydantoin

Microdialysis techniques and microdialysis-based patient-near diagnostics

This article will debate the usefulness of POCT measurements and the contribution microdialysis can make to generating valuable information. A particular theme will be the rarely considered difference between ex vivo sampling, which typically generates only a static measure of concentration, and in vivo measurements that are subject to dynamic changes due to mass transfer. Those dynamic changes provide information about the patients' physiological state.

Cytochrome P450-mediated estrogen catabolism therapeutic avenues in epilepsy

Epilepsy is a neuropsychiatric disorder, which does not have any identifiable cause. However, experimental and clinical results have asserted that the sex hormone estrogen level and endocrine system function influence the seizure and epileptic episodes. There are available drugs to control epilepsy, which passes through the metabolism process. Cytochrome P-450 family 1 (CYP1A1) is a heme-containing mono-oxygenase that are induced several folds in most of the tissues and cells contributing to their differential expression, which regulates various metabolic processes upon administration of therapeutics. CYP1A1 gene family has been found to metabolize estrogen, a female sex hormone, which plays a central role in maintaining the health of brain altering the level of estrogen active neuropsychiatric disorder like epilepsy. Hence, in this article, we endeavor to provide an opinion of estrogen, its effects on epilepsy and catamenial epilepsy, their metabolism by CYP1A1 and new way forward to differential diagnosis and clinical management of epilepsy in future.

Antiseizure, antidepressant, and antipsychotic medication prescribing in elderly nursing home residents

OBJECTIVE

The incidence of epilepsy is highest in the elderly and the prevalence of epilepsy is higher in nursing home residents than in other cohorts. Co-medications that act in the central nervous system (CNS) are frequently prescribed in this population. The objective was to identify the most commonly prescribed antiseizure drugs (ASDs) and determine the frequency of use of antipsychotic and antidepressant medications in elderly nursing home residents receiving ASDs.

METHODS

Data were obtained from a pharmacy database serving 18,752 patients in Minnesota and Wisconsin nursing homes. Prescribing information was available on ASD, antidepressant, and antipsychotic drugs on one day in October 2013. The frequency distribution by age, formulation, trademarked/generic drugs, route of administration, and multiple drug combinations were determined.

RESULTS

Overall, 66.8% of 18,752 residents received at least one CNS-active drug as classified by the Generic Product Identifier classification system. For those 65years and older, ASDs were prescribed for 14.3% residents. Gabapentin comprised 7.3%; valproate 3.0%; levetiracetam 1.8%; and phenytoin 0.9%. An antidepressant was used in 64.2% of persons prescribed an ASD. Antidepressant use varied for specific ASDs and ranged from 50 to 75%. An antipsychotic medication was used in 30% of persons prescribed an ASD and ranged from 16.8 to 54.2% for specific ASDs. Both antidepressant and antipsychotic use occurred in 22.2% of persons prescribed an ASD, respectively.

SIGNIFICANCE

The pattern of CNS-active drug use has changed from previous years in this geographic region. Use of phenytoin has declined markedly, but antidepressant use has increased substantially. The CNS side effect profile of these medications and the possible long-term consequences in this population can greatly complicate their therapy.

Antiepileptic drugs--best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies

Although no randomized studies have demonstrated a positive impact of therapeutic drug monitoring (TDM) on clinical outcome in epilepsy, evidence from nonrandomized studies and everyday clinical experience does indicate that measuring serum concentrations of old and new generation antiepileptic drugs (AEDs) can have a valuable role in guiding patient management provided that concentrations are measured with a clear indication and are interpreted critically, taking into account the whole clinical context. Situations in which AED measurements are most likely to be of benefit include (1) when a person has attained the desired clinical outcome, to establish an individual therapeutic concentration which can be used at subsequent times to assess potential causes for a change in drug response; (2) as an aid in the diagnosis of clinical toxicity; (3) to assess compliance, particularly in patients with uncontrolled seizures or breakthrough seizures; (4) to guide dosage adjustment in situations associated with increased pharmacokinetic variability (e.g., children, the elderly, patients with associated diseases, drug formulation changes); (5) when a potentially important pharmacokinetic change is anticipated (e.g., in pregnancy, or when an interacting drug is added or removed); (6) to guide dose adjustments for AEDs with dose-dependent pharmacokinetics, particularly phenytoin.

Clinically important drug interactions in epilepsy: general features and interactions between antiepileptic drugs

There are two types of interactions between drugs, pharmacokinetic and pharmacodynamic. For antiepileptic drugs (AEDs), pharmacokinetic interactions are the most notable type, but pharmacodynamic interactions involving reciprocal potentiation of pharmacological effects at the site of action are also important. By far the most important pharmacokinetic interactions are those involving cytochrome P450 isoenzymes in hepatic metabolism. Among old generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone induce the activity of several enzymes involved in drug metabolism, leading to decreased plasma concentration and reduced pharmacological effect of drugs, which are substrates of the same enzymes (eg, tiagabine, valproic acid, lamotrigine, and topiramate). In contrast, the new AEDs gabapentin, lamotrigine, levetiracetam, tiagabine, topiramate, vigabatrin, and zonisamide do not induce the metabolism of other AEDs. Interactions involving enzyme inhibition include the increase in plasma concentrations of lamotrigine and phenobarbital caused by valproic acid. Among AEDs, the least potential interaction is associated with gabapentin and levetiracetam.

Levetiracetam, a new antiepileptic agent: lack of in vitro and in vivo pharmacokinetic interaction with valproic acid

PURPOSE

The novel antiepileptic drug (AED) levetiracetam (LEV; Keppra) has a wide therapeutic index and pharmacokinetic characteristics predicting limited drug-interaction potential. It is indicated as an add-on treatment in patients with epilepsy, and thus coadministration with valproic acid (VPA) is likely. These studies were performed to determine whether coadministration of LEV with VPA might result in pharmacokinetic interactions.

METHODS

In vitro assays were performed to characterize the transformation of LEV into its main in vivo metabolite UCB L057. The reaction was examined for its sensitivity to clinically relevant concentrations of VPA. An open-label, one-way, one-sequence crossover clinical trial was conducted in 16 healthy volunteers to assess further the possibility of any relevant pharmacokinetic interaction.

RESULTS

Human whole blood and, to a lesser extent, human liver homogenates were demonstrated to hydrolyze LEV to UCB L057, its main metabolite. The reaction possibly involves type-B esterases and is not affected by 1 mM VPA (i.e., 166 microg/ml). Pharmacokinetic parameters of a single dose of LEV (1,500 mg) coadministered with steady-state concentrations of VPA (8 days of 500 mg, b.i.d.) did not differ significantly from the pharmacokinetics of LEV administered alone [area under the curve (AUC) of 397 and 400 microg/h/ml, respectively]. Furthermore, LEV did not affect the steady-state pharmacokinetics of VPA.

CONCLUSIONS

These findings suggest the absence of a pharmacokinetic interaction between VPA and LEV during short-term administration, and suggest that dose adjustment is not required when these two drugs are given together.

The importance of drug interactions in epilepsy therapy

Long-term antiepileptic drug (AED) therapy is the reality for the majority of patients diagnosed with epilepsy. One AED will usually be sufficient to control seizures effectively, but a significant proportion of patients will need to receive a multiple AED regimen. Furthermore, polytherapy may be necessary for the treatment of concomitant disease. The fact that over-the-counter drugs and nutritional supplements are increasingly being self-administered by patients also must be considered. Therefore the probability of patients with epilepsy experiencing drug interactions is high, particularly with the traditional AEDs, which are highly prone to drug interactions. Physicians prescribing AEDs to patients with epilepsy must, therefore, be aware of the potential for drug interactions and the effects (pharmacokinetic and pharmacodynamic) that can occur both during combination therapy and on drug discontinuation. Although pharmacokinetic interactions are numerous and well described, pharmacodynamic interactions are few and usually concluded by default. Perhaps the most clinically significant pharmacodynamic interaction is that of lamotrigine (LTG) and valproic acid (VPA); these drugs exhibit synergistic efficacy when coadministered in patients with refractory partial and generalised seizures. Hepatic metabolism is often the target for pharmacokinetic drug interactions, and enzyme-inducing drugs such as phenytoin (PHT), phenobarbitone (PB), and carbamazepine (CBZ) will readily enhance the metabolism of other AEDs [e.g., LTG, topiramate (TPM), and tiagabine (TGB)]. The enzyme-inducing AEDs also enhance the metabolism of many other drugs (e.g., oral contraceptives, antidepressants, and warfarin) so that therapeutic efficacy of coadministered drugs is lost unless the dosage is increased. VPA inhibits the metabolism of PB and LTG, resulting in an elevation in the plasma concentrations of the inhibited drugs and consequently an increased risk of toxicity. The inhibition of the metabolism of CBZ by VPA results in an elevation of the metabolite CBZ-epoxide, which also increases the risk of toxicity. Other examples include the inhibition of PHT and CBZ metabolism by cimetidine and CBZ metabolism by erythromycin. In recent years, a more rational approach has been taken with regard to metabolic drug interactions because of our enhanced understanding of the cytochrome P450 system that is responsible for the metabolism of many drugs, including AEDs. The review briefly discusses the mechanisms of drug interactions and then proceeds to highlight some of the more clinically relevant drug interactions between AEDs and between AEDs and non-AEDs. Understanding the fundamental principles that contribute to a drug interaction may help the physician to better anticipate a drug interaction and allow a graded and planned therapeutic response and, therefore, help to enhance the management of patients with epilepsy who may require treatment with polytherapy regimens.

3-Hydroxymethylphenytoin valproic acid ester, a new prodrug combining two anticonvulsant drugs

3-Hydroxymethylphenytoin valproic acid ester (VAL-PHT) was designed as a new prodrug combining valproic acid and phenytoin, two anticonvulsant drugs with different pharmacological profiles. The compound was hydrolyzed by rat plasma esterases in vitro but exhibited only activity in the maximal electroshock seizure test (MES test) after intraperitoneal administration to mice. The compound did not protect against pentylenetetrazole-induced seizures. It is concluded that VAL-PHT acts as a prodrug displaying the anticonvulsant profile of phenytoin.

Phenobarbitone to gabapentin: a guide to 82 years of anti-epileptic drug pharmacokinetic interactions

With the introduction of three new anti-epileptic drugs (AEDs) in the UK during the past 4 years as adjunctive add-on therapy, the possibility of AED pharmacokinetic interactions has become a relevant consideration. This review highlights the current status of AED interactions with particular emphasis on those interactions that are likely to be frequently experienced or whose outcome is potentially clinically significant.

Phenytoin concentration elevation subsequent to ranitidine administration

OBJECTIVE

To report elevated phenytoin (PHT) plasma concentrations in a patient receiving ranitidine.

CASE SUMMARY

A patient treated with PHT and ranitidine experienced elevated PHT plasma concentrations that persisted several days after PHT was discontinued. The PHT plasma concentration declined rapidly after withdrawal of ranitidine.

DISCUSSION

This is an unusual case report of elevated PHT plasma concentrations associated with concurrent ranitidine use. Ranitidine has been reported to interfere with the hepatic metabolism of other drugs. The proposed mechanism of this interaction is similar to that of other histamine 2-receptor antagonists--by binding to cytochrome P-450 hepatic mixed-function oxidase. We postulate that a small subset of patients may be susceptible to this effect of ranitidine.

CONCLUSIONS

This case was complicated by several variables that may have affected the changes observed in total PHT concentrations. However, an interaction between ranitidine and PHT should be considered, especially in a subpopulation of patients that are more susceptible to this effect. Patients using ranitidine and phenytoin concurrently should be routinely monitored.

Antiepileptic drugs. A review of clinically significant drug interactions

Approximately 20 to 30% of patients with active intractable epilepsy are commonly treated with polytherapy antiepileptic drug regimens, and these patients may experience complicated drug interactions. Furthermore, because of the long term nature of treatment, the possibility of drug interactions with drugs used for the treatment of concomitant disease is high. Classically, clinically significant drug interactions, both pharmacokinetic and pharmacodynamic, have been considered to be detrimental to the patient, necessitating dosage adjustment. However, this need not always be the case. With the introduction of new drugs (e.g. vigabatrin and lamotrigine) with known mechanisms of action, the possibility exists that these can be used synergistically. The most commonly observed clinically significant pharmacokinetic interactions can be attributed to interactions at the metabolic and serum protein binding levels. The best known examples relate to induction (e.g. phenobarbital, phenytoin, carbamazepine and primidone) or inhibition [e.g. valproic acid (sodium valproate)] of hepatic monoxygenase enzymes. The extent and direction of interactions between the different antiepileptic drugs are varied and unpredictable. Interactions in which the metabolism of phenobarbital, phenytoin or carbamazepine is inhibited are particularly important since these are commonly associated with toxicity. Some inhibitory drugs include macrolide antibiotics, chloramphenicol, cimetidine, isoniazid and numerous sulphonamides. A reduction in efficacy of antibiotic, cardiovascular, corticosteroid, oral anticoagulant and oral contraceptive drugs occurs during combination therapy with enzyme-inducing antiepileptic drugs. Discontinuation of the enzyme inducer or inhibitor will influence the concentrations of the remaining drug(s) and may necessitate dosage readjustment.

Drugs used in the management of trigeminal neuralgia

Management of trigeminal neuralgia, a severe facial pain, remains difficult. All patients are initially treated with drugs. Since the seventeenth century more than 40 different preparations have been used; some of these, although effective, had toxic side effects. The most useful drugs at present are carbamazepine, phenytoin, baclofen, and clonazepam. A new drug, oxcarbazepine, is showing therapeutic promise. The most common reason for therapeutic failure with antineuralgic drugs is inadequate dosage. We review here the pharmacokinetics, side effects, possible drug interactions, plasma and serum therapeutic concentrations, and the available formulations of each drug. On the basis of these considerations and clinical reports describing the use of these drugs, we make dosage recommendations to enable the practitioner to individualize therapeutic regimens.

Effects of discontinuation of phenytoin, carbamazepine, and valproate on concomitant antiepileptic medication

We report a prospective, controlled study of the effects of the reduction and discontinuation of phenytoin (PHT) (22 patients), carbamazepine (CBZ) (23 patients), and valproate (VPA) (25 patients) with concomitant antiepileptic drugs (AEDs). The principal changes in the serum concentrations of concomitant AEDs were (a) phenobarbital (PB) concentrations decreased by a mean of 30% on discontinuation of PHT; (b) total CBZ concentrations increased by a mean of 48% and free CBZ concentrations increased by a mean of 30% on discontinuation of PHT, with no change in CBZ-10, 11-epoxide (CBZ-E) concentrations; (c) VPA concentrations increased by a mean of 19% on discontinuation of PHT; (d) VPA concentrations increased by a mean of 42% on discontinuation of CBZ; (e) ethosuximide (ESM) concentrations increased by a mean of 48% on discontinuation of CBZ; (f) PHT concentrations decreased by a mean of 26% on discontinuation of CBZ; (g) PHT free fraction decreased from a mean of 0.11 to 0.07 on discontinuation of VPA; and (h) the mean concentrations of total and free CBZ increased by a mean of 10 and 16%, respectively, on VPA discontinuation, with a concomitant mean 24% decrease in total CBZ-E and a 22% decrease in free CBZ-E. Apart from the decrease in PB concentrations on PHT discontinuation, all significant changes had occurred by 1 week after the end of AED discontinuation. The implication for clinical practice is that a serum AED concentration at this time reflects the new steady state. Free concentrations did not add any clinically useful information to that gained from analysis of total serum concentrations.

A micromethod for the estimation of free levels of anticonvulsant drugs in serum

A micromethod for estimating free levels of phenobarbitone, phenytoin and carbamazepine in patients' sera is described. Serum samples are subjected to a process of ultrafiltration, the filtrates treated with acetonitrile and the drug concentration quantified using high performance liquid chromatography. The stability of free levels in specimens before and after storage is investigated. The method is reproducible and mean recovery exceeds 98.5% showing that there is no significant absorption of drug onto the filters used. There is no interference from other substances normally present in patients' sera and there is a good correlation between results obtained by this method and a fluorescence polarisation immunoassay with correlation coefficient between 0.975 and 0.999. Serum samples can be stored for a lengthy period before ultrafiltration without adverse effects. The relevance of the method to patient care is discussed.

Pharmacologic interactions between valproate and other drugs

Valproate is often administered with other antiepileptic drugs, a practice that can lead to clinically significant pharmacologic interactions. Concomitant administration of such enzyme-inducing antiepileptic drugs as carbamazepine, phenobarbital, primidone, or phenytoin will markedly accelerate the metabolic conversion of valproate, particularly in children. In response to the effects of enzyme induction, valproate dosage may need to be doubled to maintain therapeutic serum levels. Valproate does not appear to induce enzymatic drug metabolism, but rather acts as a metabolic inhibitor. Because of this inhibition, phenobarbital dosage must often be reduced after valproate is added to the therapeutic regimen. Valproate also may markedly increase concentrations of the active epoxide metabolite of carbamazepine. The interaction between phenytoin and valproate results primarily from displacement from plasma proteins. The resulting increase in the free fraction of phenytoin alters the relationship between total phenytoin concentration and the drug's pharmacologic effect. Thus, clinical evidence of toxicity may be present at concentrations usually considered to be in the therapeutic range.

Utility of free level monitoring of antiepileptic drugs

Criteria that were developed for monitoring free (unbound) rather than total (free plus bound) concentrations of antiepileptic drugs include extensive and variable binding to plasma proteins. Phenytoin and valproic acid belong to this category. It is shown that free drug concentration is independent of total drug concentration, whereas total drug concentration depends on free concentration and free fraction. Because antiepileptic drugs are predominantly bound to albumin, free fraction will increase in the presence of hypoalbuminemia (hepatic and renal disease, burns, and pregnancy). Free fraction also increases because of saturable binding (valproic acid) and competitive binding (valproic acid displacing phenytoin). There is suggestive evidence that side effects may be more closely related to the free, rather than to the total, plasma concentration of phenytoin. The clinical evidence that side effects or therapeutic effects are better correlated to the free, rather than the total, concentration of valproic acid or carbamazepine is not yet convincing. Knowledge of the free concentration improves our understanding of therapeutic and toxic effects of low total plasma concentrations. Further clinical trials are necessary for definitive assessment of the clinical relevance for free drug monitoring of valproic acid, carbamazepine, and phenytoin in the management of epileptic patients.

An update on sodium valproate

Sodium valproate has been in clinical use for the treatment of epilepsy in Great Britain since 1973 and in the United States since 1978. It is chemically quite different from the existing antiepileptic drugs. Although most authorities concentrate on its modification of GABAergic inhibitory transmission in the central nervous system, its mechanism of action remains obscure. It has been shown to be an effective antiepileptic drug in a wide variety of seizure types, but clinically, its major use to date has been in generalized seizures. It is particularly effective in photosensitive epilepsy and myoclonus. Most adverse reactions to sodium valproate are mild and reversible, but with increasing experience, the drug's rare, idiosyncratic, adverse effects are becoming apparent, particularly hepatotoxicity and teratogenicity. The role of therapeutic drug monitoring in the management of patients taking sodium valproate is controversial.

Metabolic interactions of phenytoin in the rat: effect of coadministration with the anticonvulsant drugs sodium valproate, sulthiame, ethosuximide or phenobarbital

Rats were administered with 50 mg/kg phenytoin (PHT), twice a day, for five consecutive days and with a second anticonvulsant drug in addition for a further five days. Analysis of 24 hr urine samples for content of unmetabolized PHT and its major metabolite 5-(p-hydroxyphenyl)-5-phenylhydantoin (pHPPH) indicates that PHT hydroxylation was significantly inhibited by sulthiame coadministration since during the test period (days 6-10) the concentrations of PHT and pHPPH in urine were significantly increased and decreased respectively. In contrast, sodium valproate, ethosuximide and phenobarbital had no significant effect on the urinary excretion pattern of PHT. These data correlate with changes in plasma and brain PHT concentrations.

Reassessment of the concept of a therapeutic range of anticonvulsant plasma levels

The significance of plasma-level measurements of anti-epileptic drugs is reviewed, especially as it applies to the treatment of childhood epilepsy. The problems associated with the determination and interpretation of 'therapeutic' levels are discussed, including the justification of applying to children the results of studies of adults. It is concluded that plasma-level determinations are of great value in specific circumstances, but that more study is needed to take into account the multiplicity of variables involved.

Drug interactions with valproic acid

Valproic acid undergoes drug-drug interactions with most of the commonly used anticonvulsants. Since it possesses a wide range of indications, concomitant use with other anticonvulsants, and hence interactions, are not infrequent. Many of these interactions are reciprocal and may have important therapeutic consequences. Valproate acts as a protein binding displacer and/or metabolic inhibitor with respect to a number of other anticonvulsants (phenobarbitone, primidone, phenytoin). Inhibition of metabolism would, in most instances, result in a decrease of the dose requirements of the affected drugs. Valproate is a low clearance drug primarily eliminated by metabolism. Its metabolism is highly inducible by some of the major anticonvulsants (e.g. carbamazepine, phenytoin). Valproate is also highly protein bound in plasma and thus is displaced by salicylates and free fatty acids. However, displacement alone, unlike induced metabolism, should not affect the drug's dose-response relationship.

Brain maturation following administration of phenobarbital, phenytoin, and sodium valproate to developing rats or to their dams: effects on synthesis of brain myelin and other subcellular membrane proteins

The anticonvulsant drugs phenobarbital, phenytoin, sodium valproate, and phenytoin-sodium valproate in combination were administered daily to (a) pregnant rats starting on the 5th day after conception, and continued through 17 days postpartum, or (b) to developing rats between 3 and 17 days of age. Each drug was prepared in water and administered at either a therapeutic dose (TD), three times therapeutic dose (3TD), or 9TD. Drug administration had no discernible effect on litter size or sex ratio in the offspring; however, phenobarbital administration to dams caused small but significant reductions in birth weights. Body weights of developing rats treated with anticonvulsant drugs either via dams of directly by intraperitoneal injection lagged behind controls. At 20-24 days of age the brain weights of the offspring of phenobarbital (9TD)-exposed dams lagged control weights by 5% whereas brain weights in the offspring of the other treated groups were indistinguishable from controls. In contrast, administration of phenobarbital directly to developing rats caused no significant brain weight deficits whereas significant deficits were observed with phenytoin (9TD), sodium valproate (9TD), and phenytoin-sodium valproate (9TD) in combination. AT 20-24 days of age the relative incorporation of radioactive leucine into purified myelin and crude nuclear proteins of drug-treated rats or the offspring of drug-treated dams was reduced by 10-20% in all cases. Dose-related differences were not observed however, and the effects of phenytoin and sodium valproate in combination approximated those of phenytoin administered alone.

Plasma protein binding of phenytoin in the aged: in vivo studies

1 The relation between plasma water phenytoin content and whole plasma phenytoin content in vivo was studied in 22 elderly patients (age range 62 to 87 years) and in 22 young patients (age range 18 to 33 years), all of whom were taking the drug for epilepsy. 2 The percentage of the drug existing unbound in plasma was slightly, but statistically significantly, higher in the elderly (12.8 +/- 1.8%) than in the young (11.1 +/- 2.5%). Reduced plasma albumin levels in the elderly (33.0 +/- 2.0 gl-1) as compared with the young (45.3 +/- 3.5 gl-1) probably contributed to the reduced protein binding in the older subjects.

Phenytoin-valproate interaction: importance of saliva monitoring in epilepsy

Sodium valproate is often used with phenytoin when epilepsy cannot be controlled by a single drug. Sodium valproate depresses phenytoin protein binding and so invalidates plasma phenytoin monitoring as a means of determining precise phenytoin dosage requirements. Plasma and saliva phenytoin and plasma valproate concentrations were measured in 42 patients with epilepsy receiving both drugs. Phenytoin protein binding was also measured by ultrafiltration in 19 of these patients and 19 patients taking phenytoin alone. Saliva phenytoin concentration bore the same close correlation to unbound (therapeutically active) phenytoin in patients receiving both drugs as it did in patients receiving phenytoin alone, whereas plasma total phenytoin did not. The same therapeutic range for saliva phenytoin (4-9 mumol/1; 1-2 microgram/ml) was therefore valid in both groups. The depression of phenytoin binding was directly related to the plasma concentration of valproate both in random samples taken from the 42 patients and in samples taken throughout the day in two of these patients. This was confirmed in vitro. Even when the concentration of valproate is known the degree of binding cannot be predicted. Saliva rather than plasma monitoring of phenytoin treatment is therefore valuable in the presence of valproate and with reduced phenytoin binding from any cause.

Pharmacokinetic interactions with antiepileptic drugs

A large number of pharmacokinetic interactions with antiepileptic drugs have been reported in recent years. Among the interactions affecting the disposition of anticonvulsants, the most important are probably those resulting in inhibition of the metabolism of phenytoin, phenobarbitone and carbamazepine. Drugs which have been shown to inhibit the metabolism of these anticonvulsants and to precipitate clinical signs of intoxication in epileptic patients include sulthiame, valproic acid, chloramphenicol, certain sulphonamides, phenylbutazone, isoniazid and propoxyphene. Interactions affecting the plasma protein binding of antiepileptic drugs are less likely to cause long-lasting alterations in response, but they are important because they change the relationship between serum drug concentrations and clinical effect. Anticonvulsant agents may induce important alterations in the pharmacokinetics of other drugs. Phenytoin and phenobarbitone may decrease the gastrointestinal absorption of frusemide and griseofulvin, respectively. Many of the drugs used in the treatment of the adult epilepsies, including phenytoin, phenobarbitone, primidone and carbamazepine, are potent inducers of the hepatic microsomal enzymes. This results in an increased rate of metabolism and decreased clinical efficacy of a number of drugs, including dicoumarol, steroid oral contraceptives, metyrapone, glucocorticoid agents, doxycycline, quinidine and vitamin D.

Depression of axonal excitability by valproate is antagonized by phenytoin

Standard electrophysiologic techniques were employed to determine the effects of the two anticonvulsants, valproic acid (VPA) and phenytoin (DPH), on the membrane excitability properties of the crayfish giant axon. VPA, 4 mM, produces a depolarization of the membrane that is associated with a decrease in the resting membrane conductance (gM). VPA also attenuates the increase in gNa and gK that are responsible for the depolarization and repolarization of the action potential; it decreases the magnitude, rate of depolarization and repolarization, and conduction velocity of the propagated action potential while increasing its duration. DPH has some effects on membrane properties that are qualitatively similar to those of VPA; 0.11 mM DPH also decreases gM, gNa, and gK. Unlike VPA, DPH does not have a significant effect on magnitude of either the resting or action potential. Pretreatment of axons with DPH reduces the effect of VPA on the magnitude, rate of depolarization and repolarization, and duration of the action potential while completely preventing the effects of VPA on resting potential, conduction velocity, and membrane conductance. These experiments and others on the effects of K(+) depolarization on membrane properties demonstrate that part, but not all, of the influence of VPA on the membrane is secondary to its depolarizing effect. The results reported here on a membrane model suggest at least part of the cellular basis for the anticonvulsant properties of VPA and DPH, alone and in combination.

Reduction of steady-state valproate levels by other antiepileptic drugs

Steady-state plasma valproate (VPA) levels were analyzed in 37 children after 6 weeks of VPA therapy. Twenty-six patients were receiving other antiepileptic drugs in addition to VPA (experimental group). Eleven patients who received VPA alone served as controls. The mean VPA dose was not statistically different for the two groups (experimental group, 35.4 mg/kg/day, 11.6 SD; control group, 31.1 mg/kg/day, SD 6.6) The mean plasma VPA level was significantly lower for the experimental group (63.0 micrograms/ml, SD 21.8) than for the control (99.3 micrograms/ml), SD 23.3) (p less than 0.01). VPA level: dose ratio (LDR) was also reduced in the experimental group (1.92, SD 0.75) as compared to controls (3.26, SD 0.65) (p less than 0.01). Within the experimental group the VPA levels and VPA LDR were significantly reduced in patients receiving either phenytoin or phenobarbital. The data suggest that other antiepileptic drugs significantly alter the steady-state level to dose relationship for VPA.

Phenytoin--valproic acid interaction in rhesus monkey

The interaction between phenytoin (PHT) and valproic acid (VPA) was investigated in the rhesus monkey. PHT was given by multiple IV bolus to reach a steady state and then continued while two successive VPA infusions were added. Plasma was assayed for total VPA and PHT as well as free PHT. The addition of VPA produced no change in total PHT levels, a 50% increase in PHT free fraction (increase in free PHT levels), and a decrease in PHT overall elimination rate constant. The increase in free fraction was also documented in a separate in vitro study. The increase in free PHT levels suggested that VPA decreased PHT intrinsic metabolic clearance. This interaction may thus involve a dual mechanism-namely protein binding displacement of PHT and inhibition of PHT metabolism. This mechanism is compatible with other studies showing that VPA is an inhibitor of drug metabolism.

Drug interactions with phenytoin

Drug interactions with phenytoin are a frequent occurrence, although their clinical relevance has often been overemphasised. Probably the most important of such interactions are those resulting in inhibition of phenytoin metabolism: due to the saturable nature of phenytoin biotransformation even minor degrees of inhibition can produce disproportionate changes in both steady-state serum concentration and the magnitude of pharmacological effect. Phenytoin has marked enzyme-inducing properties and can stimulate the metabolism of many concurrently administered drugs, thereby reducing their therapeutic efficacy. Clinically important examples of such interactions include a reduction of the anticoagulant effect of dicoumarol, a decrease in the prophylactic efficacy of the contraceptive pill and failure of response to various corticosteroid agents when administered therapeutically or diagnostically. Unless complicated by additional mechanisms, plasma protein binding interactions with phenytoin are seldom of clinical significance. However, they may alter considerably the relationship between serum drug concentration and clinical response, a possibility which needs to be taken into account when interpreting serum phenytoin levels in clinical practice.

Changes in regional brain levels of amino acid putative neurotransmitters after prolonged treatment with the anticonvulsant drugs diphenylhydantoin, phenobarbitone, sodium valproate, ethosuximide, and sulthiame in the rat

The effect of prolonged treatment (10 days) with the anticonvulsant drugs diphenylhydantoin (DPH), phenobarbitone, sodium valproate, ethosuximide and sulthiame, both singly and in combination, on regional rat brain amino acid neurotransmitter concentrations (GABA, glutamate, aspartate and taurine) were assessed. DPH had a major effect in the cerebellum and hypothalamus in that it significantly reduced cerebellar GABA, taurine and aspartate and hypothalamic GABA and aspartate. Sodium valproate significantly elevated GABA and taurine in most regions. Aspartate and glutamate were less affected. Phenobarbitone significantly elevated GABA concentrations in all brain regions, while taurine concentration was only elevated in the cerebral cortex. Ethosuximide induced changes were small compared to the other anticonvulsants while sulthiame produced complex changes. Anticonvulsant drugs administered in combination resulted in complex changes, suggesting that their mode of action is different.

Modification of phenytoin clearance by valproic acid in normal subjects

1 The effect of valproic acid on the distribution and elimination kinetics of intravenously administered phenytoin has been investigated in eight normal volunteers. 2 In each of the subjects studied the volume of distribution of phenytoin increased significantly during treatment with sodium valproate (1200 mg daily for 7 days). 3 Phenytoin clearance was markedly increased in presence of valproic acid as compared to control values (0.52 +/- 0.17 v 0.38 +/- 0.11 ml min-1 kg-1 respectively, P less than 0.02). 4 It is suggested that the increase of the volume of distribution and of the serum clearance are secondary to displacement of phenytoin from plasma protein binding sites by valproic acid.

Decreased plasma protein binding of phenytoin in patients on valproic acid

1 Plasma protein binding of phenytoin and of valproic acid were measured in ten epileptic patients on this drug combination. Ten other epileptics not on valproic acid served as controls. All patients had normal kidney function. 2 The measured free fraction of phenytoin among the patients on valproic acid ranged from 12.5 to 23.2% and after recalculation to a plasma albumin level of 45 g/l from 12.5 to 20.0 (median 15.4%). This differed significantly (P = 0.002, Mann- Whitney U-test) from the control patients where the normalized values ranged from 9.9 to 13.9% with a median value of 11.8%. 3 The measured free fractions of phenytoin and of valproic acid showed a significant correlation which, however, was due to the quantitative relation between the degree of binding of both these drugs and the concentration of plasma albumin. There was no discernable relation in this material between the free concentration of valproic acid and the free fraction of phenytoin. 4 It is concluded that patients on combined treatment with phenytoin and valproic acid have an unpredictably raised free fraction of phenytoin. This drug interaction therefore can complicate the important plasma level monitoring of phenytoin in epileptic patients unless the free concentration of this drug can be analysed or estimated.

Effect of phenytoin on protein binding of valproic acid

In vitro experiments using the equilibrium dialysis technique were performed to determine the binding of valproic acid to plasma components in the absence and presence of therapeutic concentrations of phenytoin. The free fraction of valproic acid was found to be dependent on the total valproic acid concentration. Phenytoin did not influence valproic acid protein binding.