Diphenylhydantoin and Phenobarbital Toxicity

Reactive metabolites of the anticonvulsant drugs and approaches to minimize the adverse drug reaction

Several generations of antiepileptic drugs (AEDs) are available in the market for the treatment of seizures, but these are amalgamated with acute to chronic side effects. The most common side effects of AEDs are dose-related, but some are idiosyncratic adverse drug reactions (ADRs) that transpire due to the formation of reactive metabolite (RM) after the bioactivation process. Because of the adverse reactions patients usually discontinue the medication in between the treatment. The AEDs such as valproic acid, lamotrigine, phenytoin etc., can be categorized under such types because they form the RM which may prevail with life-threatening adverse effects or immune-mediated reactions. Hepatotoxicity, teratogenicity, cutaneous hypersensitivity, dizziness, addiction, serum sickness reaction, renal calculi, metabolic acidosis are associated with the metabolites of drugs such as arene oxide, N-desmethyldiazepam, 2-(1-hydroxyethyl)-2-methylsuccinimide, 2-(sulphamoy1acetyl)-phenol, E-2-en-VPA and 4-en-VPA and carbamazepine-10,11-epoxide, etc. The major toxicities are associated with the moieties that are either capable of forming RM or the functional groups may itself be too reactive prior to the metabolism. These functional groups or fragment structures are typically known as structural alerts or toxicophores. Therefore, minimizing the bioactivation potential of lead structures in the early phases of drug discovery by a modification to low-risk drug molecules is a priority for the pharmaceutical companies. Additionally, excellent potency and pharmacokinetic (PK) behaviour help in ensuring that appropriate (low dose) candidate drugs progress into the development phase. The current review discusses about RMs in the anticonvulsant drugs along with their mechanism vis-a-vis research efforts that have been taken to minimize the toxic effects of AEDs therapy.

Drug-induced movement disorders

INTRODUCTION

Drug-induced movement disorders (DIMDs) can be elicited by several kinds of pharmaceutical agents. The major groups of offending drugs include antidepressants, antipsychotics, antiepileptics, antimicrobials, antiarrhythmics, mood stabilisers and gastrointestinal drugs among others.

AREAS COVERED

This paper reviews literature covering each movement disorder induced by commercially available pharmaceuticals. Considering the magnitude of the topic, only the most prominent examples of offending agents were reported in each paragraph paying a special attention to the brief description of the pathomechanism and therapeutic options if available.

EXPERT OPINION

As the treatment of some DIMDs is quite challenging, a preventive approach is preferable. Accordingly, the use of the offending agents should be strictly limited to appropriate indications and they should be applied in as low doses and as short duration as the patient's condition allows. As most of DIMDs are related to an unspecific adverse action of medications in the basal ganglia and the cerebellum, future research should focus on better characterisation of the neurochemical profile of the affected functional systems, in addition to the development of drugs with higher selectivity and better side-effect profile.

Use of antiepileptic drugs in hepatic and renal disease

The use of antiepileptic drugs in patients with renal or hepatic disease is common in clinical practice. Since the liver and kidney are the main organs involved in the elimination of most drugs, their dysfunction can have important effects on the disposition of antiepileptic drugs. Renal or hepatic disease can prolong the elimination of the parent drug or an active metabolite leading to accumulation and clinical toxicity. It can also affect the protein binding, distribution, and metabolism of a drug. The protein binding of anionic acidic drugs, such as phenytoin and valproate, can be reduced significantly by renal failure, causing difficulties in the interpretation of total serum concentrations commonly used in clinical practice. Dialysis can further modify the pharmacokinetic parameters or result in significant removal of the antiepileptic drugs. Antiepileptic drugs that are eliminated unchanged by the kidneys or undergo minimal metabolism include gabapentin, pregabalin, vigabatrin, and topiramate when used as monotherapy. Drugs eliminated predominantly by biotransformation include phenytoin, valproate, carbamazepine, tiagabine, and rufinamide. Drugs eliminated by a combination of renal excretion and biotransformation include levetiracetam, lacosamide, zonisamide, primidone, phenobarbital, ezogabine/retigabine, oxcarbazepine, eslicarbazepine, ethosuximide, and felbamate. Drugs in the latter group can be used cautiously in patients with either renal or liver failure. Antiepileptic drugs that are at high risk of being extracted by hemodialysis include ethosuximide, gabapentin, lacosamide, levetiracetam, pregabalin and topiramate. The use of antiepileptic drugs in the presence of hepatic or renal disease is complex and requires great familiarity with the pharmacokinetics of these agents. Closer follow-up of the patients and more frequent monitoring of serum concentrations are required to optimize clinical outcomes.

Antiepileptic drug monitoring: a reappraisal

As clinical experience with the pharmacokinetic properties and optimal dosing of the established antiepileptic drugs (AEDs) has increased, frequent monitoring of AED concentration in the blood may be less necessary than it was 15 to 25 years ago. Monitoring continues to be valuable at initiation of treatment, addition or removal of other interacting drugs, at the time of unexpected seizure breakthrough, or when symptoms suggest AED toxicity. Occasional determination to monitor compliance may also be appropriate. Blood level determinations of certain of the new, less familiar AEDs, including felbamate, lamotrigine, and oxcarbazapine, appear to be useful. However, new approaches are needed to monitor the efficacy and possible toxicity of other new AEDs for which the correlation between blood concentration of AED and clinical outcome is less clear. For administration of these AEDs, including gabapentin, tiagabine, and vigabatrin, other indirect measures, such as determination of gamma-aminobutyric acid (GABA) levels in the cerebrospinal fluid or by nuclear magnetic resonance spectroscopy, may prove useful. For monitoring compliance, alternate technologies, such as a medication-dispensing vial with an electronic memory chip, may be of clinical value. In the clinical management of patients with epilepsy, blood level monitoring plays an important role, but methods of using this monitoring have evolved with increased experience and the introduction of AEDs with new mechanisms of action.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

Presentations of acute phenytoin overdose

A 15-year-old boy ingested 19.6 g (15 g verifiable) phenytoin sodium approximately four hours before emergency department presentation. The patient survived the suicide attempt with only supportive care, despite the ingestion of 392 mg/kg and a peak serum level of 100.8 micrograms/mL. A wide spectrum of physical findings consistent with acute massive ingestion of phenytoin was noted. This case report and a review of cases reported in the English literature of acute single anticonvulsant ingestion further delineate the clinical presentation of acute phenytoin overdose.

Efficacy and safety of antiepileptic drugs: a review of controlled trials

Twenty randomized, double-blind, controlled clinical trials of antiepileptic drugs (AEDs) in mostly adult patients with mostly partial onset and/or generalized tonic-clonic seizures have been reported, with a total of 1,336 patients. None of these studies has demonstrated significant differences in antiepileptic efficacy between available antiepileptic drugs, but the results show that there are considerable individual differences between patients' responses to the same drug. While side effects are common with all of the antiepileptic drugs currently available, these are usually mild and reversible. Although some toxic effects may occur more frequently with certain drugs, there is sufficient overlap between the effects of various antiepileptic drugs that most side effects cannot be attributed with certainty to any one drug. Since side effects are generally dose-related, they can frequently be avoided or minimized by careful dosage titration and individualization of therapy.

Single-dose kinetics of primidone in acute viral hepatitis

The pharmacokinetics of primidone (PRM) after oral administration of a single 500 mg dose was studied in 7 patients with acute viral hepatitis and 7 healthy control subjects. The elimination half-life and the apparent clearance of unchanged PRM in the patients were 18.0 +/- 3.1 h and 42 +/- 14 ml X h-1 X kg-1, respectively (mean +/- SD) and did not differ significantly from the values in the controls (half-life 17.0 +/- 2.4 h; clearance 35 +/- 8 ml X h-1 X kg-1). The metabolite phenylethylmalonamide (PEMA) was detected in the serum of all normal subjects within 2-24 h. By contrast, serum levels of this metabolite were undetectable (less than 2 mumol/1) in all but one of the patients. Serum levels of phenobarbital (PB) remained below the limit of detection (less than 2 mumol/1) in all subjects. The findings indicate that accumulation of PRM with its attendant toxicity is unlikely to occur in epileptic patients who develop acute viral hepatitis, despite evidence that the metabolism of the drug is affected by this condition. The possibility of impaired conversion to PB and its implications are discussed.

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.

Correlation between age and plasma level/dosage ratio for phenobarbital in infants and children

123 plasma concentration measurements of phenobarbital were obtained from 82 children (2 months - 6 1/2 years old) at steady-state conditions. The plasma level/dosage ratio has been found to have a highly significant correlation with the age of the patient both for dosage in mg/kg and in mg/m2. The ratio increases with the increase in the age of the patient at a rate which is greater for dosages expressed on the basis of body weight. Moreover, at least for body weight related dosages, this increase is relatively high in the first year of life, becoming less marked after. Practical indications are given about the required dosage of phenobarbital in different groups of ages from 2 months to 6 1/2 years. It is recommended however to regularly measure the plasma level of the drug in infants and children treated for long periods of time.

Drug metabolism under pathological and abnormal physiological states in animals and man

The activity of microsomal drug-metabolizing enzymes is altered by several pathological or abnormal physiological states, such as changes in nutritional status, liver, heart or kidney diseases, hormonal disturbances, pregnancy, tumour-bearing state, adjuvant arthritis, changes in reticuloendothelial system and environmental factors (stress, irradiation, heavy metals). The activities of other metabolic pathways, such as glucuronidation, sulphate conjugation, acetylation and alcohol oxidation are generally affected to lesser extents. Rats are most commonly used in drug metabolism studies, and it is important to know that the activity of most of the microsomal drug-metabolizing enzymes is higher in males than in females through androgen action which is readily impaire drug-metabolizing enzymes in male rats are thus manifested by two mechanisms; one is by impairment of androgen action and the other is by depression of the basic enzymic activity. Therefore, those effects of pathological states, observed only in male rats but not in females, are generally not seen in other species of animals, including man. The effects of starvation, hyperthyroidism, adrenal insufficiency, diabetes and morphine administration are cases where changes in metabolism are due solely to impairment of androgen action. In other pathological cases, those drug-metabolizing enzymes showing sex differences are depressed more markedly in male rats than those showing no clear sex difference. The author therefore recommends the use of female rats in the evaluation of the effects of pathological states on hepatic microsomal drug-metabolizing enzymes. Generally, changes in activity of the hepatic enzymes reflect closely the changes in the rates of drug metabolism in vivo. However, the protein-binding of drugs, hepatic blood flow and renal function are also known to affect the rate of drug metabolism and excretion in vivo, and therefore changes of these factors in pathological states should also be taken into consideration.

Drugs and children: methods for therapeutic monitoring

In this era of polypharmacy, the incidence of adverse reactions due to drug therapy has increased alarmingly since the precise effects on the metabolism of a drug given in combination with other drugs can never be predicted with certainty. Inadequate therapy due to insufficient medication or to factors which diminish absorption or enhance metabolism may be equally undesirable. The consequences to patients in terms of increased morbidity and financial cost of prolonged hospitalization may be considerable. For pediatric patients, particularly in the newborn period, these hazards may be much more dangerous. There is a need for more investigation into the validity of procedures in current use for the determination of drug levels in biologic fluids and into the interpretation of the values they produce. In addition clinical chemists and clinical pharmacologists are faced with the challenge of defining those drugs for which blood level information would be advantageous and developing rapid, sensitive, and accurate assays which can be performed by the routine clinical laboratory. The day may be not too far away when a major proportion of the workload of the clinical laboratory consists of assays primarily designed as an aid to therapy rather than diagnosis.

High-speed liquid chromatographic determination of antiepileptic drugs in human plasma

A high-speed liquid chromatographic method for the simultaneous determinations of diphenylhydantoin, phenobarbital and carbamazepine in human blood plasma is presented. This method involves two step extraction procedures with chloroform and uses 2 X 50 cm long stainless steel columns packed with a anion exchange resin, with a mobile phase of 4 mM ammonium phosphate buffer solution, pH 6.2 at a flow rate of 0.40 ml/min. The results presented show linear calibration curves and quantitative determinations as low as 1.0 mug of each drug added to 0.5 ml plasma. This method has a sensitivity sufficient to detect human plasma levels after therapeutic clinical doses.

Drug interactions and clinical pharmacokinetics

Some drugs influence the gastro-intestinal absorption, distribution , metabolism or renal excretion of other drugs, i.e., processes involved in pharmacokinetic interactions. The clinical consequences of pharmacokinetic drug-drug interactions will be either an increase or a decrease in known therapeutic or toxic effects of the interacting drug. In order to evaluate the importance of drug interaction affecting gastro-intestinal absorption, it is necessary to distinguish between interactions which alter the rate of absorption of another drug and those which alter the amount of drug absorbed. Many drugs displace other drugs from their protein binding sites in vitro. This may cause an increase in the pharmacological effect of the displaced drug. However, much discrepancy exists between in vitro findings. In some cases, the enhanced effect only seems to be a temporary phenomenon. The degree of protein binding and the size of apparent volume of distribution (Vd) must also be taken into consideration. Perhaps the importance of interaction involving protein binding has been overemphasized. Barbiturates, glutethimide, rifampicin and phenytoin increase the rate of drug metabolism in man. The most important interactions reported are between oral anti-coagulants and barbiturates. After withdrawal of these hepatic microsomal enzyme inducing drugs, it takes 2 to 3 weeks before the rate of drug metabolism reaches the pretreatment level. In this period, risk of haemorrhage exists. Induction seems to be dose-dependent, but not all persons are inducible. Many drugs compete for the drug metabolising enzyme system in the liver and consequently some drugs inhibit the biotransformation of other drugs. The time course of these interactions depends on the pharmacokinetic properties of the drug involved, and these interactions also seem to be dose-dependent. The most important of such interactions, clinically involved the oral sulphonylurea hypoglycaemic drugs and the antiepilepic drug phenytoin. Drugs are eliminated by urinary excretion through three mechanisms: glomerular filtration, tubular reabsorption, and active tubular secretion. The most important interactions seem to be those involving competition for tubular secretion.

Pharmacologic therapy of ventricular arrhythmias

To treat patients with ventricular arrhythmias properly, one must characterize the arrhythmia, define the underlying heart disease and look for and treat reversible causes. When arrhythmias are suitable for pharmacologic suppression, it is necessary to predefine therapeutic goals, then carefully document that the drug accomplishes these goals. Knowledge of a drug's metabolism, excretion, active metabolites and plasma protein binding is often required for full understanding of its clinical effect. Pharmacokinetic principles require that antiarrhythmic drugs be given on a rigid schedule and that plasma drug levels be frequently determined. Use of compartment models and the principle of superposition can enable one to achieve and maintain therapeutic drug concentrations while avoiding toxic side effects. The drugs commonly used to treat arrhythmias, lidocaine, propranolol, procainamide, diphenylhydantoin and quinidine, as well as some newer agents, have specific pharmacokinetics and toxic effects that must be understood.