Plasma levels of anticonvulsants

Anticonvulsant blood levels: historical review with a pediatric focus

Epilepsy is heterogeneous and its treatment is often complicated by variable drug responses. Buchtal et al reported a close correlation between serum phenytoin levels, electroencephalographic findings, and clinical status in 1960. They suggested that physicians adjust dosage to attain a "therapeutic level." The concept was enthusiastically received. "Therapeutic serum levels" were proposed for most anticonvulsant drugs, and by 1975, most authorities believed that pharmacokinetic factors explained individual differences in drug response. However, Froscher found that measuring levels did not improve patient outcome. More recently, Schumacher's double-blind study found no correlation between phenytoin levels and seizure control or adverse effects. Pharmacodynamic variables (differences in drug responsiveness) are more important than pharmacokinetic factors for many drugs, especially receptor-active drugs. Pharmacokinetic variables were studied first, and led to a simplistic model. They are less significant than pharmacodynamic factors in the case of warfarin anticoagulation. Anticonvulsant levels can reveal noncompliance and pharmacokinetic differences. They say nothing about pharmacodynamics. Reports of "subtherapeutic levels" imply a need to increase dosage, but this is not supported by outcome data. We still lack evidence that specific drug levels are a valid intermediate target 40 years after Buchtal's paper. Responses to some anticonvulsants could depend primarily on pharmacokinetic factors, while pharmacodynamic factors could be supreme for others.

The role of therapeutic drug monitoring in improving the cost effectiveness of anticonvulsant therapy

When monitoring of the plasma concentrations of anticonvulsant drugs first came into use 25 years ago, it appeared to have a major impact on improving the effectiveness and safety of anticonvulsant therapy. However, as time has passed, prescribers have absorbed many of the lessons to be learned from the monitoring, and now apply this knowledge without necessarily monitoring plasma anticonvulsant concentrations as frequently as in the past. Therefore, the effect of the drug concentration monitoring on the cost effectiveness of anticonvulsant therapy is probably not as significant now as it originally was. In theory, drug concentration monitoring is often unlikely to decrease the cost of contemporary anticonvulsant drug therapy, but it may enhance the efficacy of the therapy. Thus, monitoring may reveal unrecognised under- or overdosage, detect failure of compliance or drug-drug interactions, or indicate when there is little point in persisting with a particular anticonvulsant drug. Despite a good deal of anecdotal testimony, surprisingly little has been published demonstrating the benefits of anticonvulsant therapeutic drug monitoring in epileptic populations. However, one study did show better rates of seizure control rates in patients monitored in the first 6 months of their epileptic disorder; but not if the monitoring began later than this.(ABSTRACT TRUNCATED AT 250 WORDS)

Salivary phenytoin levels in institutionalized epileptics

Measurement of phenytoin (PHT) levels in saliva has been advocated as a monitor of plasma levels, and therefore of effective dosage. All studies to date have been performed on outpatients. We have now analyzed whole saliva from 62 mentally retarded, institutionalized, PHT-treated persons with epilepsy. Both PHT and its major human metabolite, 5-(para-hydroxyphenyl)-5-phenylhydantoin (pHPPH) were measured using gas-liquid chromatography. We detected large inter-individual variations in saliva PHT and pHPPH levels. Data were ranked and subjected to statistical analysis according to Spearman. Significant positive correlations were detected between daily PHT dose and plasma total PHT concentration, plasma PHT and salivary PHT, and daily dose and salivary PHT level. When data were categorized and analyzed according to the type of medication being administered in addition to PHT, different results were obtained. Polypharmacy can influence drug and metabolite levels and relationships in institutionalized persons with epilepsy.

Bioavailability of phenytoin from various pharmaceutical preparations in children

In children, the blood level of phenytoin was found to be significantly higher when 100 mg capsules rather than 100 mg tablets were administered. When, on the other hand, 30 mg capsules and tablets were compared; the situation was reversed; tablets produced significantly higher blood levels of phenytoin than did the capsules. The significance and possible explantation of these findings are discussed.

Anticonvulsant level in saliva, serum, and cerebrospinal fluid

Levels of four anticonvulsant drugs were measured simultaneously in saliva, spinal fluid, and dialyzed serum, i.e., free drug in serum. The level of diphenylhydantoin and possibly of carbamazepine was the same in the three body fluids. The leves of phenobarbital was the same in spinal fluid and dialyzed serum, but was lower in saliva. The level of primidone was different in each body fluid. The technique is simple (flow of saliva stimulated when the subject chews candle was or Teflon) and will be useful to determine the level of free diphenylhydantoin and carbamazepine, which is more closely related to intoxication or drug failure than is the total level of drug.