Management of seizure disorders with anticonvulsant drugs: current concepts

Disposition, elimination, and bioavailability of phenytoin and its major metabolite in horses

OBJECTIVE

To determine pharmacokinetics and excretion of phenytoin in horses.

ANIMALS

6 adult horses.

PROCEDURE

Using a crossover design, phenytoin was administered (8.8 mg/kg of body weight, IV and PO) to 6 horses to determine bioavailability (F). Phenytoin also was administered orally twice daily for 5 days to those same 6 horses to determine steady-state concentrations and excretion patterns. Blood and urine samples were collected for analysis.

RESULTS

Mean (+/- SD) elimination half-life following a single IV or PO administration was 12.6+/-2.8 and 13.9+/-6.3 hours, respectively, and was 11.2+/-4.0 hours following twice-daily administration for 5 days. Values for F ranged from 14.5 to 84.7%. Mean peak plasma concentration (Cmax) following single oral administration was 1.8+/-0.68 microg/ml. Steady-state plasma concentrations following twice-daily administration for 5 days was 4.0+/-1.8 microg/ml. Of the 12.0+/-5.4% of the drug excreted during the 36-hour collection period, 0.78+/-0.39% was the parent drug phenytoin, and 11.2+/-5.3% was 5-(phydroxyphenyl)-5-phenylhydantoin (p-HPPH). Following twice-daily administration for 5 days, phenytoin was quantified in plasma and urine for up to 72 and 96 hours, respectively, and p-HPPH was quantified in urine for up to 144 hours after administration. This excretion pattern was not consistent in all horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Variability in F, terminal elimination-phase half-life, and Cmax following single or multiple oral administration of phenytoin was considerable. This variability makes it difficult to predict plasma concentrations in horses after phenytoin administration.

Status epilepticus in children. The acute management

Status epilepticus is a common pediatric emergency that may result in significant morbidity and mortality. This article provides a clinical update on generalized tonic-clonic status epilepticus in children and a practical approach to their initial stabilization and pharmacologic management. Only an organized approach to the initial stabilization and management of the child in status epilepticus will help prevent unnecessary complications and death.

Preliminary Nigerian experience in the use of carbamazepine in children with intractable seizures

There is insufficient clinical information about carbamazepine and other newer anticonvulsants from tropical African countries because high costs and limited availability have restricted the use of these drugs in the economically underdeveloped countries. The present communication reports the observed effects of carbamazepine in nine Nigerian children whose epileptic seizures had been intractable to various combinations of previously available anticonvulsants. In seven of these children, intractable seizures were associated with moderate to severe cognitive deficits and behavioral disturbances. Following institution of carbamazepine therapy, complete seizure control was achieved in four children, and significant reduction in seizure frequency occurred in another four. In three children complete seizure control was maintained with carbamazepine alone after withdrawal of all other medications. Among the seven children with cognitive and behavioral problems, improved seizure control was accompanied by significant psychotropic gains. No serious side effects occurred in any of the children during follow-up periods ranging from 2 to 7 years. Because of the favorable anticonvulsant and psychotropic efficacy of carbamazepine in the present study, coupled with the lack of serious side effects, it is recommended that the drug deserves wider use in African children with seizure disorders.

Immunologic aspects of phenobarbital hypersensitivity

Seven patients with hypersensitivity reactions to phenobarbital are described. Clinical manifestations consisted of fever and pruritic skin rash, often associated with adenopathy and conjunctivitis. Reexposure to phenobarbital or other anticonvulsants resulted in redevelopment of symptoms. In vitro lymphocyte stimulation studies with anticonvulsant drugs suggested a cell-mediated hypersensitivity reaction to phenobarbital.

Pharmacokinetics of phenytoin (diphenylhydantoin) in horses

The pharmacokinetics of the anti-convulsant phenytoin were investigated in clinically healthy horses after oral (p.o.) and intravenous (i.v.) administration. A single dose of phenytoin (8.8 mg/kg body weight) was given i.v. as a bolus to nine horses and one horse received 13.2 mg/kg. A two-compartment open model was used to describe the disposition of phenytoin. Four of the horses that received an i.v. dose (three at 8.8 mg/kg and one at 13.2 mg/kg) were then given the same dose 3 days later by the oral route. Phenytoin achieved a peak concentration in serum within 1-4 h after p.o. administration and was poorly absorbed with a bioavailability of 34.5 +/- 8.6%. Oral dosage regimens were calculated on the basis of a dosing interval of 8 h to provide average serum steady-state concentrations of 5 and 10 micrograms/ml for phenytoin.

Pharmacokinetics of phenytoin in children

1 Apparent Vmax and Km for phenytoin were estimated in 40 children (aged 8--33 months) and 21 adults (aged 18--66 years). 2 The derived values of Vmax and Km were used to predict the plasma and salivary concentrations of phenytoin following a change in dose. There was a highly significant correlation between observed and predicted steady-state concentration in both children and adults (P less than 0.001). 3 The apparent Km was similar in children (7.5 +/- 1.2 mg/l) and adults (9.4 +/- 2.3 mg/l). 4 Vmax differed significantly (P less than 0.001) between children (20.4 +/- 2.1 mg kg-1 day-1) and adults (8.7 +/- 0.7 mg kg-1 day-1). 5 After correction for differences in the ratio of liver weight to body weight in children and adults, Vmax was similar in the two groups.