Treatment of epilepsies

Pharmacokinetic considerations when prescribing antiepileptic drugs

Generic formulations of antiepileptic drugs

The regulatory authorities in most industrialized countries require that the generic equivalent of a standard antiepileptic drug has a bioavailability which is similar to that of a proprietary drug. Rates of absorption, however, do vary even if the extent of absorption does not. With few exceptions, generic formulations are acceptable despite the claims often made by commercial companies which purport to show otherwise. Only in the case of phenytoin, at tissue concentrations that approach saturation, do generic formulations pose particular problems. Even then the serum concentration can be monitored to provide a guide to the appropriate dosage for a particular patient.

Measurement of the serum concentration of antiepileptic drugs

Once the steady state of the drug has been reached, there is a fairly consistent relationship between the plasma and the brain concentrations. It should therefore be possible to define those plasma concentrations that are associated with the optimal clinical effects (i.e. the optimal balance between the effectiveness and the side effects). Because of the biological variation, however, such concentrations may vary from one individual to another but it is possible to develop a range of drug concentrations which are based on

statistical or population parameters. The therapeutic range for some of the commonly used antiepileptic drugs is shown below:

Drug

Therapeutic range (^mol/L)

Carbamazepine

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