The long-acting barbiturate phenobarbital was formerly one of the most widely used anticonvulsants. It’s now used less frequently because of its sedative effects. Phenobarbital is sometimes used for long-term treatment of epilepsy and is prescribed selectively to treat status epilepticus if hydantoins are ineffective.
Mephobarbital, also a long-acting barbiturate, is sometimes used as an anticonvulsant. Primidone, which is closely related chemi-cally to the barbiturates, is also used to treat chronic epilepsy.
Each barbiturate has a slightly different set of pharmacokinetic properties.
Phenobarbital is absorbed slowly but well from the GI tract. Peak plasma levels occur 8 to 12 hours after a single dose. The drug is 20% to 45% bound to serum proteins and to a similar extent to oth-er tissues, including the brain. About 75% of a phenobarbital dose is metabolized by the liver, and 25% is excreted unchanged in urine.
Almost one-half of a mephobarbital dose is absorbed from the GI tract and well distributed in body tissues. The drug is bound to tis-sue and plasma proteins. Mephobarbital undergoes extensive me-tabolism by the liver; only 1% to 2% is excreted unchanged in urine.
Approximately 60% to 80% of a primidone dose is absorbed from the GI tract and distributed evenly among body tissues. The drug is protein-bound to a small extent in the plasma. Primidone is me-tabolized by the liver to two active metabolites, phenobarbital and phenylethylmalonamide (PEMA). From 15% to 25% of primidone is excreted unchanged in urine, 15% to 25% is metabolized to pheno-barbital, and 50% to 70% is excreted in urine as PEMA. Primidone is also secreted in breast milk.
Barbiturates exhibit anticonvulsant action at doses below those that produce hypnotic effects. For this reason, they usually don’t produce addiction when used to treat epilepsy. Barbiturates ele-vate the seizure threshold by decreasing postsynaptic excitation.
Barbiturates are an effective alternative therapy for:
Barbiturates can be used alone or with other anticonvulsants. I.V. phenobarbital is also used to treat status epilepticus. The ma-jor disadvantage of using phenobarbital for status epilepticus is that it has a delayed onset of action when an immediate response is needed. Barbiturates are ineffective in treating absence seizures.
Mephobarbital has no advantage over phenobarbital and is used when the patient can’t tolerate the adverse effects of phenobarbi-tal. Because of monitoring, costs, and dosing frequency, phenobar-bital is usually tried before primidone. Primidone may be effective in patients who fail to respond to phenobarbital.
The effects of barbiturates can be reduced when they’re taken with rifampin. Here are some other drug interactions:
· The risk of toxicity increases when phenobarbital is taken with CNS depressants, valproic acid, chloramphenicol, felbamate, cimetidine, or phenytoin.
· The metabolism of corticosteroids, digoxin, and estrogens may be enhanced when these drugs are taken with phenobarbital, lead-ing to decreased effects.
· Adverse effects of tricyclic antidepres-
· sants increase when they’re taken with barbiturates.
· Evening primrose oil may increase an-ticonvulsant dosage requirements. (See Adverse reactions to barbiturates.)
Barbiturate use can decrease the effects of many drugs, including beta-adrenergic blockers, corticos-teroids, digoxin, estrogens, doxy-cycline, oral anticoagulants, hor-monal contraceptives, quinidine, phenothiazine, metronidazole, tri-cyclic antidepressants, theo-phylline, cyclosporine, carba-mazepine, felodipine, and vera-pamil.