VA Class:CN400

AHFS Class:

• Barbiturates 28:12.04

Generic Name(s):

• Primidone

Primidone, a structural analog of phenobarbital, is closely related to the barbiturate-derivative anticonvulsants.

Seizure Disorders

Primidone is used mainly in the prophylactic management of partial seizures with complex symptomatology (psychomotor seizures), and some clinicians consider it the drug of choice. Primidone is also useful in the prophylactic management of other partial seizures (e.g., those with autonomic symptoms), akinetic seizures, and tonic-clonic (grand mal) seizures, particularly tonic-clonic seizures refractory to other anticonvulsant therapy. Primidone is often used concomitantly with other anticonvulsants, especially phenytoin or phenobarbital. Some clinicians, however, do not recommend the concurrent use of primidone and phenobarbital because of possible increased sedation.

Administration

Primidone is administered orally.

Patients who are currently receiving or beginning therapy with primidone and/or any other anticonvulsant should be closely monitored for notable changes in behavior that could indicate the emergence or worsening of suicidal thoughts or behavior (suicidality) or depression.100,101,102 (See Cautions: Precautions and Contraindications, in the Anticonvulsants General Statement 28:12.)

Dosage

Seizure Disorders

Dosage of primidone must be carefully and slowly adjusted according to individual requirements and response. When primidone replaces another anticonvulsant, the dosage of primidone should be gradually increased while gradually decreasing the dosage of the drug being discontinued over a period of at least 2 weeks, in order to maintain adequate seizure control. Primidone should be withdrawn or dosage decreased slowly to avoid precipitating seizures or status epilepticus.

The usual dosage of primidone for adults and children 8 years of age and older who have received no previous treatment is 100-125 mg at bedtime for the first 3 days, 100-125 mg twice daily for days 4-6, 100-125 mg 3 times daily for days 7-9, and then a maintenance dosage of 250 mg 3 times daily. For adults and children 8 years of age and older, the usual maintenance dosage is 250 mg 3 or 4 times daily. If necessary, dosage may be increased to a maximum of 2 g daily given in divided doses.

The usual dosage of primidone for children younger than 8 years of age who have received no previous treatment is 50 mg at bedtime for the first 3 days, 50 mg twice daily for days 4-6, 100 mg twice daily for days 7-9, and then a maintenance dosage of 125-250 mg 3 times daily. For children younger than 8 years of age, the usual maintenance dosage is 125-250 mg 3 times daily or 10-25 mg/kg daily given in divided doses. Alternatively, some clinicians recommend a dosage of 1.25 g/m² daily, given in 2-4 divided doses.

Primidone shares the toxic potentials of the barbiturate-derivative anticonvulsants, and the usual precautions of anticonvulsant therapy should be observed. (See Cautions in the Anticonvulsants General Statement 28:12.) Since primidone therapy is generally prolonged, the manufacturers recommend that a complete blood count and an SMA-12 test be performed every 6 months in patients receiving the drug.

Clinicians should inform patients, their families, and caregivers about the potential for an increased risk of suicidal thinking and behavior (suicidality) associated with antiepileptic therapy.100 For a complete discussion, see Cautions: CNS Effects and Cautions: Precautions and Contraindications, in the Anticonvulsants General Statement 28:12.

Serious adverse reactions to primidone are rare, but mild adverse effects occur frequently. The most common adverse effects are drowsiness, ataxia, vertigo, lethargy, anorexia, nausea, and vomiting; ataxia and vertigo tend to disappear with continued therapy or with reduction of initial dosage.

Occasionally, primidone may cause hyperexcitability (especially in children), which may include hyperirritability; however, it is usually less severe than that associated with phenobarbital. Rarely, primidone therapy has precipitated an acute psychosis-like reaction. Other adverse reactions reported during primidone therapy are fatigue, emotional disturbances, drowsiness, diplopia, nystagmus, morbilliform rash, alopecia, edema of the eyelids, leg edema, leukopenia, eosinophilia, impotence, a malignant lymphoma-like syndrome, and a syndrome resembling systemic lupus erythematosus, all of which subsided when the drug was discontinued. Megaloblastic anemia (which responds to folic acid therapy), red cell hypoplasia, or aplasia, granulocytopenia, and agranulocytosis have occurred rarely.

Safe use of primidone during pregnancy or lactation has not been established. (See Cautions: Pregnancy and Lactation, in the Anticonvulsants General Statement 28:12.) Since primidone is apparently distributed into milk, nursing should be discontinued if excessive somnolence or drowsiness is observed in nursing infants of women receiving the drug.

Acute Toxicity

Overdosage of primidone results in symptoms similar to those of acute barbiturate intoxication. In addition, primidone crystalluria may occur and may facilitate the diagnosis. Treatment of primidone overdosage is the same as treatment of barbiturate acute toxicity. (See Acute Toxicity in the Barbiturates General Statement 28:24.04.)

Pharmacology

Primidone shares the actions of the barbiturate-derivative anticonvulsants and has sedative properties similar to phenobarbital. Primidone is effective in the management of epilepsy in subhypnotic doses.

Pharmacokinetics

Absorption

Approximately 60-80% of an oral dose of primidone is absorbed from the GI tract. Precise plasma concentrations required for therapeutic effects are unknown, but a limited number of reports indicate that serum primidone concentrations should be maintained at 5-12 mcg/mL to adequately control seizures and minimize the risk of adverse effects. Following oral administration, peak serum concentrations are reached in about 4 hours.

Distribution

Primidone is apparently distributed into milk in substantial quantities.

Elimination

One manufacturer reports that the serum half-life of primidone is about 21 hours; other investigators have reported the serum half-life to be 10-12 hours.

Primidone is slowly metabolized by the liver and slowly excreted in urine as phenylethylmalonamide (PEMA), phenobarbital, and p -hydroxyphenobarbital. Both PEMA and phenobarbital possess anticonvulsant activity; however, PEMA has only weak anticonvulsant properties and is more toxic than is primidone. In animals, PEMA potentiates the anticonvulsant activity of phenobarbital. PEMA has a half-life of 24-48 hours. In single-dose studies, phenobarbital was not detected in serum after periods of up to 48 hours following primidone administration. Patients receiving primidone on a chronic basis, however, had high serum phenobarbital concentrations (generally 2-3 times that of primidone). High serum phenobarbital concentrations during chronic administration of primidone may be the result of enzyme induction and/or differences in serum half-life of the 2 drugs. During chronic therapy, approximately 15-25% of an oral dose of primidone is excreted in urine unchanged. Approximately 15-25% of an oral dose of the drug is metabolized to phenobarbital and approximately 50-70% is excreted in urine as PEMA. Primidone is removed by hemodialysis.

It has not been established whether primidone, like phenobarbital, is a potent inducer of the enzymes involved in the metabolism of other drugs, but because the drug is chemically and pharmacologically similar to phenobarbital in addition to being metabolized to phenobarbital, this possibility should be considered.

Chemistry and Stability

Chemistry

Primidone is a structural analog of phenobarbital in which the carbonyl group at position 2 has been replaced by a methylene group. The drug is closely related to the barbiturate-derivative anticonvulsants. Primidone occurs as a white, crystalline powder and is very slightly soluble in water and slightly soluble in alcohol.

Stability

Primidone tablets should be stored in well-closed containers at a temperature less than 40°C, preferably at 15-30°C. Commercially available primidone tablets have expiration dates of 2 years following the date of manufacture.

Additional Information

For further information on chemistry, pharmacology, pharmacokinetics, uses, cautions, acute toxicity, drug interactions, and dosage and administration of primidone, see the Anticonvulsants General Statement 28:12.

Only references cited for selected revisions after 1984 are available electronically.

100. Food and Drug Administration. Information for healthcare professionals: suicidality and antiepileptic drugs. FDA Alert; 2008 Jan 31. From the FDA website: [Web].

101. Food and Drug Administration. FDA News: FDA alerts health care providers to risk of suicidal thoughts and behavior with antiepileptic medications. Rockville, MD; 2008 Jan 31. From the FDA website: [Web].

102. Food and Drug Administration. FDA Alert: Suicidality and antiepileptic drugs. Rockville, MD; 2008 Jan 31. From the FDA website: [Web].