Primidone is a first-generation barbiturate type antiepileptic medication developed for the treatment of seizures, commonly for partial and generalized seizures. Nowadays, it is prescribed not as first-choice for the treatment of psychomotor, grand mal, and focal epileptic. This drug can also be utilized in the management of tremors as since the early 80's it can be considered a valid alternative to propranolol in the treatment of essential tremors.[1][2] However, its use for this purpose is not recommended as first-line therapy.

The drug was developed by Carrington and Yule Bogue, in 1949. Subsequently, on 8 March 1954, in the United States, the Foods and Drugs Administration (FDA) endorsed this medication in the treatment of epilepsy.[3] Nevertheless, with the subsequent marketing of carbamazepine (1974, in the US), phenytoin, zonisamide, felbamate, gabapentin, lamotrigine, and vigabatrin, the role of primidone in the treatment of epilepsies has further reduced. Its use, indeed, is relegated to the treatment of forms resistant to other therapies. In particular, its indications include the treatment of grand mal and psychomotor epilepsy (temporal lobe epilepsy). 

The high efficacy of primidone in these forms has also been clinically documented in patients resistant to other therapies, suffering from idiopathic, post-traumatic forms, associated with clear signs of brain injury or with specific modifications of the EEG trace. This antiepileptic drug can also be used in the therapy of focal or Jacksonian seizures, myoclonic, and akinetic seizures.

Although primidone is currently not a first-choice drug, its pharmacokinetic and pharmacodynamic properties must be well known to clinicians, as the use of the drug can be particularly effective in specific clinical settings. For instance, QT-prolongation is a potentially fatal form of ventricular arrhythmia with an increased incidence among patients who have epilepsy. This complication may be precipitated by antiepileptic treatment. Because primidone has a favorable profile on QT-prolongation, it offers an alternative in the management of epilepsy and essential tremor when accounting for special arrhythmogenic considerations. Nevertheless, it is still unclear if the drug may be beneficial in QT-interval shortening.[4]

Primidone has also been studied for other clinical purposes. In the 1990s, researchers investigated using it in cases of theophylline-resistant neonatal apnea. Some studies, indeed, showed a possible role as an adjuvant in preventing apnea.[5] In old studies, primidone was used as an antipsychotic, antidepressant, anxiolytic drug as well as against insomnia.[6] The few data produced in the literature do not allow the drawing of definitive conclusions about the use of the drug in these particular clinical conditions.[7] On the other hand, it seems that primidone is associated with interictal depression in patients with epilepsy.[8]

Primidone appears as a white crystalline powder; it is practically insoluble in water (1/2000), slightly soluble in alcohol (1/200), while it is soluble in alkaline solutions. The basic skeleton of primidone comprises pyrimidinedione, which after metabolism, converts into phenobarbital and phenylethylmalonamide (PEMA). The chemical structure of primidone differs from phenobarbital due to the substitution in the position of the C2 of the carbonyl group with the methylene bridge. This chemical modification of the molecule led to a product with a strong anti-convulsive effect and less sedation.

The mechanism of action is not well-demarcated, but it appears to bind centrally with voltage-gated sodium channels and inhibits the monotonous firing of action potentials.[9] Also, it activates gamma-aminobutyric acid (GABA)-A receptor complex with chloride ionophore, which extends the frequency of opening of the chloride channel, causing hyperpolarization by altered the electrical activity of the nerve cell membrane.[10] The effect of primidone in treating essential tremor is not mediated by the active metabolite PEMA.[11] Interestingly, because in vivo investigations proved that primidone inhibits TRPM3 and attenuates thermal nociception, this drug could have a role in pain medicine.[12]

Pharmacokinetics

Absorption

Primidone is rapidly and completely absorbed from the gastrointestinal tract with a blood peak reached in about 3 hours; it shows a half-life of approximately 10 to 15 hours, shorter than that of the two metabolites while reaching steady-state within 1 to 2 days.[13] The half-life in elderly subjects over 75 years probably increases due to metabolic deficits. The half-life in neonates is longer than that in children/adults, probably due to a larger apparent volume of distribution and a lower metabolic capacity of these subjects.

Distribution 

The volume of distribution (Vd) is 0.8 L/Kg, while the plasma concentrations of primidone are between 5 and 12 mcg/mL; it is rapidly distributed in the tissues and shows a partial serum protein binding (about 35%); the therapeutic range is between 23 to 55 mg/mL. Primidone crosses the blood-brain barrier and the placental barrier and is also distributed in breast milk.

Metabolism

In addition to the two primary active metabolites, PEMA, and phenobarbital, it has a minor metabolite, p-hydroxyprimidone. However, primidone cannot be considered a prodrug because it has intrinsic pharmacological activity. It is subject to two main metabolisms:

• Oxidation in C2 with the formation of phenobarbital
• Cleavage of the ring in C2 with the formation of PEMA.: It is not yet known which isoform of cytochrome P 450 is involved in its metabolism.

The percentage of primidone metabolized to phenobarbital fluctuates between 15% and 25%.

Elimination

It is excreted in the urine, in unmodified form, for 40%.

Primidone is available in tablets of 50 and 250 mg in different generic formulations and under different brand names. This drug should start at the lowest possible dose, with dosing increases in a stepwise manner for minimizing adverse effects.

Grand Mal and Psychomotor Seizure

Clinicians have sued primidone for the control of grand mal and psychomotor seizure. It may be used alone or in combination with other anticonvulsants. In adults and children over eight years, 250 mg at bedtime is initiated during the first week, which is increased to 250 mg twice daily in the second week, 250 mg thrice daily in the third week, and 250 mg four times a day in the fourth week. The clinical efficacy of a dosage regimen takes several days before it can be tested.

Moreover:

• Continue similar weekly increments are necessary until seizures are under control. 
• It is not recommended to dosage greater than 2 g daily. 

In patients who are already on other anticonvulsants, the usual dosage range of primidone in adults and children eight years and older is between 125 mg and 1500 mg daily in divided doses.

Essential Tremor

Primidone has also been used effectively in the treatment of essential tremor.  The recommended initial dose for adults is 100 to 125 mg each day, with slow increments to a maintenance dose of 250 mg three times each day.[10]

The most common side effect of primidone therapy is sedation and drowsiness. Ataxia, diplopia, and nystagmus occur at the initiation of treatment. Other adverse reactions include dizziness, vertigo, epigastric pain, megaloblastic anemia, respiratory depression, polyuria, skin rash, facial edema. Hypersensitivity reactions tend to occur more frequently in patients with a history of asthma, urticaria, or angioedema. Agranulocytosis and thrombocytopenia are rare.

Long term use of barbiturates is associated with:

• Connective tissue disorder.
• There is a higher risk of decreased bone mineral density that may lead to weakened or brittle bones. In such cases, the prescriber should consider discontinuation of primidone.
• An increased risk of rickets and osteomalacia (phenobarbital) due to increased metabolism of vitamin D.[14]
• There can also be folate deficiency leading to megaloblastic anemia.

Several clinical conditions contraindicate the use of primidone.

• Primidone is contraindicated in patients who are known to be hypersensitive to barbituric acid derivatives.
• Contraindications to primidone include patients with a history of porphyria. By inducing the enzymes responsible for porphyrin synthesis, barbiturates may worsen acute porphyria.
• This drug is also contraindicated in patients with severe respiratory depression or pulmonary insufficiency, hepatic impairment, alcoholism, renal impairment, sleep apnea, suicidal potential, drug dependence, or in the presence of uncontrolled pain.[15]