Pharmacokinetics
Primidone converts to phenobarbital and PEMA; it is still unknown which exact cytochrome P450 enzymes are responsible. The phenobarbital, in turn, is metabolized to p-hydroxyphenobarbital. The rate of primidone metabolism was greatly accelerated by phenobarbital pretreatment, moderately accelerated by primidone pretreatment, and reduced by PEMA pretreatment. In 1983, a new minor metabolite, p-hydroxyprimidone, was discovered.
Primidone, carbamazepine, phenobarbital and phenytoin are among the most potent hepatic enzyme inducing drugs in existence. This enzyme induction occurs at therapeutic doses. In fact, people taking these drugs have displayed the highest degree of hepatic enzyme induction on record. In addition to being an inducer of CYP3A4, it is also an inducer of CYP1A2, which causes it to interact with substrates such as fluvoxamine, clozapine, olanzapine, and tricyclic antidepressants. Its metabolite, phenobarbital, is a substrate of CYP2C9, CYP2B6, CYP2C8, CYP2C19, CYP2A6, CYP3A5, CYP1E1, and the CYP2E subfamily. The gene expression of these isoenzymes is regulated by human pregnane receptor X (PXR) and constitutive androstane receptor (CAR). Phenobarbital induction of CYP2B6 is mediated by both. Primidone does not activate PXR.
The rate of metabolism of primidone into phenobarbital was inversely related to age; the highest rates were in the oldest patients (the maximum age being 55). People aged 70–81, relative to people aged 18–26, have decreased renal clearance of primidone, phenobarbital, and PEMA, in ascending order of significance, and that there was a greater proportion of PEMA in the urine. The clinical significance is unknown.
The percentage of primidone converted to phenobarbital has been estimated to be 5% in dogs and 15% in humans. Work done twelve years later found that the serum phenobarbital 0.111 mg/100 mL for every mg/kg of primidone ingested. Authors publishing a year earlier estimated that 24.5% of primidone was metabolized to phenobarbital. However, the patient reported by Kappy and Buckley would have had a serum level of 44.4 mg/100 mL instead of 8.5 mg/100 mL if this were true for individuals who have ingested large dose. The patient reported by Morley and Wynne would have had serum barbiturate levels of 50 mg/100 mL, which would have been fatal.
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