Plasma Levels of Primidone and Its Metabolite Phenobarbital

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Abstract
The effects of age and associated therapy on plasma primidone (PRM) and derived phenobarbital (PB) concentrations, and on plasma concentrations-to-PRM dose ratios (L/D ratio) were evaluated retrospectively from 408 consecutive PRM and derived PB determinations in 238 chronically treated epileptic patients (153 children and adolescents between 5 months and 15 years of age and 85 adults between 16 and 55 years of age). The correlation between PRM administered and both plasma PRM and derived PB levels was significant; the correlation between PRM and PB plasma levels was also significant, but the scatter of values for the linear regressions was such that the relationship had no predictive value. Significant differences in mean plasma PRM and PB L/D ratios were found between patients aged 0–3 years, 4–9 years, 10–15 years, and adults (16–55 years), with higher values in the older groups. The PB/PRM concentration ratios were significantly lower in children than in adolescents and adults. Concomitant treatment with carbamazepine affected PRM disposition and led to increased L/D ratios for PB and decreased L/D ratios for PRM, whereas phenytoin increased the L/D ratios for PB without any significant change in the L/D ratios for PRM. The variability in the results indicates the need for routine monitoring of PRM and derived PB plasma levels, particularly in pediatric populations, in order to tailor the dose to each patient.