Serum phenobarbital concentrations and timing of blood collection in epileptic dogs on chronic treatment

Aims: To assess whether there are significant changes in serum phenobarbital concentrations (SPC) within a dosing interval in epileptic dogs on chronic treatment and whether higher doses, prolonged duration of phenobarbital treatment, body weight, age, gender and concurrent treatment with potassium bromide are associated with significant changes between SPC at two (T2) and twelve (T12) hours after drug administration. Methods : Medical records of epileptic dogs on phenobarbital treatment were retrospectively analyzed. SPC were measured at T2 and T12 in 42 pairs of blood samples of dogs treated with a twice daily stable oral dose of phenobarbital for a minimum of three weeks. Results: SPC were significantly higher at T2 compared to T12 samples. There was no significant correlation between drug dosage and SPC at T2 nor at T12. The difference between T2 and T12 SPC were positively correlated with the drug dosage but not with duration of treatment, weight and age of the dogs. Significant differences in variations of SPC were not detected between males and female and between dogs receiving phenobarbital alone and those under concomitant treatment with potassium bromide. Conclusions: Timing of blood collection is important to measure SPC in dogs under long-term treatment with phenobarbital and peak and trough values should be evaluated, mainly in animals on higher doses of the drug. Further studies would be useful to detect the optimum interval between drug administration and peak sampling in dogs which have reached the steady state with therapeutic doses as happens in clinical practice.