Prospective crossover clinical trial comparing transdermal with oral phenobarbital administration in epileptic cats.

Objectives: The aim of this study was to compare serum phenobarbital concentrations, adverse events and client satisfaction during 14 weeks of transdermal vs oral phenobarbital administration to epileptic cats.

Methods: This was a prospective, fixed-order, crossover pilot study. Nine client-owned cats with presumptive or diagnosed idiopathic epilepsy were enrolled. Oral phenobarbital (PO-PB) was administered for weeks 1-14 (median starting dosage of 3.8 mg/kg [2.0-5.4 mg/kg/day] q12h); transdermal phenobarbital (TD-PB) was administered for weeks 14-28 (median starting dosage 18.8 mg/kg/day [17.6-24.0 mg/kg/day] q12h). Serum phenobarbital concentrations (S-PB) were measured at weeks 2, 14, 16 and 28. Client satisfaction questionnaires and biochemistry were evaluated at 14 and 28 weeks.

Results: Median S-PB concentrations during oral administration were 21 µg/ml (observed range 11-40 µg/ml) at week 2 and 22 µg/ml (8-35 µg/ml) at week 14, and at the higher TD dosage were 18 µg/ml (0-42 µg/ml) at week 16 and 17 µg/ml (7-50 µg/ml) at week 28. Phenobarbital concentrations were significantly correlated with PO dosage at week 2 ( = 0.75, = 0.03) but not at weeks 16 and 28. Significantly more dose adjustments were needed during the TD phase ( = 0.03), but 6/9 owners (67%) still preferred TD to PO administration. Adverse effects were mild and comparable in both groups.

Conclusions And Relevance: Therapeutic S-PB concentrations were achievable in some cats using TD-PB at 18 mg/kg/day q12h. Poor correlation between TD dosage and S-PB concentrations was observed and more dosage adjustments were required during TD administration. These findings necessitate close therapeutic drug monitoring if TD-PB is prescribed.