Pharmacokinetics of Mirtazapine and Its Main Metabolites after Single Oral Administration in Fasting/Fed Horses

Mirtazapine (MRZ) is a human antidepressant drug that is metabolized, predominantly by the cytochrome P450 enzyme system, to 8-OH mirtazapine (8-OH MRZ) and dimetilmirtazapine (DMR) metabolites. In veterinary medicine, this drug is currently administered to cats and dogs with anorexia, although it could also have applications as an antidepressant, antiemetic, and analgesic agent in these species. The aim of this study was to assess the pharmacokinetics of MRZ and its metabolites DMR and 8-OH MRZ in horses. Six healthy female horses were administered MRZ (2 mg/kg) in fasting and fed states according to a balanced crossover study design. Plasma MRZ and metabolite concentrations were evaluated by high-performance liquid chromatography fluorescence detection method. Pharmacokinetic profiles of MRZ and DMR were similar (detected from 0.5 up to 34 and 48 hours, respectively), with an MRZ AUC0-N/DMR AUC0-N ratio range varying between 1.1 and 1.7. Surprisingly, 8-OH MRZ was undetected. Most of the pharmacokinetic parameters were not altered by food, with the exception of the time required to reach maximum concentration; this showed a statistical increase in subjects in the fasting state as compared with the fed state. However, because MRZ is an active substance intended for long-term administration, the slight increase of the time required to reach maximum concentration is not considered to be of any clinical consequence. In conclusion, the pharmacokinetic parameters demonstrated in this study suggest that MRZ is suitable for oral administration in the horse. However, further investigations are required to evaluate both its safety and effectiveness in this animal species.