Pharmacokinetics of mirtazapine and its main metabolites in Beagle dogs: A pilot study

Mirtazapine (MRT) is a human antidepressant drug mainly metabolised by the cytochrome P450 enzyme system to 8-OH mirtazapine (8-OH) and dimetilmirtazapine (DMR). The drug is usually administered to dogs with anorexia according to doses extrapolated from humans, although it could also have applications as an antidepressant and analgesic in this species. The aim of this study was to assess the pharmacokinetics of MRT and its metabolites, DMT and 8-OH. Six healthy male Beagle dogs were administered MRT orally (20mg/dog) and plasma MRT and metabolite concentrations were evaluated by high performance liquid chromatography with fluorescence detection. The pharmacokinetic profiles of MRT and DMR were similar (detected from 0.25 up to 10h), while 8-OH (detected from 0.50 up to 10h) attained the highest concentrations. The mean half-life of MRT was 6.17h with a clearance of 1193mL/h/kg. The study showed that MRT has a different pharmacokinetic profile in the dog compared to other species.