In vivo and in vitro assessment of mirtazapine pharmacokinetics in cats with liver disease.

Background: Liver disease (LD) prolongs mirtazapine half-life in humans, but it is unknown if this occurs in cats with LD and healthy cats.

Hypothesis/objectives: To determine pharmacokinetics of administered orally mirtazapine in vivo and in vitro (liver microsomes) in cats with LD and healthy cats.

Animals: Eleven LD and 11 age-matched control cats.

Methods: Case-control study. Serum was obtained 1 and 4 hours (22 cats) and 24 hours (14 cats) after oral administration of 1.88 mg mirtazapine. Mirtazapine concentrations were measured by liquid chromatography with tandem mass spectrometry. Drug exposure and half-life were predicted using limited sampling modeling and estimated using noncompartmental methods. in vitro mirtazapine pharmacokinetics were assessed using liver microsomes from 3 LD cats and 4 cats without LD.

Results: There was a significant difference in time to maximum serum concentration between LD cats and control cats (median [range]: 4 [1-4] hours versus 1 [1-4] hours; P = .03). The calculated half-life of LD cats was significantly prolonged compared to controls (median [range]: 13.8 [7.9-61.4] hours versus 7.4 [6.7-9.1] hours; P < .002). Mirtazapine half-life was correlated with ALT (P = .002; r = .76), ALP (P < .0001; r = .89), and total bilirubin (P = .0008; r = .81). The rate of loss of mirtazapine was significantly different between microsomes of LD cats (-0.0022 min , CI: -0.0050 to 0.00054 min ) and cats without LD (0.01849 min , CI: -0.025 to -0.012 min ; P = .002).

Conclusions And Clinical Importance: Cats with LD might require less frequent administration of mirtazapine than normal cats.