Pharmacokinetics of isavuconazole in healthy cats after oral and intravenous administration

Background Isavuconazole is a triazole antifungal drug that has shown good efficacy in human patients. Absorption and pharmacokinetics have not been evaluated in cats. Objectives To determine the pharmacokinetics of isavuconazole in cats given a single IV or PO dose. Animals Eight healthy, adult research cats. Methods Four cats received 100 mg capsules of isavuconazole PO. Four cats received 5 mg/kg isavuconazole solution IV. Serum was collected at predetermined intervals for analysis using ultra-high performance liquid chromatography-tandem mass spectrometry. Data were analyzed using a 2-compartment uniform weighting pharmacokinetic analysis with lag time for PO administration and a 2 compartment, 1/y(2) weighting for IV administration. Predicted 24 and 48-hour dosing intervals of 100 mg isavuconazole administered PO were modeled and in vitro plasma protein binding was assessed. Results Both PO and IV drug administration resulted in high serum concentrations. Intravenous and PO formulations of isavuconazole appear to be able to be used interchangeably. Peak serum isavuconazole concentrations occurred 5 +/- 3.8 hours after PO administration with an elimination rate half-life of 66.2 +/- 55.3 hours. Intersubject variability was apparent in both the PO and IV groups. Two cats vomited 6 to 8 hours after PO administration. No adverse effects were observed in the IV group. Oral bioavailability was estimated to be approximately 88%. Serum protein binding was calculated to be approximately 99.0% +/- 0.03%. Conclusions and Clinical Importance Isavuconazole might prove to be useful in cats with fungal disease given its favorable pharmacokinetics. Additional studies on safety, efficacy, and tolerability of long-term isavuconazole use are needed.