Pharmacokinetics and Bioavailability of Single-Dose Intramuscular and Intravenous Administration of Thiafentanil in Goats (Capra hircus)

Abstract

Thiafentanil is a popular opioid agonist used for wildlife chemical immobilisation. Its effects are quickly and completely reversed by the antagonist naltrexone. Successful wildlife immobilisations using thiafentanil have been documented in a variety of wildlife species globally. The aim of this study was to describe the single-dose intramuscular (IM) and intravenous (IV) pharmacokinetics of thiafentanil in goats at a dose of 90 μg/kg using a single cross-over study. The IM dose was administered in the left Vastus lateralis. Plasma samples were collected up to 120 min after thiafentanil administration from two female and eight male adult goats. Samples were analysed by liquid chromatography–tandem mass spectrometry (LC–MS/MS). Pharmacokinetic parameters from one and two-compartment models were estimated via a Bayesian approach. The two-compartment model was preferred overall. The estimated bioavailability was 0.677 (90% Crl: 0.542–0.888), absorption rate constant (k_a) was 0.058 1/min (90% Crl: 0.045–0.115) and clearance was 29.0 mL/min/kg (90% Crl: 23.7–36.3) from this model. This study provides key pharmacokinetic data on thiafentanil, supporting a two-compartment model and offering insights into its absorption, bioavailability, and clearance when used for wildlife immobilisation.