Validation of a LC-MS/MS method to simultaneously quantify thiafentanil and naltrexone in plasma for pharmacokinetic studies in wildlife

Thiafentanil is a popular opioid agonist that is fully reversed by administering naltrexone. This agonist–antagonist combination is administered to a wide variety of wildlife species for chemical immobilisation, however plasma concentrations for thiafentanil remain unreported. This report describes a method that was developed and validated using human plasma and cross-validated for the analysis of goat plasma. Samples were extracted using a simple protein precipitation and analysed using LC-MS/MS. The assay was validated over the calibration range 4.38–1120 ng/mL for thiafentanil and 15.63–4000 ng/mL for naltrexone. The mean inter-day accuracies for QCs prepared in human plasma (n = 18) ranged from 94.8 − 103.8 % for thiafentanil and 94.8–95.9 % for naltrexone with corresponding precisions of 3.4–7.9 % and 2.8–11.4 %, respectively. The mean accuracies for QCs prepared in goat plasma (n = 6) ranged from 89.0 − 100.5 % for thiafentanil and 89.0–98.0 % for naltrexone with the associated precisions ranging from 7.1 − 11.6 % and 4.8–12.3 %, respectively. Both analytes were stable on bench for six hours and for three freeze–thaw cycles. The impact of heat-inactivation, necessary for the inactivation of potential foot-and-mouth disease, on analyte stability, matrix effect and recovery were evaluated, and a correction factor was established to determine the original analyte concentrations. The method was applied to pharmacokinetic samples collected from goats. The use of goats as a model species provides the first insight into the plasma concentrations of thiafentanil.