Comparison of liquid chromatography-mass spectrometry and radioimmunoassay for measurement of fentanyl and determination of pharmacokinetics in equine plasma.

The research study aims to ascertain the accuracy of fentanyl concentrations in horse plasma using radioimmunoassay (RIA) by comparing it with the conventional method of liquid chromatography-mass spectrometry (LC-MS).

Methods

• Horse plasma samples were tested using a solid-phase Coat-A-Count fentanyl RIA and a verified LC-MS technique.
• The fentanyl measurements obtained from both methods were compared through linear regression and pharmacokinetic analysis.
• The study also determined the cross-reactivity of the main fentanyl metabolite in horses, N-[1-(2-phenethyl-4-piperidinyl)]maloanilinic acid (PMA), with the RIA.
• The binding power of fentanyl and PMA was compared at three opioid receptor subtypes in horse cerebral cortex utilizing a radioligand binding technique.

Results

• The fentanyl concentrations determined by both RIA and LC-MS were correlated, however, RIA was found to overestimate low fentanyl concentrations and underestimate high fentanyl concentrations.
• The overestimation of low fentanyl concentrations is likely due to the 29% cross-reactivity of PMA with the RIA.
• As a result, the pharmacokinetic variables, determined from injecting an intravenous fentanyl bolus to four anesthetized horses, differed based on the analytical method used.
• Though fentanyl bound with nanomolar potency to the three receptor subtypes, PMA showed no binding activity even at micromolar concentrations.

Conclusions

• The study concluded that compared to LC-MS, the fentanyl concentrations determined by RIA in horse plasma can be misleading, especially when calculating fentanyl pharmacokinetics.