Pharmacokinetic investigations of the marker active metabolite-4-methylamino-antipyrin after intravenous and intramuscular injection of metamizole in healthy sheep

• The marker for metamizole (MT) is its metabolite 4-methylaminoantipyrine (MAA).
• MAA plasma concentrations were investigated by a HPLC method (limit of quantification 0.1 μg/mL).
• The pharmacokinetic profiles of MAA after IM and IV administrations of MT were very similar.
• The pharmacokinetic trends in sheep seem to be more similar to those reported in humans rather than in other animal species.

Metamizole (MT) is an analgesic and antipyretic drug labelled for use in humans, horses, cattle, swine, and dogs. MT is rapidly hydrolysed to the primary metabolite 4-methylaminoantipyrine (MAA). MAA is formed in much larger amounts compared to other minor metabolites, and it has been selected from the regulatory European Medicines Agency as a marker residue for MRL calculation. The aim of this research was to evaluate the pharmacokinetic profiles of MAA after 20 mg/kg MT by intravenous (IV) and intramuscular (IM) administrations in healthy sheep. Twelve sheep were randomly allocated to two equal treatment groups according to a 2 × 2 crossover study. Blood was collected at predetermined times within 36 h and plasma was analysed by a validated HPLC UV method. No behavioural changes or alterations in health parameters were observed in the IV or IM groups of animals during or after (up to 7 days) the drug administration. Plasma concentrations of MAA after IV administration of MT were detectable from 5 min to 8 h in all the sheep, they were still detectable at 10 h in two animals, and the plasma quantification of MAA was possible from 5 min to 10 h in all the animals after IM administration. The only two significantly different parameters between the groups were maximum concentration (Cmax) and time to maximum concentration (Tmax) (P < 0.01). The AUCIM/AUCIV was 1.12. The present study showed that no clinically relevant difference in the MAA was found after IM and IV administration of MT. Further studies are now necessary to assess the safety and efficacy profile of MT in sheep.