Pharmacokinetic Assessment of the Marker Active Metabolites 4-Methyl-amino-antipyrine and 4-Acetyl-amino-antipyrine After Intravenous and Intramuscular Injection of Metamizole (Dipyrone) in Healthy Donkeys

• Metamizole is metabolized in vivo in donkeys.
• 4-Methylamino-antipyrine is produced in larger amounts than 4-acetylamino-antipyrine.
• After intramuscular and IV administration of metamizole, the plasma profiles of the metabolites are different.

Metamizole (MT) is an analgesic and antipyretic drug labelled for use in humans, horses, cattle, swine, and dogs in some countries. Metamizole is rapidly hydrolyzed to the active primary metabolite 4-methyl-amino-antipyrine (MAA). MAA is formed in much larger amounts compared to other minor metabolites. Among the other secondary metabolites, 4-amino-antipyrine (AA) is also relatively active. The aim of this research was to evaluate the pharmacokinetic profiles of MAA and AA after administration of 25 mg/kg MT by intravenous (IV) and intramuscular (IM) routes in healthy donkeys. Six jennies were randomly allocated to two equally sized treatment groups according to a 2 × 2 crossover study. Blood was collected at predetermined times within 24 hours, and plasma was analyzed by a validated HPLC UV method. Plasma concentrations of MAA after IV and IM administrations of MT were detectable from 5 minutes to 10 hours in all the donkeys. Plasma concentrations of AA were detectable from 5 minutes to 8 hours, but in smaller amounts. Cmax (P < .01), AUC0-last, AUC0-∞, AUMC0-last, and MRT (P < .05) were statistically different between the IV and IM groups. The AUCIM/AUCIV ratio of MAA was 1.37. The AA concentrations were lower than those found for MAA. The AA plasma versus time curves profiles after the two routes of administration of MT were variable (within the groups) and different (between the groups). Tmax, λz, and AUC0-last were found to be statistically different between the groups (P < .05). The AUCIM AA/AUCIV AA ratio was 2.26.

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