Antinociceptive Activity of Metamizol Metabolites in a Rat Model of Arthritic Pain

Preclinical Research The aim of the present study was to evaluate the antinociceptive activity of the main metamizol (MET) metabolites, 4‐methylaminoantipyrine (MAA), 4‐aminoantipyrine (AA), 4‐formylaminoantipyrine (FAA), and 4‐acetylaminoantipyrine (AAA) using the “pain‐induced functional impairment in rat” model (PIFIR model). The antinociceptive efficacies of MAA and AA were 288.3% h and 281.1% h, respectively, close to the efficacy of MET (333.80% h). The effective dose to attain 50% of the maximum response (ED50) values for MET, MAA and AA were 126.1, 124.9, and 110.7 mg/kg, respectively. FAA and AAA were essentially inactive in this experimental model. Part of the antinociceptive effect showed by MET in this study might be attributed to the effect of the metabolites MAA and AA on cyclooxygenases COX‐1 and COX‐2 activity.