4-Methylamino antipyrine hydrochloride is an active metabolite of Metamizole. Metamizole is a pyrazolone non-steroidal anti-inflammatory drug (NSAID) and inhibits COX. Metamizole is an nonopioid analgesic drug and can be used for pain and fever[1][2][3]. 4-Methylamino antipyrine hydrochloride has analgesic, antipyretic, and relatively weak antiinflammatory properties[2].

Metamizole is a prodrug which, at room temperature and in an atmosphere with oxygen, is spontaneously, nonenzymatically converted to 4-Methylamino antipyrine. Subsequently, the N-methyl side chain of 4-Methylamino antipyrine is oxidized to yield 4-formylaminoantipyrine, which is further converted to 4-aminoantipyrine. Metamizole in aqueous solution and in the presence of oxygen consists of a group of several pyrazolone derivatives of which 4-Methylamino antipyrine is pharmacologically the most important[2].

The aim of this study is to assess the pharmacokinetics of its active metabolites 4-Methylamino antipyrine in male piglets after a single intramuscular injection of Metamizole. Eight healthy male piglets are administered Metamizole (100 mg/kg) intramuscularly. 4-Methylamino antipyrine plasma concentrations are quantitatively detectable from 0.25 to 48 h. The average maximum concentration of 4-Methylamino antipyrine is of 47.59 mg/mL. The average half-lives is 8.57 h for 4-Methylamino antipyrine[1].

[1]. Burmańczuk A, et al. Pharmacokinetic investigations of the marker active metabolites 4-methylamino-antipyrine and 4-amino-antipyrine after intramuscular injection of metamizole in healthy piglets. J Vet Pharmacol Ther. 2016 Dec;39(6):616-620. [2]. Campos C1, et al. Regulation of cyclooxygenase activity by metamizol. Eur J Pharmacol. 1999 Aug 13;378(3):339-47. [3]. Ariza A, et al. Pyrazolones metabolites are relevant for identifying selective anaphylaxis to metamizole. Sci Rep. 2016 Mar 31;6:23845.