4-Methylamino antipyrine hydrochloride is an active metabolite of Metamizole. Metamizole is a pyrazolone non-steroidal anti-inflammatory drug (NSAID) and inhibits COX. 4-Methylamino antipyrine has analgesic, antipyretic, and relatively weak anti-inflammatory properties.

At room temperature and in an atmosphere with oxygen, Metamizole 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 [2].

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 47.59 mg/mL. The average half-lives is 8.57 h for 4-Methylamino antipyrine [1].