Role of human liver microsomes in in vitro metabolism of metamizole
ABSTRACT
Metamizole or Novalgin® is a widely used well tolerated analgesic drug which is however
compromised by agranulocytosis as adverse effect. Subsequent to nonenzymatic hydrolysis, the primary
metabolic step is N-demethylation of 4-methylaminoantipyrine (4-MAA) to 4-aminoantipyrine (4-AA). The
aim of the present study was to identify the human cytochrome P-450 enzyme (CYP) mediating this reaction.
This study identified the relevant CYP using virus expressed isolated human CYP, human liver microsomes
and rat liver microsomes with chemical inhibition studies. The substrate of 4-methylaminantipyrine was
employed at six different concentrations (25, 50, 100, 400, 800 and 1200 μmol per l) with varying
concentrations of selective inhibitors of CYP1A2 (furafylline, fluvoxamine), CYP3A4 (ketoconazole),
CYP2A6 (coumarin), CYP2D6 (quinidine), CYP2C19 (omeprazole, fluvoxamine, tranylcypromine),
CYP2C9 (sulphaphenazole) and CYP1A1 (alpha-naphthoflavone). 4-MAA and 4-AA were analyzed by
HPLC and enzyme kinetic parameters (Km and Vmax) were determined by regression (Sigma plot 9.0). The Ndemethylation
of 4-MAA by microsomes prepared from baculovirus expressing human CYP was pronounced
with CYP2C19. Intrinsic clearance of the most active enzymes were 0.092, 0.027 and 0.026 for the CYP
enzymes 2C19, 2D6 and 1A2, respectively. Metabolism by human liver microsomes was strongly inhibited
by fluvoxamin, omeprazole and tranylcypromine (IC50 of 0.07, 0.07 and 0.18, respectively) but with coumarin,
sulphaphenazole, ketoconazole, moclobemid, quinidine alpha-naphthoflavone and furafylline were 0.79, 1.20,
1.36, 1.44, 3.46, 4.68 and 8.41, respectively. The enzyme CYP2C19 apparently has an important role in Ndemethylation
of 4-methylaminoantipyrine which should be further analyzed in clinical studies and which
may also be interesting concerning the agranulocytosis.
