Clinical pharmacokinetics of dipyrone and its metabolites

The pharmacokinetics of dipyrone are characterised by rapid hydrolysis to the active moiety 4-methyl-amino-antipyrine (MAA), which has 85% bioavailability after oral administration in tablet form, and takes a short time to achieve maximal systemic concentrations (tmax of 1.2 to 2.0 hours). Absolute bioavailability after intramuscular and rectal administration is 87 and 54%, respectively. MAA is further metabolised with a mean elimination half-life (t1/2) of 2.6 to 3.5 hours to 4-formyl-amino-antipyrine (FAA), which is an end-metabolite, and to 4-amino-antipyrine (AA), which is then acetylated to 4-acetyl-amino-antipyrine (AAA) by the polymorphic N-acetyl-transferase (t1/2 of AA is 3.8 hours in rapid acetylators and 5.5 hours in slow acetylators). Urinary excretion of these 4 metabolites accounts for about 60% of the administered dose of dipyrone. Protein binding of the 4 main metabolites is less than 60%. The volume of distribution of MAA is about 1.15 L/kg of lean body mass. All 4 metabolites are excreted into breast milk. A single-dose study (0.75, 1.5 and 3g) and a multiple-dose study (1g 3 times a day for 7 days) revealed nonlinear pharmacokinetics consistent with a shift of MAA metabolism from FAA to AA. Apparent MAA clearance decreased by 22% during multiple administration. MAA clearance was reduced by 33% in the elderly. In patients with cirrhosis of the liver, the apparent clearance of all metabolites is generally reduced. In patients with renal disease, apparent clearance of MAA remains unchanged, whereas elimination of the renally excreted metabolites AAA and FAA is markedly impaired. No clinically important drug interactions have thus far been recognised. Dipyrone does not affect the pharmacodynamic response to alcohol (ethanol), glibenclamide (glyburide), oral anti-coagulants or furosemide (frusemide). The low toxicity of dipyrone and its efficacy support its use in clinical practice, despite some complex aspects of its disposition.