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.

Clinical pharmacokinetics of dipyrone and its metabolites

Any drug given to a pregnant woman must be considered as possibly harmful to the fetus, since all drugs administered to the mother cross the placental membrane, although at different rates. Important physiological changes occur in pregnancy, which may influence the kinetics of drugs. Differences in gastrointestinal function are likely to alter drug absorption rates in the stomach or gut. Ventilatory alterations modify pulmonary drug absorption or elimination. Important changes in haemodynamics and alterations in body water compartments influence drug distribution and elimination. Renal drug elimination is generally enhanced, whereas hepatic drug elimination may be modified in different ways.

Drug Kinetics in Pregnancy

(1979). Pharmacokinetics of Salicylate in Man. Drug Metabolism Reviews: Vol. 9, No. 1, pp. 3-19.

Pharmacokinetics of salicylate in man.

Pharmacokinetics during pregnancy: evidence-based maternal dose formulation.

• Inhibitors of prostaglandin synthesis are increasingly recommended for delaying premature delivery. However, such inhibitors—eg, aspirin, indomethacin, or naproxen—may interfere with uterine contractility, with maternal, fetal, and neonatal platelet function, and especially with fetal circulation and postnatal adaptation. The instillation of aspirin (50 to 90 mg/kg of fetal body weight) into the rumina of fetal lambs resulted in significant increase of pulmonary arterial pressure, which was directly related to constriction of the ductus arteriosus. Such pressure elevation may act as a stimulus to increased muscular development in the small vessels of the lung, which, in turn, could interfere with the rapid reduction in pulmonary vascular resistance normally occurring after birth. The effects of acetaminophen in pregnancy and in the newborn have not been studied extensively; when ingested in usual therapeutic doses, the available data have shown no adverse effects. ( Arch Intern Med 1981;141:358-363)

https://jamanetwork.com/journals/intemed/articlepdf/600837/archinte_141_3_016.pdf

Effects of Aspirin and Acetaminophen in Pregnancy and in the Newborn

Placental passage of sodium salicylate

The pharmacokinetics of some drugs important for treatment in pregnancy were studied in several groups, women in stage III of pregnancy, women in labour and non-pregnant women as a control group.

Research on pharmacokinetics during pregnancy.

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Placental passage of sodium salicylate

Research on pharmacokinetics during pregnancy.