Antipyrine pharmacokinetics in women receiving conjugated estrogens

The pharmacokinetics of a single 1.0 to 1.2-g intravenous dose of antipyrine was studied in 22 healthy female volunteers aged 28 to 70 years (mean, 45 years). Eleven subjects had been taking a conjugated estrogen preparation for at least 3 months; the other 11 subjects who were not taking conjugated estrogens and who were matched for age, weight, and smoking patterns, served as a control group. Plasma antipyrine concentrations were determined by high-pressure liquid chromatography (HPLC) in multiple plasma samples drawn 24 to 48 hours after dosage. Mean +/- SE pharmacokinetic variables in control and conjugated-estrogen groups were volume of distribution, 0.57 +/- 0.02 versus 0.56 +/- 0.02 L/kg; elimination half-life, 11.0 +/- 0.82 versus 12.6 +/- 0.89 hours; and clearance, 0.63 +/- 0.06 versus 0.54 +/- 0.03 mL/min/kg. None of the differences was significant. Although antipyrine clearance is significantly impaired by oral contraceptives, there is no evidence of altered antipyrine pharmacokinetics from treatment with conjugated estrogens.