Plasma levels of parent drug and metabolites in the intravenous aminopyrine breath test.

[Dimethylamine-14C]-aminopyrine was given i.v. to 5 healthy volunteers and 5 medical patients in a dose of 1.67 mg/kg, containing 0.02 micro Ci/kg 14C. In 4 volunteers the experiment was repeated using oral administration of aminopyrine. Exhalation of 14CO2 was followed for 6-10 h and plasma levels of the drug and of its metabolites 4-methylamino-antipyrine, 4-amino-antipyrine, 4-acetylamino-antipyrine, 4-formylamino-antipyrine, were measured by thin-layer chromatography. The concentration-time profiles of the metabolites mostly failed to conform to a Bateman function. Areas under the curves from 1 to 6 h after dosing indicated distinct interindividual differences in metabolite patterns even in the absence of disturbed liver function, whereas the intraindividual data were closely reproducible. In most subjects, the area of formylamino-antipyrine exceeded that of the acetyl analogue. The metabolite patterns did not exhibit a consistent relationship to the ratios between 14CO2 half-life in breath and aminopyrine half-life in plasma, which varied from 1.4 to 3.2.