Kinetics of Propylthiouracil in the Elderly

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Abstract
The pharmakokinetics of propylthiouracil [an antithyroid compound] was evaluated in 9 elderly patients and compared to previous results from 6 younger subjects. By giving the drug both by the i.v. and oral route of administration it was possible to estimate the rate and extent of bioavailability. The various kinetic parameters were calculated according to a 2-compartment model by use of 2 different methods: a graphical hand drawn one and by a special developed computer program based on a least squares minimalization. While no significant differences could be demonstrated between the 2 age groups concerning volumes of distribution, clearance and extent of absorption, a large difference was found with regard to the absorption rate constant ka, which was about 3 times higher in the younger than in the elderly subjects, presumably due to a reduced gastric emptying time. Considering the comparison between the 2 methods of calculation all the kinetics parameters were similar except ka and the slow disposition rate constant .beta. which were underestimated by the graphical method. Apparently no age-dependent changes exist concerning the kinetics of propylthiouracil except for a decreased rate of absorption. Graphical methods in pharmacokinetics are useful in obtaining distribution and elimination data but seem often biased for the evaluation of absorption rate constants.

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