PHARMACOKINETICS OF ENDOGENOUS SUBSTANCES - SOME PROBLEMS AND SOME SOLUTIONS

The paper deals with the most relevant aspects related to the pharmaco kinetics of endogenous substances. Two different views are presented i n order to focus on two aspects of the problem, the physiological back ground of these substances and the need for empirical or tailored mode ls to process pharmacokinetic data. Very often endogenous substances f ollow saturable enzyme biotransformation, reversible interconversion, active and diffusional transports, renal threshold, endogenous synthes is plus dietary supply with possible balancement between these two fac tors, feedback processes, asymmetric distribution with specific body s torage, and gender differences. These mechanisms allow the body to pre serve and restore homeostatic equilibria of endogenous substances. The most relevant problem in pharmacokinetics of these substances is the presence of baseline concentration which needs to be carefully defined also for possible rhythms related to age, sex, diet, night and day pe riods. Theoretical considerations are presented for the management of pharmarokinetic analysis of these substances, which only rarely follow linear processes. Throughout the text various practical examples are considered.