Consequence of equal absorption, distribution and/or elimination rate constants

When fitting experimental data to an open one- or two-compartment model, wi th first order kinetics, it may happen that no optimized value is obtained for model parameters. Several authors pointed out that this case is especia lly encountered when absorption and elimination coefficients approach each other in a one-compartment model or when absorption and exponential elimina tion or distribution rate constants are equal in a two-compartment model. W e analyze these situations of equal coefficients here. Firstly, dealing wit h a one-compartment model, we get the concentration in the central compartm ent after a single oral dose and after successive various doses at various times (first order kinetics). Secondly, dealing with a two-compartment mode l, also for single or successive various doses, the concentration is expres sed when absorption and exponential elimination or distribution rate consta nts are equal. In all cases, the areas under concentration curves and the m ean residence time of the drug are calculated even when cancellation of one exponential term occurs. Furthermore, the concentration at steady-state is taken into account.