In a recent paper,' we reported an equation for the calculation of the fraction of the bioavailable dose remaining in the body at t,,, Fsys,l,,, assuming that the drug follows one-compartment model disposition and first-order absorption. It was shown' that this fraction is solely dependent on the ratio k,lk, where k, and k, are the first-order absorption and elimination rate constants, respectively. However, this equation can be applied only to a specific time point, 1.e. t,,,.
In the present communication equations are derived for the calculation of the fraction of the bioavailable dose, Fsys,[, remaining in the body at any time t after drug's administration. These equations can be applied to drugs obeying one compartment model kinetics assuming either firstor zero-order absorption. In addition, an alternative and simpler method of derivation of the equation relating Fsys,l, with the ratio kJk, is proposed.
In the previous communication' a lengthy procedure based on material balance relationships was used to derive equations (1) and (2) for Fsys,l, in the linear one-compartment model:
. .
Fsys,I,, =
Fsys,1,, = 0.368 where Q, = k,/k,. when k, + k, (1) when k, = k, (2) However, the fraction of the bioavailable dose, Fsys,t, remaining in the body at any time t, can be defined by the equation (3):
where C, is the plasma concentration at time t, F is the fraction of the dose absorbed, D is the administered dose and Vis the apparent volume of distribu-