Sequential first-pass elimination of a metabolite derived from a precursor.

We examined data from our previous studies in which we not only delivered perfusate containing tracer concentrations of [14C]phenacetin and its metabolite [3H]acetaminophen under constant perfusate flow (10 ml/min/liver) into the rat liver preparation just once, but also recirculated fresh reservoir perfusate containing a tracer dose of [14C]phenacetin through the same rat liver preparation. From the single-pass studies, estimates of fm, the fractional rate of conversion for [14C]phenacetin to form [14C]acetaminophen, and F(M.P), the apparent availability of [14C]acetaminophen, were obtained by determining the concentrations of [14C]acetaminophen in the perfusate before and after incubation with Glusulase. These estimates were fm = 0.871 +/- 0.16 and F(M.P) = 0.43 +/- 0.10. These and the steady-state clearance values of phenacetin (9.1 +/- 0.8 ml/min) and acetaminophen (6.7 +/- 0.7 ml/min) from the single-pass studies were used to predict the concentrations of [14C]acetaminophen in the reservoir perfusate on recirculation of [14C]phenacetin. We found that the sequential first-pass elimination of the metabolite must be considered when the metabolite is highly extracted by the liver. If we had neglected to take this into account, the fractional rate of conversion of a precursor to form a metabolite and the rate of formation of the metabolite would have been underestimated by the factor F(M.P).