Formed and preformed metabolite excretion clearances in liver,a metabolite formation organ: Studies on enalapril and enalaprilat in the single-pass and recirculating perfused rat liver

Single-pass and recirculating rat liver perfusion studies were conducted with ¹⁴C]enalapril and ³H] enalaprilat, a precursor-product pair, and the data were modeled according to a physiological model to compare the different biliary clearances for the solely formed metabolite, ¹⁴C]enalaprilat, with that of preformed ³H]enalaprilat. With single-pass perfusion, the apparent extraction ratio (or biliary clearance) of formed ¹⁴C]enalaprilat was 15-fold the extraction ratio of preformed ³H] enalaprilat, an observation attributed to the presence of a barrier for cellular entry of the metabolite. Upon recirculation of bolus doses of ¹⁴C]enalapril and ³H]enalaprilat, the biliary clearance, estimated conventionally as metabolite excretion rate/midtime metabolite concentration, for formed ¹⁴C]enalaprilat was again 10-to 15-fold higher than the biliary clearance for preformed ³H]enalaprilat, but this decayed with perfusion time and gradually approached values for preformed ³H]enalaprilat. The decreasing biliary clearance of formed enalaprilat with recirculation was explained by the dual contribution of the circulating and intrahepatic metabolite (formed from circulating drug) to excretion. Physiological modeling predicted (i) an influx barrier (from blood to cell) at the sinusoidal membrane as the rate-limiting process in the overall removal of enalaprilat, (ii) a 15-fold greater extraction ratio or biliary clearance for formed ¹⁴C]enalaprilat over ³H]enalaprilat during single-pass perfusion, and (iii) the time-dependent and declining behaviour of the biliary clearance for formed ¹⁴C]enalaprilat during recirculation of the medium. In the absence of a direct knowledge of eliminating organs in vivo, this variable pattern for excretory clearance of the formed metabolite within the organ is indicative of a metabolite formation organ.Glossary C _R denotes the reservoir concentration - C _In andC _Out,L respectively, denote the input and output concentrations. - Q _L is the total hepatic plasma flow rate. - Q _Bile is the bile flow rate - f _p and f_L denote the unbound fractions in plasma and liver tissue, respectively - C_p is the concentration in renal plasma; C_L is the concentration in liver; - C _Bile is the concentration in bile. - v _R,V _p,V _L, andV _Bile denote the reservoir plasma, hepatic plasma, tissue, and bile volumes, respectively - CL _d ⁱⁿ andCL _d ^ef denote the influx and efflux clearances, respectively - CL _int,L ^m ,L represents the hepatic metabolic intrinsic clearance of the drug - CL _int,L ^b L denotes the biliary intrinsic clearance This work was supported by the Medical Research Council of Canada. I. A. M. de Lannoy was a recipient of the Ontario Graduate Fellowship from the Ontario Ministry of Health; K. S. Pang was a recipient of the Faculty Development Award, Medical Research Council, Canada.