Extensive biliary excretion of the model opioid peptide [D.PEN^2,5]] enkephalin in rats

Purpose. This study was designed to test the hypothesis that the enzymatically stable opioid peptide, [D-pen^2,5] enkephalin (DPDPE), is excreted extensively into bile. Methods. Following an i.v, bolus dose of DPDPE (10 mg/kg) to rats, concentrations of DPDPE in serum, bile, liver homogenate and urine were measured by a novel capillary zone electrophoresis method. Data were analyzed to recover the fundamental pharmacokinetic parameters (volumes of distribution; distribution and elimination rate constants governing DPDPE systemic and biliary disposition). Parallel in vitro experiments were performed to evaluate the partitioning of DPDPE between erythrocytes and plasma, as well as to assess the degree of binding of DPDPE to serum proteins. Results. The majority of the administered dose (~ 80%) was recovered from bile as intact peptide. DPDPE disposition was best described by a two-compartment model with Michaelis-Menten elimination (K(m): 37.5 ± 11 μg/ml; V(max): 1143 ± 368 μg/min/kg) from the central compartment into bile, suggestive of an active hepatic transport system. DPDPE was associated with a distributional space of 486 ± 62 ml/kg. In vitro incubation of DPDPE with whole blood showed that ~ 65% of the peptide was associated with erythrocytes. The difference between concentrations of DPDPE in erythrocytes and plasma was statistically significant (29.2 ± 4.9 vs. 18.1 ± 3.1 μg/ml, p < 0.05), but not between whole blood and plasma (21.3 ± 2.8 vs. 18.1 ± 3.1 μg/ml, p > 0.05). Concentration-independent binding of DPDPE to serum proteins was evidenced between 10 and 100 μg/ml, with an unbound fraction of 0.517 ± 0.182. Conclusions. DPDPE undergoes extensive biliary excretion after i.v administration in rats. The apparent nonlinearity in the biliary excretion of DPDPE revealed by the pharmacokinetic modeling strongly suggests the existence of an active transport system(s) in hepatocytes which may mediate the rapid disappearance of DPDPE from the systemic circulation.