Cholyl-lysylfluorescein: synthesis, biliary excretion in vivo and during single-pass perfusion of isolated perfused rat liver.

A fluorescent bile salt, cholyl-lysylfluorescein (cholyl-lys-F), was synthesised so that it retained both an intact steroid ring and a side chain structure with an unblocked carboxyl group. Its biliary kinetics and hepatic extraction were studied in Wistar rats and in the isolated perfused rat liver, respectively. The synthetic method used excess N-epsilon-CBZ-l-lysine methyl ester hydrochloride (7 mmol) and cholic acid (5 mmol) via EEDQ with a yield of 94% for cholyl-lys. Cholyl-lys-F was synthesized employing equimolar amounts of cholyl-lys (sodium salt) and fluorescein isothiocyanate (FITC) in bicarbonate buffer (pH 9.5) over 16 h at room temperature (21 degrees C) with a yield of 70%. The fluorescent property of cholyl-lys-F was similar to fluorescein with a strong apple-green fluorescence. In bile-fistula rats under pentobarbital anaesthesia, the cumulative 20 min biliary excretion as a percentage of injected dose were as follows: cholyl-lys-F, 94.4 +/- 0.3%, [14C]cholylglycine (CG), 93.1 +/- 1.2% and fluorescein (F), 34.8 +/- 0.5. Furthermore the single-pass hepatic extraction of cholyl-lys-F was 64.1 +/- 3.9%, [14C]CG was 66.1 +/- 1.2% and F was 16.5 +/- 2%. The similarity in biliary output and hepatic extraction of cholyl-lys-F to that of the natural bile acid cholylglycine suggest that both compounds are handled in a similar fashion. The greater biliary excretion and hepatic extraction of cholyl-lys-F relative to free fluorescein further suggest that conjugation with a bile salt may be an efficient way of targeting compounds to the liver.