Hepatic clearance of gitoxin: pharmacokinetic study on rabbit isolated liver. Influence of protein binding and comparison with digoxin

Isolation, purification and structure elucidation of cyclosporin A metabolites in rabbit and man

Metabolism of cyclosporin A (CsA) was studied in a model of isolated and perfused rabbit liver, and in man. After diethyl ether extraction, CsA metabolites were separated and successfully purified by a combination of two high-performance liquid chromatographic (HPLC) procedures. A normal-phase HPLC methodology was used to separate most of the metabolites, and further separation and purification were optimized with a reversed-phase HPLC method. 27 different CsA metabolites were separated, collected, and characterized by fast atom bombardment mass spectrometry. In addition to several metabolites already described (OL-1, OL-17, OL-18 and OL-21), new compounds (of original molecular weights 1236, 1222 and 1174, and numerous structural isomers of previously reported molecular weights) were isolated from rabbit and human bile. Mass spectral analysis of two of these new metabolites strongly suggests vicinal dihydrodiol (mol. wt 1236) and N-demethylated vicinal dihydrodiol (mol. wt 1222) structures. These two new metabolites most probably derive from an epoxide as a preliminary intermediate.

A further study of the pharmacokinetics of gitoxin in rabbit isolated liver: clearance of 3H-gitoxin

Hepatic clearance of 3H-gitoxin was studied in the rabbit using an isolated perfused liver technique with an emulsion of a perfluorocarbon. The liposoluble material in the perfusion medium was extracted with dichloromethane, and gitoxin was assayed in the extract by high performance liquid chromatography. Pharmacokinetic parameters were estimated for the liposoluble (dichloromethane soluble) material in the water phase obtained by centrifugation of the emulsion, for the liposoluble material and unchanged gitoxin in the total emulsion. Distribution and elimination half-lives of the liposoluble fraction in the water phase, were estimated to be 0.47 and 4.80 hours respectively, Vd to be 148 ml.g-1 and intrinsic clearance to be 1.16 ml.min-1.g-1; these parameters were compared with those of a previous study with unlabelled gitoxin. Distribution and elimination half-lives of the liposoluble compounds in the emulsion were estimated to be 0.48 and 4.62 hours, Vd to be 47 ml.g-1 and intrinsic clearance to be 1.07 ml.min-1.g-1; these data were compared with those of the liposoluble compounds in the water phase. Distribution and elimination half-lives of unchanged gitoxin in the emulsion were estimated to be 0.22 and 0.70 hour, Vd to be 59 ml.g-1 and intrinsic clearance to be 11.4 ml.min-1.g-1; these data were compared with those of the liposoluble compounds in the emulsion. The subcellular distribution of gitoxin and its metabolites in the liver indicated that 79% of the radioactivity was found in the soluble fraction, no significant binding occurring in the mitochondrial and microsomal fractions.