Maintenance and cryopreservation of xenobiotic metabolism in precision-cut liver slices : evaluation of an alternative in vitro model to isolated hepatocytes
Enzyme-specific substrate activities were higher in Sprague Dawley rat hepatocytes than slices; testosterone (250M) hydroxylations (1.9-16.9 fold), 7-ethoxycoumarin (25M) (O-deethylation, 14.8 fold, glucuronidation, 3.1 fold), carbamazepine (50M) epoxidation (2.4 fold), styrene (2mM) diol formation (9.7 fold) and CDNB (50M)-glutathione conjugate formation (8.7 fold). Most importantly, the ratio of 7-hydroxycoumarin sulphate to glucuronide conjugation was higher in slices, in agreement with the lower rate of 7-hydroxycoumarin formation and indicative of slower diffusion of 7-ethoxycoumarin into slices. Metabolite formation was also higher in dog hepatocytes than slices, although these were thicker (436m), possibly accounting for the larger differences observed between the two in vitro models when compared with rat. In particular, CDNB-glutathione conjugate formation was 32 fold higher in hepatocytes than slices. Testosterone 6-hydroxylation in human liver was higher in isolated hepatocytes than in slices, although the differences were smaller (0.5-3 fold). 7-hydroxycoumarin glucuronidation was higher in human slices than in hepatocytes (3-4 fold), whilst 7-hydroxycoumarin formation was higher in hepatocytes than slices (2-12 fold and 6 fold, respectively). The differences between human samples could reflect the differing qualities of hepatocyte preparations. In the future, liver slices may be a means of reducing experimental animal use, whilst vitrification would allow continual availability of human liver slices for in vitro drug metabolism and toxicology studies.