Drug Targeting Organ-Specific Strategies

The mechanisms of uptake and excretion of drugs by the liver has been widely studied using
isolated hepatocytes and isolated perfused livers of rodents. The subject was extensively
discussed by Oude Elferink et al. [82], summarized in a comprehensive special issue of the
Journal of Hepatology in 1996 [83] and reviewed in several papers and chapters by our group
[84,85]. In general, the rate and mechanism of drug uptake in isolated rat hepatocytes are
very similar to those found in vivo. Only a few studies have investigated transport of drugs in
human hepatocytes, and even fewer have used liver slices. However, studies in human and rat
hepatocytes are hampered by the fact that the isolation procedures involve collagenase digestion
for the disruption of cell-to-cell contacts. Clearly, this proteolytic enzyme may also
damage plasma membranes and transport systems therein.
Jansen et al. [86] published an example of transport of a drug targeting preparation in human
and rat hepatocytes. They found that the anti-viral drug ara-AMP coupled to lactosaminated
human serum albumin, was taken up to the same extent by human and rat hepatocytes. This
is one of the few examples where an equal rate of transport was found in both species. In general
the uptake of drugs in human hepatocytes is slower than that in rat hepatocytes. In vitro–in
vivo scaling calculations [87,88] showed that these differences in the uptake rate of
drugs between rat and human cells, actually reflect inter-species differences rather than being
due to differences in viability. Moreover, the differences described in rate and mechanism
of drug transport in rat and man emphasize again that extrapolation to man of pharmacokinetic
data obtained in rat, is hazardous. Most of the current knowledge on drug transport carriers
is derived from experiments with rats. Therefore, more studies need to be carried out in
human liver preparations to further elucidate the mechanisms of drug transport in the human
liver and hence the relevance of animal data.
These results also indicate that human hepatocytes are an appropriate model in which to
study inter-species differences and the mechanisms of hepatic transport in man.
In contrast to isolated hepatocytes, liver slices retain the cellular architecture of the liver
without prior digestion with collagenase. This makes a systematic comparison of the data relating
to transport of free drugs as well as drug targeting moieties from isolated hepatocytes
and liver slices, an attractive model for studying the potential and limitations of the liver slice
model in this area of research.
12.5.2 Transport in Liver Slices
12.5 In Vitro Transport Studies 319
Mechanisms of drug uptake in liver slices were studied in vitro as early as in 1963 by
Schanker and Solomon [89]. The results obtained in these experiments are still valuable and
show that the influence of temperature, anoxia, metabolic inhibitors and substrate inhibition
can be successfully studied in this preparation. However, as mentioned before, at that time
the preparation of reproducible precision-cut slices was not feasible. Therefore, the slice incubation
technique was virtually abandoned in transport studies after the introduction of the
successful isolation of rat hepatocytes.
In order to investigate the possibilities and limitations of the use of precision-cut liver
slices prepared with a mechanical slicer in drug transport studies, different aspects of the
mechanism of uptake of several classes of drugs in human and rat liver slices were investigated
in our laboratory. Four model compounds which enter hepatocytes via entirely differ-