Bile acids and bile salts (BA/BS) are substrates of both influx and efflux transporters
on hepatocytes. Canalicular efflux transporters, such as BSEP and MRP2, are crucial
for the removal of BA/BS to the bile. Basolateral influx transporters, such as NTCP,
OATP1B1/1B3, and OSTα/β, cooperate with canalicular transporters in the transcellular
vectorial flux of BA/BS from the sinusoids to the bile. The blockage of canalicular
transporters not only impairs the bile flow but also causes the intracellular accumulation
of BA/BS in hepatocytes that contributes to, or even triggers, liver injury. In the
case of BA/BS overload, the efflux of these toxic substances back to the blood via
MRP3, MRP4, and OST α/β is considered a relief function. FXR, a key regulator of defense
against BA/BS toxicity suppresses de novo bile acid synthesis and bile acid uptake,
and promotes bile acid removal via increased efflux. In drug development, the early
testing of the inhibition of these transporters, BSEP in particular, is important
to flag compounds that could potentially inflict drug-induced liver injury (DILI).
In vitro test systems for efflux transporters employ membrane vesicles, whereas those
for influx transporters employ whole cells. Additional in vitro pharmaceutical testing
panels usually include cellular toxicity tests using hepatocytes, as well as assessments
of the mitochondrial toxicity and accumulation of reactive oxygen species (ROS). Primary
hepatocytes are the cells of choice for toxicity testing, with HepaRG cells emerging
as an alternative. Inhibition of the FXR function is also included in some testing
panels. The molecular weight and hydrophobicity of the drug, as well as the steady-state
total plasma levels, may positively correlate with the DILI potential. Depending on
the phase of drug development, the physicochemical properties, dosing, and cut-off
values of BSEP IC50 ≤ 25–50 µM or total Css,plasma/BSEP IC50 ≥ 0.1 may be an indication
for further testing to minimize the risk of DILI liability.
