Grepafloxacin (GPFX) has a comparatively greater hepatobiliary transport than other
quinolone antibiotics. The biliary excretion mechanism of GPFX was investigated in a series of in vivo and in vitro studies with Sprague-Dawley rats and the mutant strain Eisai-
hyperbilirubinemia rats (EHBR), which have a hereditary defect in their bile canalicular multispecific organic
anion transport system (
cMOAT). The biliary excretion of the parent
drug in EHBR was 38% of that in normal rats, whereas the 3-glucuronide, a main metabolite of GPFX, was scarcely excreted into the bile in EHBR. To clarify the biliary excretion mechanism of GPFX, studies of uptake by bile canalicular membrane vesicle (CMV) were performed.
ATP dependence was observed in the uptake of GPFX by CMV, although the extent was not very marked, whereas no
ATP-dependent uptake was observed by CMV prepared from EHBR. An inhibition study of the
ATP-dependent uptake of the
glutathione conjugate, 2,4-dinitrophenyl-S-glutathione (
DNP-SG), a typical substrate for
cMOAT, was performed in order to differentiate among the affinities of six
quinolone antibiotics for this transporter. All
quinolone antibiotics inhibited the
ATP-dependent uptake of
DNP-SG with different half-inhibition concentrations (IC50), and GPFX had the lowest IC50 value. The uptake of GPFX-
glucuronide by CMV from normal rats showed a marked
ATP dependence, whereas there was little
ATP-dependent uptake in EHBR. The K(m) value (7.2 microM) for the higher-affinity component of the
glucuronide uptake was comparable to the Ki value (9.2 microM) of the
glucuronide in terms of inhibition of the
ATP-dependent uptake of
DNP-SG, which indicates that
DNP-SG and the
glucuronide may share the same transporter,
cMOAT. The Ki value of the
glucuronide observed in this inhibition was less than 1/200 that of the parent, which suggests that the
glucuronide had a much higher affinity than the parent
drug. These results lead us to conclude that at least a part of the GPFX transport and a major part of its
glucuronide transport across the bile canalicular membrane are by a primary active transport mechanism mediated by
cMOAT.