Objectives: We intended to elucidate the mechanism of the molecular weight (Mw) threshold (i.e., 200 +/- 50) for appreciable hepatobiliary excretion of quaternary ammonium compounds (QACs) in rats.
Methods: We measured the effect of ion-pair complexation of QACs with taurodeoxycholate (TDC), an endogenous anionic bile salt, on the apparent partition coefficients (APC) of QACs between n-octanol and phosphate buffer, and the inhibition of organic cation transporter1 (OCT1)- and P-glycoprotein (P-gp)-mediated transport of representative substrates.
Results: By measuring the APC, we demonstrated that there is a Mw threshold of 200 +/- 50 in the ion-pair complexation of QACs with an endogenous bile salt, TDC. We also demonstrated, by measuring the inhibition of relevant transports, that a Mw threshold of 200 +/- 50 exists for the binding of QACs to canalicular P-gp, but not for sinusoidal OCT1. The Mw threshold values for ion-pair formation and P-gp binding were identical and consistent with the reported Mw threshold value for appreciable biliary excretion of QACs in rats.
Conclusions: Mw-dependent binding of QACs to canalicular P-gp contributes in part to the mechanism of the Mw threshold of 200 +/- 50. The formation of lipophilic ion-pair complexes with bile salts, followed by stronger binding to canalicular P-gp, appears to accelerate biliary excretion of QACs with a high Mw.