Increased Affinity to Canalicular P-gp via Formation of Lipophilic Ion-Pair Complexes with Endogenous Bile Salts is Associated with Mw Threshold in Hepatobiliary Excretion of Quaternary Ammonium Compounds

Abstract

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. 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. 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. 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.