In vitro studies to investigate the reasons for the low potency of cholestyramine and colestipol.

The association rates, dissociation rates, and equilibrium binding of bile acids with cholestyramine and colestipol were measured under physiological conditions with the most abundant bile acids found in humans. Cholestyramine and colestipol equilibrated with the bile acids (5 mM) within 1 h and they bound > 58% and > 17% of the bile acid, respectively, when at equilibrium with physiological concentrations of bile acid (4.3-10.1 mM). However, the conjugated trihydroxy bile acids taurocholic acid and glycocholic acid dissociated rapidly from both cholestyramine and colestipol when the sequestrants, preloaded with the bile acid, were washed with the Krebs-Henseleit buffer. The taurine-conjugated and dihydroxy bile acids dissociated more slowly from cholestyramine and colestipol than the glycine-conjugated and trihydroxy bile acids and, therefore, would be expected to avoid reabsorption to a greater extent by the terminal ileum and colon in vivo. We predict from these results that the reasons for the low potency of cholestyramine and colestipol are that they bind a relatively small proportion of the trihydroxy bile acids in the duodenum and jejunum and that all of the bile acids dissociate to varying extents from the sequestrants in the terminal ileum where the unbound bile acids are reabsorbed by the gut.