Description and simulation of a multiple mixing tank model to predict the effect of bile sequestrants on bile salt excretion.

A nonlinear, multicompartment mixing tank model based on human physiologic parameters from the literature and in vitro bile salt sequestrant binding parameters was integrated numerically to simulate bile salt excretion. The model focuses on the transit of bile salts and resin, bile salt binding, and bile salt reabsorption as a means of gaining insight into the functioning of bile sequestrants in the gastrointestinal tract and the effect of reabsorption of bile salts on the sequestering process. The series of compartments through the ileal region were tested over a range of parameter values, and the results were compared with bile salt output from ileostomy patient data to validate the model without resin. In simulations incorporating resin with a reversible binding scheme, fecal bile salt output was 2.37 (+/- 0.6) x 10(-3) mol/day compared with 2.64 (+/- 1.1) x 10(-3) mol/day for human data. Assuming irreversible bile salt binding resulted in predictions of fecal bile salt excretion greater than three times physiologic values. The results of these simulations support the hypothesis that the lack of efficacy of bile sequestrants is due to the displacement of bound bile salts from the sequestrant as a consequence of anion competition and bile salt reabsorption. Gastric emptying effects and the timing of resin doses have also been investigated with the model.