Micelles formed in the GI tract by bile acid and lecithin play an important role in oral absorption of poorly soluble drugs. In this situation, the drug molecules are present in equilibrium between the free and micellar states. In this study, the relationship between the free drug concentration and the membrane permeability of poorly soluble drugs was examined. Permeability across a Caco-2 monolayer and a dialysis membrane were measured in a side-by-side chamber system. The concentrations of sodium taurocholate (NaTC) and lecithin were varied to allow measurement of membrane permeability at different concentrations of free drugs. For troglitazone, hexylparaben, and heptylparaben, an increase in the NaTC and lecithin concentrations caused the permeability across the Caco-2 monolayer to decrease slightly, whereas the permeability across the dialysis membrane decreased markedly. In contrast, the changes in permeability of griseofulvin with an increased micelle concentration were similar for the Caco-2 monolayer and the dialysis membrane. Assuming that the permeability for the dialysis membrane reflects the free drug concentration in the medium, these results suggest that troglitazone and alkylparabens, but not griseofulvin, can partition directly from micelles to Caco-2 monolayers. This mechanism may contribute to oral absorption of drugs that are poorly soluble in water.
2009 Wiley-Liss, Inc. and the American Pharmacists Association