A METHOD FOR THE DETERMINATION OF CELLULAR PERMEABILITY COEFFICIENTS AND AQUEOUS BOUNDARY-LAYER THICKNESS IN MONOLAYERS OF INTESTINAL EPITHELIAL (CACO-2) CELLS GROWN IN PERMEABLE FILTER CHAMBERS

A linear relationship between apparent permeability coefficients (P(app)) and agitation rate was used to determine cell permeability coefficients (P(c)) and the thickness of the aqueous boundary layer (h(aq)) in monolayers of Caco-2 cells. Drugs with different physical/chemical properties were studied. P(app) values for 'highly permeable drugs' (testosterone, corticosterone, propranolol, metoprolol, warfarin) were dependent on the agitation rate i.e. on the h(aq). The permeation of testosterone was studied in detail. In the absence of agitation, the measured P(app) value of testosterone was 35.7 +/- 3.3 X 10(-6) cm/s while the P(c) value was 137.4 +/- 8.1 X 10(-6) cm/s, i.e. 3.8 times higher. P(app) values for 'less permeable drugs' (hydrocortisone, salicylic acid, sulphasalazine, mannitol) were not dependent on the agitation conditions. In the absence of agitation, h(aq) for testosterone was 1544 +/- 142-mu-m and the contribution of the resistance of the aqueous boundary layer to the total diffusional resistance was 70%. h(aq) was reduced to 128 +/- 10-mu-m when the highest agitation rate was used. Under these conditions, the contributions of the resistances of the aqueous boundary layer and the Caco-2 cell monolayers were 16 and 73%, respectively. Similar results were obtained for the other highly permeable drugs. The results indicate that the P(app) values for highly permeable drugs are controlled not only by the aqueous boundary layer but also by the Caco-2 cell monolayers.