Phenylalanine was used as ABL marker. Different flow rates in the side-by-side diffusion chamber were used to study the ABL. The filter restriction of Snapwell polycarbonate and Snapwell-Clear polyester membranes was compared. Permeability through blank filter inserts was measured to obtain Pblank for all compounds. The authors proposed that Pblank is

a combination of permeability through ABL and filter inserts (cf., Eq. (A.1)). The PABL and Pfilter were uncoupled with regression analysis of Pblank as a function of stirring rate to derive Pfilter. Consistent with our findings, the polyester membrane of Snapwell-Clear was found to restrict permeability of the highly permeable lipophilic molecule progesterone. Grouping of PABL, Selleckchem Proteasome inhibitor Pfilter and permeability buy Pictilisib through other resistances in the transport study system, designated PSYS was also practised by Carl et al. (2010). The PSYS was represented and measured as Pblank. To derive the permeability across the hCMEC/D3 cell monolayer, PSYS was subtracted from the Papp data. Subtraction of Pblank from Papp to derive Pmonolayer is appropriate if the two parameters PABL and Pfilter are the same in blank filter inserts and in the presence of the cell monolayer. However, the ABL can be thinner in blank inserts ( Hidalgo et al.,

1991). The cellular permeability coefficient, PC, was introduced through studies at different stirring rates by Karlsson and Artursson (1991). ABL also depends on the interaction between the aqueous phase and membrane surface ( Loftsson and Brewster, 2008)

including all a complex glycocalyx that differs between cell models. Hence, the interaction between the aqueous buffer and the cell membrane surface will be different from the interaction between the buffer and either coated or uncoated porous membrane surface. The Pfilter in the presence of cells will tend to be lower because tight adherence of the cells will increase the path length to accessible pores, and some pores may be occluded or restricted by fine processes extending from the basolateral membrane surface. These differences could bias calculation of the cell monolayer permeability. Pfilter will not influence the intrinsic transcellular permeability (P0) calculation if it is not a rate-limiting step. Experimental permeability data are refined to correct for ABL and eliminate the effect of paracellular permeation to derive the P0. A possible complication arises if the PABL of the compound tested is not the same as PABL of the marker used and if Ppara of the compound is not equal to the measured permeability of the paracellular marker. However, the PABL is not critical if compounds studied are moderately lipophilic when permeability is less influenced by ABL (P0 < PABL). The Ppara is minimal with use of tight monolayers. The P0 IVIVC analysis ( Fig.