Figure 5.6. Effect of DMSO on the PAMPA permeability

pharmacophore in terms of aqueous solubility. PAMPA model has a significant advantage over Caco-2 cells in this regard. Figure 5.6 shows the effect of various concentrations of DMSO on the PAMPA permeability of some probe compounds (high and low Pc compounds). Contrary to a cell-based model, where higher than 1% DMSO would compromise the cell monolayer leading to unreliable results, the PAMPA permeability was consistent in the presence of DMSO up to 10%. Having higher cosolvent concentration not only increases the percentage of compounds that can be successfully studied in a permeability assay, but it also improves the mass-balance recovery. A higher cosolvent concentration is likely to minimize the physical loss (e.g., nonspecific binding to devise and membrane) during the permeability study. Other Experimental Variability

It is well documented that the Caco-2 cell permeability of the same set of drugs obtained from different laboratories varies significantly (Walter and Kissel, 1995; Ungell, 2004; Balimane and Chong, 2005a). There are a variety of factors that can influence the outcome. Minor differences in cell culture conditions (e.g., seeding density, feeding frequency and composition of the cell media, etc.), experimental protocol (e.g., initial concentration of drugs, composition of the permeability buffer, pH and monolayer washing steps, etc.) and age of the cells (e.g., passage number, culture duration and tightness of junction) can produce dramatic differences in the permeability values. In addition, the function of drug transporters expressed in the cell-based models can fluctuate significantly with difference in culture conditions (Walter and Kissel, 1995; Anderle et al., 2004). PAMPA permeability model, on the other hand, is a relatively rigid model less prone to interlaboratory variability problems. The lipid solution used to create the bilayers membrane is the only component of the model that can incorporate fluctuations in permeability value. Moreover, since PAMPA captures only the passive diffusive permeability and is devoid of any transporter proteins, other experimental factors play a negligible role.

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