Frog intestinal sac as an in vitro method for the assessment of intestinal permeability in humans: Application to carrier transported drugs

The aim of this study was to investigate the presence of pharmaceutically relevant drug transporters in frog intestine which has been proposed as model for intestinal permeability screening assays of passively absorbed drugs in humans [Trapani, G., Franco, M., Trapani, A., Lopedota, A., Latrofa, A., Gallucci, E., Micelli, S., Liso, G., 2004. Frog intestinal sac: a new in vitro method for the assessment of intestinal permeability. J. Pharm. Sci. 93, 2909–2919]. The expression of transporters in frog intestine was supported by the following observations: (i) the involvement of purine nucleobase transport system was deduced by inhibition of acyclovir transport in the presence of adenine; (ii) baclofen or l-dopa transport was inhibited by the digitalis steroid ouabain and it may be related to the Na+ electrochemical potential difference, presumably involving amino acid transporters; (iii) the presence of proton-dependent peptide transporters was argued evaluating the effect of the pH change (from pH 5.9 to pH 7.4) on the transport of glutathione; (iv) the possible expression in the frog intestine of an efflux system distinct from P-glycoprotein (Pgp) in the benzylpenicillin transport was deduced using a glucose enriched frog Ringer with or without the known Pgp inhibitor verapamil; (v) the contribution of Pgp-mediated efflux system in determining the frog intestinal absorption of drugs was supported by the specific inhibition of cimetidine or nadolol transport in the presence of verapamil. These results indicate that pharmaceutically relevant drug transporters should be also expressed in frog intestine. In this work, an attempt was also made to compare the measured Papp values in the frog intestinal model for the aforementioned series of actively/effluxed transported drugs in humans to the corresponding literature values for the fraction absorbed. The Papp values used in these comparisons were obtained at high concentrations of drugs at which probably saturation of the carrier occurs. Interestingly, it was found that drugs that are completely absorbed had Papp values >3×10−6 cm/s, while drugs absorbed <90% had Papp values lower than 1×10−6 cm/s. In these cases, indeed, a borderline region characterized by the apparent permeability coefficient Papp value between 1×10−6 and 3×10−6 cm/s should be considered for which the prediction of the absorbed fraction after oral administration in humans become more uncertain by the frog intestinal sac system.