Dipartimento di Farmacia, Università degli Studi di Napoli Federico II, Via D. Montesano, 49 I-80131 Naples, Italy
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Graphical abstractThe relationships between data of passage through Caco-2 cultured cell lines (log Papp), taken from the literature, for 38 structurally unrelated compounds and both n-octanol lipophilicity parameters (log PN and log D7.4) and phospholipid affinity indexes were investigated. Phospholipid affinitySymbol was experimentally determined by HPLC on two different phospholipid stationary phases and the polar/electrostatic interaction component drug/phospholipids Symbol was calculated according to a method we previously proposed. Log Papp moderately related to lipophilicity values measured at pH 7.4 (log D7.4), according to a parabolic pattern, but poorly related with Symbol. Furthermore, a significant inverse linear relationship with Symbol values was only observed for the analytes with m.w. >300 Da, for which paracellular diffusion can be considered a minor transport route in vivo. Indeed, it has been reported that Caco-2 passage data also encode secondary passage mechanisms, which participate in a different extent to the jejunal absorption in vivo and cannot be directly equated to the corresponding human in situ log Peff values, unless a normalization is performed.In an attempt to elucidate this issue, 47 structurally unrelated compounds whose cultured cell line passage data were corrected for the effects of the aqueous boundary layer and paracellular permeability, so as to express transcellular intrinsic permeability, log P0Caco-2/MDCK, were also considered. Highly significant inverse linear relationships were observed between log P0Caco-2/MDCK and Symbol values from both IAM.PC.MG (r2 = 0.765) and IAM.PC.DD2 (r2 = 0.806) stationary phases whereas the relationships with either lipophilicity in n-octanol or Symbol values were very poor. The results of the present study, in complete agreement with those of our recent study on the relationships between jejunal absorption data measured in situ and Symbol values, confirm the soundness of Symbol parameters in the prediction of the intestinal absorption of drugs. From a mechanistic point of view, they suggest that the polar/electrostatic forces between drugs and phospholipids play a major role in the passage through biomembranes.