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The permeation of acamprosate is predominantly caused by paracellular diffusion across Caco-2 cell monolayers: A paracellular modelling approach.

In drug development, estimating fraction absorbed (Fa) in man for permeability limited compounds in important but challenging. To model Fa of such compounds from apparent permeabilities (Papp) across filter-grown Caco-2 cell monolayers, it is central to elucidate the intestinal permeation mechanism(s) of the compound. The present study aims to refine a computational permeability model in order to investigate the relative contribution of paracellular and transcellular routes to the Papp across Caco-2 monolayers of the permeability limited compound acamprosate having a bioavailability of ~11%. The Papp of acamprosate and of several paracellular marker molecules were measured. These Papp values were used to refine system-specific parameters of the Caco-2 monolayers, i.e. paracellular pore radius, pore capacity and potential drop. The refined parameters were subsequently used as input in modelling the permeability (Pmodelled) of the tested compounds using mathematical models collected from two published permeability models. The experimental data shows that acamprosate Papp across Caco-2 monolayers is low and similar in both transport directions. The obtained acamprosate Papp, 1.56 ± 0.28 × 10-7 cm∙s-1, is similar to the Papp of molecular markers for paracellular permeability namely mannitol, 2.72 ± 0.24 × 10-7 cm∙s-1, lucifer yellow, 1.80 ± 0.35 × 10-7 cm∙s-1 and fluorescein, 2.10 ± 0.28 × 10-7 cm∙s-1 and lower than that of atenolol 7.32 ± 0.60 × 10-7 cm∙s-1 (mean ± SEM, n = 3-6), while the end-point amount of acamprosate internalized by the cell monolayer, Qmonolayer, was lower than that of mannitol. Acamprosate did not influence the barrier function of the monolayers since it neither altered the Papp of the three paracellular markers, nor the transepithelial electrical resistance (TEER) of the cell monolayer. The Pmodelled for all the paracellular markers and acamprosate was dominated by the Ppara component and matched the experimentally obtained Papp. Furthermore, acamprosate did not inhibit the uptake of probe substrates for the solute carriers PEPT1, TAUT, PAT1, EAAT1, B0,+AT/rBAT, OATP2B1 and ASBT expressed in Caco-2 cells. Thus, the Pmodelled estimated well Ppara and the paracellular route appears to be the predominant mechanism for acamprosate Papp across Caco-2 monolayers, while the alternative transcellular routes, mediated by passive diffusion or carriers, are suggested to only play insignificant roles.