The Permeation of Acamprosate Is Predominantly Caused by Paracellular Diffusion across Caco-2 Cell Monolayers: A Paracellular Modeling Approach

In drug development, estimating fraction absorbed (F a) in man for permeability-limited compounds is important but challenging. To model F a of such compounds from apparent permeabilities (P app) 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 to investigate the relative contribution of paracellular and transcellular routes to the P app across Caco-2 monolayers of the permeability-limited compound acamprosate having a bioavailability of ∼11%. The P app values of acamprosate and of several paracellular marker molecules were measured. These P app values were used to refine system-specific parameters of the Caco-2 monolayers, that is, paracellular pore radius, pore capacity, and potential drop. The refined parameters were subsequently used as an input in modeling the permeability (P modeled) of the tested compounds using mathematical models collected from two published permeability models. The experimental data show that acamprosate P app across Caco-2 monolayers is low and similar in both transport directions. The obtained acamprosate P app, 1.56 ± 0.28 × 10 -7 cm·s -1, is similar to the P app 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, Q monolayer, was lower than that of mannitol. Acamprosate did not influence the barrier function of the monolayers since it altered neither the P app of the three paracellular markers nor the transepithelial electrical resistance (TEER) of the cell monolayer. The P modeled for all the paracellular markers and acamprosate was dominated by the P para component and matched the experimentally obtained P app. Furthermore, acamprosate did not inhibit the uptake of probe substrates for solute carriers PEPT1, TAUT, PAT1, EAAT1, B 0,+AT/rBAT, OATP2B1, and ASBT expressed in Caco-2 cells. Thus, the P modeled estimated well P para, and the paracellular route appears to be the predominant mechanism for acamprosate P app across Caco-2 monolayers, while the alternative transcellular routes, mediated by passive diffusion or carriers, are suggested to only play insignificant roles.