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.