Background: The Caco-2 monolayer has been used extensively for the high-throughput screening of drug permeability and identification of substrates, inhibitors, and inducers of intestinal transporters, especially P-glycoprotein (P-gp). Traditionally, the Caco-2 monolayer is viewed as a single barrier rather than a polarized cell monolayer consisting of metabolic enzymes that are sandwiched between two membrane barriers with distinctly different transporters. Objective: This review addressed the usefulness and limitations of the Caco-2 cell monolayer in drug discovery and mechanistic studies. Methods: This mini-review covered applications of the Caco-2 monolayer, clarified misconceptions, and critically addressed issues on data interpretation. Conclusion: The catenary model extends the usefulness of Caco-2 monolayer and provides proper mechanistic insight and data interpretation.

We demonstrate a role of the vitamin D receptor (VDR) in reducing cerebral soluble and insoluble amyloid-β (Aβ) peptides. Short-term treatment of two human amyloid precursor protein-expressing models, Tg2576 and TgCRND8 mice, with 1α,25-dihydroxyvitamin D₃[1,25(OH)₂D₃], the endogenous active ligand of VDR, resulted in higher brain P-glycoprotein (P-gp) and lower soluble Aβ levels, effects negated with coadministration of elacridar, a P-gp inhibitor. Long-term treatment of TgCRND8 mice with 1,25(OH)₂D₃during the period of plaque formation reduced soluble and insoluble plaque-associated Aβ, particularly in the hippocampus in which the VDR is abundant and P-gp induction is greatest after 1,25(OH)₂D₃treatment, and this led to improved conditioned fear memory. In mice fed a vitamin D-deficient diet, lower cerebral P-gp expression was observed, but levels were restored on replenishment with VDR ligands. The composite data suggest that the VDR is an important therapeutic target in the prevention and treatment of Alzheimer's disease.

We explored the properties of a catenary model that includes the basolateral (B), apical (A), and cellular compartments via simulations under linear and nonlinear conditions to understand the asymmetric observations arising from transporters, enzymes, and permeability in Caco-2 cells. The efflux ratio (EfR; P_app,B→A/P_app,A→B), obtained from the effective permeability from the A→B and B→A direction under linear conditions, was unity for passively permeable drugs whose transport does not involve transporters; the value was unaffected by cellular binding or metabolism, but increased with apical efflux. Metabolism was asymmetric, showing lesser metabolite accrual for the B→A than A→B direction because of inherent differences in the volumes for A and B. Moreover, the net flux (total - passive permeation) due to saturable apical efflux, absorption, or metabolism showed nonconformity to simple Michaelis-Menten kinetics against C_D,0, the loading donor concentration. EfR values differed with saturable apical efflux and metabolism (>1), as well as apical absorption (EfRs <1), but approached unity with high passive diffusive clearance (CL_d) and increasing C_D,0 at a higher degree of saturation of the process. The J_max (apparent V_max estimated for the carrier system) and \(K_{\mathrm{m}}^{{^\prime}}\) [or the \(K_{\mathrm{m}}^{{^{\prime\prime}}}\) based on a modified equation with the Hill coefficient (β)] estimates from the Eadie-Hofstee plot revealed spurious correlations with the assigned V_max and K_m. The sampling time, CL_d, and parameter space of K_m and V_max strongly influenced both the correlation and accuracy of estimates. Improved correlation was found for compounds with high CL_d. These observations showed that the catenary model is appropriate in the description of transport and metabolic data in Caco-2 cells.