Abstract
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; Papp,B→A/Papp,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 CD,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 (CLd) and increasing CD,0 at a higher degree of saturation of the process. The Jmax (apparent Vmax estimated for the carrier system) and 
[or the 
based on a modified equation with the Hill coefficient (β)] estimates from the Eadie-Hofstee plot revealed spurious correlations with the assigned Vmax and Km. The sampling time, CLd, and parameter space of Km and Vmax strongly influenced both the correlation and accuracy of estimates. Improved correlation was found for compounds with high CLd. These observations showed that the catenary model is appropriate in the description of transport and metabolic data in Caco-2 cells.
Footnotes
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This work was supported by the Canadian Institute for Health Research, Grant MOP64350.
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.107.015321.
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ABBREVIATIONS: ABC, ATP-binding cassette; Pgp, P-glycoprotein; MRP, multidrug resistance-associated protein; BCRP, breast cancerresistant protein; GF120918, N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide; A, apical; B, basolateral; CD,0, initial loading concentration in donor side at t = 0; Papp, effective permeability; EfR, efflux ratio; MK571, 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid; PPD, passive permeability; DR and Dcell, drug amounts in receiver and cell, respectively; AQ, absorptive quotient; SQ, secretory quotient; Mettotal, total amount of metabolite; ER, extraction ratio; CLint,sec, intrinsic clearance for apical efflux, mediated by transporters; CLint,met, metabolic intrinsic clearance; CLabs, intrinsic clearance for apical absorption, mediated by transporters; CLinflux and CLefflux, intrinsic clearances for basolateral influx and efflux, respectively, mediated by transporters; CLd, passive diffusion clearance common across apical and basolateral membranes; CLd4 and CLd1, passive diffusion clearance from the apical and basolateral compartments into the cell compartment, respectively; CLd3 and CLd2, passive diffusion clearance from the cell compartment into the apical and basolateral compartments, respectively; Vap and Vbaso, volumes of buffer solution in the apical and basolateral compartments, respectively; Vcell, cellular volume; Pc, Jc, and Jmax, the permeability, flux, and apparent Vmax estimated for the carrier system, respectively;
and , the apparent Kms estimated from the Eadie-Hofstee plot, with and without the Hill coefficient, respectively; fmet, fraction of dose metabolized. - Received February 23, 2007.
- Accepted October 9, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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