TABLE 1

Fitted parameter values for MDCKII-hMDR1 confluent cell monolayers

Substrate	Association to P-gp (k₁)^a	Membrane Concentration of Efflux Active P-gp [P-gp]^b	Efflux to Apical Chamber (k₂)^c	Partition Coefficients^d			Binding Constant to P-gp from Inner Apical Membrane (K_C)^e	Dissociation Constant to P-gp from Cytosol (K_D)^f	Passive Permeability Coefficient^g
Substrate	Association to P-gp (k₁)^a	Membrane Concentration of Efflux Active P-gp [P-gp]^b	Efflux to Apical Chamber (k₂)^c	K_PC	K_AO	K_BO	Binding Constant to P-gp from Inner Apical Membrane (K_C)^e	Dissociation Constant to P-gp from Cytosol (K_D)^f	At True Steady-State: P_BC; P_AC	Other Transporters: k_B; k_A
	M⁻¹s⁻¹	μM	s⁻¹				M⁻¹	μM	nm/s	s⁻¹
Amprenavir	2 × 10⁺⁹	200	150	200	150	200	1000	5	400; 400	0; 0
Digoxin	2 × 10⁺⁹	200	10	ND^h	ND^h	ND^h	ND^h	3^h	30; 30	30; 2
Loperamide	2 × 10⁺⁹	200	2	3000	700	1000	4000	0.1	350; 350	100; 0
Quinidine	2 × 10⁺⁹	200	5	700	70	100	15,000	0.1	500; 500	0; 0

ND, not determined.
↵a All values are from Acharya et al. (2008). These are median values obtained for the association rate constant fits. This is the rate constant from flip-flop across the basolateral membrane to association with the P-gp binding site.
↵b Median values for the membrane concentration of efflux active P-gp in the apical membrane inner monolayer for each of the three drugs. The units are per liter of membrane, which can be converted to P-gp per square micrometer by multiplying by 0.8. This number may be 10 to 20 times smaller than the actual surface density because only substrate released at the tips of the microvilli are likely to reach the apical chamber, rather than being absorbed back into the microvilli membrane and starting over.
↵c Median value for the efflux rate constant k₂.
↵d The partition coefficients between the cytosol and the inner plasma/apical monolayer (K_PC), between the apical outer monolayer and the apical chamber (K_AO), and between the basolateral outer monolayer and the basolateral chamber (K_BO) are shown. Partition coefficients were estimated using 0.1-μm extruded unilamellar liposomes whose lipid compositions mimic roughly the lipid compositions of the respective membrane monolayers. We only use binary and ternary lipid mixtures.
↵e The median substrate binding constant from inner apical membrane monolayer to P-gp.
↵f Dissociation constant relative to the cell cytosol, K_D = 1/(K_PCK_C).
↵g P refers to the lipid bilayer passive permeability defined as +GF120918. k is the value for the passive permeability through the other transporter for digoxin and loperamide. In reality, the passive permeability coefficients took from 15 min to 6 h to reach true steady state and P_BA was not always the same as P_AB until the steady state was reached.
↵h The mass action equations only fit the product K_CK_PC. The partition coefficient for digoxin has not been measured, so we cannot separately estimate the binding constant (K_C) to P-gp or the dissociation rate constant (k = k₁/K_C). We know that the product K_CK_PC ∼3 × 10⁵ M⁻¹ (data not shown), which yields the K_I shown.