Kinetic characterization of rat hepatic uptake of 16 actively transported drugs

To explore the determinants of hepatic uptake, 16 compounds were investigated with different physicochemical and disposition characteristics, including five statins, three sartans, saquinavir, ritonavir, erythromycin, clarithromycin, nateglinide, repaglinide, fexofenadine, and bosentan. Freshly isolated rat hepatocytes in suspension were used with the oil-spin method to generate kinetic parameters. Clearances, via passive diffusion (P(diff)) and active uptake (CL(active), characterized by maximal uptake rate and K(m)), were estimated from the initial uptake rate data over a 0.01 to 100 μM concentration range. The K(m) values had a range of 15-fold, with 10 of the 16 drugs with K(m) < 10 μM (median 6 μM). Both CL(active) and P(diff) ranged over 100-fold (median 188 and 14 μl/min/10⁶ cells). Assessment of the relative contribution of P(diff) and CL(active) indicated that, at low concentrations (approximately 0.1 μM), the active process contributes >80% to the overall uptake for 13 drugs. Although high P(diff) values were obtained for ritonavir and repaglinide, active process contributed predominantly to uptake; in contrast, high passive permeability dominates over transporter-mediated uptake for saquinavir over the full concentration range. For bosentan and erythromycin, active and passive processes were equally important. Hepatocyte-to-medium unbound concentration ratio was >10 for 9 of the 16 drugs, ranging from 2 to 494 for bosentan and atorvastatin, respectively. Some drugs showed extensive intracellular binding (fraction unbound range 0.01-0.6), which was not correlated with active uptake. LogD₇.₄ correlated significantly with P(diff) and the extent of intracellular binding but not with active uptake. This study provides systematic assessment of the role of active uptake relative to the passive process; implications of the findings are discussed.