Albumin-Mediated\nDrug Uptake by Organic Anion Transporter\n1/3 Is Real: Implications for the Prediction of Active Renal Secretion\nClearance

Modulation of the\ntransport-mediated active uptake by\nhuman serum\nalbumin (HSA) for highly protein-bound substrates has been reported\nand improved the <i>in vitro</i>-to-<i>in vivo</i> extrapolation (IVIVE) of hepatic clearance. However, evidence for\nthe relevance of such a phenomenon in the case of renal transporters\nis sparse. In this study, transport of renal organic anion transporter\n1 or 3 (OAT1/3) substrates into conditionally immortalized proximal\ntubular epithelial cells transduced with OAT1/3 was measured in the\npresence and absence of 1 and 4% HSA while keeping the unbound substrate\nconcentration constant (based on measured fraction unbound, <i>f</i><sub>u,inc</sub>). In the presence of 4% HSA, the unbound\nintrinsic active uptake clearance (CL<sub>int,u,active</sub>) of six\nhighly protein-bound substrates increased substantially relative to\nthe HSA-free control (3.5- to 122-fold for the OAT1 CL<sub>int,u,active</sub>, and up to 28-fold for the OAT3 CL<sub>int,u,active</sub>). The\nalbumin-mediated uptake effect (fold increase in CL<sub>int,u,active</sub>) was more pronounced with highly bound substrates compared to no\neffect seen for weakly protein-bound substrates adefovir (OAT1-specific)\nand oseltamivir carboxylate (OAT3-specific). The relationship between\nOAT1/3 CL<sub>int,u,active</sub> and <i>f</i><sub>u,inc</sub> agreed with the facilitated-dissociation model; a relationship was\nestablished between the albumin-mediated fold change in CL<sub>int</sub>,<sub>u,active</sub> and <i>f</i><sub>u,inc</sub> for both\nthe OAT1 and OAT3, with implications for IVIVE modeling. The relative\nactivity factor and the relative expression factor based on global\nproteomic quantification of <i>in vitro</i> OAT1/3 expression\nwere applied for IVIVE of renal clearance. The inclusion of HSA improved\nthe bottom-up prediction of the level of OAT1/3-mediated secretion\nand renal clearance (CL<sub>sec</sub> and CL<sub>r</sub>), in contrast\nto the underprediction observed with the control (HSA-free) scenario.\nFor the first time, this study confirmed the presence of the albumin-mediated\nuptake effect with renal OAT1/3 transporters; the extent of the effect\nwas more pronounced for highly protein-bound substrates. We recommend\nthe inclusion of HSA in routine <i>in vitro</i> OAT1/3 assays\ndue to considerable improvements in the IVIVE of CL<sub>sec</sub> and\nCL<sub>r</sub>.