Abstract

The purpose of the present study was to determine whether a physiologic plasma concentration of α-ketoglutarate (αKG) influences the kinetic interaction of ligands with organic anion transporter 1 (OAT1). The effect of extracellular αKG on the kinetics of para-aminohippurate (PAH) and cidofovir transport was examined along with its effect on the potency of 10 drugs in five different classes (uricosuric, nonsteroidal anti-inflammatories, loop diuretics, angiotensin II receptor antagonists, and β-lactam antibiotics) to inhibit OAT1 expressed in Chinese hamster ovary cells. Extracellular αKG competitively inhibited PAH and cidofovir transport with K_i values (∼5 μM) approximating its unbound plasma concentration (determined by equilibrium dialysis). When PAH was the substrate, extracellular αKG (5 μM) significantly increased IC₅₀ values for some inhibitors (up to 4-fold), such as probenecid, but not for others (an inhibitor-dependent effect). For some inhibitors, a significant increase in IC₅₀ value was observed when cidofovir was the substrate, but not PAH (a substrate-dependent effect). A significant increase in IC₅₀ value was also observed for inhibition of PAH transport by probenecid in renal basolateral membrane vesicles (5.2-fold). The substrate- and inhibitor-dependent effect of extracellular αKG on ligand interactions with OAT1 highlights the complexity of the OAT1 ligand-binding surface. The effect of extracellular αKG on the potency of OAT1 inhibition should be considered when assessing drug-drug interaction potential at the transporter.