Structural characterization of the urea transporter bound to the orally bioavailable inhibitor E3

Orally bioavailable inhibitors targeting the kidney urea transporter (UT) have the potential to serve as salt-sparing diuretics by employing a urea-selective diuretic mechanism of action distinct from that of diuretics targeting salt transporters. To elucidate the mechanism by which oral inhibitors interact with UTs, we solved the structure of a newly developed inhibitor, E3, with UT-A2 using cryo-electron microscopy. Through structural analysis and binding free energy calculations, we not only revealed the binding mode of E3 to UT-A2 but also clarified the structural basis by which E3 serves as a common competitive inhibitor of human, mouse and rat UT-A/UT-B. E3 exerts its inhibitory effect by competitively binding to the conserved Q-T-T-Q motif in the urea binding pockets of the transport channel. Moreover, we discovered that the BSBP region of UT can serve as a key region for enhancing the inhibitory potency of E3 with different UTs, which provides valuable structural insights for designing and modifying high-affinity UT inhibitors that act as diuretics.