Identification of human-specific amino acid residues governing atenolol transport via organic cation transporter 2

Organic cation transporter 2 (OCT2/SLC22A2) is predominantly localized on the basolateral membranes of renal tubular epithelial cells and plays a crucial role in the renal secretion of various cationic drugs. Although variations in substrate selectivity among renal organic cation transport systems across species have been reported, the characteristics of OCT2 remain unclear. In this study, we demonstrated that atenolol, a β1-selective adrenergic antagonist, is transported almost exclusively by human OCT2, contrasting with OCT2s from other selected species. Using chimeric constructs between human OCT2 (hOCT2) and the highly homologous monkey OCT2 (monOCT2), along with site-directed mutagenesis, we identified non-conserved amino acids Val⁸, Ala³¹, Ala³⁴, Tyr²²², Tyr²⁴⁵, Ala²⁷⁰, Ile³⁹⁴, and Leu⁵⁰³ as pivotal for hOCT2-mediated atenolol transport. Kinetic analysis revealed that atenolol was transported by hOCT2 with a 12-fold lower affinity than MPP⁺, a typical OCT2 substrate. The inhibitory effect of atenolol on MPP⁺ transport was 6200-fold lower than that observed for MPP⁺ on atenolol transport. Additionally, we observed weaker inhibitory effects on MPP⁺ transport compared to atenolol transport with ten different OCT2 substrates. Altogether, this study suggests that eight hOCT2-specific amino acids constitute the low-affinity recognition site for atenolol transport, indicating differences in OCT2-mediated drug elimination between humans and highly homologous monkeys. Our findings underscore the importance of understanding species-specific differences in drug transport mechanisms, shedding light on potential variations in drug disposition and aiding in drug development.