Many clinically used drugs are transported in the liver and kidney by organic cation transporters (OCT) To date, three organic cation transporters in the OCT family have been cloned and characterized (OCT1-3) Of these, OCTl appears to be an important transporter in the liver and OCT2 appears to be a major transporter in the kidney. With the availability of the cloned transporters, it is now possible to begin investigating their roles in renal and hepatic drug elimination.

The hurrrarr transporters, hOCT1 and hOCT2, share 70% sequence identity, and their predicted secondary structures, based on hydropathy analysis, are essentially the same, This might suggest that paralogous organic cation transporters such as hOCT1 and hOCT2 have similar functional charac—teristics and are functionally redundant. However, recent chimeric and mutagencsis studies of transporters have shown that changes in even one or two amino acids can dramatically alter specificity Therefore, it is reasonable to propose that these two organic cation transporter homologs serve different functions in vivo.