The excellent tumor selectivity of amino acid-based imaging probes, [¹⁸F]FAMT (3-[¹⁸F]fluoro-a-methyl-l-tyrosine) and [¹²³I]IMT (3-[¹²³I]iodo-a-methyl-l-tyrosine), is supported by their selectivity for LAT1 (L-type amino acid transporter 1) which is specifically expressed in tumor cells. However, FAMT and IMT show a physiological background in kidney. Their renal accumulation is suppressed by probenecid, suggesting a contribution of organic anion transporter OAT1. To reveal the critical moieties responsible for the interaction with OAT1, we performed a structure-activity relationship analysis of aromatic amino acid derivatives in vitro. We revealed that both halogen and hydroxyl groups on the benzene ring are critical for the interaction with OAT1. Their positions on the benzene ring also affected the interaction. In contrast, the a-methyl moiety, which is essential for the selectivity to LAT1, was dispensable for the interaction with OAT1. These results hold a significant implication for designing not only the tumor-specific imaging probes with less renal background, but also the therapeutic agents for targeted radionuclide therapy with less adverse renal damage.