Identification of Rat Monocarboxylate Transporter 9 as a Low-Affinity Transporter of 1-Methyl-4-phenylpyridinium and Amantadine

Membrane transporters affect the distribution of drugs in tissues. However, the biological roles of several orphan transporters such as monocarboxylate transporter 9 (MCT9/SLC16A9) have not been elucidated. Since the molecules responsible for the distribution of cationic compounds to the retinal, such as 1-methyl-4-phenylpyridinium (MPP⁺) and amantadine, across the blood-retinal barrier (BRB) are not yet fully known, this study aimed to clarify whether MCT9 has transport activity for cationic molecules. MPP⁺ was taken up by MCT9 cRNA-injected Xenopus laevis oocytes, with a K_m value of 24.2 mM. This MPP⁺ uptake by the MCT9 cRNA-injected oocytes was significantly reduced in Na⁺-free, low-Cl^-, and pH 5.0 uptake buffers, whereas it significantly increased in pH 8.4 uptake buffer. In addition, MCT9 cRNA-injected oocytes showed concentration-dependent amantadine uptake with a K_m value of 9.71 mM. These results indicate that MCT9 is a low-affinity transporter for MPP⁺ and amantadine, and contributes to the cellular uptake of these cationic compounds. Although mRNA expression of MCT9 was observed in rat BRB model cells, MCT9 mRNA knockdown by transfection with small interfering RNA did not change amantadine uptake. Despite the expression of MCT9 in the BRB, it is suggested that the contribution of MCT9 to amantadine transport across the BRB is minor. In conclusion, our study first identified the cationic compounds, namely, MPP⁺ and amantadine, as substrates transported by MCT9.