Limited Brain Distribution of [3R,4R,5S]-4-Acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate Phosphate (Ro 64-0802), a Pharmacologically Active Form of Oseltamivir, by Active Efflux across the Blood-Brain Barrier Mediated by Organic Anion Transporter 3 (Oat3/Slc22a8) and Multidrug Resistance-Associated Protein 4 (Mrp4/Abcc4)

[3R,4R,5S]-4-Acetamido-5-amino-3-(1-ethylpropoxy)-1-cyclohexene-1-carboxylate phosphate (Ro 64-0802) is a pharmacologically active form of the anti-influenza virus drug oseltamivir. Abnormal behavior is a suspected adverse effect of oseltamivir on the central nervous system. This study focused on the transport mechanisms of Ro 64-0802 across the blood-brain barrier (BBB). Ro 64-0802 was found to be a substrate of organic anion transporter 3 (OAT3/SLC22A8) and multidrug resistance-associated protein 4 (MRP4/ABCC4). Human embryonic kidney 293 cells expressing OAT3 exhibited a greater intracellular accumulation of Ro 64-0802 than mock-transfected cells (15 versus 1.2 μl/mg protein/10 min, respectively). The efflux of Ro 64-0802 was 3-fold greater when MRP4 was expressed in MDCKII cells and was significantly inhibited by indomethacin. After its microinjection into the cerebrum, the amount of Ro 64-0802 in brain was significantly greater in both Oat3^–/– mice and Mrp4^–/– mice compared with the corresponding wild-type mice (0.36 versus 0.080 and 0.32 versus 0.060 nmol at 120 min after injection, respectively). The brain/plasma concentration ratio (K_p,brain) of Ro 64-0802, determined in wild-type mice after subcutaneous continuous infusion for 24 h, was close to the capillary volume (approximately 10 μl/g brain). Although the K_p,brain of Ro 64-0802 was unchanged in Oat3^–/– mice, it was significantly greater in Mrp4^–/– mice (41 μl/g of brain). These results suggest that Ro 64-0802 can cross the BBB from the blood, but its brain distribution is limited by its active efflux by Mrp4 and Oat3 across the BBB. The transporter responsible for the brain uptake of Ro 64-0802 remains unknown, but Oat3 is a candidate transporter.