Abstract

The contribution of organic anion transporters to the total efflux of 17β-estradiol-d-17β-glucuronide (E₂17βG) through the blood-brain barrier (BBB) was investigated using the Brain Efflux Index method by examining the inhibitory effects of probenecid, taurocholate (TCA), p-aminohippurate (PAH), and digoxin. E₂17βG was eliminated through the BBB with a rate constant of 0.037 min⁻¹ after the microinjection into the brain. Probenecid and TCA inhibited this elimination with an IC₅₀ value of 34 and 1.8 nmol/0.5 μl of injectate, respectively, whereas PAH and digoxin reduced the total efflux to about 80 and 60% of the control value, respectively. The selectivity of these inhibitors was confirmed by examining their inhibitory effects on the transport via organic anion transporting polypeptide 1 (Oatp1), Oatp2, organic anion transporter 1 (Oat1), and Oat3 transfectants using LLC-PK1 cells as hosts. Digoxin specifically inhibited the transport via Oatp2 (K_i = 0.037 μM). TheK_i values of TCA for Oatp1 and Oatp2 (11 and 39 μM, respectively) were about 20 times lower than those for Oat1 and Oat3 (2.8 and 0.8 mM, respectively). PAH did not affect the transport via the Oatp family, but had a similar affinity for Oat1 and Oat3 (85 and 300 μM, respectively). Probenecid had a similar affinity for these transporters (Oatp1, Oatp2, Oat1, and Oat3) examined in this study. Taking the selectivity of these inhibitors into consideration, the maximum contribution made by the Oatp2 and Oat family to the total efflux of E₂17βG from the brain appears to be about 40 and 20%, respectively.