Purpose: To determine the lipophilicity trend line from the relationship between the blood-retinal barrier (BRB) permeability and the lipophilicity of permeants and compare it with that of the blood-brain barrier (BBB).
Methods: The retinal (RUI) and brain uptake index (BUI) of 26 radiolabeled compounds across the rat BRB and BBB, respectively, were measured using the carotid artery injection method.
Results: RUI was determined using 13 compounds expected to be transported from blood to the retina by passive diffusion and with a log n-octanol/Ringer distribution coefficient (DC) ranging from -2.56 to 2.48. The RUI values were correlated with the log of the DC [RUI = 46.2 × exp (0.515 × log DC), r(2) = 0.807]. A similar trend was obtained between BUI and lipophilicity. The RUI value for substrates of the influx transporters and P-glycoprotein (P-gp) was greater and smaller than the lipophilicity trend line, respectively. In contrast, [(3)H]verapamil, which is a substrate of P-gp, has a greater RUI value than the lipophilicity trend line, but not for BUI, suggesting that the BRB has an influx transport system for verapamil.
Conclusions: The lipophilicity trend line constructed from the RUI and DC values is considered to reflect the transport properties of drugs undergoing passive diffusion across the BRB.