Blood-brain barrier transport of synthetic adenosine A1 receptor agonists in vitro: structure transport relationships

Abstract Transport of 11 structurally related adenosine A 1 receptor agonists was determined in an in vitro BBB model of brain-capillary-endothelial-cells and astrocytes. Inhibitor S -(4-nitrobenzyl)-6-thioinosine (NBTI) was used to quantify the contribution of the es nucleoside transporter to the overall transport. The N 6 -substituted adenosine analogues N 6 -cyclobutyladenosine (CBA), N 6 -cyclopentyladenosine (CPA) and N 6 -cyclohexyladenosine (CHA) showed concentration-dependent clearance and their transport could be inhibited by NBTI. The V max was 1.5±0.2 pmol min −1 and the K m values were 2.2±0.2, 1.8±0.3 and 15±4 μM for CBA, CPA and CHA, respectively. Further chemical modification such as substitution in the C8-position or modification at the ribose-moiety resulted in loss of affinity for the es nucleoside transporter. Transport by passive diffusion was slow with clearances ranging from 0.21±0.01 μl min −1 for 8-(methylamino)-CPA (MCPA) to 1.8±0.18 μl min −1 for 5′-deoxy-CPA (5′dCPA). Regression analysis showed no relationship between transport clearance by passive diffusion and the GTP-shift, a non-linear relationship between the transport clearance by passive diffusion and the dynamic polar surface area ( Cl =0.469 e −0.071 DPSA ; R 2 =0.88) and a linear relationship between transport clearance and prediction of BBB transport on basis of the Abraham equation ( log Cl =1.53 log BB −1.56 ; R 2 =0.83). It is concluded that the transport of synthetic A 1 adenosine derivatives across the blood–brain barrier is generally quite slow. In addition, transport by the es nucleoside transporter may contribute to the transport of certain structurally distinct analogues.