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 Km values were 2.2+/-0.2, 1.8+/-0.3 and 15+/-4 microM 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 microl min(-1) for 8-(methylamino)-CPA (MCPA) to 1.8+/-0.18 microl 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=0.469e(-0.071DPSA); R2=0.88) and a linear relationship between transport clearance and prediction of BBB transport on basis of the Abraham equation (logCl=1.53logBB-1.56; R2=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.