Interpretation of recently presented adenosine A1 and A2 receptor binding affinity for a series of 1,3,8-trisubstituted xanthines supports a receptor binding model that predicts an orientation of receptor ligands governed by a hydrophobic binding domain, an aromatic binding domain and a ribose binding domain.We have recently proposed the three binding domain model of adenosine receptors.1~2 Features of our model are the presence of a ribose binding domain, an aromatic binding domain and a single hydrophobic binding domain in A1 and A2 receptors. This is visualized in the figure below showing the overlap of the hydrophobic groups, the common 4-membered ring aromatic and the differential location of the ribose domain of the Al selective agonist (R)-N6-(phenylisopropyl)adenosine (I), the A2 selective agonist S-N-ethyl-2-[[4-(2 carboxyethyl)phenethyl]amlno]-adenosine-5'-uronamide (2) and the At selective antagonist 1,3-dipropyl-S(R)- (phenylisopropyl)xanthine (3). 2 A recent publication has reported the synthesis and adenosine A1 and A2 receptor binding affinities for a large series of 1,3,&trisubstituted xanthines.3 It was previously known that changing the 1,3-substituen~ of ~eophylline from methyl to propyl increased the potency of the compounds at adenosine receptors4'7 but until the study by Erikson et al 3 it had not been determined whether both propyl groups are needed for activity. We now report that the three binding domain receptor model provides an explanation for the receptor binding results of the 1,3,8-trisubstituted xanthines.At the adenosine AI receptor, in comparison to the parent 8-phenyltheophylline (4a), changing either the l- or 3-methyl group to a propyl, 4b and 4c gave a lo-fold increase in potency while changing both positions (4d) gave a 20-fold increase in potency. This trend was observed with all eleven different C8 subs~~en~. At the adenosine A2 receptor, in comparison to 8-phenyltheophy~~e (4a), a change from methyi to propyl at the 3 position (4~) gave a 7-fold increase in affinity while the same change at the l-position (4b) caused no increase in A2 binding . The change in both positions (4d) gave no increase over the monosubstitution at the l-position.This trend was observed with all eleven different C8 suhstituents with respect to the ?position, however while changing the l-methyl to propyl resulted in no increase in affinity for five of the eleven series of compounds, a 2 to 3-fold increase in affinity was observed for five of the remaining six series of compounds.At the adenosine At receptor, which is visualized as having a binding domain capable of accepting 1, superimposition8 of 4d is shown in Figure 1. At the adenosine A2 receptor, which is visualized as having a binding site capable of accepting 2, superimposition of 4d is shown in Figure 2. The superimposition of Figure 1 shows that both propyls occupy an unrestricted area and propyl substitution at both positions enhances receptor affinity at the At receptor. On the other hand, Figure 2 shows that the NI propyl intrudes into the imidazole region and it is only the N3 propyl that can occupy an area that could effectively contribute to receptor affinity at the A? receptor.In summary, the three hinding domain model of adenosine receptors has explained the effect on receptor aftimty of alkyl substitution in a series of g-substituted xanthines whereby propyls at both Nl and N3 enhance affinity at At receptors while it is only propyl at N3 which enhances affinity at A2 receptors.