Abstract
A number of 3'-C-methyl analogues of selective adenosine receptor
agonists such as CPA, CHA, CCPA, 2'-Me-CCPA, NECA, and IB-MECA was
synthesized to further investigate the subdomain of the receptor
that binds the ribose moiety of the ligands. Affinity data at A(1),
A(2A), and A(3) receptors in bovine brain membranes showed that the
3'-C-modification in adenosine resulted in a decrease of the affinity
at all three receptor subtypes. When this modification was combined
with N(6)-substitution with groups that induce high potency and selectivity
at A(1) receptor, the affinity and selectivity were increased. However,
all 3'-C-methyl derivatives proved to be very less active than the
corresponding 2'-C-methyl analogues. The most active compound was
found to be 3'-Me-CPA which displayed a K(i) value of 0.35 microM
at A(1) receptor and a selectivity for A(1) vs A(2A) and A(3) receptors
higher than 28-fold. 2'-Me-CCPA was confirmed to be the most selective,
high affinity agonist so far known also at human A(1) receptor with
a K(i) value of 3.3 nM and 2903- and 341-fold selective vs human
A(2A) and A(3) receptors, respectively. In functional assay, 3'-Me-CPA,
3'-Me-CCPA, and 2-Cl-3'-Me-IB-MECA inhibited forskolin-stimulated
adenylyl cyclase activity with IC(50) values ranging from 0.3 to
4.9 microM, acting as full agonists. A rhodopsin-based model of the
bovine A(1)AR was built to rationalize the higher affinity and selectivity
of 2'-C-methyl derivatives of N(6)-substituted-adenosine compared
to that of 3'-C-methyl analogues. In the docking exploration, it
was found that 2'-Me-CCPA was able to form a number of interactions
with several polar residues in the transmembrane helices TM-3, TM-6,
and TM-7 of bA(1)AR which were not preserved in the molecular dynamics
simulation of 3'-Me-CCPA/bA(1)AR complex.
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