Xd. Ji et al., 8-(3-ISOTHIOCYANATOSTYRYL)CAFFEINE IS A SELECTIVE, IRREVERSIBLE INHIBITOR OF STRIATAL A(2)-ADENOSINE RECEPTORS, Drug development research, 29(4), 1993, pp. 292-298
8-(3-Isothiocyanatostyryl)caffeine (ISC) was synthesized and shown to
inhibit selectively the binding of [H-3]CGS 21680 (an A2a-selective ag
onist) at adenosine receptors in striatal membranes. The K(i) value at
A2a-receptors was found to be 110 nM (rat), with selectivity ratios f
or A2a versus A1-receptors in rat, guinea pig, bovine, and rabbit stri
atum of >100-fold. Preincubation of membranes with ISC caused a dose-d
ependent, irreversible antagonism of the binding of [H-3]CGS 21680, wi
th an IC50 value of 3 muM. The irreversibility is likely due to the pr
esence of the chemically reactive isothiocyanate group, since the bind
ing of the corresponding analogue in which the isothiocyanate was repl
aced with a chloro group was completely reversible. The potency of ISC
to irreversibly inhibit the binding of [H-3]CGS 21680 in several spec
ies varied in the order rat almost-equal-to guinea pig > bovine almost
-equal-to rabbit. In all four species, binding of the A1-selective ago
nist [H-3]R-N6-phenylisopropyladenosine was not diminished by pre-trea
tment with 2 muM ISC. The kinetics of irreversible inhibition of rat A
2a-receptors by 2 muM ISC gave a t1/2 of approximately 3 min. Followin
g partial inactivation, the remaining rat A2a-binding sites retained t
he same K(d) value as in control membranes for saturation by [H-3]CGS
21680. Thus, ISC appears to be a selective affinity label for A2a-vers
us A1-receptors in the brain. (C) 1993 Wiley-Liss, Inc.