A newly synthesized, chemically reactive adenosine derivative, N 6 -(3-isothiocyanatobenzyl)adenosine-5'-N-methyluronamide, was found to bind selectively to A 3 receptors. K i values for this isothiocyanate derivative in competition binding at rat brain A 1 , A 2a , and A 3 receptors were 145, 272 and 10.0 nM, respectively. A preincubation with this derivative resulted in irreversible inhibition of radioligand binding at rat A 3 receptors in membranes of transfected CHO cells or RBL-2H3 mast cells, but not at rat A 1 or A 2a receptors. The loss of binding sites for 0.1 nM [ 125 I]N 6 -(4-aminobenzyl)adenosine-5'-N-methyluronamide, a high affinity A 3 receptor radioligand, in transfected CHO cell membranes was concentration-dependent with an IC 50 of 50 nM. No change was observed in the K d value of the remaining A 3 receptor sites. The inhibition was also insensitive to theophylline (1 mM), consistent with the pharmacology of rat A 3 receptors. Structurally similar adenosine analogues lacking the chemically reactive isothiocyanate group failed to irreversibly inhibit A 3 -binding.Adenosine agonists such as NECA (5'-N-ethylcarboxamidoadenostne, 1, Figure 1) are known to facilitate the release of inflammatory mediators from mast cells (1,2). The action occurs via activation of phospholipase C and is accompanied by a rise in intracellular calcium concentration. Recently, a molecular explanation was offered for this unique action of adenosine, i.e. mast cells express the novel A 3 adenosine receptor (3), which was discovered through cloning from a rat brain cDNA library (4). Xanthines that act as antagonists at A 1 and A 2 adenosine receptors do not antagonize the response (4,5). For example, the xanthine amine congener, which has nanomolar affinity at rat brain A 1 receptors, binds to rat brain A 3 receptors with a K i value of 29 μM (6) and does not antagonize functional effects (inhibition of adenylate cyclase) of activation of rat A 3 receptors (4).A 3 adenosine receptors have been proposed as a therapuetic target in inflammatory diseases (7-9) and in diseases of the central nervous system (10,11). Adenosine is known to be a bronchoconstrictor In asthmatic patients (8,9), although the mechanism of this effects is currently unclear, since until recently selective probes have not been available. A selective A 3 antagonist might be useful as an anti-inflammatory drug (7). Chronic activation of A 3 receptors has also been proposed in treating cerebral ischemia (11).In this study we have designed a chemically-reactive receptor probe, an adenosine derivative bearing an isothiocyanate group, 4 (Figure 1)
MATERIALS AND METHODS
Synthesis[ 3 H]CGS 21680 was obtained from Dupont NEN (Boston, MA) and [ 125 I]AB-MECA was prepared as described (12). Compound 3 was prepared as described (13).N 6 -(3-Isothiocyanatobenzyl)adenosine-5'-N-methyluronamide (4)-N 6 -(3-Aminobenzyl)adenosine-5'-N-methyluronamide (3, 14.1 mg, 35.3 μmol) was partially dissolved in dry chloroform (6 mL), and saturated sodium bicarbonate solution...