THE IN-VITRO PHARMACOLOGY OF ZM-241385, A POTENT, NONXANTHINE, A(2A) SELECTIVE ADENOSINE RECEPTOR ANTAGONIST

1 This paper describes the in vitro pharmacology of ZM 241385 (4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl amino]ethyl) phenol), a novel non-xanthine adenosine receptor antagonist with selectivity for the A(2a) receptor subtype. 2 ZM 241385 had high affinity for A(2a) receptors. In rat phaeochromocytoma cell membranes, ZM 241385 displaced binding of tritiated 5'-N-ethylcarboxamidoadenosine (NECA) with a PIC50 Of 9.52, (95% confidence limits, c.l., 9.02-10.02). In guinea-pig isolated Langendorff hearts, ZM 241385 antagonized vasodilatation of the coronary bed produced by 2-chloroadenosine (2-CADO) and 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) with pA(2) values of 8.57 (c.l., 8.45-8.68) and 9.02 (c.l., 8.79-9.24) respectively. 3 ZM 241385 had low potency at A(2b) receptors and antagonized the relaxant effects of adenosine in the guinea-pig aorta with a pA(2) of 7.06, (c.l., 6.92-7.19). 4 ZM 241385 had a low affinity at Al receptors. In rat cerebral cortex membranes it displaced tritiated R-phenylisopropyladenosine (R-PIA) with a PIC50 Of 5.69 (c.l., 5.57-5.81). ZM 241385 antagonized the bradycardic action of 2-CADO in guinea-pig atria with a pA(2) of 5.95 (c.l., 5.72-6.18). 5 ZM 241385 had low affinity for A(3) receptors. At cloned rat Ag receptors expressed in chinese hamster ovary cells, it displaced iodinated aminobenzyl-5'-N-methylcarboxamido adenosine (AB-MECA) with a pIC(50) of 3.82 (c.l., 3.67-4.06). 6 ZM 241385 had no significant additional pharmacological effects on the isolated tissues used in these studies at concentrations three orders of magnitude greater than those which block A(2a) receptors. At 10 mu M it displayed only minor inhibition of the bradycardic effects in guinea-pig atria to some concentrations of carbachol. At 10 mu M, ZM 241385 had a small inhibitory effect on relaxant effects of isoprenaline in guinea-pig aortae but no effect on sodium nitrite-induced relaxation. ZM 241385 (100 mu M) was without effect on phenylephrine-induced tone in guinea-pig aortae. 7 ZM 241385 (10 mu M) had no inhibitory effect on rat hepatocyte phosphodiesterase types I, II, III and IV but caused a small inhibition of the calcium calmodulin-activated type I enzyme. 8 ZM 241385 is the most selective adenosine A(2a) receptor antagonist yet described and is therefore a useful tool for characterization of responses mediated by A(2) adenosine receptors.