Two beta-adrenergic pharmacophores on the same molecule. A set of agonist-antagonist combinations

A series of compounds containing combinations of one or two pharmacophores of the agonist type (isoproterenol) or the antagonist type (propranolol or alprenolol) on the same molecule were prepared. The pharmacophores were connected by a derivative of polyethylene glycol with an average length of six atoms (carbon and oxygen). Furthermore, compounds containing two alprenolol residues, separated by chains of average lengths of 70 or 145 atoms, were synthesized. The abilities of these compounds to interact with beta-adrenoceptors of rat heart and lung tissues were examined by measuring the following parameters: competitive binding with [3H]dihydroalprenolol, activation of adenylate cyclase, and inhibition of isoproterenol-stimulated adenylate cyclase. The affinity of the compound with two isoproterenol pharmacophores for receptor was about the same as that with one isoproterenol pharmacophore and between 30 and 200 times weaker than that of (+/-)isoproterenol. Both mono- and bis-pharmacophore compounds partially stimulated catecholamine sensitive adenylate cyclase and at high concentrations inhibited the stimulation produced by (-)isoproterenol. The affinity of the compound with antagonist (propranolol) and agonist (isoproterenol) pharmacophores on the same molecule was intermediate between that of propranolol and isoproterenol. The compound was only able to inhibit adenylate cyclase activity. Compounds containing two antagonist (alprenolol) pharmacophores bound to receptors with affinities from an order of magnitude lower to about equal to that of the compound containing one pharmacophore. When membranes were preincubated with compounds containing two antagonist pharmacophores and then washed extensively, there were persistent effects of all of these compounds on the binding constants of [3H]dihydroalprenolol. All of these compounds were only able to inhibit adenylate cyclase activity and none exhibited any subtype selectivity at beta-adrenoceptors. The results suggest that, in the beta-adrenergic system, compounds with agonist and antagonist substituents on the same molecule exhibit properties of the substituent with the higher affinity for beta-adrenoceptor, and no agonist activity is evident when two antagonist pharmacophores are linked on the same molecule. All of the above results may be explained without recourse to cross-linking of beta-adrenoceptors with two pharmacophores, a phenomenon cited in similar studies of receptors for opiates and gonadotropin-releasing hormone.