In a series of 7,8-dihydroxy-1-phenyltetrahydro-3-benzazepine dopamine receptor agonists introduction of a chloro or fluoro substituent into the 6-position increases dopaminergic potency. Also, in this series replacement of the 7-hydroxyl group with a halogen results in inversion of activity from dopamine receptor agonist to antagonist. The present study was aimed at exploring the possibility that the structure-activity observations in the 3-benzazepine series of dopaminergic agents might be extrapolated to another class of dopamine receptor agonists, the 2-aminotetralins. Thus, a series of chloro- and fluoro-substituted mono- and dihydroxylated 2-aminotetralins was prepared and evaluated for dopaminergic properties in D-1 and D-2 receptor-related tests. Introduction of a chloro substituent into the 8-position of the prototype of this series, i.e. 2-amino-6,7-dihydroxytetralin (ADTN), resulted in a compound with a high degree of selectivity for the D-1 subpopulation of dopamine receptors; it was equally or more potent than ADTN in the D-1 receptor-related tests with greatly decreased effectiveness in the tests involving D-2 receptors. A similar effect was observed with 8-fluoro-ADTN; however, the N-(4-hydroxyphenethyl)-N-propyl derivative 4g of the 8-chloro-substituted ADTN showed marked D-2 binding affinity. Conversely, introduction of a chloro substituent into the 5-position of ADTN markedly decreased D-1 receptor affinity and efficacy. This effect was not seen with the related 5-fluoro derivative, suggesting D-1 receptors are more sensitive to bulk in the 5-position of ADTN than are the D-2 receptors. Replacement of either the 6- or 7-hydroxyl groups of ADTN with a chloro or fluoro substituent, in contrast, did not parallel the response seen in the benzazepine series (i.e., the compounds uniformly demonstrated less receptor affinity and did not have dopamine receptor antagonist activity); however, the decrease in agonist potency was less marked in the case of 2-amino-6-fluoro-7-hydroxytetralins than in the chlorinated monohydroxyaminotetralins. Thus, a parallelism in structure-activity relationships in the benzazepine and aminotetralin series of dopamine receptor agonists was not observed. The differences may reflect altered modes of receptor binding in the two series.
Replacement of the catecholic hydroxyl groups of the beta-adrenergic receptor agonist 6,7-dihydroxy-1-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline (trimetoquinol) with chloro substituents results in a compound with marked beta-adrenoceptor antagonist properties. This, therefore, parallels the similar transformation of the beta-adrenoreceptor agonist isoproterenol into the antagonist dichloroisoproterenol. In a test for inhibition of isoproterenol-induced enhancement of the rate of contraction of spontaneously beating guinea pig atrial pairs the resultant 6,7-dichloro-1-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline (6b) had a KB value of (6.7 +/- 2.3) X 10(-8) M. Although this is nearly 2 orders of magnitude less potent than propranolol (KB = 6.2 X 10(-10) M in this test), this compound represents the prototype of a new class of beta-adrenergic receptor blockers, and unlike dichloroisoproterenol it is not a partial agonist. It has physicochemical properties, e.g., pKa and distribution and partition coefficients, that differ from the prototypic beta-blockers. These altered properties might impart advantageous tissue distribution and altered pharmacological properties to the new molecule. This new beta-adrenoreceptor antagonist is suggested to merit further study.
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