Structure-activity requirements for hypotension and alpha-adrenergic receptor blockade by analogues of atropine.

The hypotensive action of various antimuscarinic compounds structurally related to atropine was studied in conscious, unanesthetized rats. The alpha-adrenolytic activity of these agents was assessed both in vivo (blockade of norepinephrine-induced pressor response) and in vitro (displacement of [3H]WB-4101 binding). Benztropine, homatropine and hyoscyamine caused hypotension and produced alpha-adrenergic receptor blockade similar to atropine. Other analogues were either inactive (atroscine, scopolamine, tropic acid and tropine) or evoked nonspecific changes in blood pressure and lacked alpha-adrenolytic activity (benactyzine, eucatropine, methylatropine, methylhomatropine and methylscopolamine). Based on these data, we propose the following structure-activity relationship for hypotension and alpha-adrenolytic activity: (a) the tropine moiety is inactive unless it is attached to another group by an ester linkage, (b) chemical modification of the tropine moiety, including quaternization, decreases potency, (c) the d-stereoisomer appears to be more potent than the corresponding 1-form.