Distinctive features of rilmenidine possibly related to its selectivity for imidazoline receptors.

Rilmenidine is an oxazoline derivative with antihypertensive activity which was developed to enhance the dissociation between the hypotensive and adverse effect profile of centrally acting agents. Experimental studies have indicated that rilmenidine is selective for both alpha 2-adrenoceptors (v alpha 1) and newly discovered nonadrenergic imidazoline receptors in the brain and in the periphery. In experimental studies, rilmenidine differs from clonidine in that it is more selective for imidazoline receptors than for alpha 2-adrenoceptors; at equihypotensive doses, rilmenidine causes less bradycardia and reduction in cardiac output, less sedation, and little or no antinociceptive action compared to clonidine. The hypotensive effects of rilmenidine are antagonised by idazoxan and yohimbine, but idazoxan (imidazoline structure) is six times more potent than yohimbine (a selective alpha 2-antagonist). In isolated renal proximal tubule cells, where imidazoline binding has also been shown, rilmenidine inhibits reabsorption of sodium. Clinical studies comparing 1 mg rilmenidine with placebo demonstrated significant reductions in blood pressure (BP) (61% rilmenidine v 23% placebo normalized to 160/90 mm Hg). The reduction in BP was not associated with classical alpha 2 side effects such as dry mouth or daytime drowsiness. Compared with clonidine (0.15 to 0.3 mg), equihypotensive doses of rilmenidine (1 to 2 mg) induced two to three times less dry mouth, daytime drowsiness, and constipation; no orthostatic hypotension was reported. Methyldopa (0.5 to 1 mg) v rilmenidine (1 to 2 mg) indicated a comparable reduction of BP with significantly less weakness, drowsiness, orthostatic dizziness, and dry mouth on rilmenidine; there was no evidence of the "clonidine withdrawal syndrome" on drug withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)