Studies on the potency of various antitussive agents

Several antitussive agents were assessed for their cough-suppressant activity. Cough responses were obtained by electrically stimulating the lower brainstem, in cats lightly anesthetized with sodium pentobarbital or in unanesthetized midcollicular decerebrate preparations. Cough sounds were recorded with the aid of a microphone. The cough reactive region was concentrated in an area dorsomedial to the trigeminal tract and nucleus. The potency of these antitussive agents (dextromethorphan, RO 21-4790-001, codeine, clonazepam, diazepam and caramiphen) were determined by studying their effect on the centrally induced cough responses. Each of these agents was administered in graded doses intravenously to determine the minimal effective doses for suppressing the cough responses. They are 0.57, 2.55, 1.71, 0.048, 0.28 and 3.18 mg/kg for the above listed drugs. The results indicate that clonazepam was found to be the most potent antitussive among these agents, the mean effective dose being about 1/35 of that of codeine. The antitussive potency of benzodiazepines is not well correlated with their muscle relaxant activity. For instance, clonazepam and diazepam have the same potency in depressing polysynaptic spinal reflexes, whereas the former is six times more potent than diazepam as an antitussive. This finding indicates that clonazepam has a high specificity as an antitussive.