Although intravenous
chlorpheniramine can cause bronchodilatation, oral and parenteral
antihistamines have not proved useful in treating
asthma. Inhaled
antihistamines may cause throat irritation, but a recent study of the
antihistamine,
clemastine, showed it to be an effective
bronchodilator without
irritant effects. We have extended these studies to determine the site of action of inhaled
clemastine and to assess its potential usefulness both as a
bronchodilator and as a maintenance treatment. Eleven stable asthmatic patients received inhaled
clemastine and placebo and the effect was assessed by serial maximum expiratory flow volume (MEFV) curves breathing air and a
helium/
oxygen (He/O2) mixture. There was no significant improvement in peak flow rates during air breathing after
clemastine and no significant difference between the responses to
drug and placebo. Minor but significant changes were seen in some flow measurements on the downslope of the MEFV curve during air and He/O2 breathing, and these are tentatively ascribed to a dilating effect of
clemastine on peripheral airways where flow is laminar. Subsequent administration of inhaled
isoprenaline showed the patients to be still capable of significant bronchodilatation. The addition of
clemastine, from a pressurised
aerosol, to the patients' therapeutic regimen for two weeks was no more effective than placebo in controlling airflow obstruction, and did not reduce the need for standard
bronchodilators. In our patients
clemastine was not a clinically useful
bronchodilator either acutely or as a maintenance treatment for
asthma.