The Editorial Executive Committee welcomes letters, which should be less than 250 words. Before a decision to publish is made, letters which refer to a published article may be sent to the author for a response. Any letter may be sent to an expert for comment. When letters are published, they are usually accompanied in the same issue by their responses or comments. The Committee screens out discourteous, inaccurate or libellous statements. The letters are sub-edited before publication. Authors are required to declare any conflicts of interest. The Committee's decision on publication is final.
Letter to the editor
Editor, I wish to comment on the article 'Erythromycin and terfenadine should not be used together' (Aust Prescr 1995;18:18) by Associate Professor R. Moulds. It is well known and documented that an interaction can occur between erythromycin and terfenadine. However, more than 99% of terfenadine is converted to its carboxylic acid metabolite and other active metabolites via the cytochrome P450 system on first pass through the liver. However, when erythromycin is administered, there is competition for the isoenzyme resulting in raised levels of the parent compound terfenadine.
There is a relationship between the observed increase in QT interval and the circulating level of terfenadine, but little or no relationship with levels of the acid metabolite. The impression given by Professor Moulds' article is that the metabolite is potentially cardiotoxic. All data currently available point to the metabolite being devoid of cardiotoxic effects. In individuals who are using Teldane at the recommended dose, the parent compound is essentially undetectable in plasma, due to the extensive first pass metabolism. The metabolite is responsible for the antihistamine activity. According to the article, the metabolite has been implicated in causing arrhythmias and in the development of torsades de pointes, which is incorrect.
In the interest of concerned practitioners, I wish to have this error corrected.
Associate Medical Director
Marion Merrell Dow Australia Pty Ltd
Frenchs Forest, N.S.W.