Anti-Allergy and Anti-Asthma Drugs

Six classes of drug may be prescribed in the treatment of airway hyperreactivity and allergy. Use of the methylxanthine theophylline requires that plasma drug concentrations be monitored because of its pharmacokinetic properties and narrow therapeutic range. β2-Selective adrenergic agonists, glucocorticoids, sodium cromoglycate and the quaternary antimuscarinic ipratopium achieve specificity of drug action on the bronchi with minimal side effects by local delivery as aerosols or ‘microfine’ powders. Glucocorticoids, sodium cromoglycate and ipratropium bromide may also be applied locally to the nasal mucosa in allergic rhinitis, and sodium cromoglycate (cromolyn sodium) may be applied to the eye. The ‘antihistamines’ — H1-receptor antagonists — are not used to treat bronchial hyperreactivity but are frequently used systemically for treating allergic conditions. Two new agents, terfenadine and astemizole, appear to be specific for H1-receptors and represent a new ‘generation’ of antihistamines that produce sedation only infrequently. Terfenadine exhibits a bimodal elimination phase (slow component ± 22 hours) and astemizole has an active metabolite with a half-life of 12 days. The half-lives of most other antihistamines lie in the 4- to 8-hour range (except chlorpheniramine, which has a longer half-life). However, data reflecting disposition of these drugs in children are scanty. There is a need for more powerfully predictive pharmacokinetic approaches, which is discouraged by the widely used modelling approach combining patient and drug characteristics in single variables. Separation of these could improve extrapolation from drugs for which data are available to those for which they are not.