Family Nurse Practitioner Exam

Which medication is unlikely to interact with theophylline?

1. Erythromycin

2. Montelukast (Singulair)

3. Phenytoin sodium (Dilantin)

4. Cimetidine (Tagamet)

The correct answer is: Montelukast (Singulair)

Montelukast, marketed as Singulair, is a leukotriene receptor antagonist used primarily for the management of asthma and allergic rhinitis. Unlike the other medications listed, montelukast does not interact significantly with theophylline metabolism. Theophylline's clearance primarily takes place in the liver, where it is metabolized by cytochrome P450 enzymes, particularly CYP1A2. In contrast, erythromycin is a strong inhibitor of CYP3A4, which can significantly raise theophylline levels and lead to toxicity. Phenytoin, while primarily an inducer of hepatic enzymes, can influence the metabolism of theophylline by increasing its clearance in some circumstances, leading to subtherapeutic levels. Cimetidine, a known histamine H2-receptor antagonist, also inhibits various cytochrome P450 isoenzymes, including CYP1A2, which can elevate theophylline serum concentrations. Thus, montelukast operates through a different mechanism, and its metabolism does not interfere with theophylline's pathways, making it the least likely to interact with theophylline among the choices provided.