Drug Interactions between astemizole and sodium iodide-i-131

GENERALLY AVOID: Some beverages such as tonic water contain varying amounts of quinine. Coadministration of a single 430 mg dose of quinine has been shown to increase plasma concentrations of astemizole and its metabolite, desmethylastemizole. Elevated levels of these agents may cause a prolongation of the electrocardiographic QT interval and potentially fatal ventricular arrhythmias. Although pharmacokinetic data have indicated that the amounts of quinine in beverages (up to 80 mg quinine in 32 oz of tonic water) are not sufficient to produce a significant effect, the potential for an interaction exists if large amounts of tonic water are ingested. Also, grapefruit juice has been shown to inhibit CYP450 enzymes, which may lead to increased serum astemizole concentrations. The risk of life-threatening ventricular arrhythmias may be increased.

MANAGEMENT: Patients should be counseled to limit consumption of quinine-containing beverages and avoid grapefruit juice while they are taking astemizole.

MONITOR: Excessive intake of dietary iodine (e.g., iodised salt) may interfere with adequate uptake of radioiodide by the thyroid .

MANAGEMENT: A diet low in iodine is recommended before initiating therapy with sodium iodide I-131 or I-123.

ADJUST DOSING INTERVAL: Certain drugs may affect the pharmacokinetics, pharmacodynamics, and/or diagnostic results of radioiodides.

MANAGEMENT: Antithyroid agents such as carbimazole or propylthiouracil should generally be withheld for 1 week before administration of sodium iodide I-131 or I-123, and for a few days afterward. Salicylates, steroids, anticoagulants, antihistamines, antiparasitics, penicillins, sulfonamides, tolbutamide, thiopental, and nitroprusside should generally be withheld for 1 week. Amiodarone, lithium, and benzodiazepines should be withheld for at least 4 weeks.