Drug Interactions between fexofenadine and sodium iodide-i-131

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.

GENERALLY AVOID: Coadministration with large amounts of certain fruit juices, including grapefruit, orange and apple, may decrease the oral bioavailability of fexofenadine. The proposed mechanism is inhibition of drug efflux via intestinal organic anion transporting polypeptides (e.g., P-glycoprotein), of which fexofenadine is a substrate. In a five-way crossover study with 10 healthy volunteers, 1/4-strength grapefruit juice, grapefruit juice, orange juice and apple juice (300 mL with drug administration and 150 mL every 1/2 hour for up to 3 hours, total volume 1.2 L) reduced the mean area under the plasma concentration-time curve (AUC) of a 120 mg dose of fexofenadine by 23%, 67%, 72% and 77%, respectively, compared to water. Mean peak plasma concentration (Cmax) was similarly affected. The clinical significance of these changes is unknown. However, results from studies using histamine-induced skin wheals and flares found that the size of wheal and flare was significantly larger when fexofenadine was administered with either grapefruit or orange juices compared to water.

MANAGEMENT: To maximize plasma levels and therapeutic effects, fexofenadine should be taken with water. In addition, patients should refrain from consuming large amounts of grapefruit, orange, or apple juice.

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.