Drug Interaction Report

MONITOR: Theoretically, coadministration of trospium chloride with other drugs that are eliminated by active tubular secretion may result in increased plasma concentrations of trospium and/or the coadministered drug(s). The mechanism is competitive inhibition of renal excretion. Drugs that are thought to undergo active tubular secretion include acyclovir/valacyclovir, cidofovir, cimetidine, digoxin, flecainide, ganciclovir/valganciclovir, metformin, midodrine, morphine, pancuronium, procainamide, quinidine, ranitidine, tenofovir, triamterene, and vancomycin.

MANAGEMENT: Patients receiving trospium chloride in combination with other drugs that undergo active tubular secretion should be monitored for excessive pharmacologic effects of one or both drugs, and the dosages of the drugs adjusted if necessary.

ADJUST DOSING INTERVAL: Food may reduce the oral absorption and bioavailability of trospium chloride. According to the product labeling, administration of trospium chloride with a high fat meal reduced the peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) by 70% to 80% compared to administration while fasting.

MANAGEMENT: To ensure maximal oral absorption, trospium chloride should be administered at least 1 hour before meals or on an empty stomach. If trospium chloride is administered as a combination with xanomeline, the manufacturer recommends administering the capsules at least 1 hour before a meal or at least 2 hours after a meal. Capsules should be taken whole.

Concurrent ingestion of calcium-fortified foods (i.e., cereal, orange juice) may alter the bioavailability of gatifloxacin. The mechanism is chelation of calcium and the quinolone, resulting in decreased bioavailability. In the case of orange juice, inhibition of intestinal transport mechanisms (P-glycoprotein or organic anion-transporting polypeptides) by flavones may also be involved. Data have been conflicting: One study has reported no effect with milk coadministration. Another study reported a modest decrease in gatifloxacin bioavailability (13.5% decrease in Cmax,12% decrease in AUC, 15% increase in total clearance) when taken with 12 ounces of calcium-fortified orange juice instead of water, which could be clinically significant if the infecting organisms have borderline susceptibilities. The manufacturer states that gatifloxacin may be taken without regard to food, milk, or calcium. Clinicians should be aware of the possibility of an interaction if subtherapeutic effects are observed.