Drug Interaction Report

GENERALLY AVOID: Coadministration with inhibitors of CYP450 1A2 may increase the plasma concentrations of alosetron, which has been shown in vivo to be predominantly metabolized by the isoenzyme. When a single 1 mg dose of alosetron was administered to 40 healthy female subjects following pretreatment with the potent CYP450 1A2 inhibitor fluvoxamine (in escalating doses from 50 to 200 mg/day for 16 days) increased alosetron systemic exposure (AUC) and half-life by approximately 6-fold and 3-fold, respectively. Fluvoxamine is known to also inhibit CYP450 2C9 and 3A4, both of which have been shown in vitro to be involved in the metabolism of alosetron. However, it is uncertain to what extent inhibition of these isoenzymes actually contribute to the interaction. Concomitant administration of alosetron with less potent CYP450 1A2 inhibitors has not been evaluated.

MANAGEMENT: Because alosetron is associated with potentially serious and life-threatening, dose-related gastrointestinal adverse effects, concomitant use with CYP450 1A2 inhibitors should generally be avoided if possible (use with fluvoxamine is specifically contraindicated). If coadministration is required, it may be appropriate to initially prescribe a lower dosage of alosetron (e.g., 1 mg once a day). However, the product labeling does not offer recommendations for a dosage adjustment. Patients should be advised to immediately discontinue alosetron and notify their physician if they experience constipation or signs and symptoms of ischemic colitis such as rectal bleeding, bloody diarrhea, and new or worsening abdominal pain. Alosetron should not be resumed if ischemic colitis is diagnosed. Ischemic colitis and other serious complications such as obstruction, perforation, impaction, and toxic megacolon have resulted in hospitalization, blood transfusion, surgery, and death.

GENERALLY AVOID: Use of alcohol or products containing alcohol during nitroimidazole therapy may result in a disulfiram-like reaction in some patients. There have been a few case reports involving metronidazole, although data overall are not convincing. The presumed mechanism is inhibition of aldehyde dehydrogenase (ALDH) by metronidazole in a manner similar to disulfiram. Following ingestion of alcohol, inhibition of ALDH results in increased concentrations of acetaldehyde, the accumulation of which can produce an unpleasant physiologic response referred to as the 'disulfiram reaction'. Symptoms include flushing, throbbing in head and neck, throbbing headache, respiratory difficulty, nausea, vomiting, sweating, thirst, chest pain, palpitation, dyspnea, hyperventilation, tachycardia, hypotension, syncope, weakness, vertigo, blurred vision, and confusion. Severe reactions may result in respiratory depression, cardiovascular collapse, arrhythmia, myocardial infarction, acute congestive heart failure, unconsciousness, convulsions, and death. However, some investigators have questioned the disulfiram-like properties of metronidazole. One study found neither elevations in blood acetaldehyde nor objective or subjective signs of a disulfiram-like reaction to ethanol in six subjects treated with metronidazole (200 mg three times a day for 5 days) compared to six subjects who received placebo.

GENERALLY AVOID: The potential exists for pharmacodynamic interactions and/or toxicities between fexinidazole and herbal medicines and supplements. In addition, grapefruit and grapefruit juice may, theoretically, increase the plasma concentrations of fexinidazole and the risk of adverse effects. The mechanism is decreased clearance of fexinidazole due to inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. In general, the effect of grapefruit juice is concentration-, dose- and preparation-dependent, and can vary widely among brands. Certain preparations of grapefruit juice (e.g., high dose, double strength) have sometimes demonstrated potent inhibition of CYP450 3A4, while other preparations (e.g., low dose, single strength) have typically demonstrated moderate inhibition. Pharmacokinetic interactions involving grapefruit juice are also subject to a high degree of interpatient variability, thus the extent to which a given patient may be affected is difficult to predict.

ADJUST DOSING INTERVAL: Food significantly increases the oral absorption and bioavailability of fexinidazole. Compared with the fasted state, the systemic exposure (AUC) of fexinidazole and its metabolites (fexinidazole sulfoxide [M1], fexinidazole sulfone [M2]) were 4- to 5-fold higher following administration with food.

MANAGEMENT: To ensure maximal oral absorption, fexinidazole should be administered with food each day at about the same time of day (e.g., during or immediately after the main meal of the day). Coadministration of fexinidazole with grapefruit, grapefruit juice, or herbal medicines or supplements should be avoided. Because clear evidence is lacking concerning the safety of ethanol use during nitroimidazole therapy, patients should be apprised of the potential for interaction and instructed to avoid alcoholic beverages and products containing alcohol or propylene glycol while using oral, intravenous, or vaginal preparations of a nitroimidazole. Alcoholic beverages should not be consumed for at least 48 hours after completion of fexinidazole therapy.