Drug Interactions between allopurinol and ifosfamide

ADJUST DOSING INTERVAL: The tolerability of allopurinol may be improved by giving it after a meal. Additionally, when the dose is greater than 300 mg, dividing the total daily dose into smaller doses administered more often may be appropriate to help minimize gastrointestinal irritation.

MONITOR: Concomitant use of allopurinol with central nervous system (CNS) depressants, including alcohol, may potentiate adverse effects such as somnolence and sedation.

MANAGEMENT: To improve tolerability, some manufacturers suggest administering allopurinol after a meal. Additionally, if the daily dose is greater than 300 mg, administering allopurinol in divided doses may help reduce gastrointestinal intolerance. Patients should also be counseled to avoid or limit consumption of alcohol and to avoid activities requiring mental alertness such as driving or operating hazardous machinery until they know how the medication affects them.

GENERALLY AVOID: Grapefruit and/or grapefruit juice may reduce the efficacy of ifosfamide, whose anticancer effect is dependent on its activation to the 4-hydroxyifosfamide metabolite via CYP450 3A4. The proposed mechanism is inhibition of CYP450 3A4 metabolism by certain compounds present in grapefruit. There are no data available about the effects of grapefruit on ifosfamide. However, in a small study, 8 patients with incurable malignancies received ifosfamide 3 g/m2 by infusion with the potent CYP450 3A4 inhibitor ketoconazole 200 mg orally twice daily for 4 days starting 1 day before the ifosfamide infusion. Ketoconazole decreased the clearance of ifosfamide by 11%, decreased systemic exposure (AUC) of the active metabolite 4-hydroxyifosfamide by 30%, and increased the AUC of the inactive but potentially neurotoxic metabolite 2-dechloroethylifosfamide by 23%, as compared to control. Because pharmacokinetic interactions involving grapefruit juice are often subject to a high degree of interpatient variability, the extent to which a given patient may be affected is difficult to predict.

GENERALLY AVOID: Alcohol may potentiate the neurotoxic effects of ifosfamide. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills. In addition, ifosfamide therapy may cause gastrointestinal disorders and alcohol consumption may increase nausea and vomiting.

MANAGEMENT: Given the potential for reduced efficacy of ifosfamide and increased risk of neurotoxicity and nephrotoxicity it may be advisable for patients to avoid consumption of grapefruit, grapefruit juice, or supplements that contain grapefruit during treatment with ifosfamide. In addition, patients receiving ifosfamide should be warned of the increased risk of neurotoxicity, nausea and vomiting when used in combination with alcohol. Patients should avoid or limit the consumption of alcohol during treatment with ifosfamide.