Drug Interaction: Cyclophosphamide and Allopurinol

*Interaction detected between allopurinol and cyclophosphamide, MAJOR increased toxicity (increase myelosuppression, nausea and vomiting) as per Micromedex Online
(Micromedex® Healthcare Series [Internet database]. Greenwood Village, Colo: Thomson Reuters (Healthcare) Inc. Updated periodically).

Onset: Delayed

Severity: Major

Documentation: Good

Summary: There may be an increase in cyclophosphamide levels and prolongation in half-life of cyclophosphamide (Anon, 1974; Stolbach et al, 1982).

Literature:


The Boston Collaborative Drug Surveillance Program examined the incidence of bone marrow depression in a series of 160 patients, 95 of whom were controls, during combined allopurinol use with either cyclophosphamide or other cytotoxic drugs (Anon, 1974). Their results indicated that the frequency of bone marrow depression in those patients who received allopurinol and cyclophosphamide was 57.7% as compared to 18.8% in those who did not receive allopurinol. In addition they noted that mortality may be somewhat higher in allopurinol recipient patients.

One study has reported that concomitant allopurinol and cyclophosphamide therapy results in significant elevations of cyclophosphamide cytotoxic metabolites, which may contribute to the increased bone marrow depression observed during concomitant therapy (Witten et al, 1980).

*At our centre and in this individual case, patient was on allopurinol 100 mg daily continuously for management of gout. The patient was to start FEC. After discussing with treating physician, there was insufficient time to temporarily discontinue allopurinol without delaying treatment. Therefore, it was decided to monitor CBCs carefully - that is, weekly (at least for the first cycle). It will then be decided if allopurinol will be held at all next cycle.

One possible recommendation would be to hold allopurinol for 5 days before and 2 days after chemotherapy treatment. This is based on half-life of active metabolite of allopurinol, oxypurinol, being 18-30 hours and cyclophosphamide's 3-12 hours.