The Food and Drug Administration (FDA) has issued a safety alert regarding the use of azithromycin as a prophylactic immunomodulatory agent in patients who undergo hematopoietic stem cell transplantation (HSCT). A clinical trial investigating the long-term use of this macrolide antibiotic to prevent bronchiolitis obliterans syndrome (BOS) in patients who underwent allogeneic HSCT for hematological malignancy found that azithromycin was associated with an increased risk of cancer relapse compared with placebo.

Pfizer has terminated the ALLOZITHRO trial (N=480) due to this increased risk; researchers could not determine why the rates of cancer relapse and death were higher with azithromycin in the study. Cancer relapse was observed in 32.9% of the azithromycin-treated patients compared to 20.8% of those who received placebo. In addition, a larger number of patients died in the azithromycin group vs the placebo group (95 vs 66 patients); the 2-year survival rate was 56.6% in azithromycin-treated patients and 70.1% in placebo patients.

In a Dear Healthcare Provider letter, Pfizer stated that “long-term azithromycin exposure following HSCT may include risks that exceed the anticipated benefits.” Azithromycin is not approved by the FDA to prevent BOS and healthcare professionals are urged not to prescribe long-term azithromycin for prophylaxis of BOS to patients who undergo donor stem cell transplants. Currently, there are no known effective antibiotic treatments for BOS prophylaxis.

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This article originally appeared on MPR