Objective: To report the clinical outcome of infants whose mothers were taking azathioprine while nursing and to quantify the transfer of 6-mercaptopurine (6-MP), its active metabolite, into breast milk.
Case summary: We report on a series of 4 patients treated with azathioprine while lactating. Breast milk samples were analyzed for 6-MP in 2 of the mothers. Several milk samples per patient were analyzed; levels of 6-MP were undetectable by high performance liquid chromatography (limit of detection 5 ng/mL). Therefore, the absolute relative infant dose would have been less than 0.09% of the maternal weight-adjusted dose. No adverse effects were encountered in any of the 4 infants.
Discussion: A large number of women of reproductive age are treated with azathioprine for a range of chronic conditions that require immunosuppression, such as systemic lupus erythematosus or solid organ transplants. Similar to other antimetabolites, the drug has generally been contraindicated for use during breast-feeding because of the theoretical concern for toxicity in the nursing infant. The available literature in this area is sparse and dated. The data presented here confirm published reports of minimal 6-MP excretion into milk, suggesting that significant systemic adverse effects in the infant are unlikely.
Conclusions: Maternal azathioprine use during lactation does not appear to pose a significant immediate clinical risk to the suckling infant. Continued monitoring and long-term assessment of these infants are warranted.