Background: Thiopurines are widely used to maintain remission in inflammatory bowel disease. Treatment during pregnancy is generally recommended to improve the chance of a normal birth outcome, but advice concerning breastfeeding is conflicting. Aim To estimate the exposure of breastfed infants to 6-mercaptopurine, as a metabolite of azathioprine, from maternal milk.
Methods: Eight lactating women with inflammatory bowel disease receiving maintenance therapy with azathioprine 75-200 mg daily were studied. Milk and plasma samples were obtained 30 and 60 min after drug administration and hourly for the following 5 h.
Results: The variation in the bioavailability of the drug was reflected in a wide range of peak plasma values of 6-mercaptopurine within the first 3 h. A similar curve, but with an hour's delay and at significantly lower concentrations varying from 2-50 microg/L, was seen in maternal milk. After 6 h an average of 10% of the peak values were measured.
Conclusions: The major part of 6-mercaptopurine in breast milk is excreted within the first 4 h after drug intake. On the basis of maximum concentration measured, the infant ingests mercaptopurine of <0.008 mg/kg bodyweight/24 h. The findings confirm that breastfeeding during treatment with azathioprine seems safe and should be recommended, considering the extensive beneficial effects.