Mercaptopurine use while Breastfeeding
Drugs containing Mercaptopurine: Purinethol, Purixan
Mercaptopurine Levels and Effects while Breastfeeding
Summary of Use during Lactation
Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy. In the treatment of conditions such as ulcerative colitis and Crohn's disease, low doses of mercaptopurine (6-MP) for immunosuppression appear to be acceptable. No active metabolites of mercaptopurine were found in the blood of breastfed infants whose mothers were taking azathioprine and no adverse effects attributable to mercaptopurine or azathioprine have been noted. See the Azathioprine record for details. Mothers with decreased activity of the enzyme that detoxifies mercaptopurine metabolites may transmit higher levels of drug to their infants in breastmilk. It might be desirable to monitor exclusively breastfed infants with a complete blood count with differential, and liver function tests if mercaptopurine is used during lactation, although some authors feel that monitoring is unnecessary.
Mercaptopurine is the active metabolite of azathioprine. It is further metabolized to active metabolites including 6-methylmercaptopurine, thioguanine, 6-thioguanine nucleosides (6-TGNs) and 6-methylmercaptopurine nucleosides (6-MMPN). The enzyme thiopurine methyltransferase (TPMT) is responsible for metabolism of 6-TGNs. Deficiencies in this enzyme can lead to excessive toxicity.
Maternal Levels. Mercaptopurine milk levels were measured in 2 patients receiving azathioprine following renal transplantation. In one, peak milk levels occurred 2 and 8 hours after a 75 mg oral dose and were 3.4 and 4.5 mcg/L respectively. In the other, a peak milk level of 18 mcg/L occurred 2 hours after a 25 mg oral dose. Serum levels were not measured.
Four women receiving an immunomodulator to treat inflammatory bowel disease had metabolite levels measured in milk during the first 6 weeks postpartum. The abstract does not mention the specific drug and dose being taken, but the azathioprine metabolites 6-methylmercaptopurine (6-MMP) and 6-thioguanine nucleosides (6-TGNs) were measured. Although therapeutic levels were found in maternal serum, 6-MMP (<650 mcg/L) and 6-TGNs were undetectable (<123 mcg/L) in milk (time of collection not stated).
Infant Levels. Four infants were breastfed (3 exclusively, 1 rarely received formula) during maternal use of azathioprine orally in dosages of 1.2 to 2.1 mg/kg daily. All of the mothers and infants had the wild type TPMT *1/*1 genotype and all of the mothers had normal enzyme activity. At 3 to 3.5 months of age, all of the infants' had undetectable blood levels of 6-TGNs and 6-MMPN.
Effects in Breastfed Infants
In The Netherlands, 30 infants of mothers taking either azathioprine (n = 28) or mercaptopurine (n = 2) for inflammatory bowel disease during pregnancy and postpartum were followed at 1 to 6 years of age using a 43-item quality of life questionnaire. Of this cohort, 9 infants were breastfed for a mean of 7 months (range 3 to 13 months) No statistically significant differences were found between breastfed and formula-fed infants in any of the 12 domains of the survey.
A prospective cohort study followed pregnant women with inflammatory bowel disease throughout pregnancy and for 12 months postpartum. Women were assigned to one of the following groups: unexposed (no thiopurines or anti-TNF agents); group A (azathioprine or mercaptopurine); group B (infliximab, adalimumab or certolizumab) and group AB (both a thiopurine and an anti-TNF agent). Of 1052 women enrolled in the study, 72% breastfed their infants, although the extent and duration were not stated in the abstract. A total of 264 women were exposed to a thiopurine and 59 were exposed to a thiopurine plus an anti-TNF agent. The use of a thiopurine alone was not associated with any complication in the infants and their growth and development were normal throughout the 12 months. Infants exposed to both a thiopurine and an anti-TNF agent had a 50% increase in the number of infections between 9 and 12 months of age. The relationship of this increase with breastfeeding could not be determined from the available data.
Possible Effects on Lactation
Relevant published information was not found as of the revision date.
Alternate Drugs to Consider
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CAS Registry Number
- Antineoplastic Agents
LactMed Record Number
Information from the National Library of Medicine's LactMed Database.
Last Revision Date
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