Exposure to thiopurine drugs through breast milk is low based on metabolite concentrations in mother-infant pairs

Br J Clin Pharmacol. 2006 Oct;62(4):453-6. doi: 10.1111/j.1365-2125.2006.02639.x.


Aims: To determine infant exposure to 6-thioguanine and 6-methylmercaptopurine nucleotides (6-TGN and 6-MMPN, respectively) during maternal use of azathioprine in breastfeeding.

Methods: Mother-infant pairs provided blood for determination of 6-TGN and 6-MMPN concentrations, and TPMT genotype.

Results: Four women taking azathioprine 1.2-2.1 mg kg(-1) day(-1) and their infants were studied. All had the wild-type TPMT genotype. Maternal 6-TGN and 6-MMPN concentrations ranged from 234 to 291 and 284 to 1178 pmol per 8 x 10(8) red blood cells, respectively, and were consistent with those associated with improved therapeutic outcomes. Neither 6-TGN nor 6-MMPN was detected in any of the infants, despite a sensitive assay.

Conclusions: The data suggest that azathioprine may be 'safe' during breastfeeding in patients with the wild-type TPMT genotype ( approximately 90% of caucasian patients) taking 'normal' doses.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Azathioprine / adverse effects*
  • Azathioprine / metabolism
  • Breast Feeding / adverse effects*
  • Female
  • Humans
  • Immunosuppressive Agents / adverse effects
  • Immunosuppressive Agents / therapeutic use*
  • Infant
  • Male
  • Mercaptopurine / analogs & derivatives
  • Mercaptopurine / blood
  • Methyltransferases / genetics*
  • Mothers*
  • Thioguanine / blood


  • Immunosuppressive Agents
  • 6-methylthiopurine
  • Mercaptopurine
  • Methyltransferases
  • Thioguanine
  • Azathioprine