First-in-human pharmacokinetics of tamoxifen and its metabolites in the milk of a lactating mother: A case study

Fedro Alessandro Peccatori, Giovanni Codacci-Pisanelli, Gunnar Mellgren, Barbara Buonomo, Eleonora Baldassarre, Ernst Asbjorn Lien, Ersilia Bifulco, Steinar Hustad, Emil Zachariassen, Harriet Johansson, Thomas Helland

Research output: Contribution to journalArticlepeer-review


Background Breast cancer represents the most frequent neoplasm diagnosed in women of childbearing age. When the tumour is oestrogen receptor-positive, tamoxifen is among the recommended endocrine treatments. Lactating women are advised not to breastfeed while receiving tamoxifen. However, information about tamoxifen transfer into breast milk is lacking. Methods We measured the concentration of tamoxifen and its metabolites by liquid chromatography-tandem mass spectrometry in the milk of a nursing mother that was treated for pregnancy-associated breast cancer diagnosed a few months after delivery. She was advised not to breastfeed her child and she collected milk samples for 23 days while the baby was fed with formula. Results Tamoxifen concentrations in milk increased reaching a maximum of 214 nM. The two active metabolitesZ-4-hydroxy-tamoxifen and Z-endoxifen, could not be quantified in milk the first days after tamoxifen intake, but increased over time and reached clinically significant levels after day 18. Conclusion This study demonstrates for the first time in human that tamoxifen and its metabolites transfer into milk. Since tamoxifen has a complete oral bioavailability, a long half-life (>7 days) and may interfere with the normal development of the infant, mothers should not breastfeed during tamoxifen treatment.

Original languageEnglish
Article numbere000859
JournalESMO Open
Issue number5
Publication statusPublished - Oct 28 2020


  • breast cancer
  • breastfeeding
  • human milk
  • lactation
  • tamoxifen

ASJC Scopus subject areas

  • Oncology
  • Cancer Research


Dive into the research topics of 'First-in-human pharmacokinetics of tamoxifen and its metabolites in the milk of a lactating mother: A case study'. Together they form a unique fingerprint.

Cite this