Triple Targeting of Breast Tumors Driven by Hormonal Receptors and HER2

Mol Cancer Ther. 2022 Jan;21(1):48-57. doi: 10.1158/1535-7163.MCT-21-0098. Epub 2021 Nov 2.


Breast cancers that express hormonal receptors (HR) and HER2 display resistance to targeted therapy. Tumor-promotional signaling from the HER2 and estrogen receptor (ER) pathways converges at the cyclin D1 and cyclin-dependent kinases (CDK) 4 and 6 complex, which drives cell-cycle progression and development of therapeutic resistance. Therefore, we hypothesized that co-targeting of ER, HER2, and CDK4/6 may result in improved tumoricidal activity and suppress drug-resistant subclones that arise on therapy. We tested the activity of the triple targeted combination therapy with tucatinib (HER2 small-molecule inhibitor), palbociclib (CKD4/6 inhibitor), and fulvestrant (selective ER degrader) in HR+/HER2+ human breast tumor cell lines and xenograft models. In addition, we evaluated whether triple targeted combination prevents growth of tucatinib or palbociclib-resistant subclones in vitro and in vivo Triple targeted combination significantly reduced HR+/HER2+ tumor cell viability, clonogenic survival, and in vivo growth. Moreover, survival of HR+/HER2+ cells that were resistant to the third drug in the regimen was reduced by the other two drugs in combination. We propose that a targeted triple combination approach will be clinically effective in the treatment of otherwise drug-resistant tumors, inducing robust responses in patients.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Breast Neoplasms / drug therapy*
  • Female
  • Humans
  • Mice
  • Mice, Inbred NOD
  • Protein Kinase Inhibitors / pharmacology
  • Protein Kinase Inhibitors / therapeutic use*
  • Receptors, Estrogen / therapeutic use*


  • Protein Kinase Inhibitors
  • Receptors, Estrogen