KX-01, a Novel Src Kinase Inhibitor Directed Toward the Peptide Substrate Site, Synergizes With Tamoxifen in Estrogen Receptor α Positive Breast Cancer

Breast Cancer Res Treat. 2012 Apr;132(2):391-409. doi: 10.1007/s10549-011-1513-3. Epub 2011 Apr 21.


KX-01 is the first clinical Src inhibitor of the novel peptidomimetic class that targets the peptide substrate site of Src providing more specificity toward Src kinase. The present study was designed to evaluate the effects of KX-01 as a single agent and in combination with tamoxifen (TAM) on cell growth and apoptosis of ERα positive breast cancer in vitro and in vivo. Flow cytometry demonstrated that KX-01 induced cell cycle arrest in G2/M phase. Immunofluorescent staining for mitotic phase markers and TUNEL staining indicated that cells had arrested in the mitotic phase and mitotic arrested cells were undergoing apoptosis. KX-01 induced nuclear accumulation of cyclin B1, and activation of CDK1, MPM2, and Cdc25C that is required for progression past the G2/M checkpoint. Apoptosis resulted from activation of caspases 6, 7, 8, and 9. Combinational index analysis revealed that combinations of KX-01 with TAM resulted in synergistic growth inhibition of breast cancer cell lines. KX-01 combined with TAM resulted in decreased ERα phosphorylation at Src-regulated phosphorylation sites serines 118 and 167 that were associated with reduced ERα transcriptional activity. Orally administered KX-01 resulted in a dose dependent growth inhibition of MCF-7 tumor xenografts, and in combination with TAM exhibited synergistic growth inhibition. Immunohistochemical analysis revealed that combinational treatment reduced angiogenesis, and ERα signaling in tumors compared to either drug alone that may underlie the synergistic tumor growth inhibition. Combinations of KX-01 with endocrine therapy present a promising new strategy for clinical management of ERα positive breast cancer.

Publication types

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

MeSH terms

  • Angiogenesis Inhibitors / pharmacology
  • Animals
  • Antineoplastic Combined Chemotherapy Protocols / pharmacology*
  • Apoptosis / drug effects
  • Apoptosis Regulatory Proteins / metabolism
  • Breast Neoplasms / blood supply
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / enzymology
  • Breast Neoplasms / genetics
  • Breast Neoplasms / pathology
  • Catalytic Domain
  • Cell Cycle Proteins / metabolism
  • Cell Line, Tumor
  • Cell Proliferation / drug effects
  • Dose-Response Relationship, Drug
  • Drug Synergism
  • Estrogen Receptor Modulators / pharmacology
  • Estrogen Receptor alpha / drug effects*
  • Estrogen Receptor alpha / genetics
  • Estrogen Receptor alpha / metabolism
  • Female
  • Flow Cytometry
  • Fluorescent Antibody Technique
  • G2 Phase Cell Cycle Checkpoints / drug effects
  • Humans
  • In Situ Nick-End Labeling
  • Mice
  • Mice, Nude
  • Mitosis / drug effects
  • Neovascularization, Pathologic / enzymology
  • Neovascularization, Pathologic / prevention & control
  • Phosphorylation
  • Protein Kinase Inhibitors / pharmacology
  • Signal Transduction / drug effects
  • Tamoxifen / pharmacology
  • Time Factors
  • Transfection
  • Xenograft Model Antitumor Assays
  • src-Family Kinases / antagonists & inhibitors*
  • src-Family Kinases / metabolism


  • Angiogenesis Inhibitors
  • Apoptosis Regulatory Proteins
  • Cell Cycle Proteins
  • Estrogen Receptor Modulators
  • Estrogen Receptor alpha
  • Protein Kinase Inhibitors
  • estrogen receptor alpha, human
  • Tamoxifen
  • src-Family Kinases