In vitro and in vivo Effects of Combination of Trastuzumab (Herceptin) and Tamoxifen in Breast Cancer

Extensive interactions between estrogen receptor α (ERα) and HER2 signaling pathways have been described. Using BT-474 human breast cancer cells, we have previously shown that the combination of tamoxifen (TAM) and Herceptin results in strong synergistic growth inhibition, enhancement of G0–G1 cell cycle accumulation, inhibition of HER2 activity and a cytostatic effect without cell death. To further examine the underlying mechanism of synergy, we investigated the effect of this drug combination on ERα function and growth factor downstream signaling. TAM caused a small increase in ERα levels while Herceptin had no effect, and both drugs caused an increase in the level of Ser118-phosphorylated ERα. However, both TAM and Herceptin individually inhibited ERα transcriptional activity, although the combination did not have a greater effect than either single agent. Herceptin inhibited MAPK and Akt activity, while TAM had no effect on these either as a single agent or when added to Herceptin. Using a BALB/c athymic BT-474 in vivo xenograft model, the drug combination (Herceptin 0.3 mg/kg i.p. twice weekly, TAM 1.0 mg/mouse i.p. three times per week) showed a greater inhibition of tumor growth compared to either single agent. Tumor extracts and fixed sections were examined at the end of the treatment period for treatment-specific alterations: we noted a paradoxical proliferation-inducing effect of TAM that was reversed by the addition of Herceptin. Our results indicate that combined targeting of both peptide growth factor receptors and ERα represents a promising breast cancer treatment strategy.