Generation, characterization, and maintenance of trastuzumab-resistant HER2+ breast cancer cell lines

Trastuzumab became the therapy of choice for patients with HER2-positive breast cancer in 1998, and it has provided clinical benefit ever since. However, a significant percentage of patients show primary resistance to trastuzumab at diagnosis, and most patients with metastatic disease that initially respond to trastuzumab eventually progress (acquired resistance). Consequently, there is an urgent need to improve our knowledge of the mechanisms governing resistance, so that specific therapeutic strategies can be developed to provide improved efficacy. We generated new cell lines derived from BCCL through extended exposure to trastuzumab. Drug-conditioned populations were authenticated for their molecular profile and their resistance rate was determined. Heterogeneous HER2 amplification was observed across most of the BCCLs, ranging from cells without HER2 amplification to elevated HER2 gene copy numbers in others. Using a phospho-antibody array we analyzed the status of kinase receptors and effectors from different cellular pathways. This revealed that HER2, AKT, and S6RP presented high phosphorylation levels with specific variations between sensitive and resistant populations. In addition, differences in phosphorylation levels for several of those pathways targets were found between sensitive and resistant lines. Furthermore, a biochemical study characterized patterns of molecular alterations similar to those commonly described in breast cancer. Finally, a subcutaneous xenograft murine model confirmed the resistance to trastuzumab of the established cell line. We conclude that these resistant BCCLs can be a valuable tool to gain insight into the mechanisms of acquisition of trastuzumab resistance.