Development of the CK‐MB‐1 trastuzumab‐resistant HER2‐positive breast cancer cell line and xenograft animal models

BACKGROUND: Patients with human epidermal growth factor receptor 2 (HER2)‐positive breast cancer who fail to respond to anti‐HER2 treatments have poor prognoses. Most trastuzumab‐resistant breast cancer cell lines available from biobanks feature either phosphoinositide‐3‐kinase, catalytic, alpha (PIK3CA) mutation or the loss of phosphatase and tensin homolog (PTEN). However, PIK3CA mutations and/or PTEN loss do not account for most trastuzumab‐resistant tumors in humans. METHODS: Breast cancer cells were collected from one patient's malignant ascites. These cells were cultured and maintained to develop a stable cell line, which we named CK‐MB‐1. We used western blotting to evaluate protein expression. The PIK3CA status of CK‐MB‐1 cells was analyzed using Sanger sequencing and validated using next‐generation sequencing. In vivo, CK‐MB‐1 xenograft tumor models were developed in zebrafish and immunodeficient mice. RESULTS: CK‐MB‐1 cells maintained the major characteristics of the parental tumor including HER2 positivity and estrogen receptor negativity. The HER2 gene amplification of CK‐MB‐1 cells was detected by fluorescence in situ hybridization. The integrity of PTEN was confirmed by its positive protein expression and the absence of gene mutations. No common PIK3CA mutation was detected. Compared with the findings in two other HER2‐positive trastuzumab‐resistant cell lines, CK‐MB‐1 cells exhibited greater resistance to trastuzumab, chemotherapeutics, and small‐molecule drugs. Trastuzumab resistance in CK‐MB‐1 cells was confirmed in vivo using the NOD SCID mouse model. CONCLUSIONS: CK‐MB‐1 cells represent a stable HER2‐positive trastuzumab‐resistant breast cancer cell line. The resistance of CK‐MB‐1 cells does not originate from the PTEN or phosphoinositide 3‐kinase signaling pathway, which can provide an alternative approach for potential drugs.