Development and characterization of a novel endoxifen-resistant breast cancer cell line highlights numerous differences from tamoxifen-resistant models.

Despite the availability of effective drugs that target ERa-positive breast cancer, resistance commonly occurs, resulting in relapse, metastasis, and death. Tamoxifen remains the most commonly-prescribed endocrine therapy worldwide, and “tamoxifen resistance” has been extensively studied. However, little consideration has been given to the role of tamoxifen's primary active metabolite, endoxifen, in driving resistance mechanisms. Endoxifen is the most abundant active tamoxifen metabolite detected in patients and functions differently from the parent drug and other primary metabolites, including 4-hydroxy-tamoxifen (4HT). Many studies have shown that patients who extensively metabolize tamoxifen into endoxifen have superior outcomes relative to patients who do not, further supporting a primary role for endoxifen in driving tamoxifen responses. Therefore, “tamoxifen resistance” may be actually better modeled by “endoxifen resistance” for such patients. Here, we report the development of a novel endoxifen-resistant breast cancer cell line and have extensively compared this model to tamoxifen (4HT) and fulvestrant (ICI) resistant models. Endoxifen-resistant cells were found to be phenotypically and molecularly distinct from 4HT-resistant cells and more closely resembled ICI-resistant cells overall. Specifically, endoxifen resistance was associated with ERa and PR loss, estrogen insensitivity, EMT-like features, a unique gene signature, and striking resistance to most FDA-approved second- and third-line therapies. Given these findings, and the importance of endoxifen in the efficacy of tamoxifen therapy, our data indicate that endoxifen-resistant models may be more clinically relevant than existing models and suggest that a better understanding of endoxifen resistance could substantially improve patient care. Overall design: Examination of global gene expression changes in MCF7-derived endoxifen-resistant, 4HT-resistant, and ICI-resistant cell lines. Three replicates are provided for each of four samples: the three resistant lines as well as a long-term vehicle-treated control.