Novel Endoxifen-Resistant Breast Cancer Cell Lines Highlight Numerous Differences from Tamoxifen-Resistant Models


Tamoxifen is the oldest and most frequently used drug used for breast cancer with ERα+ (Estrogen Receptor) subtype. The most challenging issue with tamoxifen use is the development of resistance in an initially responsive breast tumor. Studying the molecular mechanisms of tamoxifen resistance will provide insight into the design of regimens to overcome tamoxifen resistance and discovery of novel therapeutic agents with a decreased chance of developing resistance as well as establishing more efficient treatment strategies.

Endoxifen is the ultimate metabolite formed by CYP2D6-catalyzed phase I metabolism of tamoxifen. Interestingly, the serum concentrations of endoxifen showed large variations among patients receiving tamoxifen therapy, implying that individual differences in metabolic profiles of tamoxifen exist and may affect the therapeutic outcomes.  Indeed, it is now believed that “tamoxifen resistance” could be better characterized by “endoxifen resistance” because it predicts better outcomes in patients.

Tamoxifen is activated by an enzyme termed as cytochrome P450. The endoxifen is generated from tamoxifen by the help of CYP2D6 enzyme. The concentrations of endoxifen in blood varies because the gene for CYP2D6 enzyme is polymorphic and could produce up to 100 different alleles producing different concentrations. The increased concentration is related with improved result and reduced concentrations are due to decreased response of drug. The molecular system of endoxifen action vary exceptionally in contrast to tamoxifen and its other metabolites like 4HT (4-Hydroxytamoxifen). However, the examined mechanism of endoxifen activity is unidentified yet.

To answer this question, Mayo clinic at Rochester scientists: Dr. Calley Jones, Professor Malayannan Subramaniam, Professor Michael Emch, Elizabeth Bruinsma, Professor James Ingle, Professor Matthew Goetz, and Professor John Hawse developed novel breast cancer cell lines that uniquely exhibit endoxifen resistance. They compared these cell lines with the “fulvestrant-resistant model” and “4HT resistant model”. Their results indicated that the new models for endoxifen resistance might be more relevant clinically than the current experimental models which would help advance patients’ care. The original research article was published in Molecular Cancer Research journal.

The research team found that the generated resistant cells contained discrete morphological changes in contrast to control cells. the 4HT resistant cells cultivated packed tightly in clusters and of smaller sizes and the ICI and endoxifen resistant cells were not packed and were of larger sizes. Without drug treatment the 4HT resistant cells produced rapidly but ICI and endoxifen ones revealed tardy proliferation. The drug resistance was shown completely in case of all three drugs against ICI and endoxifen resistant cells but in 4HT resistant cells the ICI drug was totally sensitive and for endoxifen and 4HT drugs significantly reduced antiproliferative effect was observed.

The cell migration test was also carried out to confirm resistance which revealed that all the three drugs represented the blockage of migration of MCF7 cells as control and there was no reservation of the resistant cell lines’ migration. Further in this research, the resistant cell lines’ sensitivity was found out for eight different drugs. The ICI and endoxifen resistant cell lines represented less responsiveness towards the drugs as compared to the 4HT resistant and control cell lines. However, the drug named venetoclax showed equal effectiveness in all the models. Conversely, the 4HT resistant cells represented more responsiveness towards ipatasertib and alpelisib as compared to control cells.

All these results characterize the clear differences between 4HT and endoxifen resistant cells because the latter exhibited least sensitivity towards these drugs. Moreover, the impacts of drugs extraction from the resistant cells subsequently 3 months of treatments were also analyzed. The tendencies of genes expression were unaffected and the cells with drugs withdrawn revealed different expression of genes in all the resistant cell lines.

The new study has characterized the sensitivity of endoxifen-resistant cell lines towards different drugs. Even though these drugs are effective for ER breast cancer, the differences in sensitivity are still not understood. Future studies to understand tamoxifen resistance using these new endoxifen-resistant cell lines will expand our knowledge on devising new therapy regimens and benefit the breast cancer patients.


Calley J., Subramaniam M., Emch M., Bruinsma E., Ingle J., Goetz M., Hawse J. Development and Characterization of Novel Endoxifen-Resistant Breast Cancer Cell Lines Highlight Numerous Differences from Tamoxifen-Resistant Models Mol Cancer Res. 2021;19(6):1026-1039

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