Breast Cancer: Hope in Sight for Improved Tamoxifen Therapy?

Vincent Richeux

August 24, 2023

LYON, France — A team at Lyon's Cancer Research Center (CRCL) has revealed the role of an enzyme, PRMT5, in the response to tamoxifen, a drug used to prevent relapse in premenopausal women with breast cancer. The protein could become the first predictive marker of response to tamoxifen.

Muriel Le Romancer, MD, director of research at France's Institute of Health and Medical Research, explained the issues involved in this discovery to Medscape French Edition. She jointly led this research along with Olivier Trédan, MD, PhD, oncologist at Lyon's Léon Bérard Clinic. The research concluded with the publication of a study in EMBO Molecular Medicine. The researchers both head up the CRCL's hormone resistance, methylation, and breast cancer team.

Although the enzyme's involvement in the mode of action of tamoxifen has been observed in close to 900 patients with breast cancer, these results need to be validated in other at-risk patient cohorts before the biomarker can be considered for routine use, said Le Romancer. She estimated that 2 more years of research are needed.

Medscape French Edition: Can you tell us which cases involve the use of tamoxifen and what its mode of action is?

Le Romancer: Tamoxifen is a hormone therapy used to reduce the risk of breast cancer relapse. It is prescribed to premenopausal women with hormone-sensitive cancer, which equates to roughly 25% of women with breast cancer: 15,000 women each year. The drug, which is taken every day via oral administration, is an estrogen antagonist. By binding to these receptors, it blocks estrogen from mediating its biological effect in the breasts. Aromatase inhibitors are the preferred choice in postmenopausal women, as they have been shown to be more effective. These also have an antiestrogenic effect, but by inhibiting estrogen production.

Tamoxifen therapy is prescribed for a minimum period of 5 years. Despite this, 25% of women treated with tamoxifen relapse. Tamoxifen resistance is unique in that it occurs very late on, generally 10 to 15 years after starting treatment. This means that it's really important for us to identify predictive markers of the response to hormone therapy to adapt treatment as best we can. For the moment, the only criteria used to prescribe tamoxifen are patient age and the presence of estrogen receptors within the tumor.

Medscape French Edition: Exactly how would treatment be improved if a decisive predictive marker of response to tamoxifen could be identified?

Le Romancer: Currently, when a patient's breast cancer relapses after several years of treatment with tamoxifen, we don't know if the relapse is linked to tamoxifen resistance or not. This makes it difficult to choose the right treatment to manage such relapses, which remain complicated to treat. Lots of patients die due to metastases.

By predicting the response to tamoxifen using a marker, we will be able to either use another hormone therapy to prevent the relapse or prescribe tamoxifen alongside a molecule that stops resistance from developing. We hope that this will significantly reduce the rate of relapse.

Medscape French Edition: You put forward PRMT5 as a potential predictive marker of response to tamoxifen. What makes you think it could be used in this way?

Le Romancer: Our research has allowed us to demonstrate that PRMT5, when present in the nuclei of tumor cells, is involved in the mechanisms of action of tamoxifen. Remember that estrogen receptors are located in cell nuclei. For tamoxifen to exert its antitumoral action, PRMT5, an enzyme, needs to enter the nucleus to modify the estrogen receptor. It's this modification that allows tamoxifen to inhibit tumor growth. The proliferative effect induced by the estrogens is also blocked.

The results of our study showed that high nuclear expression of PRMT5, specifically in the nuclei of breast cancer cells, is associated with a prolonged survival of tamoxifen-treated patients. Until now, we thought this enzyme had an oncogenic role when present in the cytoplasm. It turns out that it also has the opposite effect when acting within the nucleus, at least in this patient cohort: women with hormone-sensitive breast cancer treated with tamoxifen.

Medscape French Edition: What are the next steps in your research before we can begin to think about its use in clinical practice?

Le Romancer: Our next research will focus on understanding the circumstances surrounding PRMT5 entering and leaving the nucleus. We have also shown that in some patients, tamoxifen causes PRMT5 to enter the cell nucleus. This translocation is only seen in women who respond to tamoxifen, not in those who are resistant to treatment with the drug. All that remains is for us to work out how tamoxifen facilitates this translocation.

Once the elements promoting this translocation have been identified, we will be able to propose a treatment aimed at forcing the enzyme to enter the nucleus and stay there. Eventually, the idea is to combine treatment with antiestrogens with a medicinal product that promotes localization of PRMT5 in the nucleus to guarantee response to tamoxifen. It will be a few years of research before we can apply our findings to clinical practice.

Medscape French Edition: Could we use this biomarker as is just to identify tamoxifen resistance?

Le Romancer: In the short term, yes, we could use this biomarker to better guide treatment choices at time of diagnosis. We have demonstrated the role of PRMT5 in response to tamoxifen by studying two cohorts of 900 patients with breast cancer receiving treatment at the Léon Bérard Center, Lyon. Before moving on to routine testing, we need to replicate these results in other cohorts, especially in high-risk patients with, for example, greater cell proliferation or those who experience relapse.

The use of this biomarker is based on histological examination of cancer tissue. Single antibody tissue staining targeting PRMT5 reveals the localization of the enzyme in the cells and provides a score evaluating its presence in the nucleus. Using this score, it would be possible to determine the level of response to tamoxifen and decide whether the treatment should be used. This biomarker is the first of its kind undergoing validation as part of the examination of resistance to hormone therapy. We should be able to confirm the findings within the next 2 years.

Medscape French Edition: If clinical tests using this biomarker predict tamoxifen resistance, what alternative treatments are available to these patients?

Le Romancer: We could give them an aromatase inhibitor or one of the new estrogen antagonists that are currently in development. In a phase 3 study, fulvestrant (Faslodex), for example, demonstrated a significant benefit in treating women with hormone-sensitive advanced breast cancer when administered via injection. The same goes for oral treatment, elacestrant (Orserdu), which has recently been approved by the FDA. These treatments are usually deemed second line after tamoxifen, but they could certainly be used as first-line therapy in resistant patients.

Medscape French Edition: The results obtained from research into novel estrogen antagonists are certainly encouraging. Can tamoxifen retain its prominent position while still ensuring its efficacy?

Le Romancer: Keeping in mind the current trend for personalized medicine, we should keep as many treatment options open as possible. When a patient relapses, there need to be other treatments available to them. Tamoxifen has been ousted in favor of aromatase inhibitors for postmenopausal women, but it's still the gold standard for premenopausal women and has been for over 20 years. Despite having been replaced by a novel estrogen antagonist, it will still have a prominent place in the therapeutic arsenal of premenopausal women with breast cancer.

With the development of PRMT5 as a predictive biomarker, we could even see tamoxifen being proposed as first-line therapy for postmenopausal women in whom high levels of PRMT5 are found in the nuclei of their cancer cells. By predicting their response, we could achieve greater efficacy of tamoxifen, compared with aromatase inhibitors. For now, this remains a hypothesis and must be verified in further clinical studies.

This article was translated from the Medscape French Edition.


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