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.
What are the next steps in your research before we can begin to think about its use in clinical practice?
Dr. 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.
Could we use this biomarker as is just to identify tamoxifen resistance?
Dr. 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.
If clinical tests using this biomarker predict tamoxifen resistance, what alternative treatments are available to these patients?
Dr. 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 Food and Drug Administration. These treatments are usually deemed second line after tamoxifen, but they could certainly be used as first-line therapy in resistant patients.
The results obtained from research into novel estrogen antagonists are certainly encouraging. Can tamoxifen retain its prominent position while still ensuring its efficacy?
Dr. 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. A version appeared on Medscape.com.