In a study published in Molecular Therapy, the team led by Marie-Claude Bourgeois-Daigneault, a researcher at the CHUM Research Centre, identified a key mechanism of oncolytic virotherapy, paving the way for therapeutic combinations that can be brought to the clinic more quickly.

This breakthrough holds particular promise for triple-negative breast cancer, which is the most difficult form of the disease to treat. In 2021, the team proved that certain viruses capable of killing cancer cells, known as oncolytic viruses, could serve as adjuvants in vaccines. In 2025, they demonstrated for the first time in mice that a virus that’s genetically modified to strengthen the immune system increases the efficacy of anti-cancer vaccines.

Rethinking the role of hormones

Since 2013, Bourgeois-Daigneault has focused specifically on oncolytic viruses, particularly in the context of breast cancer.

“Over the years, it has become clear that not all breast cancers are sensitive to viruses,” she explained. “When I investigated the subject, I noticed that all hormone-responsive breast cancers—so-called luminal breast cancers—were highly sensitive to my virus, whereas the others were not.”

By analyzing the role of estrogen, she then demonstrated that this hormone promotes viral infection and accelerates the death of cancer cells. Estrogen blocks a natural cellular defense pathway called NF-kB, which normally protects cells against viruses. As a result, the cancer naturally becomes more sensitive to virus treatment.

Toward a new therapeutic approach

This new study also highlights a significant limitation. First-line hormone therapies that block estrogen reduce the virus’s efficacy. This finding has significant implications, as it underscores the need to consider therapeutic incompatibility when administering these viruses to patients.

By combining therapeutic viruses with drugs that block a key cellular pathway, NF-kB, this research paves the way for treatments that are more flexible, more inclusive and better tailored to real-world clinical needs. This breakthrough opens new therapeutic possibilities. Patients could continue their hormone therapy without compromising the efficacy of the virus. Better yet, this strategy could be applied to all types of breast cancer, and even to other solid tumours that are not hormone dependent.

Today, international clinical trials, including in the United States, are already evaluating therapeutic viruses in patients with breast cancer. “In the short term, our new findings help better target patient recruitment, prioritizing those with estrogen receptor-positive breast cancer, in order to optimize the effectiveness of ongoing and future studies,” explained Bourgeois-Daigneault.

Writing: Caroline Devillers