Scientists announced on Wednesday that a single dose of a synthetic molecule has eliminated small breast tumours and significantly shrunk large tumours in mice, stirring hopes for a groundbreaking single-dose treatment for breast cancer.

The molecule, named ErSO-TFPy, developed by researchers at the University of Illinois at Urbana-Champaign in the US, induces regression of tumours in multiple mouse models of estrogen receptor-positive breast cancer with a single dose, the research team reported.

"It is very rare for a compound to shrink tumours in mouse models of breast cancer, let alone completely eradicate those tumours with a single dose," Paul Hergenrother, a professor of chemistry at the UIUC who led the study, said in a media release.

Patients with estrogen-receptor-positive breast cancer, which makes up roughly 70 per cent of breast cancers, typically undergo surgery, which is followed by 5 to 10 years of treatment with adjuvant hormone therapy. The prolonged hormone treatment comes with enhanced risks of blood clots, musculoskeletal pain, sexual dysfunction, and fatigue that impairs the patient's quality of life.

Studies suggest that 20 to 30 per cent of patients on such long-term hormone therapy discontinue treatment because of the side effects. Some 30 to 50 per cent of patients on prolonged hormone therapy progress to advanced breast cancer.

Hergenrother and his colleagues in 2021 developed a synthetic molecule they called ErSO which could kill breast cancer cells but it came with undesirable side effects. Over the next three years, they tweaked the molecular structure of ErSO to design a series of derivatives that promised similar action against breast cancer cells but came with fewer side effects.

In their study, they found a derivative they named ErSO-TFPy effectively killing multiple human estrogen-positive-receptor breast cancer cells in laboratory experiments and shrinking human breast tumours transplanted into mice. A single dose of ErSO-TFPy induced complete regression of small tumours and near-complete regression of large tumours growing in mice.

"An anti-cancer regimen that consists of a single dose or a handful of doses could change the face of breast cancer treatment,” Hergenrother and his colleagues wrote, describing their findings in a paper published on Monday in ACS Central, a research journal published by the American Chemical Society. The data shows that ErSO-TFPy is "a highly effective and well-tolerated single-dose anti-tumour agent", they wrote.

The researchers have cautioned that their efficacy studies are based on a special type of mice that are immune-deficient and that advancing the molecule towards therapy will require additional toxicological and safety studies in rodents and dogs. How the molecule will behave in humans remains unknown for now.

But the researchers said, if such an anti-cancer drug that is effective with a single dose is successfully transplanted into human cancer patients, it "would provide a significant clinical benefit".