Adding a novel drug to hormone therapy can help block the return of the most common form of breast cancer by 25 per cent, according to a study.

Researchers found that patients with early hormone-receptor (HR) positive/HER2 negative breast cancer who took the combination therapy -- ribociclib, a targeted therapy drug, and hormone therapy -- had substantially longer invasive disease-free survival compared to those who were treated with the hormone therapy alone.

This was regardless of whether the cancer has spread to the lymph nodes, said the team from the University of California, Los Angeles.

The addition of the targeted therapy reduced the risk of recurrence by 25 per cent, they noted while presenting the study at the American Society of Clinical Oncology Annual Meeting.

"The findings show this combination is a treatment of choice for patients with stage 2 or stage 3 HR-positive/HER2 negative breast cancer," said Dr Dennis Slamon, chair of haematology-oncology and director of clinical and translational research at the UCLA Jonsson Comprehensive Cancer Center.

Positive/HER2-negative breast cancer is the most common subtype of the disease. The clinical trial involved 5,101 patients with stage 2 or stage 3 early HR-positive/HER2-negative breast cancer.

Patients were randomised into two arms: 2,549 were randomised to the combination treatment and 2,552 to the hormone therapy alone. Invasive disease-free survival was the primary endpoint of the study.

The median duration of study follow-up was 34 months, with a three-year and two-year duration of ribociclib completed by 20 per cent and 57 per cent of patients respectively. Invasive disease-free survival was evaluated after 426 events.

At the time of this analysis, 189 people in the ribociclib group (7.4 per cent of patients) experienced an invasive disease-free survival event, compared to 237 people in the hormone therapy alone group (9.3 per cent of patients).

The data showed a statistically significant improvement in the risk of invasive disease in favour of the combination arm. The invasive disease-free survival rates at three years were 90.4 per cent, compared to 87.1 per cent for women who were treated with only hormone therapy.

Ribociclib at 400 mg had a favourable safety profile with no new signals.

"Overall, the combination therapy showed more favourable outcomes, significantly reducing the risk of cancer returning," Slamon said. "These results should change how we evaluate and treat patients."