Thousands more women with breast cancer could benefit from a blockbuster immunotherapy drug than previously thought, research suggests.

Pembrolizumab, sold under the brand name Keytruda, targets and blocks a specific protein on the surface of certain immune cells that then seek out and destroy the cancerous cells.

In England it is offered to those with triple-negative breast cancer, one of the most aggressive forms of the disease, which accounts for about 15% of cases. However, new findings from a global trial suggest it could be effective when used more widely.

The findings will be presented at the European Breast Cancer Conference in Milan on Wednesday.

Treating a common form of breast cancer with pembrolizumab, as well as chemotherapy, before and after surgery could be effective regardless of the patient’s age or if they have gone through the menopause, researchers found.

They trialled the drug on women with early-stage breast cancer that is at high risk of recurring or spreading further, and that is oestrogen-receptor positive (ER positive) and HER2 negative.

According to Cancer Research UK, about 80 in 100 breast cancer diagnoses are ER positive.

The Keynote-756 trial has been running internationally for eight years and comprises 1,278 patients with invasive ductal carcinoma (IDC), meaning cancer had started to spread out of the milk ducts and into the surrounding breast tissues.

Patients were either treated with pembrolizumab and chemotherapy before and after surgery, or with a placebo. Researchers measured the lack of cancer signs in tissue samples, also known as a pathological complete response (PCR) rate.

Prof Javier Cortés, the director of the International Breast Cancer Centre in Barcelona, Spain, said there was a “statistically significant increase” in PCR rate among those treated with pembrolizumab.

About 24.3% of patients had no cancer cells remaining in the breast or lymph nodes compared with 15.6% of patients treated with a placebo.

Dr Simon Vincent, the director of research, support and influencing at the UK charity Breast Cancer Now, said: “This exciting study shows that adding pembrolizumab to chemotherapy before and after surgery could be more effective in eliminating cancer cells in women with the most common type of breast cancer, ER-positive HER2-negative, regardless of their age or menopausal status.

“The trial found that pembrolizumab led to more patients having no detectable cancer cells in their breast or lymph nodes when their treatment finished, although further research is needed to see whether this translates into improved survival rates and a lower likelihood of the cancer coming back.

“With over 1,000 people dying from breast cancer every month in the UK, we desperately need new and effective treatments for this disease.

“While pembrolizumab is currently used for the treatment of triple negative breast cancer, we hope the drug can be submitted to the MHRA [Medicines & Healthcare products Regulatory Agency] for licensing and assessed by Nice [National Institute for Health and Care Excellence] as soon as possible so that patients with ER-positive HER2-negative breast cancer, who may also benefit from this treatment, can access it on the NHS.”

The conference will also hear how researchers have developed a genetic test that can identify how patients with triple negative early-stage breast cancer will respond to immunotherapy drugs. It means patients who are unlikely to respond to these drugs can avoid the adverse side-effects associated with them and can be treated with other therapies.