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A new drug delays the progression of advanced breast cancer by over twice as long when used in combination with standard treatment, compared to standard treatment alone, a major clinical trial shows.

By halting tumour progression for several months, women who took the drug were able to delay moving on to chemotherapy following hormone treatment – which is effective, but can have severe side effects.

The results of the phase III trial, called PALOMA3, were presented today (Monday) at the major American Society for Clinical Oncology conference in Chicago and published in the New England Journal of Medicine.

In the trial, in women with the most common form of breast cancer, tumours treated with the combination of palbociclib and a standard hormone treatment progressed after an average of more than nine months. This compared with an average of fewer than four months with the hormone treatment alone.

The trial was led by researchers at The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, London, along with colleagues from 144 international research centres in 17 countries. It was funded by Pfizer.

It enrolled 521 women with a type of cancer called hormone-receptor positive, HER2 negative breast cancer – which accounts for around 75 per cent of cases. All women on the trial had seen their cancer get worse or relapse after first-line hormone therapy.

Palbociclib is a ‘first-in-class’ drug which has a different mechanism of action to standard drugs. It simultaneously blocks two proteins called CDK4 and CDK6 – which are involved in cell division – in cancer cells. Because it is targeted at patients’ tumours, it causes less severe side effects than traditional chemotherapy.

347 women were treated with palbociclib and a standard hormone drug called fulvestrant – which is used after earlier hormone drugs – and 174 with fulvestrant and a placebo.

It took an average of 9.2 months for cancer in women in the palbociclib group to progress – as measured by CT and MRI scans. This compared with 3.8 months for women in the placebo group.

Palbociclib was also well tolerated, with only 2.6 per cent of patients on palbociclib experiencing side effects that led them to stop taking the drug. The trial was stopped early on the advice of an independent data monitoring committee, because of the positive results.

PALOMA3 is one of the first targeted therapy-hormone drug combination studies to include a significant number of younger, pre-menopausal women – who accounted for 21 per cent of patients enrolled in the study. The drug was just as effective in these younger women.

Study leader Dr Nicholas Turner, Consultant Medical Oncologist who leads breast cancer research for the NIHR Biomedical Research Centre based at The Institute of Cancer Research, London, and The Royal Marsden NHS Foundation Trust, said:

“We were delighted to show that this new drug combination delayed the progression of the most common type of breast cancer by an average of five extra months, compared with standard treatment on its own.

“It could help a lot of women to delay the moment when they start chemotherapy, which is effective but is often a very difficult experience for women and their families to go through.”

Vikki Orvice, 52, has been on the PALOMA3 study since June 2014. She says:

“The treatment has had a very swift effect – by the end of 2014 the tumour had shrunk in size, and is still shrinking now. I am so grateful for the opportunity to be on the trial. I don’t like to think what would happen without this study and the care I have received at The Royal Marsden.”

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