Charities have welcomed the "outstanding" results of a clinical trial which shows that a drug for a type of incurable breast cancer delays the disease getting worse.
Olaparib (Lynparza) can give women living with BRCA-mutated advanced breast cancer significant extra time before their disease progresses, the study found.
Presented at the American Society of Clinical Oncology (Asco), in Chicago, the trial compared olaparib with chemotherapy for patients with HER2-negative BRCA-mutation advanced breast cancer.
At a follow-up of about 14 months, patients who received olaparib had a 42% lower chance of cancer progression than those who received chemotherapy.
The typical time to progression was seven months compared with 4.2 months for the chemotherapy group.
It is estimated that around 1,500 women a year with the difficult-to-treat cancer could benefit from the drug.
Olaparib - which is one of a new class of drugs known as PARP inhibitors - was developed after research by UK scientists at the Breast Cancer Now Research Centre at the Institute of Cancer Research in London.
The drug works by blocking a DNA repair protein known as PARP, causing breast cancer cells to die.
Baroness Delyth Morgan, chief executive of Breast Cancer Now, said: "This outstanding news represents a significant step forward in the treatment of BRCA-mutant secondary breast cancer.
"Olaparib could now become the first biologically targeted drug for a group of patients with incurable and aggressive breast cancer who currently have few treatment options.
"That it can offer around three extra months before the disease progresses compared to chemotherapy - and a better quality of life during that time - will be invaluable to so many women and their families.
"It also represents an extraordinary achievement for UK science."
In the study, led by the Memorial Sloan Kettering Cancer Centre in New York, tumours shrank in about 60% of patients who received olaparib, compared with 29% of those who received chemotherapy.
Professor Andrew Tutt, director of the Breast Cancer Now Research Centre at the Institute of Cancer Research, London, said: "It is fantastic news that olaparib delays the progression of advanced breast cancer in women who have inherited BRCA1 or BRCA2 mutations - the most common type of inherited breast cancer.
"Olaparib is already available for women with BRCA-mutant advanced ovarian cancer, and is the first drug to be approved that is directed against an inherited genetic mutation.
"We are getting much better at curing patients with breast cancer that is diagnosed early - but once the disease has spread around the body, it is much more difficult to treat.
"We need to see more innovative new cancer drugs like olaparib being developed, to give these women a better quality of life for longer."
In a second study presented at Asco, the drug Perjeta, when given after surgery to women with HER2-positive breast cancer alongside Herceptin and chemotherapy reduced the risk of breast cancer recurrence or death by 19% compared with Herceptin and chemotherapy alone.
In absolute terms, after a follow-up of four years, 92.3% of people treated with the Perjeta-based regime did not have their breast cancer return compared with 90.6% treated with Herceptin and chemotherapy.
An editorial in the New England Journal of Medicine regarded the results as a "disappointment" compared with how the drug works in advanced cancer.
In a third study, the drug abemaciclib was found to stop breast cancer patients’ disease progressing for an extra seven months on average, compared with those on the drug fulvestrant alone.
If both drugs were approved for use on the NHS in the future, an estimated 5,000 women per year in the UK could benefit.
The trial on 669 patients was for women with advanced HER2 negative, HR positive cancer that has become resistant to hormone therapy such as tamoxifen.
This type of cancer affects about 70% of women with the disease and many eventually become resistant to hormone therapy.
Women in the abemaciclib plus fulvestrant group lived for 16.4 months on average, compared with 9.3 months for those on fulvestrant alone.
Baroness Delyth Morgan, chief executive at Breast Cancer Now, said: "These very promising results suggest that abemaciclib could become a significant new treatment option for a large group of women whose breast cancer has worsened on anti-hormone therapy.
"The study shows that abemaciclib can stop patients’ breast cancers progressing for an extra seven months on average, compared with fulvestrant alone, time that could be truly invaluable to patients and their loved ones.
"Abemaciclib is one of an exciting new class of drugs, called CDK4/6 inhibitors, that aim to stop cancer cells multiplying out of control by targeting two of the crucial proteins involved.
"These drugs are proving very effective in trials for women with the most common types of locally advanced or incurable breast cancer, and we very much look forward to following their progress towards the clinic."