Almost 40% of patients who spent 12 months on the drug pembrolizumab – known as a “checkpoint inhibitor” – as part of a new study were still alive and one in 10 had not seen the cancer grow.
The 258 men with advanced prostate cancer were treated with the drug as part of a trial led by a team at the Institute of Cancer Research (ICR) in London and the Royal Marsden NHS Foundation Trust.
The results – which are to be presented at the American Society of Clinical Oncology annual meeting in Chicago – have been lauded as “amazing”.
Professor Johann de Bono, director of the drug development unit, said: “I have these men who are basically dying, with weeks to months to live, who we gave this drug to and had complete responses.
“Their cancers shrunk, disappeared actually, with minimal cancer left on scans.
“These are amazing results, and these are men whose cancers had all the treatments, they had everything possible, they’ve got no treatments left, and they are in trouble. They have very short life spans left.”
According to Prof de Bono, the results mark the first time immunotherapy has demonstrated benefits in men with prostate cancer, which kills more people in the UK than breast cancer.
But on this occasion, the researchers found that particular patients may benefit from such treatment depending on the genetic makeup of their tumours.
While only 5% of men in the trial saw their tumours shrink or disappear after treatment, many of those had mutations in genes involved in repairing DNA in their tumours.
The researchers suggest these mutating cancer cells may be easy for the immune system to recognise and attack because they look different from healthy cells.
Prof de Bono added: “We are planning a new clinical trial, specifically in men with prostate cancer whose tumours have mutations in DNA repair genes, to see if immunotherapy can become a standard part of their treatment.
“It’s exciting that immunotherapy could offer some men more time with their loved ones where they have such advanced disease that they have run out of existing treatment options.”
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But the ICR has warned that the treatment will still only work for a minority of patients, with only around 20% likely to respond to immunotherapy.
Research will now focus on identifying signs to help pick out the prostate cancer patients whose tumours are most likely to shrink after the treatment.