Some men who have exhausted all other treatment options for advanced prostate cancer could survive for at least two more years on immunotherapy, new research suggests.
Researchers found this small proportion of men, described as “super responders”, were alive and well even after the major clinical trial ended, despite having a poor prognosis before treatment.
One in 20 men with end-stage prostate cancer responded to the immunotherapy pembrolizumab.
However, the scientists say that while this number was small, these patients sometimes gained years of extra life.
According to the study published in the Journal of Clinical Oncology, the most dramatic responses were seen in patients whose tumours had mutations in genes involved in repairing DNA.
Researchers are now looking at whether this group might especially benefit from immunotherapy.
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Johann de Bono, Regius Professor of cancer research at The Institute of Cancer Research, London, said: “Our study has shown that a small proportion of men with very advanced prostate cancer are super responders to immunotherapy and could live for at least two years and possibly considerably longer.
“We don’t see much activity from the immune system in prostate tumours, so many oncologists thought immunotherapy wouldn’t work for this cancer type.
“But our study shows that a small proportion of men with end-stage cancer do respond, and crucially that some of these men do very well indeed.”
The study involved 258 men with advanced prostate cancer who had previously been treated and became resistant to androgen deprivation therapy and docetaxel chemotherapy.
Some 5 per cent of the men treated with the immunotherapy saw their tumours shrink or disappear, while 19 per cent showed some evidence of tumour response.
Among a group of 166 patients with particularly advanced disease and high levels of prostate-specific antigen (PSA), the average length of survival was 8.1 months with pembrolizumab.
Nine of these patients saw their disease disappear or partly disappear on scans.
Of these, four were super-responders who remained on treatment at the end of the study follow-up, with responses lasting for at least 22 months, scientists say.
A second group of patients whose PSA levels were lower, but whose disease had spread to the bone, lived for an average of 14.1 months.
Pembrolizumab was well tolerated, with 60 per cent of patients reporting any side effects and only 15 per cent of patients experiencing grade three to five side-effects.
Professor Paul Workman, chief executive of The Institute of Cancer Research, London, said: “Immunotherapy has had tremendous benefits for some cancer patients and it’s fantastic news that even in prostate cancer, where we don’t see much immune activity, a proportion of men are responding well to treatment.
“A limitation with immunotherapy is that there’s no good test to pick out those who are most likely to respond.
“It’s encouraging to see testing for DNA repair mutations may identify some patients who are more likely to respond, and I’m keen to see how the new, larger trial in this group of patients plays out.”
Immunotherapy uses the immune system to fight cancer, and works by helping the immune system recognise and attack cancer cells.
The phase II clinical trial was led by a team at The Institute of Cancer Research, London, and The Royal Marsden Foundation Trust, and funded by the drug’s manufacturer Merck, Sharpe & Dohme.