Terminal prostate cancer patients could get almost two more years of life with experimental immunotherapy drug, a study shows.
Men who had exhausted all other treatment options for end-stage prostate cancer trialled pembrolizumab.
Researchers described 1.6 per cent of the men as ‘super responders’ because their disease disappeared on scans after taking the experimental therapy.
They were surviving even after the clinical trial ended 22 months later, despite having a poor prognosis before treatment.
Overall, a quarter of participants showed evidence of the drug working, which could mean gaining extra time for those with little hope remaning.
Terminal prostate cancer patients could get almost two more years of life with experimental immunotherapy drug pembrolizumab (Keytruda)
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.
Pembrolizumab, marketed as Keytruda, is already used on the NHS for skin cancer, bladder cancer and lung cancer.
Immunotherapy works by helping the immune system recognise and attack cancer cells, the body’s natural way of fighting disease.
The typical treatment given to men with prostate cancer is hormone therapy, which reduces levels of the male hormones, called androgens, that lead to tumor growth.
Chemotherapy may also be given or radiotherapy for advanced or metastatic prostate cancer.
Johann de Bono, Regius Professor of cancer research at The Institute of Cancer Research, London, said: ‘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.’
‘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.’
Advanced prostate cancer is when cancer that originated in the prostate spreads to another part of the body, such as the bones or lungs.
In its early stages, prostate cancer is highly treatable, with five-year survival rates close to 100 per cent, according to The Cancer Research Institute.
Once prostate cancer has spread, however, the five-year survival rate falls to less than 30 per cent.
Scientists say there is a significant need for more effective treatment of advanced stage disease.
The study, led by The Institute of Cancer Research, London, and The Royal Marsden Foundation Trust, was a phase two clinical trial.
A total of 258 men with advanced prostate cancer were involved. They had previously been treated and became resistant to androgen deprivation therapy and docetaxel chemotherapy.
All patients received a 200mg dose of pembrolizumab every three weeks for up to 35 cycles.
One in twenty (five per cent) 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 the protein 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.
PSA is a protien which leaks from the prostate. High levels signal a man may have prostate cancer and can be seen in blood tests.
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.’
Pembrolizumab was well tolerated, with 60 per cent of patients reporting any side effects. Only 15 per cent of patients experienced severe side-effects.
The study was funded by the drug’s manufacturer Merck, Sharpe & Dohme.