A modified herpes virus has shown promise killing off cancer cells - with one patient seeing the disease vanish entirely.
Patients were injected with a drug that was a weakened form of the cold sore virus - herpes simplex - that has been modified to kill tumours.
While more research is needed, it could offer a lifeline for those living with advanced-stage cancer.
Krzysztof Wojkowski, 39, a builder from West London, went from end-of-life care to being cancer free after joining the trial.
Mr Wojkowski was diagnosed with Mucoepidermoid carcinoma, a type of salivary gland cancer, in May 2017.
Despite multiple surgeries, he was told that there were no treatment options left, before being given the opportunity to join the RP2 trial at The Royal Marsden in 2020.
He said: "I was told there were no options left for me and I was receiving end of life care, it was devastating, so it was incredible to be given the chance to join the trial at The Royal Marsden, it was my final lifeline.
"I had injections every two weeks for five weeks which completely eradicated my cancer. I've been cancer free for two years now, it's a true miracle, there is no other word to describe it.
"I've been able to work as a builder again and spend time with my family, there's nothing I can't do."
The genetically engineered virus, which is injected directly into the tumours, is designed to have dual action - it multiplies inside cancer cells to burst them from within and it also blocks a protein known as CTLA-4, releasing the brakes on the immune system and increasing its ability to kill cancer cells.
Rare to see such promise in early trials
Three out of nine patients treated with RP2 saw their tumours shrink.
Seven out of 30 patients who received both RP2 and the immunotherapy nivolumab also benefitted from treatment.
In this group, four out of nine patients with melanoma skin cancer, two out of eight patients with the eye cancer uveal melanoma, and one out of three patients with head and neck cancer saw their cancer's growth halt or shrink.
Of the seven patients receiving the combination who saw a benefit, six remained progression-free at 14 months.
It is rare to see such a good response rate in early-stage clinical trials, according to the study leader Professor Kevin Harrington, professor of biological cancer therapies at The Institute of Cancer Research, London, and consultant oncologist at The Royal Marsden NHS Foundation Trust.
He said: "Our study shows that a genetically engineered, cancer-killing virus can deliver a one-two punch against tumours - directly destroying cancer cells from within while also calling in the immune system against them.
"It is rare to see such good response rates in early-stage clinical trials, as their primary aim is to test treatment safety and they involve patients with very advanced cancers for whom current treatments have stopped working.
"Our initial trial findings suggest that a genetically engineered form of the herpes virus could potentially become a new treatment option for some patients with advanced cancers - including those who haven't responded to other forms of immunotherapy.
"I am keen to see if we continue to see benefits as we treat increased numbers of patients."
Exploiting the features of viruses
Professor Kristian Helin, Chief Executive of The Institute of Cancer Research, London, said it was possible to exploit some of the features of viruses.
They said: "Viruses are one of humanity's oldest enemies, as we have all seen over the pandemic. But our new research suggests we can exploit some of the features that make them challenging adversaries to infect and kill cancer cells.
"It's a small study but the initial findings are promising. I very much hope that as this research expands we see patients continue to benefit."