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Four-in-10 patients with a deadly skin cancer were still alive after three years when given a drug to boost their immune system, a study suggests.

The trial gave 655 patients with melanoma the drug pembrolizumab, which stops cancers evading the immune system's assault.

The data, released ahead of the world's biggest cancer conference, also showed 15% of patients had no sign of cancer.

Experts said the findings were exciting and "really a step forward".

The immune system is a powerful defence against infection. However, there are many "brakes" built in to stop it attacking our own tissues.

Cancer - which is a corrupted version of healthy tissue - can take advantage of those brakes to evade assault.

Pembrolizumab, one of a new class of immunotherapies, cuts the brake known as PD-1.

Promising early data on the drug means it is already being used by doctors around the world.

But the latest findings, to be presented at the American Society of Clinical Oncology's annual conference, suggest there is a long-term benefit.

Dr Caroline Robert, a researcher at the Gustave Roussy Institute in France, said: "Before 2011 advanced melanoma had a median overall survival of less than one year and things have changed a lot.

"What is really exciting is to see at three years the estimated survival rate is 40% and this is regardless of previous treatment."

Even patients that had tried other immunotherapies appeared to benefit in the trial.

However, patients did develop side effects including fatigue and a rash.

Dr Daniel Hayes, the president-elect of the American Society of Clinical Oncology, told the BBC News website: "This is frankly a pretty exciting abstract.

"This has been a bad disease, it's hard to treat, it's a sneaky disease and the mortality rates have been enormous so to see 40% of patients alive at three years is really a step forward.

"We're even wondering if we could use the word cure here, but it's going to take longer follow up."

Prof Peter Johnson, the chief clinician at Cancer Research UK, said: "It's fantastic news that the benefits of this drug can last for years rather than months for patients with melanoma who until recently have had limited treatment options.

"Pembrolizumab, one of several new drugs that works by unveiling cancer cells to the immune system, has already been approved for use on the NHS for patients with melanoma."

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