A new drug that targets the immune system in deadly skin cancer patients is boosting survival rates from a few months to several years.

A new kind of drug for the deadliest form of skin cancer has helped some patients survive for at least three years, a study shows.

Patients until recently faced dismal chances of living for more than a few months.

About 40 per cent of melanoma patients in the study were still alive three years later. The drug, which targets the immune system, was used to treat former President Jimmy Carter, who was diagnosed with melanoma that had spread to his brain.

"This is incredible," Dr Caroline Robert, the study's lead author, said of the results released on Wednesday. "I spend my time telling my residents that these patients would be dead if it was five years ago."

The drug, Keytruda, is among a new class of genetically engineered antibody-based medicines. They block proteins that prevent the body's disease-fighting immune system from attacking cancer cells.

The latest findings for Merck's Keytruda (kee-TROO'-duh) are among the best long-term data ever for treating melanoma that has spread to other organs, Robert and other cancer experts said.

Keytruda, also known as pembrolizumab (pem-bro-LIZ'-uh-mab,) is one of the treatments Carter, 91, received after his diagnosis last year. Last weekend he helped give an honorary humanities degree to rocker Gregg Allman at Georgia's Mercer University, where Carter is a trustee.

The new results in 655 patients are a follow-up to research that led to the 2014 approval of Keytruda for advanced melanoma.

In addition to the 40 per cent survival rate at three years, Robert said 85 patients remained cancer free.

Robert, a melanoma researcher at Gustave Roussy cancer centre near Paris, has worked as a consultant for Merck and Bristol-Myers Squibb, which makes two other immunotherapy drugs approved for advanced melanoma. The newest drug, Opdivo, targets the same protein as Keytruda, while Yervoy targets a different protein.

Dr Len Lichtenfeld, the American Cancer Society's deputy chief medical officer, said the new Keytruda research was "a big deal".

The Keytruda study was released at a news briefing organised by the American Society of Clinical Oncology in advance of the group's annual meeting next month in Chicago.

The American Cancer Society estimates that almost 77,000 people will be diagnosed with melanoma this year and that 10,000 will die from the disease. It's much less common than other skin cancers and much more aggressive and likely to spread to other organs. The earliest sign is often a large misshapen or unusually coloured mole that grows in size.

Yervoy, approved in 2011, had the longest-term data of the three drugs, with an eight-year survival rate of about 20 per cent, said Dr Thomas Gajewski, a University of Chicago immunotherapy expert.

In other immunotherapy developments, a new drug was approved Wednesday for bladder cancer, and Opdivo was approved on Tuesday for another cancer, Hodgkin lymphoma.