U.S. regulators have approved the first drug in a new class of cancer medicines that work by stimulating the immune system, a Merck drug developed for treating deadly skin cancer.

The Food and Drug Administration says it has granted accelerated approval to Merck’s Keytruda, for treating melanoma that’s spread or can’t be surgically removed, in patients previously treated with another drug.

The drug, known chemically as pembrolizumab, is part of a hot, promising new class of antibody-based drugs that work by taking a brake off the immune system so it can better recognize and attack cancer cells.

Merck’s drug is the first in the class approved in the U.S. Rival Bristol-Myers Squibb and a partner got a similar drug approved in Japan in July and are seeking U.S. approval.