Bloomberg News

Merck  (MRK - Get Report)  revealed that one of its key cancer-fighting drugs, Keytruda, failed to meet its stand-alone goal of treating an aggressive type of breast cancer. 

The drugmaker said Keytruda, among a class of medicines called PD-1 inhibitors, didn't meet the main goal of helping patients live longer, when compared to chemotherapy.

The treatment will continue to be studied in earlier stages of the disease and in combination with chemotherapy in patients with triple negative breast cancer, the company said.

Triple negative breast cancer occurs in about 10%-20% of breast cancer patients, and is more difficult to treat, the company said.

Keytruda, which brought in sales of $2.27 billion for the company in the first quarter, has U.S. approval for other forms of cancer including skin and lung cancer.