Matt Rourke, File, Associated Press
In this Thursday, Feb. 28, 2013 file photo, a Merck logo is placed on scientist's lab coat in West Point, Pa.

KENILWORTH, N.J. — Drugmaker Merck & Co. says it will advance a new cancer drug into bigger patient tests, after promising findings in an early study against a very aggressive, common type of breast cancer.

Merck says its Keytruda shrank tumors in one-third of patients, all of whom had triple-negative breast cancer that had spread. Most had worsened after multiple rounds of chemotherapy and other treatments.

The drug is in a hot new class of medicines, mostly still experimental, called immuno-oncology drugs. They harness the body's immune system to fight cancer.

Merck, based in Kenilworth, New Jersey, says it will start mid-stage patient tests of Keytruda in the first half of 2015. It's received accelerated approval for advanced melanoma and is being tested against five other cancer types besides breast cancer.