Merck's ($MRK) pembrolizumab, sold as Keytruda, showed promising signs of efficacy in a particularly deadly brand of breast cancer, early data that could expand the use for a new class of treatments expected to take the market by storm. Merck's treatment, approved to treat melanoma, charted an overall response rate of 18.5% in a study on women with triple-negative breast cancer, an aggressive form of the disease with no approved targeted therapies. The results suggest a wider use for so-called checkpoint inhibitors, a class of treatments that help expose cancer cells to the immune system's natural defenses and have thus far demonstrated efficacy in skin, lung and other cancers. Bristol-Myers Squibb ($BMY), Roche ($RHHBY) and AstraZeneca ($AZN) are among the companies developing similar therapies. More