3. Merck

Merck's landmark approval in 2015 was for immunotherapy Keytruda, which was cleared in October as a second-line treatment for the most common form of skin cancer, following its September 2014 approval in metastatic melanoma. The drug, however, remains slightly behind competitor Bristol-Myers Squibb's Opdivo. During its Q4 earnings call, Merck confirmed that results of its trial evaluating Keytruda as a stand-alone therapy in first-line lung cancer will be reported toward the middle of 2016. If the trial goes well, Keytruda could beat Opdivo to the first line in lung cancer by “as much as six months,” according to Evercore ISI analyst Mark Schoenebaum. But there's a lot going on at Merck beyond Keytruda, clearly. The company's $3.8 billion acquisition of Idenix Pharmaceuticals came to fruition in late January with the approval of Zepatier, the drugmaker's doublet hep-C therapy. In the worth-monitoring file, Merck's antibiotic Cubicin, which generated sales of $1.1 billion in 2015, could face generic competition as soon as June 2016.