While yesterday's approval of Keytruda (pembrolizumab) is a win for the melanoma community, it also marks the first approval of a new drug class called PD-1 inhibitors, which has shown promise in treating a variety of cancers, including kidney and lung cancers.After being granted breakthrough status last year and then priority review in May, the Food and Drug Administration gave Keytruda (also known as MK-3457) accelerated approval, which means the agency felt the drug's safety and potential effectiveness filled an unmet need to patients and pushed it through the pipeline much sooner--four months compared with the traditional 10 months of review. The approval marks the sixth new melanoma treatment since 2011.[Learn more about breakthrough therapy, priority review, fast track status and more here ]Keytruda, a monoclonal antibody, blocks the PD-1 pathway, which allows cancer cells to evade the immune system. The drug makes it possible for T cells to identify and attack melanoma tumors. Stay tuned to hear more about PD-1 (programmed cell death protein 1) inhibitors, as well as PD-L1 (programmed death-ligand 1) inhibitors, which works against the same cell pathway.The approval is designated for patients with advanced melanoma or whose cancer cannot be surgically removed and no longer responds to other treatments, including Yervoy (ipilumumab), another type of immunotherapy. The approval was based on two trials that showed tumor shrinkage that lasted from 1 to 8 months and that the drug was safe. In the phase 1 study, the drug appeared to work in patients with newly diagnosed melanoma as well as in those who had been heavily pretreated. Common side effects included fatigue, cough and nausea; reports of immune-related side effects were rare. When the data was originally presented at the American Society of Clinical Oncology's annual meeting in early June, researchers were very complimentary of the lack of severe side effects seen – only 4 percent of participants in the clinical study discontinued treatment because of side effects, a result many researchers hadn't seen before in a metastatic patient population. (You can read more about melanoma advancements in CURE's ASCO 2014 coverage here.)Of course, the approval is contingent on further testing. Because it was based on tumor responsiveness in a phase 1 trial, follow-up studies will need to be completed to see if there is an improvement in overall survival or other benefit.Keytruda is now one of several treatments that boosts the immune system against cancer. Additional studies are testing various combinations of these drugs, hoping that targeting different pathways may boost survival. "The news of FDA's first approval of an anti-PD-1 drug is extremely exciting and shows just how far the field has come in the last few years," says Debra Black, co-founder of the Melanoma Research Alliance, in a statement released by the organization.Update: Previous version stated that breakthrough status was granted in February. Pembrolizumab was granted breakthrough status by the FDA in April 2013.