"First checkpoint" refers to an initial designated stop or stage in a journey, task, or process where progress is assessed and evaluated. Full definition
BMS's drug, ipilimumab (Yervoy), was the first checkpoint inhibitor (a kind of cancer immunotherapy drug that essentially helps the immune system release its brake and go after tumor cells it might normally miss) to get approved in the US in 2011 for melanoma. (businessinsider.com)
In 2011, the FDA approved the first checkpoint inhibitor, and four others, including Keytruda, have followed. (scientificamerican.com)
In the mid-1990s, Allison discovered the first checkpoint inhibitor, a finding that paved the way for the development of Bristol - Myers's ipilimumab, the first drug ever shown to improve survival in patients with advanced melanoma. (uchospitals.edu)