According to the sources provided, the patent for Keytruda is set to expire on June 20, 2028 [1]. Keytruda is a blockbuster drug that has been approved to treat several types of cancer, including melanoma, non-small cell lung cancer, and head and neck cancer. It is expected to generate over $22 billion in annual sales by 2026 [2].

Merck, the manufacturer of Keytruda, is exploring ways to extend the drug's patent life. One strategy being considered is to develop a new formulation of Keytruda that can be administered as a single shot instead of the current infusion process. This could potentially extend the patent life of the drug until 2031 [3].

In summary, the patent for Keytruda is set to expire in 2028, but Merck is exploring ways to extend its patent life.

Sources:
[1] https://www.drugpatentwatch.com/p/biologics/tradename/KEYTRUDA
[2] https://www.fiercepharma.com/special-report/top-15-blockbuster-patent-expirations-coming-decade
[3] https://www.reuters.com/business/healthcare-pharmaceuticals/merck-could-keep-its-patent-edge-by-shifting-keytruda-cancer-drug-simple-shot-2022-12-02/