After reviewing the provided sources, Keytruda (pembrolizumab) was first approved by the U.S. Food and Drug Administration (FDA) on September 4, 2014 [3]. The FDA initially approved Keytruda for the treatment of advanced melanoma in adults and children aged 12 years and older [3].

Keytruda is a type of immunotherapy called a programmed death receptor-1 (PD-1) blocking antibody [1]. It works by increasing the ability of the body’s immune system to help detect and fight tumor cells [1].

Since its initial approval in 2014, Keytruda has received additional FDA approvals for various types of cancers [3]. For example, in 2017, Keytruda was approved for the treatment of certain types of metastatic non-small cell lung cancer [3]. In 2019, Keytruda was approved for the treatment of patients with recurrent locally advanced or metastatic Merkel cell carcinoma [2].

In summary, Keytruda was first approved by the FDA for the treatment of advanced melanoma in adults and children aged 12 years and older in 2014.

Sources:
[1] https://www.drugpatentwatch.com/p/biologics/tradename/KEYTRUDA
[2] https://www.cancer.gov/news-events/cancer-currents-blog/2019/pembrolizumab-merkel-cell-carcinoma-fda-approval
[3] https://www.fda.gov/drugs/resources-information-approved-drugs/oncology-cancer-hematologic-malignancies-approval-notifications