Which specific cancer saw keytruda's first efficacy?

Which cancer type was first linked to Keytruda’s efficacy?

Pembrolizumab (Keytruda) showed its first efficacy in clinical testing for melanoma. [1]

What was the early signal and where did it come from?

The earliest notable efficacy results for pembrolizumab were reported in patients with advanced (unresectable or metastatic) melanoma. [1]

Why melanoma specifically?

Melanoma was the cancer type where early pembrolizumab activity was most clearly demonstrated in initial clinical studies, before later expansion into other tumors. [1]

Sources:
1. https://www.merck.com/stories/keytruda-history-of-discovery-and-development/