Keytruda, a cancer-immunotherapy medicine, ranks among the most lucrative drugs ever sold. Since its launch in 2014 it has raked in over $130bn in sales for Merck, its American maker, including $29.5bn last year. In September last year an experimental drug did what none had done before. In late-stage trials for non-small-cell lung cancer, it nearly doubled the time patients lived without the disease worsening—to 11.1 months, compared with 5.8 months for Keytruda.
