Drug Giants Look for Growth Fuel as Older Blockbusters Show Age

The U.S.’s biggest pharmaceutical companies are beginning to prepare for a new and potentially challenging chapter after reaping the rewards of a fruitful period of cancer research that created new blockbusters.

First-quarter earnings reports on April 30 from three drug giants showed that sales growth for some of the biggest-selling medicines of recent years is beginning to mature and slow—and that companies are on the hunt for new therapies to keep their growth engines roaring.

Merck & Co.’s cancer blockbuster Keytruda brought in $2.27 billion in the first quarter, but the immuno-oncology medication’s rapid growth is beginning to moderate. The ...