RAHWAY, N.J.—Merck & Co. is making one of the biggest bets on a single drug in the pharmaceutical industry, a move that is risky but that could, if successful, pay off for the company and a broad array of cancer patients.
Merck’s Keytruda, a new type of cancer drug that harnesses patients’ immune systems to fight tumors, is already marketed to treat lung, skin, bladder and other cancers. Global sales soared to more than $3.8 billion in 2017—about 9% of Merck’s total revenue—fueled by a monthly U.S. price tag of $13,500...