Merck & Co. agreed to buy respiratory drugmaker Verona Pharma Plc for around $10 billion as part of its ongoing search for ways to fill the Keytruda-sized hole that will emerge over the next few years.

Merck said it will buy Verona Pharma for $10 billion to bolster its lung-disease business and help replace the expected loss of revenue once Keytruda patent protection lapses.