Every year, thousands of patients sit in doctors’ offices with needles in their arms receiving a dose of a wonder drug called Keytruda. The cancer medicine is one of the world’s best sellers, earning Merck $29.5bn in sales last year. But drip by drip, the US pharmaceutical company’s time is running out.

In 2028 Keytruda’s patent ends, allowing rivals to sell the same drug at a cheaper price. Investors are spooked and Merck’s shares have sunk 35 per cent over the past 12 months. While the company has been at pains to show it is prepared, Daina Graybosch, an analyst at Leerink, says the so-called patent cliff is hanging over the business. Merck has a “massive hole” in its revenue to fill. “They can’t fill it with one drug,” she adds.

Merck on Wednesday appeared to take a step towards addressing the looming Keytruda patent cliff. The company is closing in on a $10bn deal to buy London-based biotech Verona Pharma, which has an approved respiratory disease drug that analysts predict could generate peak annual sales of approaching $4bn.