LONDON — AstraZeneca announced on Thursday that a cancer drug in development had not shown as much progress as expected, a setback for the pharmaceutical company, which is grappling with weak sales and competition from generic manufacturers.

The hotly anticipated treatment for lung cancer had been expected to be a major driver of growth for AstraZeneca. Like many of its rivals, the company faces increased pressure from manufacturers that capitalize on the loss of patent protections on best-selling medicines.

The trial’s disappointing results sent shares in AstraZeneca plummeting, with the stock falling nearly 16 percent, to about $56.

The company said on Thursday that the trial, known as Mystic, had not met a “primary endpoint” and would not have met a secondary target either. Sean Bohen, executive vice president and chief medical officer at AstraZeneca, said in a statement that the results were “disappointing.”

AstraZeneca’s challenge is illustrated by the fate of the anti-cholesterol pill Crestor, which was prescribed more than 20 million times in the United States in 2015, the second-highest of any brand-name drug, according to the prescription tracker IMS Health. Crestor had been AstraZeneca’s best-selling drug, accounting for $5 billion of the company’s $23.6 billion in product sales that year.

But a generic version was approved in 2016. Since then sales of Crestor have dropped dramatically. AstraZeneca’s revenue, which was reported on Thursday, was down 11 percent in the first half of the year, to $10.5 billion.