Despite the Dow’s record-breaking performance in July, August and September are historically ranked as its worst-performing months.

LONDON — Shares in Anglo-Swedish drugmaker AstraZeneca plunged as much as 17% Thursday after a new lung cancer treatment proved less successful than the company had hoped.

At 10:43 a.m., AstraZeneca shares on the New York Stock Exchange were trading at $28.70, down about $5.24 a share (-15.4%).

AstraZeneca said human trials of the drug Imfinzi failed to meet the goal of improving progression-free survival rates in patients with non-small cell lung cancer. Investors saw the results as a major setback because the company had hoped the study, known as Mystic, would show Imfinzi to be more effective than standard chemotherapy.

“Analysts have been waiting for the numbers from the Mystic trial for months and had, admittedly tentatively, booked in billions of dollars of future sales from the combination therapy,” said Nicholas Hyett, an equity analyst at Hargreaves Lansdown. “Those health-care billions will now be going elsewhere.”

AstraZeneca reported the findings along with second-quarter financial results that were less than rosy as the loss of patent protection for two blockbuster drugs hurt sales. The company has been seeking to develop new treatments to fight off competition from generic drugmakers.

The potential for such drugs was repeatedly described as one of AstraZeneca’s strengths when it fended off a takeover bid from U.S….