Ironwood Pharmaceuticals Inc., best known for its drug to treat irritable bowel syndrome, said Tuesday it has agreed to pay AstraZeneca PLC up to $265 million for US commercial rights to a drug that treats hyperuricemia, a buildup of uric acid in the blood.

But shares of Cambridge-based Ironwood fell 5.5 percent over the day as a firm called Phase Five Research, which provides corporate research on health care and biotech companies, released a report saying it notified the Food and Drug Administration of data it said shows that Ironwood’s irritable bowel drug Linzess is the “primary suspect’’ in numerous adverse events in patients, including seven deaths.

In a statement, Ironwood said “we are evaluating the report and its methodology to assess its validity, in particular in view of significant clinical experience with Linzess.’’ The company said its own monitoring of adverse events, which it shares with the FDA, “has not identified new safety signals.’’ The number of adverse events has been consistent with those in Ironwood’s clinical studies, which led to the FDA’s approval of Linzess in 2012, the company said.

Ironwood’s licensing deal with AstraZeneca, a London-based company with a research lab in Waltham, gives the Cambridge company the rights to sell Zurampic in the United States. The drug was approved by the FDA last December to treat hyperuricemia. The condition is caused by gout, a debilitating form of inflammatory arthritis.

Under their agreement, Ironwood also agreed to pay unspecified royalties to AstraZeneca on US sales of Zurampic. The drug will continue to be manufactured by AstraZeneca.

Robert Weisman can be reached at robert.weisman@globe.com.