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U.S. Sen. Al Franken says Medtronic's handling of a study of its Infuse Bone Graft product may have "potentially skewed the risk profile" for thousands of patients and exposed "insufficient vigilance" by the Food and Drug Administration.

Franken raised both concerns in letters delivered Tuesday to Medtronic CEO Omar Ishrak and FDA Commissioner Robert Califf. Franken said he was "writing out of concern" about a Star Tribune story that appeared Sunday about the study's handling.

Medtronic ran a retrospective study of 3,647 Infuse patients from 2006-2008 but shut it down without reporting more than 1,000 "adverse events" to the government within 30 days, as the law required.

Medtronic, which acknowledges it should have reported the information promptly, says employees misfiled it. The company eventually reported the adverse events to the FDA more than five years later.

"This lack of information potentially skewed the risk profile of the device, which may have affected the treatment of thousands of additional patients," Franken wrote to Ishrak and Califf.

Medtronic and the FDA have told the Star Tribune that the adverse events in the retrospective study did not reveal new types of problems with Infuse, so no one was hurt by the reporting delay. Neither the company nor the agency has made specifics of the patient complications public.

Franken pressed Ishrak on why Medtronic's staff did not know that the adverse events in the study should be reported as soon as the company learned of them. Franken asked what training had been put in place to resolve future oversights. He also asked the CEO: "What assurances can you provide to rule out any intentional lack of reporting adverse event data by Medtronic?"

Franken's questions for Califf were equally pointed.

Medtronic provided the adverse events more than five years after they were due, "despite the fact that the [FDA] had previously issued two warning letters to Medtronic regarding the late submission of other adverse event data," Franken wrote. "Did the agency consider taking additional steps to penalize the company for noncompliance? If not, why?"

Medtronic acknowledged receipt of Franken's letter.

"The Star Tribune article makes insinuations that are false and fails to include important information regarding the [retrospective chart review] and Medtronic's actions," a spokesman said. "We look forward to further discussing these issues with Sen. Franken and his staff in our response."

An FDA spokesman said the agency would "respond directly to the senator."

Medtronic filed four individual death reports from the study in July 2013. Seven months later, the FDA posted a three-sentence summary of 1,039 other adverse events from the Infuse study, but deleted the number from public view, calling it a corporate trade secret.

Saying that patients "deserve a system that works," Franken, a member of the Senate Health, Education, Labor and Pensions Committee, asked Ishrak and Califf to provide "the range of injuries" in the data, as well as the "severity" of the adverse events and how the events "related to Infuse." He also asked how many of the patients in the study received Infuse in FDA-approved ways and how many were treated "off-label."

"The Star Tribune article suggests that we need a strong commitment from the FDA, companies, and Congress to revamp medical device surveillance in this country with a focus on improving patient safety," Franken wrote.

Timely, detailed and public reporting of patient complications is critical, said Lisa McGiffert, director of the Consumers Union Safe Patient project.

"People depend on these oversight systems like the FDA to protect them," McGiffert said. "And there is a lot of trust from the public that companies are responsible, that doctors are responsible, that this would not happen in our system."

Jim Spencer • 202-383-6123 Joe Carlson • 612-673-4779