A PET imaging agent that lights up beta-amyloid deposits and could help doctors diagnose Alzheimer's disease and the MRI contrast agent Gadovist both come under U.S. Food and Drug Administration scrutiny starting Thursday.

In supporting material made available on the FDA's website, the federal watchdog group expressed concerns over the seemingly wide variability in reader estimates for the PET scans using the agent.

And although largely supportive of Gadovist, the agency worried about labeling requirements for the drug, as it could pose an overdose risk.

In a study released Tuesday in the Journal of the American Medical Association, researchers said PET imaging scans using the agent florbetapir F-18 could help measure beta-amyloid plaque deposits in living patients.

The study compared brain scans on living patients analyzed by three nuclear medicine specialists with later autopsy results on the same patients.

The preliminary findings came from a phase 3 trial by the drug's manufacturer, Avid Radiopharmaceuticals, which was acquired last month by pharma giant Eli Lilly and Company.

The clinical utility of the PET scan using florbetapir would largely come from using a "negative" finding to accurately rule out the presence of the biomarker linked with Alzheimer's disease, the FDA said, based on Avid's filings.

But the agency wasn't fully convinced by the current results, according to documents on its website.

The FDA's main concerns were the small sample size and the variability in readers' judgments, noting that the three specialists who examined the scans in the study only gave identical ratings on five of the 29 patients in the primary arm of the study.

Taking into account all subjects under review raised other red flags, the agency said.

An FDA critic said it was "truly concerning" that for three of 147 subjects that were rated 0, that is, having "no amyloid" from the PET image, at least one other reader gave the subject a 4 -- meaning the highest amyloid rating.

"Only one of the autopsy cohort readers appeared to achieve both high sensitivity and specificity; according to the sponsor, this reader had previous experience assessing amyloid burden using a different imaging agent," the FDA noted.

In clinical practice, only one specialist would likely read the exams, the agency said.

Avid said doctors would need to be trained in interpreting the exams, and has proposed offering such training through a website, according to Reuters.

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