Eli Lilly and Co.’s experimental drug to help identify plaque in the brain tied to Alzheimer’s disease isn’t ready for approval, according to U.S. regulators.

While injections of florbetapir seem to detect beta-amyloid plaque deposits, this hasn’t been shown to be clinically useful, Food and Drug Administration staff said in a report released Tuesday. A panel of outside advisers to the agency will discuss the drug’s benefits at a Jan. 20 meeting at the agency’s headquarters in Silver Spring, Md.

Florbetapir, the first drug of its kind to be widely tested in the U.S., was acquired by Lilly in its $300 million purchase of closely held Avid Radiopharmaceuticals in December. Avid has proposed to market the drug under the brand name Amyvid for use in patients undergoing positron emission tomography, or PET, scans. Lilly needs to replace drugs representing almost half its revenue that are scheduled to lose patent protection by 2013.

“Although no significant risks associated with the use of Amyvid have been identified, no convincing evidence of benefit from Amyvid PET has been demonstrated either,” FDA staff said in a Dec. 13 memo included in the review. “At this point in time, the regulatory and secondary clinical reviewer recommends against approving the new drug application.”

The agency is scheduled to make a decision on the drug application by March 17, according to the report.

Indianapolis-based Lilly's stock has dropped 2.5 percent in the past year, underperforming a 0.7-percent decline in the 51-stock Standard & Poor’s 500 Health Care Index.

Avid had initially proposed to market florbetapir with information tying the detection of beta-amyloid plaque to Alzheimer’s disease. The company revised the application to remove the link, according to the FDA.

Alzheimer’s disease is the most common form of dementia among older people, affecting as many as 5.1 million Americans, according to the U.S. National Institutes of Health. While doctors use brain scans and mental exercises to determine when a patient probably has the disease, a definitive diagnosis can now only be made through an autopsy after death.

Florbetapir was given an expedited review under an FDA program to accelerate drugs that represent a significant advantage over current products. The medicine is also being tested in Parkinson’s disease.