WASHINGTON (Dow Jones)--A doctor who was involved in early clinical studies of prasugrel, an anti-clotting drug being developed by Eli Lilly & Co. (LLY) and Daiichi Sankyo Co. (4568.TO), said the Food and Drug Administration should stop its review of the product and require the companies to conduct additional studies of a lower dose.

The doctor, Victor Serebruany of Johns Hopkins University in Baltimore, outlined his concerns that included bleeding risks, in a letter sent to FDA Commissioner Margaret Hamburg Wednesday. Serebruany is listed as one of the inventors on a patent application for prasugrel, but later sold his rights to Eli Lilly. The application is pending.

Serebruany said the proposed 10-milligram dose of the drug causes excess bleeding and that a lower 5-milligram dose of the drug should be tested. He also questioned the data that showed prasugrel was better than another drug, Plavix, at cutting heart-attack rates.

The FDA review of prasugrel has dragged on for more than a year. An FDA spokeswoman confirmed the agency received the letter, but said federal law prohibits the agency from commenting on the status of drug applications pending at the agency. In February, an FDA panel recommended the agency approve prasugrel after a generally positive review from FDA staff.

The letter was also signed by two doctors from the consumer advocacy group Public Citizen, including Sidney Wolfe. Wolfe is the director of Public Citizen's health research group and also serves on the FDA's drug safety advisory panel.

In a statement, Lilly and Daiichi Sankyo said the companies remained "confident in the overall benefit-risk profile of prasugrel and stand behind the clinical trial design and results." The companies said they were in the "final" stages of the drug-review process with the FDA but couldn't speculate on the outcome. The main clinical trial, involving more than 13,000 patients, submitted to the FDA in support of prasugrel showed it was better than Plavix at preventing life-threatening blood clots in heart patients, but it was also associated with a higher risk of dangerous bleeding in some patients.

The FDA review of the study showed that compared with Plavix, which is comarketed by Bristol-Myers Squibb Co. (BMY) and Sanofi Aventis SA (SNY), prasugrel would prevent an additional 20 heart attacks and two cardiovascular-related deaths but would likely result in two fatal bleeding events and four life-threatening bleeding events. An agency reviewer said the drug carried a "worthwhile risk-benefit profile."

Serebruany raised several concerns about how prasugrel and Plavix were given to patients in the trial and whether it offered a fair comparison between the drugs. He also wrote the benefit of prasugrel at reducing heart attacks was driven by a reduction in non-fatal heart attacks and noted the definition of certain types of heart attacks has changed since the trial was conducted.

Last month investigators for the prasugrel trial reanalyzed the data according to the new heart-attack standards and found prasugrel still cut heart-attack rates compared to Plavix, although at somewhat lower rates than originally seen.

Lilly has said the company is studying a lower dose of prasugrel but the study won't be complete until 2011.

Serebruany also spoke at the FDA advisory panel on prasugrel and raised questions about the safety and effectiveness of the drug. At the time he said he's received grants from Lilly, Bristol and Sanofi. Serebruany also said he's been a consultant for Bristol-Myers, Sanofi, Bayer AG (BAY) and a unit of Johnson & Johnson (JNJ).

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com