AstraZeneca said on Friday that the company had received a complete response letter from the U.S. Food and Drug Administration (FDA), requesting additional analyses of clinical trial data before it can approve the drug.  But, the agency did not ask for additional clinical trials to be conducted. The delay may not be so serious, but analysts started fretting about the potential impact it could have in limiting the drug’s blockbuster potential.

The request marks a further delay for Brilinta, on which AstraZeneca is relying to drive revenue in the face of generic competition to some of its best-selling drugs. The deadline for the agency delivering its verdict had already been postponed once from September 16 and the market had been widely expecting a green light from the watchdog by December 16.

FDA requested for a more detailed analysis of the new drug Brilinta’s study against Plavix. The phase III study was called Platelet Inhibition and Patient Outcomes (PLATO). The drugs were marketed in patients with severe chest pain or earlier heart attacks. The twice-daily pill won a 7-1 recommendation from an FDA advisory panel on July 28 although all committee members had expressed concern about study results in the U.S., where many patients were also receiving high-dose aspirin.

Brilinta’s market competitors are Plavix from New York-based Bristol-Myers Squibb Co. and Paris-based Sanofi-Aventis SA, and Effient, sold by Eli Lilly & Co. For Plato 18,624 people were included. 9.8 percent of patients on Brilinta for a year had a heart attack, stroke or died from cardiovascular disease, compared with 11.7 percent of those on Plavix. Another study showed that where Plavix may be rendered ineffective in some people with a different genetic makeup, Brilinta works in all patients with equal efficacy.

Brilinta was approved this month in Europe, where it is known as Brilique. AstraZeneca will likely begin marketing the drug in the second half of 2011 once prices are negotiated with member countries.