It's time for large-cap pharma investors to mark their investing calendars. Last week Eli Lilly (NYSE: LLY) announced that the FDA had granted its top pipeline compound a priority review that could give it marketing approval in 2008.

The FDA priority review for Lilly's blood thinner, prasugrel, means that the agency should make a decision on whether to grant approval to the acute coronary syndrome treatment around June 26 (assuming a six-month review date from Lilly's Dec. 26 marketing application). This PDUFA date is the FDA's goal for making a decision on marketing approval, but the review date is not set in stone and might come before or after that date.

What makes the pending approval decision for prasugrel so interesting is that back in November, Lilly and partner Daiichi Sankyo announced somewhat mixed phase 3 data against Bristol-Myers Squibb's (NYSE: BMY) and Sanofi-Aventis' (NYSE: SNY) Plavix. For instance, in the just-announced 13,000-plus-person phase 3 trial, prasugrel outperformed Plavix on its primary efficacy endpoint (fewer deaths due to cardiac events), but it did worse on its safety endpoint (major bleeding events).

Prasugrel means a lot for Eli Lilly. Beside a long-lasting second-generation diabetes drug it is working on with partners Amylin Pharmaceuticals (Nasdaq: AMLN) and Alkermes (Nasdaq: ALKS), prasugrel represents Lilly's best opportunity to fill in the gap from lower sales of Zyprexa once it faces generic competition in the U.S. in a couple of years.

The market is huge for any drug, like (potentially) prasugrel, that treats acute coronary syndromes. For instance, sales of Plavix surpassed $1 billion in the U.S. in the most recent quarter.

I won't try to predict what the FDA will do with prasugrel. But whatever happens in June, it will likely be the most interesting FDA ruling this year -- the FDA decision could go either way.

Nothing's more exciting than a drugmaker with an upcoming binary decision. Come rate Eli Lilly in the Motley Fool's CAPS database, and see what other investors have to say about the Income Investor pick.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy that takes Vitamin C.