Arena Pharmaceuticals ( ARNA) carries a heavy burden.

Not only is the San Diego-based drug company under pressure to produce boffo results from a phase III study of its obesity drug later this month, but Arena's drug also must find a way to free itself from the dark and costly legacy of the fen-phen diet pill tragedy of the 1990s.

Arena's obesity drug is known as lorcaserin. Investors will get a first look at lorcaserin's ability to shed pounds -- and do it relatively safely --when Arena announces results from the first of two, phase III studies at the end of March. Data from the second phase III study are expected in the fall.

Arena is one of three companies with obesity drugs in ongoing phase III clinical trials ( Vivus ( VVUS) and Orexigen Therapeutics ( OREX) are the others). Of the three, Arena is probably under the most scrutiny because lorcaserin is a chemical cousin to fenfluramine, or the "fen" in the effective but unsafe fen-phen diet pill combination regimen.

People who took fen-phen lost weight, as much as 15%-20% of their body weight, in fact. But the combination, in particular, fenfluramine, caused serious damage to some people's heart valves. The drug was yanked from the market in 1997and its maker American Home Products, which later became Wyeth ( WYE), was forced to pay billions of dollars in damages to people injured by the drug.

Lorcaserin and fenfluramine both work through an effect on a receptor in the body that regulates levels of serotonin, a brain chemical that can increase feelings of satiety, or fullness.

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