Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of biopharma company Arena Pharmaceuticals (Nasdaq: ARNA) soared as much as 26% in intraday trading today as investors prepared for the FDA's decision on the company's lorcaserin obesity drug.

So what: Back in September, an FDA advisory committee advised against approving lorcaserin due primarily to safety concerns. Today, a decision is expected from the FDA on the drug. While the FDA isn't bound to follow the panel's recommendation, it seems unlikely that it will go against the decision, particularly in light of the recent decision on Abbott Labs' (NYSE: ABT) Meridia. If the FDA gives a thumbs-down on lorcaserin, it would be a huge setback for the company and likely another big dive for the stock.

Now what: In an interesting twist, a never-say-die group of shareholders has been lobbying the FDA to try and influence its decision. Perhaps the stock's action today suggests that investors think the extra push will persuade the FDA to buck the panel's decision. However, it's hard to imagine what the FDA's upside would be from ignoring the concerns of the panel and giving the drug the go-ahead. Right now, Arena looks like a big gamble -- and likely a bad one at that.

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Fool contributor Matt Koppenheffer owns shares of Abbott Labs, but does not own shares of any of the other companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool’s disclosure policy assures you no Wookiees were harmed in the making of this article.