There's no delicate way to put this: Arena Pharmaceuticals (Nasdaq: ARNA) got a poor deal when it licensed its obesity drug lorcaserin to Japanese drugmaker Eisai yesterday.

I have no problem with Eisai as a choice; the company sells Aricept with Pfizer (NYSE: PFE), and it's certainly large enough to get the job done. But the terms of the agreement are downright pathetic.

The measly $50 million milestone payment makes it pretty clear that Eisai is as worried about a potential approval as the rest of us. If the drug is approved, Arena is still only due "up to" $90 million. My guess is the "up to" implies that the milestone payment goes down if lorcaserin isn't approved on the first go-round.  Obesity is easily a multibillion-dollar market, with drugs like Abbott Labs' (NYSE: ABT) Meridia and GlaxoSmithKline's (NYSE: GSK) alli offering no real competition. So why do the milestones up to an approval only total $140 million?

What's it giving up? Arena has to pay the cost for manufacturing the drug, and will then capture a percentage of Eisai's net sales, which starts at 31.5% and steps up to a maximum of 36.5% on the portion of annual net sales above $750 million. Assuming a manufacturing cost in the low single digits, Arena will recognize about a little more than a quarter of net sales. By contrast, Gilead Sciences (Nasdaq: GILD), which also sells small-molecule drugs, pockets more than 60% of its revenue after subtracting out manufacturing and all its selling, general, and administrative costs.

There are one-time payments worth a total $1.2 billion, triggered when annual sales reach a certain level. Arena wasn't specific about exactly how the one-time payments break out, except to say that the first one occurs when annual sales reach $250 million, and the last one happens if annual sales hit $2.5 billion. The total payments are $300 million for as much as $1 billion in sales, so it's fairly back-end weighted.

Investors seem to share my feelings for the uninspiring deal. Shares went up 15% yesterday, but that's pretty much an artifact of the low market cap that Arena has now. The company gained an extra $50 million in cash, and its market cap went up by about the same amount.

We'll have to see whether VIVUS (Nasdaq: VVUS) and Orexigen Therapeutics (Nasdaq: OREX), fellow obesity-drug makers, can get better deals for their drugs, but Arena certainly hasn't set the bar very high.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a Motley Fool Inside Value pick. The Fool owns shares of GlaxoSmithKline and has a disclosure policy.