SAN DIEGO ( TheStreet) -- Arena Pharmaceuticals ( ARNA - Get Report) is getting just $50 million upfront from the Japanese drug maker Eisai in exchange for U.S. marketing rights to the company's experimental obesity drug lorcaserin. A $50 million dowry is pretty chincy given the blockbuster sales potential for weight-loss drugs. Eisai and Arena might be engaged but it seems like they've decided to wait before sleeping together. The U.S. Food and Drug Administration is already reviewing lorcaserin, with an approval decision expected on Oct. 22. At this advanced stage, Eisai would have easily paid well over $100 million in upfront rights for lorcaserin if the company were confident in the drug's approval and commercial potential. Instead, Eisai is holding off, willing to pay Arena big bucks -- upwards of $1 billion or more -- only if lorcaserin is approved and generates top-tier sales, according to terms of the agreement announced Thursday. Still, Arena deserves kudos for being the first of the obesity drug players to land a marketing partner. Investors have been watching closely to see if any the large drug companies would latch onto one of these weight-loss drugs now making their way through the U.S. Food and Drug Administration review process. Arena won this stage of the competition, while rivals Vivus ( VVUS - Get Report) and Orexigen Therapeutics ( OREX) are still single. The chief concern with lorcaserin is that patients treated with the drug in two pivotal phase III studies didn't lose much weight -- a little more than 3% on a placebo adjusted basis. The biggest positive in lorcaserin's corner is the drug's safety and tolerability, which appear better than its rivals. An FDA advisory panel will review lorcaserin on Sept. 16. Investors are reacting to Arena's partnering announcement Thursday with relatively cool detachment, likely due to Eisai's small upfront payment. Arena shares are up 62 cents, or 20%, to $3.69 in pre-market trading. The stock closed Wednesday at $3.07. -- Reported by Adam Feuerstein in Boston. Follow Adam Feuerstein on Twitter.