It's shaping up to be another eventful day in the health care industry. Here at, we'll be following the market reaction to a high-profile Food and Drug Administration approval for Pfizer (NYSE: PFE), as well as an earnings report from obesity drugmaker Arena Pharmaceuticals (NASDAQ: ARNA).

Yesterday, Pfizer's rheumatoid arthritis drug tofacitnib, now christened with the brand name Xeljanz, was handed a faster than anticipated approval by the FDA. Widely anticipated to be a blockbuster capable of more than $2 billion in sales, the drug's approval could mark the beginning of an epic battle with Abbott's (NYSE: ABT) Humira, which will soon approach annual sales near the $10 billion mark. Other drugs likely to feel the pinch include Johnson & Johnson's (NYSE: JNJ) top seller, Remicade, as well as Pfizer's own Enbrel.

While Xeljanz might cannibalize some Enbrel revenue in the near term, the net effect should be a positive one. That's because the economics of Enbrel sales are subject to a partnership with Amgen (NASDAQ: AMGN). Under the co-promotion agreement, which expires in October of next year, the two companies share Enbrel profits in the U.S. and Canada. After expiration, Pfizer's set to receive a smaller royalty for the next three years. While the effect from Xeljanz stealing Enbrel sales might mean shooting their own foot a bit in the near term, bringing Xeljanz to market ahead of the deal's expiration is incredible for the company going forward.

Moving from one lucrative therapeutic area to another potentially lucrative one, obesity drugmaker Arena Pharmaceuticals released quarterly results after yesterday's close. Since its drug Belviq isn't yet commercialized, the results were light on, well, results. The company generated a negligible amount of revenue and posted a slightly lower loss than expected.

While Arena did announce a collaboration agreement in South Korea, the real news yesterday was a sales checkup from rival VIVUS (NASDAQ: VVUS), whose obesity drug Qsymia hit the market in mid-September. Its weaker-than-expected results, along with discouraging commentary around insurance coverage, sunk all ships in the obesity space. While I didn't find anything too notable to merit a huge reaction in Arena shares today, the hangover from VIVUS' disappointing report could continue in today's session.

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