Which pharma needs its cash cow most? It's not just about numbers

We regularly rank drugs by sales: Top-selling cancer treatments, top-selling diabetes drugs, top-selling heart meds--you get the picture. And if you follow the industry, you know the world's biggest prescription products already. AbbVie's Humira, for one, which brought in $12.5 billion last year.

And if you follow AbbVie ($ABBV), you know that Humira also happens to be the company's key moneymaker, with the rest of its meds far, far behind on sales. It's more than 60% of the company's top line. With Humira's patent expiring in 2016, and biosimilars already working their way to approval, AbbVie is scrambling to prepare for that impending gap.

Other companies may not have the exact problem, but 10 companies do count on one top product to supply a major chunk of their sales, the business news site Quartz calculates. And several of those are either anticipating some serious pain from a near-term patent expiration or already suffering from competition.

Celgene ($CELG) leads that top 10, though it's not one of those facing impending decline. The California-based biotech depends on one product--its blood cancer treatment Revlimid, with $4.98 billion in 2014 sales--for 65% of its revenue. It already has a follow-up treatment on the market, Pomalyst, which is well on its way to blockbuster status. And Revlimid won't be facing head-to-head rivals anytime soon; its lock on the U.S. market doesn't expire for many years.

Then there's Gilead's ($GILD) Sovaldi, in third place. That hep C treatment racked up more than $10 billion last year, its first on the market, rocketing the company into the top 10 pharma companies by sales. And because 2014 was Sovaldi's first on the market, those sales aren't likely to slump anytime soon--though it may lose some ground to its sister med, Harvoni, which is Gilead's two-drug hep C cocktail in one pill.

Rounding out the top 5 are two meds that pose somewhat greater dangers to their owners: Biogen's ($BIIB) Avonex, a $3 billion multiple sclerosis treatment whose original patent expired in 2013, comes in fourth, with 21% of the company's sales. Avonex has a method patent that lasts through 2026, but it's been challenged in court, and might not hold up. But even if that patent falls, Biogen has a few answers. For one, its new MS pill, Tecfidera, which quickly zoomed to blockbuster status by carving out a significant chunk of market share. And then there's Plegridy, a longer-acting version of Avonex, approved last August. (Avonex, by the way, came in second on FiercePharma's list of top-selling MS meds, with Tecfidera in 7th.)

Finally, Quartz tabs Amgen's ($AMGN) Neulasta, whose $5.76 billion accounts for almost 29% of the company's sales. The white blood cell booster goes off patent this year, and a biosimilar version from Novartis ($NVS) just aced an FDA advisory panel meeting. It's expected to win agency approval later this year. But there's a hitch for Novartis: Amgen has sued the Swiss drugmaker, citing a provision in the America Invents Act, which established an approval pathway for biosimilar meds. That lawsuit--and its outcome--could tie up a biosim launch for months.

Some familiar names populate the rest of the top 10--Teva's ($TEVA) Copaxone and Sanofi's ($SNY) Lantus, for two--and the entire list of 25 runs the gamut of Big Pharma and Big Biotech.

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