GSK's Advair dogged again as Mylan launches first bioequivalent copy in U.K.

GlaxoSmithKline ($GSK) is already struggling with sales of aging respiratory superstar Advair, and Mylan ($MYL) just made its job more difficult.

On Monday, the generics maker announced it had launched the first bioequivalent copy of the British pharma's best seller in the U.K., a country that 5.4 million asthma sufferers call home. Translation: Things are about to get dicey for the pharma giant on its home turf.

The Mylan med, dubbed Sirdupla, will join a growing list of Advair generics available in Europe. Cipla's version, for one, is available in countries including Germany, Croatia and Sweden, and Novartis' AirFluSal Forspiro initially won approval in Denmark before nabbing green lights in other countries.

As a result, European Advair sales are on the decline--though it's the U.S. that's been the drugmaker's major concern. Advair's Diskus inhaler technology is still foiling U.S. copycats long after the drug's IP shield expired, but stateside sales of the med plunged 21% in Q1 on payer pressure, and that'll only get worse once generics arrive on the scene.

Glaxo has promised its newer respiratory meds--wannabe blockbusters Breo and Anoro--would pick up the slack, though they've struggled to win payers' favor, too. And since trading away its oncology assets to Novartis in favor of bulking up in the low-margin vaccines and consumer health fields, GSK has been touting a volume-based strategy as the way to return to growth.

But if that plan doesn't work out--and Advair's freefall continues to bring the company down--someone's going to pay, industry watchers predict, and many of them have their money on CEO Andrew Witty.

"Mr. Witty is running out of time," Liontrust Asset Management fund manager Stephen Bailey told Bloomberg in May. "He's either got to deliver in the next 12 months or step aside."

- read Mylan's release

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