In a win for three generic drug companies, the U.S. Court of Appeals for the Federal Circuit has invalidated key claims in Bayer AG’s patent on the brand-name oral contraceptive Yaz.

In a 14-page decision issued on Tuesday, the Federal Circuit ruled that the hormone formulation in Yaz was obvious to scientists at the time. Generic drug companies Actavis Inc., Sandoz Inc. and Lupin Ltd. had challenged Bayer’s patent through Hatch-Waxman Act litigation.

Birth control pills contain estrogen, which blocks ovulation. A 28-pill packet typically includes seven placebo pills. In 1993, Bayer sought a patent on a low-estrogen formulation with a reduced pill-free interval of only four or five days.

The patent issued in 2002, and Bayer brought Yaz to market in 2006. (Incidentally, Yaz also contains the hormone drospirenone, which has been linked to blood clots and has set off a cottage industry of personal injury litigation.)

Beginning in 2008, Actavis (formerly Watson Pharmaceuticals Inc), Sandoz, and Lupin challenged Bayer’s patent for Yaz on obviousness grounds. They pointed to six prior art references that, when read in tandem, allegedly made Bayer’s formulation seem plainly obvious.

Their claims were consolidated before U.S. District Judge Kent Dawson in Salt Lake City. Dawson sided with Bayer on summary judgment in March 2012, ruling that "the differences between the prior art and the claimed invention are substantial."

The Federal Circuit has now reversed by a 3-0 vote. In a decision penned by Circuit Judge Alan Lourie, the panel held that the prior art references dug up by the generics "go beyond just illuminating a known problem: they also expressly proposed the claimed solution."

Lawyers on the winning side of the ruling include Mark Jansen of Crowell & Moring (for Actavis); Joseph Hynds of Rothwell Figg Ernst & Manbeck (for Sandoz); and Robert Green of Leydig Voit & Mayer (for Lupin). Adam Mortara of Bartlit Beck Herman Palenchar & Scott argued for Bayer.

Johnson & Johnson had better luck recently fending off similar challenges to its own low-dose contraceptive, Ortho Tri-Cyclen Lo.