In a win for OSI Pharmaceuticals, the U.S. Court of Appeals for the Federal Circuit reversed a ruling by the Patent Trial and Review Board that overturned OSI’s patent for non-small cell lung cancer drug Tarceva (erlotinib).

PTAB conducted an inter partes review of the patent last year at the request of Apotex, which is working on a generic version of the drug. Apotex argued that two of the claims in the OSI patent were obvious.

The board agreed, finding that a person of ordinary skill could have used prior art and had a “reasonable expectation of success” without resort to the patent. But OSI challenged the PTAB ruling in June, arguing that it was not based on “substantial evidence.”

In overturning the ruling, the three-judge appeals court panel found that PTAB misread the prior works Apotex presented and found that they “do not provide substantial evidence” to support the board’s finding.