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A federal judge in Pennsylvania ruled that one of the patents held by

GlaxoSmithKline's

(GSK) - Get GlaxoSmithKline Plc Report

for its antidepressant Paxil is invalid. The ruling, which came in a motion for summary judgment in a case brought by a company seeking to market a generic version, will be appealed.

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According to GlaxoSmithKline, a judge in the U.S. District Court in eastern Pennsylvania ruled one Paxil patent was valid, one invalid, and parts of two others invalid. The judge, however, said the legal precedent for determining the validity of the claims is conflicting and "it is impossible to know which approach is correct," the company said. He encouraged the two sides to "seek clarification of the law on this issue."

Paxil is Glaxo's biggest selling drug. The suit was brought by TorPharm Pharmaceuticals, a unit of Apotex, which has been seeking marketing approval for a generic form of Paxil since 1998.

Glaxo said it "continues to believe there are significant hurdles that prevent launch of a generic Paxil" and as a result it said its existing 2002 and '03 earnings guidance remains intact. The company previously said it expects earnings per share growth of at least 10% in 2002 and in the high single digits in 2003.

Glaxo's shares were up 2.4% to $36.79 in early trading.