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Lilly ICOS, a joint venture between Pfizer's U.S. rivals Eli Lilly and Co. and ICOS Corp., had brought the case because it is trying to produce a treatment to rival Viagra, one of the world's fastest-selling drugs.

The ruling affects only Pfizer's patent in Britain, but a Pfizer spokeswoman said there was a corresponding action in the works on a similar patent for the entire European Union.

High Court judge Hugh Laddie said he found the British patent on the use of Viagra component sildenafil citrate was based on knowledge already in the public domain before the patent was taken out in 1993.

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Pfizer said in a statement it was reviewing the court decision and considering an appeal.

The ingredient is so important that Pfizer regards it as the patent "for" Viagra, Judge Laddie told the court.

Pfizer, however, pointed out its patent of the actual Viagra product, which runs until 2013, would not be affected.

Sildenafil is known as a "PDE5 inhibitor" because it helps PDE5, or the phosphodiesterase enzyme, break down the powerful relaxant guanosine monophosphate, which helps smooth muscle relaxation and enhances penile erection.

Pfizer said the ruling would allow other pharmaceutical companies to develop products using PDE5 inhibitors.

Shares in Pfizer, which became the largest drug company in the world through this year's purchase of Warner Lambert, closed at $44-13/16 in New York on Tuesday. Eli Lilly stock had ended at $87-1/4 and Nasdaq-listed ICOS at $52-9/16.

Several groups have been trying to produce a rival for Viagra to match Pfizer's success, including German drug maker Bayer AG with its Vardenafil treatment, due in 2002.

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