UPDATE 1-EU patent on Astra's Nexium upheld but amended

(Adds sales, details on challenge from ratiopharm, background)

LONDON Oct 9 (Reuters) - A European patent covering the manufacturing of AstraZeneca Plc's (AZN.L) top-selling stomach acid drug Nexium has been upheld in an amended form, the European Patent Office said on Tuesday.

The Anglo-Swedish group said it was pleased with the decision, which would protect its manufacturing process in an amended form until 2015.

Industry analysts said it remained to be seen whether the minor amendments might make it easier for generic drug firms to produce cheap copies of the medicine before then.

Nexium is AstraZeneca's most important product and the world's second best-selling medicine. Worldwide sales were $5.2 billion in 2006, with Europe accounting for $1.2 billion.

Its patent in Europe is shaky, however, since the key "composition of matter" patent covering the drug was ruled invalid by the European Patent Office last December.

Patents on the medicine have been challenged in Europe by privately held German drugmaker ratiopharm, which wants to market a generic form of Nexium.

Data exclusivity rules protect Nexium from generics until 2010 but ratiopharm or other companies might be able to launch copies immediately after that, if they can manufacture the drug in a way that does not infringe the amended process patent.

Nexium's patents in the United States -- the product's biggest market -- are also being challenged, although no date has yet been set for a trial there.