AstraZeneca PLC (AZN) Tuesday said it has yet to decide whether to contest a decision by the European Patent Office to revoke a key patent underpinning its top-selling heartburn medicine Nexium.

Last Friday, the U.K. drug maker lost a patent challenge brought by a group of generic drugmakers that challenged the validity of patent EP 1020461, which covers the oral administration of Nexium. The European Patent Office ruled that the patent covering esomeprazole magnesium was invalid and overturned it, on grounds that it lacked inventiveness.

"We will decide whether to file an appeal once we have reviewed the written reasons for the decision," an AstraZeneca spokeswoman said Tuesday in an email.

"AstraZeneca is disappointed by the decision ... but has confidence in its intellectual property portfolio protecting Nexium. This portfolio includes many patents with expiration dates ranging from 2014 through to 2019," she added.

"After the patent was granted in 2009, AstraZeneca has enforced this patent in several countries in Europe. We will continue to decide on a case by case basis whether it is appropriate to continue to assert this patent," the spokeswoman said.

The patent protection on the drug, which generated $5 billion in sales for AstraZeneca last year, has expired in a number of EU countries, but generic competition has so far has been limited.

Several generic drugmakers launched cheap copies of Nexium in Germany in late 2010, and AstraZeneca said in October that a generic had also gone on sale in Spain.

Nexium was always expected to face earlier generic competition in Europe than in the U.S., its most important market, with the timing of European launches varying from country to country.

In the U.S., cheap copies of Nexium aren't expected until 2014 following settlement deals struck between AstraZeneca and generic firms.

Still, Matrix analyst Navid Malik said the EPO's ruling on the Nexium patent "in our view leaves the door wide open for a generic to enter the market, subject to an appeal by AZN."

-By Sten Stovall, Dow Jones Newswires; +44 207 842 9292; sten.stovall@dowjones.com