July 9 (Bloomberg) -- Pfizer Inc., the world’s biggest drugmaker, failed to maintain its patent on the hypertension drug Norvasc in Canada after a federal judge granted Ratiopharm GmbH permission to sell a generic version of the medicine.

More than 7.8 million prescriptions of amlodipine besylate, marketed by Pfizer as Norvasc, are sold annually in Canada, Ulm, Germany-based Ratiopharm said today. A generic version probably will save Canadian patients about C$180 million ($155 million) a year, the closely held company said. Pfizer said it will appeal.

“We are extremely pleased at the decision by the Federal Court,” Jean-Guy Goulet, chief executive officer of Ratiopharm Canada, said in a statement. “This decision opens the amlodipine patent to generic manufacturers in Canada, the last major jurisdiction where the patent was held.”

Ratiopharm challenged the Canadian patent in 2004 and won in 2006. Pfizer successfully reversed the ruling on appeal. Ratiopharm then sued to invalidate Pfizer’s patent, with Judge Roger Hughes ruling in favor of Ratiopharm yesterday.

The three-paragraph judgment says the patent is “invalid and void” and orders the Canadian Patent Office to record an entry to that effect. Hughes didn’t release the reasons for his decision, giving the two sides 20 days to make submissions on which portions of the reasons should remain confidential.

Pfizer said in a statement that it will ask the Federal Court of Appeal of Canada to overturn the decision. The New York-based company also said that legal challenges to the patent in Canada by other drugmakers are still pending.

‘Ensuring Viability’

“Pfizer remains committed to protecting its intellectual property rights and to ensuring the long-term viability of its research and development programs,” the company said in the statement.

Pfizer fell 32 cents, or 2.2 percent, to $14.29 at 4:15 p.m. in New York Stock Exchange composite trading. The shares have dropped 19 percent this year.

Ratio-Amlodipine, Ratiopharm’s version of Norvasc, will be available in Canadian pharmacies “imminently,” the company said. Norvasc sales plunged 52 percent in the first quarter of 2008 to $513 million after generic copies of the medicine came on the U.S. market in March 2007.

The case is Ratiopharm Inc. v. Pfizer Ltd., T-1712-07, Federal Court of Canada (Toronto).

To contact the reporter on this story: Joe Schneider in Toronto at jschneider5@bloomberg.net.

Last Updated: July 9, 2009 16:40 EDT