The Supreme Court of Canada has rejected an attempt by a Toronto-based generic drug maker to prevent brand-name pharmaceutical companies from patenting specific parts of previously patented medicines.

The court ruled unanimously against Apotex Inc. and in favour of France's Sanofi-Synthelabo over a challenge involving the blood-thinner clopidogrel bisulphate. The compound is marketed as Plavix, one of the world's top-selling drugs.

Apotex argued that giving patents on components mentioned in previous patents effectively served to evergreen, or extend, the company's exclusive rights.

In this case, Sanofi first patented a whole genus of compounds useful in thinning blood as a way of treating coronary heart disease, including Plavix. In the company's second patent, it gained rights for the specific compound that made up Plavix.

At issue is whether so-called "selection patents," where rights are secured for a fraction of a larger known class of compounds, are valid.

In early March 2003, Apotex applied to get approval from the federal health ministry to market its generic version of Plavix. The Canadian company based its application on the argument that the patent held by the product's maker repeated the previous patent and the compound could've been recreated based on information from that one.

Sanofi successfully secured a Federal Court order banning the ministry from allowing the application. In 2006, the Federal Court of Appeal dismissed Apotex's attempt to overturn the decision.

In the top court's ruling, Justice Marshall Rothstein agreed with that decision.

He wrote that the Plavix compound was "not self-evident" based on the previous patent and required "inventive" steps to develop the compound.

Sanofi spent millions of dollars and several years developing Plavix as a "less toxic" and "better tolerated" alternative, Rothstein said.

"The inventor selects only a bit of the subject matter of the original genus patent because that bit does something better than and different from what was claimed in the genus patent," Rothstein wrote.

Rothstein said Apotex's concern about evergreening was not justified. He added that it is important to allow such "selection patents" because it encourages improvements by the pharmaceutical industry.

Apotex is already banned from selling its generic version of the drug in the United States after a 2007 New York court decision.

With files from Reuters