The Supreme Court on Monday declined to hear an appeal by Sanofi-Aventis SA (SNY), ending the company's six-year effort to protect a patent on its $3 billion-a-year blood thinner.

Sanofi wanted the court to declare that the patent for Lovenox was still intact despite lower court rulings saying a company scientist intentionally mislead the U.S. Patent Office about the drug. Sanofi has reason to fight hard to protect the patent as Lovenox, a version of heparin, brought the company more than $3.5 billion in sales last year.

Sanofi competitors vying to take a piece of Lovenox's market will likely welcome the court's refusal but the decision doesn't guarantee that generic versions of the blood thinner will reach consumers soon as the Food and Drug Administration hasn't approved any. Amphastar Pharmaceuticals Inc., Teva Pharmaceutical Industries Ltd. (TEVA), Hospira Inc. (HSP), Momenta Pharmaceuticals Inc. (MNTA) and a Novartis AG (NVS) unit are seeking FDA approval for generic Lovenox. Momenta and Novartis' Sandoz unit are partnering.

The court's refusal also dashes hopes that it will clarify a controversial area of patent law called inequitable conduct. Under inequitable conduct, a company's patent becomes unenforceable if a court finds that a company intentionally withheld important information from the U.S. Patent Office.

The Supreme Court for years stayed away from taking patent cases. But in recent years as patent reform legislation has stalled in Congress, the court has taken cases to overrule what it sees as poor precedents in patent law.

In the case of Sanofi, a federal appeals court last year found a company scientist intentionally withheld evidence from the Patent Office to support extending Lovenox's patent. Sanofi says federal courts have been too liberal in finding inequitable misconduct, applying it in cases where people made serious mistakes, not calculated schemes to defraud.

Weakening or ending the use of inequitable conduct as grounds for challenging drug patents has become a sticking point in pending congressional legislation to overhaul patent law. The Senate's Patent Reform Act is the focus of an expensive lobbying campaign by the brand-name pharmaceutical industry and biotechnology groups, who want to limit generic drug companies' ability to make cheaper versions of medicines.

California-based Amphastar Pharmaceuticals, which specializes in generic drugs, was the first company to file for a generic version of Lovenox. The first company to file for a generic version of a drug gets 180-days of marketing exclusivity, where it can gobble up market share before competitors can sell their products.

Amphastar's exclusivity period came and went before the FDA approved the company's generic version of Lovenox, meaning it's unclear which company will first get its product to patients first. The companies expect a decision from the FDA this year. The FDA doesn't comment on whether and when it will approve products.

-By Jared A. Favole, Dow Jones Newswires; 202.862.9207; jared.favole@dowjones.com