By Brendan Pierson

Sept 20 (Reuters) - A U.S. appeals court on Friday revived more than 500 lawsuits alleging that Merck & Co failed to warn that its osteoporosis drug Fosamax increased the risk of thigh bone fractures.

A unanimous panel of the Philadelphia-based 3rd U.S. Circuit Court of Appeals ruled that federal law did not block the plaintiffs' state law claims over the drug. Fosamax was acquired in 2021 by Organon , which agreed to indemnify Merck against liability from the lawsuits.

"We're pleased by the Third Circuit's very careful and nuanced judgment, which gives Fosamax victims an opportunity to seek justice from Merck," David Frederick of Kellogg Hansen Todd Figel & Frederick, a lawyer for the plaintiffs, said in an email.

Merck referred a request for comment to Organon, which said it "remains confident in the efficacy and safety profile of Fosamax."

About 3,115 lawsuits over Fosamax were pending against Merck in both federal and state courts around the country as of June 30, according to Organon's most recent quarterly report. The cases affected by Friday's ruling are federal cases that have been centralized in New Jersey.

The federal litigation stretches back to 2008, and has now been dismissed and revived twice. All of those decisions have hinged on the issue of federal preemption - whether the federal law governing what pharmaceutical companies are required to put on drug labels overrides, or preempts, state law claims that the label fails to warn consumers.

Merck has argued that it does, saying that in 2009 it proposed adding a warning about femur fractures but was initially rejected by the FDA. The warning was eventually added in 2011.

Now-deceased U.S. District Judge Joel Pisano in Trenton, New Jersey, ruled in Merck's favor on the preemption issue in 2013, but the 3rd Circuit overturned that ruling in 2017, concluding that issue should have been decided by a jury.

The U.S. Supreme Court in 2019 ordered the issue reconsidered once again, saying it was a legal issue for a judge, not a jury.

In 2022, U.S. District Judge Freda Wolfson, who had taken over the case, again sided with Merck on preemption.

The 3rd Circuit on Friday, however, again reversed. It said that, even though the FDA had not approved the specific warning proposed by Merck in 2009, it would not necessarily have rejected any warning about femur fractures.

It said that Wolfson's decision failed to interpret the FDA's action "in a manner that disfavors pre-emption and carries out Congress's intent to permit displacement of state law only when it is abundantly clear that it is impossible for a manufacturer to comply with both federal and state law."

Circuit Judge Kent Jordan wrote the opinion, joined by Circuit Judges Peter Phipps and Arianna Freeman.

Wolfson has since retired, and the case is now assigned in the trial court to U.S. District Judge Karen Williams. The litigation will return to that court for potential future trials.

The case is In re Fosamax (alendronate sodium) Products Liability Litigation, 3rd U.S. Circuit Court of Appeals, No. 22-3412.

For plaintiffs: David Frederick of Kellogg Hansen Todd Figel & Frederick

For Merck: Yaakov Roth of Jones Day

Read more:

Merck defeats 500 lawsuits over Fosamax bone-fracture risk warnings

Supreme Court gives Merck another shot to avoid Fosamax lawsuits

(Reporting By Brendan Pierson in New York)

((Brendan.Pierson@thomsonreuters.com; 332-219-1345 (desk); 646-306-0235 (cell);))