A U.S. appeals court has blocked Novartis AG from selling its knockoff version of an Amgen Inc. cancer supportive-care drug in the U.S. until the court resolves a legal fight between the rivals.

The U.S. Court of Appeals for the Federal Circuit in Washington on May 5 granted Amgen's request for an injunction blocking Novartis' Zarxio from the U.S. market until the court resolves Amgen's appeal of a lower-court ruling that gave the green light to Zarxio's market entry.

The new order is expected to keep Zarxio off the U.S. market at least until June 3, when oral arguments are scheduled, though resolution of the case could take longer.

Zarxio is a close copy of Amgen's Neupogen, a drug given to ward off infections in cancer patients receiving chemotherapy, and which had $839 million in U.S. sales last year. The U.S. Food and Drug Administration granted marketing approval to Zarxio in March, making it the first so-called "biosimilar" to be approved using new, abbreviated criteria for copies of biotechnology drugs.

But Novartis still hasn't started selling Zarxio in the U.S. because it agreed to delays as part of ongoing litigation with Amgen. Amgen filed a lawsuit in federal court in San Francisco last year accusing Novartis of failing to disclose certain information about its product to Amgen, which Amgen said was required under U.S. regulations. Amgen also alleged Zarxio would infringe upon a patent for Neupogen. Novartis denies the allegations.

In March, a federal judge in San Francisco denied Amgen's request for an injunction blocking Zarxio and dismissed Amgen's claims that Novartis didn't follow the rules about disclosing product information. That order didn't resolve the patent dispute, which is pending. Amgen appealed the lower-court ruling.

The drug industry and its lawyers are closely watching the Neupogen biosimilar case because the outcome could shape the path to market for a coming wave of biosimilar drugs that are expected to cost less than the original brands.

A Novartis spokesman said the company looks forward to a prompt resolution of the appeal, and to launching Zarxio "in the near future."

An Amgen spokeswoman said the company was pleased with the appellate court injunction.

Write to Peter Loftus at peter.loftus@wsj.com

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