An appeals court has blocked sales of Zarxio, a copycat version of Amgen Inc.’s blockbuster drug Neupogen.

The U.S. Court of Appeals for the Federal Circuit on Tuesday overturned a March 19 ruling by San Francisco federal court Judge Richard Seeborg, who had denied Amgen’s request for an injunction

The appeals court ruled that sales of Zarxio, made by Swiss drug maker Novartis AG, should not proceed pending the Thousand Oaks biotech’s appeal of its case.

In March, the Food and Drug Administration approved the sale of Zarxio, a biosimilar form of Neupogen, which increases the number of infection-fighting white cells in the blood.

Biosimilars are a new form of drugs that are copies of proprietary biotech drugs when patent protection expires. Because small differences in the environment or manufacturing processes may alter the final product, the drugs are called similar – but not identical – to the original drug.

Neupogen’s patents expired last year. Zarxio would be the first biosimilar of a blockbuster drug for sale in the United States. They have been available in Europe for about a decade. Neupogen had worldwide sales last year of about $1.16 billion, according to Amgen filings.

Amgen filed paperwork in February seeking a preliminary injunction to stop Novartis from selling Zarxio, arguing that Novartis hadn’t complied with the time elements specified in the law allowing biosimilars. Novartis attorneys filed counter arguments that Amgen’s lack of cooperation caused delays and the timing issues.

In a statement, Amgen said it was pleased with the appeals court’s decision.

Shares closed down $1.05, or less than 1 percent, to $157.90 on the Nasdaq.

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