Novartis' Sandoz division announced today that the US FDA has issued a complete response letter regarding the Biologics Licensing Application (BLA) for its proposed biosimilar rituximab, meaning that approval of the drug has been delayed in the country.

The letter is good news for Roche, the original marketers of Rituxan, for whom the drug generates around $4 billion of sales in the US.

"Sandoz stands behind the robust body of evidence included in the regulatory submission and is currently evaluating the content of the letter," the company said in a statement. "While disappointed, Sandoz remains committed to further discussions with FDA in order to bring this important medicine to US patients as soon as possible."

Another biosimilar Rituxan, produced by Celltrion and Teva, was rejected by the FDA last month.

Rituximab biosimilars both Celltrion and Novartis are already available in Europe and sales of Rituxan have fallen by 44 percent in the region in the first quarter of 2018.