Novartis disclosed today that its Sandoz unit has received a Complete Response Letter from the FDA turning back its application to market a biosimilar version of pegfilgrastim.

“We are working with the agency to address remaining questions,” Novartis said in a brief statement tucked inside a press release announcing second-quarter results, without detailing the concerns that prompted the letter.

Sandoz won FDA acceptance of the application in November, with the review expected to take up to 10 months.

Sandoz said its application for biosimilar pegfilgrastim was based on data from three Phase III trials. One study focused on pharmacokinetics and pharmacodynamics in healthy volunteers, whereas two other studies assessed the drug’s safety and efficacy in patients with breast cancer.

Pegfilgrastim is marketed by Amgen under the name Neulasta®. Amgen generated $4.715 billion last year, and another $1.183 billion during the first quarter, in Neulasta product sales.

Sandoz’ pegfilgrastim application was the company’s third U.S. regulatory filing and the second of 10 planned over a 3-year period.

The first of the 10 was Sandoz’ filing for biosimilar etanercept, which was approved by the FDA in October 2015. Last week, the FDA’s Arthritis Advisory Committee unanimously recommended approval of the proposed biosimilar—marketed by Amgen as Enbrel®—for all five indications of the reference product, including rheumatoid arthritis, plaque psoriasis, psoriatic psoriasis, ankylosing spondylitis, and polyarticular juvenile idiopathic arthritis.

Sandoz won the first FDA biosimilar approval last year for ZarxioTM (filgrastim-sndz), indicated for the treatment of neutropenia associated with chemotherapy. The approval included all indications in the label of reference product Neupogen®, another Amgen-marketed drug.

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