The Food and Drug Administration (FDA) has issued a Complete Response Letter (CRL) to Citius Pharmaceuticals regarding the Biologics License Application (BLA) for denileukin diftitox for the treatment of patients with relapsed or refractory cutaneous T-cell lymphoma (CTCL) after at least 1 prior systemic therapy.

Denileukin diftitox is an engineered interleukin-2 (IL-2)-diphtheria toxin fusion protein. By binding to the IL-2 receptor on the cell surface, the drug is internalized and causes diphtheria toxin fragments to inhibit protein synthesis resulting in cell death.

Denileukin diftitox was previously approved under the brand name Ontak, which was withdrawn from the market in 2014. The product being developed by Citius under the proposed brand name Lymphir™ is a reformulation of Ontak.

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The BLA submission included data from a phase 3 trial ( Identifier: NCT01871727) that assessed the safety and efficacy of the reformulated product in patients with recurrent or persistent CTCL. Sixty-nine patients were included in the primary efficacy analysis set. Findings showed the treatment was considered efficacious with an objective response rate (primary endpoint) of 36.2% (95% CI, 25.0-48.7%; 25 patients out of 69). Adverse events were consistent with those previously seen in studies with Ontak.

While no efficacy or safety issues were raised in the CRL, the FDA is requiring the Company to “incorporate enhanced product testing, and additional controls.”

“We intend to provide additional data and remain fully engaged with the FDA as we continue to work toward approval,” said Leonard Mazur, Chairman and CEO of Citius. “We remain confident in the potential of Lymphir to become an important addition to the treatment landscape for patients with relapsed or refractory CTCL and make a meaningful difference in their lives.”

This article originally appeared on MPR