(Reuters) - Privately held drug developer Enzyvant said on Thursday the U.S. Food and Drug Administration declined to approve its regenerative tissue therapy for a rare immunodeficiency disorder and raised concerns about its manufacturing.

The company was hoping to win its first approval for the therapy, RVT-802, aimed at treating congenital athymia, a disorder affecting babies born without a small gland called thymus, which produces T-cells needed to regulate the immune system.

The health regulator in a letter to Enzyvant raised questions about the manufacturing process for the treatment as well other issues based on its inspection of the manufacturing site, the company said.

“Many of these are topics that we are very aware of and we anticipated those to be post-marketing commitments, not approval requirements,” Chief Executive Officer Rachelle Jacques told Reuters on a phone call.

“The good news is there’s no requirement for us to foresee any additional animal studies or any additional clinical trials.”

The company said it had planned to manufacture its treatment through a third-party manufacturer.

If approved, RVT-802 would have been the first to win an FDA nod under the agency’s Regenerative Medicine Advanced Therapy (RMAT) designation, granted to drug developers making regenerative therapies for conditions lacking treatment options.

Babies born with congenital athymia - about 17 to 24 cases in the United States every year - usually do not survive beyond the age of two.

RVT-802 is a tissue-based therapy, manufactured by sourcing thymus tissue from infants undergoing heart surgeries unrelated to congenital athymia, and administered only once.

Ten clinical studies spanning over two decades and 85 patients were conducted at North Carolina’s Duke University, where the treatment was developed.

Enzyvant is currently owned and entirely funded by Swiss drug developer Roivant, but will become here a subsidiary of Japan-based Sumitomo Dainippon in a deal expected to close next year.

“We would anticipate that the deal would close before we could fully address the issues in the (FDA letter),” Jacques said.