FDA Clears Thymus Tissue Product to Treat Congenital Athymia

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The product, called Rethymic, is composed of human allogeneic thymus tissue that is processed and cultured and then implanted into children born without a thymus to help reconstitute immunity.

The FDA has approved the nation’s first thymus tissue product in order to treat children with congenital athymia, an ultra-rare immune disorder where they are born without a thymus.

The product, called Rethymic, is composed of human allogeneic thymus tissue that is processed and cultured and then implanted into patients to help reconstitute immunity in those who are athymic. Dosing is customized, determined by the surface area of the Rethymic slices and the body surface area of the patient.

Children born with the condition typically die within 2 years and may have repeated, often life-threatening infections because they lack adequate working T cells. The estimated incidence is 17 to 24 live births each year in the United States.

“Today’s action marks the first FDA approval of a therapy to treat this very rare and devastating disease in children,” Peter Marks, MD, PhD, director of the FDA’s Center for Biologics Evaluation and Research, said in a statement. “We remain committed to helping advance the development of safe and effective medical products for patients affected by rare diseases—an area of such critical need.”

The approval was granted to Enzyvant Therapeutics; the application was granted a rare pediatric disease voucher by the FDA.


The safety and efficacy of Rethymic were established in clinical studies that included 105 patients, aged 1 month to 16 years who each received a single administration of Rethymic, from 1993 to 2020. Rethymic improved the survival of children with congenital athymia, and most children treated with this product survived at least 2 years. Children treated with Rethymic who survive past the first year generally survive long term. Rethymic also reduced the frequency and severity of infections over time.

The most common adverse reactions in patients who received Rethymic include high blood pressure, cytokine release syndrome, low blood magnesium levels, rash, low platelets, and graft-versus-host disease.

Because Rethymic is derived from human tissue, it carries a risk of transmitting infectious disease. Based on effective donor screening procedures and product manufacturing processes, the risk of infectious disease transmission is remote, but not completely eliminated.

It takes 6 months or longer to reconstitute the immune function in treated patients. The FDA noted that until immune reconstitution occurs, patients must continue to take strict precautions to prevent infections and health care providers should treat accordingly.

The product is not indicated for the treatment of patients with severe combined immunodeficiency.