"Today's action marks the first FDA approval of a therapy to treat this very rare and devastating disease in children," said Peter Marks, M.D., Ph.D., director of the FDA's Center for Biologics Evaluation and Research. "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."

Congenital athymia is a rare immune disorder in which a child is born without a thymus – an organ that plays a critical role in helping the body learn to fight infections. Children impacted by this disease typically die within the first two years of life and may have repeated, often life-threatening infections because they lack adequate working T cells (a kind of infection-fighting white blood cell).

Rethymic is composed of human allogeneic (donor-derived) thymus tissue that is processed and cultured, and then implanted into patients to help reconstitute immunity (improve immune function) in patients who are athymic. Dosing is patient–customized, determined by the surface area of the Rethymic slices and the body surface area of the patient. Rethymic is not indicated for the treatment of patients with severe combined immunodeficiency (SCID).

The safety and efficacy of Rethymic were established in clinical studies that included 105 patients, with ages from one month to 16 years, who each received a single administration of Rethymic, from 1993 – 2020. Rethymic improved survival of children with congenital athymia, and most children treated with this product survived at least two 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 that 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 six months or longer to reconstitute the immune function in treated patients; therefore, it is important that until immune reconstitution occurs patients continue to take strict precautions to prevent infections and healthcare providers should treat accordingly.

This application was granted a rare pediatric disease voucher by the FDA. A description of rare pediatric disease designation and voucher programs can be found in the Guidance for Industry: Rare Pediatric Disease Priority Review Vouchers.

The FDA granted approval of Rethymic to Enzyvant Therapeutics, Inc.

Healthcare professionals should report all serious adverse events suspected to be associated with the use of any drug, biological product or device to FDA's MedWatch Reporting System or by calling 1-800-FDA-1088.