FDA Approves First Treatment for Rare MCT8 Deficiency, Allan-Herndon-Dudley Syndrome

The US Food and Drug Administration (FDA) has approved tiratricol (Emcitate) for treating peripheral thyrotoxicosis in adults and children with monocarboxylate transporter 8 (MCT8) deficiency, also known as Allan-Herndon-Dudley syndrome. The approval marks the first treatment specifically approved for this rare genetic disorder, offering a new therapeutic option for affected patients.

Emcitate is the first therapy approved by the FDA to treat symptoms of this very rare, devastating genetic disease. This approval reflects the FDA’s dedication to patients with rare genetic diseases, many of whom have serious unmet medical needs.

“Until now, patients living with MCT8 deficiency and their families had no FDA-approved treatment option,” said Marina Zemskova, M.D., Deputy Director of the Division of General Endocrinology in the FDA’s Center for Drug Evaluation and Research. “Emcitate’s approval reflects the FDA’s deep commitment to patients with rare conditions who are in need of meaningful treatment options.”

MCT8 deficiency is a rare genetic disorder that primarily affects males. The condition is caused by a faulty gene that provides instructions for making the MCT8 transporter, a critical protein responsible for carrying thyroid hormone into the brain. Because thyroid hormone cannot cross the blood-brain barrier without the MCT8 transporter, the brain receives too little of this hormone, while excessive levels of the hormone build up in the bloodstream. Many patients with MCT8 deficiency experience a range of debilitating effects, including the inability to walk or sit independently, absent or severely limited speech, intellectual disability, feeding difficulties and chronic stress on the heart and metabolism.

“The challenge in treating MCT8 deficiency has always been that the protein needed to deliver thyroid hormone into cells is the one that isn’t working,” said Hylton V. Joffe, M.D., MMSc, Director of the Office of Cardiology, Hematology, Endocrinology, and Nephrology in the FDA’s Center for Drug Evaluation and Research. “This drug sidesteps that problem, as its active ingredient, tiratricol, can enter cells on its own without relying on the broken transporter, leading to a decrease in the elevated blood thyroid hormone levels.”

The effectiveness of Emcitate was evaluated in two clinical studies in patients ranging in age from infants to adults, including an international, multi-center, randomized, placebo-controlled trial (NCT05579327) and a longer-term open-label study. Across both studies, patients treated with Emcitate had reductions in excess thyroid hormone levels in the bloodstream and improvements in cardiovascular and metabolic symptoms impacted by thyroid levels, such as systolic blood pressure and heart rate.

Emcitate is taken once daily as a liquid suspension, either by mouth or through a feeding tube for patients who have difficulty swallowing, making it accessible for patients with a wide range of abilities. The most common side effects were diarrhea, vomiting, rash and excessive sweating. Patients taking another thyroid medication should talk to their healthcare provider before starting Emcitate, as the two should not be used together.