The U.S. Food and Drug Administration has approved the first treatment for MCT8 deficiency, a very rare genetic disorder that can cause severe neurological, cardiovascular and metabolic complications.

The FDA approved Emcitate (tiratricol) tablets for oral suspension to treat peripheral thyrotoxicosis, the agency said in a news release. Peripheral thyrotoxicosis involves excess thyroid hormone levels in the blood that can cause symptoms including rapid heart rate, increased blood pressure and adverse effects on metabolism.

Also known as Allan-Herndon-Dudley syndrome, MCT8 deficiency 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, according to the FDA.

Because thyroid hormone cannot cross the blood-brain barrier without the MCT8 transporter, the brain receives too little of the hormone while excessive levels build up in the bloodstream. The FDA said many patients experience debilitating effects including an inability to walk or sit independently, absent or severely limited speech, intellectual disability, feeding difficulties and chronic stress on the heart and metabolism.

“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, in the news release. “Emcitate’s approval reflects the FDA’s deep commitment to patients with rare conditions who are in need of meaningful treatment options.”

The treatment is designed to bypass the faulty transporter. Tiratricol, the active ingredient in Emcitate, can enter cells without relying on the MCT8 transporter, the FDA said. This allows it to address elevated thyroid hormone levels in the bloodstream.

“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 involving patients ranging from infants to adults. The studies included an international, multicenter, 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 affected by thyroid hormone levels, including systolic blood pressure and heart rate, the FDA said.

Emcitate is taken once daily as a liquid suspension, either by mouth or through a feeding tube for patients who have difficulty swallowing. The administration method makes the treatment accessible to patients with a wide range of abilities, according to the agency.

The most common side effects were diarrhea, vomiting, rash and excessive sweating.

Patients taking another thyroid medication should talk with their health care provider before starting Emcitate because the two medications should not be used together, the FDA said.

Emcitate was granted Orphan Drug, Rare Pediatric Disease, Fast Track and Breakthrough Therapy designations, as well as Priority Review.

The FDA granted approval of Emcitate to Egetis Therapeutics US Inc.