The goal of this program is to provide expanded access (i.e., before marketing authorization) to tiratricol as treatment for patients with monocarboxylate transporter 8 deficiency (MCT8 deficiency, also known as Allan-Herndon-Dudley syndrome [AHDS]), who in their Treating Physician's opinion, could benefit from tiratricol and meet the eligibility criteria.

This will be an open-label, single-arm, multi-center EAP, designed to provide treatment access in the US to tiratricol for eligible patients with MCT8 deficiency, also known as AHDS.

Patients will undergo clinical and safety assessments before tiratricol treatment initiation, during the titration phase and then at approximately 3-6 monthly intervals or more frequently if clinically indicated, following the initiation of tiratricol treatment and until treatment is completed or discontinued.

Adverse events (AEs) and serious adverse events (SAEs) will be recorded and reported as per FDA regulations.

Inclusion Criteria:

Each patient must meet all of the following criteria to be eligible:

1. Diagnosis of MCT8 deficiency confirmed with a genetic test.
2. Either tiratricol treatment naïve, or patients who may be on a stable dose of tiratricol having transferred from the Phase 3 MCT8-2021-3 (ReTRIACt) study (NCT05579327) or prior individual investigational new drug (IND).
3. In the Treating Physician's medical opinion, the potential benefits of treatment with tiratricol outweigh the potential risks for the patient.
4. Patient or legal representative provided signed and dated informed consent to be treated with tiratricol, through this EAP.
5. Given the severity of the disease, sexual activity in these patients is deemed unlikely. However, where, at the discretion of the Treating Physician sexual activity is possible for the patient, patients must follow protocol-specified-contraception guidance.
6. Patient is approved for enrolment by the sponsor RTT.

Exclusion Criteria:

Patients who meet any of the following criteria will be excluded from the EAP:

1. Parents, legal representative or, if applicable, patients unwilling or unable to comply with the Treating Physician's treatment plan related to this EAP for any reason.

2. Major illness or recent major surgery unrelated to MCT8 deficiency (in the Treating Physician's judgement), defined as:

   • Conditions requiring repeated hospitalizations that are likely to confound ability to participate in the program.
   • Major illness in the 3 months before the screening visit that is likely to confound the ability of the patient to participate fully within the program and/or confound the assessment of serum total T3 and/or safety.
   • Major surgery within the 3 months before the screening visit, or planned to take place during the program, including but not limited to major abdominal/thoracic/neurosurgical procedures.
   • Major/minor abdominal and/or maxillofacial surgery that may inhibit the administration and/or absorption of tiratricol.

3. Patients with any contra-indication for treatment with tiratricol or any excipients in the program treatment.

4. Patients using thyroid hormone analogues- such as levothyroxine -or thionamides, such as propylthiouracil. Prior use of these drugs is not an exclusion criterion, provided the use of the medication has subsided and the thyroid hormone levels have stabilized after the cessation of these medications. For patients currently using these medications the switch to tiratricol should be made following the above and under the guidance of an endocrinologist with knowledge of MCT8 deficiency, if needed, after consultation with pharmacologist.

5. Known hypersensitivity to tiratricol including any ingredient in the pharmaceutical formulation.

6. Although very unlikely, as this is a severe X-linked disease: Women who are nursing or pregnant (or women who are planning to become pregnant during treatment with tiratricol).

7. Patients eligible for clinical trials with tiratricol.