Upon joining the study, an informed consent form must be signed and dated. This form confirms understanding and agreement to participate in the study.

Eligibility requires completion of a previous VX-670 study, specifically having received at least two doses of the study drug in Study 001, Part B.

An initial assessment will be conducted to evaluate current health status. This includes checking vital signs, performing an electrocardiogram (ECG), and conducting laboratory tests.

These assessments help ensure the safety and suitability for continued participation in the study.

The study involves receiving the medication VX-670 as a solution for injection/infusion. This is administered through a vein, a process known as intravenous administration.

The frequency and dosage of the medication will be determined by the study protocol and communicated during the study visits.

Regular visits are scheduled to monitor health and response to the medication. This includes checking for any side effects, known as adverse events, and assessing overall well-being.

Vital signs, ECGs, and laboratory tests will be repeated at these visits to ensure safety and tolerability of the treatment.

At Week 24, a muscle biopsy of the tibialis anterior muscle will be performed. This procedure involves taking a small sample of muscle tissue to study changes from the baseline.

The concentration of VX-670 and another compound, PMO-0221a, will be measured in both plasma and muscle tissues.

The study is estimated to end on November 16, 2029. Upon completion, a final assessment will be conducted to evaluate long-term safety and efficacy of the treatment.

Participants will receive information about the study results and any further steps if necessary.