Open Label Extension to Assess Long Term Safety and Efficacy of KL1333 in Patients With Primary Mitochondrial Disease

An Open-label, Single-arm Extension Study to Evaluate the Long-term Safety, Tolerability, and Efficacy of KL1333 (Napazimone) in Patients With Primary Mitochondrial Disease

The purpose of this study is to investigate if the study medicine, KL1333, is safe, well-tolerated and effective long-term in improving the symptoms of fatigue and impacts on daily living and functional capacity (physical abilities) in people with PMD.

This is a 12-month open-label extension (OLE) study to evaluate the safety, tolerability, and efficacy of KL1333 in subjects previously treated with KL1333 or placebo in Study KL1333-2020-104A (hereafter referred to as FALCON).

Subjects can be enrolled in this extension study either directly at the FALCON study completion visit (FALCON Week 48) or the safety follow-up visit (FALCON Week 53) or later in time. Subjects who previously received KL1333 in FALCON will receive retreatment whereas subjects previously treated with placebo will be treatment naïve to KL1333.

The study consists of a screening visit (if the subject rolls over directly from the FALCON study completion visit [FALCON Week 48] or the safety follow up visit [FALCON Week 53], the screening visit is coincident with the visit), a 48-week treatment course with KL1333 up to 100 mg/day, a completion visit (end of treatment; EoT) and approximately 5 weeks of follow up, including the end of study (EoS) visit. The treatment period may extend beyond 48 weeks until the study drug is commercially or otherwise available, in which case the EoT visit will occur later than Week 48. Periodic safety monitoring visits (phone visits every 4 weeks after Week 48 and clinic visits every 24 weeks after Week 48) will continue for subjects who receive KL1333 in the optional extended treatment period until the study drug is commercially or otherwise available.

Inclusion Criteria:

• Completed the FALCON study (age 18 years or older), and in the opinion of the investigator and sponsor has been compliant with the study requirements
• Willingness and ability to attend study appointments within the specified time windows
• Willingness and ability to complete electronic patient-reported outcomes
• Concomitant medications likely to remain stable throughout participation in the study where clinically possible
• Willingness to suspend treatment with idebenone during the study

Exclusion Criteria:

• The subject is, in the investigator's opinion, unlikely to comply with the protocol, e.g., due to cognitive impairment, or is unsuitable for any reason.
• Any medical, psychiatric, laboratory or other condition that may negatively affect the benefit-risk considerations of study participation or interfere with the interpretation of study results and, in the judgment of the investigator and/or the medical monitor, would make the subject inappropriate for entry into this study.

Subjects not enrolling directly at the FALCON study completion visit (FALCON Week 48) or the safety follow-up visit (FALCON Week 53) study will be required to fulfill the additional exclusion criteria below during the screening visit:

• General fatigue or muscle weakness due to causes other than mitochondrial disease, in the opinion of the investigator.