Inclusion Criteria:

- Participant agrees and is able to adhere to the study requirements for the length of
the study, including performing 6MWT, as well as the use of digital applications and
video recordings.

- Diagnosed with primary mitochondrial myopathy (PMM), consisting of the following:

- Molecular genetic abnormality (i.e., nuclear or mitochondrial) known to be
associated with causing mitochondrial dysfunction (such as, but not limited to,
mitochondrial DNA (mtDNA) single, variable deletions in chronic progressive
external ophthalmoplegia (CPEO) and Kearns-Sayre syndrome (KSS); mtDNA m.3243 A >
G common mutation in mitochondrial encephalomyopathy with lactic acidosis and
stroke-like episodes (MELAS); pathogenic nuclear or mitochondrial genome variants
demonstrated to cause primary mitochondrial disease), and

- Participant reported symptoms (i.e., muscle weakness, fatigue and exercise
intolerance) or physical examination findings of myopathy that are the
predominant symptoms of the participant's mitochondrial disorder.

- Participant has been on stable dose regimen of coenzyme Q10 (CoQ10), carnitine,
creatine or other mitochondrial disease-focused vitamins or supplemental therapies for
3 months prior to randomization and intends to stay on a stable dose for duration of
study period (for participants who take any above-mentioned medications or
supplements).

- Participant has been on stable exercise regimen within 4 weeks prior to randomization
and intends to stay on a stable regimen for duration of study period (for participants
who participate in a regular exercise regimen).

- Female participant is not pregnant and at least one of the following conditions apply:

- Not a woman of childbearing potential (WOCBP).

- WOCBP who agrees to follow the contraceptive guidance from the time of informed
consent through at least 30 days after final study treatment administration.

- Female participant must agree not to breastfeed starting at screening and throughout
the study period and for 30 days after final study treatment administration.

- Female participant must not donate ova starting at first dose of IP and throughout the
study period and for 30 days after final study treatment administration.

- Male participant with female partner(s) of childbearing potential (including
breastfeeding partner) must agree to use contraception throughout the treatment period
and for 30 days after final study treatment administration.

- Male participant must not donate sperm during the treatment period and for 30 days
after final study treatment administration.

- Male participant with pregnant partner(s) must agree to remain abstinent or use a
condom for the duration of the pregnancy throughout the study period and for 30 days
after final study treatment administration.

- Participant agrees not to participate in another interventional study while
participating in the present study.

Open-label Extension Continuation Criteria:

- Participant must meet all of the following OLE criteria at the week 52 study visit in
the treatment period to be eligible for OLE:

- Participant must continue to be able and willing to adhere to the study
requirements.

- Participant who is eligible to continue in OLE.

Exclusion Criteria:

- Participant has additional signs and/or symptoms due to non-myopathic process (e.g.,
cerebellar dysfunctions, movement disorder, peripheral neuropathy, stroke or other) or
a gait problem not attributed to the myopathy that would interfere/may in addition to
the myopathy affect the participant's performance during 6MWT or 5 times sit to stand
(5XSTS).

- Participant has received any investigational therapy within 28 days or 5 half-lives,
whichever is longer, prior to screening.

- Participant has any condition, which makes the participant unsuitable for study
participation.

- Participant has cardiac troponin I (cTnI) > upper limit of normal (ULN) at screening
and is assessed as clinically significant.

- Participant has estimated glomerular filtration rate (eGFR) calculated by the
Modification of Diet in Renal Disease equation < 60 mL/min/1.73 m^2 at screening.

- Participant has at screening: total bilirubin (TBL) > ULN or transaminase(s)
(aspartate aminotransferase [AST] or alanine aminotransferase [ALT]) > ULN in the
absence of elevations in creatine kinase (CK).

- Participant has psychiatric conditions such as schizophrenia, bipolar disorder or
major depressive disorder that has not been under control within 3 months prior to
screening.

- Participant has a history of suicide attempt, suicidal behavior or has any suicidal
ideation within 1 year prior to screening that meets criteria at a level of 4 or 5 by
using the Columbia-Suicide Severity Rating Scale (C-SSRS) or who is at significant
risk to commit suicide.

- Participant has severe behavioral or cognitive problems that preclude participation in
the study.

- Participant has undergone an in-patient hospitalization that precludes participation
in the study, within the 30 days prior to the randomization.

- Participant has a planned hospitalization or a surgical procedure during the study,
which may affect the study assessments.

- Participant has clinically significant and unstable respiratory disease and/or cardiac
disease (medical history or current clinical findings), or prior interventional
cardiac procedure (e.g., cardiac catheterization, angioplasty/percutaneous coronary
intervention, balloon valvuloplasty, etc.) within 3 months prior to randomization.

- Participant has a corrected mean QT interval using Fridericia's correction (QTcF) >
450 msec for male participants and > 480 msec for female participants at screening or
randomization. If QTcF exceeds these limits, one additional triplicate ECG can be
repeated on the same day in order to determine the participant's eligibility.

- ECG evidence of acute ischemia, atrial fibrillation or active conduction system
abnormalities with the exception of any of the following:

- First degree atrioventricular (AV)-block

- Second degree AV-block Type 1 (Mobitz Type 1/Wenckebach type)

- Right bundle branch block

- Left fascicular block

- Bi-fascicular block

- Participant requires 24/7 ventilator support (those who require nocturnal ventilation
support such continuous positive airway pressure [CPAP] or bilevel positive airway
pressure [BiPAP], due to nighttime hypoxia from chest muscle weakness or obstructive
sleep apnea are allowed).

- Participant has severe vision impairment that may interfere with their ability to
complete all study requirements.

- Participant has an intractable seizure disorder that may interfere with their ability
to complete all study requirements.

- Active malignancy or any other cancer from which the participant has been disease-free
for < 5 years, except for curative treated localized non-melanoma skin cancer (e.g.,
basal cell or squamous cell carcinoma).

- Participant has a solid organ transplant and/or is currently receiving treatment with
therapy for immunosuppression.

- Participant has severe scoliosis or kyphoscoliosis that significantly impair
respiratory capacity and pulmonary function tests or limit positioning due to pain who
would be likely to require orthopedic surgical intervention within a year after study
randomization.

- Participant has a positive for human immunodeficiency virus (HIV), hepatitis B or
hepatitis C infection at screening.

- Participant has previously received ASP0367.

- Participant has a history of active substance abuse within 1 year prior to
randomization.

- Participant has used any peroxisome proliferator-activated receptor (PPAR) ligands
such as fibrates and thiazolidinediones within 4 weeks prior to randomization.

- Participant has initiated the use of CoQ10, carnitine, creatine or other mitochondrial
disease-focused supplements within 3 months prior to study randomization.

- Participant has a known or suspected hypersensitivity to ASP0367 or any components of
the formulation used.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

Eligibility last updated 12/20/22. Questions regarding updates should be directed to the study team contact.