Study to Assess the Effects of PTC857 Treatment in Participants With Amyotrophic Lateral Sclerosis ALS

Open to people ages 18-70

• Males or females aged between 18 and 70 years with a body mass index between 18 and 35 kilograms/meters squared
• ALS with preserved function, defined as:
• Onset of the first symptom leading to the diagnosis of ALS ≥7 and ≤18 months at the time of the initial Screening Visit
• Revised EL Escorial criteria of either:

(i) Clinically definite ALS (ii) Clinically probable ALS

• A total ALSFRS-R score of at least 34 at the start of the Screening Period
• No significant respiratory compromise as evidenced by slow vital capacity ≥60%
• All concomitant medications (both prescription and over the counter) and non-pharmacologic therapy regimens should be stable and unchanged from 30 days prior to the start of the Screening Period and intend to remain stable and unchanged throughout the course of the study, with the exception of prohibited medications

• Females who are pregnant or nursing or plan to become pregnant during the study
• Participants with clinically significant gastrointestinal, renal, hepatic, neurologic, hematologic, endocrine, oncologic, pulmonary, immunologic, psychiatric, or cardiovascular/ischemic disease or any other condition that, in the opinion of the investigator, would jeopardize the safety of the participant or impact the validity of the study results
• Any clinically significant medical or psychiatric condition or medical history that, in the opinion of the investigator or the medical monitor, would interfere with the participant's ability to participate in the study or increase the risk of participation for that participant
• Current participation in any other investigational study with an investigational product or participation within 30 days prior to the start of the Screening Period or 5 half-lives of the previously taken investigational drug, whichever is longer
• Participant has previously received PTC857
• Edaravone treatment, where applicable, within 30 days prior to the start of the Screening Period