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

This study will assess the efficacy and safety of PTC857 treatment in participants diagnosed with ALS.

Participants will be randomized to 1 of the 2 treatment groups: PTC857 or matching placebo. Following successful completion of the Treatment Period, participants who enter the LTE Period, will receive open-label PTC857 for 28 weeks. Following completion of the LTE period, participants who enter the Continued LTE Period will receive open-label PTC857 for an additional 108 weeks.

Eligibility Criteria:
Key Inclusion Criteria:

* ALS with preserved function, defined as:

  1. Onset of the first symptom leading to the diagnosis of ALS ≤24 months at the time of the initial Screening Visit
  2. 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% at the start of the Screening Period
* All chronic concomitant medications (both prescription and over the counter), and non-pharmacologic therapy regimens, excluding standard-of-care therapy riluzole, edaravone, or sodium phenylbutyrate/taurursodiol, should be stable and unchanged from 14 days prior to the start of the Screening Period and intend to remain stable and unchanged throughout the course of the study
* Female participants must have a negative breast cancer imaging screening status (not considered clinically abnormal and/or requiring further evaluation/treatment) within 6 months prior to the Screening Visit, or during the Screening Period.
* Standard-of-care therapy for the treatment of ALS (riluzole, edaravone, or sodium phenylbutyrate/taurursodiol) should be stable and unchanged from 30 (-3) days prior to the start of the Screening Period and intend to remain stable and unchanged throughout the course of the study.

Key Exclusion Criteria:

* 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
* Participant is receiving a combination of edaravone and sodium phenylbutyrate/taurursodiol treatment, where applicable, within 30 days prior to the start of the Screening Period
* For female participants, any past medical history of breast cancer, regardless of remission status, or any first degree relative with history of breast cancer