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

Key Inclusion Criteria:

• ALS with preserved function, defined as:

1. Onset of the first symptom leading to the diagnosis of ALS ≤24 months at thetime 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% atthe start of the Screening Period

• All chronic concomitant medications (both prescription and over the counter), andnon-pharmacologic therapy regimens, excluding standard-of-care therapy riluzole,edaravone, or sodium phenylbutyrate/taurursodiol, should be stable and unchangedfrom 14 days prior to the start of the Screening Period and intend to remain stableand unchanged throughout the course of the study

• Female participants must have a negative breast cancer imaging screening status (notconsidered 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 sodiumphenylbutyrate/taurursodiol) should be stable and unchanged from 30 (-3) days priorto the start of the Screening Period and intend to remain stable and unchangedthroughout 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 ofthe investigator would jeopardize the safety of the participant or impact thevalidity 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 theparticipant's ability to participate in the study or increase the risk ofparticipation for that participant

• Current participation in any other investigational study with an investigationalproduct or participation within 30 days prior to the start of the Screening Periodor 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 sodiumphenylbutyrate/taurursodiol treatment, where applicable, within 30 days prior to thestart of the Screening Period

• For female participants, any past medical history of breast cancer, regardless ofremission status, or any first degree relative with history of breast cancer