A Study to Evaluate the Efficacy and Safety of Reldesemtiv in Patients With Amyotrophic Lateral Sclerosis (ALS)

Key Inclusion Criteria: * Males or Females between the ages of 18 and 80 years of age, inclusive * Diagnosis of familial or sporadic ALS (defined as meeting the laboratory-supported probable, probable, or definite criteria for ALS according to the World Federation of Neurology El Escorial criteria). Patients who meet the possible criteria are eligible if they have lower motor neuron findings; those who have purely upper motor neuron findings are ineligible. * First symptom of ALS ≤ 24 months prior to screening. The qualifying first symptoms of ALS are limited to manifestations of weakness in extremity, bulbar, or respiratory muscles. * ALSFRS-R total score ≤ 44 at screening. Patients with a total score of 45 or higher may be rescreened 60±7 days following the original screening date. * Upright FVC ≥ 65.0% of predicted for age, height, sex and ethnicity at screening according to Global Lung Initiative equation * Must be either on riluzole for ≥ 30 days prior to screening or have not taken it for at least 30 days prior to screening * Must have completed at least 2 cycles of edaravone at the time of screening or have not received it for at least 30 days prior to screening * Able to swallow whole tablets Exclusion Criteria: * eGFRCysC \< 45.0 mL/min/1.73 m2 at screening * Urine protein/creatinine ratio \> 1 mg/mg (113 mg/mmol) at screening * Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≥ 3-times the upper limit of normal (ULN) * Total bilirubin (TBL), direct or indirect bilirubin above the ULN. * Cognitive impairment, related to ALS or otherwise that impairs the patient's ability to understand and/or comply with study procedures and provide informed consent * Other medically significant neurological conditions that could interfere with the assessment of ALS symptoms, signs or progression. * Has a tracheostomy