A Study to Evaluate, Safety, Tolerability, Pharmacodynamic (PD) Markers and Pharmacokinetics (PK) of AP-101 in Participants With Amyotrophic Lateral Sclerosis (ALS)

Inclusion Criteria: * All participants must adhere to contraception restrictions * Female participants of childbearing potential must adhere to contraception restrictions * Have possible, clinically probable, clinically probable-laboratory supported or definite familial or sporadic ALS in accordance with the El-Escorial criteria or who have a diagnosis of ALS as defined by the Gold Coast Criteria; progressive motor impairment documented by history or repeated clinical examination, preceded by normal motor development, and presence of upper and lower motor neuron dysfunction in at least 1 body region or lower motor neuron dysfunction in at least 2 body regions and investigations excluding other conditions * In familial ALS participants, a confirmed pathogenic superoxide dismutase 1 (SOD1) mutation * Onset of symptoms (i.e, weakness) within past 24 months prior to screening, at the time of obtaining informed consent * Have slow vital capacity (SVC) of greater than or equal to (\> or =) 50 percentage (%) of predicted values. Participants with SVC of \<50% of predicted values may be permitted to enter the open-label extension, based on the opinion of the investigator * Absence of bilevel positive airway pressure (BiPAP)/proportional assist ventilation (PAV) \> 4 hours for symptoms attributable to ALS. Use of a CPAP for pre-existing conditions will be allowed * If on riluzole, must be on a stable dose. * If on edaravone, must have completed 2 cycles and are expected to remain on …