This research study is designed to assess the safety and tolerability of an investigational drug called AT-1501, which is a monoclonal antibody. Antibodies are Y-shaped proteins that are produced naturally by your immune system to attack and fight foreign substances that cause illness. Monoclonal antibodies are man-made proteins manufactured to serve as substitute antibodies to fight diseases. Monoclonal antibodies can restore, enhance or mimic (copy) the immune system's attack process, they can also tone down the immune system, which is how the investigational drug is thought to work. It is believed that the immune system plays a role in the development of ALS.