The purpose of this study is to investigate the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of TCD601 (siplizumab) in newly diagnosed adult ALS patients.

The purpose of this study is to investigate how safe and tolerable the study drug siplizumab is when given to adult patients with ALS. The study will examine how the drug is distributed in the body and what effects it has on different immune cells in the blood and in the fluid surrounding the motor neurons (cerebrospinal fluid). In addition, the study will monitor the progression of ALS and look for any changes related to the disease that may occur during the study.

Key Inclusion Criteria:

• Male or female patients ≥ 18 to 80 years of age.
• Diagnosis of ALS by revised El Escorial Criteria, at study entry within 24 months of first symptoms.
• Patients on existing ALS treatment must have been on a stable dose for 28 days.

Key Exclusion Criteria:

• Patient with severe systemic infections, current or within the two weeks prior to randomization.
• Subjects who, in the opinion of the investigator, are not capable of giving informed consent for the study or who are unable or unwilling to adhere to the study requirements outlined in the protocol.
• Use of other investigational products or treatment in another investigational drug study within 30 days of screening
• Pregnant or nursing (lactating) women.