As of March 13, 2019, Arsanis, Inc. was acquired by X4 Pharmaceuticals Inc., in a reverse merger transaction. Arsanis, Inc., a clinical-stage biopharmaceutical company, focuses on applying monoclonal antibody (mAb) immunotherapies to address infectious diseases in the United States. The company engages in the development of ASN500, a mAb that is in preclinical development targeting respiratory syncytial virus (RSV), a virus that can cause respiratory tract infections in young children and elderly, and immunocompromised patients. Its pipeline also comprises mAbs targeting various serious bacterial and viral pathogens, including Staphylococcus aureus and RSV. In addition, the company’s preclinical stage Gram-negative mAb programs include ASN300 for Klebsiella pneumoniae and ASN200 for Escherichia coli. Arsanis, Inc. was founded in 2010 and is headquartered in Waltham, Massachusetts.