Arsanis Inc., an infectious disease specialist developing a targeted monoclonal antibody to prevent Staphylococcus aureus pneumonia, has closed on a $45.5 million series D financing that will help it complete a phase II trial of candidate ASN-100 in high-risk mechanically ventilated patients. The new funds will also support the advancement of preclinical gram-negative programs, as well as a respiratory syncytial virus (RSV) program.