Phase III Efficacy and Safety Study of AB103 in the Treatment of Patients With Necrotizing Soft Tissue Infections

The primary hypothesis of this study is that in addition to standard of care treatment (which includes surgical intervention, antimicrobial therapy and critical care support for organ dysfunction or failure), AB103 will demonstrate a clinically significant treatment benefit over placebo. This hypothesis will be addressed by measuring the effect of AB103 on a composite of clinical parameters associated with the disease course of patients with NSTI, using a responder analysis. A responding patient must meet all 5 parameters of the composite clinical success end point, while a non-responding patient can fail by not meeting any one of the parameters. These analyses are designed to demonstrate that in addition to being safe, one dose of 0.5 mg/kg of AB103 will: Improve systemic signs of the infection by improving organ function of patients compared to placebo as measured by: * Survival at Day 28 * Modified SOFA (mSOFA) score on Day 14 and change from baseline to Day 14 ≥ 3. A Day 14 mSOFA score of ≤1 and a change from baseline (pre-treatment) to Day 14 ≥3 will be required for a patient to achieve the primary composite clinical success endpoint (NICCE) Improve the local signs of the infection, as measured by: * Reduced number of debridements, counted to Day 14. No more than 3 debridements to Day 14 will be required for a patient to achieve composite clinical success * No amputation after the first debridement (amputation on the first debridement is not considered a failure). A patient will be required to have had no amputations done after the first surgical procedure in order to achieve composite clinical success. 290 patients will be recruited into the study and randomized to receive either 0.5 mg/kg AB103 or placebo in a 1:1 ratio. Randomization will be stratified within center by the diagnosis of Fournier's Gangrene and mSOFA score category (3-4 vs \>4) at screening. The study will be conducted with interim analyses for futility at 100 patients and safety monitored by an independent Data Monitoring Board at regular planned intervals.