Study of Safety and Efficacy of ALT-100mAb in Participants With Moderate/Severe ARDS

PUERTA is a Phase 2a, randomized, double-blind, placebo-controlled study in adults with moderate to severe ARDS consequent to sepsis, septic shock, trauma, and/or bacterial or viral pneumonia who have been hospitalized. The safety and tolerability, PK, preliminary efficacy, and PD of a single infusion of ALT-100 mAb will be assessed. All participants will receive study drug within 12 hours of their ARDS diagnosis and within 4 hours of initiation of MV (mechanical ventilation). It is planned that 90 eligible participants will be randomized at a 2:1 ratio to receive a single dose of either ALT-100 mAb or placebo via IV infusion at the time the diagnosis of moderate to severe ARDS is confirmed. An additional 9 participants may be randomized if an optional cohort of low or intermediate ALT-100 mAb dose is enrolled. The study will be conducted in 2 parts: * Part A: a dose escalation phase followed by * Part B: dose expansion phase Dose Escalation (Part A) Part A will assess 2 doses of ALT-100 mAb in sequentially enrolled cohorts of up to 9 participants in each cohort. The planned doses of ALT-100 mAb are 0.4 mg/kg (Cohort 1a) and 1.0 mg/kg (Cohort 2a). Dose Expansion (Part B) Following SRC review of all data up to Day 29 from the 9 participants in each cohort in Part A, additional participants (up to 36 per dose cohort) may be enrolled into 2 dose expansion cohorts, the dose of which will be determined by the SRC. Part B will further explore the safety, preliminary efficacy, PK, and systemic biomarker profile of ALT-100 mAb. Participants enrolled in Part A may not be re-enrolled in Part B. The screening, Treatment, and Safety Follow-up schedules are the same for Part A (dose escalation) and Part B (dose expansion) cohorts.