Inclusion Criteria:
* Age ≥18 years at the time of consent.
* Acute Respiratory Distress Syndrome (ARDS) meeting the contemporary clinical definition including all of the following occurring within a 24-hour interval:
* Hypoxemia defined as PaO2/FiO2 ≤300 mmHg, or SpO2/FiO2 ≤315 if SpO2 is ≤97%;
* Bilateral pulmonary infiltrates/opacities on chest imaging consistent with pulmonary edema or fibroproliferative lung injury and not fully explained by pleural effusions, lobar collapse, or atelectasis;
* Requirement for positive pressure respiratory support, including invasive mechanical ventilation via endotracheal tube;
* Respiratory failure not fully explained by left atrial hypertension or cardiogenic pulmonary edema; if measured, pulmonary artery wedge pressure should be ≤18 mmHg.
* Late-stage, non-resolving ARDS, defined as:
* At least 7 days and no more than 28 days since ARDS onset at the time of enrollment; and
* Persistent bilateral infiltrates since ARDS onset; and
* Continued need for substantial respiratory support since ARDS onset.
* On the day of enrollment, ongoing respiratory impairment demonstrated by:
* PaO2/FiO2 ≤300 mmHg, or
* SpO2/FiO2 ≤315 if SpO2 is ≤97%.
* Subject is receiving invasive mechanical ventilation, or other protocol-defined intensive respiratory support as approved by the Sponsor/Medical Monitor.
* The treating team considers the subject an appropriate candidate for enteral administration of study drug, including by nasogastric (NG), orogastric (OG), or other enteral tube, and there is an intention to provide enteral nutrition or medication administration through the gastrointestinal tract.
* For subjects unable to provide consent, legally authorized representative (LAR) consent is obtainable in accordance with local regulations and Institutional Review Board (IRB)/Ethics Committee requirements.
* The investigator judges that the subject is likely to remain under protocol-compatible management and follow-up for the duration of the treatment and key follow-up period.
Exclusion Criteria:
* Age \<18 years.
* More than 28 days since onset of ARDS at the time of enrollment.
* ARDS that is clearly resolving rapidly, in the judgment of the investigator, such that enrollment in a 28-day rescue-therapy study is not clinically appropriate.
* Pregnancy or breastfeeding.
* Participation in another interventional investigational drug or biologic study within 30 days prior to enrollment, or within 5 half-lives of the investigational product, whichever is longer, unless approved by the Sponsor and Medical Monitor.
* Extracorporeal support for gas exchange at the time of study entry, including ECMO.
* Inability to use the gastrointestinal tract for study drug administration, including but not limited to:
* Continuous gastric suction/drainage that would preclude dosing,
* Bowel discontinuity preventing enteral delivery,
* Open abdomen,
* Dependence on total parenteral nutrition with no enteral access,
* Other conditions that, in the investigator's judgment, make enteral dosing infeasible.
* Known or suspected bowel ischemia, gastrointestinal perforation, uncontrolled GI bleeding, or other severe gastrointestinal disorder that would substantially increase risk from enteral dosing.
* Not committed to full supportive care, with the exception that subjects with a do-not-resuscitate order may be eligible if all other indicated intensive supportive treatments are being provided.
* AIDS by CDC criteria, including documented AIDS-defining illness or CD4 count \<200 cells/mm³, if known.
* Severe immunosuppression, including any of the following:
* Cytotoxic chemotherapy within 3 weeks prior to enrollment,
* High-dose corticosteroid exposure defined as cumulative prednisone ≥300 mg equivalent within 21 days prior to enrollment,
* Ongoing systemic corticosteroid therapy \>15 mg/day prednisone equivalent within 7 days prior to enrollment, unless justified as part of standard ARDS care and approved by the Medical Monitor,
* Other severe acquired or iatrogenic immunodeficiency considered clinically significant by the investigator.
* Active uncontrolled infection outside the lung that, in the judgment of the investigator, would confound safety assessment or increase subject risk.
* Severe chronic respiratory disease likely to confound ARDS assessment or outcomes, including:
* Advanced COPD requiring chronic home oxygen,
* Clinically significant pulmonary fibrosis or interstitial lung disease,
* Other severe chronic lung, chest wall, or neuromuscular disorders causing chronic respiratory failure.
* Diffuse alveolar hemorrhage due to vasculitis.
* Malignancy or other irreversible chronic condition for which estimated 6-month mortality is ≥50%, in the investigator's judgment.
* Morbid obesity or body habitus that would substantially impair protocol assessments or ventilatory interpretation, if deemed clinically significant by the investigator.
* Any condition that, in the opinion of the investigator or Medical Monitor, would make participation unsafe, interfere with interpretation of study results, or be inconsistent with study objectives.
