This is a phase I/II, randomized, double-blind, placebo-controlled clinical trial that will evaluate the safety and potential efficacy of therapy with extracellular vesicles (EVs) obtained from mesenchymal stromal cells (MSCs), patients with moderate to severe acute respiratory distress syndrome due to COVID-19 or other etiology. Participants will be allocated to receive EVs obtained from MSCs or placebo (equal volume of Plasma-Lyte A). Blinding will cover the participants, the multidisciplinary intensive care team and the investigators.

The studied population will consist of 15 patients diagnosed with acute respiratory failure syndrome admitted to the intensive care unit of Hospital São Rafael. This research aims to assess the safety and potential effectiveness of intravenous therapy using extracellular vesicles derived from mesenchymal cells in patients with moderate to severe acute respiratory distress syndrome. The treatment group will receive intravenous administration of extracellular vesicles obtained from mesenchymal cells, while the control group will receive a placebo.

EV group: will consist of 10 participants who will receive two infusions of 25 mL of the investigational product (Plasma-Lyte A solution containing EVs obtained from MSCs), intravenously, at intervals of 48 h.

Placebo group: will consist of 5 participants who will receive an equal volume of Plasma-Lyte A, intravenously, following the same schedule as the IV group: two infusions with an interval of 48 hours.

Inclusion Criteria:

• Age >= 18 years old;
• Chest CT radiological image with ground-glass opacities or chest X-ray with - bilateral infiltrates characteristic of pulmonary edema;
• In invasive mechanical ventilation with PEEP 5 cm H2O and PaO2/FiO2<250mmHg;
• Respiratory failure not explained by cardiac causes or fluid overload.

Exclusion Criteria:

• Unable to provide informed consent;
• Pregnancy or breastfeeding;
• Patients with active malignancy who have received chemotherapy in the last 2 years;
• Life expectancy of less than 6 months or in exclusive palliative care;
• Severe liver failure, with a Child-Pugh score > 12;
• Previous renal failure: patients already undergoing dialysis or patients with GFR < 30ml/min/1.73 m2
• Clinical or radiological suspicion of tuberculosis;
• Chronic respiratory failure;
• Use of ECMO;
• Moribund (high probability of death within the next 48 hours).

The treatment and placebo solutions will be indistinguishable in terms of their appearance. Stringent protocols will be established to guarantee that the physicians responsible for assessing patient safety, evaluating efficacy outcomes, and conducting laboratory analyses will be blinded to the randomization codes.