A multicenter, randomized, double-blind, placebo-controlled, phase II-III clinical trial to evaluate the efficacy and safety of the study drug in adult patients with pulmonary arterial hypertension (PAH).

The trial will evaluate the efficacy and safety of both single and multi-dose (seven-day) administrations of the study drug in three dosages: 0.03 mg/kg, 0.10 mg/kg, and 0.17 mg/kg.

The trial is an observer-blinded trial, since the appearance of the study drug and Placebo differs from each other. Patients included in the trial, as well as trial coordinators, investigators, and study center staff will be blinded. The only unblinded staff in the study centers will be those who preparing IP/Placebo and administering IP/Placebo to the study patients.

The trial will include 200 patients with PAH, who will be sequentially included in the trial in two stages.

Inclusion Criteria:

• Confirmed diagnosis of PAH II or III functional class according to WHO classification
• Symptomatic PAH group 1 according to clinical classification
• Patients who have not previously received any PAH-specific therapy OR patients who have received endothelin receptor antagonists and/or iloprost/selexipag at stable doses for at least 3 months prior to the initial right heart catheterization.
• Ability to perform reproducible 6-min walking test at the screening with a distance from 150 to 450 m.

Exclusion Criteria:

• as per the study protocol