This is a Phase 1B, randomized, participant- and investigator-blinded, placebo-controlled, multi-center clinical trial to evaluate the safety, pharmacokinetics (PK), pharmacodynamics (PD), and biomarkers of inhaled GB002 in adults with World Health Organization (WHO) Group 1 Pulmonary Arterial Hypertension (PAH).

The primary objective for this trial is to evaluate the safety and tolerability of GB002. The secondary objective for this trial is to evaluate the PK parameters of GB002. Exploratory objectives are to evaluate the PD readouts, change in WHO Group I functional class, and change in quality of life associated with GB002 treatment.

In this Phase 1B study, two dose levels of GB002 will be tested in adult participants with PAH. Four participants in cohort 1 will be randomized to receive up to two daily doses of either active drug or placebo, with 3 subjects receiving GB002 and 1 subject receiving placebo. An additional 4 participants (3 active and 1 placebo) may be added to cohort 1 at the discretion of the Data Review Committee (DRC). The dose and dosing interval (i.e., once daily or twice daily) for the second cohort will be determined by review of the safety, tolerability, and drug levels in the blood from cohort 1. Cohort 2 participants will also be randomized such that 6 subjects receive GB002 and 2 subjects receive placebo.

Eligible subjects who have completed the 2 week treatment period have the option to participate in a 24 week open label extension.

Inclusion Criteria:

1. Adult (males and females) aged 18 to 75 years (inclusive) with pulmonary arterial hypertension (PAH) (Main study)

2. A current diagnosis of symptomatic PAH classified by one of the following (Main and OLE study):

   1. Idiopathic (IPAH) or heritable pulmonary arterial hypertension (HPAH)

   2. PAH associated with one of the following connective tissue diseases (CTDs):

      systemic sclerosis, rheumatoid arthritis, mixed CTD or overlap syndrome, or systemic lupus erythematosus

   3. PAH associated with anorexigen or methamphetamine use

3. World Health Organization (WHO)/New York Heart Association (NYHA) functional class II-IV symptomatology (Main study)

Exclusion Criteria:

1. Clinically significant systemic hypertension or hypotension (Main and OLE study)
2. History of left-sided heart disease and/or clinically significant cardiac disease (Main and OLE study)
3. History of decompensated right heart failure within 30 days of screening (e.g., hospitalization for PAH or the need to add an additional PAH medication) (Main study)

NOTE: Additional inclusion/exclusion criteria may apply, per protocol.

Clinical site investigators, study personnel, and study subjects will be blinded to treatment assignment; however, the Sponsor will be unblinded.