This Phase 3, 2-part, open-label, multicenter study aims to demonstrate the safety and tolerability of L606 in patients with PAH or PH-ILD. The study will determine the short-term and long-term safety and tolerability of L606 in this patient population.

This Phase 3, 2-part, open-label, multicenter study aims to demonstrate the safety and tolerability of repeated doses of L606 in patients with PAH or PH-ILD.

Cohort A: Subjects with PAH or PH-ILD receiving prior stable doses of Tyvaso and willing to switch to L606.

Cohort B: Subjects with PAH (not initially on prostacyclin therapy) who are likely to receive clinical benefit from inhaled treprostinil based on the opinion of the investigator.

Cohort A subjects will sequentially participate in the Main Study Period (MSP) for 2 weeks and the Open Extension Period (OEP) for 46 weeks. Cohort B subjects will sequentially participate in the MSP for 12 weeks and the OEP for 36 weeks.

Key Inclusion Criteria:

1. Males and females ≥18 and ≤80 years of age.

2. Diagnosed with

   1. PAH belonging to at least 1 of the following subgroups of Group 1 pulmonary hypertension (PH) per European Society of Cardiology/European Respiratory Society Guidelines for the diagnosis and treatment of PH at least 1 year prior to screening. or
   2. PH-ILD Group 3 European Society of Cardiology/European Respiratory Society Guidelines for the diagnosis and treatment of PH. Subjects with Group 3 PH that is not related to underlying ILD are not eligible.

3. Subjects with PAH: Documentation of having PAH as confirmed by RHC meeting the following criteria:

   i. Mean PAP >20 mmHg. ii. Pulmonary arterial wedge pressure ≤15 mmHg. iii. Pulmonary vascular resistance >3 Wood units. Subjects with PH-ILD: Confirmation of the underlying ILD must be based on HRCT imaging with demonstration of diffuse parenchymal lung disease and documented by the Investigator or radiology report. Subjects may have any form of ILD or CPFE.

4. NYHA functional class II, III, or IV at the screening visit.

5. Can complete a screening 6MWD of ≥150 meters

6. For subjects with PAH: >65% of predicted and FEV1/FVC ratio >65% at screening. For subjects with PH-ILD: >40% of predicted and FEV1/FVC ratio >70% at screening.

Key Exclusion Criteria:

1. LVEF of ≤45% on a historical echocardiogram.
2. History of sleep apnea, or left-sided heart disease (including but not limited to aortic or mitral valve disease, pericardial constriction, restrictive or congestive cardiomyopathy, or coronary artery disease) per investigator's discretion.
3. Experienced an acute exacerbation of disease or hospitalization for any reason within 30 days of signing the ICF or prior to baseline.
4. Musculoskeletal disorder (eg, arthritis affecting the lower limbs, recent hip or knee joint replacement) or any disease that would likely be the primary limit to ambulation or subject is connected to a machine that is not portable enough to allow for a 6MWT.