Inclusion Criteria:

• An Institutional Review Board (IRB) approved informed consent is signed and dated by the patient prior to any study-related activities.
• The patient is 18 years of age or older.
• If the patient is a female of childbearing potential, then the patient has a negative pregnancy test at the Baseline Visit and agrees to practice adequate birth control throughout the duration of the study. If the patient is postmenopausal or has documented surgical sterilization, a pregnancy test and birth control is not necessary. It is the Investigator’s responsibility for determining whether the patient has adequate birth control for study participation.
• The patient has been diagnosed with PAH belonging to the following subgroups of the updated Nice Clinical Classification Group 1, which include:
  • Idiopathic PAH (1.1); or
  • Heritable PAH (1.2); or
  • Drug and toxin induced PAH (1.3); or
  • PAH associated with connective tissue disease (1.4.1), HIV infection (1.4.2), or congenital heart disease (1.4.4) with simple systemic-to-pulmonary shunt at least 1 year after surgical repair.
• The patient is NYHA Functional Class II - IV at Screening and:
  • has documented stable doses of Tyvaso for at least 3 months prior to screening and is willing and able to transition from their prescribed dose of inhaled therapy to study drug; or
  • has documented stable doses of no more than two approved oral therapies for at least 3 months prior to screening and is willing and able to add LIQ861 to their treatment regimen.
• The patient can complete a baseline six-minute walk distance (6MWD) ≥ 150 m.
• The patient has had evidence of FEV1 ≥ 60% and FEV1/FVC ratio ≥ 60% during the 6-month period prior to enrollment.

Exclusion Criteria:

• The patient's clinical condition is such that, in the opinion of the Investigator, they are not expected to remain clinically stable for the duration of the study.
• Patients with pulmonary hypertension (PH) in the Updated Nice Classification Groups 2-5, or PAH Group 1 subgroups not covered by the inclusion criteria (e.g., associated with portal hypertension [1.4.3] or with schistosomiasis [1.4.5]).
• The patient is currently taking oral or IV prostacyclin analogues or agonists, including treprostinil and selexipag.
• The patient has had any PAH medication (except for anticoagulants) discontinued within 14 days of Baseline.
• The patient has had a new type of chronic therapy (including but not limited to oxygen, a different class of vasodilator, diuretic, digoxin, and digitalis) for pulmonary hypertension added within 30 days of Baseline.
• The patient has uncontrolled systemic hypertension as evidenced by persistent systolic blood pressure greater than 160 mmHg or diastolic blood pressure greater than 100 mmHg.
• The patient has a history of hemodynamically significant left-sided heart disease including, but not limited to: aortic or mitral valve disease, pericardial constriction, restrictive or congestive cardiomyopathy, or coronary artery disease (CAD).
• The patient has had an atrial septostomy.
• The patient has any serious or life-threatening disease other than conditions associated with PAH.
• The patient is taking any excluded medications listed in the Investigator’s Brochure, namely inhibitors and inducers of CYP2C8.
• The patient has a hypersensitivity or allergy to any of the ingredients of LIQ861 or other clinically relevant allergies (clinical relevance per Investigator judgment).
• The patient has had a pulmonary infarction (defined as infarction in more than one lung segment documented by V/Q scan or pulmonary angiography) within two weeks of Screening.
• The patient has had a stroke or transient ischemic attack (TIA) within six months of Screening.
• The patient has evidence of an active uncontrolled sepsis or systemic infection during Screening.
• The patient is pregnant or lactating.
• The patient has any musculoskeletal disease or any other disease that would limit ambulation.
• The patient has participated in an investigational product or device study within the 30 days prior to Screening.
• The patient has current evidence of drug abuse in the opinion of the Investigator.
• The patient has severe hepatic impairment as evidenced by any history of ascites AND encephalopathy.
• The patient has severe renal impairment (eGFR < 30).

Additional Exclusion Criteria for PK Sub-Study

• The patient meets any of Primary Exclusion Criteria #1 – 19.
• The patient has moderate or severe renal impairment (eGFR < 45).