• Updated seralutinib dose from 90 mg to 60 mg to reduce capsule burden. • Updated visit schedule to occur every 6 weeks (instead of 4 weeks), increased visit windows, changed Week 8 visit to telephone contact, and clarified home-health visit options to reduce patient burden. • Added dispensation and collection of an electronic handheld device to record daily dosing. • Updated inclusion criterion to remove rheumatoid arthritis as a PAH-associated disease. • Expanded exclusion criterion to exclude subjects with a condition limiting 6MWT assessment. • Clarified male fertility assessment was optional, required 36 weeks of study participation. • Added language related to photosafety to match language in Version 5 of the Investigator’s Brochure and to align with a United Kingdom specific amendment.

• Version 3.0.1 was issued to correct a typographical error in v3.0.0. Protocol v3.0.1 replaced v3.0.0 and no patients were enrolled under v3.0.0. • Updated seralutinib dose from 60 mg to up to 90 mg (dose range of 45 mg to 90 mg BID) based on on-going safety in Phase 1 studies and projected efficacious exposure range. • Updated visit schedule to occur every 4 weeks through Week 12 for adverse event (AE) monitoring. • Added monthly pregnancy testing for Women of childbearing potential (WOCBP) at-home testing when visits were less frequent. • Added clarification that morning doses of oral PAH disease-specific background medications should be taken immediately prior to GB002 dosing in clinic on pharmacokinetic (PK) sampling days. • Added functional respiratory imaging and heart rate monitoring substudies at select sites. • Updated risk language to include embryo-fetal development as an identified risk. • Reduced maximum age inclusion criterion from 80 years of age to 75 years. • Reduced Left ventricular-end diastolic pressure (LVEDP) inclusion criterion from ≤ 15 mmHg to ≤ 12 mmHg. • Updated to indicate that if historical echocardiogram (ECHO) not available, screening ECHO may be used to establish this criterion. Expanded exclusion criterion to exclude subjects with left-sided heart disease. • Updated exclusion criteria to exclude subjects with documented uncontrolled symptomatic coronary disease and portopulmonary hypertension or Child-Pugh Class A or higher. • Expanded estimated glomerular filtration rate (eGFR) exclusion criterion from < 30 mL/min/1.73m^2 to < 45 mL/min/1.73m^2, including subjects with mild-moderate renal function impairment.

(continued) • Updated exclusion criteria to exclude inhaled marijuana product use, subjects with history of alcohol abuse and/or a positive test for drugs of abuse, subjects with milk allergy, and subjects that have any other condition or reason that (in the opinion of the Investigator or Sponsor’s Medical Monitor) would prohibit study participation. • Added stopping rules and in investigational product (IP) interruption instructions for fluid retention, neutropenia, thrombocytopenia, and electrocardiogram (ECG) abnormalities. • Added criteria for clinical worsening and updated exploratory endpoint. • Updated prohibited medications to prohibit use of medications associated with QT interval prolongation, as a cautionary measure, and added exclusionary criterion for subjects with QT corrected for heart rate by Fridericia's cube root formula (QTcF) > 480 ms. • Increased window for Week 24 RHC to up to 8 weeks beyond the Week 24 visit for subjects remaining on IP, to allow for impact of pandemic/global emergencies. • Clarified that an open-label extension study was available. A 28-day safety follow up will not be required for subjects transitioning to OLE. • Added subjects were encouraged to weigh themselves at home as part of standard of care in order to monitor for lower extremity edema. • Updated biomarker collection schedule to remove assays.

• Updated prohibited medications due to new data indicating seralutinib was not expected to have clinically significant drug-drug interactions with medications which are inhibitors of P-glycoprotein, Breast Cancer Resistance Protein, or moderate or weak cytochrome P450 isoform 3A inhibitors. • Updated prohibited medications to clarify strong cytochrome P450 isoform 3A inhibitors and washout instructions for anticoagulants. • Updated risk language to include potential risk of female reproductive organs based on nonclinical data and potential risk of bleeding to include hemoptysis. • Updated AE assessments to include review of all system organ classes (SOCs), including reproductive system. • Added clarification for the definition of Worsening Risk Score Category. • Expanded inclusion criterion to include categories of connective tissue disease-associated pulmonary arterial hypertension (CTD-APAH). • Updated inclusion criterion for pulmonary function test (PFTs) to be more informative of disease state. • Clarified exclusion criterion to note portal hypertension due to cirrhosis. • Updated exclusion criterion for alcohol use to align with Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) definition. • Removed collection of nasal mucosal fluid samples (protocol clarification letter 12APR2021); based on standard laboratory practices, added assessment of urate and activated partial thromboplastin time (APTT). • Replace measure of forced expiratory volume in 1 second (FEV1) with ratio of FEV1 to forced vital capacity (FVC). Replace requirement for carbon monoxide diffusing capacity (DLCO) with FVC.