Sotatercept in Pulmonary Arterial Hypertension

Potential participants must provide their written informed consent before any study-specific procedures. All screening procedures will be completed over two weeks during three separate visits. For participants from the Calgary site (site 2), all Visit 1 procedures except for PFT and CPET may be performed on a separate day in Calgary. Visits 2 and 3 will be performed in Edmonton (site 1). According to patient preference, visits 2 and 3 can be performed on the same day, with a minimum 3-hour break between visits. For participants from Calgary (site 2), the first drug administration will occur in Calgary within seven days of completion of Visit 3 procedures. Screening will include reviewing the participant's medical, surgical, and family history, collecting demographics, race, and ethnicity, and requesting medical records for relevant external procedures. Screening procedures include: Visit 1: * Informed consent * Inclusion/exclusion criteria * Medical history review * Physical examination * 12-lead ECG * Vital signs * Weight * WHO FC assessment * 6MWT * Pregnancy test * Hematology (complete blood count) * Urinalysis * Hemoglobin * Serum chemistry * NT-proBNP * PFT * CPET * QoL assessments * AE/SAE Review * Concomitant medication review Visit 2: * Hemoglobin * Rest/exercise DLCO, VC, DM * Rest/exercise CW-NIRS * AE/SAE Review * Concomitant medication review Visit 3: * Pregnancy test * Rest/exercise Echocardiography * AE/SAE Review * Concomitant Medication review * Study drug administration Measurements/assessments taken during the Screening Period will be recorded as the baseline values for the study assessment of endpoints. Treatment Period (Visit 4-14) All study procedures should be performed prior to the study drug administration. For participants at the Calgary site, Visits 4,8-14 occur at their home site. For participants from the Calgary site (site 2), Visit 5-7 will be performed in Edmonton (site 1). Visits 5-7 are to be performed before the administration of the 3rd dose of the study drug. According to the patient's preference, visits 6 and 7 may be combined. Visit 4 (Week 3) * Focused cardiopulmonary physical examination * Vital signs * Weight * WHO FC assessment * 6MWT * Pregnancy test * Hematology * Urinalysis * Clinical worsening assessment * AE/SAE Review * Concomitant medication review Visit 5 (Week 5-6): * PFT * CPET * QoL assessments * AE/SAE Review * Concomitant medication review Visit 6 (Week 5-6): * Hemoglobin * Rest/exercise DLCO, VC, DM * Rest/exercise CW-NIRS * AE/SAE Review * Concomitant medication review Visit 7 (Week 5-6): * Pregnancy test * Rest/exercise Echocardiography * AE/SAE Review * Concomitant Medication review Visit 8 (Week 6): * Focused cardiopulmonary physical examination * 6MWT * Vital signs * Weight * WHO FC assessment * Pregnancy test * Hematology * Urinalysis * Clinical worsening assessment * AE/SAE Review * Concomitant medication review Visit 9 (Week 9): * Focused cardiopulmonary physical examination * 6MWT * 12-lead ECG * Vital signs * Weight * WHO FC assessment * Pregnancy test * Hematology * Serum chemistry * NT-proBNP * Clinical worsening assessment * AE/SAE Review * Concomitant medication review Visit 10-14 (Week 12 - 24): * Focused cardiopulmonary physical examination * 6MWT * Vital signs * Weight * WHO FC assessment * Pregnancy test * Hematology * Urinalysis * Clinical worsening assessment * AE/SAE Review * Concomitant medication review End-of-Study Period (Visits 13-14) End-of-Study period visits will start 1 week after the last dose of the study medication and will be completed over 2 weeks. Visits 15-17 will be performed in Edmonton (site 1). Visit 16 and 17 can be performed on the same day according to patient preference. Visit 15: * Weight * Hemoglobin * PFT * CPET * QoL Assessment * AE/SAE Review * Concomitant medication review Visit 16: * Hemoglobin * Rest/exercise DLCO, VC, DM * Rest/exercise CW-NIRS * AE/SAE Review * Concomitant medication review Visit 17: * Rest/exercise echocardiography * AE/SAE Review * Concomitant medication review Follow-up period (Visit 18) A follow-up visit will occur 3 weeks after the last dose of the study medication. For Edmonton participants (site 1), Visit 18 may happen on the same day after Visit 17 is completed. Visit 18: * 6MWT * 12-lead ECG * Vital signs * WHO FC assessment * Pregnancy test * Hematology * Urinalysis * NT-proBNP sample collection * Clinical worsening assessment * AE/SAE Review * Concomitant medication review Following the last dose of study drug administration, participants have the option to continue with open label sotatercept supplied directly from the manufacturer until either the participant discontinues sotatercept or reimbursement through public or private insurance is in place. The treating physician will be responsible for ongoing monitoring and administration of sotatercept beyond the last dose.