A Phase 3, Randomized, Double-Blind, Placebo-Controlled Study to Compare the Efficacy and Safety of Sotatercept Versus Placebo When Added to Background Pulmonary Arterial Hypertension (PAH) Therapy for the Treatment of PAH

• Age ≥ 18 years

• Documented diagnostic right heart catheterization (RHC) at any time prior to screening confirming the diagnosis of World Health Organization (WHO) pulmonary arterial hypertension (PAH) Group 1 in any of the following subtypes:

‣ Idiopathic PAH

⁃ Heritable PAH

⁃ Drug/toxin-induced PAH

⁃ PAH associated with connective tissue disease

⁃ PAH associated with simple, congenital systemic to pulmonary shunts at least 1 year following repair

• Symptomatic PAH classified as WHO Functional Class (FC) II or III

• Baseline RHC performed during the Screening Period documenting a minimum pulmonary vascular resistance (PVR) of ≥ 5 Wood units (WU) and a pulmonary capillary wedge pressure (PCWP) or left ventricular end-diastolic pressure of ≤ 15 mmHg.

• On stable doses of background PAH therapy and diuretics (i.e., patient-specific dose goal for each therapy already achieved) for at least 90 days prior to screening; for infusion prostacyclins, dose adjustment within 10% of optimal dose is allowed per medical practice

‣ Background PAH therapy refers to approved PAH-specific medications and may consist of monotherapy or combination therapy with ERA, PDE5 inhibitors, soluble guanylate cyclase stimulators, and/or prostacyclin analogues or receptor agonists. Background PAH therapy should be stable at least 90 days prior to screening and remain stable throughout the study

⁃ Stable diuretic therapy is defined as no addition of a new diuretic and no switching of a pre-existent oral diuretic to parenteral administration; however, dose adjustments (up or down) in pre-existent oral diuretics are acceptable

• 6-Minute Walk Distance (6MWD) ≥ 150 and ≤ 500 m repeated twice at screening (measured at least 4 hours apart, but no longer than 1 week), and both values are within 15% of each other (calculated from the highest value)

• Females of childbearing potential must:

‣ Have 2 negative urine or serum pregnancy tests as verified by the investigator prior to starting study therapy; she must agree to ongoing urine or serum pregnancy testing during the study and until 8 weeks after the last dose of the study drug

⁃ If sexually active, have used, and agree to use, highly effective contraception without interruption, for at least 28 days prior to starting the investigational product, during the study (including dose interruptions), and for 16 weeks (112 days) after discontinuation of study treatment

⁃ Refrain from breastfeeding a child or donating blood, eggs, or ovum for the duration of the study and for at least 16 weeks (112 days) after the last dose of study treatment

⁃ Male participants must:

⁃ Agree to use a condom, defined as a male latex condom or nonlatex condom NOT made out of natural (animal) membrane (e.g., polyurethane), during sexual contact with a pregnant female or a female of childbearing potential while participating in the study, during dose interruptions and for at least 16 weeks (112 days) following investigational product discontinuation, even if he has undergone a successful vasectomy

⁃ Refrain from donating blood or sperm for the duration of the study and for 16 weeks (112 days) after the last dose of study treatment

⁃ Ability to adhere to study visit schedule and understand and comply with all protocol requirements

⁃ Ability to understand and provide written informed consent