This study is researching an experimental drug called REGN13335. The study is focused on participants with Pulmonary Arterial Hypertension (PAH). The aim of the study is to see how safe and effective REGN13335 is in participants with PAH who are taking other PAH medicines.

The study is looking at several other research questions, including:

• What side effects may happen from taking REGN13335
• How much REGN13335 is in the blood at different times
• Whether the body makes antibodies against REGN13335 (which could make REGN13335 less effective or could lead to side effects)

Key Inclusion Criteria:

1. Documented clinical diagnosis of PAH (Group 1 PH according to the 7th World Symposium on Pulmonary Hypertension (WSPH))
2. WHO functional class II or III (slight to marked limitation of functional status due to PAH)
3. Receiving background Standard Of Care (SOC) therapy for PAH on a stable dose and regimen, as determined by the investigator, as described in the protocol
4. PVR ≥400 dynes∙sec/cm^5 (5 Wood units) based on Right Heart Catheterization (RHC) during the screening period
5. Has 6MWD ≥150 and ≤550 meters repeated twice during screening as described in the protocol

Key Exclusion Criteria:

1. Has Group 2 (PH associated with left heart disease), Group 3 (PH associated with lung diseases and/or hypoxia), Group 4 (PH associated with pulmonary artery obstructions), or Group 5 (PH with unclear and/or multifactorial mechanisms) PH according to the 7th WSPH
2. Pulmonary Arterial Wedge Pressure (PAWP) >15 mm Hg by RHC during the screening period
3. History of left-sided heart disease and/or clinically significant cardiac disease, as described in the protocol
4. Obstructive lung disease defined as Forced Expiratory Volume in 1 second (FEV1)/Forced Vital Capacity <0.7 and FEV1 <70% of the predicted value as described in the protocol
5. Evidence of interstitial lung disease as defined in the protocol
6. Evidence of chronic thromboembolic pulmonary disease or acute pulmonary embolism as described in the protocol
7. Participants requiring anticoagulation and/or antiplatelet therapy for an underlying medical condition as described in the protocol
8. Has any history of intracranial bleeding or any history of elevated intracranial pressure
9. Has any history of bleeding meeting criteria as described in the protocol

Note: Other protocol-defined Inclusion/ Exclusion criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf