Study of REGN13335 in Adult Participants With Pulmonary Arterial Hypertension (PAH)

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 \