• Proportion of patients achieving adequate control of their intravascular hemolysis [ Time Frame: Week 4 through week 26 ]
  Defined as lactate dehydrogenase (LDH) ≤ 1.5 x upper limit of normal (ULN)
• Proportion of patients achieving transfusion avoidance [ Time Frame: Up to 26 weeks ]
  Defined as no post baseline transfusion of red blood cells (RBCs)

• Rate of breakthrough hemolysis [ Time Frame: Up to 26 Weeks ]
  Defined as the measurement of LDH) ≥ 2 x ULN concomitant with associated signs or symptoms at any time subsequent to an initial achievement of disease control.
• Proportion of patients achieving normalization of their intravascular hemolysis [ Time Frame: Week 4 through week 26 ]
  Defined as LDH ≤ 1.0 x ULN
• Time to first LDH ≤ 1.5 x ULN [ Time Frame: Up to week 26 ]
• Percentage of days with LDH ≤ 1.5 x ULN [ Time Frame: Week 4 through week 26 ]
• Change in LDH levels [ Time Frame: Baseline to week 26 ]
• Percent change in LDH levels [ Time Frame: Baseline to week 26 ]
• Rate of transfusion with RBCs [ Time Frame: Up to 26 weeks ]
• Number of units of transfusion with RBCs [ Time Frame: Up to 26 weeks ]
• Change in RBC hemoglobin levels [ Time Frame: Baseline to week 26 ]
• Change in free hemoglobin levels [ Time Frame: Baseline to week 26 ]
• Change in total complement hemolytic activity assay (CH50) [ Time Frame: Baseline to week 26 ]
• Percent change in CH50 [ Time Frame: Baseline to week 26 ]
• Changes in scores of patient-reported outcomes as measured by FACIT-Fatigue [ Time Frame: Baseline to week 26 ]
  The FACIT-F is a 13-item, self-reported PRO measure assessing an individual's level of fatigue during their usual daily activities over the past week. This questionnaire is part of the FACIT measurement system, a compilation of questions measuring health-related QoL in patients with cancer and other chronic illnesses. The FACIT-fatigue assesses the level of fatigue using a 4-point Likert scale ranging from 0 (not at all) to 4 (very much). Scores range from 0 to 52, with higher scores indicating greater fatigue.
• Changes in scores of patient-reported outcomes as measured by European Organization for Research and Treatment of Cancer [EORTC]- Quality of life questionnaire-core 30 (QLQ-30) [ Time Frame: Baseline to week 26 ]
  EORTC QLQ-C30 is a self-reported, 30-item generic questionnaire developed to assess 15 domains: global health status scale, five functional scales (physical, role, emotional, cognitive, and social functioning) and nine symptom scales (fatigue, nausea, vomiting, pain, dyspnea, insomnia, appetite loss, constipation, diarrhea and financial difficulties). All the scales range from 0 to 100. A high score on the functional scales represents a high level of functioning, and a high score on the symptom scales represents a high level of symptomatology. A high score on the global QOL represents a high general quality of life.
• Changes in scores of patient-reported outcomes as measured by EQ-5D-3L [ Time Frame: Baseline to week 26 ]
  The EQ-5D-3L is a self-administered standardized instrument for use as a measure of health outcome. It is a health questionnaire that consists of the EQ-5D descriptive system and the EQ visual analogue scale (EQ VAS). The EQ-5D-3L descriptive system comprises the following 5 dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension has 3 levels: no problems, some problems, extreme problems.
• Incidence and severity of treatment-emergent adverse events (TEAEs) [ Time Frame: Up to 26 weeks ]
• Safety measured by number of patients who experience abnormal laboratory values [ Time Frame: Up to 26 weeks ]
• Safety measured by number of patients who experience abnormal vital signs [ Time Frame: Up to 26 weeks ]
• Concentrations of total REGN3918 in serum [ Time Frame: Up to week 26 ]
• Incidence of treatment-emergent anti-drug antibodies to REGN3918 [ Time Frame: Up to week 26 ]

• Rate of breakthrough hemolysis [ Time Frame: Up to 26 Weeks ]
  Defined as the measurement of LDH) ≥ 2 x ULN concomitant with associated signs or symptoms at any time subsequent to an initial achievement of disease control.
• Proportion of patients achieving normalization of their intravascular hemolysis [ Time Frame: Week 4 through week 26 ]
  Defined as LDH ≤ 1.0 x ULN
• Time to first LDH ≤ 1.5 x ULN [ Time Frame: Up to week 26 ]
• Percentage of days with LDH ≤ 1.5 x ULN [ Time Frame: Week 4 through week 26 ]
• Change in LDH levels [ Time Frame: Baseline to week 26 ]
• Percent change in LDH levels [ Time Frame: Baseline to week 26 ]
• Rate of transfusion with RBCs [ Time Frame: Up to 26 weeks ]
• Number of units of transfusion with RBCs [ Time Frame: Up to 26 weeks ]
• Change in RBC hemoglobin levels [ Time Frame: Baseline to week 26 ]
• Change in free hemoglobin levels [ Time Frame: Baseline to week 26 ]
• Change in total complement hemolytic activity assay (CH50) [ Time Frame: Baseline to week 26 ]
• Percent change in CH50 [ Time Frame: Baseline to week 26 ]
• Changes in scores of patient-reported outcomes as measured by FACIT-Fatigue [ Time Frame: Baseline to week 26 ]
  The FACIT-F is a 13-item, self-reported PRO measure assessing an individual's level of fatigue during their usual daily activities over the past week. This questionnaire is part of the FACIT measurement system, a compilation of questions measuring health-related QoL in patients with cancer and other chronic illnesses. The FACIT-fatigue assesses the level of fatigue using a 4-point Likert scale ranging from 0 (not at all) to 4 (very much). Scores range from 0 to 52, with higher scores indicating greater fatigue.
• Changes in scores of patient-reported outcomes as measured by European Cooperative Oncology Group [EORTC]-QLQ-30 [ Time Frame: Baseline to week 26 ]
  EORTC QLQ-C30 is a self-reported, 30-item generic questionnaire developed to assess 15 domains: global health status scale, five functional scales (physical, role, emotional, cognitive, and social functioning) and nine symptom scales (fatigue, nausea, vomiting, pain, dyspnea, insomnia, appetite loss, constipation, diarrhea and financial difficulties).
• Changes in scores of patient-reported outcomes as measured by EQ-5D-3L [ Time Frame: Baseline to week 26 ]
  The EQ-5D-3L is a self-administered standardized instrument for use as a measure of health outcome. It is a health questionnaire that consists of the EQ-5D descriptive system and the EQ visual analogue scale (EQ VAS). The EQ-5D-3L descriptive system comprises the following 5 dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension has 3 levels: no problems, some problems, extreme problems.
• Incidence and severity of treatment-emergent adverse events (TEAEs) [ Time Frame: Up to 26 weeks ]
• Safety measured by number of patients who experience abnormal laboratory values [ Time Frame: Up to 26 weeks ]
• Safety measured by number of patients who experience abnormal vital signs [ Time Frame: Up to 26 weeks ]
• Concentrations of total REGN3918 in serum [ Time Frame: Up to week 26 ]
• Incidence of treatment-emergent anti-drug antibodies to REGN3918 [ Time Frame: Up to week 26 ]

The primary objective of the study is to demonstrate a reduction in intravascular hemolysis by REGN3918 over 26 weeks of treatment in patients with active PNH who are treatment-naive to complement inhibitor therapy or have not recently received complement inhibitor therapy.

The secondary objectives of the study are:

• To evaluate the safety and tolerability of REGN3918.
• To evaluate the effect of REGN3918 on parameters of intravascular hemolysis
• To assess the concentrations of total REGN3918 in serum.
• To evaluate the incidence of treatment-emergent anti-drug antibodies to REGN3918 over time
• To evaluate the effect of REGN3918 on patient-reported outcomes (PROs) measuring fatigue and health-related quality of life

Key Inclusion Criteria:

• Diagnosis of paroxysmal nocturnal hemoglobinuria (PNH) confirmed by high-sensitivity flow cytometry
• PNH granulocytes > 10% at screening visit
• Active disease, as defined by the presence of 1 or more PNH-related signs or symptoms (eg, fatigue, hemoglobinuria, abdominal pain, shortness of breath [dyspnea], anemia [hemoglobin <10 g/dL], history of a MAVE [including thrombosis], dysphagia, or erectile dysfunction) or history of red blood cell (RBC) transfusion due to PNH within 3 months of screening.
• Lactate dehydrogenase (LDH) level ≥ 2 × upper limit of normal (ULN) at screening visit.

Key Exclusion Criteria:

• Prior treatment with a complement inhibitor either within 6 months prior to screening visit or at any time where the patient was refractory to complement inhibitor therapy, in the opinion of the investigator (with the exception of eculizumab refractory patients due to the C5 variant R885H/C)
• History of bone marrow transplantation
• Body weight < 40 kilograms at screening visit
• Peripheral blood absolute neutrophil count (ANC) <500/μL [<0.5 x 109/L] or peripheral blood platelet count <50,000/μL
• Documented history of systemic fungal disease or unresolved tuberculosis, or evidence of active or latent tuberculosis infection (LTBI) during screening period
• Any contraindication for receiving Neisseria meningitidis vaccination and antibiotic prophylaxis therapy as recommended in the study
• Any active, ongoing infection within 2 weeks of screening or during the screening period
• Any clinically significant abnormality identified at the time of screening that in the judgment of the Investigator or any sub-Investigator would preclude safe completion of the study or constrain endpoints assessment such as major systemic diseases, or patients with short life expectancy
• Women who are pregnant, breastfeeding, or who have a positive pregnancy test at screening visit or day 1

NOTE: Other protocol defined Inclusion/Exclusion criteria apply.