REGN5381 in Adult Participants With Heart Failure With Reduced Ejection Fraction

This study is researching an experimental drug called REGN5381 (called "study drug"). The study is focused on patients with heart failure with reduced ejection fraction (ie, the heart is not functioning as well as it should). The aim of the study is to see how safe, tolerable, and effective the study drug is. The study is looking at several other research questions, including: * What side effects may happen from taking the study drug * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drug (which could make the study drug less effective or could lead to side effects)