A Study to Learn More About the Safety of a Single Dose of BIIB144 and How it is Processed in the Body of Healthy Adult Participants Ages 18 to 55 Years Old as Compared to a Placebo

In this study, researchers will learn how the body processes a study drug called BIIB144. This is a "first-in-human" study. This means that this study drug will be given to people for the first very time in this study. These studies are important because they help researchers learn about the safety of the study drug, how the body processes it, and what dose might be appropriate before testing it in larger groups. The main goal of this study is to learn more about the safety of a single dose of BIIB144 compared to a placebo. A placebo is something that looks like the study drug but does not contain any medicine. A placebo is also given in the same way as the study drug. The main question researchers want to answer in this study is: How many participants have adverse events (AEs) and serious adverse events (SAEs)? An AE is a health problem that may or may not be caused by the study drug. Researchers will also learn more about: * How the body processes BIIB144 compared to placebo. This study will be done as follows: * Participants will be screened to check if they can join the study. The screening period will be up to 28 days, after which eligible participants will check into their study research center. * Participants will stay at their study research center for about 8 days. Afterwards, they will have up to 13 visits to their study research center. * This will be a "dose escalation" study. In this kind of study, increasing amounts of study drug are given to different groups of participants. Usually, this is done until researchers find the highest dose that does not cause harmful effects. In this study, the number of dose groups and maximum dose are planned in advance. The dose escalation will stop once these planned groups are done or if safety concerns arise. * There will be a total of 6 groups. In each group, participants will receive a single dose of either BIIB144 or the placebo through an intravenous (IV) needle. * Throughout the study, participants will give blood and urine samples. Participants will also have physical exams and answer questions about how they are feeling. * Each participant will be in the study for up to 44 weeks.