A Study to Learn About the Safety of BIIB142 and How it is Processed in the Body of Healthy Adult Participants Aged 18 to 55 Years Old

In this study, researchers will learn more about the safety of BIIB142 and how it is processed in the body. This is the first time that researchers will learn about BIIB142 and how it affects people. 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 a drug during the study. An AE is considered serious when it results in death, is life-threatening, causes lasting problems, or requires hospital care. Researchers will also learn more about: • How the body processes BIIB142 This is a "dose escalation study." This is a study in which increasing amounts of the study drug are given to different groups of participants. This is done until researchers find the highest dose that does not cause harmful effects. First, participants will be screened to check if they can join the study. The screening period will be up to 28 days. This study will be split into 2 parts - Part A and Part B. During Part A: * Participants will be randomly placed into 1 of 6 groups to receive a single dose of either BIIB142 or a placebo. A placebo looks like the study drug but contains no real medicine. * Participants in Groups 1 through 5 will take either BIIB142 or the placebo without food. Participants in Group 6 will take 2 doses of their assigned treatment - once with food and once without food. * Neither the researchers nor the participants will know if the participants will receive BIIB142 or the placebo. * Participants will stay at their study research center for 5 days. They will return for another 4 visits. Each participant in Part A will be in the study for up to 58 days. During Part B: * Participants will be randomly placed into 1 of 3 groups to receive BIIB142 or the placebo. In Part B, participants will take BIIB142 or the placebo once a day for 14 days. * Neither the researchers nor the participants will know if the participants will receive BIIB142 or the placebo. * Participants will stay at their study research center for 16 days. They will return for another 4 visits. Each participant in Part B will be in the study for up to 58 days.