The primary objectives of the study are to evaluate the safety and tolerability of multiple doses of BIIB061 administered to healthy adults. The secondary objective is to determine the multiple-dose PK profile of BIIB061 in this study population.

Key Inclusion Criteria:

• Males and postmenopausal (defined as no menses for 12 months and confirmed by follicle-stimulating hormone [FSH] levels determined at screening to be in the postmenopausal range) or surgically sterile females
• Must be in good health and have normal vital signs as determined by the Investigator, based on medical history and screening evaluations.
• Must agree to abstain from alcohol ingestion for the duration of the study.
• Must agree to abstain from consumption of grapefruit or grapefruit-containing products for the duration of the study, starting at least 24 hours prior to admission to the inpatient facility.
• Must agree to abstain from taking St. John's Wort (herbal preparation) for the duration of the study
• Must agree to abstain from any vigorous exercise (as determined by the Investigator) for the duration of the study.
• Must have a body mass index of 18 to 30 kg/m2, inclusive.

Key Exclusion Criteria:

• History of or positive test result at screening for human immunodeficiency virus, hepatitis C virus antibody, or hepatitis B virus (defined as positive for hepatitis B surface antigen [HBsAg] or hepatitis B core antibody [HBcAb]).
• Presence of a metal device (e.g., any type of electronic, mechanical, or magnetic implant; cardiac pacemaker; aneurysm clips; implanted cardiac defibrillator) or potential ferromagnetic foreign body (metal slivers, metal shavings, other metal objects) that are contraindications for MRI.
• History of asthma, wheezing, allergies involving the lungs, or airway obstruction.
• History of bronchodilator or inhaler use.
• Clinically significant abnormal hematology or blood chemistry values, as determined by the Investigator.
• Inability or unwillingness to comply with study requirements.
• Other unspecified reasons that, in the opinion of the Investigator or Biogen Idec, make the subject unsuitable for enrollment.

Note: Other protocol defined inclusion/ exclusion criteria may apply.