The PSPRS is a quantitative measure of disability in participants with PSP. The PSPRS comprises 28 items in 6 areas. Six items are rated on a 3-point scale (0-2) and 22 are rated on a 5-point scale (0-4). The 6 areas are the History/Daily Activities, Mentation, Bulbar, Ocular Motor, Limb Motor, and Gait. The 28-item PSPRS total score ranges from 0 (normal) to 100. Fifteen items are selected to form a 15-item PSPRS and three domains are identified: Gait/Limb function, Ocular Motor, and Bulbar. The total 15-item PSPRS score ranges from 0 (normal) to 52. A positive change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies number of participants who had response on Week 52.

AEs: any sign, symptom, or diagnosis/disease that is unfavorable or unintended, that is new, or if pre-existing, worsens in participants administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. SAEs: an event that results in death; an event that, in the view of the investigator, places the participant at immediate risk of death (a life-threatening event); an outcome that results in a congenital anomaly/birth defect diagnosed in a child of a participant; an event that requires or prolongs inpatient hospitalization; an event that results in persistent or significant disability/incapacity.

Safety population included all randomized participants who had received at least one dose of study treatment (BIIB092 or Placebo). Participants randomized to Placebo that received at least one dose of BIIB092 2000 mg during the placebo-controlled period will be counted in the BIIB092 2000 mg group for the safety population. Three participants who received BIIB092 in Placebo group were counted in BIIB092 200 mg group.

The MDS-UPRDRS Part 2 includes 13 items assessing motor aspects of experiences of daily living (M-EDL) these include speech, saliva and drooling, chewing and swallowing, handwriting, doing hobbies and other activities, eating tasks, tremor, dressing, hygiene, turning in bed, getting out of bed, walking and balance, and freezing. All items have 5 responses with uniform anchors of 0= normal, 1= slight, 2= mild, 3= moderate, and 4= severe. Total score ranges from 0 to 52, higher score indicating severe conditions. A positive change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

The CGI-C scale measures the change in the patient's clinical status from a specific point in time. Using a 7-point scale, ranging from 1 (very much improved) to 7 (very much worse), with a score of 4 indicating no change.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

The PSP cognitive composite battery is used to identify and characterize abnormal cognitive decline in PSP participants. The PSP cognitive composite battery includes 13 sub-tests in total: 11 tests from the RBANS (only the picture naming is excluded), letter number sequencing test, and phonemic fluency test. Three domains are identified: Memory and learning, Visual-Motor function, and Working memory and Executive. A z-score transformation is applied for each component test at each visit, and the final total composite z-score is the average of the three-domain z-scores. A z-score of 0 is equal to the estimated mean adjusted by age and is considered average for this study population. Lower values are indicative of cognitive decline. A negative change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies the total number of participants analyzed in this outcome measure.

The RBANS provides both a total scale score and scores for 5 different cognitive domains. Specifically, the test measures immediate memory, visuospatial/constructional ability, language, attention, and delayed memory. Scores from all subtests are aggregated into a total composite score. RBANS data were age-normed and analyzed as index scores (also referred to as standard scores), which have a mean of 100 and a standard deviation of 15. Higher scores on each sub measure and index indicate better performance. A negative change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

The PSP-QoL is a patient-reported outcome measure developed specifically for assessing the health-related quality of life in people living with PSP. It is validated 45-item questionnaire and visual analog scale that is comprised of 2 subscales: physical health state (22 items), which covers mobility, dysarthria, dysphagia, visual disturbances, self-care, and activities of daily living, and mental health state (23 items), which covers emotional, cognitive and social functioning. Items are given a 6-reponse option format (No Problem, Slight Problem, Moderate Problem, Marked Problem, Extreme Problem and Not Applicable). The subscale results are derived by summing the respective items for that subscale and transforming the scores into a range of 0 to 100, the higher the scores indicating a greater impact of the disease on the aspect measured. The PSP-QoL also comprises of a Life Satisfaction rating gauge, which is a visual analog scale with a range of 0 (worst) to 100 (best).

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies number of participants who had response on Week 52. 'Number Analyzed' signifies the number of participants who were evaluated for the specified parameter.

The SEADL scale is a means of assessing a person's ability to perform daily activities in terms of speed and independence, with 100% indicating total independence, falling to 0%, which indicates a state of complete dependence. The individual is asked to rate his or her function using an 11-point scale (10% increments), from 100% (completely independent; able to do all chores without slowness, difficulty, or impairment; essentially normal; unaware of any difficulty) to 0% (vegetative functions such as swallowing, bladder and bowels are not functioning; bedridden). A negative change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

The Clinical Global Impression of Severity (CGI-S) Rating evaluates the severity of individual symptoms and treatment response in participants with mental disorders. The CGI-S is a 7-point scale that that requires the clinician to rate the severity of the patient's illness at the time of assessment. A rating of 1 is considered normal, or with the least severe symptoms, a rating of 7 is extremely ill, or the worst symptoms.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

Phonemic fluency is a sensitive test for assessing frontal lobe dysfunction. Participants are given a letter of the alphabet and asked to name as many words as they can that start with that letter in 1 minute. The score for each trial is auto-calculated as follows: Trial 1: Total number of correct responses for the first letter (range 0 to 40); Trial 2: Total number of correct responses for the second letter (range 0 to 40). The total score from the two trials will be used for analysis (range 0 to 80). More number of words correlates to better phonemic fluency. A negative change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

Letter number is a test of working memory which involves ordering a series of up to 8 letters and numbers in which the numbers are repeated back first in order starting with the lowest number, then followed by the letters in alphabetical order. LNS consists of 10 items and each item has 3 trials rated as Incorrect (0) or Correct (1). The LNS total raw score (range 0 to 30) is auto-calculated by summing the 10 individual item scores (range 0 to 3 for each item). Higher number of correct items correlated to better performance and a negative change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

The Color Trails test is a language free version of the Trail Making Test and was developed to allow for broader cross cultural assessment. For Part 1 (color trails test 1), the respondent uses a pencil to rapidly connect circles numbered 1-25 in sequence. For Part 2 (color trails test 2), the respondent rapidly connects number circles in sequence, but alternates between pink and yellow background. The length of time to complete each trial is recorded, along with qualitative features of performance indicative of brain dysfunction, such as near-misses, prompts, number sequence errors, and color sequence errors. Less time indicates better performance. A positive change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

The MOCA was designed as a rapid screening instrument for mild cognitive dysfunction. It assesses different cognitive domains: attention and concentration, executive function, memory, language, visuoconstructional skills, conceptual thinking, calculations, and orientation. Scores on the MOCA range from 0-30, with higher score being better performance. A negative change from baseline indicates worsening.

ITT population included randomized participants who had received at least 1 dose of blinded study treatment (BIIB092 or Placebo). 'Number of Participants Analyzed' signifies total number of participants analyzed in this outcome measure.

ADA population - subset of the safety population with at least one evaluable post-baseline evaluable ADA samples.

A 3 dimension (3D) T1-weighted MRI was performed to estimate brain volumes (e.g., ventricles, whole brain, midbrain, pons, superior cerebellar peduncle, third ventricle, and frontal lobes).

Efficacy MRI population is the subset of the ITT population who had a least one measurable brain volumetric measurement. 'Number of Participants Analyzed' signifies number of participants who had response on Week 52. 'Number Analyzed' signifies the number of participants who were evaluated for the specified parameter.