A Study to Assess Efficacy, Safety, Tolerability, and Pharmacokinetics of ABBV-8E12 in Subjects With Progressive Supranuclear Palsy (PSP).

Inclusion Criteria:

- Male or female subject with age 40 years or greater at the time of signed consent.

- Meets the criteria for possible or probable progressive supranuclear palsy
(Steele-Richardson-Olszewski Syndrome).

- Presence of PSP symptoms for less than 5 years.

- Subject is able to walk 5 steps with minimal assistance (stabilization of one arm or
use of cane/walker).

- Subject has an identified, reliable, study partner (e.g., caregiver, family member,
social worker, or friend).

Exclusion Criteria:

- Subjects who weigh less than 44 kg (97 lbs) at screening.

- MMSE score less than 15 at screening.

- Any contraindication or inability to tolerate brain magnetic resonance imaging (MRI).

- Subject resides at a skilled nursing or dementia care facility, or admission to such a
facility is planned during the study period.

- Evidence of any clinically significant neurological disorder other than PSP.

- The subject has a history of or currently has schizophrenia, schizoaffective disorder
or bipolar disorder according to DSM-V or ICD-10 criteria.

- Subject has had a significant illness or infection requiring medical intervention in
the past 30 days.