[11C]-PBR28 Positron Emission Tomography Study to Evaluate the Effect of ABT 555 in Subjects With Relapsing Forms of Multiple Sclerosis

Inclusion Criteria: Diagnosis of relapsing-remitting MS (RRMS) or relapsing secondary progressive MS (SPMS) Neurologically stable at Screening, in the investigator's judgment and not actively experiencing or recovering from a recent relapse in the 30 days preceding the Screening Visit A Kurtzke Expanded Disability Status Scale (EDSS) score of 1.0 to 6.0, inclusive at the Screening Visit High or mixed affinity binder of the TSPO, as determined by rs6971 polymorphism genotyping at screening Exclusion Criteria: Diagnosis of primary progressive or non-relapsing secondary progressive MS Smoking more than 10 cigarettes per day or use of a nicotine patch Known history of, or positive screening test result for hepatitis C virus or hepatitis B virus Varicella or herpes zoster virus infection or any severe viral infection within 6 weeks before Screening Any type of live virus vaccine from 4 weeks before randomization History of abnormal laboratory results