An Extension Study to Evaluate the Long-Term Safety and Tolerability of JNJ-54861911 in Participants in the Early Alzheimer's Disease Spectrum

The purpose of this study is to evaluate the long-term safety and tolerability of JNJ-54861911 in participants in the early Alzheimer's disease (AD \[progressive brain disease that slowly destroys memory and thinking skills, and eventually even the ability to carry out the simplest tasks\]) spectrum that have completed a Phase 1b or Phase 2 clinical trial with JNJ-54861911, who are willing to continue their assigned treatment.

Participants in the early Alzheimer's Disease (AD) spectrum, enrolled in ongoing or future clinical trials with JNJ-54861911 (Phase 1b or Phase 2 studies) will be provided the opportunity to participate in this study upon completion of their treatment period under the parent protocol. The study will consist of a Screening phase and 2 sequential treatment phases (a 12-month double-blind \[DB\] treatment phase \[placebo controlled\] and an open-label \[OL\] phase \[active\]) followed by an End-of-Treatment visit. Treatment in OL phase will continue until registration of JNJ-54861911; unless safety issues emerge as determined by the Data Review Committee (DRC) that would warrant termination of the study. Blood and cerebrospinal fluid (CSF) samples will be collected to evaluate the plasma and CSF pharmacokinetics of JNJ-54861911, as well as amyloid beta fragments. Participants' safety will be monitored throughout the study.

Eligibility Criteria:
Inclusion Criteria:

* Participants in the early Alzheimer's disease (AD) spectrum at time of enrollment under the parent protocol and according to its inclusion and exclusion criteria, must have very recently completed their treatment in a Phase 1b or Phase 2 JNJ-54861911 clinical study (example \[e.g.\], 54861911ALZ2002) under the parent protocol. Enrollment in this study should be completed (Day 1 of double-blind \[DB\] treatment phase) as soon as possible, but within 6 weeks, following completion of their treatment period under the parent protocol. If not defined under the parent protocol, completion of the treatment period is defined as having completed all study related procedures of the last visit of the treatment period under the parent protocol. A screening phase of up to 12 weeks may be allowed following written approval of the Sponsor
* Participant must be willing and able to adhere to the prohibitions and restrictions specified in this protocol
* Each Participants (or their legally acceptable representative and caregiver depending on disease state and local requirements) must sign an informed consent form (ICF) indicating that he or she understands the purpose of and procedures required for the study and are willing to participate in the study
* Participants must have a reliable informant (relative, partner, or friend). The informant must be willing to participate as a source of information and has at least weekly contact with the participant (contact can be in-person, via telephone or other audio/visual communication). The informant must have sufficient contact such that the Investigator feels he/she can provide meaningful information about the participant's daily function. If possible, an alternate informant meeting these criteria who can replace the primary informant should be identified prior to randomization

Exclusion Criteria:

* Any condition or situation which, in the opinion of the Investigator, may put the participant at significant risk, may confound the study results, or may interfere significantly with participant's participation in the study
* The use of concomitant medications known to prolong the QT/QTc interval
* Participant has a history of moderate or severe hepatic impairment or severe renal insufficiency unless completely resolved for more than a year. Participant has clinically significant ongoing hepatic, renal, cardiac, vascular, pulmonary, gastrointestinal, endocrine, hematologic, rheumatologic, psychiatric or metabolic conditions (e.g., requiring frequent monitoring or medication adjustments or is otherwise unstable)