Inclusion Criteria:
Lead-in study participants are eligible to be included in the study only if they meet the following criteria:
* Completed lead-in Protocol TB006AD2102 (Participants from both study drug and placebo groups) or are eligible for the lead-in study but were not enrolled (de novo).
* Eligibility must be reconfirmed by the investigator for participants who have a gap of more than 28 days between lead-in Protocol TB006AD2102 completion and enrolment in the current study. These participants will undergo the screening procedures in the current Open-label extension (OLE) protocol, with the exception of imaging.
* Contraceptive use by men should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
* Females must be of non-childbearing potential.
* Participants or caregiver has the ability to understand the purpose and risks of the study and provide signed and dated informed consent. Participants whose caregiver signs the informed consent must provide their assent.
* Either currently or previously (in pre-AD condition) literate and capable of reading, writing, and communicating effectively with others.
* Participants, along with the caregiver, will be compliant with study visits, procedure.
De novo participants, identified by the sponsor and referred to a participating site, are eligible to be included in the study only if all of the following criteria apply:
* Male and/or female \> 50 years of age at the time of signing the informed consent.
* Body weight of ≥ 50 kilograms (kg) and body mass index (BMI) between 18 and 35 kilograms per square meter (kg/m\^2), inclusive.
* MMSE score of 24 or less.
* Must be ambulatory.
* Clinical diagnosis of AD consistent with the following:
1. Probable AD, according to National Institute of Neurological and Communicative Disorders and Stroke - Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA).
2. Meets the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) - Criteria for Major Neurocognitive Disorder (previously dementia).
* Contraceptive use by men should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
* Females must be of non-childbearing potential.
* Participants or caregiver has the ability to understand the purpose and risks of the study and provide signed and dated informed consent. Participants whose caregiver signs the informed consent must provide their assent.
* Either currently or previously (in pre-AD condition) literate and capable of reading, writing, and communicating effectively with others.
Exclusion Criteria:
Lead-in study participants are excluded from the study if any of the following criteria apply:
* Development of an intolerable adverse event or an adverse event that was considered an important safety risk in Protocol TB006AD2102
* Any of the following emerging medical or psychiatric exclusion criteria as defined in the lead-in Protocol TB006AD2102:
1. Any medical or neurological condition other than AD that in the opinion of the investigator could be a contributing cause of the Participant's dementia
2. History within the past 6 months or evidence of clinically significant psychiatric illness like major depression, schizophrenia, or bipolar affective disorder
3. Diagnosis of a dementia-related central nervous system (CNS) disease other than AD (eg, Parkinson's Disease, Huntington's Disease, frontotemporal dementia, multi-infarct dementia, dementia with Lewy bodies, or normal pressure hydrocephalus).
4. Identification of other known cause of dementia or any other clinically significant contributing co-morbid pathologies at screening MRI
5. Any contraindications to having a brain MRI eg, pacemaker; non-MRI compatible aneurysm clips, artificial heart valves, or other metal foreign body; claustrophobia)
6. Any untreated or unstable clinically significant medical condition like hypertension, diabetes, chronic obstructive pulmonary disorder, asthma, or depression
7. Any clinically significant findings in medical examination, including physical examination, 12-lead electrocardiogram (ECG), clinical laboratory tests.
8. Undergone major surgery \<= 2 months before study drug administration.
9. Loss of more than 100 milliliters (mL) blood (eg, a blood donation) within 2 months before first study drug administration, or has received any blood, plasma, or platelet transfusions within 3 months before Day 1, or plans to donate blood during the study or within 3 months after the study.
10. Regular alcohol consumption within 6 months prior to the study defined as: an average weekly (QW) intake of \> 20 units for males or \> 16 units for females. One unit is equivalent to 8 grams (g) of alcohol.
11. Meets DSM-5 criteria for moderate or severe substance use disorder within the past 12 months, or has a positive test for substances of abuse, or has used substances, including but not limited to opiates, methadone, buprenorphine, methamphetamine, cocaine, amphetamines recreationally within the past 12 months.
12. Unable to complete this study for other reasons or the investigator believes the Participant should be excluded.
* Since participating in Protocol TB006AD2102, the participant has participated in another drug, biologic, device, or a clinical study or treatment with an investigational drug or approved therapy for investigational use.
* Any clinically significant findings in medical examination. This includes physical examination, 12-lead ECG, or clinical laboratory tests on the final visit in Protocol TB006AD2102 or on the Baseline visit in this study. Participants who are coronavirus disease of 2019 (COVID-19)-positive at Screening (or end of treatment \[EOT\] from lead-in study) must delay the start of the study until they are COVID-19-negative. They may be retested at weekly intervals.
* Participants who have undergone major surgery since enrolment in Protocol TB006AD2102 will be considered on a case-by-basis.
De Novo participants are excluded from the study if any of the following criteria apply:
* Any medical or neurological condition other than AD that in the opinion of the investigator could be a contributing cause of the participant's dementia
* History within the past 6 months or evidence of clinically significant psychiatric illness (eg, major depression, schizophrenia, or bipolar affective disorder).
* Diagnosis of a dementia-related CNS disease other than AD (eg, Parkinson's Disease, Huntington's Disease, frontotemporal dementia, multi-infarct dementia, dementia with Lewy bodies, normal pressure hydrocephalus).
* Identification of other known cause of dementia or any other clinically significant contributing co-morbid pathologies at screening MRI, in the opinion of the investigator.
* Participation in any other drug, biologic, device, or clinical study or treatment with any investigational drug or approved therapy for investigational use within 30 days (or 5 half-lives, whichever is longer) prior to screening, and/or participation in any other clinical study involving experimental medications for AD within the 60 days (or 5 half-lives, whichever is longer) prior to screening.
* Any contraindications to having a brain MRI eg, pacemaker; non-MRI-compatible aneurysm clips, artificial heart valves, or other metal foreign body; claustrophobia).
* Any untreated or unstable clinically significant medical condition (ie, hypertension, diabetes, chronic obstructive pulmonary disorder, asthma, depression, etc.) as judged by the investigator.
* Any clinically significant findings in medical examination, including physical examination, 12-lead ECG, clinical laboratory tests.
* Undergone major surgery \<= 2 months before study drug administration.
* Loss of more than 100 mL blood (eg, a blood donation) within 2 months before first study drug administration, or has received any blood, plasma, or platelet transfusions within 3 months before Day 1, or plans to donate blood during the study or within 3 months after the study.
* Recent (3-month) history of a positive COVID-19 test result or disease symptoms of COVID-19 disease such as shortness of breath, cough, rhinorrhea, and sore throat, etc.
* Known history of, or a positive test result for Hepatitis B surface antigen (HBsAg), immunoglobulin M antibody to Hepatitis B core antigen (IgM anti HBc), anti-hepatitis C virus antibodies (HCV), or human immunodeficiency virus (HIV) types 1 or 2 at screening. Participants with a documented history of treatment for Hepatitis C are otherwise eligible to participate.
* Regular alcohol consumption within 6 months prior to the study defined as: an average QW intake of \> 20 units for males or \> 16 units for females.
* Meets DSM-5 criteria for moderate or severe substance use disorder within the past 12 months, or has a positive test for substances of abuse, or has used substances, including but not limited to opiates, methadone, buprenorphine, methamphetamine, cocaine, amphetamines recreationally within the past 12 months.
* Unable to complete this study for other reasons or the investigator believes that he or she should be excluded.
