PTI-125 for Mild-to-moderate Alzheimer's Disease Patients

Inclusion Criteria: * Ages \>= 50 and \<= 85 years * Informed consent form (ICF) signed by the subject or legally acceptable representative. * Clinical diagnosis of dementia due to possible or probable Alzheimer's disease * Mini-Mental State Examination score \>= 16 and \<= 26 at screening * If female, postmenopausal for at least 1 year * Patient living at home, senior residential setting, or an institutional setting without the need for continuous (i.e. 24-h) nursing care * General health status acceptable for participation in the study * Fluency (oral and written) in English or Spanish * If receiving memantine, rivastigmine, galantamine or an AChEI, receiving a stable dose for at least 3 months. If receiving donepezil, any dose lower than 23 mg once daily. * The patient is a non-smoker for at least 3 years. * The patient or legal representative must agree to comply with the drawing of blood samples and with a lumbar puncture and the drawing of cerebrospinal fluid samples. * The patient has a ratio of total tau/Aβ42 in cerebrospinal fluid \>= 0.28. * Patient has a caregiver or legal representative responsible for administering the drug and recording the time. Exclusion Criteria: * Exposure to an experimental drug, experimental biologic or experimental medical device within the longer of 5 half-lives or 3 months before screening * Enrollment in the previous PTI-125 trial * A medical condition that would interfere with a lumbar puncture * Residence in a skilled nursing facility and requiring 24 h care. * Clinically significant laboratory test results * Clinically significant untreated hypothyroidism * Insufficiently controlled diabetes mellitus * Renal insufficiency (serum creatinine \> ULN) * Malignant tumor within 3 years before screening (except squamous and basal cell carcinoma or cervical carcinoma in situ or localized prostate cancer or localized stage 1 bladder cancer) * History of ischemic colitis or ischemic enterocolitis * Unstable medical condition that is clinically significant in the judgment of the investigator * Alanine transaminase (ALT) or aspartate transaminase (AST) \> ULN or total bilirubin \> ULN. * History of myocardial infarction or unstable angina within 6 months before screening * History of more than 1 myocardial infarction within 5 years before screening * Clinically significant cardiac arrhythmia (including atrial fibrillation), cardiomyopathy, or cardiac conduction defect (patients with a pacemaker are acceptable) * Symptomatic hypotension, or uncontrolled hypertension * Clinically significant abnormality on screening electrocardiogram (ECG), including but not necessarily limited to a confirmed corrected QT interval value \>= 450 msec for males or \>= 470 msec for females. * Stroke within 18 months before screening, or history of a stroke concomitant with onset of dementia * History of brain tumor or other clinically significant space-occupying lesion on CT or MRI * Head trauma with clinically significant loss of consciousness within 12 months before screening or concurrent with the onset of dementia * Onset of dementia secondary to cardiac arrest, surgery with general anesthesia, or resuscitation * Specific degenerative Central Nervous System disease diagnosis other than Alzheimer's disease (eg, Huntington's disease, Creutzfeld-Jacob disease, Down's syndrome, Frontotemporal Dementia, Parkinson's disease) * Wernicke's encephalopathy * Active acute or chronic Central Nervous System infection * Donepezil 23 mg quaque die currently or within 3 months prior to randomization * Discontinued AChEI \< 30 days prior to randomization * Antipsychotics; low doses are allowed only if the subject has received a stable dose for at least 3 months before randomization * Tricyclic antidepressants and monoamine oxidase inhibitors * Anxiolytics or sedative-hypnotics, including barbiturates (unless given in low doses for benign tremor); low doses of benzodiazepines and zolpidem are allowed * Immunosuppressants, including systemic corticosteroids, if taken in clinically immunosuppressive doses (Steroid use for allergy or other inflammation is permitted.) * Antiepileptic medications if taken for control of seizures * Chronic intake of opioid-containing analgesics * Sedating H1 antihistamines * Nicotine therapy (all dosage forms including a patch), varenicline (Chantix), or similar therapeutic agent within 30 days before screening * Clinically significant illness within 30 days of enrollment * History of significant neurological, hepatic, renal, endocrine, cardiovascular, gastrointestinal, pulmonary, or metabolic disease * Positive serum hepatitis B surface antigen (HBsAg) or positive hepatitis C virus HCV antibody test at screening * Positive HIV test at screening * Positive urine drug test at screening * Loss of a significant volume of blood (\> 450 mL) within 4 weeks prior to the study * Suicidality on C-SSRS at screening