Part 1 is a study with 14 Early AD and 14 Early PD patients who are randomized to 80 mg of Posiphen or placebo. Participants will undergo a Screening Visit, provide informed consent and be evaluated for eligibility per the inclusion and exclusion criteria. If enrolled, participants will proceed to the randomized treatment portions of the study. Period 1 consists of first-time dosing in clinic with administration of 80 mg of Posiphen or Placebo. Period 2 consists of an at home dosing period of 25±2 days, with daily administration of 80 mg of Posiphen or Placebo. Period 3 will be comprised of a stay at the clinical research unit where the subject will undergo study procedures that include safety assessments (AE and concomitant medication monitoring, 12-lead ECGs, clinical laboratory testing, vital signs assessments, and physical examinations), the last dose of Posiphen or Placebo, and 6 hours of blood and CSF sampling. At the end of blood/CSF sampling, the subjects will need to stay for a minimum of 1 hour of observation but may stay if necessary for observation until the following day (e.g., if the subject has blood/CSF sampling on Day 25, he/she may stay for observation until Day 26). After all end-of-study procedures are complete, the subject will be discharged to home. A 24-hour follow-up call will occur to assess the participants current condition and if there are any additional adverse events to report.

After completion of Part 1 of the study, the plasma and CSF samples will be analyzed for the biomarkers to determine if changes are needed to the biomarkers to be measured in Part 2. Since the conduct of the study in Part 2 will be identical to the conduct of the study in Part 1, recruitment will continue uninterrupted. The only potential change between Part 1 and Part 2 are the biomarkers to be measured.

Part 2 is a study with 40 Early PD patients, 10 patients each who are randomized to one of 4 treatment conditions of Posiphen (5 mg, 10 mg, 20 mg, or 40 mg). Participants will undergo a Screening Visit, provide informed consent and be evaluated for eligibility per the inclusion and exclusion criteria. If enrolled, participants will proceed to the randomized treatment portions of the study. Period 1 consists of first-time dosing in clinic with administration of 5, 10, 20, or 40mg of Posiphen. Period 2 consists of an at home dosing period of 25±2 days, with daily administration of 5, 10, 20, or 40mg of Posiphen. Period 3 will be comprised of a stay at the clinical research unit where the subject will undergo study procedures that include safety assessments (AE and concomitant medication monitoring, 12-lead ECGs, clinical laboratory testing, vital signs assessments, and physical examinations), the last dose of Posiphen or Placebo, and 6 hours of blood and CSF sampling. At the end of blood/CSF sampling, the subject will need to stay for a minimum of 1 hour of observation but may stay if necessary for observation until the following day (e.g., if the subject has blood/CSF sampling on Day 25, he/she may stay for observation until Day 26). After all end-of-study procedures are complete, the subject will be discharged to home. A 24-hour follow-up call will occur to assess the participant's current condition and if there are any additional adverse events to report.