 - Cohort 5 Elderly Placebo This single-center trial will be conducted in the United States. The overall time to participate in this study is 40 days if assigned to Cohort 1 to 4. Participants will be confined to the clinic for 12 days, and will be contacted by telephone 11 and 30 days after last dose of study drug for a follow-up assessment Days 21 and 40. 