<qa>  How many hours does ACH-0137171placebo take?Cohort 1: 300 mg ACH-0137171placebo every 12 hours 600 mgday Cohort 2: 300 mg ACH-0137171placebo every 6 hours 1200 mgday Cohort 3: 600 mg ACH-0137171placebo every 6 hours 2400 mgday 