A total of 112 healthy subjects (with an approximate 1:1 female:male ratio) are planned to be randomized.Subjects will check into the clinical research unit (CRU) on Day -1. Day 1 will be a baseline electrocardiogram (ECG) profile day (all groups). Active dosing for Group 1 with VI-0521 will commence on Day 2. VI-0521 dose will escalate every 2-3 days until steady-state is achieved on Day 10 for the therapeutic dose PHEN/TPM 7.5/46. A full ECG assessment day will be performed (all groups, blinded); enabling the comparison of VI 0521 PHEN/TPM 7.5/46 with the placebo (pooled Groups 2 and 3).Subsequently, VI-0521 doses will be escalated until subjects in Group 1 reach a dose of 22.5/138, and then continue at this dose level until Day 22. On Day 22, when the active group has reached a steady state for the supratherapeutic dose level, another ECG assessment day will be performed (all groups, blinded); enabling a comparison of supratherapeutic VI-0521 with placebo (pooled Groups 2 and 3). Groups 2 and 3 will receive VI-0521 placebo on Days 2 to 22.