This trial is an open-label, non-randomized, one-sequence crossover study. All subjects will receive an oral dose of metformin 500 mg tablet twice daily on Days 1 to 5 and on Days 30 to 34; an oral dose of sitagliptin 100 mg tablet once daily on Days 6 to 10 and on Days 35 to 39 and a single oral dose of VI-0521 (phentermine/topiramate) capsule once daily each morning at 3.75/23 mg for 2 days (Days 11-12), 7.5/46 mg for 3 days (Days 13-15), 11.25/69 mg for 3 days (Days 16-18) and 15/92 mg for 21 days (Days 19-39) plus a single oral dose of 2 g (four 500 mg) probenecid tablets on Day 29.
Subjects will remain at the site for the duration of the study (from Day -1 to Day 40). Blood samples for the determination of metformin concentrations in plasma will be collected at 0 (pre-dose), 1, 2, 3, 4, 6, 8 and 12 hours after the morning administration of metformin or metformin plus VI-0521 on Days 5 and 34. Blood samples for the determination of sitagliptin concentrations in plasma will be collected at 0 (pre-dose), 1, 2, 3, 4, 5, 7, 10, 16 and 24 hours after the administration of sitagliptin or sitagliptin plus VI-0521 on Days 10 and 39. Blood samples for the determination of phentermine and topiramate concentrations in plasma will be collected at 0 (pre-dose), 1, 2, 3, 4, 5, 7, 10, 16 and 24 hours after the administration of the investigational product(s) on Days 28, 29, 34 and 39. Pre-dose blood samples for determination of the investigational drug concentrations in plasma will be taken in the morning on Days 4 and 33 for metformin; Days 9 and 38 for sitagliptin and Days 27, 33 and 38 for phentermine and topiramate. Semen samples for determination of topiramate concentrations will be collected on Day 28 between five and seven hours after VI-0521 administration in 8 male non-vasectomized subjects to obtain at least 6 evaluable subjects.
The primary endpoints of this study will be the PK parameters AUC0-τ and Cmax of phentermine, topiramate, sitagliptin and metformin.The secondary endpoint is the topiramate concentration in semen.
