Comparison of Bioavailability of Single-Dose Extended-Release Venlafaxine Capsules in Obese Patients Before and After Gastric Bypass Surgery

Study Objective: The extended-release (ER) form of venlafaxine is preferred because of improved patient adherence, but the immediate-release (IR) form is frequently used after Roux-en-Y gastric bypass (RYGB) surgery because of concerns for malabsorption. The objective of this study was to determine whether a statistically significant and predictable change in the bioavailability of venlafaxine ER capsules occurs after RYGB. Design: Prospective nonblinded single-dose pharmacokinetic study. Setting: Clinical research unit at a large tertiary care medical practice. Patients: Ten adult pre–bariatric surgery patients who met the criteria for noncomplicated RYGB were enrolled and served as their own controls. Interventions: Patients were administered one venlafaxine ER 75-mg capsule at two visits—the first visit at least 1 week before undergoing RYGB and the second visit 3–4 months after RYGB. Blood samples were collected at predetermined intervals over 48 hours after each dose, and the pharmacokinetics of venlafaxine were measured. Measurements and Main Results: Plasma levels of venlafaxine and its primary metabolite, O-desmethylvenlafaxine (ODV), were compared at baseline and 3–4 months after RYGB. The areas under the serum concentration–time curves from 0–24 hours (AUC _0–24 ) for venlafaxine (mean ± SD 734 ± 602 vs 630 ± 553 ng·hr/ml, p=0.22) and ODV (mean ± SD 894 ± 899 vs 1083 ± 972 ng·hr/ml, p=0.07) were similar before and after RYGB. Using a bioequivalence approach, differences in pre-RYGB and post-RYGB values of AUC _0–24 , peak serum concentration, and time to peak serum concentration were not statistically significant for either venlafaxine or ODV. Conclusion: This study suggests that RYGB does not significantly alter the amount of venlafaxine or its active metabolite, ODV, absorbed from a venlafaxine ER capsule or the time over which it is absorbed.