Impact of Postural Restrictions on Tetrabenazine Pharmacokinetics in Healthy Volunteers: A Randomized Crossover Study Emphasizing Variance Minimization Strategies in Good Clinical Practice-Guided Bioequivalence Research

Background: Tetrabenazine, a VMAT2 inhibitor used for hyperkinetic disorders, shows considerable pharmacokinetic variability due to extensive first-pass metabolism. Standardization of clinical trial conditions, including posture, may reduce variability and improve bioequivalence assessments. Objective: The aim of this study was to determine the impact of postural restriction on the pharmacokinetics of tetrabenazine and its active metabolite, dihydrotetrabenazine (HTBZ), under controlled conditions. Methods: A randomized, open-label, four-period replicate crossover study enrolled 72 healthy fasted adults who received a single 25 mg tetrabenazine dose under two conditions: 4 h semirecumbent posture versus unrestricted movement. Plasma drug concentrations were measured across 36 h using validated LC–MS/MS method. Pharmacokinetic parameters were estimated via non-compartmental analysis and compared with Wilcoxon signed-rank tests. Results: Postural restriction significantly increased tetrabenazine exposure (AUC0–t: +16.4%, p < 0.0001) and half-life (p = 0.002), with a nonsignificant rise in Cmax. For HTBZ, Cmax decreased (−16.2%, p = 0.018), whereas AUC was unchanged. Parent-to-metabolite ratios increased by 24–29%. Replicate design analyses showed reduced intra-subject variability for tetrabenazine AUC with posture control (~24% vs. >28%). Simulation suggested that posture restriction could lower sample size requirements by 15–30% in two-period average bioequivalence trials. Conclusions: Maintaining a semirecumbent posture after dosing enhances tetrabenazine’s bioavailability, attenuates early metabolite formation, and reduces pharmacokinetic variability. Incorporating posture control into bioequivalence trial protocols may optimize study design, reduce participant exposure, and align with ICH-GCP ethical principles.