Relative Bioavailability of Inhaled Fluticasone Propionate and Salmeterol - is Population Pharmacokinetic Modelling a Relevant Alternative to a Non-Compartmental Approach?

Background: Conventional statistical analysis of clinical data based on standard methods suggests that fixed combinations of fluticasone propionate (FP) and salmeterol (SALM) inhaled via Wixela Methods: Three inhalations of FP/SALM were administered to healthy subjects with 7-day washouts via test and reference and delivered doses of 100/50, 250/50, or 500/50 µg (total matched doses of 300/150, 750/150, or 1500/150 µg) in studies 1, 2, and 3, respectively. Empirically derived pharmacokinetic structural models, considering random effects of subject (FP, SALM) or study (SALM), emulating linear first-order input, disposition and elimination were fitted to plasma concentrations with a frequentist approach, by study/dose of FP, and across studies of SALM, using product as categorical covariate. Bioequivalence was assessed using individually predicted relative systemic extent of bioavailability (F4_rel) and ratio of individually predicted peak concentrations (Cmax). Results: The models adequately described plasma concentration versus time data. Descriptive statistics and bioequivalence assessments (90% confidence interval generally within 0.80-1.25) reflected NCA-derived outcomes. Conclusion: Model- and NCA-based approaches to assess bioequivalence were consistent. However, a frequentist parametric approach that fully utilizes the population analysis potential and mechanistically predicts regional pulmonary deposition of FP and SALM would require a demographically more heterogenous data set and analysis including additional outcomes of parenteral and oral administration to discriminate presystemic from systemic disposition.