Stability of infliximab serum concentrations during long-term frozen storage: a paired biobank study

Background: Pharmacokinetic research of infliximab often relies on prospectively collected, long-term frozen serum samples. Although the effect of storage conditions on infliximab stability has been researched extensively, the influence of long-term storage on the reliability of infliximab concentration measurements remains unknown. Methods: In this proof-of-concept study, infliximab serum concentrations measured after 2, 5, and 8 years of standardized frozen storage at −70 °C were compared with paired samples analyzed directly after collection. Equivalence was assessed using a two one-sided tests framework with predefined margins of 80%-125%, in line with FDA equivalence guidance. Results: Equivalence was demonstrated at all three storage durations. Geometric mean ratios were 87.0% (90% CI, 80.9%-93.6%), 103.2% (90% CI, 90.3%-117.9%), and 99.3% (90% CI, 92.5%-106.6%) at 2, 5, and 8 years, respectively, all within predefined equivalence margins. A statistically significant reduction was observed in the 2-year group (P = .010), though within equivalence bounds. No concentration-dependent bias was detected (P = .926), and Spearman correlation between fresh and frozen concentrations was high (ρ = 0.908, P < .001). Conclusion: Infliximab serum concentrations remain stable after long-term frozen storage of up to 8 years under standardized conditions, supporting the validity of deferred concentration analyses in prospectively stored serum samples.