PB1183 - Performance of a Chromogenic Factor IX Activity Assay in the Recovery of Factor IX Replacement Therapies

Background: The standard treatment for patients with hemophilia B without inhibitors is intravenous FIX replacement therapy with recombinant FIX or plasma-derived FIX concentrates. Accurate measurement of factor concentrates is necessary to ensure correct dosing. For some modified rFIX products, one-stage clotting assay methods can result in different potencies depending upon the activated partial thromboplastin time (APTT) reagent. Chromogenic FIX activity assays have shown useful in monitoring select extended half-life coagulation factor replacements although limited data exists.

Aims: To evaluate the recovered activity of seven FIX replacement products including AlphaNine SD, Alprolix, BeneFIX, Idelvion, Rebinyn, Ixinity and Rixubis by a new chromogenic FIX assay.

Methods: Each replacement product was reconstituted according to the manufacturer’s instruction, then diluted with congenital FIX deficient plasma to prepare seven levels (0.05, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 IU/mL) based on labelled potencies. Ten replicates of each level were measured fresh after preparation using a new Chromogenic Factor IX assay (Precision BioLogic) on an IL ACL TOP 700 CTS analyzer. Linear regression analysis of dose-dependent recovery was performed.

Results: Using acceptance criteria of 100 ± 25 percent recovery, the Chromogenic Factor IX assay accurately quantified 6/7 products across all levels including AlphaNine, Alprolix, BeneFIX, Rebinyn, Ixinity and Rixubis. The mean FIX recovery was 96, 116, 93, 82, 117 and 102% respectively, relative to the theoretical target across all tested levels. There was an over-recovery of Idelvion (albumin fusion recombinant FIX) with a mean recovery of 153%.

Conclusion(s): Our Chromogenic Factor IX assay can be used to accurately measure FIX activity in plasma samples with AlphaNine, Alprolix, BeneFIX, Ixinity, Rebinyn and Rixubis. However, there was an overestimation of Idelvion, suggesting a chromogenic method may not be suitable for monitoring this product.