Considerations on activity assay discrepancies in factor VIII and factor IX products

Abstract

Coagulation factor VIII (FVIII) and factor IX (FIX) play pivotal roles in hemostasis, and their congenital deficiency leads to the Xlinked bleeding disorders hemophilia A and B, respectively. These conditions are managed by prophylactic and on-demand regimens using plasma-derived or recombinant FVIII and FIX products. 1 Both regimens are based on the observation that the severity of bleeding and associated arthropathy are directly related to the factor levels of the individual. 2 The goal of prophylaxis is to maintain trough factor activity levels at least 1% to 5% of normal, which has been shown to dramatically reduce the frequency of bleeding episodes and damage

Plasma Clearance of Coagulation Factor VIII and Extension of Its Half-Life for the Therapy of Hemophilia A: A Critical Review of the Current State of Research and Practice

“…Early studies reported the OC/CS assay ratio as ~0.9 for rFVIII-Fc activity measurements [94,95], whereas in subsequent studies, it was as low as ~0.5 for some reagents combinations and some patients' plasma samples [96][97][98]. Similar OC/CS ratios were found for other BDD-FVIII-based variants: turoctocog alfa, simoctog alpha, and moroctocog alfa [87,97]. This indicates that all BDD-FVIII variants may show such discrepancy, though particular assay setups may minimize it.…”

“…For the wild-type full-length (FL) rFVIII, tested versus purified pdFVIII standard, the OC and CS assays generally produce similar results, whereas these may be discrepant for a modified FVIII. In particular, deletion of the B-domain may result in lower value of FVIII activity measured by OC assay up to ~0.5-fold of the CS assay result [86,87]. The OC/CS assay discrepancy causes an uncertainty in FVIII potency assignment; thus, selection of appropriate method for that should be supported by a clinical study.…”

“…There are three current therapeutic EHL FIX variants, which are based on (i) Fc-fragment fusion (eftrenonacog alfa), (ii) pegylation (nonacog beta pegol), and (iii) albumin fusion (albutrepenonacog alfa). For measurement of FIX activity, the respective assay setups differ only by FIX depletion: (i) in plasma used for OC assay, and (ii) in solution used for CS assay step 1, containing FXIa (for FIX activation) and FVIII [87]. Similarly to EHL FVIII, the EHL FIX products are also known to have OC/CS assay results discrepancy, compared to nonmodified FIX comparator (standard).…”