Two Factor XI Concentrates Correct Impaired Thrombin Generation in Major FXI Deficiency but Are Not Equivalent in Their Effect
Authors: Pike Gillian N , Cumming Anthony M , Hay Charles R.M , Sempasa Brenda , Sutherland Megan , Thachil Jecko , Burthem John , Bolton-Maggs Paula H. B 
Published: October 2019

Abstract
Introduction: Factor XI (FXI) deficiency is an autosomally inherited bleeding disorder characterised by an increased risk of excessive bleeding following trauma or surgery. However, considerable phenotypic heterogeneity of the bleeding tendency is observed between individuals with this disorder. Treatment options for patients requiring FXI replacement are fresh frozen plasma (preferably pathogen-inactivated) or FXI concentrate. Two FXI concentrates are available: Hemoleven® ( LFB Biomedicaments, Les Ulis, France) and Factor XI Concentrate (Bio-Products Laboratory (BPL), Elstree, UK). Guidelines previously recommended a maximum replacement dose of 30 U/Kg for both concentrates but more recently a lower dose of 10-15 U/kg has been advised. The two FXI concentrates may reduce bleeding risk in FIX deficiency following surgery but indications for their use are unclear and treatment in some cases has been associated with thrombosis.
Aims: To quantify thrombin generation in major FXI deficiency (FXI:C <15 IU/dL) and to compare the in vitro effects of both FXI concentrates on thrombin generation parameters in this population with each other and with reference range values. To assess the clinical relevance of in vitro results through comparison of thrombin generation following in vitro and in vivo FXI replacement in individuals requiring surgery.
Methods: Thrombin generation (TG) was measured in controls (n=50), in individuals with FXI deficiency ( FXI < 15 IU/dL) pre and post in vitro spiking with both FXI concentrates (n=10), and in ex vivo samples following treatment with BPL FXI concentrate (n=3). Blood was drawn into S-Monovette® tubes (Sarstedt, Leicester, U.K.) containing 0.106 M trisodium citrate (1:9, V:V) and corn trypsin inhibitor (CTI) (Haematologic Technologies Inc., Essex Junction, VT, U.S.A.), at a concentration of 20 µg/mL whole blood. Thrombin generation was measured in platelet rich plasma using the Calibrated Automated Thrombography method with a tissue factor trigger of 0.5pM. Statistical analysis was performed using GraphPad prism, version 6 software package (GraphPad, san Diego, CA, USA) using student’s t-test, Mann-Whitney U test or a Wilcoxon signed rank test according to data distribution. P value <0.05 was considered significant.
Results: Major FXI deficiency (FXI:C <15 IU/dL) was associated with significantly impaired TG compared to controls, demonstrating reduced endogenous thrombin potential (ETP), peak height and velocity (all p<0.0001) and prolonged time to peak (p = 0.021). All TG parameters significantly improved from baseline with FXI replacement with both concentrates in vitro (equivalent in vivo dose 10 U/Kg). Comparison of the two FXI concentrates demonstrated that LFB Hemoleven® had greater effect on TG than BPL FXI in vitro at all doses (equivalent in vivo doses 10, 20 and 30 U/Kg): higher ETP (p < 0.0001), peak height (p < 0.01) velocity (p < 0.0002) and shorter lag time and time to peak (both p < 0.003). However, some measurements with LFB Hemoleven® exceeded the reference range. At lower doses both FXI concentrates normalised TG parameters in vitro (equivalent in vivo dose 2.5 IU/Kg LFB Hemoleven® or 5 U/Kg BPL FXI). Three patients received in vivo treatment with BPL FXI concentrate prior to surgery. TG was compared between baseline, in vitro spiked and post infusion ex vivo samples. Comparable ETP and peak height results were obtained from in vitro spiked and post infusion ex vivo samples.
Conclusions: Individuals with FXI:C levels <15 IU/dL show impaired thrombin generation. Both FXI concentrates improve thrombin generation in vitro in patients with FXI deficiency. However, when tested in vitro with the TG assay, the concentrates differ in potency and dose response and for both concentrates, doses lower than present recommendations normalised thrombin generation. Comparison of in vitro spiked and ex vivo samples suggest that in vitro results could be used to estimate an expected in vivo response to FXI replacement for the BPL product.
Acknowledgments: This work is supported by a Fellowship Project Award from the Bayer Hemophilia Awards Program, an unrestricted grant from LFB Biotechnologies and a Wycherley Fellowship grant. Hemoleven® concentrate was kindly provided by LFB Biotechnologies.

Disclosures
Pike: Bayer: Honoraria, Research Funding; LFB Biotechnologies: Honoraria, Research Funding. Bolton-Maggs:Bio-Products Laboratory (U.K.): Consultancy; LFB Biotechnologies: Consultancy.