The Function of extravascular coagulation factor IX in haemostasis

Abstract

Introduction The majority of clotting factor IX (FIX) resides extravascularly, in the subendothelial basement membrane, where it is important for haemostasis. Aim We summarize preclinical studies demonstrating extravascular FIX and its role in haemostasis and discuss clinical observations supporting this. We compare the in vivo binding of BeneFIX® and the extended half‐life FIX, Alprolix®, to extravascular type IV collagen (Col4). Methods Three mouse models of haemophilia were used: the FIX knockout as the CRM− model and two knock‐in mice, representing a CRM+ model of a commonly occurring patient mutation (FIXR333Q) or a mutation that binds poorly to Col4 (FIXK5A). The murine saphenous vein bleeding model was used to assess haemostatic competency. Clinical publications were reviewed for relevance to extravascular FIX. Results CRM status affects recovery and prophylactic efficacy. Prophylactic protection decreases ~5X faster in CRM+ animals. Extravascular haemostasis can explain unexpected breakthrough bleeding in patients treated with some EHL‐FIX therapeutics. In mice, both Alprolix® and BeneFIX® bind Col4 with similar affinities (Kd~20–40 nM) and show dose‐dependent recoveries. As expected, the concentration of binding sites in the mouse calculated for Alprolix® (574 nM) was greater than for BeneFIX® (405 nM), due to Alprolix® binding to both Col4 and the endothelial cell neonatal Fc receptor. Conclusion Preclinical and clinical results support the interpretation that FIX plays a role in haemostasis from its extravascular location. We believe that knowing the CRM status of haemophilia B patients is important for optimizing prophylactic dosing with less trial and error, thereby decreasing clinical morbidity.

Plasma factor IX: The tip of the iceberg?

“…Clinicians rely on FIX activity via an activated partial thromboplastin one‐stage clot‐based assay that is more readily available in clinical laboratories for pharmacokinetic and pharmacodynamic determinations of FIX prophylaxis. As described in this review, 4 plasma FIX activity alone may not be reflective of the extravascular FIX pool status. Moreover, there are currently no in vitro assays that directly measure the extravascular pool of FIX after SHL or EHL FIX infusion or assesses the degree of migration of extravascular FIX into the intravascular space with infusions or bleeding episodes.…”

“…Although the clinical phenotypes of both haemophilia A and B are similarly classified based on FVIII and FIX activity levels, there are significant differences in the biochemical properties of the vitamin‐K‐dependent FIX protein that differentiates its role in haemostasis 2,3 . A growing body of evidence suggests that the unique extravascular distribution of FIX may contribute to variations in the pharmacokinetic profile, bleeding phenotypes and response to factor replacement observed clinically in patients with haemophilia B in contrast to FVIII in haemophilia A 4 …”

“…2,3 A growing body of evidence suggests that the unique extravascular distribution of FIX may contribute to variations in the pharmacokinetic profile, bleeding phenotypes and response to factor replacement observed clinically in patients with haemophilia B in contrast to FVIII in haemophilia A. 4 In the article entitled "Function of Extravascular Coagulation Factor IX in Haemostasis" in this issue of Haemophilia, 4 Mann and colleagues provide an overarching review of key pre-clinical studies including various FIX murine models that ultimately elucidated FIX's ability to bind type IV collagen (or collagen IV) and has advanced our understanding of an extravascular pool of FIX that plays an important role in secondary haemostasis. Secondly, the authors discuss potential clinical implications of these studies and highlight recent clinical observations describing discrepancies between bleeding rates and plasma FIX activity levels in patients with haemophilia B treated with various modified FIX replacement products.…”

The Molecular Basis of FIX Deficiency in Hemophilia B

“…Besides the membrane-anchoring function, the Gla domain of FIX may be involved in its binding to tissue factor (TF) in the FIX/TF/FVIIa ternary complex [ 32 ], which converts mature FIX to active FIX (FIXa) by cleaving the activation peptide off ( Figure 1 ). Additionally, the Gla domain in FIXa contributes to its binding with the C2 domain of FVIIIa [ 33 , 34 ] and the collagen IV [ 6 , 35 ].…”

The B-team: Equal but different?

“…Over the last few years, I have noticed more discussion at conferences and within the literature focusing on extravascular distribution of FIX. Continued research into this area is needed in order to better optimise prophylaxis for people with haemophilia B [5] .…”