Protease domain exosites regulate extravascular binding of factor IX(a).

Background: Factor (F)IX is unique among the vitamin K-dependent coagulation factors in that a substantial portion is bound to extravascular sites.

Objectives: Determine the impact of exosites in the human FIX protease domain on clearance, tissue distribution, and activity in hemophilic mice.

Methods: Human FIX(a) variants with protease domain substitutions in the antithrombin (R150A), heparin (K126A/K132A), and FIX-Padua (R170A) exosites were evaluated in hemophilia B (zymogen) and A (protease) mice. Pharmacokinetic modeling of FIX(a) clearance, select tissue content, and in vivo activity were determined.

Results: FIX wild-type (WT) demonstrated poor plasma recovery, biphasic clearance, and heterogeneous tissue binding. FIX WT was predominantly found in liver, followed by plasma, kidney, heart, and brain. Equimolar FIX variants were similar except for FIX K126A/K132A, which demonstrated 2.3-fold higher plasma recovery and reduced liver content. FIXa WT was cleared more rapidly than zymogen, but also had poor recovery, biphasic clearance, and heterogeneous tissue binding. FIXa-Glu-Gly-Arg-chloromethyl ketone accelerated clearance from all compartments in the distribution phase but prolonged the terminal phase relative to FIXa. FIXa K126A/K132A, R150A, and R170A demonstrated 2- to 3-fold higher plasma recovery than WT, with reduced liver content for FIXa K126A/K132A. Despite reduced coagulant activity, FIX K126A/K132A and R150A demonstrated intact or enhanced potency relative to FIX WT in the saphenous vein bleeding and FeCl₃-induced carotid artery occlusion models, while FIX R170A had enhanced potency in both models.

Conclusion: Protease exosites regulate in vivo clearance, distribution, and FIX(a) activity. Human FIX(a) demonstrates extensive tissue-specific binding to extravascular sites.