Hemostasis involves the interaction of various hemostatic reaction routes finally leading to thrombus formation. Thrombi are deposits of blood components on the surface of the vascular wall that mainly consist of aggregated blood platelets and insoluble cross-linked fibrin. Fibrin formation is the result of the restricted proteolysis of fibrinogen by thrombin, a coagulation enzyme. Thrombin is the end product of the coagulation cascade, a succession of zymogen activations occurring on the surfaces of activated blood platelets and leucocytes, and a variety of vascular cells (for a survey, cf. K. G. Mann et al., Blood, 1990, Vol. 76, pp. 1-16).
A key function in the coagulation cascade resides in the activation of Factor X by the complex of activated Factor IX (Factor IXa) and activated Factor VIII (Factor VIIIa). A deficiency or a dysfunction of the components of this complex is associated with the blood disease known as hemophilia (J. E. Sadler & E. W. Davie: Hemophilia A, Hemophilia B, and von Willebrand's Disease, in G. Stamatoyannopoulos et al., (Eds.): The molecular basis of blood diseases. W.B. Saunders Co., Philadelphia, 1987, pp. 576-602). Hemophilia A is related to a deficiency of Factor VIII activity, whereas Hemophilia B is related to a Factor IX deficiency. Current treatment consists of a replacement therapy using pharmaceutical preparations comprised of the normal coagulation factor. Of these thrombopathies, Hemophilia A occurs more frequently, affecting approximately one out of 10,000 men. Replacement therapy in Hemophilia A patients involves the repeated administration of preparations containing normal Factor VIII by intravenous infusion. The interval between the infusions is a function of the degradation of the Factor VIII activity in blood circulation. The half-life of the Factor VIII activity after an infusion differs from one individual to another, ranging from 10 to 30 hours. Thus, a prophylactic therapy requires an infusion every two to three days. This constitutes a heavy load on the life of hemophilic patients, in particular, if the venous access has become difficult due to local citratization following frequent needle punctures for intravenous infusions.
It would be particularly advantageous if the frequency of infusions could be lowered by using Factor VIII having extended half-lives. The half-life of Factor VIII may be extended by interfering with the mechanism of Factor VIII degradation (clearance), for instance, by reducing the affinity of Factor VIII to receptors that are essential to its clearance, either directly by modifying Factor VIII on its binding site(s) for the clearance receptors concerned, or indirectly by using compounds interfering with the interaction of Factor VIII with those receptors. However, the design of such agents has so far been impeded by not knowing the Factor VIII clearance mechanism, the cell receptors involved in this process, and the molecular sites involved in the Factor VIII receptor interaction.
There is limited knowledge in the molecular field as to the clearance mechanism of Factor VIII. The Factor VIII protein is synthesized as a single chain polypeptide comprising 2332 amino acids and having the typical domain structure A1-A2-B-A3-C1-C2 (G. A. Vehar et al., Nature, Vol. 312, 1984, pp. 337-342; J J. Toole et al., Nature, Vol., 312, 1984, 342-347). Factor VIII enters the blood circulation as a heterodimeric complex of heavy and light chains as a result of intracellular endoproteolytic processing. The light chain comprises the amino acid residues 1649-2332 and contains the A3-C1-C2 domains. The heavy chain contains the domains A1-A2-B (residues 1-1648) and is heterogenic due to the limited proteolysis in a number of positions within the B domain. The Factor VIII heterodimer has no biological activity, but the heterodimer becomes active as a cofactor of the enzyme Factor IXa after proteolytic activation by thrombin or Factor Xa. Proteolysis affects both the heavy chain and the light chain of Factor VIII (M. J. S. H. Donath et al., J. Biol. Chem., Vol. 270, 1995, pp. 3648-3655), leading to the cleavage of an amino-terminal fragment from the light chain and a break of domain connection sites within the heavy chain (between domains A1-A2 and A2-B). The activated cofactor, Factor VIIIa, is a heterotrimer comprised of the A1 domain, the A2 domain and the light chain including domains A3-C1-C2.
It is well known in the art that the half-life of the non-activated Factor VIII heterodimer strongly depends on the presence of von Willebrand Factor, which exhibits a strong affinity to Factor VIII (yet not to Factor VIIIa) and serves as a carrier protein (J. E. Sadler and E. W. Davie: Hemophilia A, Hemophilia B and von Willebrand's disease, in G. Stamatoynnopoulos et al. (Eds.): The molecular basis of blood diseases. W.B. Saunders Co., Philadelphia, 1987, pp. 576-602). It is known that patients suffering from von Willebrand's disease type 3, who do not have a detectable von Willebrand Factor in their blood circulation, also suffer from a secondary Factor VIII deficiency. In addition, the half-life of intravenously administered Factor VIII in those patients is 2 to 4 hours, which is considerably shorter than the 10 to 30 hours observed in Hemophilia A patients.
From these findings results that Factor VIII tends to a rapid clearance from the blood circulation and that this process is to some extent inhibited by complexation with its natural carrier, von Willebrand Factor. Nevertheless, its half-life remains undesirably short.
Recently, it has been indicated in a preliminary report that Factor VIII activated by thrombin binds to low density lipoprotein receptor protein (“LRP”) (A. Yakhyaev et al., Blood, Vol. 90 (Suppl. 1), 1997, 126-I (Abstract). This abstract describes the cell absorption and the degradation of Factor VIII fragments activated by thrombin and reports that the A2 domain, unlike the two other subunits of the Factor VIIIa heterotrimer, interacts with cell-bound LRP. The authors have suggested that binding of the A2 domain to LRP further destabilizes the loose interaction of the A2 domain in the Factor VIIIa heterotrimer and thereby downwardly regulating Factor VIIIa activity.
It is known that LRP is one of the receptors that are involved in the clearance of various proteins. LRP in this field is also known as the alpha2-macroglobulin receptor, belonging to the family of low density lipoprotein (LDL) receptors. It is comprised of two non-covalently connected polypeptide chains: an alpha chain (515 kd) and a β-chain (85 kd) [for a review refer to D. K. Strickland et al., FASEB J Vol. 9, 1995, pp. 890-898]. LRP is. a multi-ligand receptor for lipoprotein and proteinase catabolism. The β-chain includes a transmembrane domain and a short cytoplasmatic tail which is essential to endocytosis. The alpha chain functions as a large ectodomain and includes three types of repeats: epidermal growth factor-like domains, Tyr-Trp-Thr-Asp sequences and LDL receptor class A domains. These class A domains are present in four separate clusters, clusters I (2 domains), II (8 domains), III (20 domains) and IV (11 domains). It has been shown that these clusters are involved in ligand binding. LRP is expressed in a plurality of tissues such as the placenta, lungs, brain, and liver. In the liver, LRP is present on parenchyma cells and Kupffer cells. Moreover, LRP is expressed in a plurality of cell types such as fibroblasts, smooth muscle cells, Leydig cells, Sertoli cells, and monocytes. The differentiation from monocytes to macrophages is associated with a drastic Increase in LRP expression. Finally, LRP is expressed also in cell types such as ape kidney cells (COS) or Chinese hamster ovary cells (CHO) (D. J. FitzGerald et al., J. Cell Biol. Vol. 129, 1995, pp. 1533-1541), which are both frequently used to express mammalian proteins including Factor VIII (R. J. Kaufman et al., Blood Coag. Fibrinol. Vol. 8 (Suppl. 2), 1997, pp. 3-14).
LRP is involved in the clearance of a diversity of ligands including proteases, inhibitors of the Kunitz type, protease serpin complexes, Upases and lipoproteins, which suggests that LRP plays an essential role in various physiological and pathophysiological clearance processes (Narita et al., Blood, Vol. 2, pp. 555-560, 1998; Orth et al., Proc. Natl. Acad. Sci., Vol. 89, pp. 7422-7426, 1992; Kounnas et al., J. Biol. Chem., Vol. 271, pp. 6523-6529, 1996). LRP's physiological importance goes back to the finding that LRP knock-out mice do not survive the embryonic stage (Herz, J. Curr. Opin. Lipidol. Vol. 4, 1993, pp. 107-113). LRP secretion may be complicated by LRP interacting with multiple ligands. Within the cell, LRP is, however, associated with its chaperone protein, the receptor-associated protein (RAP). If bound to RAP, LRP cannot interact with any of its known ligands (Herz et al., J. Biol. Chem., Vol. 266, pp. 21232-21238, 1991).
The interaction of LRP with its natural ligands may be effectively blocked by soluble LRP fragments. These fragments may be obtained by various methods known in the art, including recombinant techniques, and as such provide access to effective LRP antagonists (I. R. Horn, J. Biol. Chem., Vol. 272, 1997, pp. 13608-13613; B. Vash et al., Blood, Vol. 92, 1998, pp. 3277-3285).
In view of the typical role of LRP in the clearance of proteases, inhibitors and protease inhibitor complexes, it is to be noted that LRP also binds the activated non-enzymatic cofactor Factor VIIIa (A. Yakhyaev et al., Blood Vol. 90 (Suppl. 1), 1997, 126-1 (Abstract)). While that disclosure suggests LRP's role in the regulation of Factor VIIIa, it does not give any hint as to its role in the regulation of non-activated heterodimeric Factor VIII, although this would be of potential interest for the clearance of Factor VIII from the blood circulation—and hence the half-life of Factor VIII.
There have been several prior art attempts to enhance the pharmacokinetic profile of Factor VIII, including modifications in various regions of Factor VIII polypeptides.
WO 87/07144 describes various modifications of proteolytic interfaces comprising arginine and lysine residues, reducing the instability of the molecules for a specific protease-catalyzed cleavage, for instance the Factor VIIIa interface between Arg 1721 and Ala 1722.
WO 95/18827, WO 95/18828 and WO 95/18829 describe Factor VIII derivatives with modifications in the A2 region of the heavy chain.
WO 97/03193 discloses Factor VIII polypeptide analogs in which the modifications comprise alterations of the metal binding properties of the molecule.
WO 97/03195 describes Factor VIII:C polypeptide analogs in which modifications are provided on one or several amino acid residues adjacent an Arg residue.
EP-0 808 901 describes the construction of Factor VIII variants including at least one mutation in at least one immunodominant region of Factor VIII and the use of these Factor VIII variants in the treatment of patients with Factor VIII inhibitors. Those modifications do not result in an extended half-life or enhanced stability of the Factor VIII variant, neither in vivo nor in vitro.
In light of the prior art, none of the documents suggests that a modification in the light chain of Factor VIII will lead to a modified binding affinity relative to a cell receptor and, consequently, to a reduced clearance of the Factor VIII protein and an extended half-life and an enhanced stability of Factor VIII.