A number of viruses have been reported to cause lethal hemorrhagic disease in humans and certain other primates. These viruses include viruses from a number of viral families, which include Filoviridae, Arenaviridae, Bunyaviridae, and Flaviridae. In particular the Filoviridae, which include the Ebola and Marburg viruses, have resulted in significant morbidity and mortality in infected populations. At present there are no satisfactory disease specific therapies or vaccines for diseases caused by the Filoviridae.
Patients affected with hemorrhagic fevers develop a severe consumptive disseminated intravascular coagulation.
Disseminated intravascular coagulation (DIC) is typically characterized by wide-spread systematic activation of the coagulation cascade resulting in excess thrombin generation. In addition, activation of the fibrinolytic system coupled with the consumption of coagulation factors results in a depletion of clotting factors and degradation of platelet membrane glycoproteins.
Conventional treatment of DIC has been aimed primarily at treatment of the underlying etiologic disease process and secondarily at the coagulopathy that results in the thrombotic and hemorrhagic manifestations. Reported therapies include replacement therapy of coagulation factors by transfusion of fresh frozen plasma. Heparin has been reported as sometimes used in combination with replacement therapy.
Tissue factor (“TF”) is a 47 kDa transmembrane glycoprotein that is the major cellular trigger of blood coagulation under physiologic conditions. The factor VIIa-tissue factor (“fVII/TF”) catalytic complex is able to generate factor Xa via direct activation of factor X, and indirectly through the activation of factor IX, thus initiating thrombin generation. It has been reported that tissue factor also plays an important role in disease processes resulting from the activation of the coagulation pathway. For example, TF levels are reported to be elevated during bacterial sepsis and this is believed to contribute directly to the pathogenesis of multiple organ failure (Doshi et al., “Evolving role of tissue factor and its pathway inhibitor”, Crit. Care. Med. (2002) 30(5 Suppl): 5241–5250). In addition, a number of viruses have been reported to activate the coagulation system following infection; such activation may also be triggered by the up regulation of TF expression (Bowman, et al., “Procoagulant and inflammatory response of virus-infected monocytes”, European J. Clin. Invest. (2002) 32: 759–766; Baugh et al., “Regulation of extrinsic pathway factor Xa formation by tissue factor pathway inhibitor”, J. Biol. Chem. (1998) 273: 4378–4386; Taylor et al., “Active site inhibited factor VIIa (DEGR VIIa) attenuates the coagulant and interleukin-6 and -8, but not tumor necrosis factor, responses of the baboon to LD100 Escherichra coli”, Blood (1998) 91: 1609–1615). A variety of inflammatory stimuli, including bacterial cell products, viral infection and cytokines, have been reported to induce the expression of TF on the surface of endothelial cells and monocytes, thereby activating the coagulation pathway (Doshi, et al.).
Tissue Factor Pathway Inhibitor (TFPI) is an endogenous systemically circulating plasma protein which is said to function as a physiological anticoagulant by inhibiting VIIa/TF complexes and preventing the initiation of coagulation. TFPI contains multiple Kunitz-type protease inhibitor domains, and is the principal physiologic inhibitor of TF/FVIIa. TFPI has been reported to bind to and inhibit factor Xa directly, prior to forming a quaternary inhibitory complex with TF/FVIIa, thereby inhibiting thrombin generation (Baugh et al.). Processes for preparing recombinant TFPI have been reported. See, e.g. U.S. Pat. No. 6,300,100 to Kamel et al.
In addition to its role in initiating coagulation, the TF/FVIIa has been reported to have direct proinflammatory effects independent of the activation of coagulation in man (Taylor et al.). In experimental settings where animals were depleted of TFPI, the animals were reported to have a demonstrated sensitivity to bacterial endotoxin and a higher propensity to develop intravascular coagulation (Doshi et al.). In a lethal E. coli sepsis model in baboons, treatment with TFPI was reported to attenuate the procoagulant and inflammatory cytokine interleukin-6 (IL-6) response and to prevent mortality (Creasey et al., “Tissue factor pathway inhibitor reduces mortality from Escherichia coli septic shock”, J. Clin. Invest. (1993) 91(6): 2850–2860). However, in healthy human volunteers administered low dose bacterial endotoxin, blocking TF/FVIIa with TFPI was reported to have no impact on inflammatory cytokines, but to completely prevent endotoxin-induced activation of coagulation (deJonge et al., “Tissue factor pathway inhibitor does not influence inflammatory pathways during human endotoxemia”, J. Infect. Dis. (2001) 183(12): 1815–1818). A phase II clinical trial of recombinant TFPI (rTFPI) in patients with severe sepsis, the rTFPI group was reported to demonstrate accelerated decrease of IL-6 plasma levels (Reinhart et al., “Assessment of the safety of recombinant tissue factor pathway inhibitor in patients with severe sepsis, a multicenter randomized, placebo controlled single blind, dose escalation study”, Crit. Clin. Med. (2001) 29(11): 2081–2089). A phase III trial of trifacogin, a rTFPI, in severe sepsis failed to show a reduction in the primary end-point of 28-day all cause mortality (Doshi et al.).
Protein C (PC) is another component of the natural anticoagulant system in mammals. Unlike TFPI, which acts at the level of TF/fVIIa, the PC pathway is reported to inhibit coagulation by down regulating thrombin formation via the proteolytic inactivation of the non-enzymatic cofactors factor Va and factor VIIIa (Esmon C., “Protein C pathway in sepsis”, Annals of Medicine (2002) 34:598–605). The active component of the PC pathway is termed activated PC (aPC). aPC is formed by the action of thrombin bound to the non-enzymatic co-factor thrombomodulin. A recombinant form of aPC (drotrecogin alfa) has been reported to reduce the incidence death in patients suffering from severe bacterial sepsis (Bernard et al., “Efficacy and safety of recombinant human activated Protein C for severe sepsis”, N. Engl. J. Med. (2001) 344:699–709).
Published United States Patent Application, publication number US 2001/0028880 A1, is said to relate to the treatment of viral hemorrhagic fever with Protein C.