Extension of the circulatory half-life of recombinant ecallantide via albumin fusion without loss of anti-kallikrein activity.

Ecallantide comprises Kunitz Domain 1 of Tissue Factor Pathway Inhibitor, mutated at seven amino acid positions to inhibit plasma kallikrein (PK). It is used to treat acute hereditary angioedema (HAE). We appended hexahistidine tags to the N- or C-terminus of recombinant Ecallantide (rEcall) and expressed and purified the resulting proteins, with or without fusion to human serum albumin (HSA), using Pichia pastoris. The inhibitory constant (Ki) of rEcall-H6 or H6-rEcall for PK was not increased by albumin fusion. When 125I-labelled rEcall proteins were injected intravenously into mice, the area under the clearance curve (AUC) was significantly increased, 3.4- and 3.6-fold, for fusion proteins H6-rEcall-HSA and HSA-rEcall-H6 versus their unfused counterparts but remained 2- to 3-fold less than that of HSA-H6. The terminal half-life of H6-rEcall-HSA and HSA-H6 did not differ, although that of HSA-rEcall-H6 was significantly shorter than either other protein. Receptor Associated Protein (RAP), a Low-density lipoprotein Receptor-related Protein (LRP1) antagonist, competed H6-rEcall-HSA clearance more effectively than intravenous immunoglobulin (IVIg), a neonatal Fc receptor (FcRn) antagonist. HSA fusion decreases rEcall clearance in vivo, but LRP1-mediated clearance remains more important than FcRn-mediated recycling for rEcall fusion proteins. The properties of H6-rEcall-HSA warrant investigation in a murine model of HAE.

Correction of haemorrhagic shock-associated coagulopathy and impaired haemostasis by plasma, prothrombin complex concentrates or an activated protein C-targeted DNA aptamer in mice.

Even with extensive transfusion support, trauma-induced bleeding often leads to death. Early intervention may improve outcomes, yet which blood products, factor concentrates, or other drugs constitute optimal treatment is unclear. Patients with acute traumatic coagulopathy (ATC), arising from trauma and haemorrhagic shock, have the worst prognosis. Here, multiple interventions were compared in a mouse model of ATC. After the trauma of tissue excision, anaesthetized mice were bled to 35 mm Hg mean arterial pressure, maintained under shock for 60 min, and resuscitated with fluids equal in volume to the shed blood. Resuscitated mice were subjected to liver laceration to test haemostasis and blood loss was quantified. Saline-treated mice lost two- to three-fold more blood than sham-treated animals and were coagulopathic by prothrombin time elevation post- versus pre-procedure. Murine fresh-frozen plasma (mFFP), anti-activated protein C aptamer HS02-52G, or prothrombin complex concentrates eliminated the bleeding diathesis and coagulopathy; fibrinogen, plasminogen activator inhibitor-1, or tranexamic acid ameliorated bleeding or coagulopathy, but not both. HS02-52G and mFFP also eliminated the changes in plasma aPC and tissue plasminogen activator levels observed in saline-treated mice, as judged via microtiter plate biomarker assays. Procoagulant interventions, especially inhibiting aPC, could be beneficial in human ATC.

Selection and in vitro and in vivo characterization of a Kunitz protease inhibitor domain of protease nexin 2 variant that inhibits factor XIa without inhibiting plasmin.

The 57-amino acid Kunitz Protease Inhibitor (KPI) domain of Protease Nexin 2 inhibits Factor XIa (FXIa) and other proteases. We previously fused KPI to human serum albumin (KPIHSA). KPIHSA inhibits coagulation Factor XIa (FXIa) 6-fold more rapidly than plasmin. We screened a bacterial expression library of KPI variants randomized at M17, and selected M17D as having the highest anti-FXIa: antiplasmin activity ratio. Expressed as HSA fusion proteins in Pichia pastoris, KPIHSA and KPI(M17D)HSA inhibited FXIa indistinguishably (Ki 9 nM) but KPI(M17D)HSA lacked detectable antiplasmin activity. Purified variant and wild-type KPIHSA were expressed and injected into mice with ferric chloride-treated carotid arteries, with or without systemic administration of tissue plasminogen activator (Tenecteplase, TNKase). The time to arterial occlusion (TTO) or reperfusion (TTR) was assessed by Doppler ultrasound. TTR did not differ between mice treated with TNKase alone or with TNKase supplemented with 38 mg/kg KPI(M17D)HSA but was significantly prolonged to >60 min in all mice treated with TNKase and 38 mg/kg KPIHSA. TTO was significantly but equally prolonged by either 38 mg/kg KPIHSA or KPI(M17D)HSA versus vehicle controls. The antiplasmin activity of KPI is relevant in vivo but its elimination did not enhance counter-thrombosis by KPI.

Prothrombin, alone or in complex concentrates or plasma, reduces bleeding in a mouse model of blood exchange-induced coagulopathy.

Prothrombin complex concentrates (PCC) are fractionated plasma protein drugs that reverse warfarin anticoagulation. PCC may control more general bleeding. We sought to identify the dominant procoagulant factor in PCC in vivo. We tested PCC or coagulation factor (F) treatment in CD1 mice made coagulopathic by exchange of whole blood for washed red cells. Anesthetized mice were transfused with murine fresh-frozen plasma (mFFP), PCC, mixtures of human vitamin K-dependent proteins (VKDP) (prothrombin, FVII, FIX, or FX), or purified single human VKDP, immediately prior to tail transection (TT), liver laceration (LL), or intravascular laser injury (ILI). Plasma donor mice were treated with vehicle or control antisense oligonucleotide (ASO-CON) or ASO specific for prothrombin (FII) (ASO-FII) to yield mFFP or ASO-CON mFFP or ASO-FII mFFP. Blood losses were determined spectrophotometrically (TT) or gravimetrically (LL). Thrombus formation was quantified by intravital microscopy of laser-injured arterioles. PCC or four factor- (4F-) VKDP or prothrombin significantly reduced bleeding from TT or LL. Omission of prothrombin from 4F-VKDP significantly reduced its ability to limit bleeding. Mice transfused with ASO-FII mFFP demonstrated inferior haemostasis versus those transfused with ASO-FII following TT, LL, or ILI. Prothrombin is the dominant procoagulant component of PCC and could limit bleeding in trauma.

A factor XIa-activatable hirudin-albumin fusion protein reduces thrombosis in mice without promoting blood loss.

BACKGROUND: Hirudin is a potent thrombin inhibitor but its antithrombotic properties are offset by bleeding side-effects. Because hirudin's N-terminus must engage thrombin's active site for effective inhibition, fusing a cleavable peptide at this site may improve hirudin's risk/benefit ratio as a therapeutic agent. Previously we engineered a plasmin cleavage site (C) between human serum albumin (HSA) and hirudin variant 3 (HV3) in fusion protein HSACHV3. Because coagulation factor XI (FXI) is more involved in thrombosis than hemostasis, we hypothesized that making HV3 activity FXIa-dependent would also improve HV3's potential therapeutic profile. We combined albumin fusion for half-life extension of hirudin with positioning of an FXIa cleavage site N-terminal to HV3, and assessed in vitro and in vivo properties of this novel protein. RESULTS: FXIa cleavage site EPR was employed. Fusion protein EPR-HV3HSA but not HSAEPR-HV3 was activated by FXIa in vitro. FVIIa, FXa, FXIIa, or plasmin failed to activate EPR-HV3HSA. FXIa-cleavable EPR-HV3HSA reduced the time to occlusion of ferric chloride-treated murine arteries and reduced fibrin deposition in murine endotoxemia; noncleavable mycHV3HSA was without effect. EPR-HV3HSA elicited less blood loss than constitutively active HV3HSA in murine liver laceration or tail transection but extended bleeding time to the same extent. EPR-HV3HSA was partially activated in citrated human or murine plasma to a greater extent than HSACHV3. CONCLUSIONS: Releasing the N-terminal block to HV3 activity using FXIa was an effective way to limit hirudin's bleeding side-effects, but plasma instability of the exposed EPR blocking peptide rendered it less useful than previously described plasmin-activatable HSACHV3.

Fusion to Human Serum Albumin Extends the Circulatory Half-Life and Duration of Antithrombotic Action of the Kunitz Protease Inhibitor Domain of Protease Nexin 2.

BACKGROUND/AIMS: The Kunitz Protease Inhibitor (KPI) domain of protease nexin 2 (PN2) potently inhibits coagulation factor XIa. Recombinant KPI has been shown to inhibit thrombosis in mouse models, but its clearance from the murine circulation remains uncharacterized. The present study explored the pharmacokinetic and pharmacodynamic effects of fusing KPI to human serum albumin (HSA) in fusion protein KPIHSA. METHODS: Hexahistidine-tagged KPI (63 amino acids) and KPIHSA (656 amino acids) were expressed in Pichia pastoris yeast and purified by nickel-chelate chromatography. Clearance profiles in mice were determined, as well as the effects of KPI or KPIHSA administration on FeCl3-induced vena cava thrombus size or carotid artery time to occlusion, respectively. RESULTS: Fusion to HSA increased the mean terminal half-life of KPI by 8-fold and eliminated its interaction with the low density lipoprotein receptor-related protein. KPI and KPIHSA similarly reduced thrombus size and occlusion in both venous and arterial thrombosis models when administered at the time of injury, but only KPI was effective when administered one hour before injury. CONCLUSIONS: Albumin fusion deflects KPI from rapid in vivo clearance without impairing its antithrombotic properties and widens its potential therapeutic window.

Selection and characterization of a DNA aptamer inhibiting coagulation factor XIa.

Factor XIa (FXIa) is a serine protease that catalyzes the activation of Factor IX (FIX) in the blood coagulation cascade. FXIa and its precursor FXI are emergent therapeutic targets for the development of safer anticoagulant agents. Here, we sought a novel DNA-based agent to inhibit FXIa. Towards this goal, an 80 base, single-stranded DNA aptamer library (containing a 40 base randomized core) was screened for FXIa-binding candidates, using ten rounds of positive and negative selection. After selection, 6 of 89 different sequences inhibited FXIa-mediated chromogenic substrate S2366 cleavage. The most active anti-FXIa aptamer had a hypervariable central sequence 5'-AACCTATCGGACTATTGTTAGTGATTTTTATAGTGT-3' and was designated Factor ELeven Inhibitory APtamer (FELIAP). FELIAP, but not a scrambled aptamer control (SCRAPT), competitively inhibited FXIa-catalyzed S2366 cleavage, FIX activation, and complex formation with antithrombin. No effect of FELIAP on FXI activation was observed. FELIAP inhibited plasma clotting and thrombin generation assays to a significantly greater extent than SCRAPT. Immobilized FELIAP bound FXIa with strong affinity and an equilibrium binding constant (KD) in the low nanomolar range determined using surface plasmon resonance. FELIAP is the first FXIa-inhibitory aptamer to be described and constitutes a lead compound to develop related aptamers for in vivo use.

Phosphatidylserine externalization and procoagulant activation of erythrocytes induced by Pseudomonas aeruginosa virulence factor pyocyanin.

The opportunistic pathogen Pseudomonas aeruginosa causes a wide range of infections in multiple hosts by releasing an arsenal of virulence factors such as pyocyanin. Despite numerous reports on the pleiotropic cellular targets of pyocyanin toxicity in vivo, its impact on erythrocytes remains elusive. Erythrocytes undergo an apoptosis-like cell death called eryptosis which is characterized by cell shrinkage and phosphatidylserine (PS) externalization; this process confers a procoagulant phenotype on erythrocytes as well as fosters their phagocytosis and subsequent clearance from the circulation. Herein, we demonstrate that P. aeruginosa pyocyanin-elicited PS exposure and cell shrinkage in erythrocyte while preserving the membrane integrity. Mechanistically, exposure of erythrocytes to pyocyanin showed increased cytosolic Ca(2+) activity as well as Ca(2+) -dependent proteolytic processing of µ-calpain. Pyocyanin further up-regulated erythrocyte ceramide abundance and triggered the production of reactive oxygen species. Pyocyanin-induced increased PS externalization in erythrocytes translated into enhanced prothrombin activation and fibrin generation in plasma. As judged by carboxyfluorescein succinimidyl-ester labelling, pyocyanin-treated erythrocytes were cleared faster from the murine circulation as compared to untreated erythrocytes. Furthermore, erythrocytes incubated in plasma from patients with P. aeruginosa sepsis showed increased PS exposure as compared to erythrocytes incubated in plasma from healthy donors. In conclusion, the present study discloses the eryptosis-inducing effect of the virulence factor pyocyanin, thereby shedding light on a potentially important mechanism in the systemic complications of P. aeruginosa infection.

Fluorous Analogue of Chloramine-T: Preparation, X-ray Structure Determination, and Use as an Oxidant for Radioiodination and s-Tetrazine Synthesis.

A fluorous oxidant that can be used to introduce radioiodine into small molecules and proteins and generate iodinated tetrazines for bioorthogonal chemistry has been developed. The oxidant was prepared in 87% overall yield by combining a fluorous amine with tosyl chloride, followed by chlorination using aqueous sodium hypochlorite. A crystal structure of the oxidant, which is a fluorous analogue of chloramine-T, was obtained. The compound was shown to be stable for 7 days in EtOH and for longer than three months as a solid. The oxidant was effective at promoting the labeling of arylstannanes using [(125)I]NaI, where products were isolated in high specific activity in yields ranging from 46% to 86%. Similarly, iodinated biologically active proteins (e.g., thrombin) were successfully produced, as well as a radioiodinated tetrazine, through a concomitant oxidation-halodemetalation reaction. Because of its fluorous nature, unreacted oxidant and associated reaction byproducts can be removed quantitatively from reaction mixtures by passing solutions through fluorous solid phase extraction cartridges. This feature enables rapid and facile purification, which is critical when working with radionuclides and is similarly beneficial for general synthetic applications.

The fibrinogen but not the Factor VIII content of transfused plasma determines its effectiveness at reducing bleeding in coagulopathic mice.

BACKGROUND: The evidence supporting plasma transfusion as a means to restore hemostatic control and prevent or treat bleeding is weak, leading to uncertainties as to which proteins affect the therapeutic quality of plasma. Some regulators focus on coagulation Factor (F)VIII activity, but whether this measure reflects overall transfusable plasma efficacy is questionable. We developed a mouse model of coagulopathy in which bleeding outcomes were responsive to plasma transfusion and addressed the relative contributions of FVIII and fibrinogen (Fg) to plasma quality. STUDY DESIGN AND METHODS: Anesthetized mice were rendered coagulopathic by four rounds of exchange of whole blood for washed red blood cells (RBCs) in 5% human albumin solution (HAS), which reduced RBCs, platelets, and plasma protein levels by 55, 66, and 80% of starting levels, in a blood exchange-induced coagulopathy approach (BECA). Before tail vein transection, BECA mice were transfused with HAS, wild-type murine fresh-frozen plasma (WT mFFP), or mFFP from FVIII-/- or Fg-/- knockout mice. BECA mice were also subjected to laser-induced arteriolar injury and thrombus formation quantified by intravital microscopy. RESULTS: Transfusion of WT or FVIII-/- mFFP reduced blood loss by fourfold in BECA mice relative to HAS; Fg-/- mFFP had no effect. WT or FVIII-/- mFFP transfusion, but not that of Fg-/- mFFP, increased thrombus size in laser-injured BECA mice arterioles. Extended refrigerated storage of mFFP did not reduce its antihemorrhagic effects. CONCLUSIONS: The content of Fg, but not FVIII, determined the efficacy of plasma transfusion in coagulopathic mice.

Alpha-1 acid glycoprotein reduces hepatic leukocyte recruitment in murine models of either early endotoxemia or early sepsis.

OBJECTIVE: To characterize the effect of systemically administered AGP on early leukocyte recruitment in the livers of endotoxemic or septic mice and to determine whether this is influenced by LPS sequestration. METHODS: Endotoxemia was induced in C57Bl/6 mice via intraperitoneal injection of LPS. Sepsis was induced in mice by cecal ligation and perforation. AGP (165 mg/kg) or saline (20 mL/kg) or HAS (200 mg/kg) was administered immediately after surgery or LPS injection and the hepatic microcirculation was examined by intravital microscopy at four hour. RESULTS: Leukocyte adhesion in the PSV was reduced by treatment with AGP in mice subjected to either LPS or CLP protocols compared to either saline or HAS treatment. AGP-treated mice also had significantly higher sinusoidal flow in both models. Pre-incubation of LPS with AGP reduced the ability of LPS to recruit leukocytes to the liver microcirculation. CONCLUSIONS: AGP was more effective in limiting hepatic inflammation and maintaining perfusion than saline or HAS, in both endotoxemic and septic mice. AGP sequestration of LPS may contribute to its anti-inflammatory effects.

In vivo clearance of alpha-1 acid glycoprotein is influenced by the extent of its N-linked glycosylation and by its interaction with the vessel wall.

Alpha-1 acid glycoprotein (AGP) is a highly glycosylated plasma protein that exerts vasoprotective effects. We hypothesized that AGP's N-linked glycans govern its rate of clearance from the circulation, and followed the disappearance of different forms of radiolabeled human AGP from the plasma of rabbits and mice. Enzymatic deglycosylation of human plasma-derived AGP (pdAGP) by Peptide: N-Glycosidase F yielded a mixture of differentially deglycosylated forms (PNGase-AGP), while the introduction of five Asn to Gln mutations in recombinant Pichia pastoris-derived AGP (rAGP-N(5)Q) eliminated N-linked glycosylation. PNGase-AGP was cleared from the rabbit circulation 9-fold, and rAGP-N(5)Q, 46-fold more rapidly than pdAGP, primarily via a renal route. Pichia pastoris-derived wild-type rAGP differed from pdAGP in expressing mannose-terminated glycans, and, like neuraminidase-treated pdAGP, was more rapidly removed from the rabbit circulation than rAGP-N(5)Q. Systemic hyaluronidase treatment of mice transiently decreased pdAGP clearance. AGP administration to mice reduced vascular binding of hyaluronic acid binding protein in the liver microcirculation and increased its plasma levels. Our results support a critical role of N-linked glycosylation of AGP in regulating its in vivo clearance and an influence of a hyaluronidase-sensitive component of the vessel wall on its transendothelial passage.

Reduction of thrombus size in murine models of thrombosis following administration of recombinant α1-proteinase inhibitor mutant proteins.

The variant serpin α1-PI M358R inhibits thrombin and other proteases such as activated protein C (APC) and factor XIa. We previously described recombinant proteins HAPI M358R (α1-PI M358R containing an N-terminal extension corresponding to residues 1-75 of heparin cofactor II) and HAPI RCL5 (HAPI M358R with F352-I356 and I360 substituted for the corresponding residues of antithrombin), with enhanced selectivity for thrombin over APC inhibition. We tested the hypotheses that these recombinant proteins would limit thrombosis in three mouse models, and that the HAPI chimeric proteins would be more effective than α1-PI M358R. Recombinant serpins were purified from Escherichia coli by nickel chelate and ion exchange affinity chromatography, and administered to mice intravenously. HAPI RCL5 reduced incorporation of radiolabelled fibrin(ogen) into thrombi in the ferric chloride-injured vena cava in a dose-dependent manner; HAPI M358R was less effective and α1-PI M358R was without effect. In a model of murine endotoxaemia, HAPI RCL5 was more effective than α1-PI M358R in reducing radiolabelled fibrin(ogen) deposition in heart and kidneys; immunohistochemistry of tissue sections showed lesser staining with anti-fibrin(ogen) antibodies with both treatments. In the ferric chloride-injured murine carotid artery, administration of both recombinant serpins was equally effective in lengthening the vessel's time to occlusion. Our results show that the antithrombotic efficacy of the recombinant serpins correlates with their potency as thrombin inhibitors, since HAPI RCL5 inhibits thrombin, but not factors Xa, XIa, XIIa, or neutrophil elastase, more rapidly than α1-PI M358R.

Incorporation of albumin fusion proteins into fibrin clots in vitro and in vivo: comparison of different fusion motifs recognized by factor XIIIa.

BACKGROUND: The transglutaminase activated factor XIII (FXIIIa) acts to strengthen pathological fibrin clots and to slow their dissolution, in part by crosslinking active α(2)-antiplasmin (α(2)AP) to fibrin. We previously reported that a yeast-derived recombinant fusion protein comprising α(2)AP residues 13-42 linked to human serum albumin (HSA) weakened in vitro clots but failed to become specifically incorporated into in vivo clots. In this study, our aims were to improve both the stability and clot localization of the HSA fusion protein by replacing α(2)AP residues 13-42 with shorter sequences recognized more effectively by FXIIIa. RESULTS: Expression plasmids were prepared encoding recombinant HSA with the following N-terminal 23 residue extensions: H(6)NQEQVSPLTLLAG(4)Y (designated XL1); H(6)DQMMLPWAVTLG(4)Y (XL2); H(6)WQHKIDLPYNGAG(4)Y (XL3); and their 17 residue non-His-tagged equivalents (XL4, XL5, and XL6). The HSA moiety of XL4- to XL6-HSA proteins was C-terminally His-tagged. All chimerae were efficiently secreted from transformed Pichia pastoris yeast except XL3-HSA, and following nickel chelate affinity purification were found to be intact by amino acid sequencing, as was an N-terminally His-tagged version of α(2)AP(13-42)-HSA. Of the proteins tested, XL5-HSA was cross-linked to biotin pentylamine (BPA) most rapidly by FXIIIa, and was the most effective competitor of α(2)AP crosslinking not only to BPA but also to plasma fibrin clots. In the mouse ferric chloride vena cava thrombosis model, radiolabeled XL5-HSA was retained in the clot to a greater extent than recombinant HSA. In the rabbit jugular vein stasis thrombosis model, XL5-HSA was also retained in the clot, in a urea-insensitive manner indicative of crosslinking to fibrin, to a greater extent than recombinant HSA. CONCLUSIONS: Fusion protein XL5-HSA (DQMMLPWAVTLG4Y-HSAH6) was found to be more active as a substrate for FXIIIa-mediated transamidation than seven other candidate fusion proteins in vitro. The improved stability and reactivity of this chimeric protein was further evidenced by its incorporation into in vivo clots formed in thrombosis models in both mice and rabbits.

Comparison of methods for the purification of alpha-1 acid glycoprotein from human plasma.

Alpha-1 acid glycoprotein (AGP) is a highly glycosylated, negatively charged plasma protein suggested to have anti-inflammatory and/or immunomodulatory activities. Purification of AGP could be simplified if methods that exploit its high solubility under chemically harsh conditions could be demonstrated to leave the protein in its native conformation. Procedures involving exposure of AGP to hot phenol or sulphosalicylic acid (SSA) were compared to solely chromatographic methods. Hot phenol-purified AGP was more rapidly cleared from mice in vivo following intravenous injection than chromatographically purified AGP. In contrast, SSA-purified AGP demonstrated an identical in vivo clearance profile and circular dichroism spectrum to chromatographically purified AGP. Similarly, no differences in susceptibility to enzymatic deglycosylation or reactivity with Sambucus nigra lectin were detected between AGP purified via the two methods. Incorporation of the SSA step in the purification scheme for AGP eliminated the need for a large (4 mL resin/mL of plasma) initial chromatographic step and simplified its purification without causing any detectable distortion in the conformation of the protein. Confirmation that this procedure is nondenaturing will simplify AGP purification and investigation of its possible biological roles in laboratory animals.

A long-lasting, plasmin-activatable thrombin inhibitor aids clot lysis in vitro and does not promote bleeding in vivo.

The leech protein hirudin is a potent inhibitor of thrombin, but clinical use of recombinant hirudin is restricted by haemorrhagic risks, and complicated by hirudin's rapid clearance from the circulation. We previously employed albumin fusion to slow hirudin variant 3 (HV3) clearance. In this study, we hypothesized that reconfiguration of the chimera, appending human serum albumin (HSA) to the N-terminus of HV3, with an intervening plasmin cleavage site, would create a slowly cleared, plasmin-activatable HV3. Potential plasmin cleavage sites were screened by expression in Escherichia coli, interposed between glutathione sulfotransferase and HV3 domains. The most reactive sequence (GSGIYR-ITY) was recreated in C-terminally His-tagged albumin fusion protein HSACHV3, expressed in Pichia pastoris yeast and purified by nickel-chelate affinity chromatography. HSACHV3 showed no thrombin inhibitory activity in the absence of plasmin, but liberated active HV3 in a time- and concentration-dependent manner in its presence. In a discontinuous clot assay involving clot-bound thrombin, HSACHV3 assisted clot lysis by limiting clot extension in a tPA- and concentration-dependent manner. Similar results were obtained in plasma at higher concentrations of HSACHV3. The chimeric protein exhibited much slower clearance in mice than unfused HV3, and indistinguishable pharmacokinetics from unfused recombinant HSA. In a mouse tail transection bleeding model, doses of HSACHV3 identical to those of HV3 that elicited a four-fold increase in the volume of shed blood were without effect. Our results suggest that HSACHV3 is a fully latent, plasmin activatable, long-lasting hirudin, of potential benefit in thrombotic disorders resistant to natural or pharmacological clot lysis.

Addition of a sequence from alpha2-antiplasmin transforms human serum albumin into a blood clot component that speeds clot lysis.

BACKGROUND: The plasma protein alpha2-antiplasmin (alpha2AP) is cross-linked to fibrin in blood clots by the transglutaminase factor XIIIa, and in that location retards clot lysis. Competition for this effect could be clinically useful in patients with thrombosis. We hypothesized that fusion of N-terminal portions of alpha2-antiplasmin to human serum albumin (HSA) and production of the chimeric proteins in Pichia pastoris yeast would produce a stable and effective competitor protein. RESULTS: Fusion protein alpha2AP(13-42)-HSA was efficiently secreted from transformed yeast and purified preparations contained within a mixed population the full-length intact form, while fusions with longer alpha2AP moieties were inefficiently secreted and/or degraded. The alpha2AP(13-42)-HSA protein, but not recombinant HSA, was cross-linked to both chemical lysine donors and fibrin or fibrinogen by factor XIIIa, although with less rapid kinetics than native alpha2AP. Excess alpha2AP(13-42)-HSA competed with alpha2AP for cross-linking to chemical lysine donors more effectively than a synthetic alpha2AP(13-42) peptide, and reduced the alpha2AP-dependent resistance to fibrinolysis of plasma clots equally effectively as the peptide. Native alpha2AP was found in in vivo clots in rabbits to a greater extent than alpha2AP(13-42), however. CONCLUSION: In this first report of transfer of transglutamination substrate status from one plasma protein to another, fusion protein alpha2AP(13-42)-HSA was shown to satisfy initial requirements for a long-lasting, well-tolerated competitive inhibitor of alpha2-antiplasmin predicted to act in a clot-localized manner.

Combined administration of barbourin--albumin and hirudin--albumin fusion proteins limits fibrin(ogen) deposition on the rabbit balloon-injured aorta.

INTRODUCTION: There are continuing needs for new antithrombotic agents and procedures. We hypothesized that the slowly cleared recombinant fusion proteins barbourin--albumin (BLAH6) and hirudin--albumin (HLAH6) would be effective in limiting fibrin(ogen) and/or platelet deposition in a rabbit model of arterial injury. MATERIALS AND METHODS: Recombinant fusion proteins were expressed in Pichia pastoris fermenter cultures and purified by nickel-chelate affinity chromatography. They were injected intravenously into rabbits prior to blood sampling and platelet aggregometry, assessment of deposition of 125I-fibrin(ogen) and 51Cr-platelet onto the balloon-injured thoracic aorta, electron microscopy (EM) and immunohistochemistry of aortic sections, and determination of bleeding time following a standardized ear incision. RESULTS AND CONCLUSIONS: BLAH6 administration elicited a dose- and time-dependent inhibition of platelet aggregation in post-injection whole blood samples, and reduced both fibrin(ogen) and platelet deposition on the injured aorta, although the former effect was both more durable and more significant than the latter. In contrast, HLAH6 injection reduced fibrin(ogen) but not platelet deposition. Doses of the two proteins ineffective in preventing fibrin(ogen) deposition when given alone were effective when combined, suggesting at least additive effects. Immunohistochemistry and EM supported the radioactive deposition studies, while bleeding times were decreased with combined BLAH6 and HLAH6 administration compared to HLAH6 alone in a rabbit ear bleeding model. The data show that these fusion proteins exert an antithrombotic effect in vivo and may indicate that combined low-dose administration of antiplatelet and antithrombin agents could offer safety advantages in the treatment of thrombosis.

Recombinant albumins containing additional peptide sequences smaller than barbourin retain the ability of barbourin-albumin to inhibit platelet aggregation.

The previously described fusion protein BLAH(6) (Marques JA et al.,Thromb Haemost 2001; 86: 902-8) is a recombinant protein that combines the small disintegrin barbourin with hexahistidine-tagged rabbit serumalbumin (RSA) produced in Pichia pastoris yeast. We sought to determine: (1) if BLAH(6) was immunogenic; and (2) if its barbourin domain could be productively replaced with smaller peptides. Purified BLAH(6) was injected into rabbits, and anti-barbourin antibodies were universally detected in plasma 28 days later; BLAH(6) was, however, equally effective in reducing platelet aggregation in both naive and pre-treated rabbits. Thrombocytopenia was not observed, and complexing BLAH(6) to alpha(IIb)beta(3) had no effect on antibody detection. The barbourin moiety of BLAH(6) was replaced with each of four sequences: Pep I (VCKGDWPC); PepII (VCRGDWPC); PepIII (bar-bourin 41-54); and PepIV (LPSPGDWR). The corresponding fusion proteins were tested for their ability to inhibit ADP-induced platelet aggregation. PepIII-LAH(6) inhibited neither rabbit nor human platelets. PepI-LAH(6) and PepIV-LAH(6) inhibited rabbit platelet aggregation as effectively as BLAH(6), but PepIV-LAH(6) did not inhibit human platelet aggregation. PepI-LAH(6) and PepIILAH(6) inhibited human platelet aggregation with IC(50)s 10- and 20-fold higher than BLAH(6). Cross-immunoprecipitation assays with human platelet lysates confirmed that all proteins and peptides interacted with the platelet integrin alpha(IIb)beta(3), but with greatly varying affinities. Our results suggest that the antiplatelet activity of BLAH(6) can be retained in albumin fusion proteins in which smaller peptides replace the barbourin domain; these proteins may be less immunogenic than BLAH(6).