Characterization of a chimeric plasminogen activator consisting of a single-chain Fv fragment derived from a fibrin fragment D-dimer-specific antibody and a truncated single-chain urokinase

Engineering of plasminogen activators for targeting to thrombus and heightening thrombolytic efficacy

Thrombotic occlusion of the coronary artery, which triggers acute myocardial infarction, is one of the major causes of death in the USA. Currently, arterial occlusions are treated with intravenous plasminogen activators (PAs), which dissolve the clot by activating plasminogen. However, PAs indiscriminately generate plasmin, which depletes critical clotting factors (fibrinogen, factor V, and factor VIII), precipitates a lytic state in the blood, and produces bleeding complications in a large patient population. PAs have been extensively investigated to achieve thrombus specificity, to attenuate the bleeding risk, and to widen their clinical applications. In this review, we discuss various strategies that have been pursued since the beginning of thrombolytic therapy. We review the biotechnological approaches that have been used to develop mutant and chimeric PAs for thrombus selectivity, including the use of specific antibodies for targeting thrombi. We discuss particulate carrier-based systems and triggered-release concepts. We propose new hypotheses and strategies to spur future studies in this research arena. Overall, we describe the approaches and accomplishments in the development of patient-friendly and workable delivery systems for thrombolytic drugs.

Towards effective and safe thrombolysis and thromboprophylaxis: preclinical testing of a novel antibody-targeted recombinant plasminogen activator directed against activated platelets

RATIONALE

Fibrinolysis is a valuable alternative for the treatment of myocardial infarction when percutaneous coronary intervention is not available in a timely fashion. For acute ischemic stroke, fibrinolysis is the only treatment option with a very narrow therapeutic window. Clinically approved thrombolytics have significant drawbacks, including bleeding complications. Thus their use is highly restricted, leaving many patients untreated.

OBJECTIVE

We developed a novel targeted fibrinolytic drug that is directed against activated platelets.

METHODS AND RESULTS

We fused single-chain urokinase plasminogen activator (scuPA) to a small recombinant antibody (scFvSCE5), which targets the activated form of the platelet-integrin glycoprotein IIb/IIIa. Antibody binding and scuPA activity of this recombinant fusion protein were on par with the parent molecules. Prophylactic in vivo administration of scFvSCE5-scuPA (75 U/g body weight) prevented carotid artery occlusion after ferric chloride injury in a plasminogen-dependent process compared with saline (P<0.001), and blood flow recovery was similar to high-dose nontargeted urokinase (500 U/g body weight). Tail bleeding time was significantly prolonged with this high dose of nontargeted urokinase, but not with equally effective targeted scFvSCE5-scuPA at 75 U/g body weight. Real-time in vivo molecular ultrasound imaging demonstrates significant therapeutic reduction of thrombus size after administration of 75 U/g body weight scFvSCE5-scuPA as compared with the same dose of a mutated, nontargeting scFv-scuPA or vehicle. The ability of scFvSCE5-scuPA to lyse thrombi was lost in plasminogen-deficient mice, but could be restored by intravenous injection of plasminogen.

CONCLUSIONS

Targeting of scuPA to activated glycoprotein IIb/IIIa allows effective thrombolysis and the potential novel use as a fibrinolytic agent for thromboprophylaxis without bleeding complications.

Prophylactic thrombolysis by thrombin-activated latent prourokinase targeted to PECAM-1 in the pulmonary vasculature

A recombinant prodrug, single-chain urokinase-type plasminogen activator (scuPA) fused to an anti-PECAM-1 antibody single-chain variable fragment (anti-PECAM scFv/scuPA) targets endothelium and augments thrombolysis in the pulmonary vasculature.(1) To avoid premature activation and inactivation and to limit systemic toxicity, we replaced the native plasmin activation site in scFv/low-molecular-weight (lmw)-scuPA with a thrombin activation site, generating anti-PECAM scFv/uPA-T that (1) is latent and activated by thrombin instead of plasmin; (2) binds to PECAM-1; (3) does not consume plasma fibrinogen; (4) accumulates in mouse lungs after intravenous injection; and (5) resists PA inhibitor PAI-1 until activated by thrombin. In mouse models of pulmonary thrombosis caused by thromboplastin and ischemia-reperfusion (I/R), scFv/uPA-T provided more potent thromboprophylaxis and greater lung protection than plasmin-sensitive scFv/uPA. Endothelium-targeted thromboprophylaxis triggered by a prothrombotic enzyme illustrates a novel approach to time- and site-specific regulation of proteolytic reactions that can be modulated for therapeutic benefit.

Delivery of Anti-Platelet-Endothelial Cell Adhesion Molecule Single-Chain Variable Fragment-Urokinase Fusion Protein to the Cerebral Vasculature Lyses Arterial Clots and Attenuates Postischemic Brain Edema

Efficacy and safety of current means to prevent cerebrovascular thrombosis in patients at high risk of stroke are suboptimal. In theory, anchoring fibrinolytic plasminogen activators to the luminal surface of the cerebral endothelium might arrest formation of occlusive clots in this setting. We tested this approach using the recombinant construct antiplatelet-endothelial cell adhesion molecule (PECAM) single-chain variable fragment (scFv)-urokinase-type plasminogen activator (uPA), fusing low-molecular-weight single-chain urokinase-type plasminogen activator with a scFv of an antibody directed to the stably expressed endothelial surface determinant PECAM-1, implicated in inflammation and thrombosis. Studies in mice showed that scFv-uPA, but not unconjugated uPA 1) accumulates in the brain after intravascular injection, 2) lyses clots lodged in the cerebral arterial vasculature without hemorrhagic complications, 3) provides rapid and stable cerebral reperfusion, and 4) alleviates post-thrombotic brain edema. Effective and safe thromboprophylaxis in the cerebral arterial circulation by anti-PECAM scFv-uPA represents a prototype of a new paradigm to prevent recurrent cerebrovascular thrombosis.

Soluble expression of a strong thrombolytic pro-urokinase mutant in Escherichia coli

A recombinant pro-urokinase mutant GZ5-sPA was successfully constructed by fusion of a high fibrin-affinity fragment GZ5 to the N-terminus of the serine protease domain of pro-urokinase (sPA). The fragment GZ5 contains a tetrapeptide GPRP and a tripeptide RGD, and was synthesized in bacterial preferred expressing codons. The mutant was then fused to the C-terminus of maltose binding protein (MBP) carried by pMAL-C2x vector, and expressed in Escherichia coli strain Origami (DE3). The produced fusion protein was highly soluble in the cytoplasm of the bacteria. After being cleaved with PreScission Protease to remove MBP tag, GZ5-sPA showed a molecular weight of 31 kDa on SDS-PAGE. GZ5-sPA maintained the same epitope as wild-type pro-urokinase and possessed a thrombolytic activity three times higher than standard urokinase did after being activated as two-chain form. The results could be a clue to other complicated heterogenous proteins similar to pro-urokinase.

Endothelial targeting of a recombinant construct fusing a PECAM-1 single-chain variable antibody fragment (scFv) with prourokinase facilitates prophylactic thrombolysis in the pulmonary vasculature

Means to prevent thrombus extension and local recurrence remain suboptimal, in part because of the limited effectiveness of existing thrombolytics. In theory, plasminogen activators could be used for this purpose if they could be anchored to the vascular lumen by targeting stably expressed, noninternalized determinants such as platelet-endothelial-cell adhesion molecule 1 (PECAM-1). We designed a recombinant molecule fusing low-molecular-weight single-chain prourokinase plasminogen activator (lmw-scuPA) with a single-chain variable fragment (scFv) of a PECAM-1 antibody to generate the prodrug scFv/lmw-scuPA. Cleavage by plasmin generated fibrinolytically active 2-chain lmw-uPA. This fusion protein (1) bound specifically to PECAM-1-expressing cells; (2) was rapidly cleared from blood after intravenous injection; (3) accumulated in the lungs of wild-type C57BL6/J, but not PECAM-1 null mice; and (4) lysed pulmonary emboli formed subsequently more effectively than lmw-scuPA, thereby providing support for the concept of thromboprophylaxis using recombinant scFv-fibrinolytic fusion proteins that target endothelium.

Construction and characterization of a recombinant plasminogen activator composed of an anti-fibrin single-chain antibody and low-molecular-weight urokinase

BACKGROUND

Targeting of plasminogen activators to the fibrin component of a thrombus by antibodies directed against human fibrin can enhance their thrombolytic potency and clot specificity.

OBJECTIVES

To overcome the disadvantages of chemical conjugation, we investigated whether the recombinant fusion of a single-chain antibody and a plasminogen activator results in an active bifunctional molecule that might be useful as a therapeutic agent.

METHODS

The cDNA of low-molecular-weight single-chain urokinase-type plasminogen activator, comprising amino acids Leu144-Leu411 (scuPA(LMW)), was cloned from human endothelial cells and fused to a single-chain antibody specific for the 7 N-terminal amino acids (beta(15-22)) in the beta-chain of human fibrin (scFv(59D8)). The fusion protein was purified using affinity chromatography with the beta(15-22)-peptide of human fibrin.

RESULTS

Purified scFv(59D8)-scuPA(LMW) migrated as a 60-kDa band, which is consistent with a molecule composed of one scFv(59D8) and one scuPA(LMW) moiety. Both functions of the fusion molecule, fibrin-specific binding and plasminogen activation, were fully preserved. In human plasma clots, thrombolysis by scFv(59D8)-scuPA(LMW) is significantly faster and more potent compared with the clinically used urokinase.

CONCLUSIONS

ScFv(59D8)-scuPA(LMW) constitutes a new recombinant chimeric plasminogen activator with a significantly enhanced thrombolytic potency and relative fibrin selectivity, that can be produced with modern methods at low cost, large quantities and reproducible activity in Escherichia coli.

alpha Domain deletion converts streptokinase into a fibrin-dependent plasminogen activator through mechanisms akin to staphylokinase and tissue plasminogen activator

The mechanism of action of plasminogen (Pg) activators may affect their therapeutic properties in humans. Streptokinase (SK) is a robust Pg activator in physiologic fluids in the absence of fibrin. Deletion of a "catalytic switch" (SK residues 1-59), alters the conformation of the SK alpha domain and converts SKDelta59 into a fibrin-dependent Pg activator through unknown mechanisms. We show that the SK alpha domain binds avidly to the Pg kringle domains that maintain Glu-Pg in a tightly folded conformation. By virtue of deletion of SK residues 1-59, SKDelta59 loses the ability to unfold Glu-Pg during complex formation and becomes incapable of nonproteolytic active site formation. In this manner, SKDelta59 behaves more like staphylokinase than like SK; it requires plasmin to form a functional activator complex, and in this complex SKDelta59 does not protect plasmin from inhibition by alpha(2)-antiplasmin. At the same time, SKDelta59 is unlike staphylokinase or SK and is more like tissue Pg activator, because it is a poor activator of the tightly folded form of Glu-Pg in physiologic solutions. SKDelta59 can only activate Glu-Pg when it was unfolded by fibrin interactions or by Cl(-)-deficient buffers. Taken together, these studies indicate that an intact alpha domain confers on SK the ability to nonproteolytically activate Glu-Pg, to unfold and process Glu-Pg substrate in physiologic solutions, and to alter the substrate-inhibitor interactions of plasmin in the activator complex. The loss of an intact alpha domain makes SKDelta59 activate Pg through classical "fibrin-dependent mechanisms" (akin to both staphylokinase and tissue Pg activator) that include: 1) a marked preference for a fibrin-bound or unfolded Glu-Pg substrate, 2) a requirement for plasmin in the activator complex, and 3) the creation of an activator complex with plasmin that is readily inhibited by alpha(2)-antiplasmin.

Construction and in vitro testing of a novel fab-hirudin-based fusion protein that targets fibrin and inhibits thrombin in a factor xa-dependent manner

Fibrin targeting of the thrombin inhibitor hirudin via chemical coupling is effective in vitro and in vivo. However, since chemical coupling has limitations, a recombinant approach was taken to improve the fibrin-targeting ability of hirudin. Additionally, to activate hirudin selectively at the target area and thereby limit side effects in an in vivo setting, the authors aimed to construct an inactive precursor molecule that is converted into an active thrombin inhibitor only upon cleavage by factor Xa. Using PCR, the coding region for hirudin was fused to parts of the genomic DNA of the IgG heavy chain that was cloned from the antifibrin antibody-producing hybridoma cell line 59D8. Additionally, a factor Xa recognition site was introduced between the antibody and the hirudin sequence. The fusion construct was then transfected into a heavy-chain loss variant of the hybridoma cell line 59D8. After selection of stable hybridoma clones, the expressed fusion protein was evaluated for its molecular size (57 kd) and its binding ability to the fibrin-specific peptide Bbeta 15-22. The cleavage of the fusion protein by factor Xa was demonstrated by HPLC. The recombinant anticoagulant revealed antithrombin activity only after cleavage by factor Xa. Thus, the newly designed hirudin fusion protein revealed the anticipated functions in vitro. Further experiments are needed to prove whether this precursor anticoagulant allows a highly clot-specific and efficient thrombin inhibition in vivo.

Construction and characterization of trifunctional single-chain urokinase-type plasminogen activators

Two chimeric proteins have been constructed. One consists of four parts: a portion of the low molecular mass single-chain urokinase-type plasminogen activator (scu-PA-32K, residues 144-411), a 15-mer linker sequence, the C-terminal amino-acid sequence (residues 53-65) of hirudin (Hir), and an RGD sequence derived from the leech protein decorsin, i.e. scu-PA(32 k)-linker-Hir (residues 53-65)-RGD peptide. The other comprises two main segments: scu-PA(32 k) and hirudin into which RGDSP is inserted between its residues 33 and 34, i.e. hirudin (residues 1-33)-RGDSP-hirudin (residues 34-65)-scu-PA(32 k). These two chimeric genes were expressed in Escherichia coli, and the products were purified by Zn2+-chelating Sepharose 4B chromatography and benzamidine Sepharose 6B chromatography. Our results suggested that these two chimeric proteins not only had plasminogen-dependent fibrinolytic activity, but also possessed platelet aggregation inhibitory activity and antithrombin activity.

Construction and expression of antibody targeted plasminogen activator*

It has been known that antibody-mediated plasminogen activator will be much more specific than its parent molecular. To get a cheaper and more effective medicine for thrombolytic therapy, we used SZ51, a GMP140 specific monoclonal antibody, and a truncated single-chain urokinase to construct a novel targeted plasminogen activator. PCR was used to amplify the region of VL and VH chains from Fab of SZ51, GMP140 specific monoclonal antibody, and scu-PA-32KD(leu144-leu411) from urokinase gene, respectively. Through suitable linker and appropriate restriction sites, these fragments were joined together and inserted into the expression vector, pET-5a, via NdeI site. The recombinant protein was expressed in BL21 (DE3) plyS, a kind of E. coli. It was shown in Western-blotting and ELISA that the protein could interact with the multiple cloned antibody of urokinase. After partial purification: dialysis, Sephadex G-100, dialysis and Phenyl-Sepharose fast flow, the product had a strong fibrinolytic activity through activating plasminogen on fibrin plate. The specific activity was about 47,000 IU/mg, corresponding to 80,000 IU/mg for the part of rscu-PA-32k, and the activity could be inhibited specifically by urokinase specific antibody. Activation of plasminogen by the chimera followed Michaelis-Menten kinetics, and the K(m) was 1.08 uM.

Construction and functional evaluation of a single-chain antibody fusion protein with fibrin targeting and thrombin inhibition after activation by factor Xa

BACKGROUND

Recombinant technology was used to produce a new anticoagulant that is preferentially localized and active at the site of the clot.

METHODS AND RESULTS

The variable regions of the heavy and light chains of a fibrin-specific antibody were amplified by polymerase chain reaction (PCR) with hybridoma cDNA. To obtain a functional single-chain antibody (scFv), a linker region consisting of (Gly(4)Ser)(3) was introduced by overlap PCR. After the scFv clones were ligated with DNA encoding the pIII protein of the M13 phage, high-affinity clones were selected by 10 rounds of panning on the Bbeta15-22 peptide of fibrin (beta-peptide). Hirudin was genetically fused to the C-terminus of the variable region of the light chain. To release the functionally essential N-terminus of hirudin at the site of a blood clot, a factor Xa recognition site was introduced between scFv(59D8) and hirudin. The fusion protein was characterized by its size on SDS-PAGE (36 kDa), by Western blotting, by its cleavage into a 29-kDa (single chain alone) and 7-kDa (hirudin) fragment, by its binding to beta-peptide, and by thrombin inhibition after Xa cleavage. Finally, the fusion protein inhibited appositional growth of whole blood clots in vitro more efficiently than native hirudin.

CONCLUSIONS

A fusion protein was constructed that binds to a fibrin-specific epitope and inhibits thrombin after its activation by factor Xa. This recombinant anticoagulant effectively inhibits appositional clot growth in vitro. Its efficient and fast production at low cost should facilitate a large-scale evaluation to determine whether an effective localized antithrombin activity can be achieved without systemic bleeding complications.

A prourokinase-RGDS chimera : Construction, expression and characterization

A tetrapeptide, RGDS, was inserted into proUK kringle domain G118-L119 by the construction of a mutant proUK-RGDS gene. The gene was expressed in the baculovirus expression system. Immunoaffinity chromatography was used to purify the chimera and protein with purity over 90% was achieved. The chimera was tested for its platelet membrane binding function and showed a calcium-dependent platelet binding activity. Amidolytic activity of the chimera was tested. The result indicated that specific amidolytic activity of plasmin activated chimera was 62 000 IU/mg, comparable to the previously reported 65 355 IU/mg of plasmin activated natural proUK([1]). Activation of plasminogen by the chimera after plasmin treatment followed Michieal-Menten kinetics, and the Km was 0.97 mumol/L, which was also comparable to 1.64 mumol/L of native urokinase. The chimera also showed intensive ability to inhibit platelet aggregationin vitro. These results indicate that this chimera might be useful as a bifunctional thrombolytic agent.

Staphylokinase as a plasminogen activator component in recombinant fusion proteins

The plasminogen activator staphylokinase (SAK) is a promising thrombolytic agent for treatment of myocardial infarction. It can specifically stimulate the thrombolysis of both erythrocyte-rich and platelet-rich clots. However, SAK lacks fibrin-binding and thrombin inhibitor activities, two functions which would supplement and potentially improve its thrombolytic potency. Creating a recombinant fusion protein is one approach for combining protein domains with complementary functions. To evaluate SAK for use in a translational fusion protein, both N- and C-terminal fusions to SAK were constructed by using hirudin as a fusion partner. Recombinant fusion proteins were secreted from Bacillus subtilis and purified from culture supernatants. The rate of plasminogen activation by SAK was not altered by the presence of an additional N- or C-terminal protein sequence. However, cleavage at N-terminal lysines within SAK rendered the N-terminal fusion unstable in the presence of plasmin. The results of site-directed mutagenesis of lysine 10 and lysine 11 in SAK suggested that a plasmin-resistant variant cannot be created without interfering with the plasmin processing necessary for activation of SAK. Although putative plasmin cleavage sites are located at the C-terminal end of SAK at lysine 135 and lysine 136, these sites were resistant to plasmin cleavage in vitro. Therefore, C-terminal fusions represent stable configurations for developing improved thrombolytic agents based on SAK as the plasminogen activator component.

Enhanced thrombolytic and antithrombotic potency of a fibrin-targeted plasminogen activator in baboons

BACKGROUND

Thrombolytic therapy reduces mortality in patients with acute myocardial infarction, but significant limitations exist with the use of currently available agents. In the present report, we describe the thrombolytic and antithrombotic potencies of a hybrid recombinant plasminogen activator consisting of an antifibrin antibody 59D8 (AFA) and low-molecular-weight single-chain urokinase-type plasminogen activator (scuPA).

METHODS AND RESULTS

A thrombolysis model in which thrombi are preformed in vivo in juvenile baboons was developed to compare the potencies of AFA-scuPA, recombinant tissue plasminogen activator (rTPA), and recombinant scuPA (rscuPA) in lysing nonocclusive 111In-labeled platelet-rich arterial-type thrombi and 125I-labeled fibrin-rich venous-type thrombi. Systemic infusion of 1.89 nmol/kg AFA-scuPA produced thrombolysis that was comparable to that obtained with much higher doses of TPA (14.2 nmol/kg) and rscuPA (28.5 nmol/kg). When steady-state plasma concentrations are normalized, AFA-scuPA lyses thrombi sixfold more rapidly than scuPA and TPA (P < .001) and reduces the rate of formation more than comparable doses of rscuPA (P < .0001). At equivalent thrombolytic doses, AFA-scuPA produced fewer antihemostatic effects than either rTPA or rscuPA. Template bleeding time measurements were shorter (3.5 +/- 0.12 minutes for AFA-scuPA versus 5.3 +/- 0.36 and 5.2 +/- 0.04 minutes for rTPA and rscuPA, respectively; P < .05), alpha 2-antiplasmin consumption was less (P < .05), and D-dimer generation was lower (P < .05).

CONCLUSIONS

We conclude that antibody targeting of scuPA to fibrin increases thrombolytic and antithrombotic potencies with less impairment of hemostasis compared with rTPA and rscuPA.

Preparation and characterization of a disulfide-linked bioconjugate of annexin V with the B-chain of urokinase: an improved fibrinolytic agent targeted to phospholipid-containing thrombi

A conjugate of annexin V and the B-chain of urokinase was prepared and its fibrinolytic properties were studied. First, a mutant of annexin V was constructed with an N-terminal extension of six amino acids (Met-Ala-Cys-Asp-His-Ser) and with Cys316 mutated to Ser; this molecule was expressed in Escherichia coli. The urokinase B-chain was prepared by limited reduction of the interchain disulfide bond between the A- and B-chains of urokinase. These two molecules were then then connected by a disulfide bond and purified to yield a 1:1 stoichiometric conjugate. The conjugate had the same catalytic activity as urokinase against a synthetic substrate, Glt-Gly-Arg-MCA, and a similar plasminogen activating activity. The conjugate showed the same binding affinity for phosphatidylserine-containing membranes as annexin V. The in vitro fibrinolytic activity of the conjugates on clots prepared from platelet-rich plasma was comparable to that of urokinase. However, the conjugate showed 3-4-fold stronger in vivo thrombolytic activity than urokinase in a rat pulmonary embolism model, while having essentially the same plasma clearance rate as urokinase or B-chain. These results show that annexin V is a useful agent for targeting plasminogen activators to phospholipid-containing thrombi.

T cell-targeted immunofusion proteins from Escherichia coli

Fusion proteins between cell-targeting domains and cytotoxic proteins should be particularly effective therapeutic reagents. We constructed a family of immunofusion proteins linking humanized Fab, F(ab')2, or single chain antibody forms of the H65 antibody (which recognizes the CD5 antigen on the surface of human T cells) with the plant ribosome-inactivating protein gelonin. We reasoned that such an immunofusion would kill human target cells as efficiently as the previously described chemical conjugates of H65 and gelonin (Better M., Bernhard, S. L., Fishwild, D. M., Nolan, P. A., Bauer, R. J., Kung, A. H. C., and Carroll, S. F. (1994) J. Biol. Chem. 269, 9644-9650) if both the recognition and catalytic domains remained active, and a proper linkage between domains could be found. Immunofusion proteins were produced in Escherichia coli as secreted proteins and were recovered directly from the bacterial culture supernatant in an active form. All of the immunofusion proteins were purified by a common process and were tested for cytotoxicity toward antigen-positive human cells. A 20-60-fold range of cytotoxic activity was seen among the fusion family members, and several fusion proteins were identified which are approximately as active as effective chemical conjugates. Based on these constructs, immunofusion avidity and potency can be controlled by appropriate selection of antibody domains and ribosome-inactivating protein.

Antigen recognition and targeted delivery by the single-chain Fv

The single-chain Fv (sFv) has proven attractive for immuno-targeting, both alone and as a targeting element within sFv fusion proteins. This chapter summarizes the features of sFv proteins that have sparked this interest, starting with the conservation of Fv architecture that makes general sFv design practical. The length and composition of linkers used to bridge V domains are discussed based on the sFv literature; special emphasis is given to the (Gly4Ser)3 15-residue linker that has proven of broad utility for constructing Fv regions of antibodies and other members of the immunoglobulin superfamily. The refolding properties of sFv proteins are summarized and examples given from our laboratory. Spontaneous refolding from the fully reduced and denatured state, typified by 26-10 sFv, is contrasted with disulfide-restricted refolding, exemplified by MOPC 315 and R11D10 sFv proteins, which recover antigen binding only if their disulfides have been oxidized prior to removal of denaturant. The medical value of sFv proteins hinges on their reliability in antigen recognition and rapidity in targeted delivery. Detailed analysis of specificity and affinity of antigen binding by the 26-10 antidigoxin sFv has demonstrated very high fidelity to the binding properties of the parent 26-10 sFv. These results gave confidence to the pursuit of more complex biomedical applications of these proteins, which is indicated by our work with the R11D10 sFv for the imaging of myocardial infarctions. Diagnostic imaging and therapeutic immunotargeting by sFv present significant opportunities, particularly as a result of their pharmacokinetic properties. Intravenously administered sFv offers much faster clearance than conventional Fab fragments or intact immunoglobulin with minimal background binding.

Medical applications of single-chain antibodies

A single-chain antibody or single-chain Fv (sFv) incorporates the complete antibody binding site in a single polypeptide chain of minimal size, with an approximate molecular weight of 26,000. In antibodies, the antigen combining site is part of the Fv region, which is composed of the VH and VL variable domains on separate heavy and light chains. Efforts over nearly two decades have indicated that Fv fragments can only rarely be prepared from IgG and IgA antibodies by proteolytic dissection. Beginning in 1988, single-chain analogues of Fv fragments and their fusion proteins have been reliably generated by antibody engineering methods. The first step involves obtaining the genes encoding VH and VL domains with desired binding properties; these V genes may be isolated from a specific hybridoma cell line, selected from a combinatorial V-gene library, or made by V gene synthesis. The single-chain Fv is formed by connecting the component V genes with an oligonucleotide that encodes an appropriately designed linker peptide, such as (Gly4-Ser)3. The linker bridges the C-terminus of the first V region and N-terminus of the second, ordered as either VH-linker-VL or VL-linker-VH. In principle, the sFv binding site can faithfully replicate both the affinity and specificity of its parent antibody combining site, as demonstrated in our model studies with the 26-10 anti-digoxin sFv. Furthermore, the sFv remains stable at low concentrations that promote VH and VL dissociation from the Fv heterodimer, resulting in loss of Fv binding. Intravenously administered sFv proteins exhibit accelerated biodistribution and exceptionally fast clearance compared to IgG or Fab. These pharmacokinetic properties allow rapid imaging by sFv, which therefore may be labeled with a short-lived isotope such as Tc-99m. Expression of a single gene product from fused sFv and effector genes facilitates immunotargeting of the effector protein, as shown for single-chain Fv toxin fusion proteins.

Development of thrombolytic agents

Despite their widespread use in patients with acute myocardial infarction, all currently available thrombolytic agents suffer from a number of significant limitations, including resistance to reperfusion, the occurrence of acute coronary reocclusion and bleeding complications. Furthermore, the therapeutic use of plasminogen activators as thrombolytic agents requires intravenous infusion of relatively large amounts of material. Therefore, the quest for thrombolytic agents with a higher thrombolytic potency, specific thrombolytic activity and/or a better fibrin-selectivity continues. Several lines of research towards improvement of thrombolytic agents are being explored, including the construction of mutants and variants of plasminogen activators, chimeric plasminogen activators, conjugates of plasminogen activators with monoclonal antibodies, or plasminogen activators from animal or bacterial origin.

Biochemical characterization of single-chain chimeric plasminogen activators consisting of a single-chain Fv fragment of a fibrin-specific antibody and single-chain urokinase

K12G0S32 is a 57-kDa recombinant single-chain chimeric plasminogen activator consisting of scFv-K12Go, a single-chain variable-region antigen-binding fragment (Fv) of the monoclonal antibody MA-15C5, which is specific for fragment D-dimer of human cross-linked fibrin, and a low-molecular-mass (33 kDa) urokinase-type plasminogen activator (u-PA-33k) containing amino acids Ala132-Leu411 (Holvoet, P., Laroche, Y., Lijnen, H. R., Van Cauwenberghe, R., Demarsin, E., Brouwers, E., Matthyssens, G. & Collen D. (1991) J. Biol. Chem. 266, 19717-19724). In addition, the Arg156-Phe157 thrombin-cleavage site in the u-PA moiety of K12G0S32 is removed by substitution of Phe157 with Asp. In the present study, the fibrinolytic potency of K12G0S32, determined in a system composed of a 125I-fibrin-labeled human plasma clot submerged in citrated plasma, was found to be only twofold higher than that of intact single-chain u-Pa (rscu-PA), but 17-fold higher than that of rscu-PA(M), a variant of rscu-PA in which the thrombin-cleavage site was removed by substitution of Phe157 with Asp. The fibrinolytic potency of K12G0S32T, with an intact thrombin-cleavage site, was 6-15-fold higher than that of rscu-PA. Conversion of 1 microM single-chain K12G0S32 or rscu-PA(M) into their two-chain derivatives with plasmin occurred at a rate of 1.0 +/- 0.15 nmol.min-1.nmol plasmin-1 and 0.85 +/- 0.074 nmol.min-1.nmol plasmin-1, compared to 14 +/- 2.3 nmol.min-1.nmol plasmin-1 and 18 +/- 2.6 nM.min-1.nmol plasmin-1 for K12G0S32T and rscu-PA, respectively. Purified fragment D-dimer of human cross-linked fibrin inhibited the fibrinolytic potency of single-chain K12G0S32T, but not of two-chain K12G0S32T, in a dose-dependent manner. Furthermore, the fibrinolytic potencies of two-chain K12G0S32 and K12G0S32T were not significantly higher than those of recombinant two-chain u-PA (rtcu-PA) or of rtcu-PA(M). These findings suggest that the 59-fold increase in fibrinolytic potency of K12G0S32T, relative to that of rscu-PA(M), is due both to targeting of the activator to the clot via the single-chain Fv fragment (sixfold increase) and to a more efficient conversion of single-chain K12G0S32T to its two-chain derivative (eightfold increase). Thus, targeting to clots by means of fibrin-specific antibodies results in a significant increase of the fibrinolytic potency of single-chain but not of two-chain u-PA.(ABSTRACT TRUNCATED AT 400 WORDS)

Remaining perspectives of mutant and chimeric plasminogen activators

Potential approaches to improve thrombolytic agents comprise the construction of mutants and variants of tissue-type plasminogen activator (tPA) or of single chain urokinase-type plasminogen activator (scuPA, pro-urokinase), of chimeric plasminogen activators and of conjugates of plasminogen activators with monoclonal antibodies. tPA mutants have been constructed with altered pharmacokinetic properties or altered functional properties, including binding to and stimulation by fibrin, resistance to plasmin and to protease inhibitors. Mutants of tPA described to date, obtained by deletion/substitution of functional domains or of single amino acids, have markedly reduced clearances, but usually also reduced specific thrombolytic potencies. Mutants of scuPA with improved thrombolytic potencies have thus far not been reported. Chimeric molecules containing functional domains of both tPA and scuPA have intact enzymatic properties of uPA and some fibrin affinity of tPA. Surprisingly, chimeras endowed with fibrin affinity usually have unaltered or reduced thrombolytic potencies. However, a chimera consisting of amino acids 87-274 of tPA and amino acids 138-411 of scuPA, with negligible fibrin affinity, has a 10-fold higher thrombolytic potency than scuPA in animal models of venous thrombosis, as a result of a delayed in vivo clearance and a relatively maintained specific thrombolytic activity. Plasminogen activators conjugated with antifibrin or antiplatelet monoclonal antibodies, either chemically or by recombinant DNA technology, are targeted to blood clots, resulting in a 5- to 10-fold increased thrombolytic potency. Thus, it is possible to develop plasminogen activators with improved thrombolytic potency. Whether such agents will be clinically useful remains to be established.

Engineered antibodies as pharmacological tools

The ability to engineer the antibody molecule has now progressed to the stage where one can realistically contemplate creating a pharmacologically useful targeting molecule comprising a single-chain, minimal antibody combining site together with a domain that imparts a second functionality. It will certainly be possible in the near future to adjust the fine specificity and affinity of the antibody domain, either by in vitro selection methods or by site-directed mutagenesis based on structural criteria. The functional domain could be an enzyme, a toxin, or any other protein that suits a pharmacological purpose. In this review I have illustrated these directions primarily by summarizing work from my laboratory and those of my collaborators. The review covers single point mutations in the heavy chains of digoxin-specific antibodies that either strikingly diminish or strikingly enhance recognition of a small feature of the antigen, the position 12 OH moiety in the steroid nucleus. The principles for constructing a minimal, single-chain antigen-binding domain based on one of the digoxin-specific antibodies are also outlined, as are the principles for incorporating such domains into fusion proteins. Finally, as a practical application of antibody-targeted enzymes, the construction and evaluation of an anti-fibrin antibody-single-chain urokinase fusion protein is examined. This protein has enhanced potency and specificity both in vivo and in vitro, and it offers the promise of increased therapeutic efficacy as well as diminished toxicity.

Fusion proteins in biotechnology

Gene fusion techniques allow the production of recombinant proteins featuring the combined characteristics of the parental products. Originally, these techniques were used to probe transcriptional and translational activity, to translocate proteins across cell membranes, and to facilitate the recovery of proteins. Recently, new applications have emerged in areas such as protein refolding, immunology, drug targeting and protein display. A slightly modified version of this review is also published in Current Opinion in Structural Biology 1992, 2:569-575.

Characterization of a recombinant chimeric plasminogen activator composed of a fibrin fragment-D-dimer-specific humanized monoclonal antibody and a truncated single-chain urokinase

A recombinant chimeric plasminogen activator, MA-15C5Hu/scu-PA-32k, composed of a humanized fibrin fragment-D-dimer-specific monoclonal antibody (MA-15C5Hu) and a recombinant low-molecular-mass single-chain urokinase-type plasminogen activator, comprising amino acids Leu144-Leu411 (scu-PA-32k), was produced by cotransfecting Chinese hamster ovary (CHO) cells with the cDNA encoding the MA-15C5Hu light-chain sequence and the cDNA encoding the MA-15C5Hu heavy-chain sequence fused with the cDNA encoding scu-PA-32k. Purified MA-15C5Hu/scu-PA-32k migrated as a 215-kDa band on non-reducing SDS/PAGE, which is consistent with a molecule composed of one antibody and two scu-PA-32k moieties. However, the chimera was obtained as a mixture of single-chain u-PA-32k (37%) and amidolytically inactive (50%) and active (13%) two-chain u-PA-32k, the latter of which was removed by immunoadsorption on a monoclonal antibody specific for two-chain urokinase. The fragment-D-dimer affinity and enzymatic properties of MA-15CHu/scu-PA-32k were similar to those of MA-15C5Hu or of scu-PA-32k. In an in vitro system composed of a 125I-fibrin-labeled human plasma clot submerged in citrated human plasma, MA-15C5Hu/scu-PA-32k had a 12-fold higher fibrinolytic potency than scu-PA-32k: 50% lysis in 2 h required 0.43 +/- 0.12 micrograms u-PA-32k equivalent of the chimera/ml versus 5.4 +/- 0.3 micrograms/ml of scu-PA-32k (mean +/- SEM, n = 4). Addition of purified fibrin fragment-D dimer reduced the fibrinolytic potency of MA-15C5Hu/scu-PA-32k in a concentration-dependent way, indicating that the increased potency is the result of antibody targeting. Thus, a recombinant humanized antifibrin antibody/u-PA chimera has been obtained in which only the variable domains of the antibody moiety are of non-human origin. The chimera has intact antigen-binding capacity, u-PA enzymatic activity and a significantly increased fibrinolytic potency in a plasma medium in vitro.

Protein engineering of antibodies

This article reviews the technical advances in antibody engineering and the clinical applications of these molecules. Recombinant DNA technology facilitates the construction and expression of engineered antibodies. These novel molecules are designed to meet specific applications. Although genomic and cDNA cloning have been used widely in the past to isolate the relevant antibody V domains, at present, the PCR-based cloning is the preferred system. Bacterial and mammalian expression systems are used commonly for the production of antibodies, antibody fragments, and antibody fusion proteins. A range of chimeric antibodies with murine V domains joined to C regions from human and other species have been produced and found to exhibit the expected binding characteristics and effector functions. Humanized antibodies have been developed to minimize the HAMA response, and bifunctional immunoglobulins are being used in tumor therapy and diagnosis. Single chain antibodies and fusion proteins with antibody specificities jointed to nonimmunoglobulin sequences provide a source of antibody-like molecules with novel properties. The potential applications of minimal recognition units and antigenized antibodies are described. Combinatorial libraries produced in bacteriophage present an alternative to hybridomas for the production of antibodies with the desired antigen binding specificities. Future developments in this field are discussed also.