Targeting shear gradient activated von Willebrand factor by the novel single-chain antibody A1 reduces occlusive thrombus formation in vitro.

Intraluminal thrombus formation precipitates conditions such as acute myocardial infarction and disturbs local blood flow resulting in areas of rapidly changing blood flow velocities and steep gradients of blood shear rate. Shear rate gradients are known to be pro-thrombotic with an important role for the shear-sensitive plasma protein von Willebrand factor (VWF). Here, we developed a single-chain antibody (scFv) that targets a shear gradient specific conformation of VWF to specifically inhibit platelet adhesion at sites of SRGs but not in areas of constant shear. Microfluidic flow channels with stenotic segments were used to create shear rate gradients during blood perfusion. VWF-GPIbα interactions were increased at sites of shear rate gradients compared to constant shear rate of matched magnitude. The scFv-A1 specifically reduced VWF-GPIbα binding and thrombus formation at sites of SRGs but did not block platelet deposition and aggregation under constant shear rate in upstream sections of the channels. Significantly, the scFv A1 attenuated platelet aggregation only in the later stages of thrombus formation. In the absence of shear, direct binding of scFv-A1 to VWF could not be detected and scFV-A1 did not inhibit ristocetin induced platelet agglutination. We have exploited the pro-aggregatory effects of SRGs on VWF dependent platelet aggregation and developed the shear-gradient sensitive scFv-A1 antibody that inhibits platelet aggregation exclusively at sites of shear rate gradients. The lack of VWF inhibition in non-stenosed vessel segments places scFV-A1 in an entirely new class of anti-platelet therapy for selective blockade of pathological thrombus formation while maintaining normal haemostasis.

A versatile approach for the site-specific modification of recombinant antibodies using a combination of enzyme-mediated bioconjugation and click chemistry.

A unique two-step modular system for site-specific antibody modification and conjugation is reported. The first step of this approach uses enzymatic bioconjugation with the transpeptidase Sortase A for incorporation of strained cyclooctyne functional groups. The second step of this modular approach involves the azide-alkyne cycloaddition click reaction. The versatility of the two-step approach has been exemplified by the selective incorporation of fluorescent dyes and a positron-emitting copper-64 radiotracer for fluorescence and positron-emission tomography imaging of activated platelets, platelet aggregates, and thrombi, respectively. This flexible and versatile approach could be readily adapted to incorporate a large array of tailor-made functional groups using reliable click chemistry whilst preserving the activity of the antibody or other sensitive biological macromolecules.

Enzyme-mediated site-specific bioconjugation of metal complexes to proteins: sortase-mediated coupling of copper-64 to a single-chain antibody.

The enzyme-mediated site-specific bioconjugation of a radioactive metal complex to a single-chain antibody using the transpeptidase sortase A is reported. Cage amine sarcophagine ligands that were designed to function as substrates for the sortase A mediated bioconjugation to antibodies were synthesized and enzymatically conjugated to a single-chain variable fragment. The antibody fragment scFv(anti-LIBS) targets ligand-induced binding sites (LIBS) on the glycoprotein receptor GPIIb/IIIa, which is present on activated platelets. The immunoconjugates were radiolabeled with the positron-emitting isotope (64)Cu. The new radiolabeled conjugates were shown to bind selectively to activated platelets. The diagnostic potential of the most promising conjugate was demonstrated in an in vivo model of carotid artery thrombosis using positron emission tomography. This approach gives homogeneous products through site-specific enzyme-mediated conjugation and should be broadly applicable to other metal complexes and proteins.

Platelet-targeted thrombolysis for treatment of acute ischemic stroke.

Thrombolysis with tissue-type plasminogen activator (tPA) remains the main treatment for acute ischemic stroke. Nevertheless, tPA intervention is limited by a short therapeutic window, low recanalization rates, and a risk of intracranial hemorrhage (ICH), highlighting the clinical demand for improved thrombolytic drugs. We examined a novel thrombolytic agent termed "SCE5-scuPA," comprising a single-chain urokinase plasminogen activator (scuPA) fused with a single-chain antibody (SCE5) that targets the activated glycoprotein IIb/IIIa platelet receptor, for its effects in experimental stroke. SCE5-scuPA was first tested in a whole blood clot degradation assay to show the benefit of platelet-targeted thrombolysis. The tail bleeding time, blood clearance, and biodistribution were then determined to inform the use of SCE5-scuPA in mouse models of photothrombotic stroke and middle cerebral artery occlusion against tenecteplase. The impacts of SCE5-scuPA on motor function, ICH, blood-brain barrier (BBB) integrity, and immunosuppression were evaluated. Infarct size was measured by computed tomography imaging and magnetic resonance imaging. SCE5-scuPA enhanced clot degradation ex vivo compared with its nonplatelet-targeting control. The maximal SCE5-scuPA dose that maintained hemostasis and a rapid blood clearance was determined. SCE5-scuPA administration both before and 2 hours after photothrombotic stroke reduced the infarct volume. SCE5-scuPA also improved neurologic deficit, decreased intracerebral blood deposits, preserved the BBB, and alleviated immunosuppression poststroke. In middle cerebral artery occlusion, SCE5-scuPA did not worsen stroke outcomes or cause ICH, and it protected the BBB. Our findings support the ongoing development of platelet-targeted thrombolysis with SCE5-scuPA as a novel emergency treatment for acute ischemic stroke with a promising safety profile.

Surface Morphology, Compressive Behavior, and Energy Absorption of Graded Triply Periodic Minimal Surface 316L Steel Cellular Structures Fabricated by Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is an emerging technique for the fabrication of triply periodic minimal surface (TPMS) structures in metals. In this work, different TPMS structures such as Diamond, Gyroid, Primitive, Neovius, and Fisher-Koch S with graded relative densities are fabricated from 316L steel using LPBF. The graded TPMS samples are subjected to sandblasting to improve the surface finish before mechanical testing. Quasi-static compression tests are performed to study the deformation behavior and energy absorption capacity of TPMS structures. The results reveal superior stiffness and energy absorption capabilities for the graded TPMS samples compared to the uniform TPMS structures. The Fisher-Koch S and Primitive samples show higher strength whereas the Fisher-Koch S and Neovius samples exhibit higher elastic modulus. The Neovius type structure shows the highest energy absorption up to 50% strain among all the TPMS structures. The Gibson-Ashby coefficients are calculated for the TPMS structures, and it is found that the C2 values are in the range suggested by Gibson and Ashby while C1 values differ from the proposed range.

Low-Fouling and Biodegradable Protein-Based Particles for Thrombus Imaging.

Nanomedicine holds great promise for vascular disease diagnosis and specific therapy, yet rapid sequestration by the mononuclear phagocytic system limits the efficacy of particle-based agents. The use of low-fouling polymers, such as poly(ethylene glycol), efficiently reduces this immune recognition, but these nondegradable polymers can accumulate in the human body and may cause adverse effects after prolonged use. Thus, new particle formulations combining stealth, low immunogenicity and biocompatible features are required to enable clinical use. Here, a low-fouling particle platform is described using exclusively protein material. A recombinant protein with superior hydrophilic characteristics provided by the amino acid repeat proline, alanine, and serine (PAS) is designed and cross-linked into particles with lysine (K) and polyglutamic acid (E) using mesoporous silica templating. The obtained PASKE particles have low-fouling behavior, have a prolonged circulation time compared to albumin-based particles, and are rapidly degraded in the cell's lysosomal compartment. When labeled with near-infrared fluorescent molecules and functionalized with an anti-glycoprotein IIb/IIIa single-chain antibody targeting activated platelets, the particles show potential as a noninvasive molecular imaging tool in a mouse model of carotid artery thrombosis. The PASKE particles constitute a promising biodegradable and versatile platform for molecular imaging of vascular diseases.

Novel Thrombolytic Drug Based on Thrombin Cleavable Microplasminogen Coupled to a Single-Chain Antibody Specific for Activated GPIIb/IIIa.

BACKGROUND: Thrombolytic therapy for acute thrombosis is limited by life-threatening side effects such as major bleeding and neurotoxicity. New treatment options with enhanced fibrinolytic potential are therefore required. Here, we report the development of a new thrombolytic molecule that exploits key features of thrombosis. We designed a recombinant microplasminogen modified to be activated by the prothrombotic serine-protease thrombin (HtPlg), fused to an activation-specific anti-glycoprotein IIb/IIIa single-chain antibody (SCE5), thereby hijacking the coagulation system to initiate thrombolysis. METHODS AND RESULTS: The resulting fusion protein named SCE5-HtPlg shows in vitro targeting towards the highly abundant activated form of the fibrinogen receptor glycoprotein IIb/IIIa expressed on activated human platelets. Following thrombin formation, SCE5-HtPlg is activated to contain active microplasmin. We evaluate the effectiveness of our targeted thrombolytic construct in two models of thromboembolic disease. Administration of SCE5-HtPlg (4 µg/g body weight) resulted in effective thrombolysis 20 minutes after injection in a ferric chloride-induced model of mesenteric thrombosis (48±3% versus 92±5% for saline control, P<0.01) and also reduced emboli formation in a model of pulmonary embolism (P<0.01 versus saline). Furthermore, at these effective therapeutic doses, the SCE5-HtPlg did not prolong bleeding time compared with saline (P=0.99). CONCLUSIONS: Our novel fusion molecule is a potent and effective treatment for thrombosis that enables in vivo thrombolysis without bleeding time prolongation. The activation of this construct by thrombin generated within the clot itself rather than by a plasminogen activator, which needs to be delivered systemically, provides a novel targeted approach to improve thrombolysis.