Enzymatic single-chain antibody tagging: a universal approach to targeted molecular imaging and cell homing in cardiovascular disease.

RATIONALE: Antibody-targeted delivery of imaging agents can enhance the sensitivity and accuracy of current imaging techniques. Similarly, homing of effector cells to disease sites increases the efficacy of regenerative cell therapy while reducing the number of cells required. Currently, targeting can be achieved via chemical conjugation to specific antibodies, which typically results in the loss of antibody functionality and in severe cell damage. An ideal conjugation technique should ensure retention of antigen-binding activity and functionality of the targeted biological component. OBJECTIVE: To develop a biochemically robust, highly reproducible, and site-specific coupling method using the Staphylococcus aureus sortase A enzyme for the conjugation of a single-chain antibody (scFv) to nanoparticles and cells for molecular imaging and cell homing in cardiovascular diseases. This scFv specifically binds to activated platelets, which play a pivotal role in thrombosis, atherosclerosis, and inflammation. METHODS AND RESULTS: The conjugation procedure involves chemical and enzyme-mediated coupling steps. The scFv was successfully conjugated to iron oxide particles (contrast agents for magnetic resonance imaging) and to model cells. Conjugation efficiency ranged between 50% and 70%, and bioactivity of the scFv after coupling was preserved. The targeting of scFv-coupled cells and nanoparticles to activated platelets was strong and specific as demonstrated in in vitro static adhesion assays, in a flow chamber system, in mouse intravital microscopy, and in in vivo magnetic resonance imaging of mouse carotid arteries. CONCLUSIONS: This unique biotechnological approach provides a versatile and broadly applicable tool for procuring targeted regenerative cell therapy and targeted molecular imaging in cardiovascular and inflammatory diseases and beyond.

Scanning laser-induced endothelial injury: a standardized and reproducible thrombosis model for intravital microscopy.

Vascular injury models are indispensable for studying thrombotic processes in vivo. Amongst the available methods for inducing thrombosis, laser-induced endothelial injury (LIEI) has several unique advantages. However, a lack of methodological standardization and expensive instrumentation remain significant problems decreasing reproducibility and impeding the adoption of LIEI in the wider scientific community. In this, study, we developed a standardized protocol for scanning laser-induced endothelial injury (scanning-LIEI) of murine mesenteric veins using the intrinsic 405 nm laser of a conventional laser scanning confocal microscope. We show that our model produces thrombi with prominent core-shell architectures and minimal radiation-related fluorescence artefacts. In comparison with previous methods, the scanning-LIEI model exhibits reduced experimental variability, enabling the demonstration of dose-response effects for anti-thrombotic drugs using small animal cohorts. Scanning-LIEI using the intrinsic 405 nm laser of a confocal laser scanning microscope represents a new method to induce standardized vascular injury with improved reproducibility of thrombus formation. The reduced need for instrument customisation and user experience means that this model could be more readily adopted in the research community.

Magnetic resonance imaging contrast agent targeted toward activated platelets allows in vivo detection of thrombosis and monitoring of thrombolysis.

BACKGROUND: Platelets are the key to thrombus formation and play a role in the development of atherosclerosis. Noninvasive imaging of activated platelets would be of great clinical interest. Here, we evaluate the ability of a magnetic resonance imaging (MRI) contrast agent consisting of microparticles of iron oxide (MPIOs) and a single-chain antibody targeting ligand-induced binding sites (LIBS) on activated glycoprotein IIb/IIIa to image carotid artery thrombi and atherosclerotic plaques. METHODS AND RESULTS: Anti-LIBS antibody or control antibody was conjugated to 1-microm MPIOs (LIBS MPIO/control MPIO). Nonocclusive mural thrombi were induced in mice with 6% ferric chloride. MRI (at 9.4 T) was performed once before and repeatedly in 12-minute-long sequences after LIBS MPIO/control MPIO injection. After 36 minutes, a significant signal void, corresponding to MPIO accumulation, was observed with LIBS MPIOs but not control MPIOs (P<0.05). After thrombolysis, in LIBS MPIO-injected mice, the signal void subsided, indicating successful thrombolysis. On histology, the MPIO content of the thrombus, as well as thrombus size, correlated significantly with LIBS MPIO-induced signal void (both P<0.01). After ex vivo incubation of symptomatic human carotid plaques, MRI and histology confirmed binding to areas of platelet adhesion/aggregation for LIBS MPIOs but not for control MPIOs. CONCLUSIONS: LIBS MPIOs allow in vivo MRI of activated platelets with excellent contrast properties and monitoring of thrombolytic therapy. Furthermore, activated platelets were detected on the surface of symptomatic human carotid plaques by ex vivo MRI. This approach represents a novel noninvasive technique allowing the detection and quantification of platelet-containing thrombi.

[Helping antibodies. Targeted antithrombotic and fibrinolytic therapy]. / Antikörper als Helfer. Zielgerichtete antithrombotische und fibrinolytische Therapie.

The development of monoclonal antibodies facilitated an enormous progress in modern medicine in the last years. The targeted inhibition of defined molecular structures allows therapeutic concepts, which before were inconceivable. There are numerous antibodies in clinical use within the area of tumour therapy, chronically inflammatory diseases, transplantation, infections and also in cardiovascular medicine. Different antibody formats are used such as IgG molecules, Fab fragments and single chain antibodies. Single chain antibodies represent the smallest functional form of the antibody and are used preferentially as recombinant antibodies. The therapeutic possibilities of antibody technology are extended by fusion to radioactive or therapeutically active substances. This review focuses on the application of antibodies and fusion proteins as antithrombotic and fibrinolytic drugs. The use of antibodies allows the development of inhibitory agents with clearly defined functional properties, as for example for activation-specific GPIIb/IIIa-blockade. In addition antibodies can be used for targeting antithrombotic and fibrinolytic agents to the thrombus, allowing an effective local action with less bleeding complications.

Development and validation of a high throughput whole blood thrombolysis plate assay.

The objective of this work was to develop a high throughput assay for testing in vitro the thrombolytic activity using citrated whole blood samples, and to overcome the limitations of currently available techniques. We successfully developed a method that involves forming halo shaped, tissue factor induced, whole blood clots in 96 well plates, and then precisely measuring the thrombolysis process with a spectrophotometer plate reader. We here describe the implementation of this novel method, which we refer to as halo assay, and its validation with plasmin, urokinase and tissue plasminogen activator at different doses. The resulting data is a highly detailed thrombolysis profile, allowing comparison of different fibrinolytic agents. The time point analysis allows kinetic data to be collected and calculated to determine key parameters such as the activation time and the rate of fibrinolysis. We also assessed the capacity of the model to study the effect of clot maturation time on the fibrinolytic rate, an aspect of thrombosis rather unexplored with currently available methods, but of increasing importance in drug development. This novel thrombolysis assay could be an extremely useful research tool; to study the complex process of thrombolysis, and a valuable translational clinical tool; as a screening device to rapidly identify hypo- or hyper-fibrinolysis.

Shear-sensitive nanocapsule drug release for site-specific inhibition of occlusive thrombus formation.

Essentials Vessel stenosis due to large thrombus formation increases local shear 1-2 orders of magnitude. High shear at stenotic sites was exploited to trigger eptifibatide release from nanocapsules. Local delivery of eptifibatide prevented vessel occlusion without increased tail bleeding times. Local nanocapsule delivery of eptifibatide may be safer than systemic antiplatelet therapies. SUMMARY: Background Myocardial infarction and stroke remain the leading causes of mortality and morbidity. The major limitation of current antiplatelet therapy is that the effective concentrations are limited because of bleeding complications. Targeted delivery of antiplatelet drug to sites of thrombosis would overcome these limitations. Objectives Here, we have exploited a key biomechanical feature specific to thrombosis, i.e. significantly increased blood shear stress resulting from a reduction in the lumen of the vessel, to achieve site-directed delivery of the clinically used antiplatelet agent eptifibatide by using shear-sensitive phosphatidylcholine (PC)-based nanocapsules. Methods PC-based nanocapsules (2.8 × 1012 ) with high-dose encapsulated eptifibatide were introduced into microfluidic blood perfusion assays and into in vivo models of thrombosis and tail bleeding. Results Shear-triggered nanocapsule delivery of eptifibatide inhibited in vitro thrombus formation selectively under stenotic and high shear flow conditions above a shear rate of 1000 s-1 while leaving thrombus formation under physiologic shear rates unaffected. Thrombosis was effectively prevented in in vivo models of vessel wall damage. Importantly, mice infused with shear-sensitive antiplatelet nanocapsules did not show prolonged bleeding times. Conclusions Targeted delivery of eptifibatide by shear-sensitive nanocapsules offers site-specific antiplatelet potential, and may form a basis for developing more potent and safer antiplatelet drugs.

Immuno-magnetoliposomes targeting activated platelets as a potentially human-compatible MRI contrast agent for targeting atherothrombosis.

To detect unstable atherosclerotic plaques early and noninvasively would be of great clinical interest. Activated platelets are an interesting molecular target for detecting early lesions or unstable plaques. We therefore developed an MRI contrast agent consisting of magnetoliposomes (ML) linked to an antibody (anti-LIBS) specifically targeting the ligand-induced binding site of the activated GPIIb/IIIa receptor of platelets. ML were prepared by dual centrifugation (DC). ML pegylation up to a total PEG content of 7.5 mol% positively influenced the stability and amount of entrapped SPIOs, and also reduced SPIO-membrane interactions, while higher PEG contents destabilized PEG-ML. Stable anti-LIBS-ML with high amounts of entrapped SPIOs (∼86%, ∼0.22 mol Fe/mol liposomal lipid) and high MRI sensitivity (relaxivity r2 = 422 s(-1) mM(-1) and r2(∗) = 452 s(-1) mM(-1)) were obtained by coupling anti-LIBS to ML in a two-step post-insertion technique. We confirmed specific binding to the GPIIb/IIIa receptor's activated conformation on activated human platelets and cell lines expressing activated GPIIb/IIIa receptor ex vivo. The immuno-ML obtained in this study constitute an important step towards developing a potentially human-compatible MRI contrast agent for the timely detection of plaque rupture by targeting activated platelets.

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.

Predominantly neuronal expression of cytochrome P450 isoforms CYP3A11 and CYP3A13 in mouse brain.

Despite the very small amounts of cytochrome P450 enzymes expressed in different areas and cell populations of the brain as compared with the liver, there is significant evidence for their specific involvement in brain development, function, and plasticity. Nevertheless, the current discussion about occurrence and importance of cerebral cytochrome P450 isoforms is determined by controversial interpretations of their function in general and with respect to single isoforms. Continuing a series of publications about brain P450 isoforms, we now present evidence for the expression of cytochrome P450 3A11 and 3A13 in mouse brain. Immunocytochemical and non-radioactive in situ hybridization studies revealed identical distribution of their proteins and mRNAs throughout the brain especially in neuronal populations, and to some extent in astrocytes. The cerebral expression of these P450 isoforms was confirmed by Western blot and RNAse protection assay analysis. The well-known testosterone-metabolizing capacity and the inducibility of cytochrome P450 3a isoforms by xenobiotics as well as their presence in steroid hormone-sensitive areas and neurons (e.g. hippocampus) clarify the significance of these isoforms for impairment of steroid hormone actions by P450-inducing environmental substances. Therefore, investigation of inducible cerebral P450 isoforms which are able to metabolize xenobiotics as well as steroid hormones might help us to understand neuroendocrine regulation of brain's plasticity.

Testosterone metabolism in rat brain is differentially enhanced by phenytoin-inducible cytochrome P450 isoforms.

Many cytochrome P450 (P450) isoforms are selectively inducible by xenobiotics, e.g. pharmaceuticals like the anti-epileptic drug phenytoin. Some of these P450 enzymes are involved in the metabolism of gonadal hormones and are of great importance, especially in early brain development. In this study, the hydroxylation of testosterone by rat brain microsomes from control and phenytoin-induced animals was examined by use of high performance liquid chromatography (HPLC) provided with a photodiode array detector (PDA). In control rats, testosterone is converted by cytochrome(s) P450 to 6alpha-hydroxytestosterone (OHT) as the main metabolite and 6beta-OHT as well as androstenedione as minor metabolites. After phenytoin treatment, brain microsomes showed a strong increase of testosterone metabolism to 2alpha-, 6beta-, 16alpha-, 16beta-OHT and androstenedione, whereby 16alpha-OHT was the main degradation product. These metabolites indicated the action of isoforms of the P450 subfamilies CYP2B, CYP2C and CYP3A. Inhibition experiments with antibodies against CYP2B1/2 and with the CYP2B specific inhibitor orphenadrine indicated the occurrence of members of this subfamily which are known to catalyse the oxidation of testosterone to 16alpha-OHT, 16beta-OHT and androstenedione. Western blots revealed the phenytoin-inducible expression of CYP2B1 and the constitutive expression of CYP3A. The latter is involved in the 6beta-hydroxylation of testosterone which was found correspondingly in control microsomes. Distinct CYP2C isoforms involved in the hydroxylation of testosterone in phenytoin-induced microsomes are not yet identified. The highly increased testosterone metabolism by phenytoin-dependent induction of specific cytochrome P450 isoforms in adult rat brain illustrates the potential influence of exogenous substances on internal regulative and metabolic pathways in the brain.

Identification, induction and localization of cytochrome P450s of the 3A-subfamily in mouse brain.

Several cytochrome P450 subfamilies are inducible by specific exogenous compounds like the antiepileptic drug phenytoin. Some of these P450 enzymes are involved in the metabolism of gonadal hormones also contributing to neuronal differentiation. CYP3A enzymes have the capacity to catalyze the hydroxylation of testosterone and a wide variety of therapeutic agents, but little is known about the expression and potential function of this subfamily in mouse brain. Here, we report the identification of mouse CYP3A isoforms, their induction and localization in mouse brain. Western blot analysis with anti-CYP3A1 antibodies revealed the phenytoin-inducible expression of CYP3A in brain microsomes, and also a constitutive expression of members of this subfamily in brain mitochondria. Using RT-PCR with a consensus primer pair for known mouse liver CYP3A-isoforms we could demonstrate the expression of CYP3A11 and 3A13 mRNA in mouse brain. Finally, using double immunofluorescence labeling we analyzed the histoanatomical distribution of CYP3A throughout the brain with confocal laser scanning microscopy. We found strong immunoreactivity in neurons of hippocampus and hypothalamic areas which are sensitive to steroid hormones. CYP3A immunoreactivity was apparent also in neurons of the cerebellum, the thalamus and the olfactory bulb. Non-neuronal expression of CYP3A could be found in some astrocyte populations and in vascular as well as ventricular border lines. The presence of CYP3A predominantly in neurons but also in cells contributing to the blood-brain and blood-liquor barrier suggests important roles of this subfamily in mediation of steroid hormone action in mouse brain as well as in preventing the brain from potentially cytotoxic compounds.

Different testosterone metabolism by immortalized embryonic and postnatal hippocampal neurons from C57BL/6 mice: a crucial role for androstenedione.

Steroid hormones influence the development of undifferentiated brain during ontogenesis. In the present study we investigated the metabolic pathway of testosterone in immortalized embryonic and postnatal hippocampal neurons from C57BL/6 mice. Both cell lines are capable of metabolizing testosterone to 6alpha-hydroxytestosterone, 6beta-hydroxytestosterone and androstenedione. The formation was found to correlate with protein concentration and time of incubation. These linearities were significant for all metabolites except androstenedione that was the main metabolite in embryonic hippocampal neurons and nearly absent in postnatal neurons. Moreover, only embryonic cells react to testosterone with a decrease of beta-tubulin expression, that was a typical effect indicating induced neuronal maturation. Application of androstenedione caused the same decrease of beta-tubulin expression as testosterone did before. Our results of hippocampal testosterone metabolism in vitro confirm that not only estradiol and 5alpha-dihydrotestosterone could impact neural tissue but also androstenedione is a powerful metabolite involved in prenatal neuronal differentiation.

Expression and localization of the CYP2B subfamily predominantly in neurones of rat brain.

Despite the very small amounts of cytochrome P450 (P450, CYP) enzymes expressed in different areas and cell populations of the brain as compared with the liver, there is significant evidence for their specific involvement in brain development, function and plasticity. Nevertheless, the current discussion about occurrence and importance of cerebral cytochrome P450s is determined by inconsistent interpretations of their function in general and with respect to single isoforms. Continuing a series of publications about brain P450 isoforms, we now present evidence for the constitutive expression of CYP2B1 and CYP2B2 mRNAs in rat brain. Immunocytochemical and non-radioactive in situ hybridization studies revealed the same expression pattern throughout the brain predominantly in neuronal populations, but to some extent in astrocytes of corpus callosum and olfactory bulb. The well known testosterone-metabolizing capacity and the presence of CYP2B isoforms shown in steroid hormone-sensitive areas and neurones (e.g. hippocampus) clarify the significance of isoforms like CYP2B1 and CYP2B2 for impairment of steroid hormone actions by P450 inducing environmental substances. We argue that cerebral P450 isoforms which are induced by xenobiotics and are able to metabolize these as well as endogenous substrates help us to understand fundamental aspects of brain's functioning.

Oxidative hydrolysis of scoparone by cytochrome p450 CYP2C29 reveals a novel metabolite.

Regioselective 7-demethylation of scoparone is regularly employed as an indicator of phenobarbital-like induction of rat liver cytochrome P450 isoform CYP2B1, e.g., by the antiepileptic drug phenytoin. After induction with phenobarbital and phenytoin, a new reaction sequence catalyzed by Cyp2c29 was identified in mouse liver microsomes. Cyp2c29-dependent 6-demethylation of scoparone resulted in the formation of isoscopoletin, an intermediate which is susceptible to further oxidation. This subsequent oxidation was also catalyzed by Cyp2c29 with a K(m) of 30,31 microM and a V(max) of 3,41 microM/min x microM P450, and resulted in the formation of the new metabolite 3-[4-methoxy-p-(3,6)-benzoquinone]-2-propenoate. This novel metabolite is the product of two consecutive oxidation reactions, proceeding over isoscopoletin to a putative lactone which is accessible to immediate hydrolysis, due to the onium character of the ring oxygen. This opening of the lactone ring corresponds to an oxidative hydrolysis. Differential oxidation of scoparone can be used as a sensitive indicator for distinguishing between different cytochrome P450 isoforms.