Cell-selective intracellular delivery of a foreign enzyme to endothelium in vivo using vascular immunotargeting

Engineered Dual Antioxidant Enzyme Complexes Targeting ICAM-1 on Brain Endothelium Reduce Brain Injury-Associated Neuroinflammation

The neuroinflammatory cascade triggered by traumatic brain injury (TBI) represents a clinically important point for therapeutic intervention. Neuroinflammation generates oxidative stress in the form of high-energy reactive oxygen and nitrogen species, which are key mediators of TBI pathology. The role of the blood-brain barrier (BBB) is essential for proper neuronal function and is vulnerable to oxidative stress. Results herein explore the notion that attenuating oxidative stress at the vasculature after TBI may result in improved BBB integrity and neuroprotection. Utilizing amino-chemistry, a biological construct (designated "dual conjugate" for short) was generated by covalently binding two antioxidant enzymes (superoxide dismutase 1 (SOD-1) and catalase (CAT)) to antibodies specific for ICAM-1. Bioengineering of the conjugate preserved its targeting and enzymatic functions, as evaluated by real-time bioenergetic measurements (via the Seahorse-XF platform), in brain endothelial cells exposed to increasing concentrations of hydrogen peroxide or a superoxide anion donor. Results showed that the dual conjugate effectively mitigated the mitochondrial stress due to oxidative damage. Furthermore, dual conjugate administration also improved BBB and endothelial protection under oxidative insult in an in vitro model of TBI utilizing a software-controlled stretching device that induces a 20% in mechanical strain on the endothelial cells. Additionally, the dual conjugate was also effective in reducing indices of neuroinflammation in a controlled cortical impact (CCI)-TBI animal model. Thus, these studies provide proof of concept that targeted dual antioxidant biologicals may offer a means to regulate oxidative stress-associated cellular damage during neurotrauma.

Lipid Nanoparticles for Nucleic Acid Delivery to Endothelial Cells

Endothelial cells play critical roles in circulatory homeostasis and are also the gateway to the major organs of the body. Dysfunction, injury, and gene expression profiles of these cells can cause, or are caused by, prevalent chronic diseases such as diabetes, cardiovascular disease, and cancer. Modulation of gene expression within endothelial cells could therefore be therapeutically strategic in treating longstanding disease challenges. Lipid nanoparticles (LNP) have emerged as potent, scalable, and tunable carrier systems for delivering nucleic acids, making them attractive vehicles for gene delivery to endothelial cells. Here, we discuss the functions of endothelial cells and highlight some receptors that are upregulated during health and disease. Examples and applications of DNA, mRNA, circRNA, saRNA, siRNA, shRNA, miRNA, and ASO delivery to endothelial cells and their targets are reviewed, as well as LNP composition and morphology, formulation strategies, target proteins, and biomechanical factors that modulate endothelial cell targeting. Finally, we discuss FDA-approved LNPs as well as LNPs that have been tested in clinical trials and their challenges, and provide some perspectives as to how to surmount those challenges.

Nucleic Acid Delivery to the Vascular Endothelium

This Review examines the state-of-the-art in the delivery of nucleic acid therapies that are directed to the vascular endothelium. First, we review the most important homeostatic functions and properties of the vascular endothelium and summarize the nucleic acid tools that are currently available for gene therapy and nucleic acid delivery. Second, we consider the opportunities available with the endothelium as a therapeutic target and the experimental models that exist to evaluate the potential of those opportunities. Finally, we review the progress to date from investigations that are directly targeting the vascular endothelium: for vascular disease, for peri-transplant therapy, for angiogenic therapies, for pulmonary endothelial disease, and for the blood-brain barrier, ending with a summary of the future outlook in this field.

Expanding the arsenal against pulmonary diseases using surface-functionalized polymeric micelles: breakthroughs and bottlenecks

Pulmonary diseases such as lung cancer, asthma and tuberculosis have remained one of the common challenges globally. Polymeric micelles (PMs) have emerged as an effective technique for achieving targeted drug delivery for a local as well as a systemic effect. These PMs encapsulate and protect hydrophobic drugs, increase pulmonary targeting, decrease side effects and enhance drug efficacy through the inhalation route. In the current review, emphasis has been placed on the different barriers encountered by the drugs given via the pulmonary route and the mechanism of PMs in achieving drug targeting. The applications of PMs in different pulmonary diseases have also been discussed in detail.

Liposomal Extravasation and Accumulation in Tumors as Studied by Fluorescence Microscopy and Imaging Depend on the Fluorescent Label

Tumor trafficking of liposomes is routinely monitored via fluorescence microscopy and imaging. To investigate whether an accumulation of liposomes depends on the type of fluorescent label, we prepared PEGylated liposomes dual-labeled with indocarbocyanine lipids (ICLs: DiD or DiI) and fluorescent phospholipids (FPLs: Cy3-DSPE or Cy5-DSPE) with similar cyanine headgroups but different spectra. Using ex vivo confocal microscopy and imaging, we compared tumor extravasation and accumulation of ICLs and FPLs. After systemic injection in a syngeneic mouse model of 4T1 breast cancer, ICLs and FPLs initially colocalized in tumor blood vessels and perivascular space. At later time points, ICLs spread over a significantly larger tumor area and accumulated in tumor macrophages, whereas FPLs were mostly restricted to the vasculature with limited extravascular signal. This phenomenon was independent of liposomal composition and ICL/FPL type and was also observed in syngeneic intracranial GL261 glioma and LY2 head and neck cancer models. The dual-labeled liposomes were stable in plasma and delivered both dyes to tumors at early time points. Notably, while the level of ICLs increased over time, FPLs gradually disappeared from tumors and other organs in vivo, likely due to degradation of the phospholipid. These findings demonstrate that trafficking and stability of the label is of critical importance when assessing extravasation and accumulation of nanocarriers in tumors and other organs by fluorescence microscopy and imaging.

Systemic tumour suppression via the preferential accumulation of erythrocyte-anchored chemokine-encapsulating nanoparticles in lung metastases

Eliciting immune responses against primary tumours is hampered by their immunosuppressive microenvironment and by the greater inaccessibility of deeper intratumoural cells. However, metastatic tumour cells are exposed to highly perfused and immunoactive organs, such as the lungs. Here, by taking advantage of the preferential colocalization of intravenously administered erythrocytes with metastases in the lungs, we show that treatment with chemokine-encapsulating nanoparticles that are non-covalently anchored onto the surface of injected erythrocytes results in local and systemic tumour suppression in mouse models of lung metastasis. Such erythrocyte-anchored systemic immunotherapy led to the infiltration of effector immune cells into the lungs, in situ immunization without the need for exogenous antigens, inhibition of the progression of lung metastasis, and significantly extended animal survival and systemic immunity that suppressed the growth of distant tumours after rechallenge. Erythrocyte-mediated systemic immunotherapy may represent a general and potent strategy for cancer vaccination.

Targeting drug delivery in the vascular system: Focus on endothelium

The bloodstream is the main transporting pathway for drug delivery systems (DDS) from the site of administration to the intended site of action. In many cases, components of the vascular system represent therapeutic targets. Endothelial cells, which line the luminal surface of the vasculature, play a tripartite role of the key target, barrier, or victim of nanomedicines in the bloodstream. Circulating DDS may accumulate in the vascular areas of interest and in off-target areas via mechanisms bypassing specific molecular recognition, but using ligands of specific vascular determinant molecules enables a degree of precision, efficacy, and specificity of delivery unattainable by non-affinity DDS. Three decades of research efforts have focused on specific vascular targeting, which have yielded a multitude of DDS, many of which are currently undergoing a translational phase of development for biomedical applications, including interventions in the cardiovascular, pulmonary, and central nervous systems, regulation of endothelial functions, host defense, and permeation of vascular barriers. We discuss the design of endothelial-targeted nanocarriers, factors underlying their interactions with cells and tissues, and describe examples of their investigational use in models of acute vascular inflammation with an eye on translational challenges.

Multimodal Nanocarrier Probes Reveal Superior Biodistribution Quantification by Isotopic Analysis over Fluorescence

Absolute measurements of biodistribution are essential for understanding and optimizing the function of nanomaterials for in vivo diagnostic and therapeutic applications. Biodistribution analysis by optical imaging is desirable due to its low cost, wide accessibility, and high-throughput nature, but it is substantially less accurate than isotopic and chemical techniques. In this work, we developed multimodal probes for optical and nuclear imaging to analyze the quantitative limits of optical contrast in the red and near-infrared spectra for polysaccharide nanocarriers targeting macrophage cells. Probes incorporating three zwitterionic fluorophores together with radioactive copper distributed diffusely to optically dissimilar tissues that were either white (visceral adipose tissue) or dark red (liver and spleen) in obese rodents. We used in vivo positron emission tomography/computed tomography (PET/CT) imaging, in vivo hyperspectral tomographic fluorescence imaging, and ex vivo optical and isotopic analyses to determine correlations between optical and nuclear signals. PET imaging strongly correlated with standardized ex vivo methods for all tissue types, whereas no fluorescence signals exhibited substantial accuracy in quantification or localization in vivo. Optical imaging of resected tissues was most accurate in the 700 nm wavelength window, but only in white tissues. This work suggests that fluorescence can be used to measure diffuse probe distribution in white tissues over time or across animals, but not red tissues and not deep in the body. This work also highlights the importance of choosing validated experimental protocols and describes how optical measurements are impacted by fluorophore class and spectral properties, tissue properties, and imaging workflow.

PECAM-1 directed re-targeting of exogenous mRNA providing two orders of magnitude enhancement of vascular delivery and expression in lungs independent of apolipoprotein E-mediated uptake

Systemic administration of lipid nanoparticle (LNP)-encapsulated messenger RNA (mRNA) leads predominantly to hepatic uptake and expression. Here, we conjugated nucleoside-modified mRNA-LNPs with antibodies (Abs) specific to vascular cell adhesion molecule, PECAM-1. Systemic (intravenous) administration of Ab/LNP-mRNAs resulted in profound inhibition of hepatic uptake concomitantly with ~200-fold and 25-fold elevation of mRNA delivery and protein expression in the lungs compared to non-targeted counterparts. Unlike hepatic delivery of LNP-mRNA, Ab/LNP-mRNA is independent of apolipoprotein E. Vascular re-targeting of mRNA represents a promising, powerful, and unique approach for novel experimental and clinical interventions in organs of interest other than liver.

Acute administration of catalase targeted to ICAM-1 attenuates neuropathology in experimental traumatic brain injury

Traumatic brain injury (TBI) contributes to one third of injury related deaths in the US. Treatment strategies for TBI are supportive, and the pathophysiology is not fully understood. Secondary mechanisms of injury in TBI, such as oxidative stress and inflammation, are points at which intervention may reduce neuropathology. Evidence suggests that reactive oxygen species (ROS) propagate blood-brain barrier (BBB) hyperpermeability and inflammation following TBI. We hypothesized that targeted detoxification of ROS may improve the pathological outcomes of TBI. Following TBI, endothelial activation results in a time dependent increase in vascular expression of ICAM-1. We conjugated catalase to anti-ICAM-1 antibodies and administered the conjugate to 8 wk old C57BL/6J mice 30 min after moderate controlled cortical impact injury. Results indicate that catalase targeted to ICAM-1 reduces markers of oxidative stress, preserves BBB permeability, and attenuates neuropathological indices more effectively than non-targeted catalase and anti-ICAM-1 antibody alone. Furthermore, the study of microglia by two-photon microscopy revealed that anti-ICAM-1/catalase prevents the transition of microglia to an activated phenotype. These findings demonstrate the use of a targeted antioxidant enzyme to interfere with oxidative stress mechanisms in TBI and provide a proof-of-concept approach to improve acute TBI management that may also be applicable to other neuroinflammatory conditions.

The Future of Nanoparticle-Directed Venous Therapy

Nanoparticles, structures of less than 200 nm capable of delivering pharmacotherapeutics to sites of disease, have shown great promise for the treatment of many disease states. While no nanoparticle therapies for deep vein thrombosis are currently approved by the Food and Drug Administration, many of the unique features of these therapies have the potential to treat both deep vein thrombosis and its most significant sequela, postthrombotic syndrome, while limiting the hemorrhagic complications of current antithrombotic therapies. Nanoparticles are complex structures with several important variables that must be considered to engineer effective therapies. This article will review the structure and engineering of nanoparticles, as well as promising molecular targets for future investigation.

Targeted endothelial nanomedicine for common acute pathological conditions

Endothelium, a thin monolayer of specialized cells lining the lumen of blood vessels is the key regulatory interface between blood and tissues. Endothelial abnormalities are implicated in many diseases, including common acute conditions with high morbidity and mortality lacking therapy, in part because drugs and drug carriers have no natural endothelial affinity. Precise endothelial drug delivery may improve management of these conditions. Using ligands of molecules exposed to the bloodstream on the endothelial surface enables design of diverse targeted endothelial nanomedicine agents. Target molecules and binding epitopes must be accessible to drug carriers, carriers must be free of harmful effects, and targeting should provide desirable sub-cellular addressing of the drug cargo. The roster of current candidate target molecules for endothelial nanomedicine includes peptidases and other enzymes, cell adhesion molecules and integrins, localized in different domains of the endothelial plasmalemma and differentially distributed throughout the vasculature. Endowing carriers with an affinity to specific endothelial epitopes enables an unprecedented level of precision of control of drug delivery: binding to selected endothelial cell phenotypes, cellular addressing and duration of therapeutic effects. Features of nanocarrier design such as choice of epitope and ligand control delivery and effect of targeted endothelial nanomedicine agents. Pathological factors modulate endothelial targeting and uptake of nanocarriers. Selection of optimal binding sites and design features of nanocarriers are key controllable factors that can be iteratively engineered based on their performance from in vitro to pre-clinical in vivo experimental models. Targeted endothelial nanomedicine agents provide antioxidant, anti-inflammatory and other therapeutic effects unattainable by non-targeted counterparts in animal models of common acute severe human disease conditions. The results of animal studies provide the basis for the challenging translation endothelial nanomedicine into the clinical domain.

Vascular targeting of nanocarriers: perplexing aspects of the seemingly straightforward paradigm

Targeted nanomedicine holds promise to find clinical use in many medical areas. Endothelial cells that line the luminal surface of blood vessels represent a key target for treatment of inflammation, ischemia, thrombosis, stroke, and other neurological, cardiovascular, pulmonary, and oncological conditions. In other cases, the endothelium is a barrier for tissue penetration or a victim of adverse effects. Several endothelial surface markers including peptidases (e.g., ACE, APP, and APN) and adhesion molecules (e.g., ICAM-1 and PECAM) have been identified as key targets. Binding of nanocarriers to these molecules enables drug targeting and subsequent penetration into or across the endothelium, offering therapeutic effects that are unattainable by their nontargeted counterparts. We analyze diverse aspects of endothelial nanomedicine including (i) circulation and targeting of carriers with diverse geometries, (ii) multivalent interactions of carrier with endothelium, (iii) anchoring to multiple determinants, (iv) accessibility of binding sites and cellular response to their engagement, (v) role of cell phenotype and microenvironment in targeting, (vi) optimization of targeting by lowering carrier avidity, (vii) endocytosis of multivalent carriers via molecules not implicated in internalization of their ligands, and (viii) modulation of cellular uptake and trafficking by selection of specific epitopes on the target determinant, carrier geometry, and hydrodynamic factors. Refinement of these aspects and improving our understanding of vascular biology and pathology is likely to enable the clinical translation of vascular endothelial targeting of nanocarriers.

Endothelial targeting of nanocarriers loaded with antioxidant enzymes for protection against vascular oxidative stress and inflammation

Endothelial-targeted delivery of antioxidant enzymes, catalase and superoxide dismutase (SOD), is a promising strategy for protecting organs and tissues from inflammation and oxidative stress. Here we describe Protective Antioxidant Carriers for Endothelial Targeting (PACkET), the first carriers capable of targeted endothelial delivery of both catalase and SOD. PACkET formed through controlled precipitation loaded ~30% enzyme and protected it from proteolytic degradation, whereas attachment of PECAM monoclonal antibodies to surface of the enzyme-loaded carriers, achieved without adversely affecting their stability and functionality, provided targeting. Isotope tracing and microscopy showed that PACkET exhibited specific endothelial binding and internalization in vitro. Endothelial targeting of PACkET was validated in vivo by specific (vs IgG-control) accumulation in the pulmonary vasculature after intravenous injection achieving 33% of injected dose at 30 min. Catalase loaded PACkET protects endothelial cells from killing by H2O2 and alleviated the pulmonary edema and leukocyte infiltration in mouse model of endotoxin-induced lung injury, whereas SOD-loaded PACkET mitigated cytokine-induced endothelial pro-inflammatory activation and endotoxin-induced lung inflammation. These studies indicate that PACkET offers a modular approach for vascular targeting of therapeutic enzymes.

Delivering nanoparticles to lungs while avoiding liver and spleen through adsorption on red blood cells

Nanoparticulate drug delivery systems are one of the most widely investigated approaches for developing novel therapies for a variety of diseases. However, rapid clearance and poor targeting limit their clinical utility. Here, we describe an approach to harness the flexibility, circulation, and vascular mobility of red blood cells (RBCs) to simultaneously overcome these limitations (cellular hitchhiking). A noncovalent attachment of nanoparticles to RBCs simultaneously increases their level in blood over a 24 h period and allows transient accumulation in the lungs, while reducing their uptake by liver and spleen. RBC-adsorbed nanoparticles exhibited ∼3-fold increase in blood persistence and ∼7-fold higher accumulation in lungs. RBC-adsorbed nanoparticles improved lung/liver and lung/spleen nanoparticle accumulation by over 15-fold and 10-fold, respectively. Accumulation in lungs is attributed to mechanical transfer of particles from the RBC surface to lung endothelium. Independent tracing of both nanoparticles and RBCs in vivo confirmed that RBCs themselves do not accumulate in lungs. Attachment of anti-ICAM-1 antibody to the exposed surface of NPs that were attached to RBCs led to further increase in lung targeting and retention over 24 h. Cellular hitchhiking onto RBCs provides a new platform for improving the blood pharmacokinetics and vascular delivery of nanoparticles while simultaneously avoiding uptake by liver and spleen, thus opening the door for new applications.

Targeted Drug Delivery to Endothelial Adhesion Molecules

Endothelial cells represent important targets for therapeutic and diagnostic interventions in many cardiovascular, pulmonary, neurological, inflammatory, and metabolic diseases. Targeted delivery of drugs (especially potent and labile biotherapeutics that require specific subcellular addressing) and imaging probes to endothelium holds promise to improve management of these maladies. In order to achieve this goal, drug cargoes or their carriers including liposomes and polymeric nanoparticles are chemically conjugated or fused using recombinant techniques with affinity ligands of endothelial surface molecules. Cell adhesion molecules, constitutively expressed on the endothelial surface and exposed on the surface of pathologically altered endothelium—selectins, VCAM-1, PECAM-1, and ICAM-1—represent good determinants for such a delivery. In particular, PECAM-1 and ICAM-1 meet criteria of accessibility, safety, and relevance to the (patho)physiological context of treatment of inflammation, ischemia, and thrombosis and offer a unique combination of targeting options including surface anchoring as well as intra- and transcellular targeting, modulated by parameters of the design of drug delivery system and local biological factors including flow and endothelial phenotype. This review includes analysis of these factors and examples of targeting selected classes of therapeutics showing promising results in animal studies, supporting translational potential of these interventions.

Advanced drug delivery systems for antithrombotic agents

Despite continued achievements in antithrombotic pharmacotherapy, difficulties remain in managing patients at high risk for both thrombosis and hemorrhage. Utility of antithrombotic agents (ATAs) in these settings is restricted by inadequate pharmacokinetics and narrow therapeutic indices. Use of advanced drug delivery systems (ADDSs) may help to circumvent these problems. Various nanocarriers, affinity ligands, and polymer coatings provide ADDSs that have the potential to help optimize ATA pharmacokinetics, target drug delivery to sites of thrombosis, and sense pathologic changes in the vascular microenvironment, such as altered hemodynamic forces, expression of inflammatory markers, and structural differences between mature hemostatic and growing pathological clots. Delivery of ATAs using biomimetic synthetic carriers, host blood cells, and recombinant fusion proteins that are activated preferentially at sites of thrombus development has shown promising outcomes in preclinical models. Further development and translation of ADDSs that spare hemostatic fibrin clots hold promise for extending the utility of ATAs in the management of acute thrombotic disorders through rapid, transient, and targeted thromboprophylaxis. If the potential benefit of this technology is to be realized, a systematic and concerted effort is required to develop clinical trials and translate the use of ADDSs to the clinical arena.

Antioxidant protection by PECAM-targeted delivery of a novel NADPH-oxidase inhibitor to the endothelium in vitro and in vivo

Oxidant stress caused by pathological elevation of reactive oxygen species (ROS) production in the endothelial cells lining the vascular lumen is an important component of many vascular and pulmonary disease conditions. NADPH oxidase (NOX) activated by pathological mediators including angiotensin and cytokines is a major source of endothelial ROS. In order to intercept this pathological pathway, we have encapsulated an indirect NOX inhibitor, MJ33, into immunoliposomes (Ab-MJ33/IL) targeted to endothelial marker platelet endothelial cell adhesion molecule (PECAM-1). Ab-MJ33/IL, but not control IgG-MJ33/IL are specifically bound to endothelium and attenuated angiotensin-induced ROS production in vitro and in vivo. Additionally, Ab-MJ33/IL inhibited endothelial expression of the inflammatory marker vascular cell adhesion molecule (VCAM) in cells and animals challenged with the cytokine TNF. Furthermore, Ab-MJ33/IL alleviated pathological disruption of endothelial permeability barrier function in cells exposed to vascular endothelial growth factor (VEGF) and in the lungs of mice challenged with lipopolysaccharide (LPS). Of note, the latter beneficial effect has been achieved both by prophylactic and therapeutic injection of Ab-MJ33/IL in animals. Therefore, specific suppression of ROS production by NOX in endothelium, attainable by Ab-MJ33/IL targeting, may help deciphering mechanisms of vascular oxidative stress and inflammation, and potentially improve treatment of these conditions.

Endothelial targeting of polymeric nanoparticles stably labeled with the PET imaging radioisotope iodine-124

Targeting of therapeutics or imaging agents to the endothelium has the potential to improve specificity and effectiveness of treatment for many diseases. One strategy to achieve this goal is the use of nanoparticles (NPs) targeted to the endothelium by ligands of protein determinants present on this tissue, including cell adhesion molecules, peptidases, and cell receptors. However, detachment of the radiolabel probes from NPs poses a significant problem. In this study, we devised polymeric NPs directly labeled with radioiodine isotopes including the positron emission tomography (PET) isotope (124)I, and characterized their targeting to specific endothelial determinants. This approach provided sizable, targetable probes for specific detection of endothelial surface determinants non-invasively in live animals. Direct conjugation of radiolabel to NPs allowed for stable longitudinal tracking of tissue distribution without label detachment even in an aggressive proteolytic environment. Further, this approach permits tracking of NP pharmacokinetics in real-time and non-invasive imaging of the lung in mice using micro-PET imaging. The use of this strategy will considerably improve investigation of NP interactions with target cells and PET imaging in small animals, which ultimately can aid in the optimization of targeted drug delivery.

Targeted interception of signaling reactive oxygen species in the vascular endothelium

Reactive oxygen species (ROS) are implicated as injurious and as signaling agents in human maladies including inflammation, hyperoxia, ischemia-reperfusion and acute lung injury. ROS produced by the endothelium play an important role in vascular pathology. They quench, for example, nitric oxide, and mediate pro-inflammatory signaling. Antioxidant interventions targeted for the vascular endothelium may help to control these mechanisms. Animal studies have demonstrated superiority of targeting ROS-quenching enzymes catalase and superoxide dismutase to endothelial cells over nontargeted formulations. A diverse arsenal of targeted antioxidant formulations devised in the last decade shows promising results for specific quenching of endothelial ROS. In addition to alleviation of toxic effects of excessive ROS, these targeted interventions suppress pro-inflammatory mechanisms, including endothelial cytokine activation and barrier disruption. These interventions may prove useful in experimental biomedicine and, perhaps, in translational medicine.

Calcium condensed LABL-TAT complexes effectively target gene delivery to ICAM-1 expressing cells

Targeted gene delivery using nonviral vectors is a highly touted scheme to reduce the potential for toxic or immunological side effects by reducing dose. In previous reports, TAT polyplexes with DNA have shown relatively poor gene delivery. The transfection efficiency has been enhanced by condensing TAT/DNA complexes to a small particle size using calcium. To explore the targetability of these condensed TAT complexes, LABL peptide targeting intercellular cell-adhesion molecule-1 (ICAM-1) was conjugated to TAT peptide using a polyethylene glycol (PEG) spacer. PEGylation reduced the transfection efficiency of TAT, but TAT complexes targeting ICAM-1 expressing cells regained much of the lost transfection efficiency. Targeted block peptides properly formulated with calcium offer promise for gene delivery to ICAM-1 expressing cells at sites of injury or inflammation.

Targeted modulation of reactive oxygen species in the vascular endothelium

'Endothelial cells lining vascular luminal surface represent an important site of signaling and injurious effects of reactive oxygen species (ROS) produced by other cells and endothelium itself in ischemia, inflammation and other pathological conditions. Targeted delivery of ROS modulating enzymes conjugated with antibodies to endothelial surface molecules (vascular immunotargeting) provides site-specific interventions in the endothelial ROS, unattainable by other formulations including PEG-modified enzymes. Targeting of ROS generating enzymes (e.g., glucose oxidase) provides ROS- and site-specific models of endothelial oxidative stress, whereas targeting of antioxidant enzymes SOD and catalase offers site-specific quenching of superoxide anion and H(2)O(2). These targeted antioxidant interventions help to clarify specific role of endothelial ROS in vascular and pulmonary pathologies and provide basis for design of targeted therapeutics for treatment of these pathologies. In particular, antibody/catalase conjugates alleviate acute lung ischemia/reperfusion injury, whereas antibody/SOD conjugates inhibit ROS-mediated vasoconstriction and inflammatory endothelial signaling. Encapsulation in protease-resistant, ROS-permeable carriers targeted to endothelium prolongs protective effects of antioxidant enzymes, further diversifying the means for targeted modulation of endothelial ROS.

Modulation of endothelial targeting by size of antibody-antioxidant enzyme conjugates

Endothelial targeting of antioxidant enzymes attenuates acute vascular oxidative stress in animal studies. Superoxide dismutase (SOD) and catalase conjugated with antibodies to Platelet-Endothelial Cell Adhesion Molecule-1 (anti-PECAM/SOD and anti-PECAM/catalase) bind to endothelium, accumulate in the pulmonary vasculature, and detoxify reactive oxygen species. In order to define the role of conjugate size in the efficacy and specificity of endothelial targeting, we synthesized anti-PECAM/enzyme conjugates of controlled size (40nm-10,000nm). Binding of anti-PECAM/enzymes to endothelial cells increased with conjugate size from 300nm to 2μm (from 2.5 to 8.5% of bound fraction), and was specific, as conjugates did not bind to PECAM-negative cells. Pulmonary uptake of anti-PECAM/enzyme conjugates injected intravenously in mice also increased from 4.5 to 16% of injected dose for particles from 200 to 800nm. However, control conjugates larger than 300nm showed elevated non-specific pulmonary uptake, indicating that the targeting specificity of anti-PECAM/enzyme conjugates in vivo has a bell-shaped curve with a maximum close to 300-nm diameter. These results show that: i) the size of an antibody/enzyme conjugate modulates efficacy and specificity of targeting, and ii) a size optimum should be defined in vivo to account for parameters that are difficult to model in cell culture.

Targeting antioxidant and antithrombotic biotherapeutics to endothelium

The endothelium is one of the key targets for pharmacological interventions in oxidative stress and thrombosis, two conditions that are notoriously difficult to treat due to limited efficacy and precision of action of current drugs. Design of molecular and nano-devices that deliver potent antioxidant and antithrombotic therapeutic enzymes to the endothelium holds promise to improve the potency, localization, timing, specificity, safety, and mechanistic precision of these interventions. In particular, cell adhesion molecules expressed on the surface of resting and pathologically altered endothelial cells can be used for drug delivery to the endothelial surface (preferable for thrombolytics) and into intracellular compartments (preferable for antioxidants). Drug delivery platforms including protein conjugates, recombinant fusion constructs, and stealth polymer carriers designed to target these drugs to endothelium are reviewed in this article.

Gene expression in lung and liver after intravenous infusion of polyethylenimine complexes of Sleeping Beauty transposons

Two methods of systemic gene delivery have been extensively explored, using the mouse as a model system: hydrodynamic delivery, wherein a DNA solution equivalent in volume to 10% of the mouse weight is injected intravenously in less than 10 sec, and condensation of DNA with polyethylenimine (PEI) for standard intravenous infusion. Our goal in this study was to evaluate quantitatively the kinetics of gene expression, using these two methods for delivery of Sleeping Beauty transposons. Transposons carrying a luciferase expression cassette were injected into mice either hydrodynamically or after condensation with PEI at a PEI nitrogen-to-DNA phosphate ratio of 7. Gene expression in the lungs and liver after hydrodynamic delivery resulted in exponential decay with a half-life of about 35-40 hr between days 1 and 14 postinjection. The decay kinetics of gene expression after PEI-mediated gene delivery were more complex; an initial decay rate of 6 hr was followed by a more gradual loss of activity. Consequently, the liver became the primary site of gene expression about 4 days after injection of PEI-DNA, and by 14 days expression in the liver was 10-fold higher than in the lung. Overall levels of gene expression 2 weeks postinjection were 100- to 1000-fold lower after PEI-mediated delivery compared with hydrodynamic injection. These results provide insight into the relative effectiveness and organ specificity of these two methods of nonviral gene delivery when coupled with the Sleeping Beauty transposon system.

Targeted delivery of therapeutics to endothelium

The endothelium is a target for therapeutic and diagnostic interventions in a plethora of human disease conditions including ischemia, inflammation, edema, oxidative stress, thrombosis and hemorrhage, and metabolic and oncological diseases. Unfortunately, drugs have no affinity to the endothelium, thereby limiting the localization, timing, specificity, safety, and effectiveness of therapeutic interventions. Molecular determinants on the surface of resting and pathologically altered endothelial cells, including cell adhesion molecules, peptidases, and receptors involved in endocytosis, can be used for drug delivery to the endothelial surface and into intracellular compartments. Drug delivery platforms such as protein conjugates, recombinant fusion constructs, targeted liposomes, and stealth polymer carriers have been designed to target drugs and imaging agents to these determinants. We review endothelial target determinants and drug delivery systems, describe parameters that control the binding of drug carriers to the endothelium, and provide examples of the endothelial targeting of therapeutic enzymes designed for the treatment of acute vascular disorders including ischemia, oxidative stress, inflammation, and thrombosis.

Differential intra-endothelial delivery of polymer nanocarriers targeted to distinct PECAM-1 epitopes

Coupling drug carriers to antibodies for targeting endothelial cells (ECs) may improve treatment of vascular and pulmonary diseases. Selecting antibodies that deliver carriers to the cell surface or intracellularly may further optimize specificity of interventions. We studied antibody-directed targeting of nanocarriers to platelet-endothelial cell adhesion molecule (PECAM)-1, an endothelial glycoprotein containing 6 Ig-like extracellular domains. PECAM-1 antibodies bind to ECs without internalization, but ECs internalize by endocytosis nanocarriers carrying multiple copies of anti-PECAM (anti-PECAM/NCs). To determine whether binding and intracellular transport of anti-PECAM/NCs depend on the epitope engaged, we targeted five PECAM-1 epitopes: mAb35, mAb37 and mAb62 (membrane-distal Ig domain 1), mAbGi34 (Ig domains 2/3), and mAb4G6 (membrane-proximal Ig domain 6). The antibodies bound to ECs regardless of the epitope proximity to the plasmalemma, whereas 130 nm diameter nanocarriers only targeted effectively distal domains (mAb4G6/NCs did not bind to ECs). ECs internalized mAb35, mAb62, and mAbGi34 carriers regardless of their size (0.13 to 5 microm diameter), yet they did not internalize mAb37/NCs. After internalization, mAb62/NCs trafficked to lysosomes within 2-3 h, whereas mAb35/NCs had prolonged residence in pre-lysosomal vesicles. Therefore, endothelial binding, endocytosis, and intracellular transport of anti-PECAM/NCs are epitope-specific. This paradigm will guide the design of endothelial drug delivery systems providing specific cellular localizations.

Platelet-endothelial cell adhesion molecule-1-directed endothelial targeting of superoxide dismutase alleviates oxidative stress caused by either extracellular or intracellular superoxide

Targeting of the antioxidant enzyme catalase to endothelial cells protects against vascular oxidative stress induced by hydrogen peroxide (H(2)O(2))(Am J Physiol 285:L283-L292, 2003; Nat Biotechnol 21:392-398, 2003; Am J Physiol 293:L162-L169, 2007). However, another reactive oxygen species, superoxide anion, is also involved in many forms of vascular oxidative stress, including ischemia/reperfusion, hypertension, and inflammation. To protect endothelium against superoxide attack, we designed and tested antibody-directed targeting of superoxide dismutase (SOD) to the endothelial surface determinant, platelet-endothelial cell adhesion molecule (PECAM)-1. We synthesized anti-PECAM/SOD conjugates that retained 70% of enzymatic activity (superoxide anion dismutation) and specifically bound to endothelial cells, but not PECAM-negative cells. The effect of anti-PECAM/SOD delivery to cells was tested in two distinct models of oxidative stress induced by either extracellular or intracellular generation of superoxide anion. In the first model, anti-PECAM/SOD, but not unconjugated SOD, protected endothelial cells against injury caused by superoxide produced in the medium by hypoxanthine-xanthine oxidase. At the optimal dose, anti-PECAM/SOD provided up to 40 to 50% protection against cell death in this model. In the second model, anti-PECAM/SOD at the optimal dose provided complete protection against necrosis caused by paraquat-induced intracellular superoxide generation. Endothelial targeting of SOD represents a new molecular antioxidant approach that could be used for the management of vascular oxidative stress.

Targeted nanoparticle-based drug delivery and diagnosis

Nanotechnology, or systems/device manufacture at sizes generally ranging between 1 and 100 nm, is a multidisciplinary scientific field undergoing explosive development. The genesis of nanotechnology can be traced to advances in medicine, communications, genomics and robotics. One of the greatest values of nanotechnology will be in the development of new and effective medical treatments (i.e. nanomedicine). This review focuses on the potential of nanomedicine as it relates to the development of nanoparticles for enabling and improving the targeted delivery of therapeutic and diagnostic agents. We highlight the use of nanoparticles for specific intra-compartmental analysis using the examples of delivery to malignant cancers, to the central nervous system, and across the gastrointestinal barriers.

Factors modulating the delivery and effect of enzymatic cargo conjugated with antibodies targeted to the pulmonary endothelium

Vascular drug targeting may improve therapies, yet a thorough understanding of the factors that regulate effects of drugs directed to the endothelium is needed to translate this approach into the clinical domain. To define factors modulating the efficacy and effects of endothelial targeting, we used a model enzyme (glucose oxidase, GOX) coupled with monoclonal antibodies (anti-TM(34) or anti-TM(201)) to distinct epitopes of thrombomodulin, a surface determinant enriched in the pulmonary endothelium. GOX delivery results in conversion of glucose and oxygen into H(2)O(2) leading to lung damage, a clear physiologic endpoint. Results of in vivo studies in mice showed that the efficiency of cargo delivery and its effect are influenced by a number of factors including: 1) The level of pulmonary uptake of the targeting antibody (anti-TM(201) was more efficient than anti-TM(34)); 2) The amount of an active drug delivered to the target; 3) The amount of target antigen on the endothelium (animals with suppressed TM levels showed less targeting); and, 4) The substrate availability for the enzyme cargo in the target tissue (hyperoxia augmented GOX-induced injury). Therefore, both activities of the conjugates and biological factors control targeting and effects of enzymatic cargo. Understanding the nature of such "modulating biological factors" will hopefully allow optimization and ultimately applications of drug targeting for "individualized" pharmacotherapy.

Decline in Exogenous Gene Expression in Primate Brain Following Intravenous Administration Is Due to Plasmid Degradation

PURPOSE

Nonviral gene transfer to the brain of adult Rhesus monkeys is possible with a single intravenous administration of plasmid DNA that is encapsulated in the interior of pegylated immunoliposomes, which are targeted across membrane barriers in vivo with a monoclonal antibody to the human insulin receptor.

METHODS

The present studies measure the rate of decay of luciferase gene expression in the Rhesus monkey with luciferase enzyme assays, Southern blotting, and real-time polymerase chain reaction.

RESULTS

Luciferase enzyme activity in frontal cortex, cerebellum, and liver decays with a t(1/2) of 2.1 +/- 0.1, 2.6 +/- 0.2, and 1.7 +/- 0.01 days, respectively. Luciferase plasmid in brain and liver was detectable by Southern blotting at 2 days, but not at 7 or 14 days. The concentration of luciferase plasmid DNA in brain and liver was measured by real-time polymerase chain reaction, and decayed with t(1/2) of 1.3 +/- 0.3 and 2.7 +/- 0.5 days, respectively.

CONCLUSIONS

The maximal concentration of luciferase plasmid DNA in Rhesus monkey brain was 3-4 molecules/cell following an i.v. administration of 12 microg/kg pegylated immunoliposome encapsulated plasmid DNA. These results demonstrate that the rate of loss of exogenous gene expression in the primate in vivo correlates with the rate of DNA degradation of the exogenous plasmid DNA.

The pinpoint promise of nanoparticle-based drug delivery and molecular diagnosis

Nanotechnology, or systems/device manufacture at the molecular level, is a multidisciplinary scientific field undergoing explosive development. The genesis of nanotechnology can be traced to the promise of revolutionary advances across medicine, communications, genomics and robotics. Without doubt one of the greatest values of nanotechnology will be in the development of new and effective medical treatments (i.e., nanomedicine). This review focuses on the potential of nanomedicine as it specifically relates to (1) the development of nanoparticles for enabling and improving the targeted delivery of therapeutic agents; (2) developing novel and more effective diagnostic and screening techniques to extend the limits of molecular diagnostics providing point-of-care diagnosis and more personalized medicine.

Biomedical aspects of targeted delivery of drugs to pulmonary endothelium

Drug targeting to selected subcellular compartments of the pulmonary endothelium may optimise treatment of many diseases. This paper describes endothelial determinants that are potentially useful for such targeting, including endothelial ectopeptidases, cell adhesion molecules and novel candidates identified by high-throughput methods, as well as the means to achieve optimal subcellular targeting of drugs in the endothelium that have been explored in cell culture and animal studies. Criteria for determining the applicability for targeting include accessibility, specificity, safety and subcellular precision. The effects of endothelial delivery of therapeutic agents, including the effects mediated by the intervention in the function of the target determinants, must be characterised in the context of given pathological conditions.

Nanomedicine: current status and future prospects

Applications of nanotechnology for treatment, diagnosis, monitoring, and control of biological systems has recently been referred to as "nanomedicine" by the National Institutes of Health. Research into the rational delivery and targeting of pharmaceutical, therapeutic, and diagnostic agents is at the forefront of projects in nanomedicine. These involve the identification of precise targets (cells and receptors) related to specific clinical conditions and choice of the appropriate nanocarriers to achieve the required responses while minimizing the side effects. Mononuclear phagocytes, dendritic cells, endothelial cells, and cancers (tumor cells, as well as tumor neovasculature) are key targets. Today, nanotechnology and nanoscience approaches to particle design and formulation are beginning to expand the market for many drugs and are forming the basis for a highly profitable niche within the industry, but some predicted benefits are hyped. This article will highlight rational approaches in design and surface engineering of nanoscale vehicles and entities for site-specific drug delivery and medical imaging after parenteral administration. Potential pitfalls or side effects associated with nanoparticles are also discussed.

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.

Delivery of beta-galactosidase to mouse brain via the blood-brain barrier transferrin receptor

Enzyme replacement therapy of lysosomal storage disorders is complicated by the lack of enzyme transport across the blood-brain barrier (BBB). The present studies evaluate the delivery of a model enzyme across the BBB following enzyme conjugation to a BBB receptor-specific monoclonal antibody (mAb). Bacterial beta-galactosidase (116 kDa) was conjugated to the rat 8D3 mAb to the rat transferrin receptor (TfR) via a streptavidin-biotin linkage. The unconjugated beta-galactosidase or the beta-galactosidase-8D3 conjugate was injected intravenously in adult mice, and enzyme activity was measured at 1 and 4 h in brain and peripheral organs (liver, spleen, kidney, and heart). Unconjugated beta-galactosidase was rapidly removed from the blood compartment owing to avid uptake by liver and spleen. There was minimal uptake of the unconjugated beta-galactosidase by brain. Following conjugation of the enzyme to the 8D3 TfRmAb, there was a 10-fold increase in brain uptake of the enzyme based on measurement of enzyme activity. Histochemistry of brain showed localization of the enzyme in the intraendothelial compartment of brain following intravenous injection of the enzyme-mAb conjugate. The capillary depletion technique showed that more than 90% of the enzyme-8D3 conjugate that entered into the endothelial compartment of brain passed through the BBB to enter brain parenchyma. In conclusion, high molecular weight enzymes, such as bacterial beta-galactosidase, can be conjugated to BBB targeting antibodies for effective delivery across the BBB in vivo. Fusion proteins comprised of BBB targeting antibodies and recombinant enzymes could be therapeutic in the treatment of the brain in human lysosomal storage disorders.

Slow intracellular trafficking of catalase nanoparticles targeted to ICAM-1 protects endothelial cells from oxidative stress

Nanotechnologies promise new means for drug delivery. ICAM-1 is a good target for vascular immunotargeting of nanoparticles to the perturbed endothelium, although endothelial cells do not internalize monomeric anti-ICAM-1 antibodies. However, coupling ICAM-1 antibodies to nanoparticles creates multivalent ligands that enter cells via an amiloride-sensitive endocytic pathway that does not require clathrin or caveolin. Fluorescence microscopy revealed that internalized anti-ICAM nanoparticles are retained in a stable form in early endosomes for an unusually long time (1-2 h) and subsequently were degraded following slow transport to lysosomes. Inhibition of lysosome acidification by chloroquine delayed degradation without affecting anti-ICAM trafficking. Also, the microtubule disrupting agent nocodazole delayed degradation by inhibiting anti-ICAM nanoparticle trafficking to lysosomes. Addition of catalase to create anti-ICAM nanoparticles with antioxidant activity did not affect the mechanisms of nanoparticle uptake or trafficking. Intracellular anti-ICAM/catalase nanoparticles were active, because endothelial cells were resistant to H2O2-induced oxidative injury for 1-2 h after nanoparticle uptake. Chloroquine and nocodazole increased the duration of antioxidant protection by decreasing the extent of anti-ICAM/catalase degradation. Therefore, the unique trafficking pathway followed by internalized anti-ICAM nanoparticles seems well suited for targeted delivery of therapeutic enzymes to endothelial cells and may provide a basis for treatment of acute vascular oxidative stress.

PECAM-directed delivery of catalase to endothelium protects against pulmonary vascular oxidative stress

Targeted delivery of drugs to vascular endothelium promises more effective and specific therapies in many disease conditions, including acute lung injury (ALI). This study evaluates the therapeutic effect of drug targeting to PECAM (platelet/endothelial cell adhesion molecule-1) in vivo in the context of pulmonary oxidative stress. Endothelial injury by reactive oxygen species (e.g., H2O2) is involved in many disease conditions, including ALI/acute respiratory distress syndrome and ischemia-reperfusion. To optimize delivery of antioxidant therapeutics, we conjugated catalase with PECAM antibodies and tested properties of anti-PECAM/catalase conjugates in cell culture and mice. Anti-PECAM/catalase, but not an IgG/catalase counterpart, bound specifically to PECAM-expressing cells, augmented their H2O2-degrading capacity, and protected them against H2O2 toxicity. Anti-PECAM/catalase, but not IgG/catalase, rapidly accumulated in the lungs after intravenous injection in mice, where it was confined to the pulmonary endothelium. To test its protective effect, we employed a murine model of oxidative lung injury induced by glucose oxidase coupled with thrombomodulin antibody (anti-TM/GOX). After intravenous injection in mice, anti-TM/GOX binds to pulmonary endothelium and produces H2O2, which causes lung injury and 100% lethality within 7 h. Coinjection of anti-PECAM/catalase protected against anti-TM/GOX-induced pulmonary oxidative stress, injury, and lethality, whereas polyethylene glycol catalase or IgG/catalase conjugates afforded only marginal protective effects. This result validates vascular immunotargeting as a prospective strategy for therapeutic interventions aimed at immediate protective effects, e.g., for augmentation of antioxidant defense in the pulmonary endothelium and treatment of ALI.

ICAM-directed vascular immunotargeting of antithrombotic agents to the endothelial luminal surface

Drug targeting to a highly expressed, noninternalizable determinant up-regulated on the perturbed endothelium may help to manage inflammation and thrombosis. We tested whether inter-cellular adhesion molecule-1 (ICAM-1) targeting is suitable to deliver antithrombotic drugs to the pulmonary vascular lumen. ICAM-1 antibodies bind to the surface of endothelial cells in culture, in perfused lungs, and in vivo. Proinflammatory cytokines enhance anti-ICAM binding to the endothelium without inducing internalization. (125)I-labeled anti-ICAM and a reporter enzyme (beta-Gal) conjugated to anti-ICAM bind to endothelium and accumulate in the lungs after intravenous administration in rats and mice. Anti-ICAM is seen to localize predominantly on the luminal surface of the pulmonary endothelium by electron microscopy. We studied the pharmacological effect of ICAM-directed targeting of tissue-type plasminogen activator (tPA). Anti-ICAM/tPA, but not control IgG/tPA, conjugate accumulates in the rat lungs, where it exerts plasminogen activator activity and dissolves fibrin microemboli. Therefore, ICAM may serve as a target for drug delivery to endothelium, for example, for pulmonary thromboprophylaxis. Enhanced drug delivery to sites of inflammation and the potential anti-inflammatory effect of blocking ICAM-1 may enhance the benefit of this targeting strategy.

A novel endocytic pathway induced by clustering endothelial ICAM-1 or PECAM-1

Antibody conjugates directed against intercellular adhesion molecule (ICAM-1) or platelet-endothelial cell adhesion molecule (PECAM-1) have formed the basis for drug delivery vehicles that are specifically recognized and internalized by endothelial cells. There is increasing evidence that ICAM-1 and PECAM-1 may also play a role in cell scavenger functions and pathogen entry. To define the mechanisms that regulate ICAM-1 and PECAM-1 internalization, we examined the uptake of anti-PECAM-1 and anti-ICAM-1 conjugates by endothelial cells. We found that the conjugates must be multimeric, because monomeric anti-ICAM-1 and anti-PECAM-1 are not internalized. Newly internalized anti-ICAM-1 and anti-PECAM-1 conjugates did not colocalize with either clathrin or caveolin, and immunoconjugate internalization was not reduced by inhibitors of clathrin-mediated or caveolar endocytosis, suggesting that this is a novel endocytic pathway. Amiloride and protein kinase C (PKC) inhibitors, agents known to inhibit macropinocytosis, reduced the internalization of clustered ICAM-1 and PECAM-1. However, expression of dominant-negative dynamin-2 constructs inhibited uptake of clustered ICAM-1. Binding of anti-ICAM-1 conjugates stimulated the formation of actin stress fibers by human umbilical vein endothelial cells (HUVEC). Latrunculin, radicicol and Y27632 also inhibited internalization of clustered ICAM-1, suggesting that actin rearrangements requiring Src kinase and Rho kinase (ROCK) were required for internalization. Interestingly, these kinases are part of the signal transduction pathways that are activated when circulating leukocytes engage endothelial cell adhesion molecules, suggesting the possibility that CAM-mediated endocytosis is regulated using comparable signaling pathways.

Immunotargeting of catalase to the pulmonary endothelium alleviates oxidative stress and reduces acute lung transplantation injury

Vascular immunotargeting may facilitate the rapid and specific delivery of therapeutic agents to endothelial cells. We investigated whether targeting of an antioxidant enzyme, catalase, to the pulmonary endothelium alleviates oxidative stress in an in vivo model of lung transplantation. Intravenously injected enzymes, conjugated with an antibody to platelet-endothelial cell adhesion molecule-1, accumulate in the pulmonary vasculature and retain their activity during prolonged cold storage and transplantation. Immunotargeting of catalase to donor rats augments the antioxidant capacity of the pulmonary endothelium, reduces oxidative stress, ameliorates ischemia-reperfusion injury, prolongs the acceptable cold ischemia period of lung grafts, and improves the function of transplanted lung grafts. These findings validate the therapeutic potential of vascular immunotargeting as a drug delivery strategy to reduce endothelial injury. Potential applications of this strategy include improving the outcome of clinical lung transplantation and treating a wide variety of endothelial disorders.

In vivo phage display and vascular heterogeneity: implications for targeted medicine

The vascular endothelium expresses differential receptors depending on the functional state and tissue localization of its cells. A method to characterize this receptor heterogeneity with phage display random peptide libraries has been developed. Using this technology, several peptide ligands have been isolated that home to tissue-specific endothelial cell receptors following intravenous administration. Such peptide ligands, or antibodies directed against specific vascular receptors, can be used to target therapeutic compounds or imaging agents to endothelial cells in vitro and in vivo. Recent advances in the field include identification of novel endothelial receptors expressed differentially in normal and pathological conditions and the isolation of peptides or antibody ligands to such receptors in in vitro assays, in animal models and in a human patient. These milestones, which extend the 'functional map' of the vasculature, should lead to clinical applications in diseases such as cancer and other conditions that exhibit distinct vascular characteristics.

Platelet-endothelial cell adhesion molecule-1-directed immunotargeting to cardiopulmonary vasculature

Therapeutic molecules conjugated with antibodies to the platelet-endothelial cell adhesion molecule-1 (PECAM-1) accumulate in the pulmonary endothelium after i.v. injection in mice. In this study, we characterized PECAM-directed targeting to the lung and heart after local versus systemic intravascular administration in a large animal model, pigs. Radiolabel tracing showed that 1 h post-i.v. injection, 35% of anti-PECAM versus 2.5% of control IgG had accumulated in the lungs. Infusion of anti-PECAM via a catheter placed in the right pulmonary artery (RPA) resulted in a 3-fold elevation of the uptake in the right lower lobe and 2-fold reduction of uptake in the left lobes in the lung. Cardiac uptake of anti-PECAM was negligible after i.v. and RPA infusion. In contrast, delivery with a catheter placed in the right coronary artery (RCA) resulted in a 4-fold elevation of cardiac uptake of anti-PECAM, but not IgG, compared with i.v. injection. To estimate the targeting of an active reporter enzyme, streptavidin-conjugated beta-galactosidase (beta-Gal) was coupled to anti-PECAM or IgG (anti-PECAM/beta-Gal and IgG/beta-Gal) and injected into the RCA. Beta-Gal activity was markedly elevated in the heart and lungs (5- and 25-fold increased, respectively) after injection of anti-PECAM/beta-Gal, but not IgG/beta-Gal. Image analysis confirmed endothelial targeting of anti-PECAM/beta-Gal in the heart and lung. In summary, anti-PECAM antibody conjugates deliver agents to the pulmonary endothelium regardless of injection route, whereas local arterial infusion permits targeting to the cardiac vasculature. This paradigm may be useful for drug targeting to endothelium in lungs, heart, and possibly other organs.

Size-dependent intracellular immunotargeting of therapeutic cargoes into endothelial cells

Cell-selective intracellular targeting is a key element of more specific and safe enzyme, toxin, and gene therapies. Endothelium poorly internalizes certain candidate carriers for vascular immunotargeting, such as antibodies to platelet endothelial cell adhesion molecule 1 (PECAM-1). Conjugation of poorly internalizable antibodies with streptavidin (SA) facilitates the intracellular uptake. Although both small and large (100-nm versus 1000-nm diameter) anti-PECAM/SA-beta galactosidase (SA-beta-gal) conjugates bound selectively to PECAM-expressing cells, only small conjugates showed intracellular accumulation of active beta-gal. To study whether size of the conjugates controls the uptake, a series of anti-PECAM/SA and anti-PECAM/bead conjugates ranging from 80 nm to 5 microm in diameter were produced. Human umbilical vein endothelial cells and PECAM-transfected mesothelioma cells internalized 80- to 350-nm anti-PECAM conjugates, but not conjugates larger than 500 nm. Further, size controls intracellular targeting of active therapeutic cargoes in vitro and in vivo. Small anti-PECAM/DNA conjugates transfected target cells in culture 5-fold more effectively than their large counterpart (350- versus 4200-nm diameter). To evaluate the practical significance of the size-controlled subcellular addressing, we coupled glucose oxidase (GOX) to anti-PECAM and antithrombomodulin. Both types of conjugates had equally high pulmonary uptake after intravenous injection in mice, yet only small (200- to 250-nm), not large (600- to 700-nm), GOX conjugates caused profound oxidative vascular injury in the lungs, presumably owing to intracellular generation of H(2)O(2). Thus, engineering of affinity carriers of specific size permits intracellular delivery of active cargoes to endothelium in vitro and in vivo, a paradigm useful for the targeting of drugs, genes, and toxins.

Antiangiogenic and antivascular therapy for cancer

The great interest in the potential antineoplastic effect of targeting tumor-associated blood vessels has generated an expanding armamentarium of therapeutic tools that include antiangiogenic compounds, aimed at preventing the formation of vessels, and antivascular compounds, targeted to the existing tumor vasculature. Following promising preclinical studies, antiangiogenic drugs have rapidly gained access to clinical trials.

Lung uptake of antibodies to endothelial antigens: key determinants of vascular immunotargeting

Vascular immunotargeting is a mean for a site-selective delivery of drugs and genes to endothelium. In this study, we compared recognition of pulmonary and systemic vessels in rats by candidate carrier monoclonal antibodies (MAbs) to endothelial antigens platelet endothelial cell adhesion molecule (PECAM)-1 (CD31), intercellular adhesion molecule (ICAM)-1 (CD54), Thy-1.1 (CD90.1), angiotensin-converting enzyme (ACE; CD143), and OX-43. Tissue immunostaining showed that endothelial cells were Thy-1.1 positive in capillaries but negative in large vessels. In the lung, anti-ACE MAb provided a positive staining in 100% capillaries vs. 5–20% capillaries in other organs. Other MAbs did not discriminate between pulmonary and systemic vessels. We determined tissue uptake after infusion of 1 μg of 125I-labeled MAbs in isolated perfused lungs (IPL) or intravenously in intact rats. Uptake in IPL attained 46% of the injected dose (ID) of anti-Thy-1.1 and 20–25% ID of anti-ACE, anti-ICAM-1, and anti-OX-43 (vs. 0.5% ID of control IgG). However, after systemic injection at this dose, only anti-ACE MAb 9B9 displayed selective pulmonary uptake (16 vs. 1% ID/g in other organs). Anti-OX-43 displayed low pulmonary (0.5% ID/g) but significant splenic and cardiac uptake (7 and 2% ID/g). Anti-Thy-1.1 and anti-ICAM-1 displayed moderate pulmonary (4 and 6% ID/g, respectively) and high splenic and hepatic uptake (e.g., 18% ID/g of anti-Thy-1.1 in spleen). The lung-to-blood ratio was 5, 10, and 15 for anti-Thy-1.1, anti-ACE, and anti-ICAM-1, respectively. PECAM antibodies displayed low pulmonary uptake in perfusion (2% ID) and in vivo (3–4% ID/g). However, conjugation with streptavidin (SA) markedly augmented pulmonary uptake of anti-PECAM in perfusion (10–54% ID, depending on an antibody clone) and in vivo (up to 15% ID/g). Therefore, ACE-, Thy-1.1-, ICAM-1-, and SA-conjugated PECAM MAbs are candidate carriers for pulmonary targeting. ACE MAb offers a high selectivity of pulmonary targeting in vivo, likely because of a high content of ACE-positive capillaries in the lungs.

Targeting of superoxide dismutase and catalase to vascular endothelium

Reactive oxygen species, such as superoxide anion (O2(-)) and H2O2, cause oxidative stress in endothelial cells, a condition implicated in the pathogenesis of many cardiovascular and pulmonary diseases. Antioxidant enzymes, superoxide dismutases (SOD, converting superoxide anion into H2O2) and catalase (converting H2O2 into water), are candidate drugs for augmentation of antioxidant defenses in endothelium. However, SOD and catalase undergo fast elimination from the bloodstream, which compromises delivery and permits rather modest, if any, protection against vascular oxidative stress. Coupling of polyethylene glycol (PEG) to the enzymes and encapsulating them in liposomes increases their bioavailability and enhances their protective effect. Chemical modifications and genetic manipulations of SOD and catalase have been proposed in order to provide more effective delivery to endothelium. For example, chimeric protein constructs consisting of SOD and heparin-binding peptides have an affinity for charged components of the endothelial glycocalix. However, the problem of developing a more effective and precise delivery of the drugs to endothelial cells persists. Endothelial surface antigens may be employed to provide targeting and subcellular addressing of drugs (vascular immunotargeting strategy). Thus, SOD and catalase conjugated to antibodies directed against the constitutively expressed endothelial antigens, angiotensin-converting enzyme (ACE) and adhesion molecules (ICAM-1 or PECAM-1), bind to endothelium in intact animals after intravascular administration, accumulate in the pulmonary vasculature, enter endothelial cells and augment their antioxidant defenses. Such immunotargeting strategies may provide secondary therapeutic benefits by inhibiting the function of target antigens. For example, blocking of ICAM-1 and PECAM-1 by carrier antibodies may attenuate inflammation and leukocyte-mediated vascular damage. Additional studies in animal models of vascular oxidative stress are necessary in order to more fully characterize potential therapeutic effects and limitations of targeting of antioxidant enzymes to endothelial cells.