Targeting an adenoviral gene vector to cytokine-activated vascular endothelium via E-selectin

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.

Targeting therapeutics to endothelium: are we there yet?

Vascular endothelial cells represent an important therapeutic target in many pathologies, including inflammation, oxidative stress, and thrombosis; however, delivery of drugs to this site is often limited by the lack of specific affinity of therapeutics for these cells. Selective delivery of both small molecule drugs and therapeutic proteins to the endothelium has been achieved through the use of targeting ligands, such as monoclonal antibodies, directed against endothelial cell surface markers, particularly cell adhesion molecules (CAMs). Careful selection of target molecules and targeting agents allows for precise delivery to sites of inflammation, thereby maximizing therapeutic drug concentrations at the site of injury. A good understanding of the physiological and pathological determinants of drug and drug carrier pharmacokinetics and biodistribution may allow for a priori identification of optimal properties of drug carrier and targeting agent. Targeted delivery of therapeutics such as antioxidants and antithrombotic agents to the injured endothelium has shown efficacy in preclinical models, suggesting the potential for translation into clinical practice. As with all therapeutics, demonstration of both efficacy and safety are required for successful clinical implementation, which must be considered not only for the individual components (drug, targeting agent, etc.) but also for the sum of the parts (e.g., the drug delivery system), as unexpected toxicities may arise with complex delivery systems. While the use of endothelial targeting has not been translated into the clinic to date, the preclinical results summarized here suggest that there is hope for successful implementation of these agents in the years to come.

The new frontiers of the targeted interventions in the pulmonary vasculature: precision and safety (2017 Grover Conference Series)

The pulmonary vasculature plays an important role in many lung pathologies, such as pulmonary arterial hypertension, primary graft dysfunction of lung transplant, and acute respiratory distress syndrome. Therapy for these diseases is quite limited, largely due to dose-limiting side effects of numerous drugs that have been trialed or approved. High doses of drugs targeting the pulmonary vasculature are needed due to the lack of specific affinity of therapeutic compounds to the vasculature. To overcome this problem, the field of targeted drug delivery aims to target drugs to the pulmonary endothelial cells, especially those in pathological regions. The field uses a variety of drug delivery systems (DDSs), ranging from nano-scale drug carriers, such as liposomes, to methods of conjugating drugs to affinity moieites, such as antibodies. These DDSs can deliver small molecule drugs, protein therapeutics, and imaging agents. Here we review targeted drug delivery to the pulmonary endothelium for the treatment of pulmonary diseases. Cautionary notes are made of the risk–benefit ratio and safety—parameters one should keep in mind when developing a translational therapeutic.

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.

Endothelial nanomedicine for the treatment of pulmonary disease

INTRODUCTION

Even though pulmonary diseases are among the leading causes of morbidity and mortality in the world, exceedingly few life-prolonging therapies have been developed for these maladies. Relief may finally come from nanomedicine and targeted drug delivery.

AREAS COVERED

Here, we focus on four conditions for which the pulmonary endothelium plays a pivotal role: acute respiratory distress syndrome, primary graft dysfunction occurring immediately after lung transplantation, pulmonary arterial hypertension and pulmonary embolism. For each of these diseases, we first evaluate the targeted drug delivery approaches that have been tested in animals. Then we suggest a 'need specification' for each disease: a list of criteria (e.g., macroscale delivery method, stability, etc.) that nanomedicine agents must meet in order to warrant human clinical trials and investment from industry.

EXPERT OPINION

For the diseases profiled here, numerous nanomedicine agents have shown promise in animal models. However, to maximize the chances of creating products that reach patients, nanomedicine engineers and clinicians must work together and use each disease's need specification to guide the design of practical and effective nanomedicine agents.

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.

Genetic maneuvers to ameliorate ventricular function in heart failure: therapeutic potential and future implications

Gene therapy to treat heart failure has evolved into a growing field of investigation yielding remarkable results in preclinical models. Whether these results will persist in clinical trials remains to be seen. However, researchers still face a number of obstacles that need to be overcome before this treatment can be employed effectively. Efforts are required to identify better vectors with minimal side effects and maximal efficiency and durability. There is also a need to develop less invasive and more effective techniques to deliver these vectors. This review will discuss different methods to achieve these goals, the various pathologic mechanisms that have been targeted so far and those with strong potential for use in the future.

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.

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.

E-selectin as a target for drug delivery and molecular imaging

E-selectin, also known as CD62E, is a cell adhesion molecule expressed on endothelial cells activated by cytokines. Like other selectins, it plays an important part in inflammation and in the adhesion of metastatic cancer cells to the endothelium. E-selectin recognizes and binds to sialylated carbohydrates present on the surface proteins of certain leukocytes. E-selectin has been chosen as a target for several therapeutic and medical imaging applications, based on its expression in the vicinity of inflammation, infection or cancer. These systems for drug delivery and molecular imaging include immunoconjugates, liposomes, nanoparticles, and microparticles prepared from a wide range of starting materials including lipids, synthetic polymers, polypeptides and organo-metallic structures. After a brief introduction presenting the selectin family and their implication in physiology and pathology, this review focuses on the formulation of these new delivery systems targeting E-selectin at a molecular level.

Strategies to enhance viral penetration of solid tumors

The efficient delivery of viral vectors to tumors is an active area of investigation. A number of barriers exist that must be overcome to achieve good penetration of vectors into tumors and distribution of their effects throughout the tumor mass. Replicating oncolytic viruses have the advantage of being able to amplify the initial dose, but progeny virus are prevented from spreading because of a dense mass of tightly packed cells with a dense extracellular matrix, admixed normal stromal cells, and high interstitial pressure. Although intratumoral injection may ensure initial delivery the distribution achieved by intravenous administration may be superior and come with beneficial bystander damage to the tumor vasculature. Strategies to enhance intravenous delivery and subsequent spread of these vectors within tumors are being developed by a number of groups. Achieving the goal of efficient penetration and spread of viruses within solid tumors is a necessary prerequisite to significant improvements in virus-vectored therapy of solid tumors.

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.

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.

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.

Vascular targeting: recent advances and therapeutic perspectives

The ability to deliver therapeutics site—specifically in vivo—remains a major challenge for the treatment of malignant, inflammatory, cardiovascular, and degenerative diseases. The need to target agents safely, efficiently, and selectively has become increasingly evident because of progress in vascular targeting. The vascular endothelium is a central target for intervention, given its multiple roles in the physiology (in health) and pathophysiology (in disease) and its direct accessibility to circulating ligands. In cancer, the expression of specific molecules on the surface of vascular endothelial and perivascular cells might enable direct therapeutic targeting. The use of in vivo phage display has significantly contributed to the identification of such targets, which have been successfully used for directed vascular targeting in preclinical animal models. Several animal studies have been performed by using fused molecules between tumor endothelium-directed molecules and immunomodulatory, procoagulant, or cytotoxic molecules. In addition to delivery of therapeutic agents, vascular targeted gene therapies based on both ligand-directed delivery of gene vectors to tumor endothelium and transcriptional targeting have also emerged. In this review, we discuss ligand-directed vascular targeting strategies with an emphasis on recent developments related to phage-display-based screenings.

Tumor vascular targeting therapy with viral vectors

Tumor angiogenesis is crucial for the progression and metastasis of cancer. The vasculature of tumor tissue is different from normal vasculature. Therefore, tumor vascular targeting therapy could represent an effective therapeutic strategy with which to suppress both primary tumor growth and tumor metastasis. The use of viral vectors for tumor vascular targeting therapy is a promising strategy based on the unique properties of viral vectors. In order to circumvent the potential problems of antiviral neutralizing antibodies, poor access to extravascular tumor tissue, and toxicities to normal tissue, viral vectors need to be modified to target the tumor endothelial cells. Viral vectors that could be used for tumor vascular targeting therapy include adenoviral vectors, adeno-associated viral vectors, retroviral vectors, lentiviral vectors, measles virus, and herpes simplex viral vectors. In this review, we will summarize the strategies available for targeting viral vectors for tumor vascular targeting therapy.

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.

Cell-selective viral gene delivery vectors for the vasculature

Clinical gene therapy for cardiovascular disease remains achievable. To date, however, preclinical studies and clinical trials have highlighted shortfalls in viral gene delivery to vascular cells. These include poor efficiency, poor target tissue selectivity, the presence of pre-existing neutralizing antibodies and immunogenicity generated by the host to vectors such as adenovirus. These important issues require careful consideration when applying viral vectors for gene therapy. Each delivery vector requires precise optimization and tailoring for each disease application since parameters relating to vector : tissue exposure time, route of delivery and target cell type vary considerably. Optimization can be achieved through modification of the structure of the virus capsid proteins and expression cassette to generate vectors that are highly selective and efficient for target cell binding and entry as well as instilling transcriptional control and/or longevity on transgene expression. This ultimately will improve the efficacy and toxicity profiles of gene delivery vectors and has become a very important area in gene therapy. Here, we review recent advances in the targeting of viral gene delivery vectors to the vasculature.

Cytokine-mediated downregulation of coxsackievirus-adenovirus receptor in endothelial cells

Endothelial cells have the ability to change their complement of cell surface proteins in response to inflammatory cytokines. We hypothesized that the expression of the coxsackievirus-adenovirus receptor (CAR), a viral receptor and putative cell-cell adhesion molecule, may be altered during the response of endothelial cells to inflammation. To test this hypothesis, we evaluated CAR protein and mRNA levels in human umbilical vein endothelial cells after they were exposed to tumor necrosis factor alpha, gamma interferon, or a combination of the two cytokines. Flow cytometric and Western blot analyses indicated that cytokine treatment led to a synergistic decrease in CAR protein expression. A Western blot analysis showed that CAR levels decreased to 16% +/- 4% or 1% +/- 4% of the CAR protein levels in untreated cells with either 24 or 48 h of cytokine treatment, respectively. Quantitative reverse transcription-PCR demonstrated that the combination treatment caused CAR mRNA levels to decrease to 21% +/- 12% or 5% +/- 3% of the levels in untreated cells after a 24- or 48-h cytokine treatment, respectively. Reduced CAR expression led to a decrease in adenovirus (Ad) binding of 80% +/- 3% (compared with untreated endothelial cells), with a subsequent decrease in Ad-mediated gene transfer that was dependent on the dose and duration of cytokine treatment but not on the dose of Ad. A similar decrease in CAR protein level and susceptibility to Ad infection was observed in human microvascular endothelial cells, while CAR expression on normal human bronchial epithelial cells or A549 lung epithelial cells was less affected by cytokine treatments. Taken together, the data demonstrate that inflammatory cytokines decrease CAR mRNA and protein expression with a concomitant decrease in Ad binding, reflecting the impact of cell physiology on the function of CAR and the potential effect of inflammation on the ability of Ad to transfer genes to endothelial cells.

Designing gene delivery vectors for cardiovascular gene therapy

Genetic therapy in the cardiovascular system has been proposed for a variety of diseases ranging from prevention of vein graft failure to hypertension. Such diversity in pathogenesis requires the delivery of therapeutic genes to diverse cell types in vivo for varying lengths of time if efficient clinical therapies are to be developed. Data from extensive preclinical studies have been compiled and a certain areas have seen translation into large-scale clinical trials, with some encouraging reports. It is clear that progress within a number of disease areas is limited by a lack of suitable gene delivery vector systems through which to deliver therapeutic genes to the target site in an efficient, non-toxic manner. In general, currently available systems, including non-viral systems and viral vectors such as adenovirus (Ad) or adeno-associated virus (AAV), have a propensity to transduce non-vascular tissue with greater ease than vascular cells thereby limiting their application in cardiovascular disease. This problem has led to the development and testing of improved vector systems for cardiovascular gene delivery. Traditional viral and non-viral systems are being engineered to increase their efficiency of vascular cell transduction and diminish their affinity for other cell types through manipulation of vector:cell binding and the use of cell-selective promoters. It is envisaged that future use of such technology will substantially increase the efficacy of cardiovascular gene therapy.

Gene therapy with viral vectors

A key factor in the success of gene therapy is the development of gene delivery systems that are capable of efficient gene transfer in a broad variety of tissues, without causing any pathogenic effect. Currently, viral vectors based on many different viruses have been developed, and their performance and pathogenicity has been evaluated in animal models. The results of these studies form the basis for the first clinical trials for correcting genetic disorders using retroviral, adenoviral, and adeno-associated viral vectors. Even though the results of these trials are encouraging, vector development is still required to improve and refine future treatment of hereditary disorders.

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.

Development of vasculature targeting strategies for the treatment of cancer and chronic inflammatory diseases

Endothelial cells play a pathological role in cancer and chronic inflammation and are therefore attractive targets for therapeutic intervention. This review focuses on endothelial cell specific drug targeting strategies for the treatment of these diseases. The cellular and molecular processes involved in the activation of endothelial cells in angiogenesis and inflammation will be reviewed. Various target epitopes expressed by activated endothelium suitable for targeting purposes, design and development of drug-carrier complexes, drugs of interest which might interfere with endothelial cell activation, as well as in vitro and in vivo experimental approaches to study (intra) cellular drug delivery will be discussed.

Targeting gene therapy vectors to the vascular endothelium

The ability to deliver genes, and hence therapeutic gene over-expression site-specifically in vivo remains the major challenge for research in the field. The obligate need to target transgene expression safely, efficiently, and selectively has become increasingly evident as a result of recent events in the clinical setting. The endothelium represents an important target for gene delivery given its fundamental role in the physiology and pathophysiology of many diseases. Recently, studies demonstrating the ability to target viral vectors to the endothelium have been reported. In this review, we discuss progress to date and highlight those areas still requiring further investigation and validation.

Isolating ligands specific for human vasculature using in vivo phage selection

The endothelium lining blood vessels expresses molecules that are restricted in their expression to a particular tissue or organ. These molecules are attractive targets for therapy and diagnosis because they allow agents to be delivered specifically to the blood vessels supplying the desired tissue. However, it is difficult to identify these tissue-specific molecules because endothelium loses much of its tissue-specific nature when it is removed from the organ. This can be overcome by using in vivo phage selection - injecting libraries of phage bearing antibodies or peptides into an animal and isolating phage that bind to the relevant tissue. A variation on this approach, in which in vivo phage selection is performed in animals bearing human tissue xenografts, allows the isolation of peptides (and presumably other molecules) specific for human vasculature.

Cell physiology as a variable in gene transfer to endothelium

The accessibility, distribution, and mass of endothelial cells make this cell type an ideal target for in vivo gene transfer. Genetic modification of endothelial cells has been contemplated for a variety of therapeutic purposes, including induction of angiogenesis, prevention of restenosis following angioplasty, suppression of vessel growth in tumors, and as a source of therapeutic proteins for treatment of hereditary or acquired disorders. In targeting endothelial cells for gene transfer, the complex physiology of these cells must be taken into account. Optimizing gene transfer to endothelial cells by working in concert with endothelial cell physiology may lead to a significant decrease in dose of vector required to achieve a therapeutic result, thus increasing the safety and utility of this approach.

Endothelial cells at inflammatory sites as target for therapeutic intervention

In the course of an inflammation, vascular endothelial cells (VECs) are strongly involved in processes like leukocyte recruitment, cytokine production, and angiogenesis. Specific interference in these processes may yield great therapeutic benefit in the treatment of (chronic) inflammatory disorders. Drug targeting to VECs at inflamed sites may allow such intervention. VECs at inflamed sites represent a very well-accessible target cell population for circulating drug-targeting systems, which may also be selectively distinguished from normal VECs by the expression of several cell surface receptors involved in the inflammation. One group of specifically expressed molecules are the adhesion molecules (AMs), which have a major function in adhesion of cells to each other, to the extracellular matrix, or in the adhesion and subsequent recruitment of circulating immune cells. This review describes AMs with regard to their function in the inflammatory disease and their usefulness in functioning as a specific target receptor for drug-targeting approaches in general and with an emphasis on liposome-based drug delivery.

Antibody targeted gene transfer to endothelium

BACKGROUND

One of the drawbacks of the currently available vectors for gene therapy is the lack of selectivity in gene delivery. We have therefore investigated a strategy to generate immunoliposomes to target non-viral vectors to cell surface receptors on endothelium.

MATERIALS AND METHODS

We have developed a novel method of coupling antibodies (Abs) to liposomes complexed to DNA, using mild heat treatment to aggregate the immunoglobulin G (IgG). The interaction of plasmid DNA, liposomes and Abs was measured using a gel retardation assay and a resonant mirror biosensor. The size of the transfection complex was determined by light scattering, and the binding and internalization of the complex to cells was followed using flow cytometry. The transfection ability was tested on cell lines and primary cells in vitro and human corneal or vascular tissues ex vivo.

RESULTS

The interaction of antibodies with liposomes is relatively stable (t(1/2) congruent with 45 min). The size of the liposome, Ab and DNA complex was found to be around 500 nm in 4% BSA. The addition of anti-transferrin receptor Abs increased the internalization of the liposome-DNA complex into cells. Abs against both transferrin receptor and E-selectin were shown to augment transfection efficiency of liposomes to cell expressing the appropriate antigens. They are also shown to be efficient in mediating gene delivery to corneal and vascular tissues ex vivo.

CONCLUSIONS

We have shown that our novel vector is capable of in vitro and ex vivo gene delivery to cells and human tissues including cornea, artery and vein. In particular, an Ab against E-selectin was effective at selectively delivering genes to activated endothelial cells expressing the adhesion molecule. Such a strategy will have applications for targeting these tissues prior to transplantation or autologous grafting, and, in the longer term, may allow in vivo targeting of gene therapy to inflammatory sites.

Adenovirus and adeno-associated virus vectors

Recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) are among the most extensively used vectors in gene therapy studies to date. These two vectors share some similar features such as a broad host range and ability to infect both proliferating and quiescent cells. However, they also possess their own unique set of properties that render them particularly attractive for gene therapy applications. rAd vectors can accommodate larger inserts, mediate transient but high levels of protein expression, and can be easily produced at high titers. Development of gutted rAd vectors has further increased the cloning capacity of these vectors. The gaining popularity of rAAV use in gene therapy can be attributed to its lack of pathogenicity and added safety due to its replication defectiveness, and its ability to mediate long-term expression in a variety of tissues. Site-specific integration, as occurs with wild-type AAV, will be a unique and valuable feature if incorporated into rAAV vectors, further improving their safety. This paper describes these properties of rAd and rAAV vectors, and discusses further development and vector improvements that continue to extend the utility of these vectors, such as cell retargeting by capsid modification, differential transduction by use of serotypes, and extension of the cloning capacity of rAAV vectors by dual vector heterodimerization.

Development and use of gene transfer for treatment of cardiovascular disease

Gene therapy holds promise for the treatment of cardiovascular diseases for which effective pharmacological therapies are insufficient or unavailable. Recent studies have suggested that modification of current gene delivery systems combined with the use of efficacious therapeutic genes may ultimately be successful for clinical vascular gene therapy. Although certain applications such as vein-graft failure may be best suited for short-term transient overexpression of therapeutic genes, other disorders including human essential hypertension and atherosclerosis require sustained overexpression of genes. Hence, design and use of vector systems for delivery of genes to the required site in vivo requires careful consideration. Both viral and nonviral gene therapy vectors show low efficiency for gene transfer into vascular cells and demonstrate a lack of selectivity, as vectors have natural tropism for other cells and tissues. Recent work has focused on the design, development, and utility of vascular cell-selective gene therapy vectors for use in distinct and diverse vascular gene therapy scenarios. Using phage display technology we have isolated small peptide ligands that mediate selective binding to either vascular endothelial cells or vascular smooth muscle cells. When engineered into either adenoviral (Ad) or adeno-associated viral (AAV) vectors, candidate peptides enabled the virus to selectively bind to the desired cell type thus generating novel vascular cell-selective gene transfer. As preclinical studies have highlighted both the potential for vascular gene therapy as well as defining the potential pitfalls, the development of disease-selective gene therapeutics will increase safety and efficiency of gene therapy for future clinical use.

Strategies for muscle-specific targeting of adenoviral gene transfer vectors

Currently, adenoviral transfer of therapeutic genes such as dystrophin is hampered by low transduction efficiency of adult skeletal muscle. This is largely due to the lack of appropriate virus attachment receptors on the myofiber surface. Recent studies in transgenic mice revealed that upregulation of Coxsackie- and adenovirus receptor improves gene transfer efficiency by approximately ten-fold. Conversely, the vector load that needed to be administered to achieve sufficient gene transfer could be lowered significantly. Reduced viral vector loads may help to control virally mediated toxicity and immunogenicity. To date, there are no drugs or methods known to increase Coxsackie- and adenovirus receptor expression in skeletal muscle that would be easily applicable in humans. However, alternative strategies such as vector retargeting are currently being investigated that may allow for an increase in binding of adenoviral vectors to skeletal muscle. Recent experiments have shown that directed mutagenesis of the adenoviral fiber knob allows for a significant reduction in Coxsackie- and adenovirus receptor binding and for introduction of a new binding domain. Therefore, vector retargeting towards efficient and specific infection of skeletal muscle may be achieved by directed genetic alteration of adenoviral capsid proteins.

Targeted adenoviral vectors

Replication-defective vectors based on human adenovirus serotypes 2 and 5 (Ad2 and Ad5) possess a number of attributes which favor their use as gene delivery vehicles in gene therapy applications. However, the widespread distribution of the primary cellular receptor for Ad, the coxsackievirus and adenovirus receptor (CAR), allows Ad vectors to infect a broad range of cells in the host. Conversely, a number of tissues which represent important targets for gene therapy, such as the airway epithelium and cancer cells, are refractory to Ad infection due a paucity of CAR. Thus, there is a strong rationale for the development of CAR-independent Ad vectors capable of enhanced specificity and efficiency of gene transfer to target cells. In this article we review the approaches which have been employed to generate tropism-modified Ad vectors. These targeting strategies have led to improvements in the safety and efficacy of Ad vectors and have the potential to yield an increased therapeutic benefit in the human clinical context.

Use of phage display to identify novel peptides for targeted gene therapy

The field of gene therapy has developed at an astonishing pace over the past decade, perhaps too quickly. Clinical studies have highlighted major flaws in the ability of current vectors to deliver genes safely and effectively to patients; hence the further development of vectors is a prerequisite for future success. In this chapter we have discussed advances in development of targeted vectors through isolation of targeting moieties using phage display. The field of gene therapy will benefit considerably by the isolation and use of peptides that are effective for targeting in vivo, particularly for diseases affecting individual organs. Only when truly selective and highly efficient vectors are constructed will the tremendous potential of gene therapy be realized.

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.

Immunopathogenesis of vasculitis

Vasculitis is defined by the presence of leukocytes in the vessel wall with reactive damage to mural structures, leading to tissue ischemia and necrosis. The immunopathologic events that initiate the process of vascular inflammation and blood vessel damage are unclear. Damage of vascular endothelial cells and the recruitment and accumulation of the inflammatory infiltrate are determined by the endothelial cell and the bystanders, including the expression of adhesion molecules, the secretion of peptides and hormones, and the specific interaction with inflammatory cells. In addition to the endothelial cells, which provide costimulatory function, other cellular components and nonendothelial structures of the vessel wall are involved in controlling the inflammatory process, serve as antigen-presenting cells, and contribute with inflammatory mediators.

Prostate cancer gene therapy and the role of radiation

Even though prostate cancer is detected earlier than in the pre-PSA era, prostate cancer is the second leading cause of cancer mortality in the American male. Prostate cancer therapy is not ideal, especially for high-risk localized and metastatic cancer; therefore, investigators have sought new therapeutic modalities such as angiogenesis inhibitors, inhibitors of the cell signaling pathway, vaccines, and gene therapy. Gene therapy has emerged as potential therapy for both localized and systemic prostate cancer. Gene therapy has been shown to work supra-additively with radiation in controlling prostate cancer in vivo. With further technological advances in radiation therapy, gene therapy, and the understanding of prostate cancer biology, gene therapy will potentially have an important role in prostate cancer therapy.

New generation adenoviral vectors improve gene transfer by coxsackie and adenoviral receptor-independent cell entry

Adenoviral (Ad) vectors possess many attributes that have lead to their use as gene delivery agents in human clinical trials. However, gene transfer efficiency has generally been less than that needed for meaningful clinical responses. The restricted tropism of the virus for its native receptor, the coxsackie and adenoviral receptor (CAR), is emerging as a key limitation to the use of these agents. By developing strategies to achieve Ad infection via alternate receptor pathways, enhanced and more specific gene delivery can be achieved. This new generation of tropism-modified agents holds promise for the improved clinical utility of Ad vectors for gene therapy.

Ablating adenovirus type 5 fiber-CAR binding and HI loop insertion of the SIGYPLP peptide generate an endothelial cell-selective adenovirus

Adenovirus type 5 (Ad) based vectors transduce vascular endothelial cells (EC) and have been widely used for vascular gene transfer. However, many cell types express the Ad receptor (cox-sackievirus adenovirus receptor; CAR), preventing selective EC infection and precluding clinical use. We previously isolated the human EC-binding peptides SIGYPLP and LSNFHSS by phage display and demonstrated by means of a bispecific antibody that SIGYPLP directs efficient, high-level, EC-selective Ad-mediated gene transfer. We now generate genetically modified Ad fiber proteins with selective EC tropism by engineering these peptides into the HI loop of the Ad fiber. SIGYPLP, but not LSNFHSS, enhanced EC selectivity, demonstrating maintenance of peptide-cell binding fidelity upon incorporation into virions. Combining fiber mutations that block CAR binding (detargeting) with SIGYPLP insertion (retargeting) generated a novel Ad vector, AdKO1SIG, in a single component system. AdKO1SIG demonstrated efficient and selective tropism for EC compared with control Ad vectors. This is the first demonstration of genetic incorporation of a novel, mammalian, cell-selective ligand that retains its targeting fidelity in the Ad fiber HI loop, in combination with point mutations that abolish fiber-CAR interaction. This study demonstrates the potential for improving the cell-selectivity and safety of adenoviral vectors.

Gene therapy for acute diseases

The use of gene transfer systems to study cell function makes it apparent that overexpression of a transgene can restore or improve the function of a protein and positively influence cell function in a predetermined manner for purposes of counterbalancing cellular pathophysiology. The ability of some gene transfer vehicles to produce transgene product within hours of delivery positions gene transfer as a unique pharmaceutical administration system that can quickly affect production of biologic response modifiers in a highly compartmentalized fashion. This approach can be expected to overcome many of the adverse effects and high costs of systemic delivery of recombinant pharmaceuticals. This review highlights recent advances toward development of gene therapies for acute illnesses with particular emphasis on preclinical models of disease. In this context, a growing body of data suggests that gene therapies for polygenic and non-genetic diseases such as asthma, cardiogenic and non-cardiogenic pulmonary edema, stroke, subarachnoid hemorrhage, seizures, acute myocardial infarction, endovascular thrombosis, and infections may someday be options for the treatment of patients.

Radiation-mediated control of drug delivery

Clinical trials of radiotherapy to control drug delivery were initiated in 1999 at Vanderbilt University. The initial studies exploited the findings that platelets are activated in tumor blood vessels after high-dose irradiation as used in radiosurgery and high-dose-rate brachytherapy. Platelets labeled with 111In showed binding in tumor blood vessels. However, the platelet labeling process caused platelets to also accumulate in the spleen. That clinical trial was closed, and subsequent clinical trials targeted protein activation in irradiated tumor blood vessels. Preclinical studies showed that peptide libraries that bind within irradiated tumor blood vessels contained the peptide sequence Arg-Gln-Asp (RGD). RGD binds to integrin receptors (e.g., receptors for fibrinogen, fibronectin, and vitronectin). We found that the fibrinogen receptor (GPIIb/IIIa, alpha2bbeta3) is activated within irradiated tumor blood vessels. RGD peptidemimetics currently in clinical trials include GPIIb/IIIa antagonists and the platelet-imaging agent biapcitide. Biapcitide is an RGD mimetic that is labeled with 99Tc to allow gamma camera imaging of the biodistribution of the GPIIb/IIIa receptor in neoplasms of patients treated with radiosurgery. This study has shown that the schedule of administration of the RGD mimetic is crucial. The peptide mimetic must be administered immediately before irradiation, whereas the natural ligands to the receptor compete for biapcitide binding if biapcitide is administered after irradiation. The authors currently are conducting a dose deescalation study to determine the threshold dosage required for RGD mimetic binding to radiation activated receptor. Radiation-guided clinical trials have been initiated by use of high-dose-rate brachytherapy. In a separate trial, the pharmacokinetics of radiation-inducible gene therapy are being investigated. In this trial, the radiation-activated promoter Egr-1 regulates expression of the tumor necrosis factor alpha gene, which is administered by use of the attenuated adenovirus vector. The Ad.Egr-TNF (ADGV) gene is administered by intratumoral injection of vector followed by irradiation in patients with soft-tissue sarcomas. This review highlights recent findings in these phase I pharmacokinetic studies of radiation-controlled drug delivery systems.

Tumor angiogenesis as a therapeutic target

Angiogenesis - the formation of new blood vessels within a tumor (or many other tissue types) - has become a hotbed of pharmacological research as well as industrial drug discovery. This is the result of the efforts of a generation of scientists elucidating the complex (patho)physiological, biochemical and molecular events accompanying angiogenesis. It is estimated that >300 drug candidates are currently in various stages of testing, and it is, therefore, impossible to capture all of this in a brief review. Therefore, the emphasis here is on relatively advanced projects that are either in preclinical or clinical development, thus neglecting, to a large extent, the many exciting avenues being pursued in both academic and biotechnology laboratories. Although the potential of the approaches described cannot be overestimated, it is also important to note that there is still no drug on the market that achieves clinical benefit based on a selective modulation or inhibition of angiogenesis.

Combined transductional and transcriptional targeting improves the specificity of transgene expression in vivo

The promise of gene therapy for health care will not be realized until gene delivery systems are capable of achieving efficient, cell-specific gene delivery in vivo. Here we describe an adenoviral system for achieving cell-specific transgene expression in pulmonary endothelium. The combination of transductional targeting to a pulmonary endothelial marker (angiotensin-converting enzyme, ACE) and an endothelial-specific promoter (for vascular endothelial growth factor receptor type 1, flt-1) resulted in a synergistic, 300,000-fold improvement in the selectivity of transgene expression for lung versus the usual site of vector sequestration, the liver. This combined approach should be useful for the design of other gene delivery systems.

Targeting drug delivery to radiation-induced neoantigens in tumor microvasculature

Radiation can be used to guide drugs to specific sites such as neoplasms or aberrant blood vessels. When blood vessels are treated with ionizing radiation, they respond by expressing a number of cell adhesion molecules and receptors that participate in homeostasis. Examples of radiation-induced molecules in blood vessels include ICAM-1, E-selectin, P-selectin and the beta(3) integrin. We have observed that the endothelium and blood components respond to oxidative stress in a similar, if not identical manner in all tumor models. Although we have identified several other radiation-induced molecules within tumor blood vessels, the beta(3) target for drug delivery achieves the greatest site-specific peptide binding within irradiated tumor blood vessels. We have focused on peptides and antibodies that bind to integrin beta(3). beta(3)-binding proteins have been conjugated to fluorochromes and radionuclides to study the site specificity and microscopic distribution. We have found immunofluorescent and immunohistochemical staining of beta(3) within the lumen of blood vessels immediately following irradiation. To determine whether it is feasible to guide drug delivery to irradiated tumors, we studied ligands to alpha(2b)beta(3) (fibrinogen). Peptides within fibrinogen that bind to alpha(2b)beta(3) includes the dodecapeptide, HHLGGAKQAGDV and the RGD peptide. We utilized 131I conjugation to these ligands to study the biodistribution in tumor bearing mice. Our clinical trial consists of the RGD peptidomimetic, biapcitide, labeled with 99mTc. This study shows that it is feasible to guide drugs to human neoplasms by use of radiation-guided peptides. These studies have shown that peptides that bind to these integrins bind to tumors following exposure to ionizing radiation.

Binding properties, cell delivery, and gene transfer of adenoviral penton base displaying bacteriophage

The penton base of adenovirus mediates viral attachment to integrin receptors and particle internalisation, properties that can be exploited to reengineer prokaryotic viruses for the infection of mammalian cells. We report that filamentous phage displaying either the full-length penton base gene or a central region of 107 amino acids on their surface were able to bind, internalise, and transduce mammalian cells expressing integrin receptors. Both phage bound alphavbeta3, alphavbeta5, alpha3beta1, and alpha5beta1 integrin subtypes. Cell-binding was shown by electron microscopy; internalisation was investigated by immunofluorescence and confirmed by micropanning. As it has been described for adenovirus, pharmacologic disruption of phosphoinositide-30H kinase, but not of myosin light-chain kinase, inhibited phage internalisation. Recombinant phage encoding an eukaryotic expression cassette was able to mediate gene expression in mammalian cells. Taken together, these data open insights for the exploit of recombinant phage for integrin-targeted gene delivery.