Leveraging molecular heterogeneity of the vascular endothelium for targeted drug delivery and imaging

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.

Single Cell Analysis in Vascular Biology

The ability to quantify DNA, RNA, and protein variations at the single cell level has revolutionized our understanding of cellular heterogeneity within tissues. Via such analyses, individual cells within populations previously thought to be homogeneous can now be delineated into specific subpopulations expressing unique sets of genes, enabling specialized functions. In vascular biology, studies using single cell RNA sequencing have revealed extensive heterogeneity among endothelial and mural cells even within the same vessel, key intermediate cell types that arise during blood and lymphatic vessel development, and cell-type specific responses to disease. Thus, emerging new single cell analysis techniques are enabling vascular biologists to elucidate mechanisms of vascular development, homeostasis, and disease that were previously not possible. In this review, we will provide an overview of single cell analysis methods and highlight recent advances in vascular biology made possible through single cell RNA sequencing.

Extracellular vesicles: how they interact with endothelium, potentially contributing to metastatic cancer cell implants

Extracellular vesicles (EV) are blebs of cellular membranes, which entrap small portions of subjacent cytosol. They are released from a variety of cells, circulate in the blood for an unknown length of time and come to rest on endothelial surfaces. They contribute to an array of physiologic pathways, the complexity of which is still being investigated. They contribute to metastatic malignant cell implants and tumor-related angiogenesis, possibly abetted by the tissue factor that they carry. It is thought that the adherence of the EV to endothelium is dependent upon a combination of their P-selectin glycoprotein ligand-1 and exposed phosphatidylserine, the latter of which is normally hidden on the inner bilayer of the intact cellular membrane. This manuscript reviews what is known about EV origins, their clearance from the circulation and how they contribute to malignant cell implants upon endothelium surfaces and subsequent tumor growth.

Microenvironmental regulation of tumour angiogenesis

Tumours display considerable variation in the patterning and properties of angiogenic blood vessels, as well as in their responses to anti-angiogenic therapy. Angiogenic programming of neoplastic tissue is a multidimensional process regulated by cancer cells in concert with a variety of tumour-associated stromal cells and their bioactive products, which encompass cytokines and growth factors, the extracellular matrix and secreted microvesicles. In this Review, we discuss the extrinsic regulation of angiogenesis by the tumour microenvironment, highlighting potential vulnerabilities that could be targeted to improve the applicability and reach of anti-angiogenic cancer therapies.

Vascular-homing peptides for cancer therapy

In the past 30 years, a variety of phage libraries have been extensively utilized to identify and develop tumor homing peptides (THPs). THPs specifically bind to tumor cells or elements of the tumor microenvironment while no or low affinity to normal cells. In this regard, the efficacy of therapeutic agents in cancer therapy can be enhanced by targeting strategies based on coupling with THPs that recognize receptors expressed by tumor cells or tumor vasculature. Especially, vascular-homing peptides, targeting tumor vasculature, have their receptors expressed on or around the blood vessel including pro-angiogenic factors, metalloproteinase, integrins, fibrin-fibronectin complexes, etc. This review briefly summarizes recent studies on identification and therapeutic applications of vascular-homing peptides targeting common angiogenic markers or with unknown vascular targets in some certain types of cancers. These newly discovered vascular-homing peptides are promising candidates which could provide novel strategies for cancer therapy.

Paradigm shift in bacteriophage-mediated delivery of anticancer drugs: from targeted 'magic bullets' to self-navigated 'magic missiles'

INTRODUCTION

New phage-directed nanomedicines have emerged recently as a result of the in-depth study of the genetics and structure of filamentous phage and evolution of phage display and phage nanobiotechnology. This review focuses on the progress made in the development of the cancer-targeted nanomaterials and discusses the trends in using phage as a bioselectable molecular navigation system. Areas covered: The merging of phage display technologies with nanotechnology in recent years has proved promising in different areas of medicine and technology, such as medical diagnostics, molecular imaging, vaccine development and targeted drug/gene delivery, which is the focus of this review. The authors used data obtained from their research group and sourced using Science Citation Index (Web of Science) and NCBI PubMed search resources. Expert opinion: First attempts of adapting traditional concepts of direct targeting of tumor using phage-targeted nanomedicines has shown minimal improvements. With discovery and study of biological and technical barriers that prevent anti-tumor drug delivery, a paradigm shift from traditional drug targeting to nanomedicine navigation systems is required. The advanced bacteriophage-driven self-navigation systems are thought to overcome those barriers using more precise, localized phage selection methods, multi-targeting 'promiscuous' ligands and advanced multifunctional nanomedicine platforms.

An official American Thoracic Society workshop report: stem cells and cell therapies in lung biology and diseases

The University of Vermont College of Medicine and the Vermont Lung Center, in collaboration with the NHLBI, Alpha-1 Foundation, American Thoracic Society, European Respiratory Society, International Society for Cell Therapy, and the Pulmonary Fibrosis Foundation, convened a workshop, "Stem Cells and Cell Therapies in Lung Biology and Lung Diseases," held July 29 to August 1, 2013 at the University of Vermont. The conference objectives were to review the current understanding of the role of stem and progenitor cells in lung repair after injury and to review the current status of cell therapy and ex vivo bioengineering approaches for lung diseases. These are all rapidly expanding areas of study that both provide further insight into and challenge traditional views of mechanisms of lung repair after injury and pathogenesis of several lung diseases. The goals of the conference were to summarize the current state of the field, discuss and debate current controversies, and identify future research directions and opportunities for both basic and translational research in cell-based therapies for lung diseases. This conference was a follow-up to four previous biennial conferences held at the University of Vermont in 2005, 2007, 2009, and 2011. Each of those conferences, also sponsored by the National Institutes of Health, American Thoracic Society, and Respiratory Disease Foundations, has been important in helping guide research and funding priorities. The major conference recommendations are summarized at the end of the report and highlight both the significant progress and major challenges in these rapidly progressing fields.

Systems approaches to design of targeted therapeutic delivery

Targeted drug delivery aims to improve therapeutic effects and enable mechanisms that are not feasible for untargeted agents (e.g., due to impermeable biological barriers). To achieve targeting, a drug or its carrier should possess properties providing specific accumulation from circulation at the desired site. There are several examples of systems-inspired approaches that have been applied to achieve this goal. First, proteomics analysis of plasma membrane fraction of the vascular endothelium has identified a series of target molecules and their ligands (e.g., antibodies) that deliver conjugated cargoes to well-defined vascular cells and subcellular compartments. Second, selection of ligands binding to cells of interest using phage display libraries in vitro and in vivo has provided peptides and polypeptides that bind to normal and pathologically altered cells. Finally, large-scale high-throughput combinatorial synthesis and selection of lipid- and polymer-based nanocarriers varying their chemical components has yielded a series of carriers accumulating in diverse organs and delivering RNA interference agents to diverse cells. Together, these approaches offer a basis for systems-based design and selection of targets, targeting molecules, and targeting vehicles. Current studies focus on expanding the arsenal of these and alternative targeting strategies, devising drug delivery systems capitalizing on these strategies and evaluation of their benefit/risk ratio in adequate animal models of human diseases. These efforts, combined with better understanding of mechanisms and unintended consequences of these targeted interventions, need to be ultimately translated into industrial development and the clinical domain.

Targeting the interleukin-11 receptor α in metastatic prostate cancer: A first-in-man study

BACKGROUND

Receptors in tumor blood vessels are attractive targets for ligand-directed drug discovery and development. The authors have worked systematically to map human endothelial receptors (“vascular zip codes”) within tumors through direct peptide library selection in cancer patients. Previously, they selected a ligand-binding motif to the interleukin-11 receptor alpha (IL-11Rα) in the human vasculature.

METHODS

The authors generated a ligand-directed, peptidomimetic drug (bone metastasis-targeting peptidomimetic-11 [BMTP-11]) for IL-11Rα–based human tumor vascular targeting. Preclinical studies (efficacy/toxicity) included evaluating BMTP-11 in prostate cancer xenograft models, drug localization, targeted apoptotic effects, pharmacokinetic/pharmacodynamic analyses, and dose-range determination, including formal (good laboratory practice) toxicity across rodent and nonhuman primate species. The initial BMTP-11 clinical development also is reported based on a single-institution, open-label, first-in-class, first-in-man trial (National Clinical Trials number NCT00872157) in patients with metastatic, castrate-resistant prostate cancer.

RESULTS

BMTP-11 was preclinically promising and, thus, was chosen for clinical development in patients. Limited numbers of patients who had castrate-resistant prostate cancer with osteoblastic bone metastases were enrolled into a phase 0 trial with biology-driven endpoints. The authors demonstrated biopsy-verified localization of BMTP-11 to tumors in the bone marrow and drug-induced apoptosis in all patients. Moreover, the maximum tolerated dose was identified on a weekly schedule (20-30 mg/m²). Finally, a renal dose-limiting toxicity was determined, namely, dose-dependent, reversible nephrotoxicity with proteinuria and casts involving increased serum creatinine.

CONCLUSIONS

These biologic endpoints establish BMTP-11 as a targeted drug candidate in metastatic, castrate-resistant prostate cancer. Within a larger discovery context, the current findings indicate that functional tumor vascular ligand-receptor targeting systems may be identified through direct combinatorial selection of peptide libraries in cancer patients. Cancer 2015;121:2411–2421. © 2015 The Authors. Cancer published by Wiley Periodicals, Inc. on behalf of American Cancer Society.

The authors report on the development of a new ligand-directed peptidomimetic (termed bone metastasis-targeting peptidomimetic-11) for interleukin-11 receptor-based human vascular targeting, including the translation from preclinical studies to a first-in-class, first-in-man clinical trial in patients with metastatic, castrate-resistant prostate cancer.

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.

A novel model for evaluating therapies targeting human tumor vasculature and human cancer stem-like cells

Human tumor vessels express tumor vascular markers (TVM), proteins that are not expressed in normal blood vessels. Antibodies targeting TVMs could act as potent therapeutics. Unfortunately, preclinical in vivo studies testing anti-human TVM therapies have been difficult to do due to a lack of in vivo models with confirmed expression of human TVMs. We therefore evaluated TVM expression in a human embryonic stem cell-derived teratoma (hESCT) tumor model previously shown to have human vessels. We now report that in the presence of tumor cells, hESCT tumor vessels express human TVMs. The addition of mouse embryonic fibroblasts and human tumor endothelial cells significantly increases the number of human tumor vessels. TVM induction is mostly tumor-type-specific with ovarian cancer cells inducing primarily ovarian TVMs, whereas breast cancer cells induce breast cancer specific TVMs. We show the use of this model to test an anti-human specific TVM immunotherapeutics; anti-human Thy1 TVM immunotherapy results in central tumor necrosis and a three-fold reduction in human tumor vascular density. Finally, we tested the ability of the hESCT model, with human tumor vascular niche, to enhance the engraftment rate of primary human ovarian cancer stem-like cells (CSC). ALDH(+) CSC from patients (n = 6) engrafted in hESCT within 4 to 12 weeks whereas none engrafted in the flank. ALDH(-) ovarian cancer cells showed no engraftment in the hESCT or flank (n = 3). Thus, this model represents a useful tool to test anti-human TVM therapy and evaluate in vivo human CSC tumor biology.

Role of the retinal vascular endothelial cell in ocular disease

Retinal endothelial cells line the arborizing microvasculature that supplies and drains the neural retina. The anatomical and physiological characteristics of these endothelial cells are consistent with nutritional requirements and protection of a tissue critical to vision. On the one hand, the endothelium must ensure the supply of oxygen and other nutrients to the metabolically active retina, and allow access to circulating cells that maintain the vasculature or survey the retina for the presence of potential pathogens. On the other hand, the endothelium contributes to the blood-retinal barrier that protects the retina by excluding circulating molecular toxins, microorganisms, and pro-inflammatory leukocytes. Features required to fulfill these functions may also predispose to disease processes, such as retinal vascular leakage and neovascularization, and trafficking of microbes and inflammatory cells. Thus, the retinal endothelial cell is a key participant in retinal ischemic vasculopathies that include diabetic retinopathy and retinopathy of prematurity, and retinal inflammation or infection, as occurs in posterior uveitis. Using gene expression and proteomic profiling, it has been possible to explore the molecular phenotype of the human retinal endothelial cell and contribute to understanding of the pathogenesis of these diseases. In addition to providing support for the involvement of well-characterized endothelial molecules, profiling has the power to identify new players in retinal pathologies. Findings may have implications for the design of new biological therapies. Additional progress in this field is anticipated as other technologies, including epigenetic profiling methods, whole transcriptome shotgun sequencing, and metabolomics, are used to study the human retinal endothelial cell.

Peptide sequences mediating tropism to intact blood-brain barrier: an in vivo biodistribution study using phage display

Peptide motifs that demonstrate tropism for the blood brain barrier (BBB) are of real translational value in developing innovative delivery strategies for biological brain targeted therapies. In vivo peptide-phage display affords peptide selection against the full complement of biological markers within the correct cellular macro- and micro-environments. Here a stringent in vivo biopanning protocol was employed in the rat aimed at identifying cyclic 7-mer peptide motifs that mediate tropism to brain microvasculature. Five rounds of biopanning identified 349 unique peptide motifs in the brain tissue gray matter compartment (microvasculature and parenchyma). While in general no consensus was evident linking peptide physico-chemical properties and brain tropism, peptides bearing c-SxTSSTx-c or c-xxxSSTx-c motifs were found to be present in high abundance. Based on amino acid frequency distribution of the 349 unique peptides sequences a theoretical 'idealized' peptide pattern, c-PP(S/P)SSST-c, could be derived. For the most abundant experimental peptide sequence found in brain tissue, c-SYTSSTM-c, an in vivo pharmacokinetic and whole body tissue biodistribution study was performed. Based upon tissue exposure data (i.e. tissue AUC((0-infinity))) the sequence c-SYTSSTM-c efficiently retargeted phage virions to the brain providing an approximate 5-fold greater (P<0.05) accumulation in brain over control phage; in all other organs no significant (P>0.05) difference in tissue tropism between c-SYTSSTM-c and control phages were evident. This peptide and more generally the peptide motifs, -SxTSSTx- or -xxxSSTx-, warrant further investigation as agents mediating sequence-dependent tropism to brain microvasculature potentially able to deliver biologic cargo to the CNS.

Peptides as targeting probes against tumor vasculature for diagnosis and drug delivery

Tumor vasculature expresses a distinct set of molecule signatures on the endothelial cell surface different from the resting blood vessels of other organs and tissues in the body. This makes them an attractive target for cancer therapy and molecular imaging. The current technology using the in vivo phage display biopanning allows us to quickly isolate and identify peptides potentially homing to various tumor blood vessels. Tumor-homing peptides in conjugation with chemotherapeutic drugs or imaging contrast have been extensively tested in various preclinical and clinical studies. These tumor-homing peptides have valuable potential as targeting probes for tumor molecular imaging and drug delivery. In this review, we summarize the recent advances about the applications of tumor-homing peptides selected by in vivo phage display library screening against tumor vasculature. We also introduce the characteristics of the latest discovered tumor-penetrating peptides in their potential clinical applications.

Identification of peptides for tissue-specific delivery

Antisense-mediated exon skipping has shown to be a promising therapeutic approach and is in clinical trials for Duchenne muscular dystrophy. However, after systemic treatment the majority of the injected antisense oligonucleotides (AONs) will not end up in the intended tissue. This mistargeting of AONs might have detrimental effects, especially with long-term treatment and continuous accumulation of AONs. Further, even when no detrimental effects occur, mistargeted AONs are lost for exon skipping in the intended tissue. One way to reduce the amount of mistargeted AONs is by adding a peptide that specifically binds to and is taken up by the intended tissue. Such peptides can be found by screening phage display libraries. With in silico, in vitro, and in vivo testing, the peptides that bind the intended tissue most efficiently and most specifically can be identified.

Exercise tames the wild side of the Myc network: a hypothesis

We propose that the well-documented therapeutic actions of repeated physical activities over human lifespan are mediated by the rapidly turning over proto-oncogenic Myc (myelocytomatosis) network of transcription factors. This transcription factor network is unique in utilizing promoter and epigenomic (acetylation/deacetylation, methylation/demethylation) mechanisms for controlling genes that include those encoding intermediary metabolism (the primary source of acetyl groups), mitochondrial functions and biogenesis, and coupling their expression with regulation of cell growth and proliferation. We further propose that remote functioning of the network occurs because there are two arms of this network, which consists of driver cells (e.g., working myocytes) that metabolize carbohydrates, fats, proteins, and oxygen and produce redox-modulating metabolites such as H₂O₂, NAD⁺, and lactate. The exercise-induced products represent autocrine, paracrine, or endocrine signals for target recipient cells (e.g., aortic endothelium, hepatocytes, and pancreatic β-cells) in which the metabolic signals are coupled with genomic networks and interorgan signaling is activated. And finally, we propose that lactate, the major metabolite released from working muscles and transported into recipient cells, links the two arms of the signaling pathway. Recently discovered contributions of the Myc network in stem cell development and maintenance further suggest that regular physical activity may prevent age-related diseases such as cardiovascular pathologies, cancers, diabetes, and neurological functions through prevention of stem cell dysfunctions and depletion with aging. Hence, regular physical activities may attenuate the various deleterious effects of the Myc network on health, the wild side of the Myc-network, through modulating transcription of genes associated with glucose and energy metabolism and maintain a healthy human status.

Novel surface targets and serum biomarkers from the ovarian cancer vasculature

The molecular phenotype of tumor vasculature is different from normal vasculature, offering new opportunities for diagnosis and therapy of cancer, but the identification of tumor-restricted targets remains a challenge. We investigated 13 tumor vascular markers (TVMs) from 50 candidates identified through expression profiling of ovarian cancer vascular cells and selected to be either transmembrane or secreted, and to be either absent or expressed at low levels in normal tissues while overexpressed in tumors, based on analysis of 1,110 normal and tumor tissues from publicly available Affymetrix microarray data. Tumor-specific expression of each TVM was confirmed at the protein level in tumor tissue and/or in serum. Among the 13 TVMs, 11 were expressed on tumor vascular endothelium; the remaining 2 TVMs were expressed by tumor leukocytes. Our results demonstrate that certain transmembrane TVMs such as ADAM12 and CDCP1 are selectively expressed in tumor vasculature and represent promising targets for vascular imaging or anti-vascular therapy of epithelial ovarian cancer, while secreted or shed molecules such as TNFRSF21/DR6 can function as serum biomarkers. We have identified novel tumor-specific vasculature markers which appear promising for cancer serum diagnostics, molecular imaging and/or therapeutic targeting applications and warrant further clinical development.

On the synergistic effects of ligand-mediated and phage-intrinsic properties during in vivo selection

Phage display has been used as a powerful tool in the discovery and characterization of ligand-receptor complexes that can be utilized for therapeutic applications as well as to elucidate disease mechanisms. While the basic properties of phage itself have been well described, the behavior of phage in an in vivo setting is not as well understood due to the complexity of the system. Here, we take a dual approach in describing the biophysical mechanisms and properties that contribute to the efficacy of in vivo phage targeting. We begin by considering the interaction between phage and target by applying a kinetic model of ligand-receptor complexation and internalization. The multivalent display of peptides on the pIII capsid of phage is also discussed as an augmenting factor in the binding affinity of phage-displayed peptides to cellular targets accessible in a microenvironment of interest. Lastly, we examine the physical properties of the total phage particle that facilitate improved delivery and targeting in vivo compared to free peptides.