Targeting endothelial cells overexpressing VEGFR-2: selective toxicity of Shiga-like toxin-VEGF fusion proteins

Targeting Fluorescent Nanodiamonds to Vascular Endothelial Growth Factor Receptors in Tumor

The increased expression of vascular endothelial growth factor (VEGF) and its receptors is associated with angiogenesis in a growing tumor, presenting potential targets for tumor-selective imaging by way of targeted tracers. Though fluorescent tracers are used for targeted in vivo imaging, the lack of photostability and biocompatibility of many current fluorophores hinder their use in several applications involving long-term, continuous imaging. To address these problems, fluorescent nanodiamonds (FNDs), which exhibit infinite photostability and excellent biocompatibility, were explored as fluorophores in tracers for targeting VEGF receptors in growing tumors. To explore FND utility for imaging tumor VEGF receptors, we used click-chemistry to conjugate multiple copies of an engineered single-chain version of VEGF site-specifically derivatized with trans-cyclooctene (scVEGF-TCO) to 140 nm FND. The resulting targeting conjugates, FND-scVEGF, were then tested for functional activity of the scVEGF moieties through biochemical and tissue culture experiments and for selective tumor uptake in Balb/c mice with induced 4T1 carcinoma. We found that FND-scVEGF conjugates retain high affinity to VEGF receptors in cell culture experiments and observed preferential accumulation of FND-scVEGF in tumors relative to untargeted FND. Microspectroscopy provided unambiguous determination of FND within tissue by way of the unique spectral shape of nitrogen-vacancy induced fluorescence. These results validate and invite the use of targeted FND for diagnostic imaging and encourage further optimization of FND for fluorescence brightness.

Cell-specific targeting by engineered M13 bacteriophage expressing VEGFR2 nanobody

Objectives

Filamentous bacteriophage M13 was genetically engineered to specifically target mammalian cells for gene delivery purpose.

Materials and Methods

A vascular endothelial growth factor receptor 2 (VEGFR2)-specific nanobody was genetically fused to the capsid gene III of M13 bacteriophage (pHEN4/3VGR19). A mammalian expression construct containing Cop-green fluorescent protein (Cop-GFP), as a reporter gene, was amplified by PCR and then sub-cloned in the pHEN4/3VGR19 phagemid. The resulting construct was transfected into 293KDR cell. The recombinant phage was extracted and confirmed and then transduced into VEGFR2 expressing cell (293KDR).

Results

Seventy-two hr after transfection, green fluorescence was detected in 30% of the cells. About 1% of the cells which transduced by recombinant phages were able to express GFP.

Conclusion

It is hoped that the results from this study will help to find potential vectors to improve the efficiency of gene delivery. Taken together, we conclude that this newly-introduced vector can be used in cancer researches.

Expression of VGRNb-PE immunotoxin in transplastomic lettuce (Lactuca sativa L.)

KEY MESSAGE

This research has shown, for the first time, that plant chloroplasts are a suitable compartment for synthesizing recombinant immunotoxins and the transgenic immunotoxin efficiently causes the inhibition of VEGFR2 overexpression, cell growth and proliferation. Angiogenesis refers to the formation of new blood vessels, which resulted in the growth, invasion and metastasis of cancer. The vascular endothelial growth factor receptor 2 (VEGFR2) plays a major role in angiogenesis and blocking of its signaling inhibits neovascularization and tumor metastasis. Immunotoxins are promising therapeutics for targeted cancer therapy. They consist of an antibody linked to a protein toxin and are designed to specifically kill the tumor cells. In our previous study, VGRNb-PE immunotoxin protein containing anti-VEGFR2 nanobody fused to the truncated form of Pseudomonas exotoxin A has been established. Here, we expressed this immunotoxin in lettuce chloroplasts. Chloroplast genetic engineering offers several advantages, including high levels of transgene expression, multigene engineering in a single transformation event and maternal inheritance of the transgenes. Site specific integration of transgene into chloroplast genomes, and homoplasmy were confirmed. Immunotoxin levels reached up to 1.1% of total soluble protein or 33.7 µg per 100 mg of leaf tissue (fresh weight). We demonstrated that transgenic immunotoxin efficiently causes the inhibition of VEGFR2 overexpression, cell growth and proliferation. These results indicate that plant chloroplasts are a suitable compartment for synthesizing recombinant immunotoxins.

Development of VEGFR2-specific Nanobody Pseudomonas exotoxin A conjugated to provide efficient inhibition of tumor cell growth

Angiogenesis targeting is an attractive approach for cancer treatment. Vascular endothelial growth factor receptor 2 (VEGFR2) is such an important target that is overexpressed in tumor vasculature compared to the endothelium cells of resting blood vessels and blocking of its signaling inhibits neovascularization and tumor metastasis. Immunotoxins represent a promising group of targeted therapeutics to combat tumors. They consist of an antibody linked to a toxin and are designed to kill specifically the tumor cells. In this study, we fused a VEGFR2-specific Nanobody, the antigen-binding single-domain fragment derived from functional Heavy-chain antibody of Camelidae, to the truncated form of Pseudomonas exotoxin A and evaluated its ability to bind the VEGFR2 molecule on the cell surface. We demonstrate that this immunotoxin inhibits the proliferation of VEGFR2-expressing cells in vitro. This finding is considered to be a significant achievement in tumor therapy and it forms a basis for further studies in animal models.

Expression, purification, and characterization of a diabody against the most important angiogenesis cell receptor: Vascular endothelial growth factor receptor 2

Antibodies and their derivative fragments have long been used as tools in a variety of applications, in fundamental research work, biotechnology, diagnosis, and therapy. Camels produce single heavy-chain antibodies (VHH) in addition to usual antibodies. These minimal-sized binders are very robust and bind the antigen with high affinity in a monomeric state. Vascular endothelial growth factor recepror-2 (VEGFR2) is an important tumor-associated receptor that blockade of its signaling can lead to the inhibition of neovascularization and tumor metastasis. Here, we describe the construction, expression, and purification VEGFR2-specific Diabody. Two variable fragments of a same camel anti-VEGFR2 antibody were linked together by the upper hinge segment of antibody to make a diabody. We showed the ability of diabody to recognition of VEGFR2 on the cell surface by FACS. Diabodies can be produced in the low-cost prokaryotic expression system, so they are suitable molecules for diagnostic and therapeutic issues.

Therapeutic potential of perineural invasion, hypoxia and desmoplasia in pancreatic cancer

Pancreatic cancer is one of the most fatal human malignancies. Though a relatively rare malignancy, it remains one of the deadliest tumors, with an extremely high mortality rate. The prognosis of patients with pancreatic cancer remains poor; only patients with small tumors and complete resection have a chance of a complete cure. Pancreatic cancer responds poorly to conventional therapies, including chemotherapy and irradiation. Tumor-specific targeted therapy is a relatively recent addition to the arsenal of anti-cancer therapies. It is important to find novel targets to distinguish tumor cells from their normal counterparts in therapeutic approaches. In the past few decades, studies have revealed the molecular mechanisms of pancreatic tumorigenesis, growth, invasion and metastasis. The proteins that participate in the pathophysiological processes of pancreatic cancer might be potential targets for therapy. This review describes the main players in perineural invasion, hypoxia and desmoplasia and the molecular mechanisms of these pathophysiological processes.

Anti-angiogenesis therapy in cancer: current challenges and future perspectives

It has been nearly 9years since the FDA (Food and Drug Administration) approved the first anti-angiogenic drug (bevacizumab) for treatment of metastatic colorectal cancer. Other angiogenic inhibitors have since been approved or are in different stages of clinical trials. However, continued clinical and preclinical investigations have identified major drawbacks associated with the application of this class of agents, including inherent/acquired resistance and induction of tumor invasiveness. In addition, lack of thoroughly validated predictive biomarkers has been one of the major hurdles to stratify cancer patients and to monitor tumor progression and response to the therapy. Investigations in clinic and preclinical models have provided some molecular and cellular mechanisms for the above challenges. This review aims to provide a concise update from recent findings.

Generation and characterization of a functional Nanobody against the vascular endothelial growth factor receptor-2; angiogenesis cell receptor

Vascular endothelial growth factor receptor-2 (VEGFR2) is an important tumor-associated receptor and blockade of the VEGF receptor signaling can lead to the inhibition of neovascularization and tumor metastasis. Nanobodies are the smallest intact antigen binding fragments derived from heavy chain-only antibodies occurring in camelids. Here, we describe the identification of a VEGFR2-specific Nanobody, named 3VGR19, from dromedaries immunized with a cell line expressing high levels of VEGFR2. We demonstrate by FACS, that 3VGR19 Nanobody specifically binds VEGFR2 on the surface of 293KDR and HUVECs cells. Furthermore, the 3VGR19 Nanobody potently inhibits formation of capillary-like structures. These data show the potential of Nanobodies for the blockade of VEGFR2 signaling and provide a basis for the development of novel cancer therapeutics.

scVEGF microbubble ultrasound contrast agents: a novel probe for ultrasound molecular imaging of tumor angiogenesis

OBJECTIVE

To develop a novel microbubble (MB) ultrasound contrast agent covalently coupled to a recombinant single-chain vascular endothelial growth factor construct (scVEGF) through uniform site-specific conjugation for ultrasound imaging of tumor angiogenesis.

METHODS

Ligand conjugation to maleimide-bearing MB by thioether bonding was first validated with a fluorophore (BODIPY-cystine), and covalently bound dye was detected by fluorometry and flow cytometry. MBs were subsequently site-specifically conjugated to cysteine-containing Cys-tag in scVEGF, and bound scVEGF was quantified by enzyme-linked immunosorbent assay. Targeted adhesion of scVEGF-MB was investigated with in vitro parallel plate flow chamber assays with recombinant murine VEGFR-2 substrates and human VEGFR-2-expressing porcine endothelial cells (PAE/KDR). A wall-less ultrasound flow phantom, with flow channels coated with immobilized VEGFR-2, was used to detect adhesion of scVEGF-MB with contrast ultrasound imaging. A murine model of colon adenocarcinoma was used to assess retention of scVEGF-MB with contrast ultrasound imaging during tumor angiogenesis in vivo.

RESULTS

Proof-of-principle of ligand conjugation to maleimide-bearing MB was demonstrated with a BODIPY-cysteine fluorophore. Conjugation of BODIPY to MB saturated at 10-fold molar excess BODIPY relative to maleimide groups on MB surfaces. MB reacted with scVEGF and led to the conjugation of 1.2 × 10(5) molecules scVEGF per MB. Functional adhesion of sc-VEGF-MB was shown in parallel plate flow chamber assays. At a shear stress of 1.0 dynes/cm2, scVEGF-MB exhibited 5-fold higher adhesion to both recombinant VEGFR-2 substrates and VEGFR-2-expressing endothelial cells compared with nontargeted control MB. Additionally, scVEGF-MB targeted to immobilized VEGFR-2 in an ultrasound flow phantom showed an 8-fold increase in mean acoustic signal relative to casein-coated control channels. In an in vivo model of tumor angiogenesis, scVEGF MB showed significantly higher ultrasound contrast signal enhancement in tumors (8.46 ± 1.61 dB) compared with nontargeted control MB (1.58 ± 0.83 dB).

CONCLUSIONS

These results demonstrate the functionality of a novel scVEGF-bearing MB contrast agent, which could be useful for molecular imaging of VEGFR-2 in basic science and drug discovery research.

Specific targeting of tumor endothelial cells by a shiga-like toxin-vascular endothelial growth factor fusion protein as a novel treatment strategy for pancreatic cancer

PURPOSE

Tumor endothelial cells express vascular endothelial growth factor receptor 2 (VEGFR-2). VEGF can direct toxins to tumor vessels through VEGFR-2 for antiangiogenic therapy. This study aimed to selectively damage the VEGFR-2-overexpressing vasculature of pancreatic cancer by SLT-VEGF fusion protein comprising VEGF and the A subunit of Shiga-like toxin which inhibits protein synthesis of cells with high VEGFR-2 expression.

EXPERIMENTAL DESIGN

Expression of VEGF and VEGF receptors was evaluated in human pancreatic cancer cells (AsPC-1, HPAF-2) and in normal human endothelial cells (HUVEC) by reverse transcription-polymerase chain reaction. Cells were treated with SLT-VEGF (0.1-10 nM), and cell viability, proliferation, and endothelial tube formation were assessed. Orthotopic pancreatic cancer (AsPC-1, HPAF-2) was induced in nude mice. Animals were treated with SLT-VEGF fusion protein alone or in combination with gemcitabine. Treatment began 3 days or 6 weeks after tumor induction. Primary tumor volume and dissemination were determined after 14 weeks. Microvessel density and expression of VEGF and VEGF receptors were analyzed by immunohistochemistry.

RESULTS

SLT-VEGF did not influence proliferation of pancreatic cancer cells; HUVECs (low-level VEGFR-2) reduced their proliferation rate and tube formation but not their viability. SLT-VEGF fusion protein reduced tumor growth and dissemination, increasing 14-week survival (AsPC-1, up to 75%; HPAF-2, up to 83%). Results of gemcitabine were comparable with SLT-VEGF monotherapy. Combination partly increased the therapeutic effects in comparison to the respective monotherapies. Microvessel density was reduced in all groups. Intratumoral VEGFR-2 expression was found in endothelial but not in tumor cells.

CONCLUSIONS

SLT-VEGF is toxic for tumor vasculature rather than for normal endothelial or pancreatic cancer cells. SLT-VEGF treatment in combination with gemcitabine may provide a novel approach for pancreatic cancer.

Neuropilin-VEGF signaling pathway acts as a key modulator of vascular, lymphatic, and inflammatory cell responses of the bladder to intravesical BCG treatment

Recent findings indicate that VEGF receptors and coreceptors (neuropilins; NRP) are expressed on nonendothelial cells in human bladder urothelium, in one human bladder cancer cell line (J82), and in the mouse bladder urothelium. In addition, VEGFR1, VEGFR2, NRP1, and NRP2 expressions were upregulated in animal models of chronic bladder inflammation induced by four weekly instillations of protease-activated receptors (PAR)-activating peptides or bacillus Calmette-Guérin (BCG) into the mouse bladder. Here, we used four weekly instillations of BCG as a model for chronic bladder inflammation to further investigate whether VEGF receptors and NRPs play a role in the migration of inflammatory cells and inflammation-induced lymphangiogenesis and angiogenesis. For this purpose, we used neutralizing antibodies that were engineered to specifically block the binding of VEGF to NRP (anti-NRP1(B)) and the binding of semaphorins to NRP (anti-NRP1(A)). C57BL/6 mice received intraperitoneal injections of PBS, anti-NRP1(A)- or anti-NRP1(B)-neutralizing antibodies and then were challenged chronically with intravesical PBS or BCG. At the end of chronic challenge period, a fluorescent internalizable tracer, scVEGF/Cy5.5, was administered to all mice and near-infrared fluorescence images were obtained in vivo and in real time. BCG increased the overall accumulation of scVEGF/Cy5.5 in the urinary bladder urothelium and inflammatory cells. In addition, BCG increased the density of blood and lymphatic vessels concomitantly with an upregulation of NRP2 expression in lymphatic vessels. Treatment of the mice with NRP1-neutralizing antibodies dramatically reduced scVEGF/Cy5.5 uptake, polymorphonuclear (myeloperoxidase-positive cells) and dendritic cell (CD11c-positive cells) infiltration, and decreased the overall density of BCG-induced blood and lymphatic vessels. These results implicate NRPs as critical in vivo regulators of the vascular and inflammatory responses to the intravesical administration of BCG.

SLT-VEGF reduces lung metastases, decreases tumor recurrence, and improves survival in an orthotopic melanoma model

SLT-VEGF is a recombinant cytotoxin comprised of Shiga-like toxin (SLT) subunit A fused to human vascular endothelial growth factor (VEGF). It is highly cytotoxic to tumor endothelial cells overexpressing VEGF receptor-2 (VEGFR-2/KDR/Flk1) and inhibits the growth of primary tumors in subcutaneous models of breast and prostate cancer and inhibits metastatic dissemination in orthotopic models of pancreatic cancer. We examined the efficacy of SLT-VEGF in limiting tumor growth and metastasis in an orthotopic melanoma model, using NCR athymic nude mice inoculated with highly metastatic Line IV Cl 1 cultured human melanoma cells. Twice weekly injections of SLT-VEGF were started when tumors became palpable at one week after intradermal injection of 1 × 10⁶ cells/mouse. Despite selective depletion of VEGFR-2 overexpressing endothelial cells from the tumor vasculature, SLT-VEGF treatment did not affect tumor growth. However, after primary tumors were removed, continued SLT-VEGF treatment led to fewer tumor recurrences (p = 0.007), reduced the incidence of lung metastasis (p = 0.038), and improved survival (p = 0.002). These results suggest that SLT-VEGF is effective at the very early stages of tumor development, when selective killing of VEGFR-2 overexpressing endothelial cells can still prevent further progression. We hypothesize that SLT-VEGF could be a promising adjuvant therapy to inhibit or prevent outgrowth of metastatic foci after excision of aggressive primary melanoma lesions.

Cell-mediated Delivery and Targeted Erosion of Vascular Endothelial Growth Factor-Crosslinked Hydrogels

We have previously reported a novel polymeric delivery vehicle that is assembled via interaction between heparin and the vascular endothelial growth factor (VEGF). Here, the cell-responsiveness of this hydrogel-including the delivery of VEGF in response to VEGFR-2 overexpressing PAE/KDR cells (porcine aortic endothelial cells (PAE) equipped with the transcript for the kinase insert domain receptor (KDR)), consequent erosion of the hydrogel matrix, and cellular response-are highlighted. The release of VEGF and hydrogel erosion reached 100% only in the presence of PAE/KDR. The [PEG-LMWH/VEGF] hydrogel (PEG = poly(ethylene glycol), LMWH = low molecular weight heparin) correspondingly prompted increases in VEGFR-2 phosphorylation and proliferation of PAE/KDR cells. This study proves that growth factor-crosslinked hydrogels can liberate VEGF in response to specific receptors, causing gel erosion and desired cell responses. The promise of these approaches in therapeutic applications, including targeted delivery, is suggested.

BR55: a lipopeptide-based VEGFR2-targeted ultrasound contrast agent for molecular imaging of angiogenesis

OBJECTIVES

BR55, an ultrasound contrast agent functionalized with a heterodimer peptide targeting the vascular endothelial growth factor receptor 2 (VEGFR2), was evaluated in vitro and in vivo, demonstrating its potential for specific tumor detection.

MATERIALS AND METHODS

The targeted contrast agent was prepared by incorporation of a biospecific lipopeptide into the microbubble membrane. Experiments were performed in vitro to demonstrate the binding capacities of BR55 microbubbles on immobilized receptor proteins and on various endothelial or transfected cells expressing VEGFR2. The performance of BR55 microbubbles was compared with that of streptavidin-conjugated microbubbles targeted to the same receptor by coupling them to a biotinylated antibody. The specificity of BR55 binding to human and mouse endothelial cells was determined in competition experiments with the free lipopeptide, vascular endothelial growth factor (VEGF), or a VEGFR2-specific antibody. Molecular ultrasound imaging of VEGFR2 was performed in an orthotopic breast tumor model in rats using a nondestructive, contrast-specific imaging mode.

RESULTS

BR55 was shown to bind specifically to the immobilized recombinant VEGFR2 under flow (dynamic conditions). BR55 accumulation on the target over time was similar to that of microbubbles bearing a specific antibody. BR55 avidly bound to cells expressing VEGFR2, and the pattern of microbubble distribution was correlated with the pattern of receptor expression determined by immunocytochemistry. The binding of targeted microbubbles on cells was competed off by an excess of free lipopeptide, the natural ligand (VEGF) and by a VEGFR2-specific antibody (P < 0.001). Although selected for the human receptor, the VEGFR2-binding lipopeptide was also shown to recognize the rodent receptor. Tumor perfusion was assessed during the vascular phase of BR55, and then the malignant lesion was highlighted by specific accumulation of the targeted microbubbles on tumoral endothelium. The presence of VEGFR2 was confirmed by immunofluorescence staining of tumor cryosections.

CONCLUSIONS

VEGFR2-targeted ultrasound contrast agents such as BR55 will likely prove useful in human for the early detection of tumors as well as for the assessment of response to specific treatments.

Chaperone-targeting cytotoxin and endoplasmic reticulum stress-inducing drug synergize to kill cancer cells

Diverse physiological and therapeutic insults that increase the amount of unfolded or misfolded proteins in the endoplasmic reticulum (ER) induce the unfolded protein response, an evolutionarily conserved protective mechanism that manages ER stress. Glucose-regulated protein 78/immunoglobulin heavy-chain binding protein (GRP78/BiP) is an ER-resident protein that plays a central role in the ER stress response and is the only known substrate of the proteolytic A subunit (SubA) of a novel bacterial AB(5) toxin. Here, we report that an engineered fusion protein, epidermal growth factor (EGF)-SubA, combining EGF and SubA, is highly toxic to growing and confluent epidermal growth factor receptor-expressing cancer cells, and its cytotoxicity is mediated by a remarkably rapid cleavage of GRP78/BiP. Systemic delivery of EGF-SubA results in a significant inhibition of human breast and prostate tumor xenografts in mouse models. Furthermore, EGF-SubA dramatically increases the sensitivity of cancer cells to the ER stress-inducing drug thapsigargin, and vice versa, demonstrating the first example of mechanism-based synergism in the action of a cytotoxin and an ER-targeting drug.

Inhibition of vascular endothelial growth factor receptor signaling in angiogenic tumor vasculature

Neovascularization takes place in a large number of pathologies, including cancer. Significant effort has been invested in the development of agents that can inhibit this process, and an increasing number of such agents, known as antiangiogenic drugs, are entering clinical trials or being approved for clinical use. The key players involved in the development and maintenance of tumor neovasculature are vascular endothelial growth factor (VEGF) and its receptors (VEGFRs), and therefore VEGF/VEGFR signaling pathways have been a focus of anticancer therapies for several decades. This review focuses on two main approaches designed to selectively target VEGFRs, inhibiting VEGFR with small molecule inhibitors of receptor tyrosine kinase activity and inhibiting the binding of VEGF to VEGFRs with specific antibodies or soluble decoy VEGF receptors. The major problem with these strategies is that they appeared to be effective only in relatively small and unpredictable subsets of patients. An alternative approach would be to subvert VEGFR for intracellular delivery of cytotoxic molecules. We describe here one such molecule, SLT-VEGF, a fusion protein containing VEGF121 and the highly cytotoxic catalytic subunit of Shiga-like toxin.

Identification of oligopeptide binding to colon cancer cells separated from patients using laser capture microdissection

The development of intravascular conjugates that efficiently deliver genes or drugs to tumors is limited by the lack of efficacious targeting ligands. Small targeting peptides, such as those iterated by phage display technology, offer enormous potential for these applications. The majority of reports published to date have focused on the identification of peptides isolated for their ability to bind to human cancer cell lines in vitro, and have failed to account for the loss of polarization and de-differentiation of such cells from their in vivo state. Here, we report a novel approach for the identification of peptides capable of binding specifically to cancer cells derived from clinically resected human colon cancer. In this strategy, laser capture microdissection (LCM) is performed on a surgically resected colon cancer specimen to separate only cancer cells from the specimen. Subsequently, biopanning was performed on the LCM-selected colon cancer cells to identify peptide sequences that bound specifically to them. A peptide containing the SPT motif was selected as the most promising consensus sequence binding specifically to the LCM-selected colon cancer cells. Phage clones displaying the SPT motif demonstrated 9-fold higher binding to colon cancer cells derived from a patient than insertless phage (p < 0.05), while, recovery of the SPT phage from the colon cancer cell lines DLD-1 and HCT-15 was 7-fold higher than that of the control insertless phage (p < 0.05). The binding of SPT phage to colon cancer cells from the patient was confirmed by immunofluorescence. Additionally, a synthesized SPT-containing peptide (SPTKSNS) showed binding activity in the absence of mitogenic effects on colon cancer cells in vitro. In summary, we have introduced LCM into a biopanning procedure and identified a small peptide that binds preferentially to colon cancer cells derived from a clinically resected sample. This procedure could be applicable for the design of customized cancer cell targeting methodologies using clinical biopsy samples from human subjects.

Peptide- and protein-mediated assembly of heparinized hydrogels

Polymeric hydrogels have demonstrated significant promise in biomedical applications such as drug delivery and tissue engineering. A continued direction in hydrogel development includes the engineering of the biological responsiveness of these materials, via the inclusion of cell-binding domains and enzyme-sensitive domains. Ligand-receptor interactions offer additional opportunities in the design of responsive hydrogels, and strategies employing protein- polysaccharide interactions as a target may have unique relevance to materials intended to mimic the extracellular matrix (ECM). Accordingly, we have developed approaches for producing hydrogels via noncovalent interactions between heparin and heparin-binding peptides/proteins, and have demonstrated that such matrices are capable of both passive and receptor-mediated growth factor delivery. Further modification of these materials via the integration of these noncovalent strategies with chemical crosslinking methods will expand the range of their potential use and is under exploration. The combination of these approaches offers broad opportunities for the production of responsive matrices for biomedical applications.

Immunonanoshells for targeted photothermal ablation of tumor cells

Consisting of a silica core surrounded by a thin gold shell, nanoshells possess an optical tunability that spans the visible to the near infrared (NIR) region, a region where light penetrates tissues deeply. Conjugated with tumor-specific antibodies, NIR-absorbing immunonanoshells can preferentially bind to tumor cells. NIR light then heats the bound nanoshells, thus destroying the targeted cells. Antibodies can be consistently bound to the nanoshells via a bifunctional polyethylene glycol (PEG) linker at a density of ~150 antibodies per nanoshell. In vitro studies have confirmed the ability to selectively induce cell death with the photothermal interaction of immunonanoshells and NIR light. Prior to incubation with anti-human epidermal growth factor receptor (HER2) immunonanoshells, HER2-expressing SK-BR-3 breast carcinoma cells were seeded alone or adjacent to human dermal fibroblasts (HDFs). Anti-HER2 immunonanoshells bound to HER2-expressing cells resulted in the death of SK-BR-3 cells after NIR exposure only within the irradiated area, while HDFs remained viable after similar treatment since the immunonanoshells did not bind to these cells at high levels. Control nanoshells, conjugated with nonspecific anti-IgG or PEG, did not bind to either cell type, and cells continued to be viable after treatment with these control nanoshells and NIR irradiation.

Bacteriophage biopanning in human tumour biopsies to identify cancer-specific targeting ligands

Intravenous targeting of anticancer agents should improve both efficacy and therapeutic index. However, rational design of targeting constructs requires detailed definition of receptor targets and must take account of polarised tissue architecture that may restrict access to chosen receptors from the bloodstream. Bacteriophage biopanning provides a solution to this problem, identifying targeting sequences by functional selection rather than design, although reiterative panning in polarized human tumours has not previously been attempted. Here, we report an ex vivo, intra-arterial method for biopanning in freshly-resected human tumours, enabling reiterative selection of oligopeptide sequences capable of intravascular targeting to human colorectal tumours. Significant consensus was observed after two rounds of panning in tumours from different patients, and lead sequences demonstrated tumour targeting in samples from unrelated patients. This novel approach may be applicable to a wide range of settings, thus enabling iteration of consensus targeting sequences for tumour imaging and selective delivery of anticancer agents.

Molecular imaging of VEGF receptors in angiogenic vasculature with single-chain VEGF-based probes

We describe a new generation of protein-targeted contrast agents for multimodal imaging of the cell-surface receptors for vascular endothelial growth factor (VEGF). These receptors have a key role in angiogenesis and are important targets for drug development. Our probes are based on a single-chain recombinant VEGF expressed with a cysteine-containing tag that allows site-specific labeling with contrast agents for near-infrared fluorescence imaging, single-photon emission computed tomography or positron emission tomography. These probes retain VEGF activities in vitro and undergo selective and highly specific focal uptake into the vasculature of tumors and surrounding host tissue in vivo. The fluorescence contrast agent shows long-term persistence and co-localizes with endothelial cell markers, indicating that internalization is mediated by the receptors. We expect that multimodal imaging of VEGF receptors with these probes will be useful for clinical diagnosis and therapeutic monitoring, and will help to accelerate the development of new angiogenesis-directed drugs and treatments.

Inhibition of anthrax protective antigen outside and inside the cell

In the course of Bacillus anthracis infection, B. anthracis lethal factor (LF) and edema factor bind to a protective antigen (PA) associated with cellular receptors ANTXR1 (TEM8) or ANTXR2 (CMG2), followed by internalization of the complex via receptor-mediated endocytosis. A new group of potential antianthrax drugs, beta-cyclodextrins, has recently been described. A member of this group, per-6-(3-aminopropylthio)-beta-cyclodextrin (AmPrbetaCD), was shown to inhibit the toxicity of LF in vitro and in vivo. In order to determine which steps in lethal factor trafficking are inhibited by AmPrbetaCD, we developed two targeted fluorescent tracers based on LFn, a catalytically inactive fragment of LF: (i) LFn site specifically labeled with the fluorescent dye AlexaFluor-594 (LFn-Al), and (ii) LFn-decorated liposomes loaded with the fluorescent dye 8-hydroxypyrene-1,3,6-trisulfonic acid (LFn-Lip). Both tracers retained high affinity to PA/ANTXR complexes and were readily internalized via receptor-mediated endocytosis. Using fluorescent microscopy, we found that AmPrbetaCD inhibits receptor-mediated cell uptake but not the binding of LFn-Al to PA/ANTXR complexes, suggesting that AmPrbetaCD works outside the cell. Moreover, AmPrbetaCD and LFn-Al synergistically protect RAW 264.7 cells from PA-mediated LF toxicity, confirming that AmPrbetaCD did not affect the binding of LFn-Al to receptor-associated PA. In contrast, AmPrbetaCD did not inhibit PA-mediated internalization of LFn-Lip, suggesting that multiplexing of LFn on the liposomal surface overcomes the inhibiting effects of AmPrbetaCD. Notably, internalized LFn-Al and LFn-Lip protected cells that overexpressed anthrax receptor TEM8 from PA-induced, LF-independent toxicity, suggesting an independent mechanism for PA inhibition inside the cell. These data suggest the potential for the use of beta-cyclodextrins in combination with LFn-Lip loaded with antianthrax drugs against intracellular targets.

Self-assembled "dock and lock" system for linking payloads to targeting proteins

Random conjugation of therapeutic or diagnostic payloads to targeting proteins generates functionally heterogeneous products. Conjugation of payloads to an adapter that binds to a peptide tag engineered into a targeting protein provides an alternative strategy. To progress into clinical development, an adapter/docking tag system should include humanized components and be stable in circulation. We describe here an adapter/docking tag system based on mutated fragments of human RNase I that spontaneously bind to each other and form a conjugate with a disulfide bond between complimentary cysteine residues. This self-assembled "dock and lock" system utilizes the previously described fusion C-tag, a 1-15 aa fragment of human RNase I with the R4C amino acid substitution, and a newly engineered adapter protein (Ad-C), a 21-127-aa fragment of human RNase I with the V118C substitution. Two vastly different C-tagged recombinant proteins, human vascular endothelial growth factor (VEGF) and a 254-aa long N-terminal fragment of anthrax lethal factor (LFn), retain functional activities after spontaneous conjugation of Ad-C to N-terminal or C-terminal C-tag, respectively. Ad-C modified with pegylated phospolipid and inserted into the lipid membrane of drug-loaded liposomes (Doxil) retained the ability to conjugate C-tagged proteins, yielding targeted liposomes decorated with functionally active proteins. To further optimize the system, we engineered an adapter with an additional cysteine residue at position 88 for site-specific modification, conjugated it to C-tagged VEGF, and labeled with a near-infrared fluorescent dye Cy5.5, yielding a unique functionally active probe for in vivo molecular imaging. We expect that this self-assembled "dock and lock" system will provide new opportunities for using functionally active proteins for biomedical purposes.

Nanotechnology in cancer therapeutics: bioconjugated nanoparticles for drug delivery

Nanotechnology refers to the interactions of cellular and molecular components and engineered materials-typically, clusters of atoms, molecules, and molecular fragments into incredibly small particles-between 1 and 100 nm. Nanometer-sized particles have novel optical, electronic, and structural properties that are not available either in individual molecules or bulk solids. The concept of nanoscale devices has led to the development of biodegradable self-assembled nanoparticles, which are being engineered for the targeted delivery of anticancer drugs and imaging contrast agents. Nanoconstructs such as these should serve as customizable, targeted drug delivery vehicles capable of ferrying large doses of chemotherapeutic agents or therapeutic genes into malignant cells while sparing healthy cells. Such "smart" multifunctional nanodevices hold out the possibility of radically changing the practice of oncology, allowing easy detection and then followed by effective targeted therapeutics at the earliest stages of the disease. In this article, we briefly discuss the use of bioconjugated nanoparticles for the delivery and targeting of anticancer drugs.

Identification of oligopeptides binding to peritoneal tumors of gastric cancer

This is a report of in vivo intraperitoneal biopanning, and we successfully identified a novel peptide to target the multiple peritoneal tumors of gastric cancer. A phage display library was injected directly into the abdominal cavity of mice bearing peritoneal tumors of human gastric cancer, and phages associated with the tumors were subsequently reclaimed from isolated samples. The tumor-associated phages were amplified and the biopanning cycle was repeated five times to enrich for high affinity tumor-selective binding peptides. Finally, a tri-peptide motif, KLP, which showed homology with laminin 5 (a ligand for alpha3beta1 integrin), was identified as a binding peptide for peritoneal tumors of gastric cancer. Phage clones displaying the sequence KLP showed 64-fold higher binding to peritoneal tumors than control phage and were preferentially distributed in tumors rather than in normal organs after intraperitoneal injection into mice. In addition, the KLP phages were more likely to bind to cancer cells in malignant ascites derived from a patient with recurrent gastric cancer. Synthesized peptide containing the motif KLP (SWKLPPS) also showed a strong binding activity to peritoneal tumors without cancer growth effect. Liposomes conjugated with SWKLPPS peptide appeared significantly more often in tumors than control liposomes after intraperitoneal injection into mice. Furthermore, modification of liposomes with SWKLPPS peptide enhanced the antitumor activity of adriamycin on gastric cancer cells. The peptide motif KLP seems a potential targeting ligand for the treatment of peritoneal metastasis of gastric cancer.

In vivo tumor angiogenesis imaging with site-specific labeled 99mTc-HYNIC-VEGF

PURPOSE

We recently developed a cysteine-containing peptide tag (C-tag) that allows for site-specific modification of C-tag-containing fusion proteins with a bifunctional chelator, HYNIC (hydrazine nicotinamide)-maleimide. We then constructed and expressed C-tagged vascular endothelial growth factor (VEGF) and labeled it with HYNIC. We wished to test (99m)Tc-HYNIC-C-tagged VEGF ((99m)Tc-HYNIC-VEGF) for the imaging of tumor vasculature before and after antiangiogenic (low continuous dosing, metronomic) and tumoricidal (high-dose) cyclophosphamide treatment.

METHODS

HYNIC-maleimide was reacted with the two thiol groups of C-tagged VEGF without any effect on biologic activity in vitro. (99m)Tc-HYNIC-VEGF was prepared using tin/tricine as an exchange reagent, and injected via the tail vein (200-300 microCi, 1-2 microg protein) followed by microSPECT imaging 1 h later.

RESULTS

Sequencing analysis of HYNIC-containing peptides obtained after digestion confirmed the site-specific labeling of the two accessible thiol groups of C-tagged VEGF. Tumor vascularity was easily visualized with (99m)Tc/VEGF in Balb/c mice with 4T1 murine mammary carcinoma 10 days after implantation into the left axillary fat pad in controls (12.3+/-5.0 tumor/bkg, n=27) along with its decrease following treatment with high (150 mg/kg q.o.d. x 4; 1.14+/-0.48 tumor/bkg, n=9) or low (25 mg/kg q.d. x 7; 1.03+/-0.18 tumor/bkg, n=9) dose cyclophosphamide. Binding specificity was confirmed by observing a 75% decrease in tumor uptake of (99m)Tc/biotin-inactivated VEGF, as compared with (99m)Tc-HYNIC-VEGF.

CONCLUSION

(99m)Tc can be loaded onto C-tagged VEGF in a site-specific fashion without reducing its bioactivity. (99m)Tc-HYNIC-VEGF can be rapidly prepared for the imaging of tumor vasculature and its response to different types of chemotherapy.

Vascular endothelial growth factor selectively targets boronated dendrimers to tumor vasculature

Tumor neovasculature is a potential but, until very recently, unexplored target for boron neutron capture therapy (BNCT) of cancer. In the present report, we describe the construction of a vascular endothelial growth factor (VEGF)-containing bioconjugate that potentially could be used to target up-regulated VEGF receptors (VEGFR), which are overexpressed on tumor neovasculature. A fifth-generation polyamidoamine dendrimer containing 128 reactive amino groups was reacted with 105 to 110 decaborate molecules to produce a macromolecule with 1,050 to 1,100 boron atoms per dendrimer. This was conjugated to thiol groups of VEGF at a 4:1 molar ratio using the heterobifunctional reagent sulfo-LC-SPDP. In addition, the boronated dendrimer was tagged with a near-IR Cy5 dye to allow for near-IR fluorescent imaging of the bioconjugate in vitro and in vivo. As would be predicted, the resulting VEGF-BD/Cy5 bioconjugate was not cytotoxic to HEK293 cells engineered to express 2.5 x 10(6) VEGFR-2 per cell. Furthermore, it showed binding and activation of VEGFR-2 comparable with that of native VEGF. Internalization of VEGF-BD/Cy5 by PAE cells expressing 2.5 x 10(5) VEGFR-2 per cell was inhibited by excess VEGF, indicating a VEGFR-2-mediated mechanism of uptake. Near-IR fluorescent imaging of 4T1 mouse breast carcinoma revealed selective accumulation of VEGF-BD/Cy5, but not BD/Cy5, particularly at the tumor periphery where angiogenesis was most active. Accumulation of VEGF-BD/Cy5 in 4T1 breast carcinoma was diminished in mice pretreated with a toxin-VEGF fusion protein that selectively killed VEGFR-2-overexpressing endothelial cells. Our data lay the groundwork for future studies using the VEGF-BD/Cy5 bioconjugate as a targeting agent for BNCT of tumor neovasculature.

Boron neutron capture therapy of cancer: current status and future prospects

BACKGROUND

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when boron-10 is irradiated with low-energy thermal neutrons to yield high linear energy transfer alpha particles and recoiling lithium-7 nuclei. Clinical interest in BNCT has focused primarily on the treatment of high-grade gliomas and either cutaneous primaries or cerebral metastases of melanoma, most recently, head and neck and liver cancer. Neutron sources for BNCT currently are limited to nuclear reactors and these are available in the United States, Japan, several European countries, and Argentina. Accelerators also can be used to produce epithermal neutrons and these are being developed in several countries, but none are currently being used for BNCT.

BORON DELIVERY AGENTS

Two boron drugs have been used clinically, sodium borocaptate (Na(2)B(12)H(11)SH) and a dihydroxyboryl derivative of phenylalanine called boronophenylalanine. The major challenge in the development of boron delivery agents has been the requirement for selective tumor targeting to achieve boron concentrations ( approximately 20 microg/g tumor) sufficient to deliver therapeutic doses of radiation to the tumor with minimal normal tissue toxicity. Over the past 20 years, other classes of boron-containing compounds have been designed and synthesized that include boron-containing amino acids, biochemical precursors of nucleic acids, DNA-binding molecules, and porphyrin derivatives. High molecular weight delivery agents include monoclonal antibodies and their fragments, which can recognize a tumor-associated epitope, such as epidermal growth factor, and liposomes. However, it is unlikely that any single agent will target all or even most of the tumor cells, and most likely, combinations of agents will be required and their delivery will have to be optimized.

CLINICAL TRIALS

Current or recently completed clinical trials have been carried out in Japan, Europe, and the United States. The vast majority of patients have had high-grade gliomas. Treatment has consisted first of "debulking" surgery to remove as much of the tumor as possible, followed by BNCT at varying times after surgery. Sodium borocaptate and boronophenylalanine administered i.v. have been used as the boron delivery agents. The best survival data from these studies are at least comparable with those obtained by current standard therapy for glioblastoma multiforme, and the safety of the procedure has been established.

CONCLUSIONS

Critical issues that must be addressed include the need for more selective and effective boron delivery agents, the development of methods to provide semiquantitative estimates of tumor boron content before treatment, improvements in clinical implementation of BNCT, and a need for randomized clinical trials with an unequivocal demonstration of therapeutic efficacy. If these issues are adequately addressed, then BNCT could move forward as a treatment modality.

Gene Transfer–Mediated Pre-mRNA Segmental Trans-splicing As a Strategy to Deliver Intracellular Toxins for Cancer Therapy

Virus-mediated transfer of genes coding for intracellular toxins holds promise for cancer therapy, but the inherent toxicity of such vectors make them a risk to normal tissues and a challenge to produce due to the intrinsic dilemma that expression of toxin molecules kills producer cells. We employed pre-mRNA segmental trans-splicing (STS), in which two engineered DNA fragments coding for 5′ “donor” and 3′ “acceptor” segments of a toxin gene, respectively, are expressed by viral vectors. When co-delivered to target cells, the two vectors generate two toxin pre-mRNA fragments which are spliced by the target cell machinery to produce functional mRNA and toxin. To test this approach, we used an enzymatic fragment of Shigatoxin1A1 (STX1A1) known to provoke apoptotic cell death. Two adenovirus vectors, Shigatoxin1A1 donor (AdStx1A1Do) and Shigatoxin1A1 acceptor (AdStx1A1Ac), respectively, were used to deliver the Stx1A1 gene fragments. HeLa, HEp2, and A549 cells transfected with AdStx1A1Do and AdStx1A1Ac had a dose-dependent reduction in viability and inhibition of protein synthesis. Intratumoral injection of AdStx1A1Do and AdStx1A1Ac into preexisting HeLa, Hep2, and A549 tumors in immunodeficient mice revealed significant inhibition of tumor growth. There was no evidence of liver damage, suggesting that there was no leakage of vector or toxin from the site of injection following intratumoral injection of AdStx1A1Do and AdStx1A1Ac. These results suggest that the obstacles preventing gene transfer of intracellular toxins for local cancer therapy could be overcome by pre-mRNA segmental trans-splicing.

Assembly of targeting complexes driven by a single-chain antibody

Rapid development in design and production of recombinant antibodies and antibody fragments specific for cell surface markers opens new opportunities for targeted delivery of therapeutic or imaging agents. However, the progress in this field is slowed by inactivation of many antibodies by chemical conjugation of payloads and by lack of internalization of complexes formed on the cell surface. Here, we describe conversion of a non-internalizing single chain Fv (scFv) antibody P4G7 specific for vascular endothelial growth factor receptor 2 (VEGFR-2) into a targeting protein (Hu-P4G7) for assembly of a novel type of targeting complexes. Hu-P4G7 contains an N-terminal "docking" Hu-tag, a 15-aa fragment of human RNase I, capable of high affinity binding of S-protein fragment of human RNase I or bovine RNase A. Purified Hu-P4G7 and complexes of Hu-P4G7 with S-protein bind both soluble and full-length cellular VEGFR-2. To assemble targeted DNA delivery complexes, S-protein modified with a DNA condensing agent was "docked" to Hu-P4G7, and then loaded with luciferase plasmid DNA. As expected for a non-internalizing targeting protein, Hu-P4G7-based complexes did not deliver DNA in VEGFR-2 expressing cells. However, in the presence of vascular endothelial growth factor (VEGF), these complexes selectively delivered DNA into the cells overexpressing VEGFR-2 suggesting that even a non-internalizing scFv antibody can be used for targeted intracellular drug delivery.

Humanized docking system for assembly of targeting drug delivery complexes

Targeted drug delivery requires 'loading' drugs onto targeting proteins. Traditional technologies for loading drugs rely on chemical conjugation of drugs or drug carriers to targeting proteins. An alternative approach might rely on assembly of targeting complexes using a docking system that includes two components: a 'docking' tag fused to a targeting protein, and a 'payload' module containing an adapter protein for non-covalent binding to the docking tag. We describe here a fully humanized adapter/docking tag system based on non-covalent interaction between two fragments of human pancreatic RNase I. A 15 amino acid long N-terminal fragment of RNase I designed to serve as a docking tag, was fused to the N-terminus of human vascular endothelial growth factor that served as a targeting protein. An 18-125 and an 18-127 amino acid long fragments of RNase I were engineered, expressed and refolded into active conformations to serve as adapter proteins. Interactions between the targeting and adapter proteins were characterized using enzymatic analysis and surface plasmon resonance. Targeting DNA delivery complexes were assembled, characterized by dynamic light scattering, and found to be very effective in receptor-mediated DNA delivery.

Engineering S-protein fragments of bovine ribonuclease A for targeted drug delivery

High affinity interaction between S-protein and S-peptide fragments of bovine pancreatic RNase A has been recently used for construction of molecular vehicles for targeted drug delivery. The vehicle is assembled as a complex of drug carrier conjugated S-protein with S-peptide-tagged targeting protein. To avoid random chemical crosslinking of drug carriers to S-protein, we constructed a mutant 16-124aa fragment of RNase A in which 122ala is replaced with a cysteine residue. The mutant and the corresponding wild type fragments expressed in Escherichia coli are refolded into functional conformations only in the presence of S-peptide. After the removal of S-peptide, both fragments retain the ability to bind S-peptide and S-peptide-tagged proteins. The 122cys residue in the mutant fragment is available for site-specific conjugation.

Molecular vehicles for targeted drug delivery

Targeted drug delivery by cell-specific cytokines and antibodies promises greater drug efficacy and reduced side effects. We describe a novel strategy for assembly of drug delivery vehicles that does not require chemical modification of targeting proteins. The strategy relies on a noncovalent binding of standardized "payload" modules to targeting proteins expressed with a "docking" tag. The payload modules are constructed by linking drug carriers to an adapter protein capable of binding to a docking tag. Using fragments of bovine ribonuclease A as an adapter protein and a docking tag, we have constructed vascular endothelial growth factor (VEGF) based vehicles for gene delivery and for liposome delivery. Assembled vehicles displayed remarkable selectivity in drug delivery to cells overexpressing VEGF receptors. We expect that our strategy can be employed for targeted delivery of many therapeutic or imaging agents by different recombinant targeting proteins.