In vivo optical imaging of integrin alphaV-beta3 in mice using multivalent or monovalent cRGD targeting vectors

Effect of multimerization of a linear Arg-Gly-Asp peptide on integrin binding affinity and specificity

Multivalent interactions are frequently used to enhance ligand-receptor binding affinity. In this study, mono-, di- and trimeric Ala-Val-Thr-Gly-Arg-Gly-Asp-Ser-Tyr (AVTGRGDSY) peptides, labeled with (125)I or Cy5.5, were compared in vitro and in vivo. Using human embryonic kidney HEK293 (naturally alpha(V)-positive and beta(3)-negative), HEK293(beta(1)) (beta(1)-transfected and alpha(V)beta(3)-negative), HEK293(beta(3)) (beta(3)-transfected and strongly alpha(V)beta(3)-positive), and human glioblastoma U87MG (naturally alpha(V)beta(3)-positive) cell lines we evaluated their binding affinity and specificity. In vitro, the monomeric AVTGRGDSY showed specific binding to both HEK293(beta(1)) and HEK293(beta(3)) cells. Multimerization resulted in no change toward HEK293 cells, diminished binding with HEK293(beta(1)) cells, but substantially enhanced binding with alpha(V)beta(3)-positive HEK293(beta(3)) and U87MG cells. Moreover, multimeric AVTGRGDSY peptides were found to be nearly comparable to the same molar concentration of a well-known alpha(V)beta(3)-specific cyclo(RGDfV) (c(RGDfV)) peptide in specificity and affinity for targeting alpha(V)beta(3) integrin. Non-invasive in vivo optical imaging demonstrated that as compared to its monomeric analogue, the Cy5.5-labeled dimeric AVTGRGDSY peptide produced markedly enhanced tumor-to-background contrast in HEK293(beta(3)) tumor-bearing mice than in HEK293(beta(1)) tumor-bearing mice. In conclusion, the present study showed the difference of monomeric and multimeric linear Arg-Gly-Asp (RGD)-containing compound in integrin selectivity and affinity. Our data provide useful information for the design of novel RGD peptides.

Chemical and biological evaluations of an (111)in-labeled RGD-peptide targeting integrin alpha(V) beta(3) in a preclinical tumor model

Angiogenesis plays a central role in tumor growth and metastasis. Quantification or evaluation of angiogenesis is crucial for antiangiogenic therapeutic strategies. Since integrin alpha(v)beta(3) overexpression appears specific of angiogenesis at the adult stage, it became a target of choice over the past decade, and labeled RGD-based compounds, therefore, constitute promising agents for noninvasive tumor visualization and targeting. We evaluated the chemical and biologic properties of a new tetrameric RGD-based tracer named RAFT-RGD. RAFT-RGD was radiolabeled with indium-111, using the chelating agent [(1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid] (DOTA). Labeling reaction parameters, such as time, temperature, solvent, or molar ratio, were investigated in order to optimize the final properties of the labeled RGD peptide. A 97.7% +/- 0.7% binding efficiency was achieved. (111)In-DOTA-RAFT-RGD was injected intravenously in a cohort of alpha(v)beta(3)-positive tumor-bearing nude mice. We noninvasively visualized the in vivo distribution of the tracer, using a small-animal gamma camera. In vivo distribution and stability were also studied after organ removal. In vivo, the radiolabeled peptide showed rapid blood clearance and tumor uptake. Whole-body noninvasive planar imaging allowed tumor visualization from 1 hour postinjection. However, renal uptake must be reduced to increase the therapeutic potential of RAFT-RGD.

'In vivo' optical approaches to angiogenesis imaging

In recent years, molecular imaging gained significant importance in biomedical research. Optical imaging developed into a modality which enables the visualization and quantification of all kinds of cellular processes and cancerous cell growth in small animals. Novel gene reporter mice and cell lines and the development of targeted and cleavable fluorescent "smart" probes form a powerful imaging toolbox. The development of systems collecting tomographic bioluminescence and fluorescence data enabled even more spatial accuracy and more quantitative measurements. Here we describe various bioluminescent and fluorescent gene reporter models and probes that can be used to specifically image and quantify neovascularization or the angiogenic process itself.

Optical small animal imaging in the drug discovery process

Molecular imaging of tumors in preclinical models is of the utmost importance for developing innovative cancer treatments. This field is moving extremely rapidly, with recent advances in optical imaging technologies and sophisticated molecular probes for in vivo imaging. The aim of this review is to provide a succinct overview of the imaging modalities available for rodents and with focus on describing optical probes for cancer imaging.

Intraoperative near-infrared image-guided surgery for peritoneal carcinomatosis in a preclinical experimental model

Background

This study compared the quality of surgery performed under conventional light with near-infrared (NIR) image-guided surgery using a tumour-targeting probe and a portable clinical grade imaging device in a mouse model of peritoneal carcinomatosis.

Methods

Peritoneal carcinomatosis was induced by injection of luciferase-positive tumour cells, leading to the formation of small nodules in the peritoneal cavity. One day after intravenous injection of RAFT-c(RGDfK)4-Alexa Fluor® 700, a fluorescent tumour-targeting probe, the surgeon operated using the Fluobeam®, a portable device that illuminated the mouse with NIR light and allowed NIR vision. The quality of the surgery was evaluated using bioluminescence, a highly sensitive method that detected the remaining tumour cells, and operating time was measured.

Results

Under normal light, the surgeon detected and removed a mean(s.d.) of only 50·6(2·3) per cent of the nodules that were visible under NIR light. The duration of surgery was reduced from 19·5(3·3) min under normal light to 14·0(2·6) min when NIR light was used (P = 0·025). The sensitivity of the NIR system allowed the detection of nodules containing as few as 227 tumour cells.

Conclusion

NIR image-guided surgery improved the quality of surgery for peritoneal carcinomatosis by doubling the number of nodules detected and significantly reducing the duration of surgery.

Surgery with molecular fluorescence imaging using activatable cell-penetrating peptides decreases residual cancer and improves survival

The completeness of tumor removal during surgery is dependent on the surgeon's ability to differentiate tumor from normal tissue using subjective criteria that are not easily quantifiable. A way to objectively assess tumor margins during surgery in patients would be of great value. We have developed a method to visualize tumors during surgery using activatable cell-penetrating peptides (ACPPs), in which the fluorescently labeled, polycationic cell-penetrating peptide (CPP) is coupled via a cleavable linker to a neutralizing peptide. Upon exposure to proteases characteristic of tumor tissue, the linker is cleaved, dissociating the inhibitory peptide and allowing the CPP to bind to and enter tumor cells. In mice, xenografts stably transfected with green fluorescent protein show colocalization with the Cy5-labeled ACPPs. In the same mouse models, Cy5-labeled free ACPPs and ACPPs conjugated to dendrimers (ACPPDs) delineate the margin between tumor and adjacent tissue, resulting in improved precision of tumor resection. Surgery guided by ACPPD resulted in fewer residual cancer cells left in the animal after surgery as measured by Alu PCR. A single injection of ACPPD dually labeled with Cy5 and gadolinium chelates enabled preoperative whole-body tumor detection by MRI, intraoperative guidance by real-time fluorescence, intraoperative histological analysis of margin status by fluorescence, and postoperative MRI tumor quantification. Animals whose tumors were resected with ACPPD guidance had better long-term tumor-free survival and overall survival than animals whose tumors were resected with traditional bright-field illumination only.

Tumor targeting of functionalized lipid nanoparticles: assessment by in vivo fluorescence imaging

Lipid nanoparticles (LNP) coated by a poly(oxyethylene) polymer have been manufactured from low cost and human use-approved materials, by an easy, robust, and up-scalable process. The incorporation in the formulation of maleimide-grafted surfactants allows the functionalization of the lipid cargos by targeting ligands such as the cRGD peptide binding to alpha(v)beta(3) integrin, a well-known angiogenesis biomarker. LNP are able to encapsulate efficiently lipophilic molecules such as a fluorescent dye, allowing their in vivo tracking using fluorescence imaging. In vitro study on HEK293(beta3) cells over-expressing the alpha(v)beta(3) integrins demonstrates the functionalization, specific targeting, and internalization of cRGD-functionalized LNP in comparison with LNP-cRAD or LNP-OH used as negative controls. Following their intravenous injection in Nude mice, LNP-cRGD can accumulate actively in slow-growing HEK293(beta3) cancer xenografts, leading to tumor over skin fluorescence ratio of 1.53+/-0.07 (n=3) 24h after injection. In another fast-growing tumor model (TS/A-pc), tumor over skin fluorescence ratio is improved (2.60+/-0.48, n=3), but specificity between the different LNP functionalizations is no more observed. The different results obtained for the two tumor models are discussed in terms of active cRGD targeting and/or passive nanoparticle accumulation due to the Enhanced Permeability and Retention effect.

Labels and probes for live cell imaging: overview and selection guide

Fluorescence imaging is an important tool for molecular biology research. There is a wide array of fluorescent labels and activatable probes available for investigation of biochemical processes at a molecular level in living cells. Given the large number of potential imaging agents and numerous variables that can impact the utility of these fluorescent materials for imaging, selection of the appropriate probes can be a difficult task. In this report an overview of fluorescent imaging agents and details on their optical and physical properties that can impact their function are presented.

Near-infrared fluorescence: application to in vivo molecular imaging

Molecular imaging often relies on the use of targeted and activatable reporters to quantitate and visualize targets, biological processes, and cells in vivo. The use of optical probes with near-infrared fluorescence allows for improved photon penetration through tissue and minimizes the effects of tissue autofluorescence. There are several parameters that define the effectiveness of imaging agents in vivo. These factors include probe targeting, activation, pharmacokinetics, biocompatibility, and photophysics. Recent advances in our understanding of these variables as they pertain to the application of optical reporters for in vivo imaging are discussed in this review.

Imaging of tumor angiogenesis: functional or targeted?

OBJECTIVE

Angiogenesis--the growth of new vessels--is both a normal physiologic response and a critical step in many pathologic processes, particularly cancer. Imaging has long relied on the different enhancement characteristics of cancer compared with normal tissue; the information generated is often primarily morphologic and qualitative. However, more quantitative methods based on functional and targeted imaging have recently emerged.

CONCLUSION

In this article, we review both functional and targeted imaging techniques for assessing tumor angiogenesis.

Targeting cyclin B1 through peptide-based delivery of siRNA prevents tumour growth

The development of short interfering RNA (siRNA), has provided great hope for therapeutic targeting of specific genes responsible for patholological disorders. However, the poor cellular uptake and bioavailability of siRNA remain a major obstacle to their clinical development and most strategies that propose to improve siRNA delivery remain limited for in vivo applications. In this study, we report a novel peptide-based approach, MPG-8 an improved variant of the amphipathic peptide carrier MPG, that forms nanoparticles with siRNA and promotes their efficient delivery into primary cell lines and in vivo upon intra-tumoral injection. Moreover, we show that functionalization of this carrier with cholesterol significantly improves tissue distribution and stability of siRNA in vivo, thereby enhancing the efficiency of this technology for systemic administration following intravenous injection without triggering any non-specific inflammatory response. We have validated the therapeutic potential of this strategy for cancer treatment by targeting cyclin B1 in mouse tumour models, and demonstrate that tumour growth is compromised. The robustness of the biological response achieved through this approach, infers that MPG 8-based technology holds a strong promise for therapeutic administration of siRNA.

Drug development in oncology assisted by noninvasive optical imaging

Early and accurate detection of tumors, like the development of targeted treatments, is a major field of research in oncology. The generation of specific vectors, capable of transporting a drug or a contrast agent to the primary tumor site as well as to the remote (micro-) metastasis would be an asset for early diagnosis and cancer therapy. Our goal was to develop new treatments based on the use of tumor-targeted delivery of large biomolecules (DNA, siRNA, peptides, or nanoparticles), able to induce apoptosis while dodging the specific mechanisms developed by tumor cells to resist this programmed cell death. Nonetheless, the insufficient effectiveness of the vectorization systems is still a crucial issue. In this context, we generated new targeting vectors for drug and biomolecules delivery and developed several optical imaging systems for the follow-up and evaluation of these vectorization systems in live mice. Based on our recent work, we present a brief overview of how noninvasive optical imaging in small animals can accelerate the development of targeted therapeutics in oncology.

Clustering and internalization of integrin alphavbeta3 with a tetrameric RGD-synthetic peptide

Integrin alpha(v)beta(3) is overexpressed on neoendothelial cells and frequently on tumor cells. We have developed a peptide-like scaffold (regioselectively addressable functionalized template, RAFT), which holds four cyclo(-RGDfK-) (cRGD) motifs and proved that this molecule (called regioselectively addressable functionalized template-arginine-glycine-aspartic acid, RAFT-RGD) targets integrin alpha(v)beta(3) in vitro and in vivo. Using fluorescence correlation spectroscopy (FCS), we measured the constant of affinity (K(D)) of the RAFT-RGD for purified integrins. K(D) values rose from 3.87 nmol/l for RAFT-RGD to 41.70 nmol/l for cyclo(-RGDfK-). In addition, RAFT-RGD inhibited alpha(v)beta(3) lateral mobility in the cell membrane, probably due to the formation of integrin clusters as demonstrated by fluorescence recovery after photobleaching (FRAP). This was confirmed by electronic microscopy data, which established the formation of molecular complexes containing two integrins in the presence of RAFT-RGD but not cRGD or regioselectively addressable functionalized template-arginine-alanine- aspartic acid (RAFT-RAD). Using an enzyme-linked immunosorbent assay (ELISA), we proved that 1 micromol/l RAFT-RGD increased by 79% alpha(v)beta(3) internalization via clathrin-coated vesicles. Conversely, cRGD was internalized without modifying alpha(v)beta(3) internalization. Although RGD has been known for >20 years, this is the first study to formerly establish the relationships among multimeric presentation, increased affinity, and subsequent integrin-mediated cointernalization. These results strongly support the rationale for using multimeric RGD-peptides as targeting vectors for imaging, diagnosis, or therapy of cancers.

Synthesis and biochemical evaluation of a cyclic RGD oxorhenium complex as new ligand of alphaVbeta3 integrin

We report the design of a new ligand of integrins that might be used for the molecular imaging of tumor neoangiogenesis. For this purpose, we designed a modified RGD tripeptide bearing a N-terminal N-bis(thioethyl)glycinate (NS(2)) motif and a thioethyl moiety at the C-terminus. Simultaneous coordination of an oxorhenium core by the NS(2) and thioethyl moieties led to peptide cyclization and gave the corresponding monomers 13a and b (major isomer) resulting from the syn/anti-isomerism, along with dimers' species 16a and b. Cyclometallated peptide 13b showed the most promising activity with an IC(50) of 86 nM for integrin alpha(V)beta(3) whereas it binds integrin alpha(IIb)beta(3) with an affinity lower by an order of magnitude. Labeling with [(99m)Tc]oxotechnetium gluconate led exclusively to complex 17, the equivalent of compound 13b, which displayed satisfactory stabilities in mice plasma and towards glutathione.

Cell-penetrating and cell-targeting peptides in drug delivery

During the last decade, the potential of peptides for drug delivery into cells has been highlighted by the discovery of several cell-penetrating peptides (CPPs). CPPs are very efficient in delivering various molecules into cells. However, except in some specific cases, their lack of cell specificity remains the major drawback for their clinical development. At the same time, various peptides with specific binding activity for a given cell line (cell-targeting peptides) have also been reported in the literature. One of the goals of the next years will be to optimize the tissue and cell delivery of therapeutic molecules by means of peptides which combine both targeting and internalization advantages. In this review, we describe the main strategies that are currently in use or likely to be employed in the near future to associate both targeting and delivery properties.

Recent trends in targeted anticancer prodrug and conjugate design

Anticancer drugs are often nonselective antiproliferative agents (cytotoxins) that preferentially kill dividing cells by attacking their DNA at some level. The lack of selectivity results in significant toxicity to noncancerous proliferating cells. These toxicities along with drug resistance exhibited by the solid tumors are major therapy limiting factors that result into poor prognosis for patients. Prodrug and conjugate design involves the synthesis of inactive drug derivatives that are converted to an active form inside the body and preferably at the site of action. Classical prodrug and conjugate design have focused on the development of prodrugs that can overcome physicochemical (e.g., solubility, chemical instability) or biopharmaceutical problems (e.g., bioavailability, toxicity) associated with common anticancer drugs. The recent targeted prodrug and conjugate design, on the other hand, hinge on the selective delivery of anticancer agents to tumor tissues thereby avoiding their cytotoxic effects on noncancerous cells. Targeting strategies have attempted to take advantage of low extracellular pH, elevated enzymes in tumor tissues, the hypoxic environment inside the tumor core, and tumor-specific antigens expressed on tumor cell surfaces. The present review highlights recent trends in prodrug and conjugate rationale and design for cancer treatment. The various approaches that are currently being explored are critically analyzed and a comparative account of the advantages and disadvantages associated with each approach is presented.