Dimeric RGD Research Articles

Improving tumor uptake and excretion kinetics of 99mTc-labeled cyclic arginine-glycine-aspartic (RGD) dimers with triglycine linkers.

This report describes the synthesis of two new cyclic RGD (Arg-Gly-Asp) dimers, 3 (E[G(3)-c(RGDfK)](2)) and 4 (G(3)-E[G(3)-c(RGDfK)](2)), and their corresponding conjugates 5 (HYNIC-E[G(3)-c(RGDfK)](2): HYNIC = 6-(2-(2-sulfonatobenzaldehyde)hydrazono)nicotinyl) and 6 (HYNIC-G(3)-E[G(3)-c(RGDfK)](2)). Integrin alpha(v)beta(3) binding affinities of 5 and 6 were determined by displacement of (125)I-echistatin bound to U87MG glioma cells. (99)(m)Tc complexes 7 ([(99m)Tc(5)(tricine)(TPPTS)]: TPPTS = trisodium triphenylphosphine-3,3',3''-trisulfonate) and 8 ([(99m)Tc(6)(tricine)(TPPTS)]) were prepared in high yield and high specific activity. Biodistribution and imaging studies were performed in athymic nude mice bearing U87MG glioma and MDA-MB-435 breast cancer xenografts. It was found that G(3) linkers are particularly useful for increasing integrin alpha(v)beta(3) binding affinity of cyclic RGD dimers and improving the tumor uptake and clearance kinetic of their (99)(m)Tc radiotracers. Complex 8 is a very promising radiotracer for the early detection of integrin alpha(v)beta(3)-positive tumors and may have the potential for noninvasive monitoring of tumor growth or shrinkage during antiangiogenic treatment.

αvβ3 Integrin‐targeting radionuclide therapy and imaging with monomeric RGD peptide

AbstractThe αvβ3 integrin plays a pivotal role in angiogenesis and tumor metastasis. Angiogenic blood vessels overexpress αvβ3 integrin, as in tumor neovascularization, and αvβ3 integrin expression in other microvascular beds and organs is limited. Therefore, αvβ3 integrin is a suitable receptor for tumor‐targeting imaging and therapy. Recently, tetrameric and dimeric RGD peptides have been developed to enhance specificity to αvβ3 integrin. In comparison to the corresponding monomeric peptide, however, these peptides show high levels of accumulation in kidney and liver. The purpose of this study is to evaluate tumor‐targeting properties and the therapeutic potential of 111In‐ and 90Y‐labeled monomeric RGD peptides in BALB/c nude mice with SKOV‐3 human ovarian carcinoma tumors. DOTA‐c(RGDfK) was labeled with 111In or 90Y and purified by HPLC. A biodistribution study and scintigraphic images revealed the specific uptake to αvβ3 integrin and the rapid clearance from normal tissues. These peptides were renally excreted. At 10 min after injection of tracers, 111In‐DOTA‐c(RGDfK) and 90Y‐DOTA‐c(RGDfK) showed high uptake in tumors (7.3 ± 0.6% ID/g and 4.6 ± 0.8% ID/g, respectively) and gradually decreased over time (2.3 ± 0.4% ID/g and 1.5 ± 0.5% ID/g at 24 hr, respectively). High tumor‐to‐blood and ‐muscle ratios were obtained from these peptides. In radionuclide therapeutic study, multiple‐dose administration of 90Y‐DOTA‐c(RGDfK) (3 × 11.1 MBq) suppressed tumor growth in comparison to the control group and a single‐dose administration (11.1 MBq). Monomeric RGD peptides, 111In‐DOTA‐c(RGDfK) and 90Y‐DOTA‐c(RGDfK), could be promising tracers for αvβ3 integrin‐targeting imaging and radiotherapy. © 2008 Wiley‐Liss, Inc.

Evaluation of biodistribution and anti-tumor effect of a dimeric RGD peptide–paclitaxel conjugate in mice with breast cancer

Targeting drugs to receptors involved in tumor angiogenesis has been demonstrated as a novel and promising approach to improve cancer treatment. In this study, we evaluated the anti-tumor efficacy of a dimeric RGD peptide-paclitaxel conjugate (RGD2-PTX) in an orthotopic MDA-MB-435 breast cancer model. To assess the effect of conjugation and the presence of drug moiety on the MDA-MB-435 tumor and normal tissue uptake, the biodistribution of (3)H-RGD2-PTX was compared with that of (3)H-PTX. The treatment effect of RGD2-PTX and RGD2+PTX was measured by tumor size, (18)F-FDG/PET, (18)F-FLT/PET, and postmortem histopathology. By comparing the biodistribution of (3)H-RGD2-PTX and (3)H-PTX, we found that (3)H-RGD2-PTX had higher initial tumor exposure dose and prolonged tumor retention than (3)H-PTX. Metronomic low-dose treatment of breast cancer indicated that RGD2-PTX is significantly more effective than PTX+RGD2 combination and solvent control. Although in vivo (18)F-FLT/PET imaging and ex vivo Ki67 staining indicated little effect of the PTX-based drug on cell proliferation, (18)F-FDG/PET imaging showed significantly reduced tumor metabolism in the RGD2-PTX-treated mice versus those treated with RGD2+PTX and solvent control. Terminal uridine deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining also showed that RGD2-PTX treatment also had significantly higher cell apoptosis ratio than the other two groups. Moreover, the microvessel density was significantly reduced after RGD2-PTX treatment as determined by CD31 staining. Our results demonstrate that integrin-targeted delivery of paclitaxel allows preferential cytotoxicity to integrin-expressing tumor cells and tumor vasculature. The targeted delivery strategies developed in this study may also be applied to other chemotherapeutics for selective tumor killing.

68Ga-labeled multimeric RGD peptides for microPET imaging of integrin αvβ3 expression

We and others have reported that (18)F- and (64)Cu-labeled arginine-glycine-aspartate (RGD) peptides allow positron emission tomography (PET) quantification of integrin alpha(v)beta(3) expression in vivo. However, clinical translation of these radiotracers is partially hindered by the necessity of cyclotron facility to produce the PET isotopes. Generator-based PET isotope (68)Ga, with a half-life of 68 min and 89% positron emission, deserves special attention because of its independence of an onsite cyclotron. The goal of this study was to investigate the feasibility of (68)Ga-labeled RGD peptides for tumor imaging. Three cyclic RGD peptides, c(RGDyK) (RGD1), E[c(RGDyK)](2) (RGD2), and E{E[c(RGDyK)](2)}(2) (RGD4), were conjugated with macrocyclic chelator 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and labeled with (68)Ga. Integrin affinity and specificity of the peptide conjugates were assessed by cell-based receptor binding assay, and the tumor targeting efficacy of (68)Ga-labeled RGD peptides was evaluated in a subcutaneous U87MG glioblastoma xenograft model. U87MG cell-based receptor binding assay using (125)I-echistatin as radioligand showed that integrin affinity followed the order of NOTA-RGD4 > NOTA-RGD2 > NOTA-RGD1. All three NOTA conjugates allowed nearly quantitative (68)Ga-labeling within 10 min (12-17 MBq/nmol). Quantitative microPET imaging studies showed that (68)Ga-NOTA-RGD4 had the highest tumor uptake but also prominent activity accumulation in the kidneys. (68)Ga-NOTA-RGD2 had higher tumor uptake (e.g., 2.8 +/- 0.1%ID/g at 1 h postinjection) and similar pharmacokinetics (4.4 +/- 0.4 tumor/muscle ratio, 2.0 +/- 0.1 tumor/liver ratio, and 1.1 +/- 0.1 tumor/kidney ratio) compared with (68)Ga-NOTA-RGD1. The dimeric RGD peptide tracer (68)Ga-NOTA-RGD2 with good tumor uptake and favorable pharmacokinetics warrants further investigation for potential clinical translation to image integrin alpha(v)beta(3).

MicroPET of Tumor Integrin v 3 Expression Using 18F-Labeled PEGylated Tetrameric RGD Peptide (18F-FPRGD4)

In vivo imaging of alpha(v)beta(3) expression has important diagnostic and therapeutic applications. Multimeric cyclic RGD peptides are capable of improving the integrin alpha(v)beta(3)-binding affinity due to the polyvalency effect. Here we report an example of (18)F-labeled tetrameric RGD peptide for PET of alpha(v)beta(3) expression in both xenograft and spontaneous tumor models. The tetrameric RGD peptide E{E[c(RGDyK)](2)}(2) was derived with amino-3,6,9-trioxaundecanoic acid (mini-PEG; PEG is poly(ethylene glycol)) linker through the glutamate alpha-amino group. NH(2)-mini-PEG-E{E[c(RGDyK)](2)}(2) (PRGD4) was labeled with (18)F via the N-succinimidyl-4-(18)F-fluorobenzoate ((18)F-SFB) prosthetic group. The receptor-binding characteristics of the tetrameric RGD peptide tracer (18)F-FPRGD4 were evaluated in vitro by a cell-binding assay and in vivo by quantitative microPET imaging studies. The decay-corrected radiochemical yield for (18)F-FPRGD4 was about 15%, with a total reaction time of 180 min starting from (18)F-F(-). The PEGylation had minimal effect on integrin-binding affinity of the RGD peptide. (18)F-FPRGD4 has significantly higher tumor uptake compared with monomeric and dimeric RGD peptide tracer analogs. The receptor specificity of (18)F-FPRGD4 in vivo was confirmed by effective blocking of the uptake in both tumors and normal organs or tissues with excess c(RGDyK). The tetrameric RGD peptide tracer (18)F-FPRGD4 possessing high integrin-binding affinity and favorable biokinetics is a promising tracer for PET of integrin alpha(v)beta(3) expression in cancer and other angiogenesis related diseases.

Improved targeting of the αvβ3 integrin by multimerisation of RGD peptides

The integrin alpha(v)beta(3) is expressed on sprouting endothelial cells and on various tumour cell types. Due to the restricted expression of alpha(v)beta(3) in tumours, alpha(v)beta(3) is considered a suitable receptor for tumour targeting. In this study the alpha(v)beta(3) binding characteristics of an (111)In-labelled monomeric, dimeric and tetrameric RGD analogue were compared. A monomeric (E-c(RGDfK)), dimeric (E-[c(RGDfK)](2)), and tetrameric (E{E[c(RGDfK)](2)}(2)) RGD peptide were synthesised, conjugated with DOTA and radiolabelled with (111)In. In vitro alpha(v)beta(3) binding characteristics were determined in a competitive binding assay. In vivo alpha(v)beta(3) targeting characteristics of the compounds were assessed in mice with SK-RC-52 xenografts. The IC(50) values for DOTA-E-c(RGDfK), DOTA-E-[c(RGDfK)](2), and DOTA-E{E[c(RGDfK)](2)}(2)were 120 nM, 69.9 nM and 19.6 nM, respectively. At all time points, the tumour uptake of the dimer was significantly higher as compared to that of the monomer. At 8 h p.i., tumour uptake of the tetramer (7.40+/-1.12%ID/g) was significantly higher than that of the monomer (2.30+/-0.34%ID/g), p<0.001, and the dimer (5.17+/-1.22%ID/g), p<0.05. At 24 h p.i., the tumour uptake was significantly higher for the tetramer (6.82+/-1.41%ID/g) than for the dimer (4.22+/-0.96%ID/g), p<0.01, and the monomer (1.90+/-0.29%ID/g), p<0.001. Multimerisation of c(RGDfK) resulted in enhanced affinity for alpha(v)beta(3) as determined in vitro. Tumour uptake of a tetrameric RGD peptide was significantly higher than that of the monomeric and dimeric analogues, presumably owing to the enhanced statistical likelihood for rebinding to alpha(v)beta(3).

Binding of Small Mono- and Oligomeric Integrin Ligands to Membrane-Embedded Integrins Monitored by Surface Plasmon-Enhanced Fluorescence Spectroscopy

We recently developed a binding assay format by incorporating native transmembrane receptors into artificial phospholipid bilayers on biosensor devices for surface plasmon resonance spectroscopy. By extending the method to surface plasmon-enhanced fluorescence spectroscopy (SPFS), sensitive recording of the association of even very small ligands is enabled. Herewith, we monitored binding of synthetic mono- and oligomeric RGD-based peptides and peptidomimetics to integrins alphavbeta3 and alphavbeta5, after having confirmed correct orientation and functionality of membrane-embedded integrins. We evaluated integrin binding of RGD multimers linked together via aminohexanoic acid (Ahx) spacers and showed that the dimer revealed higher binding activity than the tetramer, followed by the RGD monomers. The peptidomimetic was also found to be highly active with a slightly higher selectivity toward alphavbeta3. The different compounds were also evaluated in in vitro cell adhesion tests for their capacity to interfere with alphavbeta3-mediated cell attachment to vitronectin. We hereby demonstrated that the different RGD monomers were similarly effective; the RGD dimer and tetramer showed comparable IC50 values, which were, however, significantly higher than those of the monomers. Best cell detachment from vitronectin was achieved by the peptidomimetic. The novel SPFS-binding assay platform proves to be a suitable, reliable, and sensitive method to monitor the binding capacity of small ligands to native transmembrane receptors, here demonstrated for integrins.

Design, Synthesis, and Evaluation of Near Infrared Fluorescent Multimeric RGD Peptides for Targeting Tumors

Molecular interactions between RGD peptides and integrins are known to mediate many biological and pathological processes. This has led to an increased interest in the development of RGD compounds with high affinity and improved selectivity for integrin receptors. In this study, we synthesized and evaluated a series of multimeric RGD compounds constructed on a dicarboxylic acid-containing near-infrared (NIR) fluorescent dye (cypate) for tumor targeting. An array of NIR fluorescent RGD compounds was prepared efficiently, including one RGD monomer (cypate-(RGD)(2)-NH(2)), two RGD dimers (cypate-(RGD)(2)-NH(2) and cypate-(RGD-NH(2))(2)), one trimer (cypate-(RGD)(3)-NH(2)), two tetramers (cypate-(RGD)(4)-NH(2) and cypate-[(RGD)(2)-NH(2)](2)), one hexamer (cypate-[(RGD)(3)-NH(2)](2)), and one octamer (cypate-[(RGD)(4)-NH(2)](2)). The binding affinity of the multimeric RGD compounds for alpha(v)beta(3) integrin receptor (ABIR) showed a remarkable increase relative to the monomer cypate-RGD-NH(2). Generally, the divalent linear arrays of the multimeric RGD units bound the ABIR with slightly higher affinity than their monovalent analogues. These results suggest that the receptor binding affinity was not only dependent on the number of RGD moieties but also on the spatial alignments of the pendant peptides. Internalization of the compounds by ABIR-positive tumor cells (A549) was monitored by NIR fluorescence microscopy. The data showed that endocytosis of the octameric RGD derivative was significantly higher by comparison to other compounds in this study. In vivo noninvasive optical imaging and biodistribution data showed that the compounds were retained in A549 tumor tissue. These results clearly demonstrated that an array of simple RGD tripeptides on a NIR fluorescent dye core can be recognized by ABIR. Optimization of the spatial alignment of the RGD moieties through careful molecular design and library construction could induce multivalent ligand-receptor interactions useful for in vivo tumor imaging and tumor-targeted therapy.

Near-Infrared Fluorescent RGD Peptides for Optical Imaging of Integrin αvβ3 Expression in Living Mice

Near-infrared fluorescence optical imaging is a powerful technique for studying diseases at the molecular level in preclinical models. We recently reported that monomeric RGD peptide c(RGDyK) conjugated to the NIR fluorescent dye specifically targets integrin receptor both in cell culture and in living subjects. In this report, Cy5.5-conjugated mono-, di-, and tetrameric RGD peptides were evaluated in a subcutaneous U87MG glioblastoma xenograft model in order to investigate the effect of multimerization of RGD peptide on integrin avidity and tumor targeting efficacy. The binding affinities of Cy5.5-conjugated RGD monomer, dimer, and tetramer for alpha(v)beta(3) integrin expressed on U87MG cell surface were determined to be 42.9 +/- 1.2, 27.5 +/- 1.2, and 12.1 +/- 1.3 nmol/L, respectively. All three peptide-dye conjugates had integrin specific uptake both in vitro and in vivo. The subcutaneous U87MG tumor can be clearly visualized with each of these three fluorescent probes. Among them, tetramer displayed highest tumor uptake and tumor-to-normal tissue ratio from 0.5 to 4 h postinjection. Tumor-to-normal tissue ratio for Cy5.5-conjugated RGD monomer, dimer, and tetramer were found to be 3.18 +/- 0.16, 2.98 +/- 0.05, and 3.63 +/- 0.09, respectively, at 4 h postinjection. These results suggest that Cy5.5-conjugated monomeric, dimeric, and tetrameric RGD peptides are all suitable for integrin expression imaging. The multmerization of RGD peptide results in moderate improvement of imaging characteristics of the tetramer, compared to that of the monomer and dimeric counterparts.

Synthesis and Biological Evaluation of Dimeric RGD Peptide−Paclitaxel Conjugate as a Model for Integrin-Targeted Drug Delivery.

Synthesis and Biological Evaluation of Dimeric RGD Peptide−Paclitaxel Conjugate as a Model for Integrin-Targeted Drug Delivery.

Synthesis and Biological Evaluation of Dimeric RGD Peptide−Paclitaxel Conjugate as a Model for Integrin-Targeted Drug Delivery

Targeting drugs to receptors involved in tumor angiogenesis is a novel and promising approach to improve cancer treatment. In this study, we evaluated the antitumor activity of paclitaxel (PTX) conjugated with a bicyclic peptide E[c(RGDyK)](2) (RGD) in a metastatic breast cancer cell line (MDA-MB-435). The cyclic RGD peptide selectively binds to alpha(v) integrin receptors that are highly expressed in metastatic cancer cells. PTX, an antimicrotubule agent, is a potent antitumor agent commonly used in the treatment of advanced metastatic breast cancer. The in vitro results showed that RGD peptide inhibited cell cycle proliferation by arresting cells in G(0)/G(1)-phase. The PTX-RGD conjugate inhibited cell proliferation with activity comparable to that observed for paclitaxel, both of which were mediated by an arrest of G(2)/M-phase of the cell cycle followed by apoptosis. Although the PTX-RGD conjugate showed slightly decreased integrin binding affinity than the unconjugated peptide, it indicated integrin specific accumulation in vivo. (125)I-Labeled PTX-RGD showed highest tumor uptake at 2 h postinjection (2.72 +/-0.16%ID/g) and best tumor/background contrast after 4 h postinjection. Our results demonstrate the potential of tumor-targeted delivery of paclitaxel based on the specific recognition of cell adhesion molecule alpha(v)beta(3) integrin to reduce toxicity and enhance selective killing of cancer cells.

MicroPET imaging of breast cancer α v-integrin expression with 64Cu-labeled dimeric RGD peptides

Alphavbeta3 and alphavbeta5 integrins are cell adhesion molecules that play a vital role in tumor angiogenesis and metastasis. The ability to visualize and quantify integrin expression in vivo will foster our understanding of the role of integrins alphavbeta3 and alphavbeta5 in tumor angiogenesis and allow for direct assessment of anti-angiogenic treatment efficacy based on integrin antagonists. This study compared the tumor targeting characteristics of two dimeric 64Cu-labeled RGD peptide agonists of alphavbeta3 integrin. Dimeric RGD peptides E[c(RGDyK)]2 and E[c(RGDfK)]2 were conjugated with 1,4,7,10-tetraazadodecane-N,N',N",N"'-tetraacetic acid (DOTA) and labeled with positron emitter 64Cu(t(1/2)=12.8 h, beta+=19%). Both 64Cu-DOTA-E[c(RGDyK)]2 and 64Cu-DOTA-E[c(RGDfK)]2 were used in biodistribution, microPET imaging and whole-body autoradiography studies in athymic female nude mice with orthotopically growing MDA-MB-435 breast carcinoma xenografts. At all time points, activity accumulation of 64)Cu-DOTA-E[c(RGDyK)]2 in tumors was significantly higher compared to the D-Phe analog. Liver uptake of the D-Tyr derivative was lower than the D-Phe derivative at early time points but the difference became marginal with time. Overall, 64Cu-DOTA-E[c(RGDyK)]2 yielded better position emission tomography (PET) images in orthotopic MDA-MB-435 bearing mice than did 64Cu-DOTA-E[c(RGDfK)]2. Both radiotracers had alphav-integrin specific tumor activity accumulation, as demonstrated by significant reduction of uptake with a coinjected blocking dose of c(RGDyK). The radiolabeled dimeric RGD peptides 64Cu-DOTA-E[c(RGDyK)]2 and 64Cu-DOTA-E[c(RGDfK)]2 have high and specific tumor uptake in a human breast cancer tumor xenograft, with the D-Tyr derivative showing better in vivo kinetics than the D-Phe derivative, most likely due to the increased hydrophilicity of the D-Tyr. Both dimeric peptides showed better tumor retention than the previously tested monomeric RGD counterparts, presumably because of bivalency and increase in apparent molecular size.

Micro-PET Imaging of α&lt;SUB&gt;v&lt;/SUB&gt;β&lt;SUB&gt;3&lt;/SUB&gt;-Integrin Expression with &lt;SUP&gt;18&lt;/SUP&gt;F-Labeled Dimeric RGD Peptide

The alphav integrins, which act as cell adhesion molecules, are closely involved with tumor invasion and angiogenesis. In particular, alphavbeta3 integrin, which is specifically expressed on proliferating endothelial cells and tumor cells, is a logical target for development of a radiotracer method to assess angiogenesis and anti-angiogenic therapy. In this study, a dimeric cyclic RGD peptide E[c(RGDyK)]2 was labeled with 18F (t(1/2) = 109.7 min) by using a prosthetic 4-[18F]fluorobenzoyl moiety to the amino group of the glutamate. The resulting [18F]FB-E[c(RGDyK)]2, with high specific activity (200-250 GBq/micromol at the end of synthesis), was administered to subcutaneous U87MG glioblastoma xenograft models for micro-PET and autoradiographic imaging as well as direct tissue sampling to assess tumor targeting efficacy and in vivo kinetics of this PET tracer. The dimeric RGD peptide demonstrated significantly higher tumor uptake and prolonged tumor retention in comparison with a monomeric RGD peptide analog [18F]FB-c(RGDyK). The dimeric RGD peptide had predominant renal excretion, whereas the monomeric analog was excreted primarily through the biliary route. Micro-PET imaging 1 hr after injection of the dimeric RGD peptide exhibited tumor to contralateral background ratio of 9.5 +/- 0.8. The synergistic effect of polyvalency and improved pharmacokinetics may be responsible for the superior imaging characteristics of [18F]FB-E[c(RGDyK)]2.