Integrin Αvβ3-positive Tumors Research Articles

Clinical Evaluation of 68Ga-FAPI-RGD for Imaging of Fibroblast Activation Protein and Integrin αvβ3 in Various Cancer Types.

Radiolabeled fibroblast activation protein (FAP) inhibitors (FAPIs) and Arg-Gly-Asp (RGD) peptides have been extensively investigated for imaging of FAP- and integrin αvβ3-positive tumors. In this study, a FAPI-RGD heterodimer was radiolabeled with 68Ga and evaluated in patients with cancer. We hypothesized that the heterodimer, recognizing both FAP and integrin αvβ3, would be advantageous because of its dual-receptor-targeting property. Methods: The effective dose of 68Ga-FAPI-RGD was evaluated in 3 healthy volunteers. The clinical feasibility of 68Ga-FAPI-RGD PET/CT was evaluated in 22 patients with various types of cancer, and the results were compared with those of 18F-FDG and 68Ga-FAPI-46. Results: 68Ga-FAPI-RGD was tolerated well, with no adverse events in any of the healthy volunteers or patients. The effective dose from 68Ga-FAPI-RGD PET/CT was 1.01 × 10-2 mSv/MBq. In clinical investigations with different types of cancer, the radiotracer uptake and tumor-to-background ratio (TBR) of primary and metastatic lesions in 68Ga-FAPI-RGD PET/CT were significantly higher than those in 18F-FDG PET/CT (primary tumors: SUVmax, 18.0 vs. 9.1 [P < 0.001], and TBR, 15.2 vs. 5.5 [P < 0.001]; lymph node metastases: SUVmax, 12.1 vs. 6.1 [P < 0.001], and TBR, 13.3 vs. 4.1 [P < 0.001]), resulting in an improved lesion detection rate and tumor delineation, particularly for the diagnosis of lymph node (99% vs. 91%) and bone (100% vs. 80%) metastases. 68Ga-FAPI-RGD PET/CT also yielded a higher radiotracer uptake and TBR than 68Ga-FAPI-46 PET/CT did. Conclusion: 68Ga-FAPI-RGD exhibited improved tumor uptake and TBR compared with 18F-FDG and 68Ga-FAPI PET/CT. This study demonstrated the safety and clinical feasibility of 68Ga-FAPI-RGD PET/CT for imaging of various types of cancer.

Influence of Linker Molecules in Hexavalent RGD Peptides on Their Multivalent Interactions with Integrin αvβ3

Multivalent RGD peptides have been used as an excellent targeting vector to integrin αvβ3-positive tumors. However, little attention has been paid to the influence of linker molecules in multivalent RGD peptides on their dissociation kinetics from tumor cells. In this study, we evaluated the dissociation kinetics of 99mTc-labeled hexavalent RGD peptides which have (CH2-CH2-O)n (n = 4, [99mTc][Tc(L1)6]+ and n = 12, [99mTc][Tc(L2)6]+) or (DPro-Gly)n (n = 1, [99mTc][Tc(L3)6]+; n = 6, [99mTc][Tc(L4)6]+; and n = 9, [99mTc][Tc(L5)6]+) as a linker molecule. The results showed that [99mTc][Tc(L4)6]+ and [99mTc][Tc(L5)6]+ displayed slower dissociation kinetics and [99mTc][Tc(L4)6]+ showed exceptionally high in vitro cellular uptake (203.1 ± 16.7% dose/mg protein) and the highest tumor to blood ratio (138.1 ± 26.3 at 4 h p.i.) in tumor bearing nude mice. These findings indicate that the use of appropriate length of (DPro-Gly)n would maximize the binding of multivalent RGD peptides to clustered integrin αvβ3.

Preliminary biological evaluation of 68Ga-labeled cyclic RGD dimer as an integrin αvβ3-targeting radiotracer for tumor PET imaging

Radiolabeled cyclic arginine-glycine-aspartic acid (RGD) peptides have been widely prepared for noninvasive monitoring of tumor angiogenesis. The aim of this study was to explore and evaluate the feasibility of a novel 68Ga-labeled RGD peptide for tumor angiogenesis imaging. [68Ga]Ga-NOTA-PEG3-β-Glu-RGD2, a cyclic RGD dimer with a symmetric β-glutamate linker, was successfully designed and radiolabeled in high radiochemical yields. A series of experiments in vivo and in vitro were investigated to perform a preliminary evaluation and confirm good target at integrin αvβ3-positive tumor cells. The results suggest that [68Ga]Ga-NOTA-PEG3-β-Glu-RGD2 may be a promising radiotracer for positron emission tomography imaging of tumor angiogenesis.

Dotted Core-Shell Nanoparticles for T1 -Weighted MRI of Tumors.

Gd-based T 1 -weighted contrast agents have dominated the magnetic resonance imaging (MRI) contrast agent market for decades. Nevertheless, they are reported to be nephrotoxic and the U.S. Food and Drug Administration has issued a general warning concerning their use. In order to reduce the risk of nephrotoxicity, the MRI performance of the Gd-based T 1 -weighted contrast agents needs to be improved to allow a much lower dosage. In this study, novel dotted core-shell nanoparticles (FeGd-HN3-RGD2) with superhigh r 1 value (70.0 mM-1 s-1 ) and very low r 2 /r 1 ratio (1.98) are developed for high-contrast T 1 -weighted MRI of tumors. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and histological analyses show good biocompatibility of FeGd-HN3-RGD2. Laser scanning confocal microscopy images and flow cytometry demonstrate active targeting to integrin αv β3 positive tumors. MRI of tumors shows high tumor ΔSNR for FeGd-HN3-RGD2 (477 ± 44%), which is about 6-7-fold higher than that of Magnevist (75 ± 11%). MRI and inductively coupled plasma results further confirm that the accumulation of FeGd-HN3-RGD2 in tumors is higher than liver and spleen due to the RGD2 targeting and small hydrodynamic particle size (8.5 nm), and FeGd-HN3-RGD2 is readily cleared from the body by renal excretion.

NGR (Asn-Gly-Arg)-targeted delivery of coagulase to tumor vasculature arrests cancer cell growth

Induction of selective thrombosis and infarction in tumor-feeding vessels represents an attractive strategy to combat cancer. Here we took advantage of the unique coagulation properties of staphylocoagulase and genetically engineered it to generate a new fusion protein with novel anti-cancer properties. This novel bi-functional protein consists of truncated coagulase (tCoa) and an NGR (GNGRAHA) motif that recognizes CD13 and αvβ3 integrin receptors, targeting it to tumor endothelial cells. Herein, we report that tCoa coupled by its C-terminus to an NGR sequence retained its normal binding activity with prothrombin and avβ3 integrins, as confirmed in silico and in vitro. Moreover, in vivo biodistribution studies demonstrated selective accumulation of FITC-labeled tCoa-NGR fusion proteins at the site of subcutaneously implanted PC3 tumor xenografts in nude mice. Notably, systemic administration of tCoa-NGR to mice bearing 4T1 mouse mammary xenografts or PC3 human prostate tumors resulted in a significant reduction in tumor growth. These anti-tumor effects were accompanied by massive thrombotic occlusion of small and large tumor vessels, tumor infarction and tumor cell death. From these findings, we propose tCoa-NGR mediated tumor infarction as a novel and promising anti-cancer strategy targeting both CD13 and integrin αvβ3 positive tumor neovasculature.

A novel integrin αvβ3-targeted isoDGR probe for SPECT/CT imaging of glioma

Objective To prepare 99Tcm-HYNIC-c(isoDGRKy) as a SPECT/CT imaging molecular probe targeting integrin αvβ3, and evaluate its biodistribution and feasibility on SPECT/CT imaging for integrin αvβ3-positive tumor in U87MG human glioma xenograft mouse models. Methods The bifunctional chelator HYNIC was conjugated to c(isoDGRKy), and tricine and TPPTS were used as coligands for 99Tcm labeling to prepare 99Tcm-HYNIC-c(isoDGRKy). The radiochemical purity and stability of the product were measured. The expression of integrin αvβ3 and binding affinity (half maximal inhibitory concentration, IC50) of c(isoDGRKy) was detected in U87MG cells by cell experiments in vitro. Biodistribution and SPECT/CT imaging of 99Tcm-HYNIC-c(isoDGRKy) including blocking experiments were performed respectively in nude mice bearing U87MG human glioma xenografts. Results The radiochemical purity of 99Tcm-HYNIC-c(isoDGRKy) was over 99%, and was still over 99% after 4 h incubation in saline at room temperature. Flow cytometry assay showed that U87MG cells were integrin αvβ3-positive (expressive rate: 70%). The IC50 of c(isoDGRKy) was 6.67×10-8 mol/L. Biodistribution results showed 99Tcm-HYNIC-c(isoDGRKy) with a rapid clearance from blood was excreted mainly via the kidneys. The 99Tcm-HYNIC-c(isoDGRKy) uptake values in U87MG tumors were (7.31±1.42) and (1.09±0.11) %ID/g at 15 and 45 min post-injection respectively, and tumor-to-muscle ratio reached 5.01±1.47 at 15 min post-injection. The tumors were clearly visualized with low background from 0.5 to 1 h post-injection in tumor bearing mice. In the blocking experiment, the tumor was barely visualized after co-injection of excess cold c(RGDfK) peptide with 99Tcm-HYNIC-c(isoDGRKy). Conclusions 99Tcm-HYNIC-c(isoDGRKy) may be easily and steadily prepared. It may be a RGD-like promising SPECT/CT imaging probe for integrin αvβ3-positive tumor. Key words: Isoaspartate-glycine-arginine; Technetium; Integrin alphaVbeta3; Glioma; Tomography, emission-computed, single-photon; Tomography, X-ray computed; Mice, nude

Abstract 2864: Tumor targeting and imaging using 64Cu labeled cyclic RGD conjugated human serum albumin via click chemistry

Abstract PURPOSE: The RGD specifically recognizes the integrin αvβ3 which is overexpressed on various malignant tumors. One major drawback of small peptide such as RGD, however, is short half-life in the blood, which greatly compromises their targeting efficacy. To improve blood circulation time and targeting efficacy, we developed cyclic RGDyK (cRGDyK) conjugated human serum albumin (HSA) using click chemistry reaction. METHODS: HSA was conjugated with DBCO-NHS (dibenzyl cyclooctyne) under physiologically favorable reaction condition for the preparation of strain promoted azide-alkyne reaction. Using this reaction group, DBCO-HSA was conjugated with N3-c RGDyK (azido cRGDyK) for integrin αvβ3 targeting, N3-FNR648 for fluorescence labeling and 64Cu labeled N3-NOTA (3-azidopropyl-NOTA) for radiolabeling. Cell binding of cRGDyK-HSA were analyzed with FNR648 labeled probes. Cellular uptake of 64Cu-cRGDyK-HSA were tested for integrin αvβ3 specific binding at cell level. PET images were acquired after tail vein injection of 64Cu-HSA, 64Cu-cRGDyK-HSA in integrin αvβ3 positive tumor (SK-OV3, ovarian cancer cell line) bearing BALB/c nude mice. PET signals were quantitatively analyzed with PET image analysis program, AMIDE. RESULTS: The number of cRGDyK on DBCO-HSA conjugates was confirmed using MALDI-TOF MS. cRGDyK-HSA was successfully conjugated with 64Cu labeled N3-NOTA. Integrin αvβ3 mRNA and protein was highly expressed in SK-OV3 cell line. At fluorescence labeled probes were treated in SK-OV3, cRGDyK-HSA were highly bound to cell membrane and this pattern were decreased with pre-treatment of excess cRGDyK. When compared to cellular uptake level, 64Cu-cRGDyK-HSA were more accumulated at integrin αvβ3 positive cells (SK-OV3) than integrin αvβ3 negative cells (P &amp;lt; 0.05) and there were no difference in 64Cu-HSA. Serial PET images were acquired 0, 4, 24, 48 hours after tail vein injection of 64Cu-cRGDyK-HSA. Radioactivity of 64Cu-cRGDyK-HSA in SK-OV3 tumor was higher than that of 64Cu-HSA. 64Cu-labeled-cRGDyK-HSA can be observed at 48 hours after injection, which shows longer circulation time in mice. CONCLUSION: We successfully conjugated cyclic RGDyK to HSA using click chemistry approach. We demonstrated that cRGDyK-HSA specifically bind to integrin αvβ3 in in vitro and in vivo model. And in animal model, 64Cu-labeled-cRGDyK-HSA can be observed at 48 hours after injection, which shows longer circulation time. Our results indicated that cRGDyK-HSA have a potential to diagnosis and therapy response monitoring of tumor expressing integrin αvβ3. Citation Format: Cho Rong Park, Myung Geun Song, Ji-Yong Park, Hyewon Youn, June-Key Chung, Jae Min Jeong, Yun-Sang Lee, Keon Wook Kang. Tumor targeting and imaging using 64Cu labeled cyclic RGD conjugated human serum albumin via click chemistry [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2864. doi:10.1158/1538-7445.AM2017-2864

A Radiofluorinated Divalent Cystine Knot Peptide for Tumor PET Imaging

A divalent knottin containing two separate integrin binding epitopes (RGD) in the adjacent loops, 3-4A, was recently developed and reported in our previous publication. In the current study, 3-4A was radiofluorinated with a 4-nitrophenyl 2-18F-fluoropropinate (18F-NFP) group and the resulting divalent positron emission tomography (PET) probe, 18F-FP–3-4A, was evaluated as a novel imaging probe to detect integrin αvβ3 positive tumors in living animals. Knottin 3-4A was synthesized by solid phase peptide synthesis, folded, and site-specifically conjugated with 18/19F-NFP to produce the fluorinated peptide 18/19F-fluoropropinate-3-4A (18/19F-FP–3-4A). The stability of 18F-FP–3-4A was tested in both phosphate buffered saline (PBS) buffer and mouse serum. Cell uptake assays of the radiolabeled peptides were performed using U87MG cells. In addition, small animal PET imaging and biodistribution studies of 18F-FP–3-4A were performed in U87MG tumor-bearing mice. The receptor targeting specificity of the radiolabeled peptide was also verified by coinjecting the probe with a blocking peptide cyclo(RGDyK). Our study showed that 18F-FP–3-4A exhibited excellent stability in PBS buffer (pH 7.4) and mouse serum. Small animal PET imaging and biodistribution data revealed that 18F-FP–3-4A exhibited rapid and good tumor uptake (3.76 ± 0.59% ID/g and 2.22 ± 0.62% ID/g at 0.5 and 1 h, respectively). 18F-FP–3-4A was rapidly cleared from the normal tissues, resulting in excellent tumor-to-normal tissue contrasts. For example, liver uptake was only 0.39 ± 0.07% ID/g and the tumor to liver ratio was 5.69 at 1 h p.i. Furthermore, coinjection of cyclo(RGDyK) with 18F-FP–3-4A significantly inhibited tumor uptake (0.41 ± 0.12 vs 1.02 ± 0.19% ID/g at 2.5 h) in U87MG xenograft models, demonstrating specific accumulation of the probe in the tumor. In summary, the divalent probe 18F-FP–3-4A is characterized by rapid and high tumor uptake and excellent tumor-to-normal tissue ratios. 18F-FP–3-4A is a highly promising knottin based PET probe for translating into clinical imaging of tumor angiogenesis.

Radiosynthesis and evaluation of 188Re-c(RGDyK)-His as a novel radiotherapeutic agent for integrin αvβ3 targeting tumour

The successes of noninvasive methods to visualize and quantify integrin αvβ3 expression in vivo have paved the way for radiolabeling anti-integrin therapy in clinic. Arginine-glycine-aspartice (RGD) peptide and related derivatives labeled with radionuclides for radio-therapy, which specifically targeting integrin αvβ3-positive tumors, could be used to treat these tumors. We have labeled c(RGDyK)-His, a RGD derivative, with 188Re and the radio-therapy efficiency has been evaluated in model nude mice. c(RGDyK)-His was labeled with 188Re by chelating with [188Re(CO)3(H2O)3]+ under a slightly basic condition. The in vitro specific binding affinity to U87 MG cell lines and the biodistribution of 188Re-c(RGDyK)-His in the animal tumor models was measured. The inhibitory effects of 188Re-c(RGDyK)-His were observed more than 1 month, and evaluated by microPET/CT imaging with 18F-FDG. Results of in vivo, cell uptake demonstrated 188Re-c(RGDyK)-His had a high specific binding affinity to receptor integrin αvβ3. In biodistribution experiment, 188Re-c(RGDyK)-His was accumulated in the tumor and cleared fast from the normal tissues. In radiotherapy study, tumor growth inhibition was significantly higher in the treatment groups than in the control groups. These studies showed that 188Re-c(RGDyK)-His could be effectively used for integrin αvβ3 targeting therapy. This may offer a potential therapeutic strategy for the treatment of integrin-positive tumors in clinic.

Comparison of two new angiogenesis PET tracers 68Ga-NODAGA-E[c(RGDyK)]2 and 64Cu-NODAGA-E[c(RGDyK)]2; in vivo imaging studies in human xenograft tumors

The aim of this study was to synthesize and perform a side-by-side comparison of two new tumor-angiogenesis PET tracers (68)Ga-NODAGA-E[c(RGDyK)](2) and (64)Cu-NODAGA-E[c(RGDyK)](2) in vivo using human xenograft tumors in mice. Human radiation burden was estimated to evaluate potential for future use as clinical PET tracers for imaging of neo-angiogenesis. A (68)Ge/(68)Ga generator was used for the synthesis of (68)Ga-NODAGA-E[c(RGDyK)](2). (68)Ga and (64)Cu labeled NODAGA-E[c(RGDyK)](2) tracers were administrated in nude mice bearing either human glioblastoma (U87MG) or human neuroendocrine (H727) xenograft tumors. PET/CT scans at 3 time points were used for calculating the tracer uptake in tumors (%ID/g), integrin αVβ3 target specificity was shown by blocking with cold NODAGA-E[c(RGDyK)](2), and biodistribution in normal organs were also examined. From biodistribution data in mice human radiation-absorbed doses were estimated using OLINDA/EXM software. (68)Ga-NODAGA-E[c(RGDyK)](2) was synthesized with a radiochemical purity of 89%-99% and a specific activity (SA) of 16-153 MBq/nmol. (64)Cu-NODAGA-E[c(RGDyK)](2) had a purity of 92%-99% and an SA of 64-78 MBq/nmol. Both tracers showed similar uptake in xenograft tumors 1h after injection (U87MG: 2.23 vs. 2.31%ID/g; H727: 1.53 vs. 1.48%ID/g). Both RGD dimers showed similar tracer uptake in non-tumoral tissues and a human radiation burden of less than 10 mSv with an administered dose of 200 MBq was estimated. (68)Ga-NODAGA-E[c(RGDyK)](2) and (64)Cu-NODAGA-E[c(RGDyK)](2) can be easily synthesized and are both promising candidates for PET imaging of integrin αVβ3 positive tumor cells. (68)Ga-NODAGA-E[c(RGDyK)](2) showed slightly more stable tumor retention. With the advantage of in-house commercially (68)Ge/(68)Ga generators, (68)Ga-NODAGA-E[c(RGDyK)](2) may be the best choice for future clinical PET imaging in humans.

Evaluating the effects of an angiogenesis inhibitor of endostar with a novel RGD dimer probe labeled by 99Tcm

Objective To study the feasibility of a novel probe 99Tcm-HYNIC-2(poly-(ethylene glycol),PEG) 4-Dimer (Dimer:E-[c (RGDfK) 2]) as a potential imaging agent for integrin αv β3 positive tumors,and also to observe the influence of an angiogenesis inhibitor,endostar,on the biodistribution and tumor uptake of the tracer in tumor bearing nude mice.Methods The expression of integrin αv β3 in human glioma cells U87MG was determined with immunofluorescence staining before and after treatment with endostar.99Tcm-HYNIC-2PEG4-Dimer was prepared and administered in U87MG tumor bearing mice in 6 h after either administration of endostar (200 μl) or saline (control group) and then biodistribution study was performed.Other 16 mice were divided into endostar treated group (20 mg/kg) and control group (saline) and then gamma imaging was performed in the two groups.Statistical significance of differences between the two groups was assessed using two-sample t test.Results Radiochemical purity of 99Tcm-HYNIC-2PEG4-Dimer was exceeded 95％.The expression of integrin αvβ3 in U87MG cell was high and gradually decreased after treatment with endostar.There was a negative dose-effect relationship between the dose of endostar and the expression of integrin αvβ3 with the peak effect at the dose of 400 μg/ml.The distribution study in vivo showed that the tracer uptake of U87MG tumors was high,but it decreased after injection of endostar.At 90 min,the ％ID/g of endostar and control groups were 1.50±0.08 and 6.27±0.33,respectively (t =40.23,P＜0.05).The average T/NT ratios of 99Tcm-HYNIC-2PEG4-Dimer uptake in the endostar and control groups were 1.02±0.11 and 2.58±0.36,respectively (t =10.25,P＜0.05).The integrin αv β3 positive expression ratios of tumor in endostar and control groups were (33.1 ±2.7) ％ and (81.5±3.2) ％,respectively (t =32.60,P＜0.05).Conclusions The novel probe 99Tcm-HYNIC-2PEG4-Dimer may be a promising radiotracer for integrin αvβ3-positive tumor imaging.It may be used for monitoring the therapeutic effect of endostar and may be potentially used for screening the candidates of anti-angiogenesis therapy. Key words: Glioma; Arg-Gly-Asp; Radionuclide imaging; Angiogenesis inhibitors ; Mice, nude

(99m)Tc-Galacto-RGD2: a novel 99mTc-labeled cyclic RGD peptide dimer useful for tumor imaging.

This study sought to evaluate [(99m)Tc(HYNIC-Galacto-RGD2)(tricine)(TPPTS)] ((99m)Tc-Galacto-RGD2: HYNIC = 6-hydrazinonicotinyl; Galacto-RGD2 = Glu[cyclo[Arg-Gly-Asp-D-Phe-Lys(SAA-PEG2-(1,2,3-triazole)-1-yl-4-methylamide)]]2 (SAA = 7-amino-L-glycero-L-galacto-2,6-anhydro-7-deoxyheptanamide, and PEG2 = 3,6-dioxaoctanoic acid); and TPPTS = trisodium triphenylphosphine-3,3',3″-trisulfonate) as a new radiotracer for tumor imaging. Galacto-RGD2 was prepared via the copper(I)-catalyzed 1,3-dipolar azide-alkyne Huisgen cycloaddition. HYNIC-Galacto-RGD2 was prepared by reacting Galacto-RGD2 with sodium succinimidyl 6-(2-(2-sulfonatobenzaldehyde)hydrazono)nicotinate (HYNIC-OSu) in the presence of diisopropylethylamine, and was evaluated for its integrin αvβ3 binding affinity against (125)I-echistatin bound to U87MG glioma cells. The IC50 value for HYNIC-Galacto-RGD2 was determined to be 20 ± 2 nM. (99m)Tc-Galacto-RGD2 was prepared in high specific activity (∼ 185 GBq/μmol) and high radiochemical purity (>95%), and was evaluated in athymic nude mice bearing U87MG glioma xenografts for its tumor-targeting capability and biodistribution. The tumor uptake of (99m)Tc-Galacto-RGD2 was 10.30 ± 1.67, 8.37 ± 2.13, 6.86 ± 1.33, and 5.61 ± 1.52%ID/g at 5, 30, 60, and 120 min p.i., respectively, which was in agreement with high integrin αvβ3 expression on glioma cells and neovasculature. Its lower uptake in intestines, lungs, and spleen suggests that (99m)Tc-Galacto-RGD2 has advantages over (99m)Tc-3P-RGD2 ([(99m)Tc(HYNIC-3P-RGD2)(tricine)(TPPTS)]: 3P-RGD2 = PEG4-E[PEG4-c(RGDfK)]2; PEG4 = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) for imaging tumors in the chest and abdominal regions. U87MG tumors were readily detected by SPECT and the tumor uptake of (99m)Tc-Galacto-RGD2 was integrin αvβ3-specific. (99m)Tc-Galacto-RGD2 also had very high metabolic stability. On the basis of results from this study, it was concluded that (99m)Tc-Galacto-RGD2 is an excellent radiotracer for imaging integrin αvβ3-positive tumors and related metastases.

Imaging integrin αvβ3 positive glioma with a novel RGD dimer probe and the impact of antiangiogenic agent (Endostar) on its tumor uptake

Integrin αvβ3 has been recognized to play an important role in angiogenesis, tumor growth and metastasis. It will be of interest to apply this promising target for tumor imaging and visualization of tumor angiogenesis in vivo. In this study, a novel integrin αvβ3 targetting imaging probe, 99mTc-HYNIC-E[c(RGDfK)]2, was used to investigate the glioma uptake in vitro and in vivo before and after treatment with an antiangiogenic agent, endostar. The results indicated that U87MG glioma cells have high expression of integrin αvβ3 and special uptake of 99mTc-HYNIC-E[c(RGDfK)]2 both in cell line and in tumor xenograft. The endostatin analogue endostar can inhibit the expression of integrin αvβ3 receptors in both U87MG cells in vitro and glioma tissues, which suggested that integrin pathway may play a role in antiangiogenic effect of Endostar. 99mTc-HYNIC-E[c(RGDfK)]2 may be a promising molecular imaging probe for integrin αvβ3 positive tumor imaging and open up the possibility to establish an molecular imaging modality for assessment of tomor antiangiogenic therapy.

Evaluation of 99mTc-Labeled Cyclic RGD Dimers: Impact of Cyclic RGD Peptides and 99mTc Chelates on Biological Properties

The main objective of this study is to explore the impact of cyclic RGD peptides and (99m)Tc chelates on biological properties of (99m)Tc radiotracers. Cyclic RGD peptide conjugates, HYNIC-K(NIC)-RGD(2) (HYNIC = 6-hydrazinonicotinyl; RGD(2) = E[c(RGDfK)](2) and NIC = nicotinyl), HYNIC-K(NIC)-3G-RGD(2) (3G-RGD(2) = Gly-Gly-Gly-E[Gly-Gly-Gly-c(RGDfK)](2)), and HYNIC-K(NIC)-3P-RGD(2) (3P-RGD(2) = PEG(4)-E[PEG(4)-c(RGDfK)](2)), were prepared. Macrocyclic (99m)Tc complexes [(99m)Tc(HYNIC-K(NIC)-RGD(2))(tricine)] (1), [(99m)Tc(HYNIC-K(NIC)-3G-RGD(2))(tricine)] (2), and [(99m)Tc(HYNIC-K(NIC)-3P-RGD(2))(tricine)] (3) were evaluated for their biodistribution and tumor-targeting capability in athymic nude mice bearing MDA-MB-435 human breast tumor xenografts. It was found that 1, 2, and 3 could be prepared with high specific activity (∼111 GBq/μmol). All three (99m)Tc radiotracers have two major isomers, which show almost identical uptake in tumors and normal organs. Replacing the bulky and highly charged [(99m)Tc(HYNIC)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3″-trisulfonate) with a smaller [(99m)Tc(HYNIC-K(NIC))(tricine)] resulted in less uptake in the kidneys and lungs for 3. Surprisingly, all three (99m)Tc radiotracers shared a similar tumor uptake (1, 5.73 ± 0.40%ID/g; 2, 5.24 ± 1.09%ID/g; and 3, 4.94 ± 1.71%ID/g) at 60 min p.i. The metabolic stability of (99m)Tc radiotracers depends on cyclic RGD peptides (3P-RGD(2) > 3G-RGD(2) ∼ RGD(2)) and (99m)Tc chelates ([(99m)Tc(HYNIC)(tricine)(TPPTS)] > [(99m)Tc(HYNIC-K(NIC))(tricine)]). Immunohistochemical studies revealed a linear relationship between the α(v)β(3) expression levels and tumor uptake or tumor/muscle ratios of 3, suggesting that 3 is useful for monitoring the tumor α(v)β(3) expression. Complex 3 is a very attractive radiotracer for detection of integrin α(v)β(3)-positive tumors.

I111n-Labeled Cystine-Knot Peptides Based on the Agouti-Related Protein for Targeting Tumor Angiogenesis

Agouti-related protein (AgRP) is a 4-kDa cystine-knot peptide of human origin with four disulfide bonds and four solvent-exposed loops. The cell adhesion receptor integrin αvβ3 is an important tumor angiogenesis factor that determines the invasiveness and metastatic ability of many malignant tumors. AgRP mutants have been engineered to bind to integrin αvβ3 with high affinity and specificity using directed evolution. Here, AgRP mutants 7C and 6E were radiolabeled with 111In and evaluated for in vivo targeting of tumor integrin αvβ3 receptors. AgRP peptides were conjugated to the metal chelator 1, 4, 7, 10-tetra-azacyclododecane- N, N′, N″, N‴-tetraacetic acid (DOTA) and radiolabeled with 111In. The stability of the radiopeptides 111In-DOTA-AgRP-7C and 111In-DOTA-AgRP-6E was tested in phosphate-buffered saline (PBS) and mouse serum, respectively. Cell uptake assays of the radiolabeled peptides were performed in U87MG cell lines. Biodistribution studies were performed to evaluate the in vivo performance of the two resulting probes using mice bearing integrin-expressing U87MG xenograft tumors. Both AgRP peptides were easily labeled with 111In in high yield and radiochemical purity (>99%). The two probes exhibited high stability in phosphate-buffered saline and mouse serum. Compared with 111In-DOTA-AgRP-6E, 111In-DOTA-AgRP-7C showed increased U87MG tumor uptake and longer tumor retention (5.74 ± 1.60 and 1.29 ± 0.02%ID/g at 0.5 and 24 h, resp.), which was consistent with measurements of cell uptake. Moreover, the tumor uptake of 111In-DOTA-AgRP-7C was specifically inhibited by coinjection with an excess of the integrin-binding peptidomimetic c(RGDyK). Thus, 111In-DOTA-AgRP-7C is a promising probe for targeting integrin αvβ3 positive tumors in living subjects.

Specific Targeting of Human Integrin αvβ3 with 111In-Labeled Abegrin™ in Nude Mouse Models

The cell adhesion molecule integrin α(v)β(3) is an important player in the process of tumor angiogenesis and metastasis. Abegrin™, a fully humanized anti-integrin α(v)β(3) monoclonal antibody, was currently in clinical trials for cancer therapy. Herein, we labeled Abegrin™ with (111)In, evaluated the in vitro and in vivo characteristics, and investigated whether the expression of integrin α(v)β(3) in tumors could be imaged with (111)In-labeled Abegrin™. The binding affinity and specificity of Abegrin™ was analyzed using U87MG glioblastoma cells. Abegrin™ was coupled with 1,4,7,10-tetraazadodecane-N,N',N",N'″-tetraacetic acid (DOTA) for (111)In radiolabeling. γ Imaging of (111)In-DOTA-Abegrin™ was carried out in nude mice bearing both integrin α(v)β(3)-positive U87MG and integrin α(v)β(3)-negative HT-29 tumors. Biodistribution and blocking studies of (111)In-DOTA-Abegrin™ were investigated in U87MG tumor-bearing nude mice. Abegrin™ exhibited high-binding affinity to human integrin α(v)β(3) expressed on U87MG cells (K (d) of 0.35 ± 0.06 nM). The antibody retained antigen-binding affinity/specificity after DOTA conjugation. γ Imaging showed that the tumor uptake of (111)In-DOTA-Abegrin™ in integrin α(v)β(3)-positive U87MG tumors was much higher than that in integrin α(v)β(3)-negative HT-29 tumors. In the HT-29 tumors, Abegrin™ was mainly nonspecifically accumulated around the blood vessels, while in the U87MG tumors, besides the nonspecific tumor retention, Abegrin™ also specifically bound the human integrin α(v)β(3) expressed on the tumor cells. Biodistribution and blocking studies exhibited that the U87MG tumor uptake of (111)In-DOTA-Abegrin™ decreased from 14.12 ± 0.44 to 6.93 ± 0.94 percentage of injected dose per gram of tissue after coinjection of excess dose of cold Abegrin™, which confirmed the in vivo integrin α(v)β(3) binding specificity of (111)In-DOTA-Abegrin™. Abegrin™ showed specific binding to human integrin α(v)β(3) expressed on the tumor cells. (111)In-DOTA-Abegrin™ can specifically target the human integrin α(v)β(3) expression in the nude mouse model. (111)In-DOTA-Abegrin™ has a potential for clinical translation as an agent for integrin α(v)β(3)-positive tumor imaging, evaluating tumor angiogenic status and monitoring the therapeutic efficacy of Abegrin™-based cancer therapy.

Radiolabeled Cyclic RGD Peptides as Integrin αvβ3-Targeted Radiotracers: Maximizing Binding Affinity via Bivalency

Integrin alpha(v)beta(3) plays a significant role in tumor angiogenesis and is a receptor for the extracellular matrix proteins with the exposed arginine-glycine-aspartic (RGD) tripeptide sequence. These include vitronectin, fibronectin, fibrinogen, lamin, collagen, Von Willibrand's factor, osteoponin, and adenovirus particles. Integrin alpha(v)beta(3) is expressed at low levels on epithelial cells and mature endothelial cells, but it is overexpressed on the activated endothelial cells of tumor neovasculature and some tumor cells. The restricted expression of integrin alpha(v)beta(3) during tumor growth, invasion, and metastasis presents an interesting molecular target for both early detection and treatment of rapidly growing solid tumors. Over the past decade, many radiolabeled linear and cyclic RGD peptide antagonists have been evaluated as integrin alpha(v)beta(3)-targeted radiotracers. Significant progress has been made on their use for imaging integrin alpha(v)beta(3)-positive tumors by SPECT or PET. Among the radiotracers evaluated in preclinical tumor-bearing models, [18F]Galacto-RGD (2-[18F]fluoropropanamide c(RGDfK(SAA); SAA = 7-amino-L-glyero-L-galacto-2,6-anhydro-7-deoxyheptanamide) and [18F]-AH111585 are currently under clinical investigation for visualization of integrin alpha(v)beta(3) expression in cancer patients. However, their low tumor uptake, high cost, and lack of preparative modules for routine radiosynthesis will limit their continued clinical application. Thus, there is a continuing need for more efficient integrin alpha(v)beta(3)-targeted radiotracers that are readily prepared from a kit formulation without further postlabeling purification. This article will focus on different approaches to maximize the targeting capability of cyclic RGD peptides and to improve the radiotracer excretion kinetics from noncancerous organs. Improvement of tumor uptake and tumor-to-background ratios is important for early detection of integrin alpha(v)beta(3)-positive tumors and/or noninvasive monitoring of therapeutic efficacy of antiangiogenic therapy.

In vitro and In vivo Characterization of 64Cu-Labeled AbegrinTM, a Humanized Monoclonal Antibody against Integrin αvβ3

Abegrin (MEDI-522 or Vitaxin), a humanized monoclonal antibody against human integrin alpha(v)beta(3), is in clinical trials for cancer therapy. In vivo imaging using Abegrin-based probes is needed for better treatment monitoring and dose optimization. Here, we conjugated Abegrin with macrocyclic chelating agent 1,4,7,10-tetra-azacylododecane N,N',N'',N'''-tetraacetic (DOTA) at five different DOTA/Abegrin ratios. The conjugates were labeled with (64)Cu (half-life = 12.7 hours) and tested in three human (U87MG, MDA-MB-435, and PC-3) and one mouse (GL-26) tumor models. The in vitro and in vivo effects of these (64)Cu-DOTA-Abegrin conjugates were evaluated. The number of DOTA per Abegrin varied from 1.65 +/- 0.32 to 38.53 +/- 5.71 and the radiolabeling yield varied from 5.20 +/- 3.16% to 88.12 +/- 6.98% (based on 2 mCi (64)Cu per 50 microg DOTA-Abegrin conjugate). No significant difference in radioimmunoreactivity was found among these conjugates (between 59.78 +/- 1.33 % and 71.13 +/- 2.58 %). Micro-positron emission tomography studies revealed that (64)Cu-DOTA-Abegrin (1,000:1) had the highest tumor activity accumulation (49.41 +/- 4.54% injected dose/g at 71-hour postinjection for U87MG tumor). The receptor specificity of (64)Cu-DOTA-Abegrin was confirmed by effective blocking of MDA-MB-435 tumor uptake with coadministration of nonradioactive Abegrin. (64)Cu-DOTA-IgG exhibited background level tumor uptake at all time points examined. Integrin alpha(v)beta(3)-specific tumor imaging using (64)Cu-DOTA-Abegrin may be translated into the clinic to characterize the pharmacokinetics, tumor targeting efficacy, dose optimization, and dose interval of Abegrin and/or Abegrin conjugates. Chemotherapeutics or radiotherapeutics using Abegrin as the delivering vehicle may also be effective in treating integrin alpha(v)beta(3)-positive tumors.