Impact of bifunctional chelators on biological properties of 111In-labeled cyclic peptide RGD dimers

EGFR- and Integrin αVβ3-Targeting Peptides as Potential Radiometal-Labeled Radiopharmaceuticals for Cancer Theranostics

The burgeoning field of cancer theranostics has witnessed advancements through the development of targeted molecular agents, particularly peptides. These agents exploit the overexpression or mutations of specific receptors, such as the Epidermal Growth Factor receptor (EGFR) and αVβ3 integrin, which are pivotal in tumor growth, angiogenesis, and metastasis. Despite the extensive research into and promising outcomes associated with antibody-based therapies, peptides offer a compelling alternative due to their smaller size, ease of modification, and rapid bioavailability, factors which potentially enhance tumor penetration and reduce systemic toxicity. However, the application of peptides in clinical settings has challenges. Their lower binding affinity and rapid clearance from the bloodstream compared to antibodies often limit their therapeutic efficacy and diagnostic accuracy. This overview sets the stage for a comprehensive review of the current research landscape as it relates to EGFR- and integrin αVβ3-targeting peptides. We aim to delve into their synthesis, radiolabeling techniques, and preclinical and clinical evaluations, highlighting their potential and limitations in cancer theranostics. This review not only synthesizes the extant literature to outline the advancements in peptide-based agents targeting EGFR and integrin αVβ3 but also identifies critical gaps that could inform future research directions. By addressing these gaps, we contribute to the broader discourse on enhancing the diagnostic precision and therapeutic outcomes of cancer treatments.

The effects of novel macrocyclic chelates on the targeting properties of the 68Ga-labeled Gastrin releasing peptide receptor antagonist RM2

BACKGROUND

The gastrin-releasing peptide receptor (GRPr) is a molecular target for the visualization of prostate cancer. Bombesin (BN) analogs are short peptides with a high affinity for GRPr. RM2 is a bombesin-based antagonist. It has been demonstrated that RM2 have superior in vivo biodistribution and targeting properties than high-affinity receptor agonists. This study developed new RM2-like antagonists by introducing the novel bifunctional chelators AAZTA5 and DATA5m to RM2.

RESULTS

The effects of different macrocyclic chelating groups on drug targeting properties and the possibility of preparing 68Ga-radiopharmaceuticals in a kit-based protocol were investigated using 68Ga-labeled entities. Both new RM2 variants were labelled with 68Ga3+ resulting in high yields, stability, and low molarity of the ligand. DATA5m-RM2 and AAZTA5-RM2 incorporated 68Ga3+ nearly quantitatively at room temperature within 3-5 min, and the labelling yield for 68Ga-DOTA-RM2 was approximately 10% under the same conditions. 68Ga-AAZTA5-RM2 showed stronger hydrophilicity according to partition coefficient. Although the maximal cellular uptake values of the three compounds were similar, 68Ga-AAZTA5-RM2 and 68Ga-DATA5m-RM2 peaked more rapidly. Biodistribution studies showed high and specific tumor uptake, with a maximum of 9.12 ± 0.81 percentage injected activity per gram of tissue (%ID/g) for 68Ga-DATA5m-RM2 and 7.82 ± 0.61%ID/g for 68Ga-AAZTA5-RM2 at 30 min after injection.

CONCLUSIONS

The conditions for complexation of DATA5m-RM2 and AAZTA5-RM2 with gallium-68 are milder, faster and require less amount of precursors than DOTA-RM2. Chelators had an evident influence on the pharmacokinetics and targeting properties of 68Ga-X-RM2 derivatives. Positively charged 68Ga-DATA5m-RM2 provided a high tumor uptake, high image contrast and good capability of targeting GRPr.

Methods for Radiolabelling Nanoparticles: SPECT Use (Part 1)

The use of nanoparticles (NPs) is rapidly increasing in nuclear medicine (NM) for diagnostic and therapeutic purposes. Their wide use is due to their chemical–physical characteristics and possibility to deliver several molecules. NPs can be synthetised by organic and/or inorganic materials and they can have different size, shape, chemical composition, and charge. These factors influence their biodistribution, clearance, and targeting ability in vivo. NPs can be designed to encapsulate inside the core or bind to the surface several molecules, including radionuclides, for different clinical applications. Either diagnostic or therapeutic radioactive NPs can be synthetised, making a so-called theragnostic tool. To date, there are several methods for radiolabelling NPs that vary depending on both the physical and chemical properties of the NPs and on the isotope used. In this review, we analysed and compared different methods for radiolabelling NPs for single-photon emission computed tomography (SPECT) use.

High spatio-temporal resolution measurement of A1 R and A2A R interactions combined with Iem-spFRET and E-FRET methods

A₁ R-A_2A R heterodimers regulate striatal glutamatergic neurotransmission. However, few researches about kinetics have been reported. Here, we combined Iem-spFRET and E-FRET to investigate the kinetics of A₁ R and A_2A R interaction. Iem-spFRET obtains the energy transfer efficiency of the whole cell. E-FRET gets energy transfer efficiency with high spatial resolution, whereas, it was prone to biases because background was easily selected due to manual operation. To study the interaction with high spatio-temporal resolution, Iem-spFRET was used to correct the deviation of E-FRET. In this paper, A₁ R and A_2A R interaction was monitored, and the changes of FRET efficiency of the whole or/and partial cell membrane were described. The results showed that activation of A₁ R or A_2A R leads to rapid aggregation, inhibition of A₁ R or A_2A R leads to slow segregation, and the interaction is reversible. These results demonstrated that combination of Iem-spFRET and E-FRET could measure A₁ R and A_2A R interaction with high spatio-temporal resolution.

Multifunctional self-assembled peptide nanoparticles for multimodal imaging-guided enhanced theranostic applications against glioblastoma multiforme

The synthesis of self-assembled peptide nanoparticles using a facile one-pot synthesis approach is gaining increasing attention, allowing therapy in combination with diagnosis. Their drawback is limited diagnostic potential, which can be improved after necessary modifications and efficacious functionalization. Herein, a cyclic heptapeptide having the Arg-Gly-Asp-Lys-Leu-Ala-Lys sequence was modified by conjugation of the ε-amino group of the terminal lysine residue with diethylenetriamine pentaacetic acid (DTPA) as a bifunctional chelating agent (BFC) for radiolabeling with a γ-emitting radionuclide (^99mTc, half-life 6.01 h; energy 140 keV). Further, the free amino group of the middle lysine residue was successfully conjugated with near-infrared fluorescence (NIRF) dye Cyanine5.5 N-succinimidyl ester (Ex/Em = 670/701 nm) by a co-assembly method to form newly designed novel NIRF dye conjugated self-assembled peptide-DTPA (Cy5.5@SAPD) nanoparticles. The fluorescent nanoparticle formation was confirmed by using a fluorescence spectrophotometer (Ex/Em = 650/701 nm), and the transmission electron microscope (TEM) images showed a size of ∼ 40 nm with a lattice fringe distance of 0.294 nm. Cytotoxicity and confocal laser scanning microscopy (CLSM) studies showed that these nanoparticles possess a high affinity for the α_vβ₃-integrin receptor overexpressed on brain tumor glioblastoma with an EC₅₀ = 20 μM. Moreover, these nanoparticles were observed to have potential to internalize into U87MG cells more prominently than HEK-293 cancer cells and induce apoptosis. The apoptosis assay showed 79.5% apoptotic cells after 24 h treatment of Cy5.5@SAPD nanoparticles. Additionally, these nanoparticles were also radiolabeled with ^99mTc for the single photon emission computed tomography (SPECT) imaging study in tumor-bearing female Balb/c mice. The excellent imaging feature of Cy5.5@SAPD-^99mTc nanoparticles as a multimodal (SPECT/NIRF) diagnostic probe, as well as noteworthy therapeutic potential was observed. The results suggested that our newly designed novel dual-targeting dual-imaging nanoparticles may serve as an admirable theranostic probe to treat brain tumor glioblastoma multiforme.

Hybridization of tumor homing and mitochondria-targeting peptide domains to design novel dual-imaging self-assembled peptide nanoparticles for theranostic applications

A novel hybridized dual-targeting peptide-based nanoprobe was successfully designed by using the cyclic heptapeptide. This peptide has Arg-Gly-Asp-Lys-Leu-Ala-Lys sequence, in which the RGD homing motif and KALK mitochondria-targeting motif were linked via amide bond. The designed peptide probe was further modified through covalent linkage to induce dual-imaging functionality, and self-assembled to form spherical nanoparticles. The novel Cy5.5-SAPD-^99mTc nanoparticles were tested for in vitro cytotoxicity, cellular uptake, and apoptosis-inducing functionalities. The cellular internalization, enhanced cytotoxicity and selective receptor binding capabilities against U87MG cells, excellent dual-imaging potential, improved apoptosis-inducing feature by damaging mitochondria, and in vivo preclinical investigations suggested that our newly designed novel hybridized peptide-based dual-imaging nanoparticles may serve as an admirable theranostic probe to treat brain tumor glioblastoma multiforme. This study describes the development of dual-targeting self-assembled peptide nanoparticles followed by modifications using NIRF dye and radiolabeled with ^99mTc for dual-imaging and enhanced therapeutic efficacy against brain tumor.

Radiolabeled cyclic RGD peptides as radiotracers for tumor imaging

The integrin family comprises 24 transmembrane receptors, each a heterodimeric combination of one of 18α and one of 8β subunits. Their main function is to integrate the cell adhesion and interaction with the extracellular microenvironment with the intracellular signaling and cytoskeletal rearrangement through transmitting signals across the cell membrane upon ligand binding. Integrin α_vβ₃ is a receptor for the extracellular matrix proteins containing arginine–glycine–aspartic (RGD) tripeptide sequence. The α_vβ₃ is generally expressed in low levels on the epithelial cells and mature endothelial cells, but it is highly expressed in many solid tumors. The α_vβ₃ levels correlate well with the potential for tumor metastasis and aggressiveness, which make it an important biological target for development of antiangiogenic drugs, and molecular imaging probes for early tumor diagnosis. Over the last decade, many radiolabeled cyclic RGD peptides have been evaluated as radiotracers for imaging tumors by SPECT or PET. Even though they are called “α_vβ₃-targeted” radiotracers, the radiolabeled cyclic RGD peptides are also able to bind α_vβ₅, α₅β₁, α₆β₄, α₄β₁, and α_vβ₆ integrins, which may help enhance their tumor uptake due to the “increased receptor population.” This article will use the multimeric cyclic RGD peptides as examples to illustrate basic principles for development of integrin-targeted radiotracers and focus on different approaches to maximize their tumor uptake and T/B ratios. It will also discuss important assays for pre-clinical evaluations of the integrin-targeted radiotracers, and their potential applications as molecular imaging tools for noninvasive monitoring of tumor metastasis and early detection of the tumor response to antiangiogenic therapy.

Comparison of biological properties of 99mTc-labeled cyclic RGD Peptide trimer and dimer useful as SPECT radiotracers for tumor imaging

INTRODUCTION

This study sought to evaluate a ^99mTc-labeled trimeric cyclic RGD peptide (^99mTc-4P-RGD₃) as the new radiotracer for tumor imaging. The objective was to compare its biological properties with those of ^99mTc-3P-RGD₂ in the same animal model.

METHODS

HYNIC-4P-RGD₃ was prepared by reacting 4P-RGD₃ with excess HYNIC-OSu in the presence of diisopropylethylamine. ^99mTc-4P-RGD₃ was prepared using a kit formulation, and evaluated for its tumor-targeting capability and biodistribution properties in the BALB/c nude mice with U87MG human glioma xenografts. Planar and SPECT imaging studies were performed in athymic nude mice with U87MG glioma xenografts. For comparison purpose, ^99mTc-3P-RGD₂ (a α_vβ₃-targeted radiotracer currently under clinical evaluation for tumor imaging in cancer patients) was also evaluated in the same animal models. Blocking experiments were used to demonstrate the α_vβ₃ specificity of ^99mTc-4P-RGD₃.

RESULTS

^99mTc-4P-RGD₃ was prepared with >95% RCP and high specific activity (~200GBq/μmol). ^99mTc-4P-RGD₃ and ^99mTc-3P-RGD₂ shared almost identical tumor uptake and similar biodistribution properties. ^99mTc-4P-RGD₃ had higher uptake than ^99mTc-3P-RGD₂ in the intestines and kidneys; but it showed better metabolic stability. The U87MG tumors were clearly visualized by SPECT with excellent contrast with ^99mTc-4P-RGD₃ and ^99mTc-3P-RGD₂.

CONCLUSION

Increasing peptide multiplicity from 3P-RGD₂ to 4P-RGD₃ offers no advantages with respect to the tumor-targeting capability. ^99mTc-4P-RGD₃ is as good a SPECT radiotracer as ^99mTc-3P-RGD₂ for imaging α_vβ₃-positive tumors.

p-SCN-Bn-HOPO: A Superior Bifunctional Chelator for (89)Zr ImmunoPET

Zirconium-89 has an ideal half-life for use in antibody-based PET imaging; however, when used with the chelator DFO, there is an accumulation of radioactivity in the bone, suggesting that the (89)Zr(4+) cation is being released in vivo. Therefore, a more robust chelator for (89)Zr could reduce the in vivo release and the dose to nontarget tissues. Evaluation of the ligand 3,4,3-(LI-1,2-HOPO) demonstrated efficient binding of (89)Zr(4+) and high stability; therefore, we developed a bifunctional derivative, p-SCN-Bn-HOPO, for conjugation to an antibody. A Zr-HOPO crystal structure was obtained showing that the Zr is fully coordinated by the octadentate HOPO ligand, as expected, forming a stable complex. p-SCN-Bn-HOPO was synthesized through a novel pathway. Both p-SCN-Bn-HOPO and p-SCN-Bn-DFO were conjugated to trastuzumab and radiolabeled with (89)Zr. Both complexes labeled efficiently and achieved specific activities of approximately 2 mCi/mg. PET imaging studies in nude mice with BT474 tumors (n = 4) showed good tumor uptake for both compounds, but with a marked decrease in bone uptake for the (89)Zr-HOPO-trastuzumab images. Biodistribution data confirmed the lower bone activity, measuring 17.0%ID/g in the bone at 336 h for (89)Zr-DFO-trastuzumab while (89)Zr-HOPO-trastuzumab only had 2.4%ID/g. We successfully synthesized p-SCN-Bn-HOPO, a bifunctional derivative of 3,4,3-(LI-1,2-HOPO) as a potential chelator for (89)Zr. In vivo studies demonstrate the successful use of (89)Zr-HOPO-trastuzumab to image BT474 breast cancer with low background, good tumor to organ contrast, and, importantly, very low bone uptake. The reduced bone uptake seen with (89)Zr-HOPO-trastuzumab suggests superior stability of the (89)Zr-HOPO complex.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

Comparison of biological properties of (111)In-labeled dimeric cyclic RGD peptides

INTRODUCTION

In this study two (111)In-labeled dimeric cyclic RGD peptides, (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2), were evaluated as radiotracers for breast tumor imaging. The objective was to evaluate the impact of SAA, PEG2 and 1,2,3-triazole linkers as compare to PEG4 on the tumor uptake and excretion kinetics of (111)In radiotracers.

METHODS

DOTA-Galacto-RGD2 was prepared by conjugation of Galacto-RGD2 with DOTA-OSu in the presence of diisopropylethylamine. Its integrin αvβ3 binding affinity was determined using a whole-cell displacement assay against (125)I-echistatin bound to U87MG glioma cells, and was compared with those of c(RGDfK), DOTA-3P-RGD2 and DOTA-3P-RGK2 (a nonsense peptide conjugate with "scrambled" RGK sequences). (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) were prepared and evaluated for their tumor-targeting capability and biodistribution properties in athymic nude mice bearing MDA-MB-435 breast tumor xenografts. Planar imaging studies were performed to demonstrate the utility of (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) for breast tumor imaging.

RESULTS

IC50 values of DOTA-Galacto-RGD2, DOTA-3P-RGD2, and DOTA-3P-RGK2 were calculated to be 27±2, 29±4, 596±48nM, respectively. The tumor uptake values of (111)In(DOTA-Galacto-RGD2) (6.79±0.98, 6.56±0.56, 4.17±0.61 and 1.09±0.13 %ID/g at 1, 4, 24 and 72hours p.i., respectively) were almost identical to those of (111)In(DOTA-3P-RGD2) (6.17±1.65, 5.94±0.84, 3.40±0.50 and 0.99±0.20 %ID/g, respectively). (111)In(DOTA-Galacto-RGD2) had a faster clearance from blood and muscle than (111)In(DOTA-3P-RGD2), leading to higher tumor/blood and tumor/muscle ratios. (111)In(DOTA-3P-RGD2) had lower liver uptake and better tumor/liver ratios than (111)In(DOTA-Galacto-RGD2). The tumor uptake of (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) was both integrin αvβ3 and RGD-specific. Imaging data suggest that (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) are useful as radiotracers for imaging integrin αvβ3-positive breast tumors.

CONCLUSION

The results from this study suggest that replacing PEG4 linkers between two RGD moieties with a pair of SAA, PEG2 and 1,2,3-triazole groups has little impact on integrin αvβ3 binding affinity and tumor uptake of (111)In-labeled dimeric cyclic RGD peptides. Despite the subtle differences in their excretion kinetics from noncancerous tissues, (111)In(DOTA-Galacto-RGD2) and (111)In(DOTA-3P-RGD2) are useful radiotracers for imaging integrin αvβ3-positive breast tumors.

Impact of multiple negative charges on blood clearance and biodistribution characteristics of 99mTc-labeled dimeric cyclic RGD peptides

This study sought to evaluate the impact of multiple negative charges on blood clearance kinetics and biodistribution properties of ^99mTc-labeled RGD peptide dimers. Bioconjugates HYNIC-P6G-RGD₂ and HYNIC-P6D-RGD₂ were prepared by reacting P6G-RGD₂ and P6D-RGD₂, respectively, with excess HYNIC-OSu in the presence of diisopropylethylamine. Their IC₅₀ values were determined to be 31 ± 5 and 41 ± 6 nM, respectively, against ¹²⁵I-echistatin bound to U87MG glioma cells in a whole-cell displacement assay. Complexes [^99mTc(HYNIC-P6G-RGD₂)(tricine)(TPPTS)] (^99mTc-P6G-RGD₂) and [^99mTc(HYNIC-P6D-RGD₂)(tricine)(TPPTS)] (^99mTc-P6D-RGD₂) were prepared in high radiochemical purity (RCP > 95%) and specific activity (37–110 GBq/μmol). They were evaluated in athymic nude mice bearing U87MG glioma xenografts for their biodistribution. The most significant difference between ^99mTc-P6D-RGD₂ and ^99mTc-P6G-RGD₂ was their blood radioactivity levels and tumor uptake. The initial blood radioactivity level for ^99mTc-P6D-RGD₂ (4.71 ± 1.00%ID/g) was ∼5× higher than that of ^99mTc-P6G-RGD₂ (0.88 ± 0.05%ID/g), but this difference disappeared at 60 min p.i. ^99mTc-P6D-RGD₂ had much lower tumor uptake (2.20–3.11%ID/g) than ^99mTc-P6G-RGD₂ (7.82–9.27%ID/g) over a 2 h period. Since HYNIC-P6D-RGD₂ and HYNIC-P6G-RGD₂ shared a similar integrin α_vβ₃ binding affinity (41 ± 6 nM versus 31 ± 5 nM), the difference in their blood activity and tumor uptake is most likely related to the nine negative charges and high protein binding of ^99mTc-P6D-RGD₂. Despite its low uptake in U87MG tumors, the tumor uptake of ^99mTc-P6D-RGD₂ was integrin α_vβ₃-specific. SPECT/CT studies were performed using ^99mTc-P6G-RGD₂ in athymic nude mice bearing U87MG glioma and MDA-MB-231 breast cancer xenografts. The SPECT/CT data demonstrated the tumor-targeting capability of ^99mTc-P6G-RGD₂, and its tumor uptake depends on the integrin α_vβ₃ expression levels on tumor cells and neovasculature. It was concluded that the multiple negative charges have a significant impact on the blood clearance kinetics and tumor uptake of ^99mTc-labeled dimeric cyclic RGD peptides.

Tumour targeting with radiometals for diagnosis and therapy

Use of radiometals in nuclear oncology is a rapidly growing field and encompasses a broad spectrum of radiotracers for imaging via PET (positron emission tomography) or SPECT (single-photon emission computed tomography) and therapy via α, β(-), or Auger electron emission. This feature article opens with a brief introduction to the imaging and therapy modalities exploited in nuclear medicine, followed by a discussion of the multi-component strategy used in radiopharmaceutical development, known as the bifunctional chelate (BFC) method. The modular assembly is dissected into its individual components and each is discussed separately. The concepts and knowledge unique to metal-based designs are outlined, giving insight into how these radiopharmaceuticals are evaluated for use in vivo. Imaging nuclides (64)Cu, (68)Ga, (86)Y, (89)Zr, and (111)In, and therapeutic nuclides (90)Y, (177)Lu, (225)Ac, (213)Bi, (188)Re, and (212)Pb will be the focus herein. Finally, key examples have been extracted from the literature to give the reader a sense of breadth of the field.

(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.

Evaluation of K(HYNIC)(2) as a bifunctional chelator for (99m)Tc-labeling of small biomolecules

This study sought to evaluate K(HYNIC)(2) (K = lysine and HYNIC = 6-hydrazinonicotinyl) as a bifunctional chelator for (99m)Tc-labeling of biomolecule. In this study, four K(HYNIC)(2)-conjugated cyclic RGD peptides, K(HYNIC)(2)-RGD(2) (RGD(2) = E[c(RGDfK)](2)), K(HYNIC)(2)-3G-RGD(2) (3G-RGD(2) = Gly-Gly-Gly-E[Gly-Gly-Gly-c(RGDfK)](2)), K(HYNIC)(2)-2P-RGD(2) (2P-RGD(2) = E[PEG4-c(RGDfK)](2), and PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid), and K(HYNIC)(2)-3P-RGD(2) (3P-RGD(2) = PEG4-E[PEG4-c(RGDfK)]2) were prepared, and evaluated for their integrin αvβ3 binding affinity. IC(50) values were determined to be 47 ± 2, 35 ± 2, 37 ± 2, 85 ± 2, and 422 ± 15 nM for K(HYNIC)(2)-2P-RGD(2), K(HYNIC)(2)-3P-RGD(2), K(HYNIC)(2)-3G-RGD(2), K(HYNIC)(2)-RGD(2), and c(RGDyK), respectively, against (125)I-echistatin bound to U87MG cells. Macrocyclic complexes [(99m)Tc(K(HYNIC)(2)-RGD(2))(tricine)] (1), [(99m)Tc(K(HYNIC)(2)-3G-RGD(2))(tricine)] (2), [(99m)Tc(K(HYNIC)(2)-2P-RGD(2))(tricine)] (3), and [(99m)Tc(K(HYNIC)(2)-3P-RGD(2))(tricine)] (4) were prepared, and evaluated in athymic nude mice bearing U87MG glioma xenografts for their tumor targeting capability and biodistribution. It was found that 1-4 all had high solution stability and more than two isomers, as evidenced by the presence of multiple radiometric peaks in their radio-HPLC chromatograms. The tumor uptake of 1-4 was 3.78 ± 0.81, 7.46 ± 1.68, 9.74 ± 1.65, and 8.59 ± 1.52%ID/g, respectively, which was completely consistent with trend of integrin α(v)β(3) binding affinity for cyclic RGD peptides. Replacing [(99m)Tc(HYNIC)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3"-trisulfonate) with [(99m)Tc(K(HYNIC)(2))(tricine)] had little impact on radiotracer tumor uptake; but it had significant effect on the uptake of radiotracer in kidneys, lungs, and spleen. The tumor was clearly visualized by SPECT/CT with excellent contrast in a glioma-bearing mouse administered with 4. K(HYNIC)(2) would be particularly useful for (99m)Tc-labeling of small biomolecules with one or more disulfide linkages.

Evaluation of In-Labeled Cyclic RGD Peptides: Effects of Peptide and Linker Multiplicity on Their Tumor Uptake, Excretion Kinetics and Metabolic Stability

Purpose: The purpose of this study was to demonstrate the valence of cyclic RGD peptides, P-RGD (PEG₄-c(RGDfK): PEG₄ = 15-amino-4,710,13-tetraoxapentadecanoic acid), P-RGD₂ (PEG₄-E[c(RGDfK)]₂, 2P-RGD₄ (E{PEG₄-E[c(RGDfK)]₂}₂, 2P4G-RGD₄ (E{PEG₄-E[G₃-c(RGDfK)]₂}₂: G₃ = Gly-Gly-Gly) and 6P-RGD₄ (E{PEG₄-E[PEG₄-c(RGDfK)]₂}₂) in binding to integrin α_vβ₃, and to assess the impact of peptide and linker multiplicity on biodistribution properties, excretion kinetics and metabolic stability of their corresponding ¹¹¹In radiotracers.

Methods: Five new RGD peptide conjugates (DOTA-P-RGD (DOTA =1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracetic acid), DOTA-P-RGD₂, DOTA-2P-RGD₄, DOTA-2P4G-RGD₄, DOTA-6P-RGD₄), and their ¹¹¹In complexes were prepared. The integrin α_vβ₃ binding affinity of cyclic RGD conjugates were determined by a competitive displacement assay against ¹²⁵I-c(RGDyK) bound to U87MG human glioma cells. Biodistribution, planar imaging and metabolism studies were performed in athymic nude mice bearing U87MG human glioma xenografts.

Results: The integrin α_vβ₃ binding affinity of RGD conjugates follows the order of: DOTA-6P-RGD₄ (IC₅₀ = 0.3 ± 0.1 nM) ~ DOTA-2P4G-RGD₄ (IC₅₀ = 0.2 ± 0.1 nM) ~ DOTA-2P-RGD₄ (IC₅₀ = 0.5 ± 0.1 nM) > DOTA-3P-RGD₂ (DOTA-PEG₄-E[PEG₄-c(RGDfK)]₂: IC₅₀ = 1.5 ± 0.2 nM) > DOTA-P-RGD₂ (IC₅₀ = 5.0 ± 1.0 nM) >> DOTA-P-RGD (IC₅₀ = 44.3 ± 3.5 nM) ~ c(RGDfK) (IC₅₀ = 49.9 ± 5.5 nM) >> DOTA-6P-RGK₄ (IC₅₀ = 437 ± 35 nM). The fact that DOTA-6P-RGK₄ had much lower integrin α_vβ₃ binding affinity than DOTA-6P-RGD₄ suggests that the binding of DOTA-6P-RGD₄ to integrin α_vβ₃ is RGD-specific. This conclusion is consistent with the lower tumor uptake for ¹¹¹In(DOTA-6P-RGK₄) than that for ¹¹¹In(DOTA-6P-RGD₄). It was also found that the G₃ and PEG₄ linkers between RGD motifs have a significant impact on the integrin α_vβ₃-targeting capability, biodistribution characteristics, excretion kinetics and metabolic stability of ¹¹¹In-labeled cyclic RGD peptides.

Conclusion: On the basis of their integrin α_vβ₃ binding affinity and tumor uptake of their corresponding ¹¹¹In radiotracers, it was conclude that 2P-RGD₄, 2P4G-RGD₄ and 6P-RGD₄ are most likely bivalent in binding to integrin α_vβ₃, and extra RGD motifs might contribute to the long tumor retention times of ¹¹¹In(DOTA-2P-RGD₄), ¹¹¹In(DOTA-2P4G-RGD₄) and ¹¹¹In(DOTA-6P-RGD₄) than that of ¹¹¹In(DOTA-3P-RGD₃) at 72 h p.i. Among the ¹¹¹In-labeled cyclic RGD tetramers evaluated in the glioma model, ¹¹¹In(DOTA-2P4G-RGD₄) has very high tumor uptake with the best tumor/kidney and tumor/liver ratios, suggesting that ⁹⁰Y(DOTA-2P4G-RGD₄) and ¹⁷⁷Lu(DOTA-2P4G-RGD₄) might have the potential for targeted radiotherapy of integrin α_vβ₃-positive tumors.

Radiolabeled Cyclic RGD Peptides as Radiotracers for Imaging Tumors and Thrombosis by SPECT

The integrin family is a group of transmembrane glycoprotein comprised of 19 α- and 8 β-subunits that are expressed in 25 different α/β heterodimeric combinations on the cell surface. Integrins play critical roles in many physiological processes, including cell attachment, proliferation, bone remodeling, and wound healing. Integrins also contribute to pathological events such as thrombosis, atherosclerosis, tumor invasion, angiogenesis and metastasis, infection by pathogenic microorganisms, and immune dysfunction. Among 25 members of the integrin family, the α(v)β(3) is studied most extensively for its role of tumor growth, progression and angiogenesis. In contrast, the α(IIb)β(3 )is expressed exclusively on platelets, facilitates the intercellular bidirectional signaling ("inside-out" and "outside-in") and allows the aggregation of platelets during vascular injury. The α(IIb)β(3) plays an important role in thrombosis by its activation and binding to fibrinogen especially in arterial thrombosis due to the high blood flow rate. In the resting state, the α(IIb)β(3) on platelets does not bind to fibrinogen; on activation, the conformation of platelet is altered and the binding sites of α(IIb)β(3 )are exposed for fibrinogen to crosslink platelets. Over the last two decades, integrins have been proposed as the molecular targets for diagnosis and therapy of cancer, thrombosis and other diseases. Several excellent review articles have appeared recently to cover a broad range of topics related to the integrin-targeted radiotracers and their nuclear medicine applications in tumor imaging by single photon emission computed tomography (SPECT) or a positron-emitting radionuclide for positron emission tomography (PET). This review will focus on recent developments of α(v)β(3)-targeted radiotracers for imaging tumors and the use of α(IIb)β(3)-targeted radiotracers for thrombosis imaging, and discuss different approaches to maximize the targeting capability of cyclic RGD peptides and improve the radiotracer excretion kinetics from non-cancerous organs. Improvement of target uptake and target-to-background ratios is critically important for target-specific radiotracers.

Molecular imaging of angiogenesis with SPECT

Single-photon emission computed tomography (SPECT) and position emission tomography (PET) are the two main imaging modalities in nuclear medicine. SPECT imaging is more widely available than PET imaging and the radionuclides used for SPECT are easier to prepare and usually have a longer half-life than those used for PET. In addition, SPECT is a less expensive technique than PET. Commonly used gamma emitters are: (99m)Tc (E(max) 141 keV, T (1/2) 6.02 h), (123)I (E(max) 529 keV, T (1/2) 13.0 h) and (111)In (E(max) 245 keV, T (1/2) 67.2 h). Compared to clinical SPECT, PET has a higher spatial resolution and the possibility to more accurately estimate the in vivo concentration of a tracer. In preclinical imaging, the situation is quite different. The resolution of microSPECT cameras (<0.5 mm) is higher than that of microPET cameras (>1.5 mm). In this report, studies on new radiolabelled tracers for SPECT imaging of angiogenesis in tumours are reviewed.