Imaging of angiogenesis: from morphology to molecules and from bench to bedside

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.

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.

A Cy5.5-labeled phage-displayed peptide probe for near-infrared fluorescence imaging of tumor vasculature in living mice

Near-infrared (NIR) fluorescence optical imaging is an emerging imaging technique for studying diseases at the molecular level. Optical imaging with a NIR emitting fluorophore for targeting tumor vasculature offers a noninvasive method for early detection of tumor angiogenesis and efficient monitoring of response to anti-tumor vasculature therapy. The previous in vitro results demonstrated that the GX1 peptide, identified by phage-display technology, is a tumor vasculature endothelium-specific ligand. In this report, Cy5.5-conjugated GX1 peptide was evaluated in a subcutaneous U87MG glioblastoma xenograft model to investigate tumor-targeting efficacy. The in vitro flow cytometry results revealed dose-dependent binding of Cy5.5-GX1 peptide to U87MG glioma cells. In vivo optical imaging with the Cy5.5-GX1 probe exhibited rapid U87MG tumor targeting at 0.5 h p.i., and high tumor-to-background contrast at 4 h p.i. Tumor specificity of Cy5.5-GX1 was confirmed by effective blocking of tumor uptake in the presence of unlabeled GX1 peptide (20 mg/kg). Ex vivo imaging further confirmed in vivo imaging findings, and demonstrated that Cy5.5-GX1 has a tumor-to-muscle ratio (15.21 ± 0.84) at 24 h p.i. for the non-blocked group and significantly decreased ratio (6.95 ± 0.75) for the blocked group. In conclusion, our studies suggest that Cy5.5-GX1 is a promising molecular probe for optical imaging of tumor vasculature.