Monitoring tumor response to linifanib therapy with SPECT/CT using the integrin αvβ3-targeted radiotracer 99mTc-3P-RGD2

Achievement in active agent structures as a power tools in tumor angiogenesis imaging

According to World Health Organization (WHO) cancer is the second most important cause of death globally. Because angiogenesis is considered as an essential process of growth, proliferation and tumor progression, within this review we decided to shade light on recent development of chemical compounds which play a significant role in its imaging and monitoring. Indeed, the review gives insight about the current achievements of active agents structures involved in imaging techniques such as: positron emission computed tomography (PET), magnetic resonance imaging (MRI) and single photon emission computed tomography (SPECT), as well as combination PET/MRI and PET/CT. The review aims to provide the journal audience with a comprehensive and in-deep understanding of chemistry policy in tumor angiogenesis imaging.

99mTc-3PRGD2 SPECT Predicts the Outcome of Endostar and Cisplatin Therapy in Xenograft Animals

Aims:

Our study was designed to investigate the usefulness of ^99mTc-3PRGD₂ single-photon emission computed tomography (SPECT) for noninvasively monitoring the response of integrin α_vβ₃ expression to antiangiogenic treatment with endostar and cisplatin in xenograft animals.

Methods:

^99mTc-3PRGD₂ SPECT imaging was performed at days 0, 7, 14, and 21. Tumors were harvested at all imaging time points for Western blotting and histopathological analysis.

Result:

In ^99mTc-3PRGD₂ SPECT imaging, the radioactivity accumulation of NaCl group rised gradually in the first half and dispersed on day 21 due to the necrosis of the tumor. While the radioactivity accumulation of treated groups gradually decreased throughout the course. The downtrend of tumor to nontumor ratio in endostar-treated group was more remarkable than cisplatin-treated group. The expression of intergrin α_vβ₃ of treated groups was lower than NaCl group from day 14. The expression of intergrin α_vβ₃ of endostar-treated group was significantly lower than cisplatin-treated group from baseline onward.

Conclusion:

It’s demonstrated that the ^99mTc-3PRGD₂ could noninvasively visualize and semiquantify tumor angiogenesis in the xenograft model and monitor the response to the antiangiogenic therapy of endostar and cisplatin effectively. It also can predict the outcome of endostar and cisplatin therapy in xenograft animals.

99mTc-3PRGD2 single-photon emission computed tomography/computed tomography for the diagnosis of choroidal melanoma: A preliminary STROBE-compliant observational study

Recent successes in monitoring and diagnosing a variety of tumors using Tc-PEG4-E[PEG4-c(RGDfK)]2 (Tc-3PRGD2) single-photon emission computed tomography (SPECT) imaging encouraged us to expand the use of this tracer. This case-control study aimed to evaluate the feasibility of Tc-3PRGD2 imaging for detecting choroidal melanoma (CM) and for monitoring tumor response to plaque brachytherapy (PB). Ten consecutive patients with CM who underwent Tc-3PRGD2 imaging before and 3 months after PB were reviewed. The tumor-to-occipital bone (T/O) and mirrored contralateral normal tissue-to-occipital bone (N/O) ratios were calculated by region of interest analysis at baseline and 3 months post-PB. T/O values were compared between patients with CM with comorbid secondary retinal detachment (RD) and those without RD. The relationship between T/O value and tumor volume was also investigated. Tc-3PRGD2 SPECT/CT showed focal uptake in CM. The mean T/O ratio before PB was 1.90 ± 1.26 and the mean N/O ratio was 0.80 ± 0.21 (P = .02). The Tc-3PRGD2 concentrations in 5 patients with CM with RD were higher (T/O = 2.69 ± 1.39) than in those without secondary RD (T/O = 1.10 ± 0.18) (P = .008). T/O ratios at 3 months post-PB were significantly lower than that at baseline (1.23 ± 0.59, P = .03). There was a linear relationship between T/O and tumor volume (y-hat = 0.028 + 0.003x, R = 0.768, P = .001). The 95% confidence interval for the (T/O)/volume ratio was 0.002 to 0.005. Tc-3PRGD2 imaging is a feasible modality for the diagnosis of CM. Furthermore, follow-up for at least 20 months after PB indicated that coanalysis of Tc-3PRGD2 imaging and tumor volume may provide a promising prognostic predictor in patients with CM.

An optimized LC-MS/MS method for determination of HYNIC-3PRGD2, a new promising imaging agent for tumor targeting, in rat plasma and its application

HYNIC-3PRGD₂ is used to prepare a new ^99mTc-radiolabeled tracer. HYNIC-3PRGD₂, which has a high binding affinity for the integrin α_vβ₃ due to its special structure, has become a promising tumor imaging agent for diagnosis and monitor of the clinical response to therapeutic effects of anti-tumor agents. Here, we developed and validated a method for determination of HYNIC-3PRGD₂ concentration in rat plasma using ultra-high performance liquid chromatography-tandem mass spectrometry system. Following sample extraction by methanol precipitation, satisfactory separation through chromatography was achieved on an hydrophilic reverse-phase C₁₈ column AQ (2.1 mm × 100 mm, 2.7 μm) at a flow rate of 0.2 mL·min^-1 with an gradient elution using mobile phase consisting of ultrapure water and acetonitrile fortified with 0.1% formic acid respectively. The calibration curve was developed over a linear range of 3.125-100 ng·mL^-1 with the lower limit of quantification of 3.125 ng·mL^-1. The HYNIC-3PRGD₂ and its internal standard c(RGDfK)(RK5) were detected and quantified with the multiple reaction monitoring (MRM) mode on a triple-quadrupole tandem mass spectrometer. This method was successfully validated and applied for pharmacokinetic evaluation of HYNIC-3PRGD₂ during pre-clinical experiments.

Cyclic RGD peptide-modified liposomal drug delivery system for targeted oral apatinib administration: enhanced cellular uptake and improved therapeutic effects

Apatinib is an oral tyrosine kinase inhibitor, which selectively targets vascular endothelial growth factor receptor 2 and has the potential to treat many tumors therapeutically. Cyclic arginylglycylaspartic acid (cRGD)- and polyethylene glycol (PEG)-modified liposomes (cRGD-Lipo-PEG) were constructed to act as a targeted delivery system for the delivery of apatinib to the human colonic cancer cell line, HCT116. These cRGD-modified liposomes specifically recognized integrin α_vβ₃ and exhibited greater uptake efficiency with respect to delivering liposomes into HCT116 cells when compared to nontargeted liposomes (Lipo-PEG), as well as greater death of tumor cells and apoptosis. The mechanism by which cRGD-Lipo-PEG targets cells was elucidated further with competition assays. To determine the anticancer efficacy in vivo, nude mice were implanted with HCT116 xenografts and treated with apatinib-loaded liposomes or free apatinib intravenously or via intragastric administration. The active and passive targeting of cRGD-Lipo-PEG led to significant tumor treatment targeting ability, better inhibition of tumor growth, and less toxicity when compared with treatments using uncombined apatinib. The results presented strongly support the case for cRGD-Lipo-PEG representing a targeted delivery system for apatinib in the treatment of colonic cancer.

Simultaneous SPECT imaging of multi-targets to assist in identifying hepatic lesions

Molecular imaging technique is an attractive tool to detect liver disease at early stage. This study aims to develop a simultaneous dual-isotope single photon emission computed tomography (SPECT)/CT imaging method to assist diagnosis of hepatic tumor and liver fibrosis. Animal models of liver fibrosis and orthotopic human hepatocellular carcinoma (HCC) were established. The tracers of ¹³¹I-NGA and ^99mTc-3P-RGD₂ were selected to target asialoglycoprotein receptor (ASGPR) on the hepatocytes and integrin α_vβ₃ receptor in tumor or fibrotic liver, respectively. SPECT imaging and biodistribution study were carried out to verify the feasibility and superiority. As expected, ^99mTc-3P-RGD₂ had the ability to evaluate liver fibrosis and detect tumor lesions. ¹³¹I-NGA showed that it was effective in assessing the anatomy and function of the liver. In synchronized dual-isotope SPECT/CT imaging, clear fusion images can be got within 30 minutes for diagnosing liver fibrosis and liver cancer. This new developed imaging approach enables the acquisition of different physiological information for diagnosing liver fibrosis, liver cancer and evaluating residual functional liver volume simultaneously. So synchronized dual-isotope SPECT/CT imaging with ^99mTc-3P-RGD₂ and ¹³¹I-NGA is an effective approach to detect liver disease, especially liver fibrosis and liver cancer.

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.

Biometabolic Distribution of 99mTc-3PRGD2 and Its Potential Value in Monitoring Chemotherapeutic Effects

Previous studies have reported that 99mTc-3PRGD2 is an excellent tumor imaging agent that showed a good correlation with integrin αvβ3, a main factor of tumor-induced angiogenesis. In this study, we investigated the biometabolic distribution characteristics of 99mTc-3PRGD2 with a continuous dynamic acquisition mode to explore the potential value of 99mTc-3PRGD2 in monitoring chemotherapeutic effects in VX2 tumor models. Eighteen rabbits with 27 implanted VX2 squamous cell tumors were randomly divided into a nontreated control group (NTG, n = 8; 12 tumors) and a treatment group (TG, n = 10; 15 tumors). 99mTc-3PRGD2 imaging was performed prior to cisplatin injection and repeated on days 0, 1, 7, and 14 postinjection. Continuous dynamic scanning up to 30 minutes; static imaging at 0.5 hours, 1 hour, and 3 hours; and single-photon emission computed tomography/computed tomography (SPECT/CT)-integrated imaging at 3 hours post-99mTc-3PRGD2 injection were performed. The peak time (time to reach peak in dynamic curve), tumor to normal (T/N) ratios, and their change rates relative to pretherapy were calculated. Autoradiography, hematoxylin-eosin (H&E) staining, and CD31 and integrin αv immunohistochemical staining were examined. VX2 tumors were clearly visualized at 3 hours post-99mTc-3PRGD2 injection. Tumors in the TG shrank significantly on day 7 after cisplatin administration (p < .05). The half-life (t1/2) of the radiotracer in heart, liver, and tumor in the NTG were 3.43 ± 0.94 minutes, 13.41 ± 9.17 minutes, and 70.83 ± 33.37 minutes, respectively. The peak time was delayed in the TG immediately and continuously after cisplatin administration compared to the peak time in the NTG. The T/N values and their change rates decreased significantly in the TG compared to the NTG after therapy (p < .05). The immunostained areas were significantly decreased in the TG (p < .05) compared to the NTG. 99mTc-3PRGD2 was an excellent imaging agent for demonstrating tumor angiogenesis. The peak time, T/N values, and their change rates were sensitive parameters to monitor early chemotherapeutic effects. Due to the specific target mechanism and the cost-effective value of 99mTc-3PRGD2, 99mTc-3PRGD2 SPECT imaging may have potential in detecting the therapeutic effects of anticancer 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.

Exploration of peptide T7 and its derivative as integrin αvβ3-targeted imaging agents

OBJECTIVE

The aim of the present study was to develop potential candidates of integrin αvβ3-targeted imaging agent, which can facilitate the diagnosis and treatment of malignant solid tumors.

METHODS

Peptides derived from tumstatin, named T7 and T7-6H, were derivatized to contain histidine in the C-terminus of their sequence and were labeled with (99m)Tc via nitrido and carbonyl precursors. The radiochemical purity and stability of (99m)Tc-labeled T7 and T7-6H were characterized by thin-layer chromatography. The whole body biodistribution was studied in NCI-H157-bearing BALB/c nude mice.

RESULTS

The (99m)Tc-labeled T7 and T7-6H showed adequate in vitro stability, with a high radiochemical purity of over 90%. The dissociation constant (Kd) value of the (99m)Tc-labeled T7 and T7-6H ranged from 68.5 nM to 140.8 nM in U251 and NCI-H157 cell lines. (99m)Tc-labeled T7 and T7-6H showed no significant difference of biodistribution in mice. Furthermore, both T7 and T7-6H exhibited a poor blood-brain barrier penetration and a transient accumulation in lung; the uptake in tumor tissues was significantly higher than in muscle tissue, with a ratio of 5.8.

CONCLUSION

(99m)Tc-labeled T7 and T7-6H can be regarded as promising single-photon emission computed tomography probes for imaging integrin αvβ3, and need to be further studied for noninvasive detection of tumors.

FITC-conjugated cyclic RGD peptides as fluorescent probes for staining integrin αvβ3/αvβ5 in tumor tissues

This study sought to evaluate FITC-conjugated cyclic RGD peptides (FITC-RGD₂, FITC-3P-RGD₂, and FITC-Galacto-RGD₂) as fluorescent probes for in vitro assays of integrin α_vβ₃/α_vβ₅ expression in tumor tissues. FITC-RGD₂, FITC-3P-RGD₂, and FITC-Galacto-RGD₂ were prepared, and their integrin α_vβ₃/α_vβ₅ binding affinity was determined using the displacement assay against ¹²⁵I-echistatin bound to U87MG glioma cells. IC₅₀ values of FITC-Galacto-RGD₂, FITC-3P-RGD₂, and FITC-RGD₂ were calculated to be 28 ± 8, 32 ± 7, and 89 ± 17 nM, respectively. The integrin α_vβ₃/α_vβ₅ binding affinity followed a general trend: FITC-Galacto-RGD₂ ∼ FITC-3P-RGD₂ > FITC-RGD₂. The xenografted tumor-bearing models were established by subcutaneous injection of 5 × 10⁶ tumor cells into shoulder flank (U87MG, A549, HT29, and PC-3) or mammary fat pad (MDA-MB-435) of each athymic nude mouse. Three to six weeks after inoculation, the tumor size was 0.1–0.3 g. Tumors were harvested for integrin α_vβ₃/α_vβ₅ staining, as well as hematoxylin and eosin (H&E) staining. Six human carcinoma tissues (colon cancer, pancreatic cancer, lung adenocarcinoma, squamous cell lung cancer, gastric cancer, and esophageal cancer) were obtained from recently diagnosed cancer patients. Human carcinoma slides were deparaffinized in xylene, rehydrated with ethanol, and then used for integrin α_vβ₃/α_vβ₅ staining, as well as H&E staining. It was found that the tumor staining procedures with FITC-conjugated cyclic RGD peptides were much simpler than those with the fluorescence-labeled integrin α_vβ₃ antibodies. Since FITC-RGD₂, FITC-3P-RGD₂, and FITC-Galacto-RGD₂ were able to co-localize with the fluorescence-labeled integrin β₃ antibody, their tumor localization and tumor cell binding are integrin α_vβ₃-specific. Quantification of the fluorescent intensity in five xenografted tumors (U87MG, MDA-MB-435, A549, HT29, and PC-3) and six human carcinoma tissues revealed an excellent linear relationship between the relative integrin α_vβ₃/α_vβ₅ expression levels determined with FITC-Galacto-RGD₂ and those obtained with the fluorescence-labeled anti-human integrin β₃ antibody. There was also an excellent linear relationship between the tumor uptake (%ID/g) of ^99mTc-3P-RGD₂ (an integrin α_vβ₃/α_vβ₅-targeted radiotracer) and the relative integrin α_vβ₃/α_vβ₅ expression levels from the quantification of fluorescent intensity in the tumor tissues stained with FITC-Galacto-RGD₂. These results suggest that FITC-conjugated cyclic RGD peptides might be useful to correlate the in vitro findings with the in vivo imaging data from an integrin α_vβ₃/α_vβ₅-targeted radiotracer. The results from this study clearly showed that the FITC-conjugated cyclic RGD peptides (particularly FITC-3P-RGD₂ and FITC-Galacto-RGD₂) are useful fluorescent probes for assaying relative integrin α_vβ₃/α_vβ₅ expression levels in tumor tissues.

(99m)Tc-3P-RGD2 micro-single-photon emission computed tomography/computed tomography provides a rational basis for integrin αvβ3-targeted therapy

PURPOSE

This study was to demonstrate the utility of (99m)Tc-3P-RGD2 micro-single-photon emission computed tomography/computed tomography (SPECT/CT) for the integrin αvβ3 expression quantification in NCI-H446 and A549 lung cancer xenografts.

MATERIALS AND METHODS

(99m)Tc-3P-RGD2 was prepared with high radiochemical purity (97%±2%) and showing high in vitro stability. The in vitro affinities of (99m)Tc-3P-RGD2 to NCI-H446 and A549 tumor cells were analyzed with γ-counter, while the in vivo uptakes in NCI-H446 and A549 xenografts were evaluated with micro-SPECT/CT. The region of interest was drawn over the tumor site and contralateral muscle on the SPECT/CT image, and the tumor to nontumor (T/NT) ratio was calculated to estimate αvβ3 expression and tumor uptake. The expressions of integrin αvβ3 in vitro and in vivo were analyzed using a flow cytometer and immunofluorescence.

RESULTS

Micro-SPECT/CT demonstrated focal uptake in the tumors. T/NT ratio in NCI-H446 xenografts was significantly higher compared with the A549 tumor model, as 5.92±0.82 and 3.62±0.91, respectively, with p<0.05. In addition, integrin αvβ3 expression in NCI-H446 cells was significantly higher compared with the A549 cells, which was consistent with the imaging data. A linear relationship was observed between (99m)Tc-3P-RGD2 uptake and αvβ3 expression (R(2)=0.7667, p<0.001).

CONCLUSION

(99m)Tc-3P-RGD2 SPECT/CT could be used to quantify integrin αvβ3 expression within tumors, providing a rational basis for integrin αvβ3-targeted cancer therapy.

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.

Molecular imaging agents for SPECT (and SPECT/CT)

The development of hybrid single photon emission computed tomography/computed tomography (SPECT/CT) cameras has increased the diagnostic value of many existing single photon radiopharmaceuticals. Precise anatomical localization of lesions greatly increases diagnostic confidence in bone imaging of the extremities, infection imaging, sentinel lymph node localization, and imaging in other areas. Accurate anatomical localization is particularly important prior to surgery, especially involving the parathyroid glands and sentinel lymph node procedures. SPECT/CT plays a role in characterization of lesions, particularly in bone scintigraphy and radioiodine imaging of metastatic thyroid cancer. In the development of novel tracers, SPECT/CT is particularly important in monitoring response to therapies that do not result in an early change in lesion size. Preclinical SPECT/CT devices, which actually have spatial resolution superior to PET/CT devices, have become essential in characterization of the biodistribution and tissue kinetics of novel tracers, allowing coregistration of serial studies within the same animals, which serves both to reduce biological variability and reduce the number of animals required. In conclusion, SPECT/CT increases the utility of existing radiopharmaceuticals and plays a pivotal role in the evaluation of novel tracers.

Integrin α(v)β₃-targeted radiotracer (99m)Tc-3P-RGD₂ useful for noninvasive monitoring of breast tumor response to antiangiogenic linifanib therapy but not anti-integrin α(v)β₃ RGD₂ therapy

Purpose: ^99mTc-3P-RGD₂ is a ^99mTc-labeled dimeric cyclic RGD peptide that binds to integrin α_vβ₃ with high affinity and specificity. The purpose of this study was to demonstrate the utility of ^99mTc-3P-RGD₂ SPECT/CT (single photon emission computed tomography/computed tomography) as a molecular imaging tool for noninvasive monitoring breast tumor early response to antiangiogenesis therapy with linifanib, and to illustrate its limitations in monitoring the efficacy of anti-α_vβ₃ treatment.

Methods: To support SPECT/CT imaging, biodistribution and therapy studies, the xenografted breast cancer model was established by subcutaneous injection of 5 × 10⁶ MDA-MB-435 cells into the fat pad of each athymic nude mouse. Linifanib (ABT-869) was used as antiangiogenesis agent. The tumor volume was 180 ± 90 mm³ on the day (-1 day) before baseline SPECT/CT. Each animal was treated twice daily with vehicle or 12.5 mg/kg linifanib. Longitudinal ^99mTc-3P-RGD₂ SPECT/CT imaging was performed on days -1, 1, 4 and 11. Tumors were harvested at each time point for pathological analysis of hematoxylin and eosin (H&E) and immunohistochemistry (IHC). Tumor uptake of ^99mTc-3P-RGD₂ was calculated from SPECT/CT quantification. When cyclic peptide E[c(RGDfK)]₂ (RGD₂) was used as the anti-α_vβ₃ agent, SPECT/CT images were obtained only at 7 and 21 days after last RGD₂ dose.

Results: The tumor uptake of ^99mTc-3P-RGD₂ from SPECT/CT quantification was almost identical to that from biodistribution. There was a dramatic reduction in both %ID and %ID/cm³ tumor uptake of ^99mTc-3P-RGD₂ during the first 24 hours of linifanib therapy. The therapeutic effect of linifanib was on both tumor cells and vasculature, as determined by IHC analysis of integrin α_vβ₃ and CD31. Changes in tumor vasculature were further confirmed by pathological H&E analysis of tumor tissues. While its %ID tumor uptake increased steadily in vehicle-treated group, the %ID tumor uptake of ^99mTc-3P-RGD₂ decreased in linifanib-treated group slowly over the 11-day study period. The degree of tumor response to linifanib therapy correlated well to the integrin α_vβ₃ expression levels before linifanib therapy.

Conclusion: ^99mTc-3P-RGD₂ is an excellent radiotracer for monitoring integrin α_vβ₃ expression during and after linifanib therapy. ^99mTc-3P-RGD₂ SPECT/CT is an useful molecular imaging tool for patient selection before antiangiogenic and anti-α_vβ₃ therapy; but it would be difficult to use ^99mTc-3P-RGD₂ for accurate and noninvasive monitoring of early tumor response to anti-α_vβ₃ therapy.