99mTc-3P4-RGD2 scintimammography in the assessment of breast lesions: comparative study with 99mTc-MIBI

Application of Breast Scintigraphy for Patients with Suspicious (Breast Imaging-Reporting and Data System IV) Breast Lesions

Background

The surgery for a breast imaging-reporting and data system (BIRADS) IV lesions needs imaging or pathology supporting data. The roll of breast scintigraphy for this purpose is unclear.

Materials and Methods

In a prospective design, 16 patients with 25 BIRADS IV lesions who were scheduled for surgery were included. Before the surgery, breast scintigraphy was done using a nondedicated dual head gamma camera in the prone position employing a shaped foam pad providing imaging at breast pendulous position. Twenty mCi^99m Tc methoxy-isobutyl-isonitrile was injected and two 15 and 60-min delayed imaging were done (anterior, bilateral, and single photon emission computed tomography [SPECT] projections). Pathology reports were collected and tumor to nontumor uptake ratio (T/NT) was analyzed, accordingly.

Results

Out of all lesions, 12 were malignant (invasive ductal and lobular carcinoma ductal carcinoma in situ). At 15 min, T/NT was insignificantly higher in the malignant compared to benign lesions (22.8 ± 23.9 vs. 10.1 ± 10.1; P = 0.109). The optimal T/NT cutoff for discrimination of malignant and benign lesions was 20. Only 1 out of 13 benign lesions presented uptake >20 (7.7%; false-positive rate; P = 0.047). The diagnostic accuracy, sensitivity, and specificity for T/NT calculated at 0.68, 0.42, and 0.92, respectively. The T/NT at 60 min remained unchanged for either benign or malignant lesions (22.3 ± 30.2 vs. 11.7 ± 17.1; P = 0.296).

Conclusions

Breast scintigraphy with general purpose gamma camera employing SPECT imaging may assist the selection of BIRADS IV lesions in need for surgery. All uptake positive cases should undergo surgery and decision for uptake negative cases should be made based on other data.

Advances in Clinical Oncology Research on 99mTc-3PRGD2 SPECT Imaging

The integrin alpha(α)v beta(β)3 receptor is ubiquitous in malignant tumors and has a certain level of specificity for tumors. Technetium-99m hydrazinonicotinamide-dimeric cyclic arginyl-glycyl-aspartic acid peptide with three polyethylene glycol spacers (99mTc-3PRGD2) can bind specifically to the integrin αvβ3 receptor with high selectivity and strong affinity. Thus, it can specifically mark tumors and regions with angiogenesis for tumor detection and be used in single-photon emission computed tomography (SPECT) imaging. This modality has good application value for diagnosing and treating tumor lesions, such as those in the lung, breast, esophagus, head, and neck. This review provides an overview of the current clinical research progress of 99mTc-3PRGD2 SPECT imaging for tumor lesions, including for the diagnosis and differential diagnosis of tumors in different body parts, evaluation of related metastases, and evaluation of efficacy. In addition, the future clinical application prospects and possibilities of 99mTc-3PRGD2 SPECT imaging are further discussed.

Radiopharmaceuticals for PET and SPECT Imaging: A Literature Review over the Last Decade

Positron emission tomography (PET) uses radioactive tracers and enables the functional imaging of several metabolic processes, blood flow measurements, regional chemical composition, and/or chemical absorption. Depending on the targeted processes within the living organism, different tracers are used for various medical conditions, such as cancer, particular brain pathologies, cardiac events, and bone lesions, where the most commonly used tracers are radiolabeled with 18F (e.g., [¹⁸F]-FDG and NA [¹⁸F]). Oxygen-15 isotope is mostly involved in blood flow measurements, whereas a wide array of ¹¹C-based compounds have also been developed for neuronal disorders according to the affected neuroreceptors, prostate cancer, and lung carcinomas. In contrast, the single-photon emission computed tomography (SPECT) technique uses gamma-emitting radioisotopes and can be used to diagnose strokes, seizures, bone illnesses, and infections by gauging the blood flow and radio distribution within tissues and organs. The radioisotopes typically used in SPECT imaging are iodine-123, technetium-99m, xenon-133, thallium-201, and indium-111. This systematic review article aims to clarify and disseminate the available scientific literature focused on PET/SPECT radiotracers and to provide an overview of the conducted research within the past decade, with an additional focus on the novel radiopharmaceuticals developed for medical imaging.

99mTC-sestamibi breast imaging: Current status, new ideas and future perspectives

Here we proposed the most recent innovations in the use of Breast Specific Gamma Imaging with ⁹⁹mTc-sestamibi for the management of breast cancer patients. To this end, we reported the recent discoveries concerning: a) the implementation of both instrumental devices and software, b) the biological mechanisms involved in the ⁹⁹mTc-sestamibi uptake in breast cancer cells, c) the evaluation of Breast Specific Gamma Imaging with ⁹⁹mTc-sestamibi as predictive markers of metastatic diseases. In this last case, we also reported preliminary data about the capability of Breast Specific Gamma Imaging with ⁹⁹mTc-sestamibi to identify breast cancer lesions with high propensity to form bone metastatic lesions due to the presence of Breast Osteoblast-Like Cells.

Comparison of [99mTc]3PRGD2 Imaging and [18F]FDG PET/CT in Breast Cancer and Expression of Integrin αvβ3 in Breast Cancer Vascular Endothelial Cells

PURPOSE

This study aimed to investigate the value of ^99mtechnetium-three polyethylene glycol spacers-arginine-glycine-aspartic acid ([^99mTc]3PRGD₂) imaging in diagnosis and staging of breast cancer compared with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) imaging, and to explore the expression of integrin α_vβ₃ in tumor vascular endothelial cells.

PROCEDURES

Forty-two women with suspected breast cancer underwent both [^99mTc]3PRGD₂ imaging and [¹⁸F]FDG imaging. Visual analysis was used to assess primary breast lesion, axillary lymph node, and distant metastasis. The tumor-blood (T/B) ratios from [^99mTc]3PRGD₂ imaging and the maximum standardized uptake value (SUVmax) from [¹⁸F]FDG imaging were analyzed for breast lesions. Integrin α_vβ₃ was analyzed through immunohistochemistry.

RESULTS

Forty-five breast lesions were found (malignant, n = 38; benign, n = 7). The sensitivity, specificity, and accuracy of [^99mTc]3PRGD₂ and [¹⁸F]FDG imaging in visual analysis for the breast lesion were 97.4, 87.5, and 95.6 % and 97.4, 71.4, and 93.3 %, respectively (P > 0.05). For semi-quantitative analysis, no significant difference of the area under the curves (AUC) was found in the imaging using the two radiopharmaceuticals (0.880 and 0.955; Z = 0.88, P > 0.05). The sensitivity, specificity, and accuracy for axillary lymph node metastasis with [^99mTc]3PRGD₂ and [¹⁸F]FDG were 78.05, 99.36, and 94.92 % and 85.37, 98.72, and 95.64 %, respectively (P > 0.05). Nine patients with distant metastases were all detected with the two radiopharmaceuticals. The expression of integrin α_vβ₃ was correlated with [^99mTc]3PRGD₂ uptake (r = 0.582, P = 0.001), which were significantly higher in the HER2-positive and stage III-IV patients (P < 0.05).

CONCLUSIONS

The prospective study demonstrated that [^99mTc]3PRGD₂ imaging seems to be valuable for diagnosis of breast cancer and its staging. It may be less sensitive for detecting small lymph node metastatic lesions when compared with [¹⁸F]FDG imaging. Integrin α_vβ₃ in tumor microvessels was associated with the breast cancer subtype and its staging.

[99mTc]3PRGD2 for integrin receptor imaging of esophageal cancer: a comparative study with [18F]FDG PET/CT

OBJECTIVES

This study was designed to investigate the efficacy of ^99mtechnetium-three polyethylene glycol spacers-arginine-glycine-aspartic acid ([^99mTc]3PRGD₂) in the evaluation of patients with esophageal cancer.

METHODS

Twenty-nine patients with a suspected esophageal lesion and for whom definite pathological diagnosis was finally obtained were recruited. Whole-body planar scanning and chest single-photon-emission computed tomography/computed tomography (SPECT/CT) were performed at 30 and 40 min, respectively, after intravenous injection of 11.1 MBq/kg of [^99mTc]3PRGD₂. 2-Deoxy-2-[¹⁸F] fluoro-D-glucose ([¹⁸F]FDG) positron-emission tomography/computed tomography (PET/CT) was performed in 1 week. The tumor-to-background ratio (T/B) and the maximum standard uptake value (SUVmax) were calculated for semi-quantitative and quantitative analyses. Integrin α_vβ₃ was analyzed through immunohistochemistry.

RESULTS

The T/B, SUVmax and expression of integrin α_vβ₃ in malignant lesions were higher than those in benign lesions (t = 3.691, P = 0.001; t = 8.271, P = 0.000; t = 3.632, P = 0.001, respectively). There was a significant correlation between T/B and SUVmax in esophageal lesions (r = 0.660, P = 0.000). The expression of integrin α_vβ₃ was correlated with [^99mTc]3PRGD₂ uptake (r = 0.782, P = 0.000). In visual analysis, the sensitivity and accuracy of the whole-body planar RGD scan were lower than those of the chest SPECT/CT RGD scan and the [¹⁸F]FDG PET/CT scan (x² = 6.769, P = 0.022). The sensitivity, specificity and accuracy of the chest SPECT/CT RGD scan were similar to those of the [¹⁸F] FDG PET/CT scan. In semi-quantitative analysis, the sensitivity, specificity and accuracy of chest SPECT/CT RGD from the receiver operating characteristic (ROC) analysis were 87%, 100% and 94%, respectively [cutoff = 3.1 of T/B, area under the curve (AUC) = 0.957]. Thirteen patients (30 lymph nodes) and 16 patients (105 lymph nodes) were suspected to have lymph node metastases based on the RGD and FDG scans, respectively.

CONCLUSION

This prospective study demonstrated that [^99mTc]3PRGD₂ imaging is valuable for the diagnosis and staging of esophageal cancer. It may be less sensitive than [¹⁸F]FDG imaging for detecting metastatic lesions in small lymph nodes. The T/B value was correlated with the expression of integrin α_vβ₃. NIH CLINICALTRIALS.GOV: NCT 02744729.

Tumor targeting with 99m Tc radiolabeled peptides: Clinical application and recent development

Targeting overexpressed receptors on the cancer cells with radiolabeled peptides has become very important in nuclear oncology in the recent years. Peptides are small and have easy preparation and easy radiolabeling protocol with no side-effect and toxicity. These properties made them a valuable tool for tumor targeting. Based on the successful imaging of neuroendocrine tumors with ¹¹¹ In-octreotide, other receptor-targeting peptides such as bombesin (BBN), cholecystokinin/gastrin analogues, neurotensin analogues, glucagon-like peptide-1, and RGD peptides are currently under development or undergoing clinical trials. The most frequently used radionuclides for tumor imaging are ^99m Tc and ¹¹¹ In for single-photon emission computed tomography and ⁶⁸ Ga and ¹⁸ F for positron emission tomography imaging. This review presents some of the ^99m Tc-labeled peptides, with regard to their potential for radionuclide imaging of tumors in clinical and preclinical application.

Advantages of 99mTc-3PRGD2 SPECT over CT in the preoperative assessment of lymph node metastasis in patients with esophageal cancer

BACKGROUND

Our study was designed to compare the diagnostic efficacies of integrated ^99mTc-HYNIC-PEG4-E[PEG4-c(RGDfK)]2 (^99mTc-3PRGD₂) single-photon emission computed tomography (SPECT) images and computed tomography (CT) images in lymph node metastasis in the patients with esophageal cancer.

METHODS

From September 2015 and May 2018, 32 patients with histologically proven primary esophageal carcinoma underwent both ^99mTc-3PRGD₂ SPECT and CT scans followed by esophagectomy with lymph node dissection. The results of reviewing ^99mTc-3PRGD₂ SPECT and CT images for the lymph node metastasis were compared in relation with pathologic findings.

RESULTS

During surgery, a total of 168 lymph nodes were dissected in 32 patients, of which 42 node groups in 18 patients were malignant on histologic examination. Preoperative nodal staging was compared with postoperative histopathological staging, The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of ^99mTc-3PRGD₂ SPECT for lymph nodes were 80.95%, 86.51%, 85.12%, 66.67%, and 93.16% on per-node basis, respectively; compared with 59.52%, 73.02%, 69.64%, 42.37%, and 84.40% for CT (p = 0.034, 0.008, 0.005, 0.011, and 0.038, respectively). 70.59% (12/17) false-negative interpretations and 50% (17/34) false-positive interpretations on CT were corrected by ^99mTc-3PRGD₂ SPECT. 37.5% false-negative interpretations on ^99mTc-3PRGD₂ SPECT were corrected by CT. 11.90% (5/42) positive lymph nodes and 13.49% (17/126) negative nodes at pathology were incorrectly diagnosed both by ^99mTc-3PRGD₂ SPECT and CT. The accuracy of ^99mTc-3PRGD₂ SPECT (87.50%, 28/32) was significantly higher than that of CT (62.50, 20/32; p = 0.022) on per-patient basis. ^99mTc-3PRGD₂ SPECT showed significantly higher sensitivity and accuracy in the neck and upper thoracic regions than CT. For nodal staging, ^99mTc-3PRGD₂ SPECT was correct in 78.12% (25/32) of the patients, whereas CT was correct in 53.12% (17/32), p = 0.037.

CONCLUSION

^99mTc-3PRGD₂ SPECT is more accurate than CT for preoperative assessment of lymph node metastasis in esophageal cancer and may be helpful in determining the therapeutic plan.

The role of radionuclide probes for monitoring anti-tumor drugs efficacy: A brief review

Despite recent advances in the development of new therapeutic agents and diagnostic imaging modalities, cancer is still one of the main causes of death worldwide. A better understanding of the molecular signature of cancer has promoted the development of a new generation of anti-cancer drugs and diagnostic agents that specifically target molecular components such as genes, ligands, receptors and signaling pathways. However, intrinsic heterogeneity of tumors has hampered the overall success of target therapies even among patients with similar tumor types but unpredictable different responses to therapy. In this sense, post-treatment response monitoring becomes indispensable and nuclear medicine imaging modalities could provide the tools for an early indication of therapeutic efficacy. Herein, we briefly discuss the current role of PET and SPECT imaging in monitoring cancer therapy together with an update on the current radiolabeled probes that are currently investigated for tumor therapy response assessment.

Evaluation of Tc-99m-3PRGD2 Integrin Receptor Imaging in the Differential Diagnosis of Breast Lesions and Comparison With Mammography

PURPOSE

The aims of this study were to evaluate and compare efficacies of Tc-99m-3PRGD2 integrin receptor imaging under variety of conditions for the diagnosis of breast lesions, in addition to comparison with mammography.

MATERIALS AND METHODS

Seventy-two female patients with established breast lesions were recruited. All patients were examined by Tc-99m-3PRGD2 integrin receptor imaging and mammography. Whole-body scan and SPECT/CT were acquired at dual time points of 2 and 4 h after injection using standard protocol. The processed images were evaluated by visual and semi-quantitative analysis. Mammography was performed using up and down and internal and external oblique views. The gold standard of diagnosis was based on histopathological findings.

RESULTS

Sensitivity greater than 85.0% and accuracy greater than 80.0% were observed under any technical method. For dense mammary gland, the sensitivity, specificity, and accuracy of Tc-99m-3PRGD2 SPECT/CT 4-h imaging and mammography were 95.2, 75.0, and 90.7%, and 71.4, 58.3, and 68.5% respectively. Combined two methods' sensitivity, specificity, and accuracy for detection of breast cancer can reach 98.3, 86.7, and 96.0%.

CONCLUSIONS

Tc-99m-3PRGD2-based molecular imaging is a sensitive method for the differential diagnosis of breast lesions. Particularly, Tc-99m-3PRGD2-SPECT/CT has better diagnostic value in dense mammary gland as compared with mammography. Combining two methods can significantly improve the diagnostic efficiency.

Comparison of Tc-99m maraciclatide and Tc-99m sestamibi molecular breast imaging in patients with suspected breast cancer

Background

Molecular breast imaging (MBI) performed with ^99mTc sestamibi has been shown to be a valuable technique for the detection of breast cancer. Alternative radiotracers such as ^99mTc maraciclatide may offer improved uptake in breast lesions. The purpose of this study was to compare relative performance of ^99mTc sestamibi and ^99mTc maraciclatide in patients with suspected breast cancer, using a high-resolution dedicated gamma camera for MBI. Women with breast lesions suspicious for malignancy were recruited to undergo two MBI examinations—one with ^99mTc sestamibi and one with ^99mTc maraciclatide. A radiologist interpreted MBI studies in a randomized, blinded fashion to assign an assessment score (1–5) and measured lesion size. Lesion-to-background (L/B) ratio was measured with region-of-interest analysis.

Results

Among 39 analyzable patients, 21 malignant tumors were identified in 21 patients. Eighteen of 21 tumors (86%) were seen on ^99mTc sestamibi MBI and 19 of 21 (90%) were seen on ^99mTc maraciclatide MBI (p = 1). Tumor extent measured with both radiopharmaceuticals correlated strongly with pathologic size (^99mTc sestamibi, r = 0.84; ^99mTc maraciclatide, r = 0.81). The L/B ratio in detected breast cancers was similar for the two radiopharmaceuticals: 1.55 ± 0.36 (mean ± S.D.) for ^99mTc sestamibi and 1.62 ± 0.37 (mean ± S.D.) for ^99mTc maraciclatide (p = 0.53). No correlation was found between the L/B ratio and molecular subtype for ^99mTc sestamibi (r_s = 0.12, p = 0.63) or ^99mTc maraciclatide (r_s = −0.12, p = 0.64). Of 20 benign lesions, 10 (50%) were seen on ^99mTc sestamibi and 9 of 20 (45%) were seen on ^99mTc maraciclatide images (p = 0.1). The average L/B ratio for benign lesions was 1.34 ±0.40 (mean ±S.D.) for ^99mTc sestamibi and 1.41 ±0.52 (mean ±S.D.) for ^99mTc maraciclatide (p = 0.75). Overall diagnostic performance was similar for both radiopharmaceuticals. AUC from ROC analysis was 0.83 for ^99mTc sestamibi and 0.87 for ^99mTc maraciclatide (p = 0.64).

Conclusions

^99mTc maraciclatide offered comparable lesion uptake to ^99mTc sestamibi, in both malignant and benign lesions. There was good correlation between lesion extent and uptake measured from both radiopharmaceuticals. ^99mTc maraciclatide offered a marginal (but not significant) improvement in sensitivity over ^99mTc sestamibi. Our findings did not support an association between the uptake of either radiopharmaceutical and tumor molecular subtype.

Trial registration

ClinicalTrials.gov, NCT00888589

(99m)Tc-3PRGD2 SPECT/CT Imaging for Monitoring Early Response of EGFR-TKIs Therapy in Patients with Advanced-Stage Lung Adenocarcinoma

OBJECTIVE

This study was aimed to assess the efficacy of (99m)Tc-3PRGD2 imaging for evaluating both early treatment response to epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) and prognosis in advanced-stage lung adenocarcinoma.

MATERIAL AND METHODS

Eighteen patients with lung adenocarcinoma were enrolled for EGFR-TKIs therapy. (99m)Tc-3PRGD2 SPECT/CT and planar imaging were performed pre- and post-therapy. The tumor to nontumor (T/NT) ratio and percentage change in T/NT ratio were assessed for the treatment response. Receiver operator characteristic (ROC) analysis was utilized to analyze the power of identifying responders based on the changes in T/NT ratios.

RESULTS

After treatment, 10 patients had partial response (PR), and 6 patients stable disease (SD), while 2 patients progressive disease (PD). The mean changes in T/NT ratios on SPECT/CT and planar images in PR group were 35.8% and 15.0% and in SD group were 8.9% and 0.7%, while in PD group were 76.1% and 18.7%, respectively. For ROC analysis, using a cutoff value of 23.8% decrease in T/NT ratio on SPECT/CT images, the sensitivity and specificity in identifying responders were 80.0% and 87.5%, respectively. The median progression-free survival (PFS) for patients with responders and nonresponders (on (99m)Tc-3PRGD2 SPECT/CT) was 18 months (95% CI 5.8-30.2 months) and 7 months (95% CI 5.2-8.8 months), respectively (p = 0.006).

CONCLUSION

(99m)Tc-3PRGD2 imaging can evaluate the early response to EGFR-targeted therapy and predict the PFS of lung adenocarcinoma patients.

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.

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.