Synthesis and evaluation of Tc-99m-labeled RRL-containing peptide as a non-invasive tumor imaging agent in a mouse fibrosarcoma model

Synthesis and Evaluation of Technetium-99m-Labeled pH (Low) Insertion Peptide Variant 7 for Early Diagnosis of MDA-MB-231 Triple-Negative Breast Cancer by Targeting the Tumor Microenvironment

Objective

To prepare technetium-99m (^99mTc)-labeled pH (low) insertion peptide variant 7 [pHLIP (Var7)] and carry out small-animal single-photon-emission computed tomography (SPECT)/computed tomography (CT) imaging of tumor-bearing nude mice in vivo to study its value in the early diagnosis of triple-negative breast cancer (TNBC).

Methods

The pHLIP (Var7) sequence was synthesized via solid-phase peptide synthesis. Four amino acids, Gly-(D)-Ala-Gly-Gly, were attached to the N-terminus of pHLIP (Var7) to form a strong chelating group containing an N4 structure. The peptide was labeled with ^99mTc using a direct labeling method. We determined the in vitro binding fraction of ^99mTc-pHLIP (Var7) to MDA-MB-231 cells. Serial biodistribution studies and small-animal SPECT/CT imaging in MDA-MB-231 TNBC-bearing mice were performed using ^99mTc-pHLIP (Var7).

Results

The radiochemical yield and purity of ^99mTc-pHLIP (Var7) were 99.49 ± 0.17% and 99.63 ± 0.44%, respectively. The radiochemical purity was still more than 96% after 24 h in serum. The binding fraction of ^99mTc-pHLIP (Var7) to MDA-MB-231 cells continuously increased in an acidic environment and was significantly higher than the cell-binding fraction (P < 0.01) at pH = 7.4 and the cell-binding fraction (P < 0.01) of ^99mTc-kVar7 at different pH values (pH = 6.0, 6.5, 7.0 and 7.4) at each time point (P < 0.01). The distribution of ^99mTc-pHLIP (Var7) in tumors at each time point was significantly greater than that of ^99mTc-kVar7 (P < 0.01). SPECT/CT imaging was largely consistent with the biodistribution results; the tumor was clearly imaged at each time point after injection of ^99mTc-pHLIP (Var7) but could not be imaged after injection of ^99mTc-kVar7.

Conclusion

^99mTc-pHLIP (Var7) showed a high radiochemical yield and stability and was highly concentrated in tumor tissues. Although there was strong radioactive background in the abdomen of tumor-bearing nude mice, it did not hinder early diagnosis of TNBC.

Arginine-Arginine-Leucine Peptide Targeting Heat Shock Protein 70 for Cancer Imaging

Arg-Arg-Leu (RRL) is a potent tumor-homing tripeptide. However, the binding target is unclear. In this study, we intended to identify the binding target of RRL and evaluate the tumor targeting of ^99mTc-MAG₃-RRL in vivo. Biotin-RRL, 5-TAMRA-RRL, and ^99mTc-MAG₃-RRL were designed to trace the binding target and tumor lesion. Immunoprecipitation-mass spectrometry was conducted to identify the candidate proteins and determination of the subcellular localization was also performed. A pull-down assay was performed to demonstrate the immunoprecipitate. Fluorescence colocalization and cell uptake assays were performed to elucidate the correlation between the selected binding protein and RRL, and the internalization mechanism of RRL. Biodistribution and in vivo imaging were performed to evaluate the tumor accumulation and targeting of ^99mTc-MAG₃-RRL. The target for RRL was screened to be heat shock protein 70 (HSP70). The prominent uptake distribution of RRL was concentrated in the membrane and cytoplasm. A pull-down assay demonstrated the existence of HSP70 in the biotin-RRL captured complex. Regarding fluorescence colocalization and cell uptake assays, RRL may interact with HSP70 at the nucleotide-binding domain (NBD). Clathrin-dependent endocytosis and macropinocytosis could be a vital internalization mechanism of RRL. In vivo imaging and biodistribution both demonstrated that ^99mTc-MAG₃-RRL can trace tumors with satisfactory accumulation in hepatoma xenograft mice. The radioactive signals accumulated in tumor lesions can be blocked by VER-155008, which can bind to the NBD of HSP70. Our findings revealed that RRL may interact with HSP70 and that ^99mTc-MAG₃-RRL could be a prospective probe for visualizing overexpressed HSP70 tumor sections.

A novel dual-modality imaging agent targeting folate receptor of tumor for molecular imaging and fluorescence-guided surgery

OBJECTIVE

Folate receptor (FR) is an ideal target for cancer imaging because it is frequently overexpressed in major types of human tumor, whereas its expression in normal organs is highly limited. Combining nuclear and fluorescence-imaging techniques provides a novel approach for cancer imaging and monitoring the surgery. The objective of this study was to report the synthesis and characteristics of a dual-modality imaging agent, Tc-99m Folate-Gly-His-Glu-Gly-Glu-Cys-Gly-Lys(-5-carboxy-X-rhodamine)-NH₂ (Folate-ECG-ROX), and verify its feasibility as both molecular imaging agent and intra-operative guidance.

METHODS

Folate-ECG-ROX was synthesized using Fmoc solid-phase peptide synthesis. Radiolabeling of Folate-ECG-ROX with Tc-99m was done using ligand exchange via tartrate. Binding affinity and in vitro cellular uptake studies were performed. Gamma camera imaging, biodistribution and ex vivo imaging studies were performed using KB and HT-1080 tumor-bearing murine models. Tumor tissue slides were prepared and analyzed with immunohistochemistry staining and confocal microscopy. Surgical removal of tumor nodules in murine models with peritoneal carcinomatosis was performed under the fluorescence-imaging system.

RESULTS

After radiolabeling procedures with Tc-99m, Tc-99m Folate-ECG-ROX complexes were prepared in high yield (> 97%). The binding affinity value (K_d) of Tc-99m Folate-ECG-ROX for KB cells was estimated to be 6.9 ± 0.9 nM. In gamma camera imaging, tumor to normal muscle uptake ratio of Tc-99m Folate-ECG-ROX increased with time (3.4 ± 0.4, 4.4 ± 0.7, and 6.6 ± 0.8 at 1, 2, and 3 h, respectively). In biodistribution study, %IA/g for KB tumor was 2.50 ± 0.80 and 4.08 ± 1.16 at 1 and 3 h, respectively. Confocal microscopy with immunohistochemistry staining detected strong Tc-99m Folate-ECG-ROX fluorescence within KB tumor tissue which is correlating with the fluorescent activity of anti-FR antibody. Under real-time optical imaging, the removal of visible nodules was successfully performed.

CONCLUSIONS

In vivo and in vitro studies revealed substantial and specific uptake of Tc-99m Folate-ECG-ROX in FR-positive tumors. Thus, Tc-99m Folate-ECG-ROX could provide both pre-operative molecular imaging and fluorescence image-guidance for tumor.

Tc-99m and Fluorescence-Labeled Anti-Flt1 Peptide as a Multimodal Tumor Imaging Agent Targeting Vascular Endothelial Growth Factor-Receptor 1

Purpose

We developed a Tc-99m and fluorescence-labeled peptide, Tc-99m TAMRA-GHEG-ECG-GNQWFI, to target tumor cells, and evaluated the diagnostic performance as a dual-modality imaging agent for tumor in a murine model.

Methods

TAMRA-GHEG-ECG-GNQWFI was synthesized using Fmoc solid-phase peptide synthesis. Radiolabeling of TAMRA-GHEG-ECG-GNQWFI with Tc-99m was done using ligand exchange via tartrate. Binding affinity and in vitro cellular uptake studies were performed. Gamma camera imaging, biodistribution, and ex vivo imaging studies were performed in murine models with U87MG tumors. Tumor tissue slides were prepared and analyzed with immunohistochemistry using confocal microscopy.

Results

After radiolabeling procedures with Tc-99m, Tc-99m TAMRA-GHEG-ECG-GNQWFI complexes were prepared in high yield (> 95%). The K _d of Tc-99m TAMRA-GHEG-ECG-GNQWFI determined by saturation binding was 29.5 ± 4.5 nM. Confocal microscopy images of U87MG cells incubated with TAMRA-GHEG-ECG-GNQWFI showed strong fluorescence in the cytoplasm. Gamma camera imaging revealed substantial uptake of Tc-99m TAMRA-GHEG-ECG-GNQWFI in tumors. Tumor uptake was effectively blocked by the co-injection of an excess concentration of GNQWFI. Specific uptake of Tc-99m TAMRA-GHEG-ECG-GNQWFI was assessed by biodistribution, ex vivo imaging, and immunohistochemistry stain studies.

Conclusions

In vivo and in vitro studies revealed substantial and specific uptake of Tc-99m TAMRA-GHEG-ECG-GNQWFI in tumor cells. Tc-99m TAMRA-GHEG-ECG-GNQWFI could be a good candidate dual-modality imaging agent for tumors.

Noninvasive PET Imaging of a Ga-68-Radiolabeled RRL-Derived Peptide in Hepatocarcinoma Murine Models

PURPOSE

Tc-99m- and I-131-labeled arginine-arginine-leucine (RRL) peptides have shown the feasibility of tumor imaging in our previous studies. However, there have been no reports using RRL peptide for positron emission tomography (PET) imaging. In this study, RRL was radiolabeled with Ga-68 under optimized reaction conditions to develop a better specific and effective tumor imaging agent.

PROCEDURES

RRL was synthesized and conjugated to a bifunctional chelating agent (DOTA-NHS), then radiolabeled with Ga-68. Labeling yield was optimized by varying pH, temperature, and reaction time and the stability was evaluated in human fresh serum. Cellular uptakes of [⁶⁸Ga]DOTA-RRL and FITC-conjugated RRL in HepG2 cells were evaluated using a gamma counter, confocal microscopy, and flow cytometry. PET images and biodistribution were performed in HepG2 tumor-bearing mice after injection of [⁶⁸Ga]DOTA-RRL or [⁶⁸Ga]GaCl₃ at different time points. Further, blocking study was investigated using cold RRL.

RESULTS

The labeling yield of [⁶⁸Ga]DOTA-RRL was 80.6 ± 3.9 % with a pH of 3.5-4.5 at 100 °C for 15 min. The cellular uptake of [⁶⁸Ga]DOTA-RRL in HepG2 cells was significantly higher than that of [⁶⁸Ga]GaCl₃ (P < 0.05). Moreover, the high fluorescent affinity of FITC-conjugated RRL in HepG2 cells was shown using confocal microscopy and flow cytometry. After injection of [⁶⁸Ga]DOTA-RRL in HepG2 tumor-bearing mice, tumor regions exhibited high radioactive accumulation over 120 min and the highest uptake at 30 min. After blocked with cold RRL, HepG2 tumors could not be visualized. [⁶⁸Ga]GaCl₃ was unable to show tumor images at any time point. The biodistribution results confirmed the PET imaging and blocking results.

CONCLUSIONS

Our study successfully prepared [⁶⁸Ga]DOTA-RRL with a high labeling yield under the optimized reaction conditions and demonstrated its potential role as a PET imaging agent for tumor-targeted diagnosis.

A novel Tc-99m and fluorescence labeled peptide as a multimodal imaging agent for targeting angiogenesis in a murine hindlimb ischemia model

The serine-aspartic acid-valine (SDV) peptide binds specifically to integrin αvβ3. We developed a Tc-99m and TAMRA labeled peptide, Tc-99m SDV-ECG-K-TAMRA for multimodal imaging of angiogenesis. Tc-99m SDV-ECG-K-TAMRA was prepared in high yield (>96%) and showed low cytotoxicity. Tc-99m tetrofosmin images 1 week after operation, revealed significantly decreased perfusion of the ischemic hindlimb, and the perfusion recovered gradually for 4 weeks. In contrast, Tc-99m SDV-ECG-K-TAMRA uptake was maximal 1 week after the operation (ischemic-to-non-ischemic uptake ratio =5.03±1.01) and decreased gradually. The ischemic-to-non-ischemic ratio of Tc-99m SDV-ECG-K-TAMRA and Tc-99m tetrofosmin was strongly negatively correlated (r =-0.94). A postmortem analysis revealed increased angiogenesis markers and uptake of Tc-99m SDV-ECG-K-TAMRA by ischemic tissue. Our in vivo and in vitro studies revealed substantial uptake of Tc-99m SDV-ECG-K-TAMRA by ischemic tissue. Tc-99m SDV-ECG-K-TAMRA could be a good candidate dual-modality imaging agent to assess angiogenesis.

A novel Tc-99 m and fluorescence labeled peptide as a multimodal imaging agent for targeting angiogenesis in a murine tumor model

The serine-aspartic acid-valine (SDV) peptide binds specifically to integrin α_V β₃ . In the present study, we successfully developed a TAMRA-GHEG-ECG-SDV peptide labeled with both Tc-99 m and TAMRA to target the integrin α_V β₃ of tumor cells; furthermore, we evaluated the diagnostic performance of Tc-99 m TAMRA-GHEG-ECG-SDV as a dual-modality imaging agent for tumor of the murine model. TAMRA-GHEG-ECG-SDV was synthesized using Fmoc solid-phase peptide synthesis. Radiolabeling of TAMRA-GHEG-ECG-SDV with Tc-99 m was done using ligand exchange methods. Labeling stability and cytotoxicity studies were performed. Gamma camera imaging, biodistribution and ex vivo imaging studies were performed in murine models with HT-1080 and HT-29 tumors. A tumor tissue slide was prepared and analyzed using confocal microscopy. After radiolabeling procedures with Tc-99 m, the Tc-99 m TAMRA-GHEG-ECG-SDV complexes were prepared in high yield (>99%). In the gamma camera imaging study, a substantial uptake of Tc-99 m TAMRA-GHEG-ECG-SDV into HT-1080 tumor (integrin α_V β₃ positive) and low uptake of Tc-99 m TAMRA-GHEG-ECG-SDV into HT-29 tumor (integrin α_V β₃ negative) were demonstrated. A competition study revealed that HT-1080 tumor uptake was effectively blocked by the co-injection of an excess concentration of SDV. Specific uptake of Tc-99 m TAMRA-GHEG-ECG-SDV was confirmed by biodistribution, ex vivo imaging and confocal microscopy studies. Our in vivo and in vitro studies revealed substantial uptake of Tc-99 m TAMRA-GHEG-ECG-SDV in the integrin α_V β₃ -positive tumor. Tc-99 m TAMRA-GHEG-ECG-SDV could be a good candidate for a dual-modality imaging agent targeting tumor angiogenesis. Copyright © 2016 John Wiley & Sons, Ltd.

Tc-99m Glu-Cys-Gly-His-Gly-Lys (ECG-HGK), a novel Tc-99m labeled hexapeptide for molecular tumor imaging

Domain 5 of kinin-free high molecular weight kininogen inhibits the adhesion of many tumor cell lines, and it has been reported that the histidine-glycine-lysine (HGK)-rich region might be responsible for inhibition of cell adhesion. The authors developed HGK-containing hexapeptide, glutamic acid-cysteine-glycine (ECG)-HGK, and evaluated the utility of Tc-99m ECG-HGK for tumor imaging. Hexapeptide, ECG-HGK was synthesized using Fmoc solid-phase peptide synthesis. Radiolabeling efficiency was evaluated. The uptake of Tc-99m ECG-HGK within HT-1080 cells was evaluated in vitro. In HT-1080 tumor-bearing mice, gamma imaging and biodistribution studies were performed. The complexes Tc-99m ECG-HGK was prepared in high yield. The uptake of Tc-99m ECG-HGK within the HT-1080 tumor cells had been demonstrated by in vitro studies. The gamma camera imaging in the murine model showed that Tc-99m ECG-HGK was accumulated substantially in the HT-1080 tumor (tumor-to-muscle ratio = 5.7 ± 1.4 at 4 h), and the tumoral uptake was blocked by the co-injection of excess HGK (tumor-to-muscle ratio = 2.8 ± 0.6 at 4 h). In the present study, Tc-99m ECG-HGK was developed as a new tumor imaging agents. Our in vitro and in vivo studies revealed specific function of Tc-99m ECG-HGK for tumor imaging.