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Medicine

Preparing a 68Ga-labeled Arginine Glycine Aspartate (RGD)-peptide for Angiogenesis

Published: January 7, 2019 doi: 10.3791/58218
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1, 1, 1, 1, 1, 1
1Division of Applied RI, Korea Institute of Radiological and Medical Sciences

Summary

The αvβ3 integrin is a type of adhesion protein that is highly expressed on activated endothelial cells undergoing angiogenesis. Thus, evaluating the integrity of the integrin is of great interest in oncology. Here, we introduce a method to prepare 68Ga-labeled radiopeptides and a method to assess its biological effectiveness.

Abstract

The αvβ3 integrin is a heterodimeric adhesion molecule involved in tumor cell migration and angiogenesis. The integrin is overexpressed in angiogenic tumor endothelial cells, where it typically has a low concentration. This specific expression of αvβ3 makes it a valid biomarker for antiangiogenic and imaging drugs. As a functional imaging modality, positron emission tomography (PET) provides information about biochemical and physiological changes in vivo, due to its unique high sensitivity at the nanomolar scale. Hence, radiometal-based PET radiopharmaceuticals have received great attention for the non-invasive quantification of tumor angiogenesis. This paper provides a systemic protocol to prepare a new radiometal-labeled peptide for the evaluation of angiogenesis. This protocol contains information about radiochemical reliability, lipophilicity, cell uptake, serum stability, and pharmacokinetic properties. The 68Ga-RGD-peptide is one of the representative PET ligands toward αvβ3 integrin. Here, we introduce a protocol to prepare a 68Ga-RGD-peptide and the evaluation of its biological efficacy.

Introduction

Angiogenesis is a biological process that is characterized by the development of new blood vessels. Among many angiogenetic factors, αvβ3 integrin is associated with invasiveness, because the integrin is highly expressed in angiogenic tumor vessels but is absent in normal tissue1.

Radiolabeled receptor-binding peptides with the arginine glycine aspartate (RGD) domain, which has a high affinity toward αvβ3 integrin receptors, are considered promising angiogenesis imaging agents2,3,4,5,6,7. Several radiopharmaceuticals have been created for PET and its biological properties have been validated in various animal models8,9,10,11. In terms of a radionuclide, 68Ga has several advantages over other radioisotopes. Firstly, it has a high accessibility for users and is economically advantageous because a cyclotron is not required. Secondly, 68Ga-based radiopharmaceuticals produce high spatial resolution compared with single-photon emission computed tomography (SPECT), allowing more accurate quantification. Lastly, the 67.71 minutes half-life of 68Ga may be sufficient for the preparation of small peptides or proteins.

To produce a stable complex with 68Ga, many chelators have been developed. Representative chelators are 1,4,8,11-tetraazacyclotetradecanetetraacetic acid (TETA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), diethylenetriaminepentaacetic acid (DTPA), and N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED). NOTA has been reported to form a highly stable complex with 68Ga (log stability constant 30.98)12,13,14.

The purpose of the present study is to provide a concise protocol for the development of a new radiopeptide (Figure 1). As an example, we prepare 68Ga-labeled RGD-peptides and present methods for the biological evaluation of these analogues in a xenograft model.

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Protocol

All animal experiments were conducted in compliance with the Guidelines for the Care and Use of Research Animals under protocols approved by the Korea Institute of Radiological and Medical Sciences Animal Studies Committee. All reagents and solvents were purchased and used without further purification. NOTA-RGD-peptides were prepared according to literature methods15.

CAUTION: 68Ga emits both positron and gamma rays. All experiments, including direct or indirect contact with radioactive substances, must be undertaken by trained and permitted personnel only. When handling radioactive materials, proper protective equipment, shielding, radiation dosimeter badge and rings, and a survey meter should be used.

1. Radiolabeling RGD-peptides with 68GaCl3

Note: 68Ga (t1/2 = 68 min, β+ = 89%, and EC = 11%) was obtained from the 68Ga/68Ge generator.

  1. Elute the 68GaCl3 from the generator with 4 mL of 0.05 M HCl.
  2. Purge with nitrogen gas at 80 °C for 30 min to dry 68GaCl3 (333 kBq, 1 mL) in a 5 mL reaction vial.
  3. Add a solution of RGD-peptide (100 µg)in 1 M sodium acetate (100 µL, pH 5 - 6) to the reaction vial containing 68GaCl3 from step 1.2.
  4. Heat the reaction mixture at 80 °C for 5 min. Then, cool it down to room temperature.
  5. Purify the crude product with high-performance liquid chromatography (HPLC). Use the following system: a C-18 column, a flow rate of 0.5 mL/min, a gradient slope of acetonitrile of 1.17%/min (5% - 40% in 30 min), and elution components: A = 0.1% trifluoroacetic acid (TFA) in acetonitrile, B = 0.1% TFA in water.
    NOTE: The HPLC is equipped with a photodiode array detector and a radioactivity detector. The 68Ga-RGD-peptide was collected at a retention time of 12.5 min (Figure 2).
  6. Purify the resulting 68Ga-RGD-peptide using a solid phase extraction system.
    1. Pass the solution through a C18 reverse-phase cartridge and wash with 2 mL of saline.
    2. Elute 68Ga-RGD-peptide with 0.7 mL of 95% ethanol. Remove the solvent at 80 °C under nitrogen gas for 20 min and reconstitute with phosphate-buffered saline (PBS) before use.
    3. Filter the radiolabeled product through a 0.22 µm sterile filter and formulate in 1 mL of sterile saline solution.
  7. Check the radiochemical yield by radio-thin-layer chromatography (TLC).
    1. Spot 1 µL on an instant thin layer chromatography plate (ITLC, 10 cm in length). Develop the plate in a chamber containing the eluent (aqueous 0.1 M citric acid, pH 5.0) until 9 cm away from the spot.
      NOTE: The retention factor for 68Ga-RGD-peptide is 0 and the retention factor for unreacted 68Ga3+ is 1.
  8. Calculate the final specific activity from the ratio of radioactivity corresponding to the non-radioactivity as MBq/nmol.
    NOTE: After the injection of 100 µL of the formulated 68Ga-RGD-peptide to HPLC, the amount of non-radioactive component was calculated from the standard calibration curve using nonradioactive Ga-RGD-peptide.

2. In Vitro Cellular Uptake

Note: Uppsala 87 Malignant Glioma (U87MG) human glioblastoma cells were grown in Dulbecco's modified Eagle's media (DMEM), supplemented with 10% fetal bovine serum and 1% penicillin-streptomycin. Cells were grown in 150 mm dishes at 37 °C in a humidified atmosphere of 5% CO2. Cells were harvested or split by trypsinization: 0.25% (w/v) trypsin and 0.02% (w/v) ethylenediaminetetraacetic acid (EDTA) in PBS at 37 °C for 3 - 5 min.

  1. Seed U87MG cells into 6-well plates at a density of 1 x 106 cells/well.
  2. Incubate the cells with 68Ga-RGD-peptide (111 kBq) at 37 °C for 30, 60, 90, and 120 min. Prepare samples in triplicate.
  3. Wash the cells 2x with 2 mL of PBS and harvest by trypsinization. Use 0.25% (w/v) trypsin and 0.02% (w/v) ethylenediaminetetraacetic acid (EDTA) in PBS at 37 °C for 3 - 5 min.
  4. Collect the cell suspension (500 µL) and measure in a γ-counter.
  5. Calculate the percent uptake of the compound by the cells by % (counts in cells/total counts).

3. In Vitro Serum Stability

  1. Add 500 µL of freshly prepared mouse serum, 500 µL of human serum, and 500 µL of PBS. Incubate the mixture at 37 °C for 2 h.
  2. Evaluate by ITLC at the specified time intervals (30, 60, 90, and 120 min). Spot 1 - 2 µL aliquot of the mixture to the ITLC plate (mobile phase: 0.1 M citric acid). Develop the plate as in step 1.7.
    NOTE: 68Ga3+ is expected to move with the solvent front, whereas the labeled compound will remain at the origin.

4. Determination of Lipophilicity

  1. Add 68Ga-RGD-peptide (3.7 MBq, 3.7 µL) to the octanol-PBS system (1:1, v/v, total 1 mL).
  2. Mix the vials vigorously for 5 min at room temperature and centrifuge at 10,000 x g for 5 min at room temperature.
  3. Take 100 µL samples from each layer and measure the radioactivity with a γ-counter. The reported log P value is based on the average of three samples.

5. Tumor Model

Note: BALB/c nude mice (6 - 8 weeks old, female, n = 23) were used for this study. The mice were subsequently used for PET studies (n = 3) and biodistribution (n = 20) when the tumor volumes reached 200 - 300 mm3 (1 - 2 weeks after implantation).

  1. Load tumor cells into 28 G, 1/2 inch insulin syringes.
  2. Inject U87MG cells (5 x 106) in 100 µL of PBS into the left arm region.
  3. Anesthetize the mouse with 2% isoflurane in oxygen gas during cell injection.
    1. Ensure that the mouse has been anesthetized by the loss of the pedal withdrawal reflex following pinching with forceps between the toes of the right hind foot. Do not leave an animal unattended until it has regained sufficient consciousness to maintain sternal recumbency.

6. In Vivo Quantification of αvβ3 Integrin Using PET

  1. Anesthetize the mice with 2% isoflurane in oxygen.
    1. Ensure that the mouse has been anesthetized by the loss of the pedal withdrawal reflex following pinching with forceps between the toes of the right hind foot. Do not leave an animal unattended until it has regained sufficient consciousness to maintain sternal recumbency.
  2. Place the head in the center of the PET gantry.
  3. Intravenously administer the 68Ga-RGD-peptide solution (7.4 MBq, 200 µL) to the xenograft mouse model via the tail vein for 1 min.
  4. At the same time, perform a PET scan in list mode (dynamic scan) for 150 min.
    NOTE: The raw PET data were reconstructed by a user-defined time frame (i.e., every 30 min). After the PET scan, a micro-computed tomography (CT) scan (50 kVp of X-ray, 0.16 mA) was conducted for attenuation correction.

7. Ex Vivo Biodistribution

  1. Inject 68Ga-RGD-peptide (0.37 MBq, 200 µL) into the tail vein of the xenograft mouse model. Anesthetize the mouse with 2% isoflurane in oxygen gas during the injections.
    Note: BALB/c nude mice, as described in section 5, were divided into four groups and sacrificed at different time points (n = 5 per group).
  2. Wake the mice immediately after the administration of 68Ga-RGD-peptide and sacrifice them at 30, 60, 90, and 120 min postinjection with carbon dioxide euthanasia.
    NOTE: The tissues of interest were extracted. Selected targets were the blood, muscle, heart, lung, liver, spleen, stomach, intestine, kidneys, bone, and tumor.
  3. Weigh the tissue and measure the radioactivity with a γ-counter.
    NOTE: Results were expressed as the percentage injected dose per gram of tissue (% ID/g).

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Representative Results

The chelation of 68GaCl3 with the NOTA-RGD-peptide was straightforward, and the radiolabeling yield was 99%. Reaction impurities were successfully removed as shown in Figure 2. The radiochemical purity of 68Ga-RGD-peptide was greater than 99%, and specific activity at the end of the synthesis was 90 - 130 MBq/nmol (Figure 3).

The cell uptake values for 68Ga-RGD-peptide were 1.49%, 0.85%, 0.36%, and 0.39% at 30, 60, 90, and 120 min, respectively. Serum stability showed that 68Ga-RGD-peptide remained almost intact after 2 h of incubation with human or mouse serum as well as PBS (> 92% stability at 2 h). The partition coefficient (log P) was 2.96, indicating high lipophilicity. PET showed an initial high uptake in the major organs, including the liver, kidney, heart, muscle, and tumor. However, in the late period (90 - 150 min), the tumor region was clearly visualized. The tumor-to-muscle ratio at 90 min was 17.57 and remained unchanged, indicating kinetic stability. The ex vivo biodistribution showed that the accumulated radioactivity in the tumor was 6.19, 4.96, 4.44, and 4.39 (% ID/g) at 30, 60, 90, and 120 min, respectively. The results of the ex vivo experiment were in accordance with the in vivo PET findings (Figure 4).

Figure 1
Figure 1: Flow diagram of the experimental procedures. This figure shows a schematic overview of the development of radiopharmaceutical. Please click here to view a larger version of this figure.

Figure 2
Figure 2: Purification of 68Ga-RGD-peptide by HPLC. Blue is radioactivity signal and black is ultraviolet (UV) signal. The UV wavelength is 314 nm. The X-axis is time and the Y-axis is absorbance unit (AU). The 68Ga-RGD-peptide has 12.4 min of retention time. Please click here to view a larger version of this figure.

Figure 3
Figure 3: Structure of 68Ga-RGD-peptide and its radiochemical purity. The ITLC of 68Ga-RGD-peptide showed high radiochemical purity. Please click here to view a larger version of this figure.

Figure 4
Figure 4: PET imaging (upper) and ex vivo biodistribution data for 68Ga-RGD-peptide (lower). PET data were expressed on the SUV scale from 0 to 5. Biodistribution data shown are the mean ± the standard deviation from five mice at each time point. Please click here to view a larger version of this figure.

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Discussion

In the present study, we introduced a protocol to prepare a radiopeptide targeting αvβ3 integrin and its biological evaluation. Traditional drug development involves a complicated procedure. It requires a large quantity of reference material and a relatively long evaluation time. Although the suggested methodology cannot replace the delicate evaluation process, this system can be used for screening purposes. This proposed system would considerably reduce the time and cost.

Over the past decade, many radiolabeled RGD-peptides have been extensively studied as radiotracers for imaging tumors16. To obtain promising radiopharmaceuticals for clinical trials, systemic approaches for drug development should be provided. Radiochemical feasibility, high selectivity-affinity to the target, metabolic stability, and proper pharmacokinetics are four major concerns. For a routine PET study, a reasonable radiochemical yield ensures the reliability of the radiopharmaceuticals. The issues of high affinity (> nM) and selectivity (> 100x) to the target protein are also satisfied. In terms of pharmacokinetics, the candidate PET tracer is rapidly excreted from the non-target tissue and has a long retention time in the tumor, allowing a high target-to-reference ratio. Candidate radiopharmaceuticals should not have troublesome metabolites in vivo that could increase non-specific binding and provide low contrast imaging. It is important to assess the comprehensive characteristics because each term influences the other properties, which are not independent.

The radiopeptide introduced in this research has suitable drug-like properties. The 68Ga-RGD-peptide has a high radiochemical yield of 99%, metabolic stability, and proper lipophilicity. In the in vivo experiment, the radiopeptide exhibited high selectivity (tumor-to-reference ratio = 17.57), and the ex vivo biodistribution data also showed significant tumor uptake (up to 6.19% ID/g).

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Disclosures

The authors have nothing to disclose.

Acknowledgments

This work was supported by a Nuclear Research and Development Program of the National Research Foundation of Korea (NRF) grant funded by the Korean government (No. 2017M2A2A6A02019904).

Materials

Name Company Catalog Number Comments
68Ga/68Ge generator ITG Company - 10 mCi 
Hydrogen chloride solution Sigma-aldrich 84429
Sodium acetate Sigma-aldrich S2889
C18 reverse-phase cartridge Waters WAT020515
0.22-μm sterile filter Milllipore SLGV033RS
Radio-TLC scanner Bioscan AR2000
ITLC paper Agilent SGI001
Citric acid Sigma-aldrich 251275
HPLC Waters - Waters 1525 system containing binary pump, photo diode array (Waters 2998), radioactivity detector (Raytest, Gabi)
Acetonitrile J.T. Baker 14-650-359
Trifluoroacetic acid Sigma-aldrich 302031
Dulbecco's modified Eagle media  Thermo fisher scientific 11965092
fetal bovine serum Thermo fisher scientific 16000044
T175 flasks  Corning CLS431080
Trypsin-EDTA (0.25%) Thermo fisher scientific 25200072
penicillin-streptomycin Thermo fisher scientific 15240112
γ-counter Perkin Elmer - 1480 Wizard 3
Insunlin syringe Becton Dickinson 326105
Synringe pump Harvard Apparatus 70-4500
micro-PET/CT Siemens Inveon -

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References

  1. Friedlander, M., et al. Definition of Two Angiogenic Pathways by Distinct alpha v integrins. Science. 270 (5241), 1500-1502 (1995).
  2. Janssen, M. L., et al. Tumor Targeting with Radiolabeled alpha v beta 3 Integrin Binding Peptides in a Nude Mouse Model. Cancer Research. 62, 6146-6151 (2002).
  3. Kok, R. J., et al. Preparation and functional evaluation of RGD-modified proteins as αvβ3 integrin directed therapeutics. Bioconjugate Chemistry. 13 (1), 128-135 (2002).
  4. Garanger, E., et al. New multifunctional molecular conjugate vector for targeting, imaging, and therapy of tumors. Molecular Therapy. 12 (6), 1168-1175 (2005).
  5. Dijkgraaf, I., et al. PET imaging of αvβ3 integrin expression in tumours with 68Ga-labelled mono-, di- and tetrameric RGD peptides. European Journal of Nuclear Medicine and Molecular Imaging. 38 (1), 128-137 (2011).
  6. Liu, Z., et al. 68Ga-labeled cyclic RGD dimers with Gly3and PEG4linkers: Promising agents for tumor integrin αvβ3 PET imaging. European Journal of Nuclear Medicine and Molecular Imaging. 36 (6), 947-957 (2009).
  7. Li, Z. B., Chen, K., Chen, X. 68Ga-labeled multimeric RGD peptides for microPET imaging of integrin αvβ3expression. European Journal of Nuclear Medicine and Molecular Imaging. 35 (6), 1100-1108 (2008).
  8. Liu, S., et al. Isomerism and solution dynamics of 90Y-labeled DTPA-biomolecule conjugates. Bioconjugate Chemistry. 12 (1), 84-91 (2001).
  9. Haubner, R., et al. Glycosylated RGD-containing peptides: tracer for tumor targeting and angiogenesis imaging with improved biokinetics. Journal of Nuclear Medicine. 42 (2), 326-336 (2001).
  10. Sivolapenko, G. B., et al. Imaging of metastatic melanoma utilising a technetium-99m labelled RGD-containing synthetic peptide. Euroean Journal of Nuclear Medicine. 25 (10), 1383-1389 (1998).
  11. Haubner, R., et al. Noninvasive Imaging of αvβ3 Integrin Expression Using 18 F-labeled RGD-containing Glycopeptide and Positron Emission Tomography. Cancer Research. 61, 1781-1785 (2001).
  12. Clarke, E. T., Martell, A. E. Stabilities of trivalent metal ion complexes of the tetraacetate derivatives of 12-, 13- and 14-membered tetraazamacrocycles. Inorganica Chimica Acta. 190 (1), 37-46 (1991).
  13. Clarke, E. T., Martell, A. E. Stabilities of the Fe(III), Ga(III) and In(III) chelates of N,N′,N″-triazacyclononanetriacetic acid. Inorganica Chimica Acta. 181 (2), 273-280 (1991).
  14. Shetty, D., Lee, Y. S., Jeong, J. M. 68Ga-labeled radiopharmaceuticals for positron emission tomography. Nuclear Medicine Molecular Imaging. 44 (4), 233-240 (2010).
  15. Shin, U. C., et al. Synthesis and Preliminary Evaluation of 68Ga-NOTA-Biphenyl-c(RGDyK) for the Quantification of Integrin αvβ3. Bulletin of the Korean Chemical Society. 38 (12), 1415-1418 (2017).
  16. Cai, W., Chen, X. Multimodality Molecular Imaging of Tumor Angiogenesis. Journal of Nuclear Medicine. 49, suppl2 113-128 (2008).

Tags

68Ga-labeled Arginine Glycine Aspartate (RGD)-peptide Angiogenesis Preparation Method Biological Evaluation Radiometal-labeled Peptide Hydrochloric Acid Gallium Trichloride Gallium Germanium Generator Nitrogen Gas Sodium Acetate High Performance Liquid Chromatography C18 Column Reverse-phased Cartridge Saline Ethanol Reconstitution PBS Sterile Strainer
Preparing a <sup>68</sup>Ga-labeled Arginine Glycine Aspartate (RGD)-peptide for Angiogenesis
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Cite this Article

Jung, K. H., Lee, Y. J., Kim, J. Y., … More

Jung, K. H., Lee, Y. J., Kim, J. Y., Lee, K. C., Park, J. A., Choi, J. Y. Preparing a 68Ga-labeled Arginine Glycine Aspartate (RGD)-peptide for Angiogenesis. J. Vis. Exp. (143), e58218, doi:10.3791/58218 (2019).

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