Background

[PubMed]

Endothelial cells are important in inflammatory responses (1, 2). The presence of bacterial lipopolysaccharide (LPS), virus growth, inflammation, and tissue injury increase secretion of tumor necrosis factor α (TNFα), interleukin-1 (IL-1), and other cytokines and chemokines. Emigration of leukocytes from the blood is dependent on their ability to roll along endothelial cell surfaces and subsequently adhere to endothelial cell surfaces. Inflammatory mediators and cytokines induce chemokine secretion from endothelial cells and other vascular cells, and they also increase their expression of cell-surface adhesion molecules, such as intracellular adhesion molecule-1, vascular cell adhesion molecule-1, integrins, and selectins. Chemokines are chemotactic toward leukocytes and toward sites of inflammation and tissue injury. The movement of leukocytes through endothelial junctions into the extravascular space are highly orchestrated through various interactions with different adhesion molecules on endothelial cells (3).

Liposomes are double-membrane lipid vesicles (4). They have been widely studied as important carriers in controlling the spatial and temporal distribution of drug molecules or other bioactive molecules for targeted therapy; for example, they have been used as universal carriers of tumor chemotherapeutic agents, as antigen carriers to stimulate immune response, as carriers of nucleic acid for gene therapy, and as carriers of antibiotics for infectious disease treatment (5). Marik et al. (6) developed liposomes by incorporation of [¹⁸F]fluorodipalmitin ([¹⁸F]FDP) into the phospholipid bilayer to minimize internalization and metabolism in cells. Therefore, the images obtained using [¹⁸F]FDP-liposomes could be more reflective of the biodistribution of long-circulating liposomes. The linear peptide Cys-Arg-Pro-Pro-Arg (CRPPR) has been found to specifically bind to heart endothelium using phage display screenings. Zhang et al. (7) coupled polyethylene glycol-stearic acid to CRPPR to form the lipo-PEG-CRPPR peptide (LPP), which was later incorporated into [¹⁸F]FDP-liposomes. The resulting CRPPR-[¹⁸F]FDP-liposomes were found to home to mouse hearts as determined with dynamic positron emission tomography.

Synthesis

[PubMed]

CRPPR was prepared by standard solid-phase synthesis (7). Polyethylene glycol (PEG; molecular weight, 1,200 (m = 1), 2,400 (m = 2), or 3,600 Da (m = 3)) was coupled onto the peptidyl resin followed by Fmoc-Lys(Fmoc)-OH and stearic acid. LLP was cleaved from the resin and purified with high-performance liquid chromatography with a 20% yield. LLP was dimerized in 0.01 M ammonium bicarbonate. [¹⁸F]FDP was synthesized via nucleophilic substitution of the p-toluenesulfonyl moiety of 3-tosyl-1,2-dipalmitoyl glycerol with [¹⁸F]F^– as reported previously (6). [¹⁸F]KF/Kryptofix 2.2.2/K₂CO₃ and 3-tosyl-1,2-dipalmitoyl glycerol were heated in acetonitrile to 100°C for 20 min with a decay-corrected yield of 43 ± 10% (n = 12). [¹⁸F]FDP was purified with chromatography and exhibited a radiochemical purity of 99%, a total synthesis time of 60 min, and a specific activity >111 GBq/mmol (3 Ci/mmol). Long-circulating radiolabeled PEG-coated liposomes were prepared by adding a CH₂Cl₂ solution of [¹⁸F]FDP to a mixture of dimerized LLP, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(PEG)-2000] (DSPE-PEG2000) (7). After evaporation of the solvent, phosphate-buffered saline (PBS) was added to the residue and sonicated at 60°C for 1 min. The CRPPR-[¹⁸F]FDP-liposomes were purified with gel filtration. The size of CRPPR-3-[¹⁸F]FDP-liposomes was 110 ± 38 nm with a molar ratio of CRPPR-3:DSPE-PEG2000:DPPC (6%:6%:88%). Radiochemical yield and specific activity of CRPPR-3-[¹⁸F]FDP-liposomes were not reported.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Flow cytometry analysis showed that both endothelial and melanoma cells incubated with CRPPR-3-liposomes containing calcein exhibited higher fluorescence intensity than cells incubated with non-targeted liposomes (7).

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

R01 CA103828, R24 CA110804

References

1.

Cybulsky M.I. , Gimbrone M.A. Endothelial expression of a mononuclear leukocyte adhesion molecule during atherogenesis. Science. 1991; 251 (4995):788–91. [PubMed: 1990440]

2.

Lowe J.B. Glycosylation in the control of selectin counter-receptor structure and function. Immunol Rev. 2002; 186 :19–36. [PubMed: 12234359]

3.

Vanderslice P. , Woodside D.G. Integrin antagonists as therapeutics for inflammatory diseases. Expert Opin Investig Drugs. 2006; 15 (10):1235–55. [PubMed: 16989599]

4.

Torchilin V.P. Liposomes as delivery agents for medical imaging. Mol Med Today. 1996; 2 (6):242–9. [PubMed: 8796897]

5.

Torchilin V.P. Targeted pharmaceutical nanocarriers for cancer therapy and imaging. Aaps J. 2007; 9 (2):E128–47. [PMC free article: PMC2751402] [PubMed: 17614355]

6.

Marik J. , Tartis M.S. , Zhang H. , Fung J.Y. , Kheirolomoom A. , Sutcliffe J.L. , Ferrara K.W. Long-circulating liposomes radiolabeled with [18F]fluorodipalmitin ([18F]FDP). Nucl Med Biol. 2007; 34 (2):165–71. [PMC free article: PMC1849971] [PubMed: 17307124]

7.

Zhang H. , Kusunose J. , Kheirolomoom A. , Seo J.W. , Qi J. , Watson K.D. , Lindfors H.A. , Ruoslahti E. , Sutcliffe J.L. , Ferrara K.W. Dynamic imaging of arginine-rich heart-targeted vehicles in a mouse model. Biomaterials. 2008; 29 (12):1976–88. [PMC free article: PMC2475513] [PubMed: 18255141]