Influence of PEGylation and RGD loading on the targeting properties of radiolabeled liposomal nanoparticles

Abstract

Purpose

Liposomes have been proposed to be a means of selectively targeting cancer sites for diagnostic and therapeutic applications. The focus of this work was the evaluation of radiolabeled PEGylated liposomes derivatized with varying amounts of a cyclic arginyl–glycyl–aspartic acid (RGD) peptide. RGD peptides are known to bind to α_vβ₃ integrin receptors overexpressed during tumor-induced angiogenesis.

Methods

Several liposomal nanoparticles carrying the RGD peptide targeting sequence (RLPs) were synthesized using a combination of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, cholesterol, diethylenetriaminepentaacetic acid-derivatized lipids for radiolabeling, a polyethylene glycol (PEG) building block, and a lipid-based RGD building block. Relative amounts of RGD and PEG building blocks were varied. In vitro binding affinities were determined using isolated α_vβ₃ integrin receptors incubated with different concentrations of RLPs in competition with iodine-125-labeled cyclo-(-RGDyV-). Binding of the indium-111-labeled RLPs was also evaluated. Biodistribution and micro single photon emission computed tomography/computed tomography imaging studies were performed in nude mice using different tumor xenograft models.

Results

RLPs were labeled with indium-111 with high radiochemical yields. In vitro binding studies of RLPs with different RGD/PEG loading revealed good binding to isolated receptors, which was dependent on the extent of RGD and PEG loading. Binding increased with higher RGD loading, whereas reduced binding was found with higher PEG loading. Biodistribution showed increased circulating time for PEGylated RLPs, but no dependence on RGD loading. Both biodistribution and micro single photon emission computed tomography/computed tomography imaging studies revealed low, nonspecific tumor uptake values.

Conclusion

In this study, RLPs for targeting angiogenesis were described. Even though good binding to α_vβ₃ integrin receptors was found in vitro, the balance between PEGylation and RGD loading clearly requires optimization to achieve targeting in vivo. These data form the basis for future development and provide a platform for the investigation of multimodal approaches.

Keywords:

• liposomes
• RGD peptides
• α_vβ₃ integrin receptors
• angiogenesis
• tumor targeting

Acknowledgments

Dr David A Cheresh and the Scripps Research Institute (La Jolla, CA) are acknowledged for providing the human melanoma M21 and M21-L cells. Jerome Burnet, Ciara Finucane, and Julie Foster assisted in the microSPECT/CT imaging studies carried out at the Center for Molecular Oncology, Barts Cancer Institute, Queen Mary University of London. The authors would like to express their thanks to Glen Perera, Institute of Pharmacy, University of Innsbruck, for assistance with the zeta potential measurements. The Austrian Nano-Initiative cofinanced this work as part of the Nano-Health project (number 0200), the subprojects NANO-NUC were financed by the Austrian FWF (Fonds zur Forderung der Wissenschaftlichen Forschung; number N208-NAN), and NANO-LIPO was financed by the Austrian Research Promotion Agency FFG (project Nano-Health number 819721).

Disclosure

The authors report no conflicts of interest in this work.