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Theranostics 2018; 8(21):5828-5841. doi:10.7150/thno.26706 This issue Cite

Research Paper

Remote loading of liposomes with a ¹²⁴I-radioiodinated compound and their in vivo evaluation by PET/CT in a murine tumor model

Gokce Engudar^1,5, Henrik Schaarup-Jensen^1,5, Frederikke P. Fliedner², Anders E. Hansen^2,3,5, Paul Kempen^3,5, Rasmus I. Jølck^3,5, Andreas Kjaer², Thomas L. Andresen^3,5, Mads H. Clausen^1,5, Andreas I. Jensen^4,5, Jonas R. Henriksen^3,5✉

1. Department of Chemistry, Technical University of Denmark, 2800 Lyngby, Denmark.
2. Cluster for Molecular Imaging, Dept. of Biomedical Sciences and Dept. of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet and University of Copenhagen, 2200 Copenhagen.
3. Department of Micro- and Nanotechnology, Technical University of Denmark, 2800 Lyngby, Denmark.
4. Center for Nuclear Technologies, Technical University of Denmark, 4000 Roskilde, Denmark.
5. Center for Nanomedicine and Theranostics, Denmark.
*Corresponding author: Jonas R. Henriksen, jhen@nanotech.dtu.dk

✉ Corresponding author:

Abstract

Long circulating liposomes entrapping iodinated and radioiodinated compounds offer a highly versatile theranostic platform. Here we report a new methodology for efficient and high-yield loading of such compounds into liposomes, enabling CT/SPECT/PET imaging and ¹³¹I-radiotherapy.

Methods: The CT contrast agent diatrizoate was synthetically functionalized with a primary amine, which enabled its remote loading into PEGylated liposomes by either an ammonium sulfate- or a citrate-based pH transmembrane gradient. Further, the amino-diatrizoate was radiolabeled with either ¹²⁴I (t_1/2 = 4.18 days) for PET or ¹²⁵I (t_1/2 = 59.5 days) for SPECT, through an aromatic Finkelstein reaction.

Results: Quantitative loading efficiencies (>99%) were achieved at optimized conditions. The ¹²⁴I-labeled compound was remote-loaded into liposomes, with an overall radiolabeling efficiency of 77 ± 1%, and imaged in vivo in a CT26 murine colon cancer tumor model by PET/CT. A prolonged blood circulation half-life of 19.5 h was observed for the radiolabeled liposomes, whereas injections of the free compound were rapidly cleared. Lower accumulation was observed in the spleen, liver, kidney and tumor than what is usually seen for long-circulating liposomes.

Conclusion: The lower accumulation was interpreted as release of the tracer from the liposomes within these organs after accumulation. These results may guide the design of systems for controlled release of remote loadable drugs from liposomes.

Keywords: remote loading, pH-gradient liposomes, transmembrane ammonium sulfate, iodinated imaging agents, I-124, PET imaging

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