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¹⁸⁶Re-Liposome Labeling Using ¹⁸⁶Re-SNS/S Complexes: In Vitro Stability, Imaging, and Biodistribution in Rats

¹ Department of Radiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas
² Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas

Liposomes are important carriers for controlling the spatial and temporal distribution of drug molecules or other bioactive molecules. Radiolabeled liposomes have potential applications in diagnostic imaging and radionuclide therapy. The purpose of this study was to develop a practical method for labeling liposomes with therapeutic rhenium radionuclides, using ¹⁸⁶Re as an example. Methods: An SNS pattern ligand, N,N-bis(2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA), and an S pattern ligand, benzene thiol (BT), were used to make 2 kinds of ¹⁸⁶Re-SNS/S complexes, ¹⁸⁶Re-BMEDA and ¹⁸⁶Re-BMEDA + BT. These ¹⁸⁶Re-SNS/S complexes were mixed with neutral liposomes encapsulating cysteine or (NH₄)₂SO₄ to prepare ¹⁸⁶Re-liposomes. The in vitro labeling stability of ¹⁸⁶Re-liposomes was investigated by incubation in 50% fetal bovine serum/50% phosphate-buffered saline, pH 7.4, at 37°C. Rat distribution studies of ¹⁸⁶Re-liposomes after intravenous injection were also performed. Results: The labeling efficiencies of ¹⁸⁶Re-liposomes were 52.9%–81.3% depending on the ¹⁸⁶Re-SNS/S complex chosen and whether cysteine- or (NH₄)₂SO₄-encapsulated liposomes were used. ¹⁸⁶Re-(NH₄)₂SO₄ liposomes labeled with ¹⁸⁶Re-BMEDA had the best in vitro labeling stability in serum with 89.8% ± 3.1% of the radioactivity associated with liposomes at 24 h and 76.2% ± 5.1% at 96 h. A specific activity of 1.85 GBq (50 mCi) of ¹⁸⁶Re per 50 mg of phospholipid could be achieved with good labeling stability. Biodistributions were followed for 72 h and showed good in vivo stability for ¹⁸⁶Re-liposomes that was characterized by a slow blood clearance and a gradually increasing spleen accumulation. ¹⁸⁶Re-BMEDA alone had fast blood clearance and no accumulation in spleen. Conclusion: A practical method for labeling liposomes with ¹⁸⁶Re using ¹⁸⁶Re-SNS/S complexes is described. The labeled ¹⁸⁶Re-liposomes were stable in serum and in vivo and could potentially be useful for radionuclide therapy.

Key Words: liposomes • ¹⁸⁶Re • SNS/S complexes • radiolabeling • biodistribution • radionuclide therapy

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