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 (186)Re 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 (186)Re-SNS/S complexes, (186)Re-BMEDA and (186)Re-BMEDA + BT. These (186)Re-SNS/S complexes were mixed with neutral liposomes encapsulating cysteine or (NH(4))(2)SO(4) to prepare (186)Re-liposomes. The in vitro labeling stability of (186)Re-liposomes was investigated by incubation in 50% fetal bovine serum/50% phosphate-buffered saline, pH 7.4, at 37 degrees C. Rat distribution studies of (186)Re-liposomes after intravenous injection were also performed.
Results: The labeling efficiencies of (186)Re-liposomes were 52.9%-81.3% depending on the (186)Re-SNS/S complex chosen and whether cysteine- or (NH(4))(2)SO(4)-encapsulated liposomes were used. (186)Re-(NH(4))(2)SO(4) liposomes labeled with (186)Re-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 (186)Re 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 (186)Re-liposomes that was characterized by a slow blood clearance and a gradually increasing spleen accumulation. (186)Re-BMEDA alone had fast blood clearance and no accumulation in spleen.
Conclusion: A practical method for labeling liposomes with (186)Re using (186)Re-SNS/S complexes is described. The labeled (186)Re-liposomes were stable in serum and in vivo and could potentially be useful for radionuclide therapy.