Preparation of long circulating 18F-labeled liposomes via conventional maleimide-thiol coupling for PET imaging

Objectives Liposomes are tiny unilamellar vesicles comprised of phospholipid bilayers with an aqueous interior. Long-circulating [18F]liposomes may be useful blood-pool agents for PET imaging[1], but are difficult to prepare practically through an intermediate 18F-lipid. The goal of this study was to therefore develop a method to enable the direct fluorine-18 radiolabeling of preformed liposomes.

Methods A fluorine-18 labeled 2-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)ethanethiol was prepared by the reaction of its trityl protected mesyl precursor with [18F]TBAF followed by deprotection of the thiol group with trifluoroacetic acid. Maleimide-PEG liposomes (diameter of 70 nm) were obtained by sonication of a lipid mixture consisting of DPPC, cholesterol and DSPE-PEG2000 (61:30:9) in acetate buffer (pH 6.5) followed by continuous high-pressure extrusion. Final conjugation of the maleimide-PEG liposome with [18F]thiol in 0.1M acetate buffer (0.1% EDTA), pH 6.5-8.0, in the presence of TCEP for 20 min gave the [18F]liposome. The product was purified by a mini-PD MiniTrap G-25 column. The pharmacokinetics and imaging potential of these batches were evaluated in rats using micro PET imaging.

Results The conjugation yield determined by HPLC was 74-92%. DLS analysis of the [18F]liposomes indicated that coupling with [18F]thiol did not have any major effect on size (diameter of 75 nm). The overall radiochemical yield of the reaction was 10-25% (n= 12, uncorrected) in a 180 minute radiolabeling time. The images obtained from the rats had high blood retention over 4 h and were comparable to images obtained from previous synthetic methods.

Conclusions We have successfully radiolabeled a preformed liposome via conjugation of a maleimide-PEG-functionalized liposome with the thiol group of 2-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)ethanethiol.

Research Support This study was funded by the intramural program of the National Institutes of Health