This study evaluated two Tc99m-liposome formulations as potential blood-pool agents in comparison with standard 99mTc-red cells and 99mTc-human serum albumin (HSA). 40 rabbits: Liposomes with no surface modification or coated with polyethylene glycol (PEG) were labeled with 99mTc using the lipophilic chelator, HMPAO. Autologous red cells were labeled with 99nTc using in vitro or in vivo techniques. Technetium-99m-HSA was supplied commercially. Rabbits were injected intravenously with 99mTcliposomes, 99mTcred cells or 99mTc-HSA.Static images were acquired and blood samples collected. Results: Technetium-99m-liposome images showed prominent blood-pool activity compared to lung and liver activities, which were similar to those acquired for 99mTc-red cells, but better than 99mTc-HSA. Heart-to-lung ratios were not significantly different between 99mTc-liposome as compared to 99mTcred cells. The ratios were higher than for 99mTc-HSA. Heart-to-liver ratios were higher for PEG 99mTc-liposomes than they were for neutral 99mTc-liposomes and 99mTc-HSA, but were not significantly different than 99mTc-red cells. Bladder activities for both 99mTc-liposome was 3-6 times lower than for the other radiopharmaceuticals.PEG 99mTcliposomes remained in circulation 1.6 times longer than any of the other agents.Conclusions: Technetium-99m-liposomes, independent of surface modification, had excellent in vivo stability when compared to 99mTc-red cells and 99mTc-HSA. PEG 99mTc-liposomes are better than neutral 99mTc-liposomes due to lower liver background activity.
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