Biodistribution studies of liposome encapsulated hemoglobin (LEH) studied with a newly developed 99m-technetium liposome label

A new method has been developed to label preformed liposomes with 99m-technetium (99m-Tc) using hexamethylpropylenamine oxime (HMPAO). 99m-Tc is an ideal isotope for performing noninvasive dynamic biodistribution studies. This labeling method results in a high labeling efficiency (>95%) and is stable as determined by both in vitro and in vivo studies. In vitro studies indicate that glutathione contained in LEH is important in the labeling process with 99m-Tc-HMPAO. In vivo studies were performed on 7 rabbits with dynamic scintigraphic 1 minute images performed from 1-120 minutes. Delayed images were performed at 20 hours followed by sacrifice and organ counting. Dynamic images reveal a gradual deposition of the LEH in the liver and spleen. Twenty-hour biodistributions revealed 50% of LEH remaining in the blood, 17% in the liver, 15% in the spleen, 3% in the lungs, 3% in muscle with trace amounts in the brain, kidneys and heart. Doses per gram were highest in the spleen with 10% of the injected dose per gram of spleen vs. 0.2% per gram in liver. This labeling technique is an effective method for noninvasively monitoring dynamic changes in liposome biodistribution and can be used to study effects of various liposome modifications on biodistribution.