Use of (99m)Tc-labeled liposomes encapsulating blue dye for identification of the sentinel lymph node

Colloidal radiopharmaceuticals are commonly used in combination with blue dye for localization of the sentinel node. Liposomes are colloidal particles composed of spontaneously forming lipid spheres that can carry a wide variety of diagnostic and therapeutic agents. Conventional liposomes are poorly retained in lymph nodes (<2% of the subcutaneously injected dose). We have previously described a system for increasing the retention of liposomes in the lymph nodes by approximately 7-fold. This system is comprised of subcutaneously injected biotin-coated liposomes, followed by an adjacent injection of avidin. When the avidin moves into the lymphatic vessels, it causes aggregation of the biotin-coated liposomes that are also in the process of migrating through the lymphatic vessels. These aggregated liposomes become entrapped in the next encountered lymph node. In this study, we use this novel lymph node delivery system with liposomes that encapsulate blue dye, resulting in intense blue staining of the sentinel node. These liposomes can also be labeled with (99m)Tc, permitting scintigraphic imaging and radioguided probe localization of the sentinel node.

Liposomes coated with biotin and coencapsulating blue dye and glutathione were labeled with (99m)Tc using hexamethylpropyleneamine oxime. Rabbits were subcutaneously administered 0.3 mL (99m)Tc-biotin-liposomes containing blue dye in both hind feet, followed by a subcutaneous injection (0.3 mL) of 5 mg avidin in only one hind foot (experimental). The other hind foot served as a control.

Labeling efficiencies (mean +/- SEM) for liposomes encapsulating blue dye were 92.1% +/- 1.9%. Necropsy at 24 h revealed that the popliteal node on the experimental leg receiving the avidin was intensely blue stained compared with virtually no blue coloration of the control node. Tissue counts of these nodes were 12.2 +/- 1.5 percentage injected dose (%ID) in the experimental node compared with 1.2 +/- 0.1 %ID in the control nodes (P<0.0001).

Biotin-liposomes encapsulating blue dye can be successfully labeled with (99m)Tc, providing a convenient option for the visualization and radiolocalization of the sentinel node. This biotin-liposome/avidin system may also have potential for the delivery of therapeutic drugs and radiopharmaceuticals to lymph nodes.