Polymer-derivatized technetium 99mTc-labeled liposomal blood pool agents for nuclear medicine applications

By using the lipophilic chelator, dipalmitoylphosphatidylethanolamine-diethylenetriaminetetraacetic acid (DPPE-DTTA), lipid vesicles may be prepared labeled on their surface with technetium 99m. When technetium-labeled vesicles were injected intravenously into rabbits, the half-life for clearance of the label from the circulation was less than 30 min. By further incorporating a synthetic phosphatidylethanolamine-monomethoxypoly(ethylene glycol) 5000 conjugate (PE-MPEG) the circulation half-life of the radiolabel was increased, liver uptake decreased and exchange of technetium from the vesicle surface suppressed, depending upon both the DPPE-DTTA and PE-MPEG content. For vesicles containing 20 mol% DPPE-DTTA, incorporation of PE-MPEG had no effect upon the circulation half-life of the radiolabel, however, for vesicles containing 2 mol% DPPE-DTTA, incorporation of more than 4 mol% PE-MPEG increased the circulation half-life of the label to more than 12 h. Less than 2 mol% PE-MPEG or 8 mol% ganglioside GM1 were, however, ineffective at increasing the circulation half-life of surface-bound technetium. It was shown that unilamellar lipid vesicles with DPPE-DTTA can be lyophilized in the presence of external sucrose, subsequently rehydrated with no change in vesicle size and labeled with technetium. It is suggested that polymer-derivatized, technetium-labeled vesicles may prove a useful substitute for technetium-labeled red blood cells as a vascular marker in various nuclear medicine procedures and that lyophilization/rehydration provides a possible route to realization of such vesicles in a pharmaceutically useful form.