Enhanced loading efficiency and retention of 225Ac in rigid liposomes for potential targeted therapy of micrometastases

Targeted alpha-particle emitters are promising therapeutics for micrometastatic disease. Actinium-225 has a 10-day half-life and generates a total of four alpha-particles per parent decay renderin Ac-225 an attractive candidate for alpha-therapy. For cancer cells with low surface expression levels of molecular targets, targeting strategies Ac-225 using radiolabeled carriers of low specific rachoactivities (such as antibodies) may not deliver enough a-particle emitters at the targeted cancer cells to result in killing. We previously proposed and showed using passive Ac-225 entrapment that liposomes can stably retain encapsulated Ac-225 for long time periods, and that antibody-conjugated liposomes (immunoliposomes) with encapsulated Ac-225 can specifically target and become internalized by cancer cells. However, to enable therapeutic use of Ac-225-containing liposomes, high activities of Ac-225 need to be stably encapsulated into liposomes. In this study, various conditions for active loading of Ac-225 in preformed liposomes (ionophore-type, encapsulated buffer solution, and loading time) were evaluated, and liposomes with up to 73 +/- 9% of the initial activity of Ac-225 (0.2-200 mu Ci) were developed. Retention of radioactive contents by liposomes was evaluated at 37 degrees C in phosphate buffer and in serum-supplemented media. The main fraction of released Ac-225 from liposomes occurs within the first two hours of incubation. Beyond this two hour point, the encapsulated radioactivity is released from liposomes slowly with an approximate half-life of the order of several days. In some cases, after 30 days, Ac-225 retention as high as 81 +/- 7% of the initially encapsulated radioactivity was achieved. The Ac-225 loading protocol was also applied to immunoliposome loading without significant loss of targeting efficacy. Liposomes with surface-conjugated antibodies that are loaded with Ac-225 overcome the limitations of low specific activity for molecular carriers and are expected to be therapeutically useful against tumor cells having a low antigen density.