(119c) Anti-PSMA Liposomes Loaded With Actinium-225 for Potential Targeted Antivascular Alpha-Particle Therapy of Cancer

Objectives. We study targeted liposomes loaded with the alpha-particle generator Actinium-225 (²²⁵Ac) to selectively kill prostate-specific-membrane-antigen-(PSMA)-expressing cells with the aim to evaluate their potential for targeted antivascular alpha-radiotherapy.  This is a bottom-up design and the rationale for the chosen components is the following: The targeted moiety is PSMA which is unique to human tumor vasculature for several different types of primary human cancers. The therapeutic radionuclide, ²²⁵Ac, emits a total of four alpha-particles per decay providing highly lethal and localized irradiation of targeted cells with minimal exposure of surrounding healthy tissues. And, finally, the delivery carrier is a lipid-based nanocarrier (a liposome). Liposomes can be loaded with high radioactivities, and, in terms of toxicities, liposomes can be easily engineered (by altering their size and surface characteristics) to become rapidly cleared from circulation with the aim to shift the distribution of normal organ toxicities away from the kidneys (the dose limiting organ in the case of ²²⁵Ac-labeled antibodies) and toward the spleen and the liver.

Methods. Actinium-225 was loaded in preformed PEGylated liposomes encapsulating DOTA using the A23187 ionophore. Different anti-PSMA ligands (antibodies, a urea-based ligand, and A10 aptamer) were conjugated to liposomes. The targeting selectivity, extent of internalization and killing efficacy of liposomes were evaluated on monolayers of prostate cancer cells intrinsically expressing PSMA (human LNCaP and rat Mat-Lu cells) and on monolayers of HUVEC induced to express PSMA (induced HUVEC) in static incubation conditions and under flow at a 15 s^-1 shear rate using a parallel-plate flow chamber. The effect of liposomes’ surface functionalization and length of incubation on the observed killing efficacy was evaluated.

Results. The loading efficiency of ²²⁵Ac into preformed liposomes ranged from 58 % to 85 % of introduced radioactivity. The conjugation reactions resulted in approximately 17 antibodies and 9 A10 aptamers per liposome. The density of approximately 300 urea-based ligands per liposome was measured to exhibit optimum behavior. Anti-PSMA-Ab-labeled liposomes display the highest levels of total specific binding to all cell lines followed by the urea-based labeled liposomes and then by the A10 aptamer-labeled liposomes. Specific cell association of targeted liposomes increases with incubation time and decreases by two-to-three fold upon introduction of the flow field. The cytotoxicity studies, which are currently in progress, demonstrate that the radiolabeled anti-PSMA-Ab-labeled liposomes exhibit LD₅₀ values comparable to the corresponding radiolabeled antibody.

Conclusions. Our studies demonstrate that liposomes targeted to PSMA via various ligands and loaded with ²²⁵Ac selectively bind, become internalized and kill PSMA-expressing cells including endothelial cells induced to express PSMA. These findings combined with the unique ability of liposomes to be easily tuned - in terms of size and surface modification - for optimum biodistributions suggest the potential of PSMA targeting liposomes encapsulating alpha-particle emitters for selective antivascular alpha-radiotherapy.