Anti-Prostate-Specific Membrane Antigen Liposomes Loaded with 225Ac for Potential Targeted Antivascular α-Particle Therapy of Cancer

This study evaluates targeted liposomes loaded with the alpha-particle generator Ac-225 to selectively kill prostate-specific membrane antigen (PSMA)-expressing cells with the aim to assess their potential for targeted antivascular radiotherapy. Methods: In this study, PEGylated liposomes were loaded with Ac-225 and labeled with the mouse antihuman PSMA J591 antibody or with the A10 PSMA aptamer. 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). Results: The loading efficiency of 225Ac into preformed liposomes ranged from 58.0% +/- 4.6% to 85.6% +/- 11.7% of introduced radioactivity. The conjugation reactions resulted in approximately 17 +/- 2 J591 antibodies and 9 +/- 2 A10 aptamers per liposome. The average size of liposomes, 107 +/- 2 nm in diameter, was not affected by conjugation or loading. LNCaP cells exhibit 2:1:0.5 relative PSMA expression, compared with MatLu and induced HUVEC, respectively, based on flow cytometry detecting association of the J591 antibody. J591-labeled liposomes display higher levels of total specific binding to all cell lines than A10 aptamer-labeled liposomes. Specific cell association of targeted liposomes increases with incubation time. Cytotoxicity studies demonstrate that radiolabeled J591-labeled liposomes are most cytotoxic, with median lethal dose values, after 24 h of incubation, equal to 1.96 (5.3 x 10(-5)), 2.92 x 10(2) (7.9 x 10(-3)), and 2.33 x 10(1) Bq/mL (6.3 x 10(-4) mu Ci/mL) for LNCaP, Mat-Lu, and induced HUVEC, respectively, which are comparable to the values for the radiolabeled J591 antibody. For A10 aptamer-labeled liposomes, the corresponding values are 3.70 x 10(1) (1.0 x 10(-3)), 1.85 x 10(3) (5.0 x 10(-2)), and 4.07 x 10(3) Bq/mL (1.1 x 10(-1) mu Ci/mL), respectively. Conclusion: Our studies demonstrate that anti-PSMA-targeted liposomes loaded with Ac-225 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 optimizing biodistributions-suggest the potential of PSMA-targeting liposomes encapsulating alpha-particle emitters for selective antivascular alpha radiotherapy.