Mitochondria-tropic radioconjugates to enhance the therapeutic potential of terbium-161

Strategies that focus on delivering Auger electron emitters to highly radiosensitive intracellular targets-such as the nucleus, cell membrane, or mitochondria-are gaining attention. Targeting these organelles could enhance therapeutic efficacy while minimizing off-target toxicity by allowing lower administered doses. In this context, this study explores the therapeutic potential of ¹⁶¹Tb-labeled radiocomplexes that integrate the mitochondria-targeting triphenylphosphonium (TPP) moiety with a prostate-specific membrane antigen (PSMA) targeting vector. The goal is to assess these dual-targeted radiocomplexes for their ability to deliver conversion electrons (CE) and Auger electrons (AEs) to prostate cancer (PCa) cells, specifically targeting the mitochondria to enhance therapeutic efficacy.

Two novel radiocomplexes, [¹⁶¹Tb]Tb-TPP-PSMA and [¹⁶¹Tb]Tb-TPP-G₃-PSMA, were synthesized with high radiochemical yield and purity. The proposed structures were validated using HPLC and ESI-MS analysis, with their ^natTb counterparts serving as reference compounds. In vitro experiments included cellular uptake, internalization, mitochondrial uptake, and DNA damage assays in PSMA-positive PCa cell lines. Clonogenic assays were performed to evaluate cell survival post-treatment. In vivo studies were conducted using SCID/Beige mice bearing PCa xenografts and involved µSPECT/CT imaging and radiometabolite analysis to evaluate biodistribution, pharmacokinetics, tumor uptake and in vivo stability of the radiocomplexes. Both [¹⁶¹Tb]Tb-TPP-PSMA and [¹⁶¹Tb]Tb-TPP-G₃-PSMA showed high radiochemical stability and were efficiently internalized by PSMA-positive cells, while showing minimal uptake in PSMA-negative cells. These dual-targeted radiocomplexes demonstrated significantly higher mitochondrial uptake compared to the non-TPP-containing [¹⁶¹Tb]Tb-PSMA-617, leading to increased DNA damage and enhanced radiocytotoxicity. In vivo, the dual-targeted complexes demonstrated PSMA-specific tumor uptake and pharmacokinetics comparable to [¹⁶¹Tb]Tb-PSMA-617, with effective clearance from non-target tissues.

The TPP-modified ¹⁶¹Tb-radiocomplexes effectively targeted the mitochondria of PSMA-positive PCa cells, leading to increased DNA damage and reduced cell viability compared to single-targeted radiocomplexes. These findings suggest that dual-targeting strategies, which combine PSMA and mitochondrial targeting, can enhance the therapeutic potential of radiopharmaceuticals for prostate cancer treatment.