Synthesis and preclinical evaluation of PSMA-Targeted ¹¹¹In-Radioconjugates containing a Mitochondria-Tropic Triphenylphosphonium carrier

Nuclear DNA is the canonical target for biological damage induced by Auger electrons (AE) in the context of targeted radionuclide therapy (TRT) of cancer, but other subcellular components might also be relevant for this purpose, such as the energized mitochondria of tumor cells. Having this in mind, we have synthesized novel DOTA-based chelators carrying a prostate-specific membrane antigen (PSMA) inhibitor and a triphenyl phosphonium (TPP) group that were used to obtain dual-targeted ¹¹¹In-radioconjugates ([¹¹¹In]In-TPP-DOTAGA-PSMA and [¹¹¹In]In-TPP-DOTAGA-G₃-PSMA), aiming to promote a selective uptake of an AE-emitter radiometal (¹¹¹In) by PSMA+ prostate cancer (PCa) cells and an enhanced accumulation in the mitochondria. These dual-targeted ¹¹¹In-radiocomplexes are highly stable under physiological conditions and in cell culture media. The complexes showed relatively similar binding affinities toward the PSMA compared to the reference tracer [¹¹¹In]In-PSMA-617, in line with their high cellular uptake and internalization in PSMA+ PCa cells. The complexes compromised cell survival in a dose-dependent manner and in the case of [¹¹¹In]In-TPP-DOTAGA-G₃-PSMA to a higher extent than observed for the single-targeted congener [¹¹¹In]In-PSMA-617. μSPECT imaging studies in PSMA+ PCa xenografts showed that the TPP pharmacophore did not interfere with the excellent in vivo tumor uptake of the “golden standard” [¹¹¹In]In-PSMA-617, although it led to a higher kidney retention. Such kidney retention does not necessarily compromise their usefulness as radiotherapeutics due to the short tissue range of the Auger/conversion electrons emitted by ¹¹¹In. Overall, our results provide valuable insights into the potential use of mitochondrial targeting by PSMA-based radiocomplexes for efficient use of AE-emitting radionuclides in TRT, giving impetus to extend the studies to other AE-emitting trivalent radiometals (e.g., ¹⁶¹Tb or ¹⁶⁵Er) and to further optimize the designed dual-targeting constructs.