Towards Targeted Alpha Therapy with Actinium-225: Chelators for Mild Condition Radiolabeling and Targeting PSMA—A Proof of Concept Study

SIMPLE SUMMARY: In nuclear medicine, therapeutic methods are used increasingly, in which tumors are destroyed by ionizing radiation that cannot or can only be treated insufficiently by other methods like surgery or chemotherapy. Targeted alpha therapy (TAT) is a promising method with increasing importance that facilitates new treatment options for advanced and late-stage cancer diseases. The effectiveness of alpha-emitting radionuclides is characterized by a higher linear energy transfer and a higher biological efficacy, compared to therapeutic approaches with beta emitters. (225)Ac is an alpha emitter with favorable nuclear properties for radiopharmaceutical applications. The aim of our research was to find a universal chelator that enables the attachment of sensitive bio(macro)molecules and allows (225)Ac-radiolabeling under mild conditions. An aza-macrocycle-derived mcp chelator with functional groups for universal connection of biomolecules using convenient click chemistry was developed for the (225)Ac-labeling. The resulting (225)Ac-radioconjugates were analyzed in vitro and in vivo, showing a high receptor affinity on tumor cells and a high tumor accumulation in tumor-bearing mice. ABSTRACT: Currently, targeted alpha therapy is one of the most investigated topics in radiopharmaceutical cancer management. Especially, the alpha emitter (225)Ac has excellent nuclear properties and is gaining increasing popularity for the treatment of various tumor entities. We herein report on the synthesis of two universal (225)Ac-chelators for mild condition radiolabeling and binding to conjugate molecules of pharmacological interest via the copper-mediated click chemistry. A convenient radiolabeling procedure was investigated as well as the complex stability proved for both chelators and two PSMA (prostate-specific membrane antigen)-targeting model radioconjugates. Studies regarding affinity and cell survival were performed on LNCaP cells followed by biodistribution studies, which were performed using LNCaP tumor-bearing mice. High efficiency radiolabeling for all conjugates was demonstrated. Cell binding studies revealed a fourfold lower cell affinity for the PSMA radioconjugate with one targeting motif compared to the radioconjugate owing two targeting motifs. Additionally, these differences were verified by in vitro cell survival evaluation and biodistribution studies, both showing a higher cell killing efficiency for the same dose, a higher tumor uptake (15%ID/g) and a rapid whole body clearance after 24 h. The synthesized chelators will overcome obstacles of lacking stability and worse labeling needs regarding (225)Ac complexation using the DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid) chelator. Moreover, the universal functionalization expands the coverage of these chelators in combination with any sensitive bio(macro)molecule, thus improving treatment of any addressable tumor target.