In Vitro Radiobiological Evaluation of [⁶⁴Cu]CuCl₂ for Theranostic Applications

Background/Objectives: Intrinsic genetic instability and the marked heterogeneity of malignant cell populations represent significant clinical challenges in oncology, often limiting the efficacy of conventional receptor-targeted and antigen-based therapies. To overcome these limitations, [⁶⁴Cu]CuCl₂ has emerged as a particularly promising theranostic agent because it combines PET imaging (β⁺ emission) with therapeutic effects (β^- particles and Auger electrons). In particular, Auger electrons, when delivered to the cell nucleus, induce severe DNA damage due to their high linear energy transfer and very short tissue range. This work aimed to deepen existing preclinical knowledge by providing a comprehensive in vitro analysis of the interactions of [⁶⁴Cu]CuCl₂ with various human cancer cell lines-specifically, the breast adenocarcinoma (MDAf-MB-231) and gastric carcinoma (NCI-N87) cell lines-and a healthy control (IMR-90 normal human fetal lung fibroblasts). Methods: We focused on evaluating cellular uptake, subcellular localization, impact on metabolic activity, and induction of apoptosis. Cell lines (MDA-MB-231, NCI-N87, IMR-90) were exposed to increasing activities of [⁶⁴Cu]CuCl₂ (10, 100, and 250 µCi/mL). Uptake was assessed in both nuclear and cytoplasmic compartments after 4 h. Metabolic activity and apoptosis/necrosis were evaluated at 96 and 120 h post-treatment. Results: Tumor cell lines demonstrated significantly higher [⁶⁴Cu]CuCl₂ uptake, particularly at the nuclear level, compared to healthy controls. A marked decrease in metabolic activity and an increase in apoptosis were observed in MDA-MB-231 and NCI-N87 cells (from 50% to 90% and 5% to 60% apoptosis, respectively). In contrast, IMR-90 cells exhibited minimal cytotoxic response (≤20%), suggesting a preferential response in the malignant cell models tested. Conclusions: [⁶⁴Cu]CuCl₂ induced distinct patterns of intracellular accumulation and biological response among the investigated cell models, with cancer cells displaying greater nuclear uptake and apoptotic susceptibility than non-malignant cells. These findings provide a high-resolution radiobiological baseline and microdosimetric validation, supporting the rigorous design of future, dedicated in vivo preclinical investigations to evaluate the translational potential of ionic [⁶⁴Cu]CuCl₂.