(1) Background: In neuroendocrine tumors (NETs), somatostatin receptor subtype 2 is highly expressed, which can be targeted by a radioactive ligand such as [¹⁷⁷Lu]Lu-1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴,-tetraacetic acid-[Tyr³,Thr⁸]-octreotide (¹⁷⁷Lu-DOTA-TOC) and, more recently, by a lead specific chelator (PSC) containing ^203/212Pb-PSC-PEG₂-TOC (PSC-TOC). The molar activity (A_M) can play a crucial role in tumor uptake, especially in receptor-mediated uptake, such as in NETs. Therefore, an investigation of the influence of different molar activities of ^203/212Pb-PSC-TOC on cell uptake was investigated. (2) Methods: Optimized radiolabeling of ^203/212Pb-PSC-TOC was performed with 50 µg of precursor in a NaAc/AcOH buffer at pH 5.3–5.5 within 15–45 min at 95° C. Cell uptake was studied in AR42 J, HEK293 sst2, and ZR75-1 cells. (3) Results: ^203/212Pb-PSC-TOC was radiolabeled with high radiochemical purity >95% and high radiochemical yield >95%, with A_M ranging from 0.2 to 61.6 MBq/nmol. The cell uptake of ²⁰³Pb-PSC-TOC (A_M = 38 MBq/nmol) was highest in AR42 J (17.9%), moderate in HEK293 sstr (9.1%) and lowest in ZR75-1 (0.6%). Cell uptake increased with the level of A_M. (4) Conclusions: A moderate A_M of 15–40 MBq/nmol showed the highest cell uptake. No uptake limitation was found in the first 24–48 h. Further escalation experiments with even higher A_M should be performed in the future. It was shown that A_M plays an important role because of its direct dependence on the cellular uptake levels, possibly due to less receptor saturation with non-radioactive ligands at higher A_M.

This work investigates the proposed enhanced efficacy of photodynamic therapy (PDT) by activating photosensitizers (PSs) with Cherenkov light (CL). The approaches of Yoon et al. to test the effect of CL with external radiation were taken up and refined. The results were used to transfer the applied scheme from external radiation therapy to radionuclide therapy in nuclear medicine. Here, the CL for the activation of the PSs (psoralen and trioxsalen) is generated by the ionizing radiation from rhenium-188 (a high-energy beta-emitter, Re-188). In vitro cell survival studies were performed on FaDu, B16 and 4T1 cells. A characterization of the PSs (absorbance measurement and gel electrophoresis) and the CL produced by Re-188 (luminescence measurement) was performed as well as a comparison of clonogenic assays with and without PSs. The methods of Yoon et al. were reproduced with a beam line at our facility to validate their results. In our studies with different concentrations of PS and considering the negative controls without PS, the statements of Yoon et al. regarding the positive effect of CL could not be confirmed. There are slight differences in survival fractions, but they are not significant when considering the differences in the controls. Gel electrophoresis showed a dominance of trioxsalen over psoralen in conclusion of single and double strand breaks in plasmid DNA, suggesting a superiority of trioxsalen as a PS (when irradiated with UVA). In addition, absorption measurements showed that these PSs do not need to be shielded from ambient light during the experiment. An observational test setup for a PDT nuclear medicine approach was found. The CL spectrum of Re-188 was measured. Fluctuating inconclusive results from clonogenic assays were found.

Background: Energy delivered at different wavelengths causes different types of damage to DNA. Methods: PC-3, FaDu, 4T1 and B16-F10 cells were irradiated with different wavelengths of ultraviolet light (UVA/UVC) and ionizing radiation (X-ray). Furthermore, different photosensitizers (ortho-iodo-Hoechst33258/psoralen/trioxsalen) were tested for their amplifying effect. Survival fraction and damage analysis using the $\mathrm{\gamma }$H2A.X assay (double-strand breaks) and the ELISA assay (cyclobutane pyrimidine dimers) were compared. Results: The PC-3 cells were found to be the most sensitive cells to the treatment strategies used. FaDu and PC-3 showed a strong sensitivity to UVA. Analysis of the damage showed that the cell lines exhibited different sensitivities. Conclusions: Thus, an enhancing effect of photosensitizers (PS) in combination with UVA could be demonstrated in some cases. However, this is cell- and dose-dependent.