(1) Background: Prostate-specific membrane antigen (PSMA) has been extensively studied in the last decade. It became a promising biological target in the diagnosis and therapy of PSMA-expressing cancer diseases. Although there are several radiolabeled PSMA inhibitors available, the search for new compounds with improved pharmacokinetic properties and simplified synthesis is still ongoing. In this study, we developed PSMA ligands with two different hybrid chelators and a modified linker. Both compounds have displayed a promising pharmacokinetic profile. (2) Methods: DATA^5m.SA.KuE and AAZTA⁵.SA.KuE were synthesized. DATA^5m.SA.KuE was labeled with gallium-68 and radiochemical yields of various amounts of precursor at different temperatures were determined. Complex stability in phosphate-buffered saline (PBS) and human serum (HS) was examined at 37 °C. Binding affinity and internalization ratio were determined in in vitro assays using PSMA-positive LNCaP cells. Tumor accumulation and biodistribution were evaluated in vivo and ex vivo using an LNCaP Balb/c nude mouse model. All experiments were conducted with PSMA-11 as reference. (3) Results: DATA^5m.SA.KuE was synthesized successfully. AAZTA⁵.SA.KuE was synthesized and labeled according to the literature. Radiolabeling of DATA^5m.SA.KuE with gallium-68 was performed in ammonium acetate buffer (1 M, pH 5.5). High radiochemical yields (>98%) were obtained with 5 nmol at 70 °C, 15 nmol at 50 °C, and 60 nmol (50 µg) at room temperature. [⁶⁸Ga]Ga-DATA^5m.SA.KuE was stable in human serum as well as in PBS after 120 min. PSMA binding affinities of AAZTA⁵.SA.KuE and DATA^5m.SA.KuE were in the nanomolar range. PSMA-specific internalization ratio was comparable to PSMA-11. In vivo and ex vivo studies of [¹⁷⁷Lu]Lu-AAZTA⁵.SA.KuE, [⁴⁴Sc]Sc-AAZTA⁵.SA.KuE and [⁶⁸Ga]Ga-DATA^5m.SA.KuE displayed specific accumulation in the tumor along with fast clearance and reduced off-target uptake. (4) Conclusions: Both KuE-conjugates showed promising properties especially in vivo allowing for translational theranostic use.

Ziel/Aim The aim of this work was to evaluate the labelling of the PSMA-inhibitor AAZTA⁵.SA.KuE with ¹⁷⁷Lu as well as with ⁶⁸Ga and ⁴⁴Sc. Furthermore, its stability should be investigated in vitro and first small animal in vivo studies should be carried out. Methodik/Methods The chelator AAZTA⁵ was synthesized in eight steps and conjugated to the KuE (Lysine-urea-glutamate) target vector, using squaric acid as coupling agent in additional two steps. Labeling was performed with gallium-68, scandium-44 and lutetium-177 and radiochemical yields of different amounts of tracer substance were determined using different labeling conditions. In vitro stability in phosphate-buffered saline and human serum was examined at 37 °C and first in vivo SPECT studies were performed with NMRI-Foxn1^nu/nu mice with a PC3-PSCA-PSMA tumor. Ergebnisse/Results AAZTA⁵.SA.KuE was synthesized with an overall yield of 0.5 % after eleven steps. Radiolabeling of AAZTA⁵.SA.KuE reached RCY of 95 to 99 % after one minute for labeling with gallium-68 and lutetium-177 at ambient temperature. The labeling with scandium-44 gives a RCY of 89.9 ± 0.3 % after 30 minutes at room temperature. All radiometal-ligand complexes were sufficiently stable in phosphate-buffered saline and human serum over 2 h for gallium-68, 24 h for scandium-44 and 48 h for lutetium-177, respectively. First in vivo-SPECT studies with [¹⁷⁷Lu]Lu- AAZTA⁵.SA.KuE were performed and indicate a good distribution leading to an almost exclusive accumulation of [¹⁷⁷Lu]Lu- AAZTA⁵.SA.KuE in the tumor after 48 h. Schlussfolgerungen/Conclusions In addition to the simple, mild and versatile labelling of AAZTA5.SA.PSMA, the in vivo application of the 177-lutetium labelled derivative was also investigated and indicated for the first time.