Albumin-Binding PSMA Radioligands: Impact of Minimal Structural Changes on the Tissue Distribution Profile

The concept of using ibuprofen as an albumin-binding entity was recently demonstrated by the development of [(177)Lu]Lu-Ibu-PSMA-01. In the present study, we designed a novel ibuprofen-containing radioligand (Ibu-PSMA-02) with subtle structural changes regarding the linker entity in order to investigate a potential impact on the in vitro and in vivo properties. Ibu-PSMA-02 was prepared using solid-phase synthesis techniques and labeled with lutetium-177. [(177)Lu]Lu-Ibu-PSMA-02 was evaluated in vitro with regard to its plasma protein-binding properties, PSMA affinity and uptake into PSMA-expressing PC-3 PIP tumor cells. The tissue distribution profile of [(177)Lu]Lu-Ibu-PSMA-02 was assessed in tumor-bearing mice and dose estimations were performed. The in vitro characteristics of [(177)Lu]Lu-Ibu-PSMA-02 were similar to those previously obtained for [(177)Lu]Lu-Ibu-PSMA-01 with respect to plasma protein-binding, PSMA affinity and tumor cell uptake. The in vivo studies revealed, however, an unprecedentedly high uptake of [(177)Lu]Lu-Ibu-PSMA-02 in PC-3 PIP tumors, resulting in an increased absorbed tumor dose of 7.7 Gy/MBq as compared to 5.1 Gy/MBq calculated for [(177)Lu]Lu-Ibu-PSMA-01. As a consequence of the high tumor accumulation, [(177)Lu]Lu-Ibu-PSMA-02 showed higher tumor-to-background ratios than [(177)Lu]Lu-Ibu-PSMA-01. This study exemplified that smallest structural changes in the linker entity of PSMA radioligands may have a significant impact on their pharmacokinetic profiles and, thus, may be applied as a means for ligand design optimization.