Gd-based MR contrast agents for molecular imaging of prostate-specific membrane antigen

Objectives PSMA is an attractive target for imaging and therapy of prostate cancer. Previously, we demonstrated successful PET and SPECT imaging using radiolabeled, urea-based PSMA inhibitors in mice and man. We hypothesized that PSMA could also serve as a suitable target for MR-based molecular imaging because of the high target concentration (~3 μM/cell volume) as well as the extra-cellular location of the ligand binding site. We have synthesized three new Gd-based contrast agents to evaluate relaxometric properties of the agents in solution, in prostate cancer cells and in an experimental mouse xenograft model to investigate the potential of PSMA-based MR imaging.

Methods PSMA binding affinities of the agents were evaluated by a competitive protein binding assay. For cell uptake and imaging studies, isogenic PC3 PSMA+ PIP and PSMA- flu cells were incubated with 50 µM agent at 37°C for 4h. ICP-MS was done to determine [Gd] in cells. MR imaging were done on a 9.4 T magnet at rt.

Results Agents demonstrated high binding affinities to PSMA (K_i), Gd1 (0.1 nM) Gd2 (11 nM) and Gd3 (3 nM) and relaxitivities of 2.95, 12.47 and 8.97 mM^-1 sec^-1, respectively. T₁-weighted images of PSMA+ PIP cells pellets following incubation with Gd3, displayed significant MR contrast enhancement in compared to the unlabeled cells as well as to the PSMA- flu cell pellets. Post-image ICP-MS analyses of the PSMA+ cell pellets revealed the highest amount of Gd for Gd3 (22.82 mM) followed by Gd2 (12.5 mM) then Gd1 (7.2 mM). Cell internalization studies for Gd1 and Gd2 revealed that > 80% of the total cell uptake of each agent was internalized. In contrast, Gd3 displayed ~ 48 % cell membrane-bound uptake. In vivo MR imaging of Gd3 in mice bearing PIP and flu tumors showed ~ 36 % enhancement in PIP tumor at 0.5h and remained high ~ 25% until 3h, whereas flu tumor showed rapid decay in signal intensity after an initial ~ 24% enhancement at 0.5h.

Conclusions These results indicate that Gd3 enables PSMA-targeted MR imaging in vivo.

Research Support K25CA148901, U54CA151838, R01CA134675