Summary

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are overexpressed in prostate cancer (PCa) cells and are promising targets for molecular imaging methods used for diagnosis. Novel heterodimer - containing PSMA inhibitor and GRPR antagonist - has been demonstrated to bind specifically to both proteins with concomitant low uptake in normal tissues. In the current study, chemical structure of the heterodimer was modified in order to improve affinity towards PCa cells and binding characteristics were analysed. Tumor-bearing mice were injected with 111In-labeled heterodimer (BQ7812). In vivo biodistribution was investigated on harvested organs and also with SPECT/CT imaging. Quantitative analysis together with in vitro tests revealed that modifications in the molecular design resulted in 10-fold improved affnity towards PSMA and high activity uptake in tumors.

Results from nanoScan SPECT/CT

For the SPECT/CT studies, BALB/c nu/nu mice implanted with PC3-pip (isogenic human prostate carcinoma) cells were injected with 830kBq 111In-BQ7812. Groups were also co-injected with non-labeled GRPR antagonist and/or non-labeled PSMA-11 to block GRPR and/or PSMA to prove binding specificity. Imaging of the non-blocked group was performed at 1 and 3h pi and for the GRPR/PSMA-blocked group at 1h pi.

Result revealed that:

• images are in good agreement with the ex vivo analysis: tumor could be visualized already at 1h pi. and the only healthy organs with high activity uptake at this time point were the kidneys (Figure 1.A)
• Co‐injection of non-labeled PSMA-11 and NOTA-PEG4-RM26 resulted in a decreased kidney uptake and a negligible activity uptake in the tumor (Figure 1.B)
• activity cleared from healthy organs and blood with time, leading to an improved imaging contrast at 3h pi. (Figure 1.C)
• Together with the results from the in vitro and in vivo specificity tests, confirmed the specific binding of [111In]In-BQ7812 to both PSMA and GRPR

Full article on mdpi.com