Bispecific GRPR-Antagonistic Anti-PSMA/GRPR Heterodimer for PET and SPECT Diagnostic Imaging of Prostate Cancer

Simultaneous targeting of the prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) could improve the diagnostic accuracy in prostate cancer (PCa). The aim of this study was to develop a PSMA/GRPR-targeting bispecific heterodimer for SPECT and positron emission tomography (PET) diagnostic imaging of PCa. The heterodimer NOTA-DUPA-RM26 was produced by manual solid-phase peptide synthesis. NOTA-DUPA-RM26 was labeled with (111)In and (68)Ga, with yields >98%, and demonstrated a high stability and binding specificity to PSMA and GRPR. IC(50) values for (nat)In-NOTA-DUPA-RM26 were 4 ± 1 nM towards GRPR and 824 ± 230 nM towards PSMA. An in vivo binding specificity 1 h pi of (111)In-NOTA-DUPA-RM26 in PC3-PIP-xenografted mice demonstrated partially blockable tumor uptake when co-injected with an excess of PSMA- or GRPR-targeting agents. Simultaneous co-injection of both agents induced pronounced blocking. The biodistribution of (111)In-NOTA-DUPA-RM26 and (68)Ga-NOTA-DUPA-RM26 revealed fast activity clearance from the blood and normal organs via the kidneys. Tumor uptake exceeded normal organ uptake for both analogs 1 h pi. (68)Ga-NOTA-DUPA-RM26 had a significantly lower tumor uptake (8 ± 2%ID/g) compared to (111)In-NOTA-DUPA-RM26 (12 ± 2%ID/g) 1 h pi. Tumor-to-organ ratios increased 3 h pi, but decreased 24 h pi, for (111)In-NOTA-DUPA-RM26. MicroPET/CT and microSPECT/CT scans confirmed biodistribution data, suggesting that (68)Ga-NOTA-DUPA-RM26 and (111)In-NOTA-DUPA-RM26 are suitable candidates for the imaging of GRPR and PSMA expression in PCa shortly after administration.