Development of Bispecific PSMA/GRPr Targeting Radioligands with Optimized Pharmacokinetics for PET Imaging of Prostate Cancer

A series of novel low-molecular weight bispecific radioligands were developed, which were able to target the prostate-specific membrane antigen (PSMA) and the gastrin releasing peptide receptor (GRPr), both expressed on prostate cancer cells. These bispecific radiotracers combined the peptidomimetic urea-based pseudo-irreversible inhibitor of PSMA: Glu-ureido-Lys with the bombesin (BN) analogue: H₂N-PEG₂-[D-Tyr⁶, β-Ala¹¹, Thi¹³, Nle¹⁴]BN(6–14), which binds to GRPr with high affinity and specificity. The two pharmacophores were linked together through the chelating agent HBED-CC and spacers made of positively charged His (H) and negatively charged Glu (E): -(HE)_n-, (n=0-3) amino acids. The positron emitter ⁶⁸Ga (t_1/2 = 68 min, β⁺ 88 %, E_β+ max. 1.9 MeV) was used for the radiolabelling of the bispecific radioligands and preliminary pharmacological data were collected from in vitro assays on prostate cancer cell lines (PC-3, AR42J, LNCaP) and in vivo experiments in normal and tumor bearing mice (biodistribution and small animal PET imaging studies). The new bispecific ligands in vitro showed binding affinities, which essentially matched the ones of the respective monomers, while in vivo they were able to target both PSMA (LNCaP) and GRPr (PC-3) positive tumors. In addition the charged -(HE)_n-, (n=1-3), linkers improved the tracer’s pharmacokinetics by significantly reducing the normal organ uptake (i.e. kidney and spleen) and by increasing the tumor to-background ratio. In conclusion, the bispecific (PSMA and GRPr) targeting ligands, developed in this study could be considered as novel radiotracer candidates for more sensitive PET/CT-imaging of prostate cancer (PCa) in future clinical application.