Imaging of prostate cancer: optimizing affinity to prostate specific membrane antigen by spacer modifications in a tumor spheroid model.

Early diagnosis of prostate cancer (PCa) is crucial for staging, treatment and management of patients. Prostate specific membrane antigen (PSMA), highly over-expressed on PCa cells, is an excellent target for selective imaging of PCa. In recent years, various scaffolds have been explored as potential carriers to target diagnostic and therapeutic agents to PSMA tumour cells. Numerous fluorescent or radioisotope probes linked via a peptide linker have been developed that selectively binds to PCa cells. However, there are very few reports that examine the effects of chemical modifications in the peptide linker of an imaging probe on its affinity to PSMA protein. This report systematically investigates the impact of hydrophobic aromatic moieties in the peptide linker on PSMA affinity and performance. For this, a series of fluorescent bioconjugates - with different aromatic spacers were designed, synthesized, and their interactions within the PSMA pocket were first analysed . Cell uptake studies were then performed for - in PSMA cell lines and 3D tumour models . Binding affinity values of - were found to be in the range of 36 to 157.9 nM, and with three aromatic groups in the spacer exhibit highest affinity (K = 36 nM) compared to which is devoid of aromatic groups. These studies suggest that aromatic groups in the spacer region can significantly affect deep tissue imaging of fluorescent bioconjugates. Bioconjugate can be a promising diagnostic tool, and conjugation to near-infrared agents would further its applications in deep-tissue imaging and surgery. Communicated by Ramaswamy H. Sarma.