In vitro and in vivo comparative study of a novel ⁶⁸Ga-labeled PSMA-targeted inhibitor and ⁶⁸Ga-PSMA-11

⁶⁸Ga-radiolabeled small molecules that specifically target prostate-specific membrane antigen (PSMA) have been extensively investigated, and some of these tracers have been used in the diagnosis of prostate cancer via ⁶⁸Ga-positron emission tomography (⁶⁸Ga-PET). Nevertheless, current ⁶⁸Ga-labeled radiotracers show only fair detection rates for metastatic prostate cancer lesions, especially those with lower levels of prostate specific antigen (PSA), which often occurs in the biochemical recurrence of prostate cancer. The goal of this study was to design and synthesize a new PSMA-targeted radiotracer, ⁶⁸Ga-SC691, with high affinity for prostate cancer cells and excellent pharmacokinetics. To this end, structural optimization was carried out on the bifunctional group, target motif, and linker while the high affinity targeting scaffold remained. To explore its potential in the clinic, a comparative study was further performed in vitro and in vivo between ⁶⁸Ga-SC691 and ⁶⁸Ga-PSMA-11, a clinically approved tracer for PSMA-positive prostate cancer. SC691 was radiolabeled to provide ⁶⁸Ga-SC691 in 99% radiolabeling yield under mild conditions. High uptake and a high internalization ratio into LNCaP cells were observed in in vitro studies. In vivo studies showed that ⁶⁸Ga-SC691 had favorable biodistribution properties and could specifically accumulate on PSMA-positive LNCaP xenografts visualized by micro-PET/CT. This radiotracer showed excellent PET imaging quality and comparable, if not higher, uptake in LNCaP xenografts than ⁶⁸Ga-PSMA-11.