Synthesis and Preclinical Evaluation of a Bispecific PSMA-617/RM2 Heterodimer Targeting Prostate Cancer

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) have been used for diagnostic molecular imaging/therapy of prostate cancer (PCa). To address tumor heterogeneity, we synthesized and evaluated a bispecific PSMA/GRPR ligand (3) combining PSMA-617 (1) and the GRPR antagonist RM2 (2) with the radiometal chelator DOTA. 3 was radiolabeled with ⁶⁸Ga ([⁶⁸Ga]Ga-3) and ¹⁷⁷Lu ([¹⁷⁷Lu]Lu-3). [⁶⁸Ga]Ga-3 was tested in the following PCa cell lines for receptor affinity, time kinetic cell-binding/specificity, and cell-internalization: PC-3 and LNCaP. Compared to the monomers (1 and 2), ligand 3 showed specific cell binding, similar receptor affinities, and higher lipophilicity, while its internalization rates and cell-binding were superior. Docking calculations showed that 3 can have binding interactions of PSMA-617 (1) inside the PSMA receptor funnel and RM2 (2) inside the GRPR. In vivo biodistribution studies for [⁶⁸Ga]Ga-3 showed dual targeting for PSMA(+) and GRPR(+) tumors and higher tumor uptake, faster pharmacokinetic, and lower kidney uptake compared to 1 and 2.