Zwitterionic Modification of PSMA Ligands Reduces Off-Target Binding and Tissue Retention.

Off-target tissue retention is a serious limitation for prostate-specific membrane antigen (PSMA)-targeted drugs. This study addresses key questions regarding the role of zwitterionic modifications in PSMA-ligand design for tissue distribution. A series of fluorescent PSMA-ligands was synthesized and evaluated with respect to PSMA-binding, tumor uptake, and biodistribution in cell experiments and in mice. The data revealed that the introduction of two zwitterionic groups into the linker domain of the PSMA-specific conjugates was particularly advantageous. The resulting compound 10 combined high and specific PSMA-binding affinity (IC50 = 4.39 ± 1.69 nM) and good uptake in tumor cells and tumor xenografts with extremely low off-target tissue retention. A major practical advantage of this strategy is its simple synthetic realization using solid-phase peptide synthesis with commercial building blocks and their modification using click-chemistry. Zwitterionization is therefore easily transferable to other targeting vectors and alternative effector molecules, for example in radiopharmaceuticals.