Discovery of PSMA-specific peptide ligands for targeted drug delivery

Prostate-specific membrane antigen (PSMA) has been widely used as a biomarker and targeting receptor for prostate cancer therapy because of its overexpression in most prostate cancer cells. In this study, a novel combinatorial phage biopanning procedure was developed to discover PSMA-specific peptides that can be potentially used as ligands for targeted drug delivery to prostate cancer cells. Five rounds of biopanning against recombinant human PSMA extracellular domain (ECD), PSMA-positive LNCaP cells, and LNCaP xenografts in nude mice were conducted. Various affinity assays were conducted to identify high-affinity peptides for PSMA ECD and PSMA-positive prostate cancer cells. Among them, the GTI peptide shows the highest affinity as well as specificity to PSMA in prostate cancer cells. The apparent K_d values of the GTI peptide to PSMA-positive LNCaP and C4-2 cells are 8.22μM and 8.91μM, respectively. The GTI peptide can specifically deliver the proapoptotic peptide _D(KLAKLAK)₂ to LNCaP cells to induce cell death. In the biodistribution study, the GTI peptide shows the highest uptake in C4-2 xenografts, while its uptake in other major organs, such as the liver and spleen, are either low or negligible. Compared to its scrambled control (random permutation of the GTI peptide), the GTI peptide exhibits higher and more specific uptake in C4-2 xenografts. All the results suggest that the GTI peptide is a potentially promising ligand for PSMA-targeted drug delivery for prostate cancer therapy.