Theranostic iRGD peptide containing cisplatin prodrug: Dual-cargo tumor penetration for improved imaging and therapy

In theranostics, peptide-based platforms have widely been exploited owing to their unique biological functions and chemical versatilities. As a tumor-homing ligand, internalizing RGD peptide (iRGD), composed of a tumor-targeting sequence (RGD) and a cell-penetrating C-end Rule (CendR) motif, is known to facilitate the tumor-specific delivery of payloads that are covalently conjugated on its N-terminal fragment or co-administered without any covalent linkages. However, theranostic uses of the iRGD-based platform remain in its infancy with its full potential unexplored; for instance, detailed mechanism of iRGD fragmentation during internalization, strategies for the tumor-specific release of payloads from iRGD and the role of the C-terminal iRGD fragment in delivery have yet to be revealed. In this study, we designed a dual-channel fluorescent cyclic iRGD (TAMRA-iRGDC-Cy5.5) to track each of the N- and C-terminal fragments separately during the tumor internalization process. It turned out that both fragments undergo translocation into cancer cells together and are localized within endosomal-lysosomal compartments. The resulting co-internalization of both iRGD fragments allowed us to develop a new theranostic peptide platform (Cy5.5-iRGDC-Pt(IV)) by conjugating a fluorescent dye and a cisplatin prodrug on each terminus of cyclic iRGD for simultaneous cancer-targeted imaging and therapy. Compared to a control peptide having a non-iRGD sequence, the Cy5.5-iRGDC-Pt(IV) did not only provide a better tumor imaging contrast but also induced tumor-specific apoptosis leading to efficacious tumor suppression. Besides the outstanding cancer imaging and therapeutic performance, the Cy5.5-iRGDC-Pt(IV) revealed negligible systemic toxicity, holding potential to be applied for theranostic uses.