Rationale Design of a Zn(II)-Coordinated, Cell Penetrating, and Functionalized Tetrapeptide Self-Assembly for Delivery of Chemotherapeutic Drugs to Folic Acid Receptor-Expressing Cancer Cells

Design and development of functionalized and biocompatible drug delivery systems (DDS) for site-specific release of small molecules is emerging as a means to target disease sites while sparing healthy tissue. Cell penetrating short peptides capable of self-assembly and drug encapsulation represent one scaffold with which selective DDSs can be rationally designed due to their chemical diversity, biocompatibility, tunable bioactivity, ease of functionality, and high loading capacity. Herein, we designed and synthesized two tetra peptides, BOC-YWWD (PB1 with Trp-Trp-Sequence) and BOC-WYWD (PB2 without Trp-Trp sequence). Structural rigidification of these two peptides with Zn(II) in the self-assembled state were characterized by the density functional theory (DFT) method and demonstrated to shift self-assembly of their characteristic emission from the ultraviolet to visible range allowing for visualization of cellular entry. PB1-Zn, unlike PB2-Zn, exhibits cell penetrating capabilities and is photo and thermally stable and biocompatible. Self-assembled PB1-Zn effectively encapsulated the chemotherapeutic drug Doxorubicin (Dox) and facilitated intracellular drug delivery. To test the utility of PB1-Zn as a DDS, we chemically modified PB1-Zn with folic acid to target folate receptor α (FLOR1), commonly overexpressed on the surface of cancer cells. In HeLa cervical cancer cells, this chemical conjugation with folic acid significantly improved the ability of Dox to activate the pro-apoptotic DNA damage response and trigger oxidative stress and mitochondrial dysfunction critical for the cancer cell killing actions of the drug. However, PB1-Zn failed to facilitate Dox delivery into the lung cancer epithelial cell line, A549, which does not express high levels of FLOR1. Our results represent an important proof of concept describing the fabrication of fluorescent Zn(II) coordinated, self-assembled short peptides containing the sequential Trp-Trp unit that may be used to develop superior imaging reagents and site-specific DDSs.