A Rational design of multi-functional nanoplatform: Fluorescent-based "off-on" theranostic gold nanoparticles modified with D-α-Tocopherol succinate.

It is crucial to develop nanocarrier systems to detect and treat drug-resistant micro tumors to prevent recurrence and/or metastasis of cancer. Due to their exceptional features such as biocompatibility, easy surface modification, serving as imaging and therapeutic agent, gold nanoparticles (AuNPs) draw attention as theranostic agents. It is beneficial to combine AuNPs with a second imaging and/or treatment modality such as photodynamic therapy (PDT). PDT is a non-mutagenic treatment approach in which photosensitizer is activated with light, generating reactive oxygen species and/or free radicals to destroy tumor cells. With the aim of developing "off-on" theranostic system, citrate stabilized spherical 13 nm AuNPs were densely coated with polyethylene glycol (PEG). To advance the theranostic feature of PEGylated AuNPs, they were further functionalized with FDA-Approved photosensitizer, Verteporfin (BPD-MA). Due to static quenching between BPD-MA and AuNPs as well as in between nearby BPD-MA molecules, the fluorescence of the ground state complex is quenched and the system is in "off" state. When BPD-MA molecules are cleaved from the AuNPs surface and diffuse away, fluorescence is recovered. Consequently, the system switches to the "on" state. Among the various mole ratios of BPD-MA carrying conjugates prepared, the most promising candidate was selected based on stability, quenching factor, and fluorescence recovery rate. The conjugate was further decorated with D-α-Tocopherol succinate (VitES) to increase the therapeutic efficacy of the theranostic agent via enhancing cellular uptake. Our results showed that it was possible to achieve as high as 80 times fluorescence quenching when the system was "off". As the system switched from "off" to "on" state, 51% of the fluorescence was recovered. When BPD-MA was immobilized on the PEGylated AuNPs, the phototoxic effect of BPD-MA increased twice against the MCF-7 cell line. Moreover, the developed system showed four times more phototoxicity than BPD-MA alone after it was decorated with VitES. Since the developed system is capable of dual imaging (computed tomography and fluorescence) and dual treatment (PDT and hyperthermia), it potentially offers superior imaging and therapy options for various types of in vitro/in vivo applications.