Theranostic Nanoplatforms Based on Novel Core-Shell-Shell Upconversion Nanoparticles for Imaging, Photodynamic Therapy and Chemotherapy

Abstract

Upconversion nanoparticles (UCNPs) are a type of lanthanide-doped nanoparticles that are widely used for bioimaging, drug delivery, gene delivery, photodynamic therapy (PDT) and photothermal therapy (PTT). The lanthanide¬doped UCNPs can absorb two or more low-energy photons and emit a high-energy photon. Thus the UCNPs can offer visible and even ultraviolet emission under near infrared (NIR) irradiation. Traditional Yb3+-sensitized UCNPs are excited at 980 nm, at which wavelength water also has a strong absorption, so it can lead to an overheating effect as well as low tissue penetration depth.

Herein, a NaYbF4@Na(Yb,Gd)F4@NaGdF4 core-shell-shell nanoparticle host was employed to perform the upconversion process. The novel Nd3+ ion-doped core-shell-shell UCNPs were thus developed to solve the overheating issue from traditional 980 nm-excited UCNPs. The Nd3+ ion doped in the core served as the harvesting layer which can be excited at 808 nm. The absorbed energy transferred to the inner shell layer Er3+ ion (upconverting ions) to give upconversion emissions at 550 and 660 nm, which can be used for photodynamic therapy and bioimaging to construct a theranostic nanoplatform.

In the first part, the core-shell-shell UCNPs were conjugated with photosensitizer (PS) to perform photodynamic therapy under 808 nm to avoid the overheating effect. The UCNPs were firstly transferred into water by simple ligand exchange of AEP (2-aminoethly dihydrogen phosphate) with abundant amino groups on the surface for the following modifications of photosensitizer (PS) and hydrophilic polymer. Chlorin e6 (Ce6), a second-generation photosensitizer with a maximum absorption at 660 nm, was chosen as photosensitizer. Poly(ethylene glycol) (PEG) and folic acid (FA) as a targeting moiety for the folate receptor (FR) positive cells were further conjugated on these nanoparticles. The nanoplatform acts as a theranostic nanoplatform for simultaneous bioimaging and PDT. Our nanoplatform can obtain high energy transfer efficiency and generate cytotoxic singlet oxygen under 808 nm irradiation. The nanomedicine can effectively kill cells in concentration-, time-, and receptor-dependent manners. In addition, at 808 nm the absorption of water is minimized, thus the light can get deeper tissue penetration compared with 980 nm. Our nanoplatform is still able to efficiently generate singlet oxygen beneath 15-mm thickness of muscle tissue under 808 nm irradiation but 980 nm irradiation cannot.

In the second part, the UCNPs were functionalized with a transactivator of transcription (TAT) peptide (GRKKRRQRRRPQ), a cell-penetrating peptide, to achieve mitochondria-targeting properties and to perform PDT with improved effect. Besides the cell specificity of a specific type of cancer cells used for PDT, the subcellular targeting ability is also a key component for PDT efficiency. Subcellular targets for PS such as mitochondria, lysosomes, plasma membrane, and nuclei of tumor cells can all be potential PDT targets and affect the PDT efficiency to some degree. Thus, we further study the cellular targeting properties by using a specific peptide (TAT peptide) to functionalize the UCNPs system used for PDT. We simply used C18PMH polymer to directly stabilize the oleic acid-capped UCNPs (OA-UCNPs) and transfer into water and physiological environment. Then the hydrophobic photosensitizer of pyropheophorbide a (Ppa) is chosen to load in the hydrophobic area in the C18PMH-UCNPs. The TAT peptide was subsequently conjugated on the nanoparticles for the mitochondria-targeting ability. The nanomedicine shows high PDT efficiency under NIR irradiation and the mitochondria target properties can improve the PDT efficiency by the introduction of TAT peptide, revealing that mitochondria is a key subcellular target site in cancer cells for the UCNPs-based PDT nanoplatform. The mitochondria health condition and the colocalization of the nanoparticles, Ppa, and the mitochondria were further confirmed to reveal the mechanism of this NIR-induced PDT effect.

In the third part, we discussed the combined therapies of platinum chemotherapy and photodynamic therapy simultaneously against cisplatin resistance. The PDT may encounter low oxygen availability in cells. Thus it could limit the PDT efficiency. Chemotherapy is frequently used and has excellent anticancer effectiveness. Thus we also try to combine chemotherapy and photodynamic therapy in the UCNPs system to achieve a synergistic effect through Pt chemotherapy and PDT. The UCNPs were first modified by poly(acrylic acid) (PAA) to yield a PAA-UCNPs. Through this process the nanoparticles are well dispersed in water and physiological conditions. The carboxylic acid groups on the surface are further used to functionalize with anticancer Pt(IV) prodrugs, rose bengal (RB), and PEG polymer. The combined Pt chemotherapy and photodynamic therapy showed a synergistic anticancer effect and the nanomedicine also showed an efficient anticancer effect in cisplatin-resistant cancer cells, showing that it is a good strategy to combat drug resistance by the combined therapies. The mechanisms for chemotherapy and photodynamic therapy were further confirmed by the Pt uptake and cellular reactive oxygen species (ROS) generation. This combined therapy of thermotherapy and photodynamic therapy based on UCNPs provides a promising theranostic nanoplatform for anticancer therapy and against drug resistance with NIR-excited bioimaging ability.

In summary, the novel core-shell-shell upconversion nanoparticles utilizing 808 nm excitation significantly avoid overheating effects from traditional 980¬nm excited UCNPs. This novel type of UCNPs was modified with photosensitizer to perform UCNP-based PDT. The nanoplatform can be irradiated by 808 nm for a long time with negligible overheating effects. In addition, the 808-nm excited UCNPs can obtain higher tissue penetration depth. The nanoplatform can still generate singlet oxygen under 15-mm thickness of muscle tissue under 808 nm irradiation, but not for 980 nm. The nanoplatform functionalized with folic acid can significantly kill cancer cells in concentration-, time-, and receptor-dependent manners. When conjugated with TAT peptides, the nanoplatform can specifically target mitochondria, which is an important subcellular site for PDT. This mitochondria-targeting nanoplatform gets improved PDT effect under 808 nm irradiation. Furthermore, the combination of PDT and Pt chemotherapy based on UCNPs can provide a synergistic anticancer effect. The combined therapy is effective in cisplatin-resistant cells, and thus our strategy provides a promising approach to solve drug resistance issues. Additionally, the upconversion luminescence emission serves as a bioimaging tool under NIR excitation.

Date of Award	20 Mar 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Guangyu ZHU (Supervisor)