1. INTRODUCTION
Photodynamic therapy (PDT) is increasingly recognized as a promising treatment for a variety of cancers due to its low cost, highly localized and specific tumor treatments, fewer side effects as compared with radiation therapy and chemotherapy, and minimal trauma to organism tissue (Ackroyd, Kelty, Brown, & Reed, 2001; Dolmans, Fukumura, & Jain, 2003; Dougherty et al., 1998; Levy & Obochi, 1996). PDT is a powerful noninvasive therapeutic technique for a range of diseases including cancers, based on the photochemical reactions mediated by the interaction of photosensitizers (PSs) with specific light and molecular oxygen. Upon irradiation at the appropriate wavelength, the PSs become excited and transfer energy to oxygen in the surrounding tissue, generating highly reactive oxygen species (ROS) such as singlet oxygen (1O2). The ROS moieties can react with biological molecules, resulting in an irreversible oxidative tissue damage and cell death (Allison et al., 2005; DeRosa & Crutchley, 2002; Dougherty, 1987; Wilson, 2002).
However, the principal problem limiting the use of many current PS in PDT is the low water solubility. These hydrophobic PSs could form aggregates in aqueous solution, which would reduce the1O2 quantum yield and affect the therapeutic efficiency of PDT. Additionally, because of their low water solubility, these PSs are difficult to prepare as pharmaceutical formulations for parenteral administration and cannot be directly injected intravenously (Konan, Gurny, & Allemann, 2002; B. H. Li et al., 2007). To overcome these limitations, various nanoscale drug carriers such as micelles (Woodburn & Kessel, 1994; G. D. Zhang et al., 2003), liposomes (Ferro, Ricchelli, Mancini, Tognon, & Jori, 2006), dendrimers (Kim, Lee, Lee, Kim, & Kim, 2007), gold nanoparticles (Hone et al., 2002), mesoporous materials (Ideta et al., 2005) and carbon nanotubes (Liu et al., 2007; J. Wang, Liu, & Jan, 2004; Woolley, Guillemette, Cheung, Housman, & Lieber, 2000) have been explored as PS delivery systems in cancer therapy.
In addition, another main challenge for PDT is efficient treatment of cancers at a deep tissue level. However, the PSs used in conventional PDT are mostly excited by visible or even UV light, which has limited penetration depth due to the light absorption and scattering by biological tissues. PDT has been generally applicable to tumors on or just under the skin or on the lining of internal organs or cavities but does not produce effective therapeutic effects when treating large and deep-seated tumors (Detty, Gibson, & Wagner, 2004; C. Wang, Tao, Cheng, & Liu, 2011).
Near-infrared (NIR) light is referred to as the “optical window” of the biological tissues due to the minimal light absorption and scattering. Compared with the UV or visible light, NIR shows larger penetration distance in tissue, lower photodamage effects and higher signal-to-noise ratio (Du et al., 2010; Zhou et al., 2011). However, the current PSs for clinical usage, which can be efficiently activated by NIR light, remain rare.
Upconversion is an optical process that involves the conversion of lower-energy photons into higher-energy photons (Dong, Sun, & Yan, 2015; X. M. Li, Zhang, & Zhao, 2013; Zhou, Liu, & Li, 2012). Especially, lanthanide ion-doped upconversion nanoparticles (UCNPs) exhibit unique luminescent properties, including the ability to convert NIR long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. This process can further activate the PSs attached to nanoparticles to produce ROS. The advent of UCNPs would open a new pathway to full utilization of current and commercially available PSs upon NIR irradiation (Chatterjee, Gnanasammandhan, & Zhang, 2010; F. Wang, Banerjee, Liu, Chen, & Liu, 2010; P. Zhang, Steelant, Kumar, & Scholfield, 2007). In particular, UCNPs with a hexagonal phase have been demonstrated to be the best NIR-to-visible nanotransducers, which could provide the highest photon upconversion efficiency (Dong et al., 2015; X. M. Li et al., 2013; Zhou et al., 2012).
Recently, the UCNP-based PS delivery system for PDT has widely attracted interest from scientists, as it shows potential to overcome the above mentioned drawbacks of current PDT. However, there are still technical difficulties in the practical application of UCNP-based PS carriers. Also, the strategy of a UCNP-based theranostic system with a tumor-targeting ligand for selective PS delivery has not been reported much.
Therefore, we aimed to develop a NIR-regulated theranostic system based on hexagonal-phase UCNPs for tumor-targeted PDT and fluorescence imaging as shown in Figure 1. In this study, we optimized the hydrothermal synthesis procedure to produce NaYF4:Yb/Er UCNPs with uniform size, hexagonal phase, and strong fluorescent intensity. In order to increase the aqueous solubility of UCNPs and introduce functional moieties into the surface of UCNPs for subsequent biological functionalization, folic acid-polyethylene glycol-poly(aspartic acid-hydrazone)-dihydrolipoic acid (FA-PEAH) polymer chains were conjugated. Then, a derivative of chlorophyll a, pheophorbide a(Pha), was conjugated to the side chain of FA-PEAH copolymer via an acid-labile hydrazone bond that is stable at physiological pH (7.0-7.4), but degraded at the lower pH (4.0-6.0) of the endosomal/lysosomal compartments. The size, size distribution, elemental composition, crystalline morphology, and luminescence properties of UCNPs were determined. To assess the potential of FA-PEAH-UCNPs-Pha as a NIR-triggered theranostic system, in vitro cellular localization and phototoxicity effects of UCNP-based nanocarriers were also investigated.