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.