ABSTRACT
4. Other Smart Materials for CDR 1 Albumins
Albumin, a family of globular proteins, presents advantages of great stability in vivo, no toxicity and non-antigenic to the body. The main albumins investigated as vehicles are obtained mainly from egg white (ovalbumin), bovine serum (bovine serum albumin) and human serum (human serum albumin). As albumin contains charged amino acids, a signifi cant amount of drugs can be incorporated into the albumin by electrostatic interactions.
More importantly, the amino acids and carboxylic groups provide active sites for functionalization, which achieve the design of smart albumin vehicle for CDR.
Shen et al. fabricated a thermo-responsive albumin vector, which could selectively accumulate and release payload Rose Bengal (RB) in the solid tumor site with local higher temperature. The drug carrier was constructed by modifi cation thermo-sensitive PNIPAM derivatives on the surface of albumin with RB loaded inside covalently. Below 37ºC, the polymers on surface were hydrophilic and expanded, inhibiting the release of RB. When the temperature increased to 42ºC, PINIPAM polymers shrank, which allowed the release of RB (Shen et al. 2008). Interestingly, albumin was recently reported to be incorporated with magnetic particles for magnetically responsive drug delivery and release (Zeybek et al. 2014).
This albumin vehicle loaded with DOX displayed higher cytotoxicity to cells cancer cells than free DOX, which could be regulated under magnetic fi eld manner.
Protein nanoparticles have a bright future in the controlled delivery of therapeutic agents due to their biodegradability, biocompatibility, and possibility of covalent derivatization with drug targeting ligands and trigger-sensitive molecules. At the present stage, developing smart albumins with precise controllability is highly required.
4.2 Liposomes
Liposome is an artificial-prepared spherical vesicle composed of phospholipid-enriched phospholipids bilayer surrounding an aqueous interior core, which is formed spontaneously when amphiphilic lipids are dispersed in water. Liposomes were fi rst described in 1961 by British hematologists Bangham, at the Babraham Institute, in Cambridge, and soon proposed in the fi eld of CDR due to its advantageous properties, such as biocompatibility, biodegradability and little or no antigenic reaction. The assembled core-shell structure can load hydrophobic anticancer drugs and hydrophilic molecules in the phospholipid bilayer and aqueous core, respectively.
However, the early studies found that lyposomes were rapidly cleared in the blood circulation by phagocytic cells before they accumulated at the tumor site through EPR effect. The problem was resolved after the development of PEGylation technology, which coated liposomes with hydrophilic PEG to reduce immunogenicity and prolong circulation. The long-circulating liposomes could deliver anticancer drugs to tumor sites.
In order to optimize the drug delivery in time, location, and amount, considerable efforts have been dedicated to the design and construction of new-generation of liposome-vectors, i.e., smart liposome nanomaterials, which could be triggered by stimulus, such as temperature, pH, enzymes, and light. In 2012, Mo et al. reported a multistage pH-responsive liposomal system based on zwitterionic oligopeptide liposomes (HHG2C18-L).
Oligopeptide liposomes (HHG2C18-L) consist of soy phosphatidylocholine (SPC), cholesterol, and a synthetic lipid (1,5-dioctadecyl-L-glutamyl 2-histidyl-hexahdrobezoic acid, HHG2C18). The special design of HHG2C18-L would undergo multistage pH change in the process from physiological blood to cytoplasmic inside tumor cells. Typically, the negative surface change of liposomes in physiological blood would reverse to be positive in tumor environment, which was helpful for the internalization of liposomes. Then in endosome, the subsequent pH-response induced by the imidazole group of histidine facilitated the proton infl ux, which led to the endosomal bursting. At the same time, the hydrolysis of hexa hydrogenzoic amide yielded a stronger positive surface charge of HHG2C18-L, which was easy to accumulate at the target mitochondria by electrostatic interaction in the fi nal stage. This intelligent liposome provided a safe and effi cient
strategy to deliver and release of drugs in target sites (Mo et al. 2012).
Agarwal et al. reported a NIR-triggered release liposomal formulation to delivery and release DOX in tumors. The liposome incorporated with GNRs was more effi ciently accumulated in the tumor site and released the chemotherapeutic DOX upon the NIR trigger, leading to the signifi cant increase in effi cacy (Agarwal et al. 2011).
Liposomes gained extensive research in the past decades, which are the fi rst nanomedicine formulations developed from concept to clinical applications. But that’s not the end. Smart liposomal release systems with targeting delivery drug and controlled release of drug under triggers are still in the early stage of development.
4.3 Graphene
Graphene is a sheet of two-dimensional carbon atoms with distinct electrical, mechanical and thermal properties, which have been employed widely in electronics, photonics, and biomedicine. Geim and coworkers fi rst obtained the single layered graphene in 2004, which won the Nobel Prize in Physics in 2010 for the groundbreaking experiment. Now, graphene is an important new addition to the carbon family materials. Usually, the fabrication of graphene can be divided to bottom-up approaches, such as chemical vapor deposition, solvothermal and organic synthesis, and up-down approaches, such as repeated exfoliation methods. With the extremely high surface area and function ability, graphene and the derivatives, especially for GO and reduced graphene oxide (rGO), have gained lots of interests in the biomedicine since 2008. Although investigations indicated the graphene or GO displayed toxicity, the GO-derivatives exhibited no signifi cant side effect to cells and animals in the tested dosage after coating. Specifi cally, the surface modifi cation and intrinsic high NIR absorbance encourage the design of smart nanomaterials based on graphene for delivery and targeted release of drugs upon NIR trigger for disease treatment.
The 2D nanostructure of graphene with the presence of delocalized surface π electrons in plane can be utilized for effective drug loading via hydrophobic interactions or π-π stacking. Additionally, large surface area of graphene allows for high density bio-functionalization via both covalent and non-covalent surface modifi cation, which could attain the delivery and release of drugs triggered by stimulus for treatment of cancers. Smart graphene-based nanomaterials have been applied in CDR since 2008.
For example, Kurapati and Raichur demonstrated a NIR-triggered GO composites carrier for DOX release (Fig. 12a). The smart nanovector/
formulation was composed of GO and poly(allylamine hydrochlorid) (PAH) with DOX entrapped in the core. The GO exhibited strong absorbance in NIR region, which converted light to heat effi ciently. Upon NIR irradiation,
the absorbance of light would result in the rupture of GO-PAH capsules and the release of DOX (Kurapati and Raichur 2013). Wang et al. demonstrated a multifunctional graphene nanomaterial for targeted therapy of Glioma (Fig. 12b) (Wang et al. 2013). The material was formed by mesoporous silica-coated graphene, which was further modifi ed with glioma-targeting ligands (IP). This graphene vector possessed enlarged surface area, active target ability, and synergistic control of release, which was triggered by NIR light, and pH stimuli. Yang et al. reported as hybrid formulation formed by GO and INOPs via wet chemical precipitation method. The magnetic hybrids loaded with DOX can be dispersed uniformly in aqueous solution
Figure 12. Graphene derivative materials for CDR. (a) NIR-responsive graphene oxide capsule for controlled DOX release (Kurapati and Raichur 2013), Copyright 2013 Royal Society of Chemistry. (b) Graphene-Fe3O4 composite for targeting deliver and release of anticancer drug triggered by light and pH (Wang et al. 2013). Copyright 2013 American Chemical Society.
Before NIR-laser irradiation
1064 nm Laser
Dox
G
b a
GS GSP
GSPI Laser-triggered capsule Doxorubicin loaded capsule
After NIR-laser irradiation
CTAB PEG IP
DOX TEOS+APTES
GSPID Photothermal therapy
Chemotherapy NIR
pH NIR
Heat
and aggregate in acidic medium. The moving of materials was under the control of magnetic fi eld. This pH-triggered magnetic behavior of GO-Fe3O4 nanoparticle hybrids can be exploited potentially for CDR (Yang et al. 2009).
As emerging materials for biomedical application, graphene-based nanomaterials have attracted recent interests. Now these materials have created excitement amongst biomedical scientists. Remarkable progress in fabrication and functionalization of graphene materials has provided considerable opportunities for exploring their use in drug/gene delivery.
Lots of challenges have to be faced before their clinical application, including the graphene-cell interaction, in vivo effi cacy in animal. The future emphasis on tissue distribution, mechanisms of clearance and toxicity is required to understand their true potential.
4.4 Au nanoparticles
Au nanoparticles (AuNPs), including nanospheres, nanorods, and nanocages, are of great interests for biomedical applications due to their unique properties, e.g., the localized surface plasmon resonance and NIR absorbance. Since 2005, AuNPs have been studied widely as vectors for delivery and release of drugs for disease treatment. In addition, AuNPs could be fabricated with controlled size from 1–150 nm, which is non- toxic, and favorable for endocytosis by cells. AuNPs becoming an excellent platform to design smart vectors for CDR are also attributed to the functional versatility, which could achieve the modifi cation on the surface of AuNPs with stimuli-sensitive molecules. The loading of drugs can be accomplished through non-covalent interaction or covalent conjugation. The application of AuNPs as promising vectors for CDR is a rapidly expanding fi eld. In addition, the Au core in essence is non-toxic, biocompatible, and inert, which is critical for biomedical use.
In 2006, Hong et al. designed a GSH-responsive AuNPs vector (Fig. 13a). The 2 nm AuNPs were decorated with a tetra(ethylene glycol)- lyated cationic ligand (TTMA) thought Au-S bonds, which could improve the effi ciency of cell uptake. A thiolated Bodipy dye (HSBOP) as the cargo was loaded by covalent interaction, which could be released in the presence of GSH (Hong et al. 2006). Xiao et al. constructed a novel AuNPs vector delivering DOX to the target site, which was then triggered to release upon NIR (Fig. 13b). The vector was composed of GNRs, complementary DNA with one pre-conjugated targeting ligand, and PEG. DOX was loaded by DNA strands. PEG modifi cation improved the surface property and prolonged the circulation time. Upon NIR irradiation, the absorbance of AuNRs would transduce the light to heat, which denatured the DNA assembly, resulting in the release of DOX (Xiao et al. 2012).
AuNPs show great potential for the creation of smart vectors for biomedical applications. Their stability, tunable surface flexibility, low toxicity, and optical property offer many possibilities for further development of CDR systems. Future investigations of these systems will go forward to understand fully their interactions with the immune system, and fi nally construction of smart formulation and the in vivo studies.
4.5 Metal-Organic Framework Nanomaterials
Metal-organic frameworks (MOFs) are compounds consisting of metal ions or clusters coordinated to polydentate bridging organic ligands to form one-, two-, or three-dimensional porous structures, which has been proposed for applications since it was discovered by Robson in 1989 (Pollack et al. 1989),
Figure 13. Smart AuNPs for CDR. (a) Glutathione-triggered AuNPs vehicle (Hong et al.
2006), Copyright 2006 American Chemical Society. (b) NIR-responsive AuNPs vehicle (Xiao et al. 2012).
including catalysis, separation, gas storage, nonlinear optics and sensing.
Recently, these materials have been scaled down to nanometer sizes, and become appealing for the biomedical applications due to their advantages over conventional nanomaterials. First, MOFs can be designed in diversity with different composition, shape, and pore size. MOFs are biodegradable, which could be cleared quickly by the body. In addition, the combined metal ions or organic ligands may be designed to play specifi c functions, such as MRI. Generally, the preparations of MOFs with homogeneous and stable structure can be divided into four methods: hydrosolvothermal, reverse- phase microemulsion, sonochemical synthesis, and nanoprecipitation.
Therapeutic agents are loaded into the nanostructures by direct incorporation or postsynthesis loading (Fig. 14). In the direct incorporation
Figure 14. MOFs fabrication (a) and the strategies of loading drugs by non-covalent interaction (b1) and covalent incorporation (b2) (Della Rocca et al. 2011). Copyright 2011 American Chemical Society.
Metal ions
b1)
b2) a)
+
Self-assembly
Organic bridging ligands
Biomedically relevant building blocks
NMOFs for imaging and therapy
non-covalent encapsulation
covalent attachment
strategy, biomedical functional agents are the structure blocks, i.e., either the metal connection points, or the bridging ligand. In the postsynthesis loading strategy, loaded agents are attached by covalent bonding or non- covalent interactions. In addition, as the drug vector, the modifi cation, including silica coating and polymer decoration, is important for MOFs, which could improve the stability of structure in physiological circumstance.
The modifi cation could bring the MOFs with intelligent property, which is the new promising way for treating cancer. For example, An et al. developed a MOFs vector assembled by zinc-adeninate columnar secondary units.
This MOFs vector could storage cationic drug via cation exchange with dimethylammonium cations. Exogenous cations could triggered the release of loaded drug from the pores (An et al. 2009).
The development of smart MOFs as drug delivery vectors is in its infancy. Nonetheless, MOFs have already shown great promise as a new nanocarrier platform owing to unique properties. In future, studies should be performed on the construction of smart MOFs, which could achieve the targeted delivery of drug by triggers. At the same time, more strategies will be utilized to modify MOFs to improve the biocompatibility and prolong the circulation in blood. Although there are many reports on in vitro effi cacy of MOFs, no systematic studies on in vivo effi cacy have been performed yet. Such systematic in vivo studies are critical for optimization the performance of MOFs.