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Theranostics 2012; 2(3):238-250. doi:10.7150/thno.3509 This issue Cite

Review

Applications of Functionalized Fullerenes in Tumor Theranostics

Zhiyun Chen, Lijing Ma, Ying Liu , Chunying Chen

CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology, Beijing 100190, China.

✉ Corresponding author: Dr. Ying Liu, CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology (NCNST), No.11, 1st North Street, Zhongguancun, Beijing 100190, China. Tel: +86-10-82545526; Fax: +86-10-62656765; E-mail: liuycn. More

Abstract

Functionalized fullerenes with specific physicochemical properties have been developed for cancer diagnosis and therapy. Notably, metallofullerene is a new class of magnetic resonance imaging (MRI) contrast-enhancing agent, and may have promising applications for clinical diagnosis. Polyhydroxylated and carboxyl fullerenes have been applied to photoacoustic imaging. Moreover, in recent years, functionalized fullerenes have shown potential in tumor therapies, such as photodynamic therapy, photothermal treatment, radiotherapy and chemotherapeutics. Their antitumor effects may be associated with the modulation of oxidative stress, anti-angiogenesis, and immunostimulatory activity. While various types of novel nanoparticle agents have been exploited in tumor theranostics, their distribution, metabolism and toxicity in organisms have also been a source of concern among researchers. The present review summarizes the potential of fullerenes as tumor theranostics agents and their possible underlying mechanisms are discussed.

Keywords: Functionalized fullerenes, nanoparticles, cancer diagnosis, cancer therapy, theranostics.

Introduction

Buckminsterfullerene (C₆₀) is a carbon allotrope discovered in 1985 [1]. Since then, fullerenes have attracted much attention and been widely investigated. After the main problem of poor solubility was solved by chemical or supramolecular approaches (usually a chemical modification on the surface of the sphere), various functionalized fullerenes were synthesized to achieve promising results [2]. For example, the empty interior cavity of the fullerene molecule makes it an ideal site for containment of a metal atom to form metallofullerene. This special structure of metallofullerene shows advantages compared to the metal itself or metal chelate complexes. One of their most important and novel electronic properties is the so-called intra-fullerene electron transfer from the encaged metal atom(s) to the fullerene cage(s), which has been suggested to use in MRI [3]. Most likely due to its unique chemical structure, C₆₀ possesses interesting photo-physical properties and generates reactive oxygen species (ROS) by exposure to visible light, making it a potentially strong agent for photodynamic therapy in biological systems [4].

The specific physicochemical properties of fullerenes promoted the development of tumor theranostic; consequently, functionalized fullerenes are currently under extensive investigation due to their potential application to tumor imaging and therapy (Table 1).

Functionalized fullerenes for tumor imaging

Magnetic resonance imaging

Among many imaging strategies, such as optical imaging (OI), computed tomography (CT), MRI, ultrasound (US), and radionuclide imaging with positron emission tomography (PET) or single photon emission computed tomography (SPECT), MRI is a versatile, non-invasive medical diagnostic technique that can provide both physiological and anatomical information, while not requiring the use of ionizing radiation. In addition, in many applications it is often the most sensitive method available to depict soft tissues. Therefore, it has been extensively used for brain and central nervous system imaging, for assessing cardiac function and for detecting abnormal tissues such as tumors [5, 6]. However, the difference between MR images of normal and abnormal tissues is often not large enough; therefore, specific exogenous contrast agents are often required to increase MR image quality and obtain higher resolution and sensitivity.

Paramagnetic metal ions, especially gadolinium (Gd³⁺), have been proposed as MR contrast agents. Conventional T1 contrast agents have been developed in the form of more stable and less toxic metallochelates, including gadolinium(III) diethyltriaminepentaacetic acid (Gd(III)-DTPA) and gadolinium-tetraazacyclododecanetetraacetic acid (Gd(III)-DOTA), which are currently marketed as Omniscan® and ProHance®, respectively [6, 7]. However, for Gd³⁺ chelate complexes, the main limitation is the release of metal ions in vivo during metabolic processes and the subsequent toxicity. Since gadolinium-containing metallofullerenes were first used as excellent contrast agents for MRI with relatively high spatiotemporal resolution, they have attracted much attention for their potential application in clinical diagnosis. The Gd³⁺ ion was encapsulated in the fullerene cage, which is a structure that preserves the properties of the metal ion, avoids any leakage and thus prevents its dissociation in vivo [7]. After specific chemical modifications, various Gd@C_n (n=60, 82) derivatives have been developed and widely explored (Figure 1A, B and C) [8, 9].

Gd@C₈₂(OH)_n has shown a water proton relaxivity twenty times higher than the commercially available Gd-DTPA. After intravenous (iv) administration of Gd@C₈₂(OH)₄₀ at a dose of 1/20 of the typical clinical dose of Gd-DTPA, a higher signal enhancement was measured in vivo (Figure 2) [10]. To achieve better bio-distribution, modification of the fullerene cages with biologically active groups has often been proposed. For example, the organic phosphonate modified Gd@C₈₂O₂(OH)₁₆(C(PO₃Et₂)₂)₁₀ has exhibited high affinity to bone [11].

 Figure 1 

Depiction of (A) Gd@C₆₀[C(COOH)₂]₁₀ [8], (B) Gd@C₆₀(OH)_x [8], (C) Gd@C₈₂(OH)₂₂ [9], (D) C₆₀(OH)_x [38], (E) C₆₀[C(COOH)₂]₂ [39]. Adapted with permission from [8].

 Figure 2 

(A) T1-weighted MRI of (a)Gd@C₈₂(OH)₄₀ and Gd-DTPA phantom at a concentration of 0.05, 0.1, and 0.2 mmol Gd/L, (B) CDF1 mice before and 30 min after i.v. administration of Gd@C₈₂(OH)₄₀ via tail vein as a dose of 5 μmol Gd/kg [10].

For a long time, the greatest obstacle to the practical application of Gd@C₈₂ was its low production yield and requirement for potentially laborious HPLC purification. Recently, without the complicated process of purification, Grushko et al. tested the MRI efficiency of a mixture of gadolinium endometallofullerenes (with high content of more than 80 mol.%) and empty fullerenes, and found that the T1-weighted signal intensity was 15~30 times higher than the commercial “Gadovist” contrast agent [12].

After the Gd@C₈₂ contrast agent was developed, another type of gadofullerene derivative, Gd@C₆₀[C(COOH)₂]₁₀ was also found to possess a higher relaxivity (4.6 mM^-1 s^-1 at 20 MHz and 40 °C) compared to that of commercial Gd³⁺ chelate. It was used as an efficient cellular magnetic resonance imaging label in vitro and a 2.5-fold signal enhancement was shown in T1-weighted MRI of labeled cells compared to control cells. The cellular uptake characteristics of this gadofullerene were significant and nonspecific, and an excellent labeling efficiency (98-100%) was achieved without any adjunctive agent, compared to Gd@C₈₂(OH)₄₀. Furthermore, Gd@C₆₀[C(COOH)₂]₁₀ was the first highly soluble, air-stable discrete molecule based on the more abundant M@C₆₀ fraction of metallofullerenes, obtaining by a non-chromatographic procedure [13, 14].

The Gd₃N@C₈₀ fullerene exhibiting the highest relaxivity has seen wider use because of the presence of three Gd³⁺ ions per molecule. Functionalized trimetallic nitride endohedral metallofullerene species Gd₃N@C₈₀ [DiPEG5000(OH)_x] has been used to perform high water hydrogen MR imaging relaxivity (R1) when used at a lower concentration, with an equivalent efficiency compared with commercial agents. After it was directly infused into a tumor bearing brain, improved tumor delineation was possible in comparison with the intravenously injected conventional Gd³⁺ chelate [15]. Zhang et al. also compared the ¹H MRI relaxivities of Gd₃N@C₈₀ fullerenes with PEG of four different molecular weights (350-5000 Da), and found that 350/750 Da PEG derivatives exhibited the highest relaxivity compared to longer chain length ones, 5000 Da PEG derivatives [16]. At nearly the same time, Gd₃N@C₈₀(OH)₂₆(CH₂CH₂COOM)₁₆ (M=Na or H) was readily synthesized by Shu et al. with high yield. The longitudinal and transverse relaxivities R1 and R2 were 50 times larger than those of Gd³⁺ poly(aminocarboxylate) complexes, such as commercial Omniscan and Magnevist. Furthermore, the new derivatives were more compact, diffused more slowly and lingered for a longer time than commercial agents and Gd₃N@C₈₀[PEG5000(OH)_x] [17]. These results gave greater impetus to the development of metallofullerenes as MRI contrast agents.

In the previously mentioned studies, most imaging was obtained by directly administering fullerenes into tumor tissue, therefore tumor-targeting is certainly a subsequent challenge for tumor imaging. Antibodies of the green fluorescent protein (anti-GFP) were conjugated to endohedral gadofulleride, Gd@C₈₂O₆(OH)₁₆-(NHCH₂CH₂COOH)₈, as a model for “tumor-targeted” imaging agents, and could be detected by using a fluorescent GFP probe. Results revealed that the antibody activity was well preserved after the conjugation, and the MRI activity was higher than that of unconjugated gadofulleride [18]. Recently, a more encouraging result was obtained. A carboxyl functionalized metallofullerene Gd₃N@C₈₀(OH)₂₆(CH₂CH₂COOH)₁₆ was successfully conjugated to an interleukin-13 (IL-13) peptide, the receptor of which was expressed on glioblastoma multiforme cells. This preliminary finding exhibited the potential of functionalized gadolinium metallofullerenes for tumor targeting and imaging [19].

Photoacoustic imaging

Recently, an attractive new therapeutic concept called 'acoustic-explosion' was suggested for some functionalized fullerenes, polyhydroxy fullerenes (PHF; C₆₀(OH)_xO_yNa_z) and carboxy fullerenes (CF; C₆₀(C(COOH)₂)₃). Concomitant with the exposure to low-intensity (< 10² W cm^-2) continuous-wave laser irradiation in the presence or absence of oxygen, sustained glowing of functionalized fullerenes was observed. When this process was performed in cancer cells, a faint pop was heard following irradiation, indicating the photoacoustic properties of functionalized fullerenes. To investigate their potential for cancer imaging, researchers then used a mechanical scanning photoacoustic system with a single transducer to collect the acoustic signal, and embedded it in a phantom together with functionalized fullerenes. After laser irradiation in a water tank, high contrast photoacoustic images of functionalized fullerenes were obtained. Similarly, the photoacoustic imaging experiment yielded promising results in tumor-bearing nude mice. Excellent contrast between tumor and non-tumor regions was shown after intratumoral injection of PHF, and the laser energy used was only one-third of the maximum permissible exposure level of 29.5 mJ cm⁻² for a 785 nm pulsed laser as established by the American National Standards Institute [20, 21]. This new finding suggests potential applications of functionalized fullerenes for obtaining photoacoustic images (Figure 3).

 Figure 3 

(A) Visible light emission from functionalized fullerenes irradiated with a 785-nm laser [21]. (B) Photoacoustic imaging and 2-D signal mapping of tumors in living mice before (Left) and after (Right) intratumoral injection of polyhydroxy fullerenes [20].

Functionalized fullerenes for tumor therapy

Photodynamic therapy

Photodynamic therapy (PDT) is a non-invasive treatment for various types of tumor. The combination of a photosensitizing agent and focused irradiation is used to elicit specific, controlled production of reactive oxygen species in a localized area, leading to cell destruction through several pathways. By using highly focused light irradiation, photodynamic therapy has the potential to act specifically at the desired site of action with low levels of collateral damage to healthy cells [22]. Fullerenes can efficiently generate reactive oxygen species when exposed to visible light, which means it may be an effective cytotoxic agent. The anatomical characteristics of tumor, such as hyperpermeable vasculature and an immature lymph system, make large-sized substances accumulate and remain longer in tumor tissues than in normal tissues. After the ideal molecular size, water-solubility, and tumor targeting were developed through various modifications, functionalized fullerenes are now available for cancer PDT both in tumor cells and tumor-bearing mice [23-25]. Many researches have been carried out to demonstrate this potential application of functionalized fullerenes.

When C₆₀ was conjugated with polyethylene glycol (PEG), pullulan, etc., strong tumor suppression was shown after the conjugate was injected intravenously into tumor-bearing mice coupled with light irradiation [26, 27]. To confirm tumor uptake of C₆₀(OH)_x, ¹²⁵I-labeled fullerenol, ¹²⁵I-C₆₀(OH)_x were administered to five tumor-bearing mice. The accumulation ratios of ¹²⁵I-C₆₀(OH)_x in tumor compared to normal muscle tissue (T/N) and blood (T/B) at different time points revealed that C₆₀(OH)_x could be accumulated in several tumors readily and persistently, especially in mouse H22 hepatocarcinoma, suggesting that it could be used as a good photosensitizer in photodynamic tumor therapy [28]. To elevate the PDT effect by controlling the treatment time, Gd³⁺-chelated C₆₀-PEG (C₆₀-PEG-Gd) was obtained from mixing gadolinium acetate solution with C₆₀-PEG-DTPA, which was prepared by introducing diethylenetriaminepentaacetic acid (DTPA) to the terminal group of C₆₀-PEG. The MRI activity was introduced into the C₆₀-PEG of PDT photosensitizers by the chelate incorporation of Gd³⁺ ions and provided a non-invasive way to detect the time course of C₆₀-PEG accumulating in the tumor tissue. There was good agreement between the time profile of the PDT effect and the detection of a positive MRI signal. These results indicated that C₆₀ could be a potent photosensitizing agent for photodynamic tumor therapy after proper modifications [29].

Two oxidative mechanisms are considered to be mainly implicated in the photodamage of cells. One is free radicals and the other is singlet molecular oxygen (¹O₂), generated through electron transfer and energy transfer, respectively. These two mechanisms may be determined by some important factors. For example, C₆₀ induced DNA cleavage occurs mainly through reduced active oxygen radicals such as superoxide anion radical (O₂^—·) and hydroxyl radical (·OH) under physiological conditions, while ¹O₂ was generated effectively in nonpolar solvents [30]. Besides, aggregation is a well-accepted factor that could deactivate the excited electronic states of photosensitizers and cause further loss of photoreactivity. Zhao et al. found that γ-cyclodextrin bicapped pristine C₆₀ [(γ-CyD)₂/C₆₀] was in a monomeric state in water whereas C₆₀(OH)₂₄ was aggregated. It could be used to explain the phototoxicity induced by (γ-CyD)₂/C₆₀ toward human keratinocytes. The toxicity was mainly mediated by singlet oxygen and much higher than its water-soluble derivative C₆₀(OH)₂₄ [31, 32]. Compared to Type 2 photochemistry mechanism, Type 1 is more depend on the surrounding oxygen concentration; thus, some functionalized fullerenes acting through the Type 1 pathway may be relatively more effective in hypoxic tumors. C₆₀-porphyrin could induce HepG2 human larynx-carcinoma cell death through either a ¹O₂-mediated photoreaction process or a free radical mechanism under anaerobic conditions [33]. N-methylpyrrolidinium-fullerene (BB4) was found to be a powerful photosensitizer with the ability to kill cancer cells in vitro and in a mouse model of disseminated peritoneal carcinomatosis after illumination with white light. BB4 can induce apoptosis by producing Type 1 ROS, rather than singlet oxygen, i.e. a Type 2 mechanism [34, 35].

Photothermal treatment

Recently, certain functionalized fullerenes, especially polyhydroxy fullerenes (PHF) and carboxy fullerenes (CF) were found to be heated to their ignition temperature by exposure to low-intensity (< 10² W cm^-2) continuous-wave laser irradiation, which was likely because of distortion of the symmetrical cage structure. This heating property would be extremely advantageous for cancer therapy, when a negatively charged PHF coating on silica nanoparticles functionalized with positively charged amine groups was dosed to A549 cells and localized destruction of cells was induced by irradiation with a near-infrared laser [21].

Moreover, when tumors were exposed to near-infrared light after injected with PHF nanoparticles, they decreased in cross-sectional area by an average of 32% within 2 hours of treatment, with only a blister visible 20 hours post-treatment. In addition, a concomitant faint audible sound was heard accompanied by bubble formation and bursting of the irradiated cells after exposure to a 785 nm laser. Thus, the rapid tumor shrinkage may be explained by both photothermal ablation and “acoustic-explosion” mechanisms (Figure 4) [20].

 Figure 4 

Treatment of a mice tumor with polyhydroxy fullerenes. (A) Experimental setup for photothermal ablation of tumor with CP-0.25 (chitosan-polyhydroxy fullerenes) nanoparticles. (B) Photographs of mouse tumor 2 hours (Left) and 20 hours (Right) after laser irradiation, only a blister was visible after 20 hours (within dotted lines). (C) Histological sections of remaining tumor stained with hematoxylin and eosin demonstrate areas of necrosis [20].

Therapeutic Nuclear Medicine

While the potential of metallofullerenes on MRI was developed vigorously, research on another application of functionalized fullerenes to therapeutic nuclear medicine was also performed. Based on a similar principle, metallic radionuclides encapsulated in fullerenes can be protected in the empty carbon cage stably, eliminating any undesired toxicity induced by the leakage and catabolism of administered radionuclides.

The availability of radio-fullerenes in therapeutic nuclear medicine was then proven by distribution and stability evaluations. Diener et al. prepared the α-emitting ²¹²Pb@C₆₀ malonic acids for the first time and investigated the bio-distribution of the untargeted water-soluble radio-fullerene in mice and its stability during β^--decay of ²¹²Pb to ²¹²Bi. It was shown that ²¹²Pb@C₆₀ malonate prevented the ²¹²Pb from accumulation in bone, attenuated the myelotoxicity of ²¹²Pb, and exhibited a rather slow clearance. This radio-fullerene also appeared stable in solution (about 36% of the ²¹²Bi released) [36]. The above results suggested that water-soluble fullerenes might be suitable as cancer radiopharmaceuticals.

Effective targeted delivery of radionuclides is the primary challenge in radio-immunotherapy (RIT). By conjugating the radio-fullerenes with an antibody that is specific for receptors expressed preferentially on the cancer cells, radioisotopes can be delivered to cancer cells. After encapsulation of the β-emitter ¹⁷⁷Lu in a fullerene cage, an IL-13 peptide that targets an over-expressed receptor in glioblastoma multiforme tumors was successfully conjugated to the ¹⁷⁷Lu_xLu_(3-x)N@C₈₀ [37]. However, the efficiency of these radio-fullerenes for tumor targeting RIT requires further investigation in the living body or tumor cells.

Chemotherapeutics

Besides surgery, chemotherapeutics is another principal method for tumor therapy. However, the high toxicity and easily developed drug resistance considerably confine the chemotherapeutic effect. In recent years, functionalized fullerenes have been found to have some good characteristics to aid with antitumor treatments, and some of them have exhibited the potential for tumor inhibition (Figure 1C, D and E) [9, 38, 39]. In 2005, we found that gadolinium endohedral metallofullerenol (Gd@C₈₂(OH)_n nanoparticles) can efficiently inhibit the growth of murine H22 hepatoma without obviously adverse effects on important organs, having nearly no direct toxic effect to tumor cells [40], which is an advantage over conventional anti-tumor drugs. Zhu et al. [41] also tested the tumor-inhibitory effect of C₆₀(OH)_x on the same model. In the C₆₀(OH)_x-treated group, significant tumor inhibition rates and reduced damage to liver were shown, in accordance to the histological results of inhibited tumor infiltration. Additionally, the anti-metastatic activities of fullerenol C₆₀(OH)₂₀ was observed in cancer metastasis models [42]. Moreover, some fullerenes could increase the chemo-sensitization of tumor cells to chemotherapeutic agents and result in antitumor drug-mediated cell death, especially in some drug-resistant cancer cells [43, 44]. Thereafter, great attention was focused on the underlying mechanisms of the antitumor activity of fullerenes as follows:

1) Anti-angiogenesis

Angiogenesis is one of the most important factors for the progression of tumor, by supplying tumors with oxygen and nutrients [45]. As tumors can generate their own blood vessels from the surrounding vasculature [46, 47], anti-angiogenesis treatments can inhibit growth and metastasis of various solid tumors [48].

Meng et al. investigated the anti-angiogenic activity of Gd@C₈₂(OH)₂₂ and found that the nanoparticle can simultaneously down-regulate more than 10 angiogenic factors, which was confirmed in vivo by decreasing tumor microvessel density and lowering the nutrient supply to tumor tissues. A similar result was also obtained in another anti-angiogenesis study of C₆₀(OH)₂₀ (Figure 5) [42, 49].

2) Modulation of oxidative stress

ROS such as superoxide radical anion, hydrogen peroxide, singlet oxygen, and hydroxyl radical, which can cause oxidative stress, are considered to be an important part in the mechanism of carcinogenesis. ROS can induce lipid peroxidation, protein fragmentation, DNA damage, damage to tumor-suppressor genes and enhance the expression of proto-oncogenes. It has been reported that various carcinoma cells synthesize ROS at an elevated rate in vitro, and many tumors are under persistent oxidative stress in vivo [42]. As water-soluble fullerenes showed high inhibitory activity on many kinds of tumor growth in mice, and it is well established that fullerenes and their derivatives possess a unique capacity for scavenging ROS [50], the relationship between the two functions of the nanoparticles was investigated.

The ROS scavenging capability of [Gd@C₈₂(OH)₂₂]_n was verified by electron spin resonance (ESR) [51], as well as in tumor tissues of C₆₀(OH)₂₀ treated mice [42], which meant that the antitumor mechanism of fullerenes may be related to its capacity to modulate oxidative stress in tumor-bearing models. On the other hand, Injac et al. found that fullerenol C₆₀(OH)₂₄ had protective effects on the primary organs (heart, liver, lung and kidney) against chronic toxicity induced by doxorubicin, and thus may be useful as a potential organo-protector for anticancer therapy [52-55]. These results may increase the feasibility of applying fullerenes to cancer chemotherapeutics.

3) Immunological activity

Our previous studies have demonstrated that water-soluble C₆₀(OH)₂₀ and Gd@C₈₂(OH)₂₂ nanoparticles have specific immunomodulatory effects on T cells and macrophages, and up-regulated the immune response in vivo, including the polarization of the cytokine balance towards Th1 (T-helper cell type 1) cytokines, which particularly increase TNF-α production, a very important factor helping to scavenge and kill tumor cells [9, 38]. These results indicated effective immunological mechanisms of functionalized fullerenes (Figure 6).

 Figure 5 

Mechanism by which C₆₀(OH)₂₀ suppresses carcinoma metastasis in vivo. (A) (A-C) Photos of lungs after soaking in Bouin's solution showing spontaneous pulmonary breast cancer metastases (white arrows). (B) Administration of C₆₀(OH)₂₀ can decrease oxidative stress, the content of angiogenic factors(TNF-α, VEGF and PDGF) and neovascularization in tumor tissues, inhibition of which diminish transmission of cancer cells to lung tissues [42].

 Figure 6 

Possible immune-associated pathways by which Gd@C₈₂(OH)₂₂ nanoparticles inhibit the growth of tumors. (A) In comparison to the untreated saline group, IFN-γ and TNF-α expression levels in mouse tumor tissues increased markedly in the group treated with 0.5 mmol/kg/day Gd@C₈₂(OH)₂₂ nanoparticles. (B)The Gd@C₈₂(OH)₂₂ nanoparticles injected in the abdominal cavity are mostly engulfed by macrophages and other phagocytes through phagocytosis, whilst a few enter the blood directly through the peritoneum or mesentery. The Gd@C₈₂(OH)₂₂ nanoparticles stimulate macrophages and T cells to release several kinds of cytokines, such as, IL-2, IL-4, IL-5, TNF-α and IFN-γ, which then triggers a series of signal pathways of immune responses and possibly promotes tumor cell apoptosis [9].

Zhang et al. found that Gd@C₈₂(OH)₂₂ could effectively activate dendritic cells (DCs) and macrophages, which are the most important antigen presenting cells (APC), as well as up-regulate the expression of co-stimulatory molecules and MHC molecules, thus enhancing the APC function of DCs and macrophages as illustrated by their capacity to activate allogeneic T cells. More evidence was also given to demonstrate the immune activity of Gd@C₈₂(OH)₂₂, which could increase the production of Th1 cytokines and promote macrophages secreting pro-inflammatory cytokines such as IL-6 and TNF-α [56, 57].

Moreover, there are few reports about the antigenicity of C₆₀ derivatives. Chen et al. found that mice immunized with the fullerene-bovine thyroglobulin conjugates generated a population of fullerene-specific antibodies of the IgG isotype, including a subpopulation that was cross-reacted with a C₇₀ fullerene [58]. The specificity of the monoclonal antibody for the C₆₀ fullerene was determined by competitive inhibition, and the anti-C₆₀ antibodies were able to interact with single walled carbon nanotubes, which was imaged using atomic force microscopy [59, 60].

4) Overcoming tumor resistance to chemotherapeutic drugs

For cisplatin-resistant cancer (CP-r) cells, Gd@C₈₂(OH)₂₂ alone had no obvious inhibition on tumor growth, although it has been indicated that the metallofullerene could improve the sensitivity of CP-r tumors to cisplatin, as an effective inhibition of CP-r tumors growth was observed following cisplatin plus nanoparticle treatment in vitro and in vivo. Transferrin-labeled experiments have shown that less intracellular transferrin is distributed in CP-r cells than in CP-s cells, indicating a possible defective endocytosis of the CP-r cells. Since nanoparticles have been used as drugs or gene carriers for cancer treatment, it was posited that [Gd@C₈₂(OH)₂₂]_n nanoparticles reverse tumor resistance by enhancing the endocytosis of cisplatin via nanoparticle-mediated penetration through the plasma membrane of the CP-r cells, and this hypothesis was confirmed by increased cisplatin adducts in NP and CP-treated CP-r cells measured by ICP-MS. Restoring defective endocytosis and other related possible mechanisms for overcoming tumor resistance to cisplatin by using fullerenes could lead to new chemotherapies for cancer [43].

5) Autophagy-mediated chemo-sensitization

The main therapeutic goal of cancer treatment is to trigger tumor-selective cell death. One of the cellular pathways receiving much attention in recent years is autophagy, a highly-regulated intracellular process for the degradation of long-lived proteins and damaged organelles. Autophagy can be either a cell death pathway or a cell survival pathway depending on many factors such as the degree of cell damage and intactness of apoptotic pathways [61]. Undoubtedly, promoting autophagy-mediated cell apoptosis is of most concern for tumor therapy.

Zhang et al. have performed comprehensive work on the modulation of autophagy in cancer cells by fullerene C₆₀ nanocrystals. They found that HeLa cells treated with nano-C₆₀ exhibited autophagic features, such as LC3 puncta formation, and many more autophagic vacuoles including autophagosomes and autolysosomes. Nano-C₆₀ at non-cytotoxic concentrations sensitized chemotherapeutic killing of cancer cells in a photo-enhanced and ROS-dependent pathway, which was further verified via GFP-LC3 dot formation that was effectively reduced by free radical scavengers in the dark treatments [44]. A much greater potential in the induction of autophagy by another C₆₀ derivative, C₆₀(Nd), was also reported [62]. All above results prompt that fullerenes might be available to be an efficient chemo-sensitizer in cancer therapy.

Bio-distribution and Toxicity

Although the diverse potential applications of fullerene and its derivatives have been exploited in cancer diagnosis and therapy, it is necessary to consider their biocompatibility and toxicity for organisms prior to their widespread utilization and production. Since fullerene derivatives with different modifications have distinct biological behaviors, understanding these will help in the safe development of fullerene-based drugs [63, 64].

Bio-distribution and excretion

The dynamic distribution of fullerenes within the body is influenced by several factors, including the particle size, various modifications, and administration pathways. Studying these factors can help to identify potential target tissues of fullerene and assess the toxicity at particular target sites.

Unlike oral administration, whereby ¹⁴C-labeled C₆₀ is not efficiently absorbed and excreted primarily in the feces, C₆₀ injected intravenously was cleared from the blood circulation rapidly, accumulated mainly within the liver and remained there for a long time, and then was cleared slowly from the liver (nearly all was eliminated 13 days after injection). All these results suggest that the liver is a potential target for fullerene accumulation and toxicity contributes to the metabolism of C₆₀ (C₆₀ metabolites were identified within the liver) and facilitates the removal of fullerenes from the body within the feces [65-67]. By comparison, the water-soluble C₆₀ had a wider tissue distribution, which might be attributed to its more hydrophilic water-soluble character. ^99mTc-C₆₀(OH)_x injected into mice through tail veins was quickly distributed to all organs except for tissues with limited blood flow, such as muscle. It remained throughout the 48 h of the study, particularly in the kidneys, bone, spleen and liver, and displayed a slow clearance rate except for bone, which showed slightly increasing accumulation within 24 h [68]. This phenomenon was further demonstrated by studying the distribution of ¹²⁵I-C₆₀(OH)_x, which was rapidly distributed to all organs (mainly in the kidneys, liver, bone, spleen and stomach) through the blood circulation and especially exhibited a rapid and high uptake in kidneys, followed by a fast excretion into urine. Likewise, ¹²⁵I-C₆₀(OH)_x was not found in the brain, suggesting it could not penetrate the blood-brain barrier (BBB), which was different from the C₆₀ modified with dipolar trimethylenemethane [28].

Similarly, Cagle et al. [69] found that ¹⁶⁶Ho@C₈₂(OH)_y was distributed throughout the entire body, except for the brain and fat tissues, as early as 1 h after the tail-vein injection, which was different from the finding with metal chelate (Na₂[¹⁶⁶Ho(DTPA)(H₂O)]). Only the concentrations of ¹⁶⁶Ho in liver and bone increased from 1 to 4 h, especially in bone, which subsequently displayed a slight increase in concentration over 48 h (Figure 7A). These results were consistent with the observations for the bio-distribution of [Gd@C₈₂(OH)₂₂]_n particles. Unlike GdCl₃, [Gd@C₈₂(OH)₂₂]_n was delivered to almost all observed tissues, but accumulated mostly in bone, suggesting that the fullerene cage could not be destroyed in organisms and that the internal Gd³⁺ was well protected in the cage. The concentration of Gd in the brain was close to the baseline, suggesting that the [Gd@C₈₂(OH)₂₂]_n nanoparticles could not pass through the BBB either (Figure 7B) [70]. The similar results for the bio-distributions of C₆₀(OH)_x and C₈₂(OH)_y showed that the biological properties would not be expected to change by incorporation of a metal atom within a fullerene cage. Thus, distribution and biocompatibility of fullerenes were usually assessed through ip and iv injections, and chemical modification to the fullerene cages may play an important role in determining their distribution and biocompatibility.

 Figure 7 

(A) Biodistribution of ¹⁶⁶Ho_x@C₈₂(OH)_y in BALB/c mice at 1, 4, 24, and 48 h after injection. The animals were sacrificed, and organs were sampled at 1, 4, 24, and 48 h, followed by measurement of the ¹⁶⁶Ho 80.5 KeV γ-emission [69]. (B) Biodistribution of [Gd@C₈₂(OH)₂₂]_n nanoparticles in tumor-bearing mice (expressed as concentrations of gadolinium element, ng Gd/g wet weight) [70].

Toxicity evaluation

To understand the toxicity of C₆₀, it is usually necessary to investigate the lethality, gross or microscopic lesions, histopathological change, etc. However, other biological responses, such as stimulation of the immune system, regulation of oxidative stress, and changes on the molecular level induced by fullerenes, should generally be considered as well.

Following the inhalation pathway, C₆₀ and its derivates can be partly deposited in the lungs and internalized by alveolar macrophages; a pulmonary inflammatory response was elicited for a high exposure dose of fullerenols (10 mg/kg) and fullerenols at low concentrations (1 mg/kg) exhibited an anti-inflammatory property. Although slight changes in gene expression level were observed, no obviously toxicity was reported [71, 72]. Only when water-soluble polyalkylsulfonated C₆₀ was administrated at extremely high dose (LD₅₀=600 mg/kg) in an acute toxicity study or at a moderately high dose (up to 60 mg/kg) via ip injection, observable pathological or lethal toxicity was induced. However, in subsequent study of oral exposure, fullerenes were considered to be nontoxic at a dose of 50 mg/kg, which was close to that of ip administration [73]. Even when the dose was increased to 2,000 mg/kg, fullerite (a mixture of C₆₀ and C₇₀) has not shown to cause any acute toxicity. The possible reason is that fullerenes are not effectively absorbed through the alimentary tract [74]. Toxicity assessment of fullerenes should therefore be considered comprehensively, including the exposure routes and the delivered dose, etc.

After fullerenes enter into the body, they may be internalized by different types of cells, and induce various changes inside the cells, such as viability, proliferation, inflammation responses, and oxidative responses. Therefore, the cytotoxicity assessment of fullerenes has been one of the most important aspects to consider when assessing the toxicity of fullerenes [75]. Many related studies have been performed on various cell lines. C₆₀ and quaternary ammonium derivatized C₆₀ inhibited keratinocytes proliferation at a concentration of 2 μM and over a period of 8 days, which is relatively long [66]. In both human skin (HDF) and liver carcinoma (HepG2) cells, water-soluble fullerene species are less cytotoxic than pristine C₆₀, while C₆₀(OH)₂₄ showed no cytotoxicity up to its limits of solubility, suggesting the functional groups on the surface of a fullerene molecule can dramatically decrease the toxicity. However, C₆₀(OH)₂₄ was found to induce cytotoxic morphological changes within the HUVEC cell line in a dose-dependent way, and only maximal doses of fullerenes caused cytotoxic injury and/or death and inhibited cell growth [76]. In nearly every fullerene-related anti-tumor study, fullerenes were not found to have a direct influence on tumor cell viability [9, 38, 40, 42]. The diverse toxicity results of fullerenes reveal that the sensitivity of various tissues or cells to the nanoparticles can be different. Therefore, thorough toxicity evaluation of fullerenes before their clinical application is necessary.

Perspectives

Due to their attractive physical, chemical, and biological properties and the availability of various methods for surface modification, fullerenes and their derivatives have been studied for their numerous biomedical applications, not only in tumor diagnosis and therapy, but also as an anti-HIV agent [77, 78], antimicrobial photosensitizer [79, 80], antioxidants [52, 81, 82], and gene vector [83-85]. Some properties involved in these applications might create new insights into fullerenes as anti-tumor therapeutics, for example, in gene therapy. However, since the underlying mechanisms related to their apparent activities are not yet clear, further research towards a better understanding is urgently needed to investigate the biocompatibility and safe application of functionalized fullerenes as diagnostic tools and therapeutic agents.

Acknowledgements

We thank the Ministry of Science and Technology of China (National Basic Research Program: 2011CB933401 and 2010CB934004) and the National Science Foundation of China (21001034 and 10975040) for financial support.

Conflict of Interest

The authors have declared that no conflict of interest exists.

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Author contact

Corresponding author: Dr. Ying Liu, CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology (NCNST), No.11, 1st North Street, Zhongguancun, Beijing 100190, China. Tel: +86-10-82545526; Fax: +86-10-62656765; E-mail: liuycn.

Received 2011-9-13
Accepted 2011-11-2
Published 2012-3-1

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