1.1. Current strategies

Surgery is one of the oldest treatments for cancer. Surgical procedures may be curative, reconstructive or palliative. The aim of curative surgery is the removal of a tumor and in some cases surrounding tissues. Depending on the size and the location of the tumor, curative surgery can affect the functionality of organs or cause permanent disfiguration. Palliative surgery aims to relieve tumor side effects and restore functionality of the tissue. Reconstructive surgery is used to restore function or correct disfigurement. Sometimes surgery may be done to confirm the presence of cancer. Overall difficulties in surgery and later complications for patients let to the development of other types of cancer treatment.

Another treatment for cancer is radiation. In this treatment method, high-energy rays are used to destroy cancer cells and inhibit the proliferation of them. The goal of radiation is to cure or eliminate the symptoms. Radiation therapy may be delivered both internally or externally. In the external scheme, high-energy rays emit from a device and passing the skin and targeting the underlying tissue. Internal radiation therapy that is also known as brachytherapy involves placing small amounts of radioactive material inside the tissue [4]. Limitations in selectiveness of radiation therapy are the most problematic feature of this therapy which can cause serious damages in structure and function of surrounding tissues depending on the dosage of radiation and the part of body receiving the radiation.

1.2. Promised strategies

Chemotherapy is one of the most common treatments that involves using of drugs to terminate cancer cells and prevent their proliferation. Chemotherapeutic strategies usually include administration of high doses of drugs which can also affect normal cells; especially the cells that grow and divide rapidly such as the skin cells, hair follicle cells, and bone marrow cells. So the side effects of this approach include hair loss, bone marrow depression resulting anemia [5], nausea, vomiting, etc. A newer procedure for cancer therapy is immunotherapy, whose aim is to improve immune system so that it recognizes cancer cells as foreign objects and eliminate them. Many types of immunotherapy are used to treat cancer such as monoclonal antibodies, cytokines, and vaccines. Immunotherapy has the advantage of good selectivity, but unfortunately, hypersensitivity and allergic reaction are probable which can cause harmful effects [6].

Since one of the most significant problems of cancer treatment methods is the low selectivity of the techniques, many types of research have focused on developing targeted drug delivery systems (DDS). In this field, many types of carriers have been found to be advantageous. Among them, nanoparticles [7], polymers [8], [9] and liposomal structures [10] are notable. A schematic representation of the mentioned vectors is available in Fig. 1, Fig. 2, Fig. 3.

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Fig. 1. Carbon nanotubes: a kind of nanoparticle vectors [11]. Reprinted with permission from Elsevier.

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Fig. 2. A polymeric nanohybrid device vector [9]. Reprinted with permission from Elsevier.

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Fig. 3. Liposomal vector [12]. Reprinted with permission from Elsevier.

1.3. Nanomaterials in cancer therapy and diagnosis

Different types of nanomaterials have been involved in cancer therapy and diagnosis [13]. For example, the Quantum dots have been implicated in cancer detection through the MR and PET imaging methods [14]. Nano-bubbles are used in early cancer detection, besides in gene therapy and microsurgery [15]. Liposomes have shown promising results in targeted delivery of anti-cancer drugs [16]. Nevertheless, the swift removal of these particles by macrophages limits the usage of unmodified liposomes [17]. Fullerenes have been used in cancer therapy through the photodynamic therapy methods [18]. However, the toxic effects of fullerenes (especially C60) limit its diverse usage for patients [19]. It has also been shown that graphene can have a potential use in cancer therapy and diagnosis. Similarly, in the case of graphene cytocompatibility and biocompatibility issues, further investigations and modifications on these nanoparticles have been suggested [20], [21]. Another important nanomaterial in the cancer-related researches is carbon nanotubes, which are the target of this review article.

Carbon nanotubes (CNTs) are one of the most commonly used nanoparticles for cancer therapy. CNTs are categorized into three main types of single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs) and double-wall carbon nanotubes (DWCNTs) [22]. CNTs are utilized in both cancer diagnosis and therapy; one of the first reviews in this matter was written by Yang et al. [23] in which CNT was admired as an improving agent for cancer therapy and diagnosis because of properties like large surface, conjugation ability and encapsulation of drugs. Focusing on the conjugation and hybridization of the CNTs some articles has reviewed the in-vivo effects of cancer. In 2010, Zhang et al. have focused on reviewing the utilization of CNTs for in-vivo administration and suggested no significant toxicity in well-functionalized nanotubes [24]. While some others covered the in-vitro studies, Kesharwani et al. have reviewed the studies regarding analyzing the efficiency of the CNTs on cell lines [25]. CNTs present the selectivity ability by being functionalized with different kinds of molecules and subsequently making them sufficient for drug delivery systems. Some authors have reviewed the targeting capabilities of nanotubes, nanohybrids and polymeric nanoparticlesand their usage as drug delivery vectors [9], [26], [27], [28], [29], [30]. CNTs are fabricated by different processes classified to physical, chemical and miscellaneous processes. The physical methods include the arc discharge and laser ablation which can be used to produce both SWCNTs and MWCNTs with different diameters. The chemical schemes, including vapor deposition, high pressure carbon monoxide reaction and cobalt molybdenum process are also commonly used to produce different types of CNTs. Finally, miscellaneous methods such as the helium arc discharge, electrolysis and flame synthesis are less used for production of CNTs. Although, the helium arc discharge method seems to be a potential for the commercial applications [29]. The most important limitation regarding to CNT utilization is their extreme hydrophobicity causing these insoluble particles to pack together in water. Another major problem about CNTs is the inherited cytotoxic properties of these particles. Although some researchers reported no cytotoxicity effect due to CNTs [31], some others stated different levels of cytotoxicity on different cells [32], [33]. However, modifications on CNTs can eliminate some of these limitations. By functionalization of different chemical groups on CNTs, the hydrophobicity and toxicity levels could be lowered. Indeed, the chemical accessibility of CNT surface is one of the most important advantages of using these nanomaterials in researches. Other advantages of employment of CNTs include their permeability to cell and nucleus membranes, remarkable photo thermal properties, and the ability of drug delivery.

Some review also covered the studies regarding CNT mediated ablation of cancer cells focusing on the improvements achieved by the usage of CNTs [34]. This review highlights and focuses new studies since 2011 and also covers some important studies of earlier since 2005 regarding the utilization of CNTs in the cancer fields, discussing the advanced diagnosis methods as well as therapy strategies. The diagnosis methods include the development of biosensors for cancer biomarkers as well as cancer-specific gene detection methods. In the case of therapy, both targeted DDS methodology, and ablation methods are covered. The studies are categorized by the investigated cancer type in each chapter and, in the case of larger chapters, diagnosis methods are separated from therapy strategies for a better review.

2.1. CNT mediated detection of breast cancer

Methods relating to breast cancer detection have been studied for several years. In 2004, Sirdeshmukh et al. [35] investigated utilization of antibody-CNTs for detection of primary monoclonal mouse immunoglobulin (Ig), which is a breast cancer biomarker. Their results showed a noticeable decrease in the conductance of CNTs which were incubated with primary mouse Ig. In another research, SWCNTs were conjugated to antibodies specific for breast cancer surface receptors. The binding degree of CNTs to antibodies was 67-88% which is claimed to be the cause of usage of surfactant in the separation of CNTs leaving more available active sites [36]. In another work, Teker [37] first prepared the biosensors with a similar approach to the later study by conjugating insulin-like-growth factor 1 receptor specific antibody (IGF1R-Ab) and non-specific monoclonal Ab to produce two different biosensors. The conjugations mentioned above caused a decrease in conductance of the both CNTs. The Subsequent exposure to MCF7 and BT474 breast cancer cells increased the conductance only in IGF1R-Ab conjugated CNTs indicating specific detection of cancer cells by the device which is shown in Fig. 4. It is also notable that the increased conductance of MCF7 was greater than BT474 probably because the MCF7 cells express IGF1R more than BT474 cells.

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Fig. 4. An antibody-based biosensor for detection of BT474 and MCF7 cells [37]. Reprinted with permission from Elsevier.

Polyethylene glycol (PEG) was also used for SWCNT functionalization in a study by Ou et al. [38]. They functionalized SWCNTs with phospholipids bearing PEG and also with integrin αvβ3 to modulate the targeting system. Their results showed a successful entry of CNTs in the U87MG cells by integrin-mediated endocytosis and sufficient accumulation for imaging and drug delivery purposes. Jones et al. [39] developed a radioimmunoassay like method by mediation of CNTs for detection of IGF-1 for breast cancer diagnosis. The results of their investigation confirmed a good correlation between in-vivo serum IGF-1 detection by radioimmunoassay and the novel CNT mediated method. Furthermore, it is described that the new method needs lesser serum (100-fold) than the radioimmunoassay. In a study, isoforms of cytochrome p450 were conjugated on MWCNTs to detect anti-cancer drugs in human serum. The conjugated CNT was able to detect drugs in a range of 8-925 nA/μM in both PBS and human serum. Another advantage of the described method is to detect pairs of the drugs which can be helpful in the case of mixture drug treatments [40]. In a similar approach SWCNTs were conjugated to antibodies specific for epithelial cell adhesion molecule (EpCAM) in order to cancer cell detection. Since the EpCAM is highly expressed on MCF-7 cancer cells and not on normal cells, the results of Raman spectroscopy after 30min of incubation with anti-EpCAM-SWCNTs could distinct tumors from normal fibroblastic cells [41]. In a similar research Khosravi et al. [42] developed carbon nanotube-antibody microarray which was capable of breast cancer cell diagnosis from buffy coat. Li et al. [43] applied SWCNTs in a different way for cancer detection. They conjugated a specific sequence which is complementary to breast cancer 1 (BRCA1) gene. After exposure to the patient's sample, the rate of hybridization was analyzed for diagnosis of breast cancer.

CNTs also have become important in imaging methodology of breast cancer. In an investigation in 2009, researchers utilized SWCNTs as a contrast agent in microwave imaging because SWCNTs are accumulated in cancer tissues more than normal tissues. They analyzed different concentrations of SWCNTs in a tissue-mimicking phantom and indicated that in concentration less than 0.5% (by weight), significant differences could be observed [44]. In another study by Gidcumb et al. [45], CNTs were used for improvement of X-ray imaging. It was shown that utilizing a CNT array field emission x-ray source, could increase spatial resolution and reduce the imaging time. It was also indicated that contributing CNTs in the imaging method caused production of a higher current, which could reduce the timing of the technique even more.

Faraj et al. [46] also improved an imaging technique by the use of SWCNTs. They functionalized SWCNTs with endoglin (CD105) antibody to increase the targeting features and then loaded with iron oxide nanoparticles to enhance the magnetic detection of the construct. The aforementioned nanostructure was analyzed in-vivo for optimizing a non-invasive MRI protocol. According to their results, the efficiency of the magnetic targeting and delivery of the CNTs was much higher than the traditional MRI. Also, the biocompatibility test confirmed the safety of the method. In a different study, the targeting properties of MWCNTs were improved with functionalization by glucose amine. For analysis, two type of functionalization was performed one covalently and the other non-covalently. The potential of both CNTs was assessed in-vitro and in-vivo and results indicated a higher blood circulation time, late urinary clearance, lower tissue retention and higher accumulation in cancer cells by the non-covalently functionalized CNT [47]. In 2016, a research was conducted in which CD44 antibodies were conjugated on SWCNTs and evaluated by MRI for localization in tumors in-vivo. The results showed that the anti-CD44-SWCNT was co-localized with CD44 receptors and a significantly higher accumulation of the CNTs was observed in breast cancer stem cells (CSC) which was predictable as the breast CSCs express CD44 more than other cancer cells [48].

2.2. CNT mediated treatment of breast cancer

2.2.1. Targeted drug delivery

In an investigation in 2009, ricin a chain protein (RTA) was conjugated to MWCNTs and transported to live cells to achieve higher cell mortality than the usage of RTA alone. It was indicated that cell death rates were much higher with MWCNT-RTA than the RTA alone especially for L-929, MCF-7, HeLa and COS-7 cells. In HeLa cell, the MWCNT-RTA could cause a 75% cell death. Furthermore, HER3 was coupled with the mentioned CNT to selectively cause cell death among HeLa cells [49]. In a different research, doxorubicin was loaded on MWCNTs which were also conjugated to folic acid. Then in-vitro drug release, erythrocyte toxicity, ex-vivo cytotoxicity and cell uptake of CNTs were analyzed on MCF-7 cells. The results showed a higher efficiency of the nanoconstructs in the suppression of tumor growth, and also the in-vivo tests indicated an increased survival rate among CNT treated rats [50]. In another investigation, SWCNTs were loaded with paclitaxel (PTX) with the use of a lipid chain which was conjugated to the drug and connected to CNTs through hydrophobic interactions. Then folic acid (FA) also was added on SWCNTs for increasing the entry to the cells. According to the in-vitro results the FA-FWCNT-lipid-PTX has a higher efficiency of 78.5% cytotoxicity against 31.6% for PTX alone. They also analyzed the nanoconstructs in-vivo in xenograft mice by measuring tumor size which confirmed the results of the in-vitro analysis. The decreased tumor size was compared in the case of treatment with FA-FWCNT-lipid-PTX and Taxol and the results are indicated in Fig. 5 [51].

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Fig. 5. Effect of using nanomaterial in inhibition of tumor growth in xenograft mice [51]. Reprinted with permission from Elsevier.

Arora et al. [52] analyzed anticancer properties of docetaxel (DTX) conjugated to MWCNTs. They measured the internalization of the DTX-MWCNT by flow cytometry, confocal and transmission electron microscopy and also the cytotoxicity by MTT assay. The results of their analysis showed increased efficacy of the DTX-MWCNT in comparison to DTX alone in both internalization and cytotoxicity to cancer cells. In another study, the SWCNTs were utilized as a vector for delivery of doxorubicin (DOX) using antibody and magnetic targeting. The SWCNT was conjugate to endoglin/CD105 antibody and also to iron-oxide to increase the targeting properties of the nanotubes and subsequently decrease the side effects. The results indicated a significant increase in apoptosis rate, DNA damage and oxidative stress in the cancer cell. MRI and BLI imaging also showed that the tumor growth was inhibited by the administration of the nanoconstructs [53]. In a study, the effects of CNTs were measured on the efficiency of paclitaxel. According to the findings of this investigation when paclitaxel was combined with oxygen-CNT (oxygen carbon nanotubes) the inhibitory effects of the drug was significantly increased and the tumor weight in tumor-bearing mice models was decreased too [54]. Mitoxantrone (MTO) is another anticancer drug which was analyzed alongside CNTs. MTO was conjugated to MWCNT and assessed for cytotoxicity in MDA231 and NIF3T3 cell lines in-vitro. The results confirmed an increased efficacy of killing properties of MTO in both cell lines [55]. Shao et al. also showed that usage of SWCNTs as nanocarriers for paclitaxel could increase its efficiency on MCF-7 cell lines. According to their in-vitro studies, conjugating HSA on SWCNTs and then combining the former with PTX causes greater cell internalization because of the high affinity of PTX to HAS [56].

CNTs also were used as vectors for DNA or SiRNA for cancer therapy in some researches. In one particular study, SWCNTs were chemically functionalized by DSPE-PEG-Amin which binds to SiRNA via disulfide bonds. The mentioned SiRNAs are silencer of genes related to proliferation properties of cancer cells, so delivery of them can cause a reduction in cell proliferation of cancer cells. The results indicated successful delivery of siRNAs by the developed complexes with an efficiency of 83.55% [57]. In another similar study, siRNAs against EpCAM (epithelial cell adhesion molecule) was selected for the survey as it is overexpressed in CSC cancer cells. Then the siRNAs were attached to piperazine-polyethyleneimine conjugated SWCNTs and analyzed for transfection and apoptosis rates. Administration of the developed construct caused a 20% increase in apoptosis induction in MCF-7 cell line and not in other cells with low EpCAM expression [58]. In another investigation SWCNTs were conjugated to polyethylene glycol (PEG) and polyethyleneimine (PEI) to form a DNA delivery vector for gene therapy. Then they were loaded with 5TR1 aptamer and exposed to breast cancer cells. The results of their investigation showed an increased transfection activity (8.5-10 folds) when the genes were conjugated to SWCNTs [59].

3.1. CNT mediated detection of prostate cancer

Detection of Prostate Specific Antigen (PSA) is one of the most clinical approaches for the prostate cancer diagnosis. Gong et al. [69] (2006) introduced a new strategy for PSA detection by the usage of CNTs. They designed an amplified electrochemical immunoassay by using SWCNTs which has a sensitivity for detection of 0.004 ng/ml PSA in the only 10 μl of serum. In another research, MWCNTs were utilized to produce tower like structures as electrodes which were then conjugated to Au particles for detection of LNCaP (a prostate cancer cell line). After preparing the electrodes, they were implanted into polydimethylsiloxane channels and tested for deionized water, buffer solution and LNCaP serum by the electrochemical impedance spectroscopy. The results indicated a significant difference between above solutions, suggesting a likely use of this type of electrodes for the cell-based biosensors [70].

In a study by Kim et al. [71] the sensitivity of SWCNT-based electrodes was increased by the enrollment of spacer and linker molecules. Linkers (1-pyrenebutanoic acid succinimidyl ester) are conjugated to gold particles, enabling the PSA to bind and cause a gating effect and changing electrodes impedance. While spacers (1-pyrenbutanol) are not able to conjugate to gold particles, leaving a space between some linkers, different electrodes were made by changing the linker to spacer ratio to achieve the best ratio for the highest sensitivity. According to their results, electrodes which were made by using only linkers (1:0 ratio) were able to detect PSA in concentrations above 500 ng/ml. But electrodes which had a 1:3 ratio of linkers to spacers were more sensitive and could detect PSA in lower concentrations down to 1 ng/ml. it was also mentioned that the latter electrodes were able to block non-target proteins in human serum.

For the purpose of measuring multiple prostate cancer biomarkers at ones, an SWCNT- based array was designed in an investigation. The array was designed to detect PSA, prostate-specific membrane antigen (PSMA), platelet factor-4 (PF-4), and interleukin-6 (IL-6) as prostate cancer biomarkers in a sandwich immunoassay method by the use of Horseradish Peroxidase (HRP). Clinical human serum was analyzed to measure the four biomarkers, and results confirm the detection of the difference between patient's and control's serum in ELIZA [72]. In a similar research by Wan et al. [73] an MWCNT based array was designed to detect using the same sandwich scheme but using a universal multi-labeled nanoprobe. Using the latter probe, they could higher the sensitivity of their array to detect 5 pg/ml of PSA and 8 pg/ml of IL-8 in patient's serum. Salami et al. [74] also developed an electrochemical immunosensor for the use of PSA antibody conjugated on MWCNTs. Their method shows a linear increase in immunosensor current for PSA concentrations ranged between 0.2–1 ng/ml and 1–40 ng/ml.

In 2012, Lerner et al. [75] developed a strategy for detection of osteopontin (OPN), another prostate cancer biomarker. To do so, a genetically engineered protein, named single-chain variable fragment (scFV) was developed and then attached on a CNT field-effect transistor for analysis. Their technique was capable of detecting 1 pg/ml of OPN in ELIZA immunoassay. Since they used the high-affinity specific antibody for OPN, exposure to even high concentrations of BSA did not induce any response on the device. They also tested the specificity of the scFV by comparing the results of a mixture of proteins and OPN to a solution containing only OPN. The comparison showed no significant difference in the results confirming high functionality of this method. Another similar study also involved monoclonal OPN antibodies for prostate cancer detection. These OPN antibodies were covalently bound to SWCNTs which were deposited between gold/indium electrodes. The designed biosensor was analyzed against 1 pg/ml to 1 μm/ml concentrations of OPN in PBS and human serum, indicating linear behaviors. These results were solid when OPN concentrations were measured against another test protein confirming the specificity of the method for OPN. The final results of this investigations show the limit of detection for OPN is 0.3 pg/ml [76]; a graphical abstract of this study is provided in Fig. 6.

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Fig. 6. A graphical abstract describing the functionalization of OPN specific antibodies on SWCNTs for detection of existence of OPN in different concentration. [76]. Reprinted with permission from Elsevier.

In another electrochemical sensor development in research, a label-free microRNA was conjugated on MWCNTs. The mention microRNA was complementary for miR-141, a microRNA biomarker of prostate cancer, and would make a detectable signal current if to two microRNAs are attached to each other. Exposure to other non-complementary microRNAs caused to signal indicating the good specificity of the method. The minimum sample concentration for this method was estimated to be 8 fM, suggesting the likely use of this method for human serum analysis [77]. DNA strands were also used as biomarkers in another investigation by Shobha and Muniraj [78] (2014) in which were conjugated both on MWCNTs and SWCNTs. The SWCNTs are selected as a more suitable base for the biosensor as they have higher current carrying capabilities. In this research, three DNA strands were analyzed: ssDNA, dsDNA, and PNA (peptide nucleic acid). Results over ally show that dsDNAs are better biomarkers for more sensitive detection of prostate cancer. Evolving DNAs as biomarkers for prostate cancer is not limited to their sequences, and the hypermethylation of these molecules can also be used as biomarkers of early prostate cancer. In another research, hypermethylation of Glutathione S-transferase p-1 (GSTP-1) gene was analyzed via DNA hybridization in MWCNT based biosensor. The conjugation of DNA probes on MWCNTs was done by passive absorption and then target samples were exposed on the biosensor for complementation and detection of a signal by the device. According to their results, this biosensor is as much sensitive to detect picomolar ranges of hypermethylated GSTP-1 [79].