Advances in Cancer Nanotheranostics for Experimental and Personalized Medicine E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Sprache: Englisch
Nanotheranostics is a recent medical field which integrates diagnostic imaging protocols and therapeutic functions to monitor real time drug release in the body and distribution to the target site. The combined processes allow technicians to observe the effectiveness of a specifically designed drug candidate and predict its possible side effects. All these features help clinicians in optimizing treatment options for cancer and other diseases for the individual patient. Current research is tailored to individual therapy because each drug may display a variety of responses depending on variations in an individual’s genetics and subsequently, their clinical biochemistry. Many tumors are still challenging for therapists in terms of available treatment and nanotheranostic strategies may help them to combat cancer more efficiently.
Advances in Cancer Nanotheranostics for Experimental and Personalized Medicine presents information about current theranostic technologies in use at clinics and recent research on nanotheranostic applications, with a focus on cancer treatment. Information is presented in seven organized chapters that cover the basics of cancer nanotheranostics, tumor microenvironmental factors, gene therapy and gene delivery concepts, and the combined application of diagnostic imaging with cancer chemotherapy. A chapter focusing on the role of non-coding MRNAs in breast cancer carcinogenesis is also included, giving readers a glimpse of the complexities in the molecular biology of cancer which drive the need for new theranostic technologies. The book is of interest to medical professionals (including oncologists and specialists in internal medicine), diagnostic imaging technicians, and researchers in the fields of pharmacology, molecular biology and nuclear medicine.
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Seitenzahl: 356
Veröffentlichungsjahr: 2020
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FOREWORD
Oncologic drug development focused on single target-single drug strategy for several years. However, cancer cells are genius! They can bypass inhibitor perturbations by using alternative routes. Further, human genome project results indicated that four letter-alphabet is not as simple as central dogma; only few percent of the gene sequences are transcribed and translated. Rest of the genome function is involved in uncharacterized biochemical pathways and cell biology. And epigenetics and metabolites add more complexity to the understanding of molecular mechanisms in detail. This unknown mechanism makes cancer cell genius!!! But a great endeavor of eminent scientists and continuous research to elucidate pathways both in cellular and tissue levels make drug design more effective every day.
To control the effect of several cellular factors, the new trend in contemporary drug design is to employ drug cocktail that will synergistically act on these factors and proper oncologic drug targeting to eliminate off-targeting. For this purpose, nanocarriers have been designed to deliver drugs to tumor microenvironment not only to treat the tumor but prevent its metastasis.
Biomimicking is an old fashioned yet excellent method in disease treatment. Macrophages target cancer cells and using this biomimick bullet macromolecule with oncologic drug cargo serves as a fine treatment strategy. The biomimick bullet cargo can be a wide range of molecules from small to large molecules.
Targeting with nanotheranostic carriers provides specific delivery to cancer microenvironment. However, cancer cells so called “the other strategies” yet to be elucidated. For example, transformation of a healthy cell to cancer cell may enhance inner and surface signaling molecules and may introduce new set of metabolites. Further, some of the non-coding gene’s expression increases during this transformation. Altogether, cellular and tissue level characterization of oncologic pathways may help our understanding of tumor biology. Currently, cancer nanotheranostics tries to find a short cut for experimental and personalized medicine in cancer treatment. This book covers recent advances in this field.
PREFACE
Personalized medicine with novel therapeutic approaches provides direct targeting of macromolecules with contemporary drug delivery systems for treatment of severe diseases, including cancer. Nanotheranostic design offers increased bioavailability of the drugs through controlled release and distribution. Nanotheranostics also integrates diagnostic test with treatment of the disease. Recent advances in cancer studies revealed new genetic elements and factors that affect theranostic drug targeting approaches. Also, several tumors are challenging, and new treatment modalities are required. Molecular level mechanisms provide valuable information for therapy and innovative design for treatment. Several creative approaches have been proposed for theranostic therapy. For this reason, an updated approach over in vivo and translational properties of nanotheranostics with special emphasis on cancer will widen the scope of the readers/researchers with this book.
Chapter 1 despite significant advances in cancer therapy, many tumors are still challenging, and novel strategies are essential for treatment. Nanotheranostics use nanotechnology for diagnosis and therapy of cancer. Recent advancement in nanotechnology has provided novel types of nanomaterials composed of either organic- or polymer-based nanoparticles. Small alterations and modifications transform this carrier system with unique properties and optimize drug delivery and release. This chapter provides overview in cancer nanotheranostics field.
Chapter 2 overviews tumor microenvironment as prelude. This site regulates tumor progression and metastasis. Non-cellular components in this environment such as cytokines, chemokines, growth factors, inflammatory and matrix remodeling enzymes shape the progression of the disease by mediating the communication taking place between the tumor itself and its surrounding. This may prevent the benefits of therapeutical strategies. The chapter focuses on understanding the function and mechanism of these non-cellular components in the environment to elucidate obstacles in the treatment of cancer.
Chapter 3 covers immune system employment in fighting cancer cells to prevent tumor development. Immunotherapies are innovative cancer treatment. Nanomedical formulations modulate macrophages which can influence the tumor microenvironment, since macrophages target tumor environment. Macrophage may be used as trojan horse and its cargo may mediate gene and/or protein expression in the treatment regime. This section discusses improvements in cancer immunotherapies through this biological strategy.
Chapter 4 Gene and genome modification tools allow gene therapy through alteration of malignant genes and editing mutations for correction of errors. These innovative technologies deliver therapeutic nucleic acids to cells and tissues. Therefore, the success of gene therapy formulation is proportional to efficient delivery of the carrier and its nucleic acid cargo to a specific target and proper cellular uptake. The platforms have been developed for higher loading capacity, and low immunogenicity and toxicity. In chapter 4, the authors provide a review on different gene delivery vectors and platforms at the nanoscale.
Chapter 5 Oncology research applications may not yet fully suppress cancer-based mortalities and morbidities. Conventional therapeutic approaches have limitations as most research depends on coding genes. Human genome sequencing revealed that only 2-3 percent of the genome codes for genes and proteins however the rest is unknown. Further, heterogeneity among malignant tumors lead obstacles. Therefore, “precision medicine” in oncology and its extrapolation to “personalized treatment” for each cancer patient is essential. The chapter covers non-coding RNAs as biopharmaceutical tools in oncology. The new trend in drug design is covered in this section.
Chapter 6 Nanoparticles are convenient carrier systems based on their plasmonic and magnetic properties, active surface areas and various physicochemical properties. Development of therapeutic nanoparticles provides imaging modalities such as magnetic resonance imaging, radionuclide-based imaging; positron emission tomography and single-photon emission computed tomography and X-ray-computed tomography. Methodology and applications of the techniques are explained thoroughly in this chapter.
Chapter 7 covers practical clinical applications in chemotherapy and nuclear medicine. The simultaneous yield of imaging in radiologic and nuclear medicine applications and therapeutic agents offers diagnosis and treatment effectiveness in real-time.
This book covers recent advancements both in applied and in clinical research. Since targeting small organic molecules are common, the book mainly focused on DNA, protein and immunotherapy on cancer. Different applications for cancer treatment are in progress but basic strategies are similar. We hope this book will help not only early career scientists but also will help experienced researchers to widen the scope of their projects.
List of Contributors
Cancer Nanotheranostics
Ezgi Nurdan Yenilmez Tunoglu1,Berçem Yeman1,Merve Biçen1,Servet Tunoglu2,Yousef Rasmi3,Yusuf Tutar1,4,*Abstract
Molecular profiling of diseases identifies specific cancer-causing genes and associated networks. Administered drug displays different therapeutic efficiency depending on individual cancer subtype and therapeutic responses. Personalized medicine helps designing treatment methods for individual patients with distinct diseases. For complete understanding of patient’s pathophysiology, different omics data types are integrated. These data can be derived from whole-exome sequencing, metabolomics, pharmacogenomics, and proteomics. Pharmacogenomics deals with the interaction of drug and patient’s genetic make-up and metabolomics reveals custom regulation of biochemical pathways in patients. Transcriptomics and proteomics analyze organism tissue or cell type in cancer and play even more relevant role in personalized medicine. Since associated genetic anomalities and metabolic profiles influence therapy response, a continuous evolution of cancer nanotheranostics helps preventing and treating the disease more precisely.
INTRODUCTION
There is a six million nucleotide difference between two individual’s genome and even twins have 35 intergenerational mutational nucleotide differences [1]. Epigenetic alterations as well as gene duplication or deletion like unequal crossing over alter genomic content during the course of individual life cycle.
Therefore, nanotheranostic approaches require delivery of individually adapted medicine based on genetic profiles of cancer patients. High-throughput technologies in oncology provide genomic analysis to be used for guidance of individualized medical treatment.
Initial studies of individualized treatment started with the Human Genome Project (HGP). HGP has helped to make use of the relationship between drug and target and improve its efficacy and safety. In this concept, patients are individually treated taking their unique genomic profiles into consideration. Even though the genomic profiles of different cells of a patient are the same, their expression profiles are clearly different. This difference is taken into account in personalized medicine along with the patient’s disease history. On the other hand, traditional treatment methods have overlooked the genetic variability among patients and focused on a reactive approach based on population-based conclusions. During the physical examination; symptoms described by patients, medications taken, and biopsy outcomes are taken into account to decide on the traditional method to be implemented [2].
A new concept, P4 medicine, was first introduced in 2011 by Hood et al. as a systems approach including predictive, preventive, personalized, and participatory features of medicine. This approach puts emphasis on the patient instead of the disease itself, making it a proactive discipline rather than reactive [3]. P4 medicine does not focus just on genomic data but also involves data from DNA (together with epigenetic changes), RNA, protein, metabolite, cell, and tissue level. Using the data gathered, it is possible to personalize any form of treatment for any form of disease. It is important to have as much knowledge as possible of each patient’s and tumor’s genetic backgrounds to increase the efficiency of targeted treatment. When the two pieces of information are evaluated with regard to one another, it is also possible to determine the risk groups for specific disease types. Overall, personalized medicine aims to design the right treatment for the right patient at the right time and the right dose.
Oncology in Brief
Our body is a living and growing system that contains billions of cells that perform many functions such as metabolism, transport, secretion, reproduction and mobility. Growth and development occur as a result of the growth of newly formed cells and their transformation into different types of tissues. The branch of oncology is interested in cancer and the biochemistry of cancer cells is different. There are three different types of cells in our body: static cells (differentiated cells), growing cells (undifferentiated cells) and regenerating cells (stem cells). In contrast to these cell types, cancer cells do not have a growth-inhibiting control mechanism as in normal cells. Therefore, it is possible to compare cancer cells to uncontrolled stem cells. Tumors can be malignant or benign tumors. Cancer occurs due to many factors. In addition to genetic factors, many environmental factors such as UV light, X-rays, chemicals and tobacco products can cause cancer. In order to define cancer, it is necessary to understand cancer genetics. There are three types of genes in cancer genetics: Oncogenes, tumor suppressors and DNA repair genes. The normal form of an oncogene is defined as a proto-oncogene. Proto-oncogenes are converted to oncogenes by mutation. Tumor suppressors produce proteins that avoid cell division and cause cell death. Genes that prevent cancer-causing mutations are DNA repair genes. Occasionally, a virus-induced mechanism inserts nucleotides into or near a proto-oncogene and transform it to an oncogene. This results in uncontrolled cell growth. A single oncogene is usually not adequate to cause cancer. Cancer-related genes [4] serve as a biomarker in the definition of cancer (Table 1).
Another term in cancer biology is “angiogenesis”. Angiogenesis is the process by which new capillary blood vessels are formed to supplement blood cells with nutrients and oxygen. Without angiogenesis, tumors cannot exceed half the size of one mm. In the treatment of cancer, surgical intervention, radiotherapy, chemotherapy, and immunotherapy methods are used.
The cancer pathway is a system of regulation in which activation or inactivation by a genetic or epigenetic mutation is required for the development of cancer. Janus Kinases (JAKS), the JAKS/STAT pathway generated by signal transducers and transcription activators (STATs), play an important role in mediating cell fate such as apoptosis, differentiation and proliferation in response to growth factor and cytokines. Disruption of the JAK/STAT signaling pathway contributes to tumorigenesis. STAT3 is active in more than 50% of lung and breast cancer tumors; in over 95% of head and neck cancers. The JAK/STAT signaling pathway also regulates the cellular response to cytokines and attenuated STAT signaling. The notch signaling pathway plays an important role in tissue homeostasis. The notch can inhibit the spread of cellular differentiation within a tissue. T-cell acute leukemia is a type of blood cancer that results from the unlimited proliferation of immature T-cells. Abnormal Notch signal is not only observed in this type of cancer, but also in breast cancer, ovarian cancer and brain tumors. It is possible that the signals in the Notch signaling pathway may enhance cell proliferation by downstream activation of the transcription factor C-myc, where impaired expressions are observed in many cancer types. The RAS-Mitogen-activated Protein Kinase (MAPK) signaling pathway constitutes an important part of the translation of signals from cytokines and growth factors. Mutations in this signaling pathway occurred in approximately 45% of colon cancer and approximately 90% of pancreatic cancer. Similar to the MAPK signaling pathway, the phosphatidylinositol 3-kinase/AKT (PI3K/AKT) signaling pathway responds to various extra and intracellular signals transmitted by hormonal receptors, tyrosine-kinase-bound receptors, and intracellular factors. The PI3K/AKT signaling pathway is active in many types of cancer. Activation of this signaling pathway promotes cell survival and proliferation. The nuclear factor kappa B (NF-kB) signaling pathway regulates genes involved in key cellular processes such as proliferation, stress response, hereditary immunity, and inflammation. Signals in this signaling pathway are activated by many extracellular factors such as tumor necrosis factor, interleukin, growth factors, bacterial and viral infections, oxidative stress, and pharmaceutical compounds. Disruption of this pathway results in malignant tumors in human B cells. The Wnt signaling pathway consists of calcium, planar polarity and standard portion. Distortion in the standard part results in colon cancer and breast cancer. Homeostatic displacement occurs in many epithelia of the human body. The Wnt signaling path plays an important role in this process. Abnormal Wnt signaling results in chronic and acute myeloid leukemia. The TGF-β signaling pathway was first discovered in tumors as an anti-proliferation signal that controls tissue proliferation and provides tissue homeostasis. Similar to the Wnt signal path, this path includes SMAD1/5/8, SMAD2/3, and TAB/TAK. Activation of this signaling pathway takes place with TGF-β ligands that bind to the extracellular portion of TGF-β receptors. Mutations, reduced regulation of TGF-β receptors, inactivation of SMAD4 are found in many types of cancer. Inactivation of SMAD4 results in approximately 53% pancreatic adenocarcinomas [5].
CD36 (platelet integral membrane glycoprotein IV) is known as a suitable receptor for thrombospondin-1 (TSP-1). TSP-1 protease activity in extracellular matrices and platelet granules is involved in TGF-β activation, regulation of neurite outgrowth and angiogenesis, as well as cell addition, mobility, proliferation. Lipid metabolism has attracted interest from researchers in this area in terms of tumor onset, development and important role in metastasis. CD36 can be used as an important cancer-targeted biomarker in lipid homeostasis, angiogenesis, immune response, adhesion, and metastasis in cancer. CD36 plays an important role in regulating endothelial cell function in multiple cancer types, such as brain tumor, colorectal and breast cancer. High density lipoprotein (HDL) is known to have anticancer effects, while low density lipoprotein (LDL) cannot be ruled out. CD36 has been reflected in studies where it acts as a scavenger regarding LDL. The deterioration of lipid metabolism and inflammation causes oxidative stress to produce oxidized LDL (oxLDL). A high-fat diet can cause cancer. OxLDL levels were higher in patients with cancer (breast, ovarian) than non-cancer patients. CD36 and LOX-1 (lectin-like receptor) are important in the uptake of oxLDL. Cholesterol homeostasis is maintained in part by cells that express the radical scavenging receptors (CD36) then absorb oxLDL, which are then converted to oxysterol ligands of nuclear liver X receptors. Heterodimers of activated liver X receptors (LXR) target genes with the LXR element containing ATP binding transporters leading to cholesterol efflux to HDL or cholesterol release by the intestines. While oxLDL can reduce chemodynamic sensitivity to drugs such as cisplatin, statin therapy can lower serum oxLDL. Also, statins are important in the regulation of radical scavenging receptors in oxidative pathways [6-11].
Autophagy is a catabolic process that is maintained by the vesicle and maintains homeostatic functions such as protein degradation and organelle turnover, which is degraded in the lysosome. This mechanism eliminates hazardous compounds such as cytostatic compounds and harmful organelles [6-11]. A disorder in autophagy will cause tumor growth. Many autophagy-related inhibitors can be used to inhibit the growth of tumors [12]. However, tumor heterogeneity, quantitative degree of autophagy, and duration of drug administration are the points to be considered in this case.
A high enough de novo biosynthesis rate in cancer may not always be possible; for example, in solid tumors, enlargement and inadequate vascularization may limit glucose and oxygen delivery. If oxygen is limited, the activation of the hypoxia inducible factor 1 (HIF1) pathway may increase survival [6-11].
Effective cancer treatment is still a major challenge for modern medicine. Nanotechnology has tremendous potential to improve cancer treatment. Lipid-based formulations, poly (ethylene glycol) (PEG), polyamidoamine (PAMAM), dextran-based platforms, gold NPs, quantum dots can be used as drug delivery systems [13]. Multidrug resistance is an important concern in cancer [14]. There are studies where siRNA and anticancer drugs are given to cancer cells simultaneously. While siRNA silences the relevant genes involved in drug resistance, accumulation of anticancer drug in cancer cells gives chemotherapeutic results following the release of siRNA.
Genomic Era
Human Genome Project (HGP) put a new dimension to patient therapy, disease diagnostic, and drug design research. Advances in technology, from past to present, have allowed the analysis of complex biological systems and, accurate identification of early diagnostic factors of diseases. Sequencing technologies have helped to interpret the genetic code of various organisms. The knowledge gained after the completion of HGP has altered the perspective of genomics. Genomic approaches focus on the diagnosis of diseases in order to predict the risk of patients for various diseases. Cancer-specific mutations can be an example of genomic approaches in the clinic. As an example, in patients with non-polyposis colorectal cancer, Lynch Syndrome is caused by mutations in DNA mismatch repair genes like MLH1, PMS1 [15]. Mutations in BRCA1 and BRCA2 genes often cause hereditary ovary and breast cancer [16]. Even though different individuals may have the same tumor type, they may have different mutations such as single nucleotide polymorphism (SNP), deletion/insertion and copy number variations. All these differences have led researchers to identify early indicators of cancer. In the light of new technologies, genetic mapping plays a critical role of deciding on treatment options that vary among patients. Next Generation Sequencing (NGS) allows sequencing individual tumor DNA, including single-cell level sequencing as well. Analyzing the data set generated from sequencing points out the number and profile of somatic mutations in a patient that can be different. Therefore, personalized medicine catches growing attention, and has become nearly necessary for individual treatment planning. Genomic technologies in pharmacology are used to predict individual responses to any given drug. Until today, many pharmacogenomic tests have been developed for clinical use. For example, maintenance of oral anticoagulant warfarin dose is associated with 2 genes, CYP2C9 and VKORC1 [17]. Consideration of the CYP2C9 genotype together with VKORC1 helps determine the necessary warfarin dose. Despite the accumulated genomic information, many obstacles must be handled to use this information in medicinal applications.
Transcriptome Analysis
Since DNA sequencing and microarray technologies advanced, profiling and analyzing the transcriptome have become a useful tool to determine molecular mechanisms underlying cancer development and progression. Growing transcriptome dataset has allowed us to associate between DNA sequence variations and gene expression changes. Until today, Gene Expression Omnibus (GEO) database has collected approximately 800K sets of transcriptomic data. Numerous researches have shown the gene expression pattern that can guide clinicians to predict treatment responses. As a result of these datasets, many clinical tests have improved the prediction of prognosis and relapse risk for patients with a variety of cancers, such as breast, colorectal, and non-small lung cancer [18-20].
Recently, extended studies at the single-cell level have helped increase our knowledge of cell or tumor complexity in cancer and this may influence clinicians to decide on treatment options in terms of personalized medicine.
Epigenomic Regulations
Even though differences in somatic mutations, germline factors, and gene expression profiles help us understand the characterization of cancer cells or tumors, in recent years, researchers have focused on how gene expressions are regulated in tissues and cells. This regulation mechanism called epigenetics plays an important role in tumor formation and growth. Epigenetic regulations clarify the alterations in DNA methylation, histone modification, and non-coding RNA function (miRNAs, lnRNAs) [21-23]. These differences are thought to affect cancer drug response. Some research is ongoing for epigenetic mechanisms in cancer. For example, methylation status of MGMT promotor is evaluated to see whether temozolamide (a DNA alkylating agent) will be an efficient drug for treatment of glioblastoma [24]. Elevated levels of miRNA-449a have been revealed to increase the survival rate in chemotherapy-treated triple-negative breast cancer patients [25]. As a histone deacetylase 8 inhibitor, PCI-34051 induced apoptosis in a calcium-mediated manner and has shown promising results in preclinical studies for treating T-cell malignancies [26]. In conclusion, all these epigenetic alterations may help understand the drug resistance mechanisms in patients undergoing chemotherapy.
Proteomic Approaches
Although genomics and transcriptomics are very useful tools, they are not enough to clarify the mechanisms underlying human diseases. Therefore, we need to do further studies to explore cellular mechanisms. Central dogma of molecular biology explains clearly that RNAs are transcribed from DNA, proteins are translated from RNA. Proteins, the translational products of RNA, are the main components of cellular functions. Posttranslational modifications and conformational folding are mostly required for cellular activity or signalization in different pathways. Since analytical and diagnostic techniques advanced, proteomics has started to be used in clinical practice more often. In oncology, drugs used to target key proteins like EGFR, VEGF, MAPK, and PI3K and affect their target directly at the protein level not at genomic or transcriptomic level [27]. Phosphorylation status of proteins determines their role in cellular functions and is among the most important issues in oncogenic transformation. In clinical level, use of proteins is more advantageous than other cellular components due to their stable and robust characteristics. At this point, profiling of proteins appears as a useful tool that may help diagnose diseases and determine treatment responses. Most proteomics methodologies are based on quantification and identification of individual proteins via Enzyme-Linked Immuno Sorbent Assay (ELISA), Mass Spectroscopy (MS), Nuclear Magnetic Resonance (NMR), and X-ray crystallography. Recently, several studies have used MS technologies to determine biomarkers specific to ovarian and breast cancers [28, 29]. With growing understanding of alterations at the proteomic level, researchers have focused on investigating the differences of individual proteins.
Metabolomics
Metabolomics is a method that emerged after other “omics”. Advances in analytical devices and data interpretation have led to rapid development of the metabolic field. NMR, GC-MS and LC-MS techniques are used in metabolomics investigations. These techniques should be evaluated together for the detection of metabolites such as organic acids, amino acids, and lipids in targeted or untargeted metabolomics studies [30].
In case of an identical drug treatment against same cases, examination of the responses may show different individual susceptibility to the disease based on differences in their genetic or metabolic backgrounds. Therefore, it is important to define biomarkers in order to understand the sensitivity of patients against diseases in order to develop personalized treatments.
Specific targeting of a macromolecule or a receptor in cellular milieu is often difficult and off-target effects may lower the efficiency of the therapy. Therefore, directing special cargo-drugs to specific targets is often achieved by NPs. These molecules precisely transport their cargo to the final destination.
Targeted Delivery Through NPs
Personalized medicine is an umbrella term that holds targeted therapy beneath it. As one the leading causes of death worldwide, cancer is widely used in research hoping to reveal new treatment methods. The need for targeted therapy stems from the side effects and limitations of the traditional treatments currently in use. NPs (NPs) can be especially used for targeted delivery of drugs to regions that standard drugs cannot easily reach. In this manner, it is important to aim at disrupted pathways or their components related to the specific disease [2]. Upon specifically targeting cancerous cells, the efficiency of drug delivery is increased while the side effects on healthy cells are diminished [31]. Other than therapeutic drugs; miRNA, siRNA, DNA, plasmids, and oligonucleotides can be carried as well [32].
Targeting the cancer cells with NPs may be via passive or active targeting. In passive targeting, tumor properties come to forth. The leaky tumor vasculature and poor lymphatic drainage lead to enhanced permeability and retention (EPR) effect. NPs are naturally carried to tumor core, vasculature and microenvironment, taking advantage of the EPR effect. Smaller NPs are more successful in passive targeting in terms of longer circulation time, higher accumulation in the tumor, and decreased elimination from the system [31]. However, the complexity of the tumor microenvironment is a disadvantage since every tumor is different and so are their EPR effects [33].
Active targeting is used to target specific cancer cells by means of targeting moieties such as monoclonal antibodies, proteins, polymers or aptamers. Certain receptors are specifically overexpressed in cancer cells, which are therefore easily targeted by their specific ligands. Once attached to these receptors, ligands help the accumulation of NPs in the tumor and release their drug content. Specific targeting of any tumor cell receptor is an advantage, however, more research should be done in the area as in vivo results are not as successful as in vitro results [34].
Types of NPs include; synthetic and natural polymers, inorganic NPs, lipid-based NPs, liposomes, dendrimers, mesoporous silica NPs, nanoshells, quantum dots, metallic NPs, nanohybrids, viral nanocarriers, carbon nanocarriers [32, 33]. They have different abilities for drug loading and can be used in combination with approved traditional treatment methods to carry the advantages one step further. When designing NPs, it is important to consider their biocompatibility, drug encapsulation efficiency, and ability to prevent premature release before reaching their target [33]. Choosing the type of NP is also important as their preparation techniques also vary [32].
Using NPs, it is possible to come up with new treatment formulations. Depending on the type of NP used, induced cytotoxic treatments may be designed, consequently increasing the therapeutic index by promoting fast and efficient treatments. By conjugating a photosensitizer, reactive oxygen species can be produced by radiation to kill cancer cells through photodynamic therapy. In the case of metallic NPs, radiation produces heat using surface plasmon resonance to kill cancer cells with photothermal therapy. In a similar concept to photothermal therapy, magnetic hyperthermia also kills cancer cells with the heat produced by magnetic NPs through magnetic radiation [33].
Drug delivery has multiple challenges that may be overcome with the help of NPs. Uptake of cells is improved, especially in the case of charge-related challenges, when the agent being carried is negatively charged, it slows down the interaction with the cell membrane that is of the same change. Cell-specific ligands may be conjugated to NPs to enhance target specificity. Agents carried are protected from elimination by the macrophages of the reticuloendothelial system (RES) as they are covered and therefore unrecognized and this also prevents immune system activation. Additionally, if the carried agents are nucleic acids, they can escape from nuclease degradation when sheltered by the NP [32].
NPs in cancer therapy have various advantages that count as desirable over the traditional methods. Firstly, their size is ideally small enough to move around the leaky tumor vasculature and big enough not to be eliminated by the system. NP size is one of the important features to be taken into account when considering the success of drug delivery. It should be able to penetrate into the tumor for successful on-target delivery, because the failure of penetration prevents drug delivery into necessary tissue. Studies investigating the effect of particle size on tumor penetrating ability have shown that the two features are inversely proportional. While larger particles accumulate around the blood vessels of the tumor, smaller particles are found throughout the tissue indicating better penetration [35-37]. Comparing the time needed for penetration, larger particles also take a while to be distributed within the tumor, but smaller particles can penetrate and distribute within the tumor at ease [38, 39].
NPs can be designed specifically to carry multiple drugs at once for a synergistic effect. In a recent study, biodegradable PGLA (poly(D,L)-lactide-co-glycolide; an FDA-approved polymer) nanofibers able to improve drug delivery across the blood-brain barrier (BBB) were designed [40]. They were loaded with four different drugs (carmustine, irinotecan, cisplatin, and combretastatin) to be released sequentially in the brains of rats with one of the most aggressive primary tumors; glioblastoma multiforme (GBM). After the combined treatment with drugs for different antitumor mechanisms targeting different properties, there was no inflammation developed and drugs were successfully released in high concentrations over the course of two months. Such successful experimental designs indicate that it is possible to target multiple hallmarks of cancer and prevent drug resistance using chemotherapeutic agents with different antitumor activity.
The surface property of NPs is another factor affecting the efficiency of drug delivery. Once introduced into biological fluids, the NP surface is swiftly covered by plasma proteins, in other words, opsonized, to give them an identity to be recognized by the members of the immune system [41]. Additionally, NPs with hydrophobic surfaces tend to aggregate, which also increases the risk of opsonization. Since the formation of this protein corona (PC) may lead to the induction of immune response, surface functionalization is an ideal option to reduce the binding of serum proteins and activation of the immune response [42]. Due to its hydrophilic property, polyethylene glycol (PEG) reduces the chances of NP opsonization and clearance. It is important to ensure the circulation of NP in the bloodstream and increase its half-life in order to increase its stability required for desired targeting [31]. However, studies in the past several years suggest that certain modifications of the PC may open a new window towards therapeutics of different cancers and neurodegenerative diseases [42]. In addition to functionalization, surface properties of NPs can also be used for conjugation of ligands for specific cell types. In the case of cancer, ligands against receptors overexpressed on cancer cells can be conjugated for specific targeting diseased cells and avoiding off-targeting of healthy cells.
Even though a challenging approach, targeted therapy using NPs is a promising novel alternative over traditional cancer treatment methods. NPs owe their efficiency mainly to their small size, surface, and structure properties. Efficiency may be taken even further when these properties are adapted to traditional methods by designing NPs accordingly.
Although the NP approach is promising and has been studied for quite a time, there are issues that should be taken into account. A major drawback of NP research is that studies have mostly focused on in vitro models and there is a lack of adequate investigations in in vivo. More effort should be given to in vivo work to figure out whether the same findings are relevant in both cases.
