**3. Limitations and opportunities of aptamers**

Around 50 million people worldwide have Alzheimer's disease, and more than 10 million people have Parkinson's disease. The most common type of cancer in children younger than 19 years is brain tumors and central nervous system tumors and is the leading cause of cancer-related deaths in children under 14 years of age in the United States. Although antibody drugs have taken major steps in cancer therapies, the passage of these drugs through the blood-brain barrier and proper cleansing of the brain limit the use of antibody drugs for the treatment of neurological diseases.

Aptamer technology has emerged into almost every field in the life sciences since its inauguration 25 years ago. To date, a wide variety of aptamers have been selected and characterized and also allowed to bind to a wide variety of target molecules. Aptamers are suitable for diagnostic and therapeutic applications because of their unique properties. In this chapter, we explained that because of the specific properties of aptamers, it can be a valuable tool for basic research in the field of neuroscience. In the future, neurobiology approaches will reveal a number of interesting target proteins in which specific and potent inhibitors are required. Analysis of target proteins in neurons and neuronal systems requires identification and availability of specific inhibitory compounds. Thus, the identification of new aptamers may be crucial for the timely acquisition of new inhibitors for the treatment of disorders in the neurological system. Together, we believe that aptamers will be a valuable research tool for neurological studies and that data from new studies in the field of aptamer and neuroscience will reveal the full potential of

*Aptamers and Possible Effects on Neurodegeneration DOI: http://dx.doi.org/10.5772/intechopen.89621*

aptamers.

**Author details**

Fatma Söylemez<sup>1</sup>

Turkey

**281**

Mersin University, Mersin, Turkey

provided the original work is properly cited.

\* and Çağatay Han Türkseven<sup>2</sup>

\*Address all correspondence to: soylemez\_fatma@yahoo.com

1 Department of Food Processing, Vocational School of Technical Sciences,

2 Department of Biophysics, Faculty of Medicine, Mersin University, Mersin,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Aptamers have a number of advantages, such as their high specificity and affinity, their enzymatic or chemical production, and their high reliability and their renewability in simple ways. Furthermore, they have a higher inhibitory potential than monoclonal antibodies and have the possibility of wider chemical modification as they can be synthesized enzymatically or chemically. Aptamers can also remain stable in a wide variety of buffer environments without loss of activity and are more resistant to harsh processes such as physical or chemical denaturation. Since aptamers are developed completely in vitro without the need for cells or animal immune systems, aptamer production offers a wide variety of binding options. Aptamers can be produced against a large number of targets, such as molecules that are toxic to the cell, targets where an immune response does not occur, and compounds that are soluble only in solvents other than water [35]. Given the chemical and physical properties of aptamers, it is unlikely that they will enter the brain via paracellular aqueous pathways or transcellular lipophilic pathways. However, the aptamer may enter the brain by any of the pathways of adsorptive-mediated transcytosis and channels and/or receptors for absorption or liquid-phase pinocytosis. A recent study has shown that a quadruplex DNA aptamer binds to nucleotide by micropinocytosis [36]. Cheng et al. identified an aptamer that can enter brain endothelial cells under physiological conditions, and in vivo, into the brain parenchyma [36]. This development has shed light on the use of aptamers in the investigation of neurological diseases.

As all technologies and classes of substances, the use of aptamers has certain limitations as well as advantages. In vitro selection experiments are the major obstacle to success, which means whether a specific combination of the target protein and the nucleic acid library will produce an aptamer. Essentially, this limitation binds to a limited number of nucleotide groups and building blocks from which a nucleic acid library is made. The four canonical nucleotides, together with ribose (in the case of RNA) or deoxyribose (in the case of DNA) and phosphate backbone, are highly limited chemical diversity in comparison to 21 proteinogenic amino acids that are capable of forming a large number of molecular interactions and properties such as polar, charged, basic, acidic, aromatic, and aliphatic. Therefore, the success rate of in vitro selection experiments for protein targets was found to be 30% [37].

The inability of aptamers to cross cell membranes autonomously (e.g., passive diffusion represents a further limitation in their applicability and is mainly attributed to their macromolecules and polyanionic nature). Nevertheless, several options are available to overcome this restraint (e.g., transfection with liposomes, through plasmid or viral delivery or using nanoparticles). In experiments with isolated cells and cell culture, intracellular distribution of aptamers can be achieved, but this is more complicated in vivo, especially when the targeted tissue is CNS. The barrier (bloodbrain barrier) cannot passively pass aptamers and other macromolecules. However, by performing in vivo experiments, aptamers that cross the blood-brain barrier in mice and penetrate into the brain parenchyma could be identified [36].

#### *Aptamers and Possible Effects on Neurodegeneration DOI: http://dx.doi.org/10.5772/intechopen.89621*

**3. Limitations and opportunities of aptamers**

*Neuroprotection - New Approaches and Prospects*

logical diseases.

gation of neurological diseases.

to be 30% [37].

**280**

Around 50 million people worldwide have Alzheimer's disease, and more than 10 million people have Parkinson's disease. The most common type of cancer in children younger than 19 years is brain tumors and central nervous system tumors and is the leading cause of cancer-related deaths in children under 14 years of age in the United States. Although antibody drugs have taken major steps in cancer therapies, the passage of these drugs through the blood-brain barrier and proper cleansing of the brain limit the use of antibody drugs for the treatment of neuro-

Aptamers have a number of advantages, such as their high specificity and affin-

ity, their enzymatic or chemical production, and their high reliability and their renewability in simple ways. Furthermore, they have a higher inhibitory potential than monoclonal antibodies and have the possibility of wider chemical modification as they can be synthesized enzymatically or chemically. Aptamers can also remain stable in a wide variety of buffer environments without loss of activity and are more resistant to harsh processes such as physical or chemical denaturation. Since aptamers are developed completely in vitro without the need for cells or animal immune systems, aptamer production offers a wide variety of binding options. Aptamers can be produced against a large number of targets, such as molecules that are toxic to the cell, targets where an immune response does not occur, and compounds that are soluble only in solvents other than water [35]. Given the chemical and physical properties of aptamers, it is unlikely that they will enter the brain via paracellular aqueous pathways or transcellular lipophilic pathways. However, the aptamer may enter the brain by any of the pathways of adsorptive-mediated transcytosis and channels and/or receptors for absorption or liquid-phase pinocytosis. A recent study has shown that a quadruplex DNA aptamer binds to nucleotide by micropinocytosis [36]. Cheng et al. identified an aptamer that can enter brain endothelial cells under physiological conditions, and in vivo, into the brain parenchyma [36]. This development has shed light on the use of aptamers in the investi-

As all technologies and classes of substances, the use of aptamers has certain limitations as well as advantages. In vitro selection experiments are the major obstacle to success, which means whether a specific combination of the target protein and the nucleic acid library will produce an aptamer. Essentially, this limitation binds to a limited number of nucleotide groups and building blocks from which a nucleic acid library is made. The four canonical nucleotides, together with ribose (in the case of RNA) or deoxyribose (in the case of DNA) and phosphate backbone, are highly limited chemical diversity in comparison to 21 proteinogenic amino acids that are capable of forming a large number of molecular interactions and properties such as polar, charged, basic, acidic, aromatic, and aliphatic. Therefore, the success rate of in vitro selection experiments for protein targets was found

The inability of aptamers to cross cell membranes autonomously (e.g., passive diffusion represents a further limitation in their applicability and is mainly attributed to their macromolecules and polyanionic nature). Nevertheless, several options are available to overcome this restraint (e.g., transfection with liposomes, through plasmid or viral delivery or using nanoparticles). In experiments with isolated cells and cell culture, intracellular distribution of aptamers can be achieved, but this is more complicated in vivo, especially when the targeted tissue is CNS. The barrier (bloodbrain barrier) cannot passively pass aptamers and other macromolecules. However, by performing in vivo experiments, aptamers that cross the blood-brain barrier in

mice and penetrate into the brain parenchyma could be identified [36].

Aptamer technology has emerged into almost every field in the life sciences since its inauguration 25 years ago. To date, a wide variety of aptamers have been selected and characterized and also allowed to bind to a wide variety of target molecules. Aptamers are suitable for diagnostic and therapeutic applications because of their unique properties. In this chapter, we explained that because of the specific properties of aptamers, it can be a valuable tool for basic research in the field of neuroscience. In the future, neurobiology approaches will reveal a number of interesting target proteins in which specific and potent inhibitors are required. Analysis of target proteins in neurons and neuronal systems requires identification and availability of specific inhibitory compounds. Thus, the identification of new aptamers may be crucial for the timely acquisition of new inhibitors for the treatment of disorders in the neurological system. Together, we believe that aptamers will be a valuable research tool for neurological studies and that data from new studies in the field of aptamer and neuroscience will reveal the full potential of aptamers.
