**10. Conclusion**

Since the inception of aptamer technology, this field has significantly evolved with its foray into a wide variety of applications, including targeted delivery. Aptamers caught the attention of researchers due to their unique advantages for use in therapeutics in nanomedicine due to their small size that gives higher penetration in tissues as compared to antibodies, ease of synthesis, high specificity and affinity that is comparable with antibodies and ease of chemical modifications that allowed

**15**

*Aptamers for Targeted Delivery: Current Challenges and Future Opportunities*

important aspect of treating lysosomal storage diseases.

determine their efficiency at the clinical level.

We would like to thank Lakshmi Narayanan Gopalan for his assistance in the preparation of figures. The salary support for this work was provided by Indo-Finland research grant (IV4SP-FinSynBio-Synthetic Biology), funded by Department of Biotechnology, Government of India (Grant number BT/IN/

**Acknowledgements**

Finland/29/MN/2013).

easy conjugation with a variety of molecules like siRNA/miRNAs, proteins, peptides, antibody, drugs and nanoparticles. However, there have also been challenges in the application of aptamers due to their unique characteristics, *viz.*, small size that lead to quick renal filtration, serum instability and lack of diversity in aptamer library. It is important to note that each of these issues has been addressed in different possible ways. Quick renal filtration of aptamers was tackled by conjugation with high molecular weight moieties, e.g., PEG. Similarly, serum instability was dealt with introduction of modifications to the nucleic acid backbone and modifications at 5′- or 3′-end of the oligonucleotide. The diversity of aptamer library was increased by introduction of modified bases, and this lead to the development of mutant polymerases which were capable of amplifying these unnatural aptamers. Endosomal escape of cell-internalizing aptamers is a very important issue that needs to be addressed for clinical translation. Conjugation of aptamer with endosome escaping peptides will facilitate this issue. But lysosomal localization of aptamers is not always a problem, since lysosomal delivery of proteins or drugs is an

Efforts to up the speed of aptamer selection for industrializing this technology were achieved by development of high-throughput SELEX, and CE-SELEX which brought down the duration of selection process from months or weeks to just a few days. Additionally, the development of next-generation sequencing platform has been a boon for aptamer selection since it allowed assay of millions of aptamer sequences at one go, and alleviated the need to perform several rounds of selection that was earlier a norm. It significantly reduced the amount of time and resources that went in the selection of aptamers. From the point of selectionologists, it is quite important to characterize aptamers in the best possible way, i.e., keep right controls and perform vigorous validation experiments to make sure that aptamers coming out of a lab are robust, and work in hands of different people. Development of working reliable aptamers will pave their way for targeted delivery of therapeutics. Macugen was the first FDA approved aptamer drug that appeared in market for clinical use in 2005. No other aptamer drug has entered the market since then. Several aptamers underwent clinical trials in the last 2 decades as listed in earlier reviews [5, 97, 98]. However, many of them have failed [5, 98]. In many cases, the failure in clinical trials is not due to lack of aptamer specificity or affinity to the target, but because they were unable to provide better treatment over the existing state-of-the-art treatment regimens. However, it is a common phenomenon that more drugs fail during clinical trails than pass it. In the past 2 decades the advances described in this chapter have facilitated in the development of stabilized aptamers that have entered clinical trials. In future, application of aptamers in targeted delivery and therapeutics is only poised to increase due to the establishment of ancillary technologies that facilitate faster and more stable aptamer development. Due to this, a number of aptamers are entering clinical trials. In summary, aptamermediated targeted delivery remains a work in progress; however, with bright results at *in vivo* level and several aptamer drugs in clinical trials, the coming decade will

*DOI: http://dx.doi.org/10.5772/intechopen.84217*

#### *Aptamers for Targeted Delivery: Current Challenges and Future Opportunities DOI: http://dx.doi.org/10.5772/intechopen.84217*

easy conjugation with a variety of molecules like siRNA/miRNAs, proteins, peptides, antibody, drugs and nanoparticles. However, there have also been challenges in the application of aptamers due to their unique characteristics, *viz.*, small size that lead to quick renal filtration, serum instability and lack of diversity in aptamer library. It is important to note that each of these issues has been addressed in different possible ways. Quick renal filtration of aptamers was tackled by conjugation with high molecular weight moieties, e.g., PEG. Similarly, serum instability was dealt with introduction of modifications to the nucleic acid backbone and modifications at 5′- or 3′-end of the oligonucleotide. The diversity of aptamer library was increased by introduction of modified bases, and this lead to the development of mutant polymerases which were capable of amplifying these unnatural aptamers. Endosomal escape of cell-internalizing aptamers is a very important issue that needs to be addressed for clinical translation. Conjugation of aptamer with endosome escaping peptides will facilitate this issue. But lysosomal localization of aptamers is not always a problem, since lysosomal delivery of proteins or drugs is an important aspect of treating lysosomal storage diseases.

Efforts to up the speed of aptamer selection for industrializing this technology were achieved by development of high-throughput SELEX, and CE-SELEX which brought down the duration of selection process from months or weeks to just a few days. Additionally, the development of next-generation sequencing platform has been a boon for aptamer selection since it allowed assay of millions of aptamer sequences at one go, and alleviated the need to perform several rounds of selection that was earlier a norm. It significantly reduced the amount of time and resources that went in the selection of aptamers. From the point of selectionologists, it is quite important to characterize aptamers in the best possible way, i.e., keep right controls and perform vigorous validation experiments to make sure that aptamers coming out of a lab are robust, and work in hands of different people. Development of working reliable aptamers will pave their way for targeted delivery of therapeutics.

Macugen was the first FDA approved aptamer drug that appeared in market for clinical use in 2005. No other aptamer drug has entered the market since then. Several aptamers underwent clinical trials in the last 2 decades as listed in earlier reviews [5, 97, 98]. However, many of them have failed [5, 98]. In many cases, the failure in clinical trials is not due to lack of aptamer specificity or affinity to the target, but because they were unable to provide better treatment over the existing state-of-the-art treatment regimens. However, it is a common phenomenon that more drugs fail during clinical trails than pass it. In the past 2 decades the advances described in this chapter have facilitated in the development of stabilized aptamers that have entered clinical trials. In future, application of aptamers in targeted delivery and therapeutics is only poised to increase due to the establishment of ancillary technologies that facilitate faster and more stable aptamer development. Due to this, a number of aptamers are entering clinical trials. In summary, aptamermediated targeted delivery remains a work in progress; however, with bright results at *in vivo* level and several aptamer drugs in clinical trials, the coming decade will determine their efficiency at the clinical level.
