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      Toward the Selection of Cell Targeting Aptamers with Extended Biological Functionalities to Facilitate Endosomal Escape of Cargoes

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          Over the past decades there have been exciting and rapid developments of highly specific molecules to bind cancer antigens that are overexpressed on the surfaces of malignant cells. Nanomedicine aims to exploit these ligands to generate nanoscale platforms for targeted cancer therapy, and to do so with negligible off-target effects. Aptamers are structured nucleic acids that bind to defined molecular targets ranging from small molecules and proteins to whole cells or viruses. They are selected through an iterative process of amplification and enrichment called SELEX (systematic evolution of ligands by exponential enrichment), in which a combinatorial oligonucleotide library is exposed to the target of interest for several repetitive rounds. Nucleic acid ligands able to bind and internalize into malignant cells have been extensively used as tools for targeted delivery of therapeutic payloads both in vitro and in vivo. However, current cell targeting aptamer platforms suffer from limitations that have slowed their translation to the clinic. This is especially true for applications in which the cargo must reach the cytosol to exert its biological activity, as only a small percentage of the endocytosed cargo is typically able to translocate into the cytosol. Innovative technologies and selection strategies are required to enhance cytoplasmic delivery. In this review, we describe current selection methods used to generate aptamers that target cancer cells, and we highlight some of the factors that affect productive endosomal escape of cargoes. We also give an overview of the most promising strategies utilized to improve and monitor endosomal escape of therapeutic cargoes. The methods we highlight exploit tools and technologies that can potentially be incorporated in the SELEX process. Innovative selection protocols may identify aptamers with extended biological functionalities that allow effective cytosolic translocation of therapeutics. This in turn may facilitate successful translation of these platforms into clinical applications.

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          Most cited references 63

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          Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase.

          High-affinity nucleic acid ligands for a protein were isolated by a procedure that depends on alternate cycles of ligand selection from pools of variant sequences and amplification of the bound species. Multiple rounds exponentially enrich the population for the highest affinity species that can be clonally isolated and characterized. In particular one eight-base region of an RNA that interacts with the T4 DNA polymerase was chosen and randomized. Two different sequences were selected by this procedure from the calculated pool of 65,536 species. One is the wild-type sequence found in the bacteriophage mRNA; one is varied from wild type at four positions. The binding constants of these two RNA's to T4 DNA polymerase are equivalent. These protocols with minimal modification can yield high-affinity ligands for any protein that binds nucleic acids as part of its function; high-affinity ligands could conceivably be developed for any target molecule.
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            The past 20 years have seen enormous progress in the understanding of the mechanisms used by the enteric bacterium Escherichia coli to promote protein folding, support protein translocation and handle protein misfolding. Insights from these studies have been exploited to tackle the problems of inclusion body formation, proteolytic degradation and disulfide bond generation that have long impeded the production of complex heterologous proteins in a properly folded and biologically active form. The application of this information to industrial processes, together with emerging strategies for creating designer folding modulators and performing glycosylation all but guarantee that E. coli will remain an important host for the production of both commodity and high value added proteins.
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              Ligand-targeted therapeutics in anticancer therapy.

               T Allen (2002)
              Cytotoxic chemotherapy or radiotherapy of cancer is limited by serious, sometimes life-threatening, side effects that arise from toxicities to sensitive normal cells because the therapies are not selective for malignant cells. So how can selectivity be improved? One strategy is to couple the therapeutics to antibodies or other ligands that recognize tumour-associated antigens. This increases the exposure of the malignant cells, and reduces the exposure of normal cells, to the ligand-targeted therapeutics.

                Author and article information

                24 August 2017
                September 2017
                : 5
                : 3
                [1 ]Department of Biochemistry, University of Missouri, Columbia, MO 65211, USA; kdtkc3@
                [2 ]Bond Life Sciences Center, University of Missouri, Columbia, MO 65211, USA
                [3 ]Department of Molecular Microbiology & Immunology, University of Missouri, Columbia, MO 65212, USA
                [4 ]Department of Bioengineering, University of Missouri, Columbia, MO 65211, USA
                Author notes
                [* ]Correspondence: porcianid@ (D.P.); burkedh@ (D.H.B.); Tel.: +1-573-882-8989 (D.P. & D.H.B.); Fax: +1-573-884-3087 (D.P. & D.H.B.)

                These authors contributed equally to this work.

                © 2017 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (



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