Using aptamers to visualize and capture cancer cells

DNA-Based Nanomaterials as Drug Delivery Platforms for Increasing the Effect of Drugs in Tumors

DNA nanotechnology has significantly advanced and might be used in biomedical applications, drug delivery, and cancer treatment during the past few decades. DNA nanomaterials are widely used in biomedical research involving biosensing, bioimaging, and drug delivery since they are remarkably addressable and biocompatible. Gradually, modified nucleic acids have begun to be employed to construct multifunctional DNA nanostructures with a variety of architectural designs. Aptamers are single-stranded nucleic acids (both DNAs and RNAs) capable of self-pairing to acquire secondary structure and of specifically binding with the target. Diagnosis and tumor therapy are prospective fields in which aptamers can be applied. Many DNA nanomaterials with three-dimensional structures have been studied as drug delivery systems for different anticancer medications or gene therapy agents. Different chemical alterations can be employed to construct a wide range of modified DNA nanostructures. Chemically altered DNA-based nanomaterials are useful for drug delivery because of their improved stability and inclusion of functional groups. In this work, the most common oligonucleotide nanomaterials were reviewed as modern drug delivery systems in tumor cells.

DNA aptamers targeting Leishmania infantum H3 protein as potential diagnostic tools

Leishmaniasis is a disease caused by a parasite of the genus Leishmania that affects millions of people worldwide. These parasites are characterized by the presence of a DNA-containing granule, the kinetoplastid, located in the single mitochondrion at the base of the cell's flagellum. Interestingly, these flagellates do not condense chromatin during mitosis, possibly due to the specific molecular features of their histones. Although histones are extremely conserved proteins, kinetoplastid core histone sequences diverge significantly from those of higher eukaryotes. This divergence makes kinetoplastid core histones potential diagnostic and/or therapeutic targets. Aptamers are short single-stranded nucleic acids that are able to recognize target molecules with high affinity and specificity. Their binding capacity is a consequence of the particular three-dimensional structure acquired depending on their sequence. These molecules are currently used for detection, diagnosis and therapeutic purpose. Starting from a previously obtained ssDNA aptamer population against rLiH3 protein we have isolated two individual aptamers, AptLiH3#4 and AptLiH3#10. Next, we have performed ELONA, Western blot and slot blot assays to establish aptamer specificity and affinity for LiH3 histone. In addition, ELONA assays using peptides corresponding to overlapped sequences of LiH3 were made to map the aptamers:LiH3 interaction. Finally, different assays using aptamers were performed in order to evaluate the possibility of using these aptamers as sensing molecule to recognize the endogenous protein LiH3. Our results indicate that both aptamers have high affinity and specificity for the target and are able to detect the endogenous LiH3 histone protein in promastigotes lysates. In silico analysis reveals that these two aptamers have different potential secondary structure among them, however, both of them are able to recognize the same peptide sequences present in the protein. In conclusion, our findings indicate that these aptamers could be used for LiH3 histone detection and, in consequence, as potential biosensing molecules in a diagnostic tool for leishmaniasis.

Competition-Enhanced Ligand Selection to Identify DNA Aptamers

Competition-enhanced ligand screening (CompELS) was employed to rapidly screen through large DNA libraries to identify single-stranded, oligonucleotide-based ligands called aptamers that bind to a nonbiological target. This previously unreported aptamer screening approach involves the repeated introduction of unenriched random sequence populations during the biopanning process, but avoids iterative elution and polymerase chain reaction (PCR) amplification steps inherent to traditional SELEX (systematic evolution of ligands by exponential enrichment) screening. In this study, 25 aptamers were identified against a gold surface via CompELS and evaluated to identify patterns in primary structures and predicted secondary structures. Following a final one-round competition experiment with the 25 identified aptamers, one particular aptamer sequence (1N) emerged as the most competitive adsorbate species for the gold substrate. Binding analysis indicated at least an order of magnitude difference in the binding affinity of 1N ( K_d = 5.6 × 10^-10 M) compared to five other high affinity aptamer candidates ( K_d = 10^-8-10^-9 M) from identical secondary structure families. Collectively, these studies introduce a rapid, reliable screening and ranking platform along with a classification scheme well-suited for identifying and characterizing aptamers for nonbiological as well as biological targets.

Induction of potent apoptosis by an anti-CD20 aptamer via the crosslink of membrane CD20 on non-Hodgkin's lymphoma cells

An anti-CD20 DNA aptamer was successfully generated by cell-SELEX, the crosslink of which can induce potent apoptosis in target cells.

Ultrasensitive electrochemical aptasensor based on sandwich architecture for selective label-free detection of colorectal cancer (CT26) cells

Colorectal cancer is one of the most common cancers in the world and has no effective treatment. Therefore, development of new methods for early diagnosis is instantly required. Biological recognition probes such as synthetic receptor and aptamer is one of the candidate recognition layers to detect important biomolecules. In this work, an electrochemical aptasensor was developed by fabricating an aptamer-cell-aptamer sandwich architecture on an SBA-15-3-aminopropyltriethoxysilane (SBA-15-pr-NH₂) and Au nanoparticles (AuNPs) modified graphite screen printed electrode (GSPE) surface for the selective, label-free detection of CT26 cancer cells. Based on the incubation of the thiolated aptamer with CT26 cells, the electron-transfer resistance of Fe (CN)₆^3-/4- redox couple increased considerably on the aptasensor surface. The results obtained from cyclic voltammetry and electrochemical impedance spectroscopy studies showed that the fabricated aptasensor can specifically identify CT26 cells in the concentration ranges of 10-1.0×10⁵cells/mL and 1.0×10⁵-6.0×10⁶ cells/mL, respectively, with a detection limit of 2cells/mL. Applying the thiol terminated aptamer (5TR1) as a recognition layer led to a sensor with high affinity for CT26 cancer cells, compared to control cancer cells of AGS cells, VERO Cells, PC3 cells and SKOV-3 cells. Therefore a simple, rapid, label free, inexpensive, excellent, sensitive and selective electrochemical aptasensor based on sandwich architecture was developed for detection of CT26 Cells.

Generating Cell Targeting Aptamers for Nanotheranostics Using Cell-SELEX

Detecting and understanding changes in cell conditions on the molecular level is of great importance for the accurate diagnosis and timely therapy of diseases. Cell-based SELEX (Systematic Evolution of Ligands by EXponential enrichment), a foundational technology used to generate highly-specific, cell-targeting aptamers, has been increasingly employed in studies of molecular medicine, including biomarker discovery and early diagnosis/targeting therapy of cancer. In this review, we begin with a mechanical description of the cell-SELEX process, covering aptamer selection, identification and identification, and aptamer characterization; following this introduction is a comprehensive discussion of the potential for aptamers as targeting moieties in the construction of various nanotheranostics. Challenges and prospects for cell-SELEX and aptamer-based nanotheranostic are also discussed.

Applications of aptamers in targeted imaging: state of the art

Aptamers are single-stranded oligonucleotides with high affinity and specificity to the target molecules or cells, thus they can serve as an important category of molecular targeting ligand. Since their discovery, aptamers have been rapidly translated into clinical practice. The strong target affinity/selectivity, cost-effectivity, chemical versatility and safety of aptamers are superior to traditional peptides- or proteins-based ligands which make them unique choices for molecular imaging. Therefore, aptamers are considered to be extremely useful to guide various imaging contrast agents to the target tissues or cells for optical, magnetic resonance, nuclear, computed tomography, ultrasound and multimodality imaging. This review aims to provide an overview of aptamers' advantages as targeting ligands and their application in targeted imaging. Further research in synthesis of new types of aptamers and their conjugation with new categories of contrast agents is required to develop clinically translatable aptamer-based imaging agents which will eventually result in improved patient care.

Multimodal imaging probe for targeting cancer cells using uMUC-1 aptamer

For adequate cancer therapy, newer imaging modalities with more specific ligands for unique targets are crucial. Underglycosylated mucin-1 (uMUC-1) antigen is an early marker of tumor development and is widely overexpressed on most tumors. A combination of nanotechnology with optical, radionuclide, and magnetic resonance (MR) imaging has great potential to improve cancer diagnosis and therapy. In this study, a multimodal nanoparticle imaging system was developed that can be used for optical, MR and positron emission tomography (PET) imaging. Cobalt ferrite magnetic nanoparticles surrounded by fluorescent rhodamine (designated MF) within a silica shell matrix were conjugated with an aptamer targeting uMUC-1 (designated MF-uMUC-1) and further labeled by (68)Ga (designated MFR-uMUC-1) with the help of a p-SCN-bn-NOTA chelating agent, resulting in single multimodal nanoparticles. The resultant nanoparticles are spherical and monodispersed, as revealed by transmission electron microscopy. The MFR-uMUC-1 nanoparticle showed specific and dose-dependent fluorescent, radioisotope and MR signals targeting BT-20 cells expressing uMUC-1. In vivo targeting and multimodal imaging in tumor-bearing nude mice also showed great specificity for targeting cancers with MFR-uMUC-1. The MFR-uMUC-1 probe could be used as a single multimodal probe to visualize cancer cells by means of optical, radionuclide and MR imaging.

Cell-SELEX-based aptamer-conjugated nanomaterials for cancer diagnosis and therapy

Nucleic acid aptamers, which are generated by a novel technique called SELEX (systematic evolution of ligands by exponential enrichment), have recently attracted significant attention in the field of early detection and treatment of cancer based on their numerous merits, such as high affinity, high specificity, small size, little immunogenicity, stable structures, and ease of chemical modification. Furthermore, aptamers can gain more flexibility as cancer cell targeting tools when conjugated to nanomaterials, including metallic nanoparticles, carbon nanomaterials, DNA nanodevices, and polymeric nanoparticles. We discuss the progress achieved in cancer diagnosis and therapy through the conjugation of cell-SELEX-based aptamers with different nanomaterials.

Detection and characterization of cancer cells and pathogenic bacteria using aptamer-based nano-conjugates

Detection and characterization of cells using aptamers and aptamer-conjugated nanoprobes has evolved a great deal over the past few decades. This evolution has been driven by the easy selection of aptamers via in vitro cell-SELEX, permitting sensitive discrimination between target and normal cells, which includes pathogenic prokaryotic and cancerous eukaryotic cells. Additionally, when the aptamer-based strategies are used in conjunction with nanomaterials, there is the potential for cell targeting and therapeutic effects with improved specificity and sensitivity. Here we review recent advances in aptamer-based nano-conjugates and their applications for detecting cancer cells and pathogenic bacteria. The multidisciplinary research utilized in this field will play an increasingly significant role in clinical medicine and drug discovery.

Identification of RNA aptamers that internalize into HPV-16 E6/E7 transformed tonsillar epithelial cells

Human papillomavirus type 16 (HPV-16) associated oropharyngeal cancers are on a significant increase and better therapeutic strategies are needed. The HPV-16 oncogenes E6 and E7 are expressed in HPV-associated cancers and are able to transform human tonsillar epithelial cells (HTECs). We used cell-Systematic Evolution of Ligands by Exponential Enrichment (SELEX) to select for RNA aptamers that entered into HPV-16 E6/E7-HTECs. After 12 rounds of cell-SELEX, a pool of aptamers was obtained that had significantly greater internalization capacity (~5-fold) into E6/E7-HTECs as compared to primary HTECs or fibroblasts. Analysis of individual aptamers from the pool indicated variable internalization into E6/E7-HTECs (1-8-fold as compared to a negative control). Most of the individual aptamers internalized into E6/E7 and primary HTECs with similar efficiency, while one aptamer exhibited ~3-fold better internalization into E6/E7-HTECs. Aptamers that internalize into cells may be useful for delivering therapeutic agents to HPV-16 associated malignancies.

Aptamer--nanoparticle-based chemiluminescence for p53 protein

A simple colorimetric biosensing technique based on the interaction of gold nanoparticles (AuNPs) with the aptamer was developed for detection of p53, a tumor suppressor protein, in the current study. Aggregation of AuNPs was induced by desorption of the p53 binding RNA aptamer from the surface of AuNPs as a result of the aptamer target interaction leading to the color change of AuNPs from red to purple. The detection limit of p53 protein by the colorimetric approach was 0.1 ng/ml after successful optimization of the amount of aptamer, AuNPs, salts, and incubation time. Furthermore, the catalytic activity of the aggregated AuNPs was greatly enhanced by chemiluminescence (CL) reaction, where the detection limit was enhanced to 10 pg/ml with a regression coefficient of R2 = 0.9907. Here the sensitivity was increased by 10-fold compared with the AuNP-based colorimetric method. Hence, the sensitivity of detection was increased by employing CL, by using the catalytic activity of aggregated AuNPs, on the luminol-hydrogen peroxide reaction. Thus, the combination of colorimetric and CL-based aptasensor can be of great advantage in increasing the sensitivity of detection for any target analyte.

Cancerous ovarian stem cells: obscure targets for therapy but relevant to chemoresistance

Chemotherapy with platinum and taxanes is the first line of treatment for all epithelial ovarian cancer (EOC) patients after debulking surgery. Even though the treatment is initially effective in 80% of patients, recurrent cancer is inevitable in the vast majority of cases. Emerging evidence suggests that some tumor cells can survive chemotherapy by activating the self-renewal pathways resulting in tumor progression and clinical recurrence. These defined population of cells commonly termed as "cancer stem cells" (CSC) may generate the bulk of the tumor by using differentiating pathways. These cells have been shown to be resistant to chemotherapy and, to have enhanced tumor initiating abilities, suggesting CSCs as potential targets for treatment. Recent studies have introduced a new paradigm in ovarian carcinogenesis which proposes in situ carcinoma at the fimbrial end of the fallopian tube to generate high-grade serous ovarian carcinomas, in contrast to ovarian cortical inclusion cysts (CIC) which produce borderline and low grade serous, mucinous, endometrioid, and clear cell carcinomas. This review summarizes recent advances in our understanding of the cellular origin of EOC and the molecular mechanisms defining the basis of CSC in EOC progression and chemoresistance. Using a model ovarian cancer cell line, we highlight the role of CSC in response to chemotherapy, and relate how CSCs may impact on chemoresistance and ultimately recurrence. We also propose the molecular targeting of CSCs and suggest ways that may improve the efficacy of current chemotherapeutic regimens needed for the management of this disease.

DNA aptamers selectively target Leishmania infantum H2A protein

Parasites of the genus Leishmania produce leishmaniasis which affects millions people around the world. Understanding the molecular characteristics of the parasite can increase the knowledge about the mechanisms underlying disease development and progression. Thus, the study of the molecular features of histones has been considered of particular interest because Leishmania does not condense the chromatin during mitosis and, consequently, a different role for these proteins in the biology of the parasite can be expected. Furthermore, the sequence divergences in the amino and in the carboxy-terminal domains of the kinetoplastid core histones convert them in potential diagnostic and/or therapeutics targets. Aptamers are oligonucleotide ligands that are selected in vitro by their affinity and specificity for the target as a consequence of the particular tertiary structure that they are able to acquire depending on their sequence. Development of high-affinity molecules with the ability to recognize specifically Leishmania histones is essential for the progress of this kind of study. Two aptamers which specifically recognize Leishmania infantum H2A histone were cloned from a previously obtained ssDNA enriched population. These aptamers were sequenced and subjected to an in silico analysis. ELONA, slot blot and Western blot were performed to establish aptamer affinity and specificity for LiH2A histone and ELONA assays using peptides corresponding to overlapped sequences of LiH2A were made mapping the aptamers:LiH2A interaction. As "proofs of concept", aptamers were used to determine the number of parasites in an ELONA platform and to purify LiH2A from complex mixtures. The aptamers showed different secondary structures among them; however, both of them were able to recognize the same peptides located in a side of the protein. In addition, we demonstrate that these aptamers are useful for LiH2A identification and also may be of potential application as diagnostic system and as a laboratory tool with purification purpose.

Improving the diagnosis and management of neuroendocrine tumors: utilizing new advances in biomarker and molecular imaging science

Neuroendocrine tumors (NET) are malignant solid tumors that arise in hormone-secreting tissue of the diffuse neuroendocrine system or endocrine glands. Although traditionally understood to be a rare disease, the incidence and prevalence of NET have increased greatly in the past 3 decades. However, during this time, progress in diagnosis and outcome of NET has generally been modest. In order to achieve improved outcome in NET, a better understanding of NET biology combined with more reliable serum markers and better techniques to identify tumor localization and small lesions are needed. Although some NET biomarkers exist, sensitive and specific markers that predict tumor growth and behavior are generally lacking. In addition, the integration of new molecular imaging technologies in patient diagnosis and follow-up has the potential to enhance care. To discuss developments and issues required to improve diagnostics and management of NET patients, with specific focus on the latest advances in molecular imaging and biomarker science, 17 global leaders in the fields of NET, molecular imaging and biomarker technology gathered to participate in a 2-day meeting hosted by Prof. Kjell Öberg at the University of Uppsala in Sweden. During this time, findings were presented regarding methods with potential prognostic and treatment applications in NET or other types of cancers. This paper describes the symposium presentations and resulting discussions.

Sensitive detection of human breast cancer cells based on aptamer-cell-aptamer sandwich architecture

Breast cancer is one of the most critical threats to the health of women, and the development of new methods for early diagnosis is urgently required, so this paper reports a method to detect Michigan cancer foundation-7 (MCF-7) human breast cancer cells with considerable sensitivity and selectivity by using electrochemical technique. In this method, a mucin 1 (MUC1)-binding aptamer is adopted to recognize MCF-7 human breast cancer cells, while enzyme labeling is employed to produce amplified catalytic signals. The molecular recognition and the signal amplification are elaborately integrated by fabricating an aptamer-cell-aptamer sandwich architecture on an electrode surface, thus a biosensor for the detection of MCF-7 is fabricated based on the architecture. The detection range can be from 100 to 1×10(7) cells, and the detection limit can be as low as 100 cells. The method is also cost-effective and conveniently operated, implying potential help for the development of early diagnosis of breast cancer.

Selection of DNA aptamers against glioblastoma cells with high affinity and specificity

Background

Glioblastoma is the most common and most lethal form of brain tumor in human. Unfortunately, there is still no effective therapy to this fatal disease and the median survival is generally less than one year from the time of diagnosis. Discovery of ligands that can bind specifically to this type of tumor cells will be of great significance to develop early molecular imaging, targeted delivery and guided surgery methods to battle this type of brain tumor.

Methodology/Principal Findings

We discovered two target-specific aptamers named GBM128 and GBM131 against cultured human glioblastoma cell line U118-MG after 30 rounds selection by a method called cell-based Systematic Evolution of Ligands by EXponential enrichment (cell-SELEX). These two aptamers have high affinity and specificity against target glioblastoma cells. They neither recognize normal astraglial cells, nor do they recognize other normal and cancer cell lines tested. Clinical tissues were also tested and the results showed that these two aptamers can bind to different clinical glioma tissues but not normal brain tissues. More importantly, binding affinity and selectivity of these two aptamers were retained in complicated biological environment.

Conclusion/Significance

The selected aptamers could be used to identify specific glioblastoma biomarkers. Methods of molecular imaging, targeted drug delivery, ligand guided surgery can be further developed based on these ligands for early detection, targeted therapy, and guided surgery of glioblastoma leading to effective treatment of glioblastoma.

Aptamer-containing surfaces for selective capture of CD4 expressing cells

Aptamers have recently emerged as an excellent alternative to antibodies because of their inherent stability and ease of modification. In this paper, we describe the development of an aptamer-based surface for capture of cells expressing CD4 antigen. The glass or silicon surfaces were modified with amine-terminated silanes and then modified with thiolated RNA aptamer against CD4. Modification of the surface was first characterized by ellipsometry to demonstrate assembly of biointerface components and to show specific capture of recombinant CD4 protein. Subsequently, surfaces were challenged with model lymphocytes (cell lines) that were either positive or negative for CD4 antigen. Our experiments show that aptamer-functionalized surfaces have similar capture efficiency to substrates containing anti-CD4 antibody. To mimick capture of specific T-cells from a complex cell mixture, aptamer-modified surfaces were exposed to binary mixtures containing Molt-3 cells (CD4+) spiked into Daudi B cells (CD4-). 94% purity of CD4 cells was observed on aptamer-containing surfaces from an initial fraction of 15% of CD4. Given the importance of CD4 cell enumeration in HIV/AIDS diagnosis and monitoring, aptamer-based devices may offer an opportunity for novel cell detection strategies and may yield more robust and less expensive blood analysis devices in the future.

Target binding improves relaxivity in aptamer-gadolinium conjugates

MRI contrast agents (CA) have been heavily used over the past several decades to enhance the diagnostic value of the obtained images. From a design perspective, two avenues to improve the efficacy of contrast agents are readily evident: optimization of magnetic properties of the CA, and optimization of the pharmacokinetics and distribution of the CA in the patient. Contrast agents consisting of DNA aptamer-gadolinium(III) conjugates provide a single system in which these factors can be addressed simultaneously. In this proof-of-concept study, the 15mer thrombin aptamer was conjugated to diethylenetriaminepentaacetic (DTPA) dianhydride to form a monoamide derivative of the linear open-chain chelate present in the commonly used contrast agent Magnevist(®). The stability of the conjugated DNA aptamer-DTPA-Gd(III) chelate in a transmetallation study using Zn(II) was found to be similar to that reported for DTPA-Gd(III). Relaxivity enhancements of 35 ± 4 and 20 ± 1 % were observed in the presence of thrombin compared to a control protein at fields of 9.4 and 1.5 T, respectively. The inclusion of spacers between the aptamer and the DTPA to eliminate possible steric effects was also investigated but not found to improve the relaxation enhancement achieved in comparison to the unaltered aptamer conjugate.

Polyvalent nucleic acid aptamers and modulation of their activity: a focus on the thrombin binding aptamer

Nucleic acid-based aptamers can be selected from combinatorial libraries of synthetic oligonucleotides to bind, with affinity and specificity similar to antibodies, a wide range of biomedically relevant targets. Compared to protein therapeutics, aptamers exhibit significant advantages in terms of size, non-immunogenicity and wide synthetic accessibility. Various chemical modifications have been introduced in the natural oligonucleotide backbone of aptamers in order to increase their half-life, as well as their pharmacological properties. Very effective alternative approaches, devised in order to improve both the aptamer activity and stability, were based on the design of polyvalent aptamers, able to establish multivalent interactions with the target: thus, multiple copies of an aptamer can be assembled on the same molecular- or nanomaterial-based scaffold. In the present review, the thrombin binding aptamers (TBAs) are analyzed as a model system to study multiple-aptamer constructs aimed at improving their anticoagulation activity in terms of binding to the target and stability to enzymatic degradation. Indeed - even if the large number of chemically modified TBAs investigated in the last 20 years has led to encouraging results - a significant progress has been obtained only recently with bivalent or engineered dendritic TBA aptamers, or assemblies of TBAs on nanoparticles and DNA nanostructures. Furthermore, the modulation of the aptamers activity by means of tailored drug-active reversal agents, especially in the field of anticoagulant aptamers, as well as the reversibility of the TBA activity through the use of antidotes, such as porphyrins, complementary oligonucleotides or of external stimuli, are discussed.

Recent developments in protein and cell-targeted aptamer selection and applications

Because of their easily modified chemical structures and wide range of targets, aptamers are ideal candidates for various applications, such as biomarker discovery, target diagnosis, molecular imaging, and drug delivery. Aptamers are oligonucleotide sequences that can bind to their targets specifically via unique three dimensional (3-D) structures. Usually, aptamers are obtained from repeated rounds of in vitro or in vivo selection termed SELEX (Systematic Evolution of Ligands by EXponential enrichment), which can generate aptamers with high affinity and specificity for many kinds of targets, such as biomedically important proteins and even cancer cells. In this review, some basic principles and recent developments in the design of SELEX process are discussed, hopefully to provide some guidelines towards performing more efficient aptamer isolation procedures. Moreover, the biomedical and bioanalytical applications of aptamers are further reviewed, based on some smart biochemical modifications of these oligonucleotide structures.

RNA aptamer against a cancer stem cell marker epithelial cell adhesion molecule

The lack of a specific targeting strategy against cancer stem cells in current cancer treatment regimens is at least partly responsible for life-threatening cytotoxicity for patients undergoing traditional chemotherapy. An effective cancer stem cell targeting system is urgently required for the next generation of cancer medicine. Epithelial cell adhesion molecule (EpCAM) is overexpressed in most solid cancers and it has recently been identified as a cancer stem cell marker. In this study, we isolated a 40-base RNA aptamer that binds to EpCAM from a random oligonucleotide library using systematic evolution of ligands by exponential enrichment. The aptamer was further truncated to 19 bases. This 19-nt RNA aptamer interacts specifically with a number of live human cancer cells derived from breast, colorectal, and gastric cancers that express EpCAM, but not with those not expressing EpCAM, as analyzed using flow cytometry and confocal microscopy. The binding affinity of the EpCAM RNA aptamer to human cancer cells is approximately 55 nM. Importantly, this EpCAM RNA aptamer is efficiently internalized after binding to cell surface EpCAM. To our knowledge, this is the first RNA aptamer against a cancer stem cell surface marker being developed. Such cancer stem cell aptamers will greatly facilitate the development of novel targeted nanomedicine and molecular imaging agents for cancer theranostics.