Derivatizations of Sgc8-c aptamer to prepare metallic radiopharmaceuticals as imaging diagnostic agents: Syntheses, isolations, and physicochemical characterizations

Targeted-Lymphoma Drug Delivery System Based on the Sgc8-c Aptamer

Aptamers are emerging as a promising new class of functional nucleic acids because they can specifically bind to any target with high affinity and be easily modified chemically with different pharmacophoric subunits for therapy. The truncated aptamer, Sgc8-c, binds to tyrosine-protein kinase-like 7 receptor, a promising cancer therapeutic target, allowing the recognition of haemato-oncological malignancies, among others. We have previously developed aptamer-drug conjugates by chemical synthesis, hybridizing Sgc8-c and dasatinib, a drug proposed for lymphoma chemotherapy. One of the best-characterised Sgc8-c-dasatinib hybrids, namely Sgc8-c-carb-da, was capable of releasing dasatinib at an endosomal-pH. Herein, we probed the therapeutic potential of this aptamer-drug conjugate. Sgc8-c-carb-da specifically inhibited murine A20 B lymphocyte growth and produced cell death, mainly by late apoptosis and necrosis. In addition, Sgc8-c-carb-da generated an arrest in cell proliferation, with a cell cycle arrest in the Sub-G1-peak. The mitochondrial potential was altered accordingly to these pathways. Moreover, using an in vitro cell-targeting assay that mimics in vivo conditions, we showed that Sgc8-c-carb-da displayed higher (2.5-fold) cytotoxic effects than dasatinib. These findings provide proof-of-concept of the therapeutic value of Sgc8-c-carb-da for lymphoma, creating new opportunities for the chemical synthesis of targeted biotherapeutics.

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

In Vivo Evaluation of Sgc8-c Aptamer as a Molecular Imaging Probe for Colon Cancer in a Mouse Xenograft Model

Recent biotechnological applications in the field of clinical oncology led to the identification of new biomarkers as molecular targets of cancer, and to broad developments in the field of personalized medicine. Aptamers are oligonucleotides (ssDNA or RNA) that are selected to specifically recognize a molecular target with high affinity and specificity. Based on this, new horizons for their use as molecular imaging probes are being explored. The objective of this work was to evaluate the Sgc8-c aptamer conjugated with Alexa Fluor 647 fluorophore as an imaging probe in a colon tumor xenograft mouse model, with potential application in molecular imaging. In this study, the LS174T cell line was used to induce colorectal adenocarcinoma in nude mice. After confirmation of PTK7 overexpression by immunohistochemistry, in vivo studies were performed. Pharmacokinetic, in vivo and ex vivo biodistribution imaging, and a competition assay were evaluated by fluorescence imaging. In vivo visualization of the probe in the tumors was assessed two hours after aptamer probe administration, exhibiting excellent tumor-to-background ratios in biodistribution studies and high specificity in the competition test. Our results demonstrated the functionality of Scg8-c as an imaging probe for colon cancer, with potential clinical applications.

T908 Polymeric Micelles Improved the Uptake of Sgc8-c Aptamer Probe in Tumor-Bearing Mice: A Co-Association Study between the Probe and Preformed Nanostructures

Aptamers are oligonucleotides that have the characteristic of recognizing a target with high affinity and specificity. Based on our previous studies, the aptamer probe Sgc8-c-Alexa647 is a promising tool for molecular imaging of PTK7, which is an interesting biomarker in cancer. In order to improve the delivery of this probe as well as create a novel drug delivery nanosystem targeted to the PTK7 receptor, we evaluate the co-association between the probe and preformed nanostructures. In this work, preformed pegylated liposomes (PPL) and linear and branched pristine polymeric micelles (PMs), based on PEO–PPO–PEO triblock copolymers were used: poloxamer F127^® and poloxamines T1307^® and T908^®. For it, Sgc8-c-Alexa647 and its co-association with the different nanostructures was exhaustively analyzed. DLS analysis showed nanometric sizes, and TEM and AFM showed notable differences between free- and co-associated probe. Likewise, all nanosystems were evaluated on A20 lymphoma cell line overexpressing PTK7, and the confocal microscopy images showed distinctness in cellular uptake. Finally, the biodistribution in BALB/c mice bearing lymphoma-tumor and pharmacokinetic study revealed an encouraging profile for T908-probe. All data obtained from this work suggested that PMs and, more specifically T908 ones, are good candidates to improve the pharmacokinetics and the tumor uptake of aptamer-based probes.

Metastatic and non-metastatic melanoma imaging using Sgc8-c aptamer PTK7-recognizer

Melanoma is one of the most aggressive and deadly skin cancers, and although histopathological criteria are used for its prognosis, biomarkers are necessary to identify the different evolution stages. The applications of molecular imaging include the in vivo diagnosis of cancer with probes that recognize the tumor-biomarkers specific expression allowing external image acquisitions and evaluation of the biological process in quali-quantitative ways. Aptamers are oligonucleotides that recognize targets with high affinity and specificity presenting advantages that make them interesting molecular imaging probes. Sgc8-c (DNA-aptamer) selectively recognizes PTK7-receptor overexpressed in various types of tumors. Herein, Sgc8-c was evaluated, for the first time, in a metastatic melanoma model as molecular imaging probe for in vivo diagnostic, as well as in a non-metastatic melanoma model. Firstly, two probes, radio- and fluorescent-probe, were in vitro evaluated verifying the high specific PTK7 recognition and its internalization in tumor cells by the endosomal route. Secondly, in vivo proof of concept was performed in animal tumor models. In addition, they have rapid clearance from blood exhibiting excellent target (tumor)/non-target organ ratios. Furthermore, optimal biodistribution was observed 24 h after probes injections accumulating almost exclusively in the tumor tissue. Sgc8-c is a potential tool for their specific use in the early detection of melanoma.

Sgc8-c Aptamer as a Potential Theranostic Agent for Hemato-Oncological Malignancies

Background: Aptamers represent an emerging class of oligonucleotides that have the ability to bind ligands with high affinity. Sgc8-c aptamer recognizes PTK7, a member of the catalytically defective receptor protein tyrosine kinase family that is upregulated in various cancers, including hemato-oncological malignancies. Herein, an Sgc8-c-NOTA-radiolabeled probe was prepared for theranostic purpose. Materials and Methods: In this work, an Sgc8-c-radiolabeled probe against PTK7 was prepared, and biological evaluations-pharmacokinetic studies, biodistribution analysis, and in vivo molecular imaging-were performed. To obtain the radiolabeled probe, a modified 5'-amino-derivative of the Sgc8-c aptamer was bound to the metal chelator NOTA, and subsequently labeled with ⁶⁷Ga with high yield and radiochemical purity. The precursor, Sgc8-c-NOTA, the radio probe Sgc8-c-NOTA-⁶⁷Ga, and its nonradioactive complex, Sgc8-c-NOTA-^69/71Ga, were purified by reverse-phase high-performance liquid chromatography and characterized by electrospray ionization mass spectrometry. The binding ability of Sgc8-c-NOTA-⁶⁷Ga was studied in vitro against purified PTK7 receptor. In addition, the binding was also evidenced against the hemato-oncological A20 cell line, derived from B lymphocytes, and the corresponding A20-green fluorescent protein (GFP)-transfected cells. The proof of concept was performed on A20-GFP tumor-bearing mice, in which the biodistribution of the radiolabeled probe was evaluated through imaging, using X-ray, fluorescence, and γ modalities. The specific uptake of the probe was confirmed by blocking with the Sgc8-c aptamer in an in vivo competition assay. Results: The biodistribution results showed considerable uptake in tumor since 2 h, with highest at 48 h postinjection. However, the blood and muscle ID/g (injected dose per gram of tissue) activities were decreasing with time and tumor/no-target ratios increasing to 20 at 24 h postinjection. These results are consistent with the in vivo images. Conclusions: This study supports the utility of Sgc8-c-NOTA radiolabeled as a theranostic agent.

Aptamers in Diagnostic and Molecular Imaging Applications

The origin of the term diagnostic comes from the Greek word gnosis, meaning "to know." In medicine, a diagnostic can predict the pathology risk, disease status, treatment, and prognosis, even following therapy. An early and correct diagnosis is necessary for an efficient treatment. Moreover, it is possible to predict if and why a therapy will be successful or fail, enabling the timely application of alternative therapeutic strategies. Available diagnostics are due to the advances in biotechnology; however, more sensitive, low-cost, and noninvasive methodologies are still a challenge. Knowledge about molecular characteristics provide personalized information, which is the goal of future medicine. Today, multiple diagnostic techniques have emerged, with which it is possible to distinguish molecular patterns.In this way, aptamers are the perfect tools to recognize molecular targets and can be easily modified to confer additional functions. Their versatile characteristics and low cost make aptamers ideal for diagnostic applications.This chapter is a review of aptamer-based diagnostics in biomedicine, with a special focus on probe design and molecular imaging. Graphical Abstract.

Radiolabeling of Nucleic Acid Aptamers for Highly Sensitive Disease-Specific Molecular Imaging

Aptamers are short single-stranded DNA or RNA oligonucleotide ligand molecules with a unique three-dimensional shape, capable of binding to a defined molecular target with high affinity and specificity. Since their discovery, aptamers have been developed for various applications, including molecular imaging, particularly nuclear imaging that holds the highest potential for the clinical translation of aptamer-based molecular imaging probes. Their easy laboratory production without any batch-to-batch variations, their high stability, their small size with no immunogenicity and toxicity, and their flexibility to incorporate various functionalities without compromising the target binding affinity and specificity make aptamers an attractive class of targeted-imaging agents. Aptamer technology has been utilized in nuclear medicine imaging techniques, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), as highly sensitive and accurate biomedical imaging modalities towards clinical diagnostic applications. However, for aptamer-targeted PET and SPECT imaging, conjugation of appropriate radionuclides to aptamers is crucial. This review summarizes various strategies to link the radionuclides to chemically modified aptamers to accomplish aptamer-targeted PET and SPECT imaging.