Peptide aptamers: recent developments for cancer therapy

Targeted Delivery Methods for Anticancer Drugs

Simple Summary

The current main technological strategies for the delivery of anticancer drugs are discussed herein. This comprehensive review may help researchers design suitable delivery systems.

Abstract

Several drug-delivery systems have been reported on and often successfully applied in cancer therapy. Cell-targeted delivery can reduce the overall toxicity of cytotoxic drugs and increase their effectiveness and selectivity. Besides traditional liposomal and micellar formulations, various nanocarrier systems have recently become the focus of developmental interest. This review discusses the preparation and targeting techniques as well as the properties of several liposome-, micelle-, solid-lipid nanoparticle-, dendrimer-, gold-, and magnetic-nanoparticle-based delivery systems. Approaches for targeted drug delivery and systems for drug release under a range of stimuli are also discussed.

Report on biopharmaceutical profile of recent biotherapeutics and insilco docking studies on target bindings of known aptamer biotherapeutics

Accumulated Toxicity, disease recurrence and drug resistivity problems have been observed due to the synthetic and semisynthetic therapeutic practices, which alternatively led to focus on Bio-therapeutics production than xenobiotics. Quick plasma clearance and high potency are the reasons for trending research with huge pharma market of numerous Bio-therapeutics than ever before. Researchers proved that most of the nano and micro Bio-therapeutics have multiple beneficial therapeutic effects. We have analyzed the past, and present scenario of some notable clinically approved Bio-therapeutics to identify the future formulation needs with advanced techniques. Protein-related drugs are the foremost Bio-therapeutics such as antibodies, enzymes, and short, fragmented polypeptides show aggregation properties during storage, naked peptide moieties are resisted by the polar cell membrane, and also the antidrug antibodies were reported. Even though Nucleic acid nano-bodies are excellent target binders than proteins, they had only a few minutes of half-life. Maintaining homogeneousness upon storage of Bio-therapeutics is still a significant challenge in industrial-scale formulation. Notably, plant systems are identified as most useful cost-effective hosts to produce human enzymes than animal systems without any possible viral loads. Irrespective of numerous advancements in routes of administration and additives, subcutaneous is still a golden one to achieve better dynamics. Additionally, the interactions and effective bonds made by each class of well-known aptamer biotherapeutics which are considered as future drugs were studied.

Advancing Aptamers as Molecular Probes for Cancer Theranostic Applications-The Role of Molecular Dynamics Simulation

Theranostics cover emerging technologies for cell biomarking for disease diagnosis and targeted introduction of drug ingredients to specific malignant sites. Theranostics development has become a significant biomedical research endeavor for effective diagnosis and treatment of diseases, especially cancer. An efficient biomarking and targeted delivery strategy for theranostic applications requires effective molecular coupling of binding ligands with high affinities to specific receptors on the cancer cell surface. Bioaffinity offers a unique mechanism to bind specific target and receptor molecules from a range of non-targets. The binding efficacy depends on the specificity of the affinity ligand toward the target molecule even at low concentrations. Aptamers are fragments of genetic materials, peptides, or oligonucleotides which possess enhanced specificity in targeting desired cell surface receptor molecules. Aptamer-target binding results from several inter-molecular interactions including hydrogen bond formation, aromatic stacking of flat moieties, hydrophobic interaction, electrostatic, and van der Waals interactions. Advancements in Systematic Evolution of Ligands by Exponential Enrichment (SELEX) assay has created the opportunity to artificially generate aptamers that specifically bind to desired cancer and tumor surface receptors with high affinities. This article discusses the potential application of molecular dynamics (MD) simulation to advance aptamer-mediated receptor targeting in targeted cancer therapy. MD simulation offers real-time analysis of the molecular drivers of the aptamer-receptor binding and generate optimal receptor binding conditions for theranostic applications. The article also provides an overview of different cancer types with focus on receptor biomarking and targeted treatment approaches, conventional molecular probes, and aptamers that have been explored for cancer cells targeting.

Cancer therapeutic approach based on conformational stabilization of mutant p53 protein by small peptides

The p53 tumor suppressor serves as a major barrier against malignant transformation. Over 50% of tumors inactivate p53 by point mutations in its DNA binding domain. Most mutations destabilize p53 protein folding, causing its partial denaturation at physiological temperature. Thus a high proportion of human tumors overexpress a potential potent tumor suppressor in a non-functional, misfolded form. The equilibrium between the properly folded and misfolded states of p53 may be affected by molecules that interact with p53, stabilizing its native folding and restoring wild type p53 activity to cancer cells. To select for mutant p53 (mutp53) reactivating peptides, we adopted the phage display technology, allowing interactions between mutp53 and random peptide libraries presented on phages and enriching for phage that favor the correctly folded p53 conformation. We obtained a large database of potential reactivating peptides. Lead peptides were synthesized and analyzed for their ability to restore proper p53 folding and activity. Remarkably, many enriched peptides corresponded to known p53-binding proteins, including RAD9. Importantly, lead peptides elicited dramatic regression of aggressive tumors in mouse xenograft models. Such peptides might serve as novel agents for human cancer therapy.

Nucleic acid aptamer-mediated drug delivery for targeted cancer therapy

Aptamers are emerging as promising therapeutic agents and recognition elements. In particular, cell-SELEX (systematic evolution of ligands by exponential enrichment) allows in vitro selection of aptamers selective to whole cells without prior knowledge of the molecular signatures on the cell surface. The advantage of aptamers is their high affinitiy and binding specificity towards the target. This Minireview focuses on single-stranded (ss) oligonucleotide (DNA or RNA)-based aptamers as cancer therapeutics/theranostics. Specifically, aptamer-nanomaterial conjugates, aptamer-drug conjugates, targeted phototherapy and targeted biotherapy are covered in detail. In reviewing the literature, the potential of aptamers as delivery systems for therapeutic and imaging applications in cancer is clear, however, major challenges remain to be resolved, such as the poorly understood pharmacokinetics, toxicity and off-target effects, before they can be fully exploited in a clinical setting.

Sensitive and selective detection of thrombin by using a cyclic peptide as affinity ligand

Here we describe a sensitive assay for thrombin by using a high binding-affinity cyclic peptide against thrombin as affinity ligand. The cyclic peptide is immobilized on the magnetic beads or microplates to selectively capture thrombin. The enriched thrombin then catalyzes the cleavage of a substrate of thrombin to a detectable product. The detection of thrombin is finally achieved by measuring the generated product. This assay enables the detection of thrombin at tens fM in 100 µL of sample solution when fluorogenic substrate was applied, while detection limits reached pM level when chromogenic substrate was used. Thrombin in plasma sample can be detected with this assay. This cyclic peptide affinity ligand shows potentials for thrombin analysis in other detection formats.

A membrane penetrating peptide aptamer inhibits STAT3 function and suppresses the growth of STAT3 addicted tumor cells

Cancer cells are characterized by the aberrant activation of signaling pathways governing proliferation, survival, angiogenesis, migration and immune evasion. These processes are partially regulated by the transcription factor STAT3. This factor is inappropriately activated in diverse tumor types. Since tumor cells can become dependent on its persistent activation, STAT3 is a favorable drug target. Here, we describe the functional characterization of the recombinant STAT3 inhibitor, rS3-PA. This inhibitor is based on a 20 amino acid peptide which specifically interacts with the dimerization domain of STAT3. It is integrated into a thioredoxin scaffold and fused to a protein transduction domain. Protein gel blot and immunofluorescence analyses showed that rS3-PA is efficiently taken up by cells via an endocytosis independent mechanism. Intracellularly, it reduces the phosphorylation of STAT3 and enhances its degradation. This leads to the downregulation of STAT3 target gene expression on the mRNA and protein levels. Subsequently, tumor cell proliferation, survival and migration and the induction of angiogenesis are inhibited. In contrast, normal cells remain unaffected. Systemic administration of rS3-PA at doses of 7.5 mg/kg reduced P-STAT3 levels and significantly inhibited tumor growth up to 35% in a glioblastoma xenograft mouse model.

Aptamer-derived peptide inhibitors of Rho guanine nucleotide exchange factors

Small G proteins of the Rho family and their activators the guanine nucleotide exchange factors (RhoGEFs) regulate essential cellular functions and their deregulation has been associated with an amazing variety of human disorders, including cancer, inflammation, vascular diseases, and mental retardation. Rho GTPases and RhoGEFs therefore represent important targets for inhibition, not only in basic research but also for therapeutic purposes, and strategies to inhibit their function are actively being sought. Our lab has been very active in this field and has used the peptide aptamer technology to develop the first RhoGEF inhibitor, using the RhoGEF Trio as a model. Trio function has been described mainly in cell motility and axon growth in the nervous system via Rac1 GTPase activation, but recent findings suggest it to play also a role in the aggressive phenotype of various cancers, making it an attractive target for drug discovery. The object of this chapter is to demonstrate that targeting a RhoGEF using the peptide aptamer technology represents a valid and efficient approach to inhibit cellular processes in which Rho GTPase activity is upregulated. This is illustrated here by the first description of a peptide inhibitor of the oncogenic RhoGEF Tgat, TRIP(E32G), which is functional in vivo. On a long-term perspective, these peptide inhibitors can also serve as therapeutic tools or as guides for the discovery of small-molecule drugs, using an aptamer displacement screen.

Potential of phage-displayed peptide library technology to identify functional targeting peptides

Combinatorial peptide library technology is a valuable resource for drug discovery and development. Several peptide drugs developed through phage-displayed peptide library technology are presently in clinical trials and the authors envision that phage-displayed peptide library technology will assist in the discovery and development of many more. This review attempts to compile and summarize recent literature on targeting peptides developed through peptide library technology, with special emphasis on novel peptides with targeting capacity evaluated in vivo.

Random mutagenesis of peptide aptamers as an optimization strategy for inhibitor screening

Accumulating work over the past decade has shown that peptide aptamer screening represents a valid strategy for inhibitor identification that can be applied to a variety of different targets. Because of the screening method in cells and the highly combinatorial libraries available, this approach yields rapidly highly specific candidate inhibitors. Once a hit peptide has been identified, its interaction strength and affinity towards its target protein can be optimized even more, in order to increase its inhibition efficiency when subsequently applied in vivo. A condition to a successful optimization is that gain of inhibition strength should not result in loss of specificity. Here we present a simple method for peptide aptamer optimization, which can be achieved by PCR-based random mutagenesis combined with a selection screen in yeast using a strong selective drug. The rationale of this approach, which has proven valid and efficient, is that stronger interaction in yeast will also lead to stronger inhibition. Our optimization method is effective, without loss of specificity, which is of a great importance for the discovery of inhibitors that target specific protein-protein interactions.

Aptamers as therapeutics in cardiovascular diseases

With many advantages over other therapeutic agents such as monoclonal antibodies, aptamers have recently emerged as a novel and powerful class of ligands with excellent potential for diagnostic and therapeutic applications. Typically generated through Systematic Evolution of Ligands by EXponential enrichment (SELEX), aptamers have been selected against a wide range of targets such as proteins, phospholipids, sugars, nucleic acids, as well as whole cells. DNA/RNA aptamers are single-stranded DNA/RNA oligonucleotides (with a molecular weight of 5-40 kDa) that can fold into well-defined 3D structures and bind to their target molecules with high affinity and specificity. A number of strategies have been adopted to synthesize aptamers with enhanced in vitro/in vivo stability, aiming at potential therapeutic/diagnostic applications in the clinic. In cardiovascular diseases, aptamers can be developed into therapeutic agents as anti-thrombotics, anti-coagulants, among others. This review focuses on aptamers that were selected against various molecular targets involved in cardiovascular diseases: von Willebrand factor (vWF), thrombin, factor IX, phospholamban, P-selectin, platelet-derived growth factor, integrin α(v)β(3), CXCL10, vasopressin, among others. With continued effort in the development of aptamer-based therapeutics, aptamers will find their niches in cardiovascular diseases and significantly impact clinical patient management.

Targeting cancer with peptide aptamers

A major endeavour in cancer chemotherapy is to develop agents that specifically target a biomolecule of interest. There are two main classes of targeting agents: small molecules and biologics. Among biologics (e.g.: antibodies), DNA, RNA but also peptide aptamers are relatively recent agents. Peptide aptamers are seldom described but represent attractive agents that can inhibit a growing panel of oncotargets including Heat Shock Proteins. Potential pitfalls and coming challenges towards successful clinical trials are presented such as optimizing the delivery of peptide aptamers thanks to Nanotechnology.

Nucleic acid and peptide aptamers: fundamentals and bioanalytical aspects

In recent years new nucleic acid and protein-based combinatorial molecules have attracted the attention of researchers working in various areas of science, ranging from medicine to analytical chemistry. These molecules, called aptamers, have been proposed as alternatives to antibodies in many different applications. The aim of this Review is to illustrate the peculiarities of these combinatorial molecules which have initially been explored for their importance in molecular medicine, but have enormous potential in other biotechnological fields historically dominated by antibodies, such as bioassays. A description of these molecules is given, and the methods for their selection and production are also summarized. Moreover, critical aspects related to these molecules are discussed.

Peptide aptamers for small molecule drug discovery

Peptide aptamers have primarily been used as research tools to manipulate protein function and study regulatory networks. However, they also find multiple applications in therapeutic research, from target identification and validation to drug discovery. Because of their unbiased combinatorial nature, peptide aptamers interrogate the biological significance of numerous molecular surfaces on target proteins. Their use enables the identification and validation of some of these surfaces as interesting therapeutic targets to pursue. Peptide aptamers can subsequently be used to guide the discovery of small molecule drugs specific for these molecular surfaces.Here, we present a high-throughput screening assay that identifies small molecules that displace interactions between proteins and their cognate peptide aptamers. AptaScreen is a duplex yeast two-hybrid assay featuring two luciferase reporter genes. It can be performed in 96- or 384-well plates and can be fully automated.

The function of Stat3 in tumor cells and their microenvironment

Stat3 was initially recognized as a transcription factor and mediates the nuclear action of many different cytokines and growth factors. In addition to its roles in normal cell function, the inappropriate activation of Stat3 in tumor cells has attracted the attention of tumor biologists and has led to the consideration of Stat3 as a drug target. The induction of Stat3 activity under physiological circumstances is transient and many different levels of activation and deactivation have been defined. In addition to kinases and phosphatases, the SOCS proteins and the PIAS proteins have been recognized as negatively regulating components, which fine-tune the extent and the duration of Stat3 function. Its nuclear cytoplasmic shuttling is exquisitely regulated and adds to the complexity of Stat3 action. Newly discovered associations with cytoplasmic molecules suggest functions outside the conventional transcriptional regulation context. High molecular weight transcription complexes suggest that Stat3 might assume roles in transcriptional induction as well as in transcriptional suppression. The aberrant activation in tumor cells and the central function of Stat3 in the communication between cells of the immune system and tumor cells are of great interest for translational research projects and innovative drug development.

Split-intein mediated circular ligation used in the synthesis of cyclic peptide libraries in E. coli

Recent advances in chemical biology and the advantages presented by in vivo screening have highlighted the need for a robust and flexible biologically synthesized small-molecule library. Herein we describe a method for the biosynthesis of cyclic peptide libraries of up to 10(8) members in Escherichia coli using split-intein circular ligation of peptides and proteins (SICLOPPS). The method utilizes split-intein chemistry to cyclize randomized peptide sequences. The cyclic peptide library can potentially be of any size and the peptide itself may contain unlimited random residues. However, the library size is limited by the transformation efficiency of E. coli and random residues are generally limited to five, but additional amino acids can be used in the cyclic peptide backbone, varying the structure and ring size of the cyclic peptide. SICLOPPS libraries have been combined with a bacterial reverse two-hybrid system in our labs and used in the identification of inhibitors of several protein-protein interactions. This protocol is expected to take around 3-4 weeks to implement.

Alternative non-antibody scaffolds for molecular recognition

Originally proposed one decade ago, the idea of engineering proteins outside the immunoglobulin family for novel binding functions has evolved as a powerful technology. Several classes of protein scaffolds proved to yield reagents with specificities and affinities in a range that was previously considered unique to antibodies. Such engineered protein scaffolds are usually obtained by designing a random library with mutagenesis focused at a loop region or at an otherwise permissible surface area and by selection of variants against a given target via phage display or related techniques. Whereas a plethora of protein scaffolds has meanwhile been proposed, only few of them were actually demonstrated to yield specificities towards different kinds of targets and to offer practical benefits such as robustness, smaller size, and ease of expression that justify their use as a true alternative to conventional antibodies or their recombinant fragments. Currently, the most promising scaffolds with broader applicability are protein A, the lipocalins, a fibronectin domain, an ankyrin consensus repeat domain, and thioredoxin. Corresponding binding proteins are not only of interest as research reagents or for separation in biotechnology but also as potential biopharmaceuticals, especially in the areas of cancer, autoimmune and infectious diseases as well as for in vivo diagnostics. The medical prospects have boosted high commercial expectations, and many of the promising scaffolds are under development by biotech start-up companies. Although some issues still have to be addressed, for example immunogenicity, effector functions, and plasma half-life in the context of therapeutic use or low-cost high-throughput selection for applications in proteomics research, it has become clear that scaffold-derived binding proteins will play an increasing role in biotechnology and medicine.

Peptide Aptamer-mediated Inhibition of Target Proteins by Sequestration into Aggresomes

Peptide aptamers (PAs) can be employed to block the intracellular function of target proteins. Little is known about the mechanism of PA-mediated protein inhibition. Here, we generated PAs that specifically bound to the duck hepatitis B virus (HBV) core protein. Among them, PA34 strongly blocked duck HBV replication by inhibiting viral capsid formation. We found that PA34 led to a dramatic intracellular redistribution of its target protein into perinuclear inclusion bodies, which exhibit the typical characteristics of aggresomes. As a result, the core protein is efficiently removed from the viral life cycle. Corresponding findings were obtained for bioactive PAs that bind to the HBV core protein or to the human papillomavirus-16 (HPV16) E6 protein, respectively. The observation that PAs induce the specific sequestration of bound proteins into aggresomes defines a novel mechanism as to how this new class of intracellular inhibitors blocks the function of their target proteins.

Selective delivery of therapeutic agents for the diagnosis and treatment of cancer

Research activity aimed towards achieving specific and targeted delivery of cancer therapeutics has expanded tremendously in the last decade, resulting in new ways of directing drugs to tumours, as well as new types of drugs. The available strategies exploit differences in the nature of normal and cancer cells and their microenvironment. The discovery and validation of cancer-associated markers, as well as corresponding ligands, is pivotal for developing selective delivery technology for cancer. Although most current clinical trials are either monoclonal antibody- or gene-based, methodological advances in combinatorial libraries of peptides, single chain variable fragments and small organic molecules are expected to change this scenario in the near future. Nanotechnology platforms today allow systematic and modular combinations of therapeutic agents and tumour-binding moieties that may generate novel, personalised agents for selective delivery in cancer. This paper discusses recent developments and future prospects of targeted delivery technologies in the management of cancer.

Current strategies for the development of peptide-based anti-cancer therapeutics

The completion of the human genome sequence and the development of new techniques, which allow the visualisation of comprehensive gene expression patterns, has led to the identification of a large number of gene products differentially expressed in tumours and corresponding normal tissues. The task at hand is the sorting of these genes into correlative and causative ones. Correlative genes are merely changed as a consequence of transformation and have no decisive effects upon transformation. In contrast, causative genes play a direct role in the process of cellular transformation and the maintenance of the transformed state, which can be exploited for therapeutic purposes. Oncogenes and tumour suppressor genes are prime targets for the development of new inhibitors and gene therapeutic strategies. However, many target oncogene products do not exhibit enzymatic activity that can be inhibited by conventional small molecular weight compounds. They exert their functions through regulated protein-protein or protein-DNA interactions and might require other compounds for efficient interference with such functions. Peptides are emerging as a novel class of drugs for cancer therapy, which could fulfil these tasks. Peptide therapy aims at the specific inhibition of inappropriately activated oncogenes. This review will focus on the selection procedures, which can be employed to identify useful peptides for the treatment of cancer. Before peptide-based therapeutics can become useful, it will be necessary to increase their stability by modifications or the use of scaffolds. Additionally, various delivery methods including liposomes and particularly the use of protein transduction domains (PTDs) have to be explored. These strategies will yield highly specific and more effective peptides and improve the potential of peptide-based anti-cancer therapeutics.