Development of next generation adeno-associated viral vectors capable of selective tropism and efficient gene delivery

Site-specific tethering nanobodies on recombinant adeno-associated virus vectors for retargeted gene therapy

Recombinant adeno-associated viruses (rAAVs) have been extensively studied for decades as carriers for delivering therapeutic genes. However, designing rAAV vectors with selective tropism for specific cell types and tissues has remained challenging. Here, we introduce a strategy for redirecting rAAV by attaching nanobodies with desired tropism at specific sites, effectively replacing the original tropism. To demonstrate this concept, we initially modified the genetic code of rAAV2 to introduce an azido-containing unnatural amino acid at a precise site within the capsid protein. Following a screening process, we identified a critical site (N587+1) where the introduction of unnatural amino acid eliminated the natural tropism of rAAV2. Subsequently, we successfully redirected rAAV2 by conjugating various nanobodies at the N587+1 site, using click and SpyTag-Spycatcher chemistries to form nanobody-AAV conjugates (NACs). By investigating the relationship between NACs quantity and effect and optimizing the linker between rAAV2 and the nanobody using a cathepsin B-susceptible valine-citrulline (VC) dipeptide, we significantly improved gene delivery efficiency both in vitro and in vivo. This enhancement can be attributed to the facilitated endosomal escape of rAAV2. Our method offers an exciting avenue for the rational modification of rAAV2 as a retargeting vehicle, providing a convenient platform for precisely engineering various rAAV2 vectors for both basic research and therapeutic applications. STATEMENT OF SIGNIFICANCE: AAVs hold great promise in the treatment of genetic diseases, but their clinical use has been limited by off-target transduction and efficiency. Here, we report a strategy to construct NACs by conjugating a nanobody or scFv to an rAAV capsid site, specifically via biorthogonal click chemistry and a spy-spycatcher reaction. We explored the structure-effect and quantity-effect relationships of NACs and then optimized the transduction efficiency by introducing a valine-citrulline peptide linker. This approach provides a biocompatible method for rational modification of rAAV as a retargeting platform without structural disruption of the virus or alteration of the binding capacity of the nanobody, with potential utility across a broad spectrum of applications in targeted imaging and gene delivery.

Brain gliomas: Diagnostic and therapeutic issues and the prospects of drug-targeted nano-delivery technology

Glioma is the most common intracranial malignant tumor, with severe difficulty in treatment and a low patient survival rate. Due to the heterogeneity and invasiveness of tumors, lack of personalized clinical treatment design, and physiological barriers, it is often difficult to accurately distinguish gliomas, which dramatically affects the subsequent diagnosis, imaging treatment, and prognosis. Fortunately, nano-delivery systems have demonstrated unprecedented capabilities in diagnosing and treating gliomas in recent years. They have been modified and surface modified to efficiently traverse BBB/BBTB, target lesion sites, and intelligently release therapeutic or contrast agents, thereby achieving precise imaging and treatment. In this review, we focus on nano-delivery systems. Firstly, we provide an overview of the standard and emerging diagnostic and treatment technologies for glioma in clinical practice. After induction and analysis, we focus on summarizing the delivery methods of drug delivery systems, the design of nanoparticles, and their new advances in glioma imaging and treatment in recent years. Finally, we discussed the prospects and potential challenges of drug-delivery systems in diagnosing and treating glioma.

Chemical approaches to probe and engineer AAV vectors

Adeno-associated virus (AAV) has emerged as the most promising vector for in vivo human gene therapy, with several therapeutic approvals in the last few years and countless more under development. Underlying this remarkable success are several attractive features that AAV offers, including lack of pathogenicity, low immunogenicity, long-term gene expression without genomic integration, the ability to infect both dividing and non-dividing cells, etc. However, the commonly used wild-type AAV capsids in therapeutic development present significant challenges, including inadequate tissue specificity and the need for large doses to attain therapeutic effectiveness, raising safety concerns. Additionally, significant preexisting adaptive immunity against most natural capsids, and the development of such anti-capsid immunity after the first treatment, represent major challenges. Strategies to engineer the AAV capsid are critically needed to address these challenges and unlock the full promise of AAV gene therapy. Chemical modification of the AAV capsid has recently emerged as a powerful new approach to engineer its properties. Unlike genetic strategies, which can be more disruptive to the delicate capsid assembly and packaging processes, "late-stage" chemical modification of the assembled capsid-whether at natural amino acid residues or site-specifically installed noncanonical amino acid residues-often enables a versatile approach to introducing new properties to the capsid. This review summarizes the significant recent progress in AAV capsid engineering strategies, with a particular focus on chemical modifications in advancing the next generation of AAV-based gene therapies.

Precise Manipulation of the Site and Stoichiometry of Capsid Modification Enables Optimization of Functional Adeno-Associated Virus Conjugates

The ability to engineer adeno-associated virus (AAV) vectors for targeted transduction of specific cell types is critically important to fully harness their potential for human gene therapy. A promising approach to achieve this objective involves chemically attaching retargeting ligands onto the virus capsid. Site-specific incorporation of a bioorthogonal noncanonical amino acid (ncAA) into the AAV capsid proteins provides a particularly attractive strategy to introduce such modifications with exquisite precision. In this study, we show that using ncAA mutagenesis, it is possible to systematically alter the attachment site of a retargeting ligand (cyclic-RGD) on the AAV capsid to create diverse conjugate architectures and that the site of attachment heavily impacts the retargeting efficiency. We further demonstrate that the performance of these AAV conjugates is highly sensitive to the stoichiometry of capsid labeling (labels per capsid), with an intermediate labeling density providing optimal activity for cRGD-mediated retargeting. Finally, we developed a technology to more precisely control the number of attachment sites per AAV capsid by selectively incorporating an ncAA into the minor capsid proteins with high fidelity and efficiency, such that AAV conjugates with varying stoichiometry can be synthesized. Together, this platform provides unparalleled control over the site and stoichiometry of capsid modification, which will enable the development of next-generation AAV vectors tailored with desirable attributes.

Non-canonical amino acid incorporation into AAV5 capsid enhances lung transduction in mice

Gene therapy using recombinant adeno-associated virus (rAAV) relies on safe, efficient, and precise in vivo gene delivery that is largely dependent on the AAV capsid. The proteinaceous capsid is highly amenable to engineering using a variety of approaches, and most resulting capsids carry substitutions or insertions comprised of natural amino acids. Here, we incorporated a non-canonical amino acid (ncAA), Nε-2-azideoethyloxycarbonyl-L-lysine (also known as NAEK), into the AAV5 capsid using genetic code expansion, and serendipitously found that several NAEK-AAV5 vectors transduced various cell lines more efficiently than the parental rAAV5. Furthermore, one NAEK-AAV5 vector showed lung-specific transduction enhancement following systemic or intranasal delivery in mice. Structural modeling suggests that the long side chain of NAEK may impact on the 3-fold protrusion on the capsid surface that plays a key role in tropism, thereby modulating vector transduction. Recent advances in genetic code expansion have generated synthetic proteins carrying an increasing number of ncAAs that possess diverse biological properties. Our study suggests that ncAA incorporation into the AAV capsid may confer novel vector properties, opening a new and complementary avenue to gene therapy vector discovery.

Applications of synthetic biology in medical and pharmaceutical fields

Synthetic biology aims to design or assemble existing bioparts or bio-components for useful bioproperties. During the past decades, progresses have been made to build delicate biocircuits, standardized biological building blocks and to develop various genomic/metabolic engineering tools and approaches. Medical and pharmaceutical demands have also pushed the development of synthetic biology, including integration of heterologous pathways into designer cells to efficiently produce medical agents, enhanced yields of natural products in cell growth media to equal or higher than that of the extracts from plants or fungi, constructions of novel genetic circuits for tumor targeting, controllable releases of therapeutic agents in response to specific biomarkers to fight diseases such as diabetes and cancers. Besides, new strategies are developed to treat complex immune diseases, infectious diseases and metabolic disorders that are hard to cure via traditional approaches. In general, synthetic biology brings new capabilities to medical and pharmaceutical researches. This review summarizes the timeline of synthetic biology developments, the past and present of synthetic biology for microbial productions of pharmaceutics, engineered cells equipped with synthetic DNA circuits for diagnosis and therapies, live and auto-assemblied biomaterials for medical treatments, cell-free synthetic biology in medical and pharmaceutical fields, and DNA engineering approaches with potentials for biomedical applications.

Aptamer-programmable adeno-associated viral vectors as a novel platform for cell-specific gene transfer

Adeno-associated viruses (AAVs) are commonly used for in vivo gene therapy. Nevertheless, the wide tropism that characterizes these vectors limits specific targeting to a particular cell type or tissue. Here, we developed new chemically modified AAV vectors (Nε-AAVs) displaying a single site substitution on the capsid surface for post-production vector engineering through biorthogonal copper-free click chemistry. We were able to identify AAV vectors that would tolerate the unnatural amino acid substitution on the capsid without disrupting their packaging efficiency. We functionalized the Nε-AAVs through conjugation with DNA (AS1411) or RNA (E3) aptamers or with a folic acid moiety (FA). E3-, AS1411-, and FA-AAVs showed on average a 3- to 9-fold increase in transduction compared with their non-conjugated counterparts in different cancer cell lines. Using specific competitors, we established ligand-specific transduction. In vivo studies confirmed the selective uptake of FA-AAV and AS1411-AAV without off-target transduction in peripheral organs. Overall, the high versatility of these novel Nε-AAVs might pave the way to tailoring gene therapy vectors toward specific types of cells both for ex vivo and in vivo applications.

AAV vectors applied to the treatment of CNS disorders: Clinical status and challenges

In recent years, adeno-associated virus (AAV) has become the most important vector for central nervous system (CNS) gene therapy. AAV has already shown promising results in the clinic, for several CNS diseases that cannot be treated with drugs, including neurodegenerative diseases, neuromuscular diseases, and lysosomal storage disorders. Currently, three of the four commercially available AAV-based drugs focus on neurological disorders, including Upstaza for aromatic l-amino acid decarboxylase deficiency, Luxturna for hereditary retinal dystrophy, and Zolgensma for spinal muscular atrophy. All these studies have provided paradigms for AAV-based therapeutic intervention platforms. AAV gene therapy, with its dual promise of targeting disease etiology and enabling 'long-term correction' of disease processes, has the advantages of immune privilege, high delivery efficiency, tissue specificity, and cell tropism in the CNS. Although AAV-based gene therapy has been shown to be effective in most CNS clinical trials, limitations have been observed in its clinical applications, which are often associated with side effects. In this review, we summarized the therapeutic progress, challenges, limitations, and solutions for AAV-based gene therapy in 14 types of CNS diseases. We focused on viral vector technologies, delivery routes, immunosuppression, and other relevant clinical factors. We also attempted to integrate several hurdles faced in clinical and preclinical studies with their solutions, to seek the best path forward for the application of AAV-based gene therapy in the context of CNS diseases. We hope that these thoughtful recommendations will contribute to the efficient translation of preclinical studies and wide application of clinical trials.

Overcoming the Challenges Imposed by Humoral Immunity to AAV Vectors to Achieve Safe and Efficient Gene Transfer in Seropositive Patients

One of the major goals of in vivo gene transfer is to achieve long-term expression of therapeutic transgenes in terminally differentiated cells. The extensive clinical experience and the recent approval of Luxturna® (Spark Therapeutics, now Roche) and Zolgensma® (AveXis, now Novartis) place vectors derived from adeno-associated viruses (AAV) among the best options for gene transfer in multiple tissues. Despite these successes, limitations remain to the application of this therapeutic modality in a wider population. AAV was originally identified as a promising virus to derive gene therapy vectors because, despite infecting humans, it was not associated with any evident disease. Thee large proportion of AAV infections in the human population is now revealing as a limitation because after exposure to wild-type AAV, anti-AAV antibodies develops and may neutralize the vectors derived from the virus. Injection of AAV in humans is generally well-tolerated although the immune system can activate after the recognition of AAV vectors capsid and genome. The formation of high-titer neutralizing antibodies to AAV after the first injection precludes vector re-administration. Thus, both pre-existing and post-treatment humoral responses to AAV vectors greatly limit a wider application of this gene transfer modality. Different methods were suggested to overcome this limitation. The extensive preclinical data available and the large clinical experience in the control of AAV vectors immunogenicity are key to clinical translation and to demonstrate the safety and efficacy of these methods and ultimately bring a curative treatment to patients.

Customized blood-brain barrier shuttle peptide to increase AAV9 vector crossing the BBB and augment transduction in the brain

Recombinant adeno-associated virus (rAAV) vectors have been widely used as favored delivery vehicles for the treatment of inherited diseases in clinical trials, including neurological diseases. However, the noninvasive systemic delivery of rAAV to the central nervous system is severely hampered by the blood-brain barrier (BBB). Several approaches have been exploited to enhance AAV vector brain transduction after systemic administration, including genetic modification of AAV capsids and physical methods. However, these approaches are not always predictive of desirable outcomes in humans and induce complications. It is imperative to explore novel strategies to increase the ability of AAV9 to cross the BBB for enhanced brain transduction. Herein, we have conducted a combinatorial in vivo/in vitro phage display library screening in mouse brains and purified AAV9 virions to identify a customized BBB shuttle peptide, designated as PB5-3. The PB5-3 peptide specifically bound to AAV9 virions and enhanced widespread transduction of AAV9 in mouse brains, especially in neuronal cells, after systemic administration. Further study demonstrated that systemic administration of AAV9 vectors encoding IDUA complexed with PB5-3 increased the phenotypic correction in the brains of MPS I mice. Mechanistic studies revealed that the PB5-3 peptide effectively increased AAV9 trafficking and transcytosis efficiency in the human BBB model hCMEC/D3 cell line but did not interfere with AAV9 binding to the receptor terminal N-linked galactosylated glycans. Additionally, the PB5-3 peptide slowed the clearance of AAV9 from blood without hepatic toxicity. This study highlights, for the first time, the potential of this combinatorial approach for the isolation of peptides that interact with specific AAV vectors for enhanced and targeted AAV transduction. This promising approach will open new combined therapeutic avenues and shed light on the potential applications of peptides for the treatment of human diseases in future clinical trials with AAV vector-mediated gene delivery.

Advances in immunotherapy for glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common and aggressive malignant brain tumor of the central nervous system and has a very poor prognosis. The current standard of care for patients with GBM involves surgical resection, radiotherapy, and chemotherapy. Unfortunately, conventional therapies have not resulted in significant improvements in the survival outcomes of patients with GBM; therefore, the overall mortality rate remains high. Immunotherapy is a type of cancer treatment that helps the immune system to fight cancer and has shown success in different types of aggressive cancers. Recently, healthcare providers have been actively investigating various immunotherapeutic approaches to treat GBM. We reviewed the most promising immunotherapy candidates for glioblastoma that have achieved encouraging results in clinical trials, focusing on immune checkpoint inhibitors, oncolytic viruses, nonreplicating viral vectors, and chimeric antigen receptor (CAR) immunotherapies.

Chemical Modifications of the Capsid for Redirecting and Improving the Efficacy of Adeno-Associated Virus Vectors

Adeno-associated virus (AAV) vector-directed gene therapy is one of the most exciting modalities of biotechnology as more applications enter clinical stage. Although AAV vectors generally feature low toxicity, high stability, and long-lasting transgene expression, potential challenging issues of AAV include high vector dose, limited tissue tropism, and the host immune response and inflammation, which are all related to the capsid protein. To overcome these challenges, various strategies have been developed to engineer AAV capsids. Apart from widely employed genetic engineering of capsid protein, powerful and versatile chemical modification strategies are underexploited. This minireview summarizes recent advances and our perspectives for future direction in AAV capsid chemical modification to enhance its therapeutic use for gene therapy.

Amber codon is genetically unstable in generation of premature termination codon (PTC)-harbouring Foot-and-mouth disease virus (FMDV) via genetic code expansion

The foot-and-mouth disease virus (FMDV) is the causative agent of FMD, a highly infectious and devastating viral disease of domestic and wild cloven-hoofed animals. FMD affects livestock and animal products' national and international trade, causing severe economic losses and social consequences. Currently, inactivated vaccines play a vital role in FMD control, but they have several limitations. The genetic code expansion technology provides powerful strategies for generating premature termination codon (PTC)-harbouring virus as a live but replication-incompetent viral vaccine. However, this technology has not been explored for the design and development of new FMD vaccines. In this study, we first expanded the genetic code of the FMDV genome via a transgenic cell line containing an orthogonal translation machinery. We demonstrated that the transgenic cells stably integrated the orthogonal pyltRNA/pylRS pair into the genome and enabled efficient, homogeneous incorporation of unnatural amino acids into target proteins in mammalian cells. Next, we constructed 129 single-PTC FMDV mutants and four dual-PTC FMDV mutants after considering the tolerance, location, and potential functions of those mutated sites. Amber stop codons individually substituted the selected amino acid codons in four viral proteins (3D, L, VP1, and VP4) of FMDV. We successfully rescued PTC-FMDV mutants, but the amber codon unexpectedly showed a highly degree of mutation rate during PTC-FMDV packaging and replication. Our findings highlight that the genetic code expansion technology for the generation of PTC-FMD vaccines needs to be further improved and that the genetic stability of amber codons during the packaging and replication of FMDV is a concern.

Manufacturing Challenges and Rational Formulation Development for AAV Viral Vectors

Adeno-associated virus (AAV) has emerged as a leading platform for gene delivery for treating various diseases due to its excellent safety profile and efficient transduction to various target tissues. However, the large-scale production and long-term storage of viral vectors is not efficient resulting in lower yields, moderate purity, and shorter shelf-life compared to recombinant protein therapeutics. This review provides a comprehensive analysis of upstream, downstream and formulation unit operation challenges encountered during AAV vector manufacturing, and discusses how desired product quality attributes can be maintained throughout product shelf-life by understanding the degradation mechanisms and formulation strategies. The mechanisms of various physical and chemical instabilities that the viral vector may encounter during its production and shelf-life because of various stressed conditions such as thermal, shear, freeze-thaw, and light exposure are highlighted. The role of buffer, pH, excipients, and impurities on the stability of viral vectors is also discussed. As such, the aim of this review is to outline the tools and a potential roadmap for improving the quality of AAV-based drug products by stressing the need for a mechanistic understanding of the involved processes.

Clinically Explored Virus-Based Therapies for the Treatment of Recurrent High-Grade Glioma in Adults

As new treatment modalities are being explored in neuro-oncology, viruses are emerging as a promising class of therapeutics. Virotherapy consists of the introduction of either wild-type or engineered viruses to the site of disease, where they exert an antitumor effect. These viruses can either be non-lytic, in which case they are used to deliver gene therapy, or lytic, which induces tumor cell lysis and subsequent host immunologic response. Replication-competent viruses can then go on to further infect and lyse neighboring glioma cells. This treatment paradigm is being explored extensively in both preclinical and clinical studies for a variety of indications. Virus-based therapies are advantageous due to the natural susceptibility of glioma cells to viral infection, which improves therapeutic selectivity. Furthermore, lytic viruses expose glioma antigens to the host immune system and subsequently stimulate an immune response that specifically targets tumor cells. This review surveys the current landscape of oncolytic virotherapy clinical trials in high-grade glioma, summarizes preclinical experiences, identifies challenges associated with this modality across multiple trials, and highlights the potential to integrate this therapeutic strategy into promising combinatory approaches.

Genetic code expansion in mammalian cells: A plasmid system comparison

Genetic code expansion with unnatural amino acids (UAAs) has significantly broadened the chemical repertoire of proteins. Applications of this method in mammalian cells include probing of molecular interactions, conditional control of biological processes, and new strategies for therapeutics and vaccines. A number of methods have been developed for transient UAA mutagenesis in mammalian cells, each with unique features and advantages. All have in common a need to deliver genes encoding additional protein biosynthetic machinery (an orthogonal tRNA/tRNA synthetase pair) and a gene for the protein of interest. In this study, we present a comparative evaluation of select plasmid-based genetic code expansion systems and a detailed analysis of suppression efficiency with different UAAs and in different cell lines.

Chemical modification of the adeno-associated virus capsid to improve gene delivery

Gene delivery vectors based on adeno-associated virus (AAV) are highly promising due to several desirable features of this parent virus, including a lack of pathogenicity, efficient infection of dividing and non-dividing cells and sustained maintenance of the viral genome. However, the conclusion from clinical data using these vectors is that there is a need to develop new AAVs with a higher transduction efficiency and specificity for relevant target tissues. To overcome these limitations, we chemically modified the surface of the capsid of AAV vectors. These modifications were achieved by chemical coupling of a ligand by the formation of a thiourea functionality between the amino group of the capsid proteins and the reactive isothiocyanate motif incorporated into the ligand. This strategy does not require genetic engineering of the capsid sequence. The proof of concept was first evidenced using a fluorophore (FITC). Next, we coupled the N-acetylgalactosamine ligand onto the surface of the AAV capsid for asialoglycoprotein receptor-mediated hepatocyte-targeted delivery. Chemically-modified capsids also showed reduced interactions with neutralizing antibodies. Taken together, our findings reveal the possibility of creating a specific engineered platform for targeting AAVs via chemical coupling.

Bioconjugated AAV vectors, achieved by coupling of ligands on amino groups of the capsid, are of great interest for gene delivery. Chemical modifications can be used to enhance cell tropism and to decrease interactions with neutralizing antibodies.

Viral vector: potential therapeutic for glioblastoma multiforme

Glioblastoma multiforme is a highly malignant primary brain tumour found in adults and is highlighted as the most devastating among all the other grades of glioma. Well-established standard treatment methods, such as chemotherapy, radiation and surgery, have resulted in modest improvement in the survival of patients. Hence, the arduous search for novel treatments backed by advancements in molecular biology still persists. Glioblastoma has many distinctive characteristics, which makes it a potential candidate for gene therapy. Gene therapy involves the delivery of genetic material of therapeutic use into tumour cells, which produces a specific antitumour response. Moreover, viruses stimulate a vigorous cytotoxic effect, they are easily modifiable and the inherent property of horizontal transfer of genetic material makes them valuable tools for genetic engineering. In this review, we have enlisted the various viral vectors employed in gene therapy for glioblastoma.

Using genetically incorporated unnatural amino acids to control protein functions in mammalian cells

Genetic code expansion allows unnatural (non-canonical) amino acid incorporation into proteins of interest by repurposing the cellular translation machinery. The development of this technique has enabled site-specific incorporation of many structurally and chemically diverse amino acids, facilitating a plethora of applications, including protein imaging, engineering, mechanistic and structural investigations, and functional regulation. Particularly, genetic code expansion provides great tools to study mammalian proteins, of which dysregulations often have important implications in health. In recent years, a series of methods has been developed to modulate protein function through genetically incorporated unnatural amino acids. In this review, we will first discuss the basic concept of genetic code expansion and give an up-to-date list of amino acids that can be incorporated into proteins in mammalian cells. We then focus on the use of unnatural amino acids to activate, inhibit, or reversibly modulate protein function by translational, optical or chemical control. The features of each approach will also be highlighted.

Adeno-associated virus vector as a platform for gene therapy delivery

Adeno-associated virus (AAV) vectors are the leading platform for gene delivery for the treatment of a variety of human diseases. Recent advances in developing clinically desirable AAV capsids, optimizing genome designs and harnessing revolutionary biotechnologies have contributed substantially to the growth of the gene therapy field. Preclinical and clinical successes in AAV-mediated gene replacement, gene silencing and gene editing have helped AAV gain popularity as the ideal therapeutic vector, with two AAV-based therapeutics gaining regulatory approval in Europe or the United States. Continued study of AAV biology and increased understanding of the associated therapeutic challenges and limitations will build the foundation for future clinical success.

RNA-Guided Adenosine Deaminases: Advances and Challenges for Therapeutic RNA Editing

Targeted transcriptome engineering, in contrast to genome engineering, offers a complementary and potentially tunable and reversible strategy for cellular engineering. In this regard, adenosine to inosine (A-to-I) RNA base editing was recently engineered to make programmable base conversions on target RNAs. Similar to the DNA base editing technology, A-to-I RNA editing may offer an attractive alternative in a therapeutic setting, especially for the correction of point mutations. This Perspective introduces five currently characterized RNA editing systems and serves as a reader's guide for implementing an appropriate RNA editing strategy for applications in research or therapeutics.

Therapeutic applications of genetic code expansion

In nature, a limited, conservative set of amino acids are utilized to synthesize proteins. Genetic code expansion technique reassigns codons and incorporates noncanonical amino acids (ncAAs) through orthogonal aminoacyl-tRNA synthetase (aaRS)/tRNA pairs. The past decade has witnessed the rapid growth in diversity and scope for therapeutic applications of this technology. Here, we provided an update on the recent progress using genetic code expansion in the following areas: antibody-drug conjugates (ADCs), bispecific antibodies (BsAb), immunotherapies, long-lasting protein therapeutics, biosynthesized peptides, engineered viruses and cells, as well as other therapeutic related applications, where the technique was used to elucidate the mechanisms of biotherapeutics and drug targets.

Synthesis at the interface of virology and genetic code expansion

How a virus efficiently invades its host cell and masterfully engineers its properties provides valuable lessons and resources for the emerging discipline of synthetic biology, which seeks to create engineered biological systems with novel functions. Recently, the toolbox of synthetic biology has also been enriched by the genetic code expansion technology, which has provided access to a large assortment of unnatural amino acids with novel chemical functionalities that can be site-specifically incorporated into proteins in living cells. The synergistic interplay of these two disciplines holds much promise to advance their individual progress, while creating new paradigms for synthetic biology. In this review we seek to provide an account of the recent advances at the interface of these two research areas.

Redirecting Killer T Cells through Incorporation of Azido Sugars for Tethering Ligands

The genetic expression of chimeric antigen receptors (CARs) on the surfaces of T cells enables the redirection of T cell specificity. To enhance the versatility of T cells as tumor-specific killers, we developed a nongenetic approach by which azide-containing sialic acids were metabolically incorporated into T cells to modify cellular sialyl glycans. After successful display of these moieties on the T cells, small-molecule ligands such as RGD and folate (as proof-of-concept, rather than supersized antibodies) were clicked orthogonally, leading to highly selective time- and dose-dependent cytotoxicity to integrin α_v β₃ - and folate-receptor-positive cells, respectively. This chemical approach provides a facile platform for rational design of tumor-specific cytotoxic T cells for targeted immunotherapy.

Site-Specific PEGylated Adeno-Associated Viruses with Increased Serum Stability and Reduced Immunogenicity

Adeno-associated virus (AAV) is one of the most extensively studied and utilized viral vectors in clinical gene transfer research. However, the serum instability and immunogenicity of AAV vectors significantly limit their application. Here, we endeavored to overcome these limitations by developing a straightforward approach for site-specific PEGylation of AAV via genetic code expansion. This technique includes incorporation of the azide moiety into the AAV capsid protein followed by orthogonal and stoichiometric conjugation of a variety of polyethylene glycols (PEGs) through click chemistry. Using this approach, only the chosen site(s) was consistently PEGylated under mild conditions, preventing nonselective conjugation. Upon a series of in vitro examinations, AAVs conjugated with 20-kD PEG at sites Q325+1, S452+1, and R585+1 showed a 1.7- to 2.4-fold stability improvement in pooled human serum and a nearly twofold reduction in antibody recognition. Subsequent animal research on Sprague Dawley rats displayed a promising 20% reduction in antibody inducement and a higher virus titer in the blood. Together, our data demonstrate successful protection of an AAV vector from antibody neutralization and blood clearance, thereby increasing the efficiency of therapeutic gene delivery.

Synthesis of Site-Specific Radiolabeled Antibodies for Radioimmunotherapy via Genetic Code Expansion

Radioimmunotherapy (RIT) delivers radioisotopes to antigen-expressing cells via monoantibodies for the imaging of lesions or medical therapy. The chelates are typically conjugated to the antibody through cysteine or lysine residues, resulting in heterogeneous chelate-to-antibody ratios and various conjugation sites. To overcome this heterogeneity, we have developed an approach for site-specific radiolabeling of antibodies by combination of genetic code expansion and click chemistry. As a proof-of-concept study, model systems including anti-CD20 antibody rituximab, positron-emitting isotope ⁶⁴Cu, and a newly synthesized bifunctional linker (4-dibenzocyclooctynol-1,4,7,10-tetraazacyclotetradecane-1,4,7,10-tetraacetic acid, DIBO-DOTA) were used. The approach consists of three steps: (1) site-specific incorporation of an azido group-bearing amino acid (NEAK) via the genetic code expansion technique at the defined sites of the antibody as a "chemical handle"; (2) site-specific and quantitative conjugation of bifunctional linkers with the antibodies under a mild condition; and (3) radiolabeling of the chelate-modified antibodies with the appropriate isotope. We used heavy-chain A122NEAK rituximab as proof-of-concept and obtained a homogeneous radioconjugate with precisely two chelates per antibody, incorporated only at the chosen sites. The conjugation did not alter the binding and pharmacokinetics of the rituximab, as indicated by in vitro assays and in vivo PET imaging. We believe our research is a good supplement to the genetic code expansion technique for the development of novel radioimmunoconjugates.

Cancer suicide gene therapy: a patent review

INTRODUCTION

Cancer is considered the second leading cause of death worldwide despite the progress made in early detection and advances in classical therapies. Advancing in the fight against cancer requires the development of novel strategies, and the suicide gene transfer to tumor cells is providing new possibilities for cancer therapy.

AREAS COVERED

In this manuscript, authors present an overview of suicide gene systems and the latest innovations done to enhance cancer suicide gene therapy strategies by i) improving vectors for targeted gene delivery using tissue specific promoter and receptors; ii) modification of the tropism; and iii) combining suicide genes and/or classical therapies for cancer. Finally, the authors highlight the main challenges to be addressed in the future.

EXPERT OPINION

Even if many efforts are needed for suicide gene therapy to be a real alternative for cancer treatment, we believe that the significant progress made in the knowledge of cancer biology and characterization of cancer stem cells accompanied by the development of novel targeted vectors will enhance the effectiveness of this type of therapeutic strategy. Moreover, combined with current treatments, suicide gene therapy will improve the clinical outcome of patients with cancer in the future.

Data on interaction between adeno-associated virus and U87 cell via cRGD chemical modification

RGD tripeptide is a specific, high-affinity ligand for integrin, which is highly expressed in cancer cells. We previously reported that cRGD chemically modified AAV2 (AAV2^{N587+1/azido+RGD}) showed significantly enhanced infectivity compared to RGD genetically inserted AAV2 (AAV2^N587+RGD) (10.1016/j.biomaterials.2015.11.066) [1]. Herein we provide the binding ability analysis of RGD modified AAV2 and U87 cell by flow cytometry and the theoretical working model of RGD–αvβ3 integrin interaction.