Endostatin gene therapy delivered by attenuated Salmonella typhimurium in murine tumor models

Matrikines in the skin: Origin, effects, and therapeutic potential

The extracellular matrix (ECM) represents a complex multi-component environment that has a decisive influence on the biomechanical properties of tissues and organs. Depending on the tissue, ECM components are subject to a homeostasis of synthesis and degradation, a subtle interplay that is influenced by external factors and the intrinsic aging process and is often disturbed in pathologies. Upon proteolytic cleavage of ECM proteins, small bioactive peptides termed matrikines can be formed. These bioactive peptides play a crucial role in cell signaling and contribute to the dynamic regulation of both physiological and pathological processes such as tissue remodeling and repair as well as inflammatory responses. In the skin, matrikines exert an influence for instance on cell adhesion, migration, and proliferation as well as vasodilation, angiogenesis and protein expression. Due to their manifold functions, matrikines represent promising leads for developing new therapeutic options for the treatment of skin diseases. This review article gives a comprehensive overview on matrikines in the skin, including their origin in the dermal ECM, their biological effects and therapeutic potential for the treatment of skin pathologies such as melanoma, chronic wounds and inflammatory skin diseases or for their use in anti-aging cosmeceuticals.

Synergistic Brilliance: Engineered Bacteria and Nanomedicine Unite in Cancer Therapy

Engineered bacteria are widely used in cancer treatment because live facultative/obligate anaerobes can selectively proliferate at tumor sites and reach hypoxic regions, thereby causing nutritional competition, enhancing immune responses, and producing anticancer microbial agents in situ to suppress tumor growth. Despite the unique advantages of bacteria-based cancer biotherapy, the insufficient treatment efficiency limits its application in the complete ablation of malignant tumors. The combination of nanomedicine and engineered bacteria has attracted increasing attention owing to their striking synergistic effects in cancer treatment. Engineered bacteria that function as natural vehicles can effectively deliver nanomedicines to tumor sites. Moreover, bacteria provide an opportunity to enhance nanomedicines by modulating the TME and producing substrates to support nanomedicine-mediated anticancer reactions. Nanomedicine exhibits excellent optical, magnetic, acoustic, and catalytic properties, and plays an important role in promoting bacteria-mediated biotherapies. The synergistic anticancer effects of engineered bacteria and nanomedicines in cancer therapy are comprehensively summarized in this review. Attention is paid not only to the fabrication of nanobiohybrid composites, but also to the interpromotion mechanism between engineered bacteria and nanomedicine in cancer therapy. Additionally, recent advances in engineered bacteria-synergized multimodal cancer therapies are highlighted.

A Review of the Use of Native and Engineered Probiotics for Colorectal Cancer Therapy

Colorectal cancer (CRC) is a serious global health concern, and researchers have been investigating different strategies to prevent, treat, or support conventional therapies for CRC. This review article comprehensively covers CRC therapy involving wild-type bacteria, including probiotics and oncolytic bacteria as well as genetically modified bacteria. Given the close relationship between CRC and the gut microbiota, it is crucial to compile and present a comprehensive overview of bacterial therapies used in the context of colorectal cancer. It is evident that the use of native and engineered probiotics for colorectal cancer therapy necessitates research focused on enhancing the therapeutic properties of probiotic strains.. Genetically engineered probiotics might be designed to produce particular molecules or to target cancer cells more effectively and cure CRC patients.

Emerging therapeutic approaches for peritoneal metastases from gastrointestinal cancers

Peritoneal metastases from gastrointestinal malignancies present difficult management decisions, with options consisting primarily of systemic chemotherapy or major surgery with or without hyperthermic intraperitoneal chemotherapy. Current research is investigating expanding therapeutic modalities, and the aim of this review is to provide an overview of the existing and emerging therapies for the peritoneal metastases from gastrointestinal cancers, primarily through the recent literature (2015 and newer). These include the current data with systemic therapy and cytoreduction with hyperthermic intraperitoneal or pressurized intraperitoneal aerosol chemotherapy, as well as novel promising modalities under investigation, including dominating oncolytic viral therapy and adoptive cellular, biologic, and bacteria therapy, or nanotechnology. The novel diverse strategies, although preliminary and preclinical in murine models, individually and collectively contribute to the treatment of peritoneal metastases, offering hope for improved outcomes and quality of life. We foresee that these evolving treatment approaches will facilitate the transfer of knowledge and data among studies and advance discovery of new drugs and optimized treatments for patients with peritoneal metastases.

Potential role of intratumor bacteria outside the gastrointestinal tract: More than passengers

INTRODUCTION

Tumor-associated bacteria and gut microbiota have gained significant attention in recent years due to their potential role in cancer development and therapeutic response. This review aims to discuss the contributions of intratumor bacteria outside the gastrointestinal tract, in addition to exploring the mechanisms, functions, and implications of these bacteria in cancer therapy.

METHODS

We reviewed current literature on intratumor bacteria and their impact on tumorigenesis, progression, metastasis, drug resistance, and anti-tumor immune modulation. Additionally, we examined techniques used to detect intratumor bacteria, precautions necessary when handling low microbial biomass tumor samples, and the recent progress in bacterial manipulation for tumor treatment.

RESULTS

Research indicates that each type of cancer uniquely interacts with its microbiome, and bacteria can be detected even in non-gastrointestinal tumors with low bacterial abundance. Intracellular bacteria have the potential to regulate tumor cells' biological behavior and contribute to critical aspects of tumor development. Furthermore, bacterial-based anti-tumor therapies have shown promising results in cancer treatment.

CONCLUSIONS

Understanding the complex interactions between intratumor bacteria and tumor cells could lead to the development of more precise cancer treatment strategies. Further research into non-gastrointestinal tumor-associated bacteria is needed to identify new therapeutic approaches and expand our knowledge of the microbiota's role in cancer biology.

Synthetic engineered bacteria for cancer therapy

INTRODUCTION

Cancer mortality worldwide highlights the urgency for advanced therapeutic methods to fill the gaps in conventional cancer therapies. Bacteriotherapy is showing great potential in tumor regression due to the motility and colonization tendencies of bacteria. However, the complicated in vivo environment and tumor pathogenesis hamper the therapeutic outcomes. Synthetic engineering methods endow bacteria with flexible abilities both at the extracellular and intracellular levels to meet treatment requirements. In this review, we introduce synthetic engineering methods for bacterial modifications. We highlight the recent progress in engineered bacteria and explore how these synthetic methods endow bacteria with superior abilities in cancer therapy. The current clinical translations are further discussed. Overall, this review may shed light on the advancement of engineered bacteria for cancer therapy.

AREAS COVERED

Recent progress in synthetic methods for bacterial engineering and specific examples of their applications in cancer therapy are discussed in this review.

EXPERT OPINION

Bacteriotherapy bridges the gaps of conventional cancer therapies through the natural motility and colonization tendency of bacteria, as well as their synthetic engineering. Nevertheless, to fulfill the bacteriotherapy potential and move into clinical trials, more research focusing on its safety concerns should be conducted.

Living Gut Bacteria Functionalized with Gold Nanoclusters and Drug for Facile Cancer Theranostics

Bacbots are potent self-propelling vehicles for targeted therapy that can be guided by chemical and biochemical stimuli of the host. In addition, they can be guided externally by the use of magnetic field or other physical forces. The challenge is to incorporate drugs and diagnostic tools in living bacteria with retention of theranostic activity until reaching the targets and easy clearance of the remainder following the treatment. We report that living Lactobacillus rhamnosus, when functionalized with photoluminescent Au nanoclusters and the anticancer drug methotrexate, was cytotoxic to monolayer and spheroids of cancer cells (HeLa and HT29) even at a low dose of bacteria used (10⁷ cfu/mL). The observed cell death was nearly 90% in HeLa spheroids and 70% in HT29 spheroids. Further, functionalization of the bacterial surface with the nanoclusters helped incorporate the drug onto their cell surfaces. The drug and nanocluster-loaded bacteria annihilated the cells and the spheroids in a rather short time (6 h) that revealed the specificity and effectiveness of the bacbots. The bacbots exhibited synergistic toxicity on the cells as their effect was more than the drug and the bacteria individually. This higher toxicity could be associated with elevated levels of reactive oxygen species generated in the bacbot-treated cells. The multifunctional bacbots reported here provide an option for guided therapy with the natural variant of the human gut-friendly living bacteria without the need for attenuation or genetic modification.

The role of angiogenesis in melanoma: Clinical treatments and future expectations

The incidence of melanoma has increased rapidly over the past few decades, with mortality accounting for more than 75% of all skin cancers. The high metastatic potential of Melanoma is an essential factor in its high mortality. Vascular angiogenic system has been proved to be crucial for the metastasis of melanoma. An in-depth understanding of angiogenesis will be of great benefit to melanoma treatment and may promote the development of melanoma therapies. This review summarizes the recent advances and challenges of anti-angiogenic agents, including monoclonal antibodies, tyrosine kinase inhibitors, human recombinant Endostatin, and traditional Chinese herbal medicine. We hope to provide a better understanding of the mechanisms, clinical research progress, and future research directions of melanoma.

Anti-Tumor Effects of Engineered VNP20009-Abvec-Igκ-mPD-1 Strain in Melanoma Mice via Combining the Oncolytic Therapy and Immunotherapy

Programmed cell death protein 1/Programmed cell death ligand 1 (PD-1/PD-L1) immune checkpoint inhibitors are the most promising treatments for malignant tumors currently, but the low response rate limits their further clinical utilization. To address this problem, our group constructed an engineered strain of VNP20009-Abvec-Igκ-mPD-1 [V-A-mPD-1 (mPD-1, murine PD-1)] to combine oncolytic bacterial therapy with immunotherapy. Further, we evaluated its growth performance and mPD-1 expression ability in vitro while establishing the melanoma mice model to explore its potential anti-cancer effects in tumor therapy. Our results indicated that the V-A-mPD-1 strain has superior growth performance and can invade B16F10 melanoma cells and express PD-1. In addition, in the melanoma mice model, we observed a marked reduction in tumor volume and the formation of a larger necrotic area. V-A-mPD-1 administration resulted in a high expression of mPD-1 at the tumor site, inhibiting tumor cell proliferation via the down-regulation of the expression of rat sarcoma (Ras), phosphorylated mitogen-activated protein kinase (p-MEK)/MEK, and phosphorylated extracellular signal-regulated kinase (p-ERK)/ERK expression significantly inhibited tumor cell proliferation. Tumor cell apoptosis was promoted by down-regulating phosphoinositide 3 kinase (PI3K) and protein kinase B (AKT) signaling pathways, as evidenced by an increased Bcl-2-associated X protein/B cell lymphoma-2 (Bax/Bcl-2) expression ratio. Meanwhile, the expression levels of systemic inflammatory cytokines, such as interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α), were substantially reduced. In conclusion, our research demonstrated that V-A-mPD-1 has an excellent anti-tumor effect, prompting that the combined application of microbial therapy and immunotherapy is a feasible cancer treatment strategy.

A self-guidance biological hybrid drug delivery system driven by anaerobes to inhibit the proliferation and metastasis of colon cancer

Colorectal cancer is often accompanied by multiple organ metastasis. Anaerobic Bifidobacterium Infantis (BI) bacterial can selectively grow in hypoxic colorectal tumor microenvironment (TME), to own the natural advantage of preferentially colorectal tumor targeting. Herein, a self-guidance biological hybrid drug delivery system (BI-ES-FeAlg/DOX) based on BI was constructed to inhibit the proliferation and metastasis of colon cancer. Results demonstrated that BI-ES-FeAlg/DOX could overcome physical barriers to target and accumulate in colon tumor tissues. Then DOX was released to kill tumor cells along with the phase transition (solid to liquid) of FeAlg hydrogel, due to Fe³⁺ was reduced to Fe²⁺by intracellular GSH. Meanwhile, BI-ES selectively colonized into tumors and expressed endostatin (ES) protein to down-regulate VEGF and bFGF expression, exerting anti-angiogenic effect. Moreover, FeAlg catalyzed H₂O₂ in the local tumor to generate cytotoxic ·OH, further enhancing the antitumor effect. The pharmacodynamic result in AOM/DSS model proved that BI-ES-FeAlg/DOX had the best therapeutic effect, with the final V/V₀ of 2.19 ± 0.57, which was significantly lower than the other groups. Meanwhile, on CT-26 tumor-bearing model, it also showed an outstanding anti-tumor effect with inhibition rate of 82.12% ± 3.08%. In addition, lung metastases decreased significantly in tumor metastasis model after BI-ES-FeAlg/DOX treatment.

Microbial cancer therapeutics: A promising approach

The success of conventional cancer therapeutics is hindered by associated dreadful side-effects of antibiotic resistance and the dearth of antitumor drugs' selectivity and specificity. Hence, the conceptual evolution of anti-cancerous therapeutic agents that selectively target cancer cells without impacting the healthy cells or tissues, has led to a new wave of scientific interest in microbial-derived bioactive molecules. Such strategic solutions may pave the way to surmount the shortcomings of conventional therapies and raise the potential and hope for the cure of wide range of cancer in a selective manner. This review aims to provide a comprehensive summary of anti-carcinogenic properties and underlying mechanisms of bioactive molecules of microbial origin, and discuss the current challenges and effective therapeutic application of combinatorial strategies to attain minimal systemic side-effects.

Bacteria-mediated cancer therapy: A versatile bio-sapper with translational potential

Bacteria are important symbionts for humans, which sustain substantial influences on our health. Interestingly, some bastrains have been identified to have therapeutic applications, notably for antitumor activity. Thereby, oncologists have developed various therapeutic models and investigated the potential antitumor mechanisms for bacteria-mediated cancer therapy (BCT). Even though BCT has a long history and exhibits remarkable therapeutic efficacy in pre-clinical animal models, its clinical translation still lags and requires further breakthroughs. This review aims to focus on the established strains of therapeutic bacteria and their antitumor mechanisms, including the stimulation of host immune responses, direct cytotoxicity, the interference on cellular signal transduction, extracellular matrix remodeling, neoangiogenesis, and metabolism, as well as vehicles for drug delivery and gene therapy. Moreover, a brief discussion is proposed regarding the important future directions for this fantastic research field of BCT at the end of this review.

Recombinant Attenuated Salmonella enterica as a Delivery System of Heterologous Molecules in Cancer Therapy

Simple Summary

Cancer is among the main causes of death of millions of individuals worldwide. Although survival has improved with conventional treatments, the appearance of resistant cancer cells leads to patient relapses. It is, therefore, necessary to find new antitumor therapies that can completely eradicate transformed cells. Bacteria-based tumor therapy represents a promising alternative treatment, particularly the use of live-attenuated Salmonella enterica, with its potential use as a delivery system of antitumor heterologous molecules such as tumor-associated antigens, cytotoxic molecules, immunomodulatory molecules, pro-apoptotic proteins, nucleic acids, and nanoparticles. In this review, we present the state of the art of current preclinical and clinical research on the use of Salmonella enterica as a potential therapeutic ally in the war against cancer.

Abstract

Over a century ago, bacterial extracts were found to be useful in cancer therapy, but this treatment modality was obviated for decades. Currently, in spite of the development and advances in chemotherapies and radiotherapy, failure of these conventional treatments still represents a major issue in the complete eradication of tumor cells and has led to renewed approaches with bacteria-based tumor therapy as an alternative treatment. In this context, live-attenuated bacteria, particularly Salmonella enterica, have demonstrated tumor selectivity, intrinsic oncolytic activity, and the ability to induce innate or specific antitumor immune responses. Moreover, Salmonella enterica also has strong potential as a delivery system of tumor-associated antigens, cytotoxic molecules, immunomodulatory molecules, pro-apoptotic proteins, and nucleic acids into eukaryotic cells, in a process known as bactofection and antitumor nanoparticles. In this review, we present the state of the art of current preclinical and clinical research on the use of Salmonella enterica as a potential therapeutic ally in the war against cancer.

Advances in Salmonella Typhimurium-based drug delivery system for cancer therapy

The clinical application of bacteria-mediated immune therapy dates back over a century ago. In recent years, these strategies have advanced greatly with the rapid development of synthetic biology and nanotechnology. Several bacterial therapies have been developed allowing for more effective treatments for cancers, and Salmonella is one of the most studied bacterial species. Here, we review the advances in the bioengineered and functionalized Salmonella Typhimurium strains as drug delivery carries, including the various genetic circuits for programing these bacteria, the surface modification strategies using nanoparticles or other therapeutic agents for richer and broader features, and the bacterial component-based vehicles for cancer immunotherapy. This review will include the promises and challenges of these optimized Salmonella-based delivery systems and their related clinical trials. Ultimately, we hope to provide a spark of thought in the field of drug delivery and find important crosstalk between bacteria-mediated therapy and other different forms of treatments.

Engineered microbial systems for advanced drug delivery

The human body is a natural habitat for a multitude of microorganisms, with bacteria being the major constituent of the microbiota. These bacteria colonize discrete anatomical locations that provide suitable conditions for their survival. Many bacterial species, both symbiotic and pathogenic, interact with the host via biochemical signaling. Based on these attributes, commensal and attenuated pathogenic bacteria have been engineered to deliver therapeutic molecules to target specific diseases. Recent advances in synthetic biology have enabled us to perform complex genetic modifications in live bacteria and bacteria-derived particles, which simulate micron or submicron lipid-based vectors, for the targeted delivery of therapeutic agents. In this review, we highlight various examples of engineered bacteria or bacteria-derived particles that encapsulate, secrete, or surface-display therapeutic molecules for the treatment or prevention of various diseases. The review highlights recent studies on (i) the production of therapeutics by microbial cell factories, (ii) disease-triggered release of therapeutics by sense and respond systems, (iii) bacteria targeting tumor hypoxia, and (iv) bacteria-derived particles as chassis for drug delivery. In addition, we discuss the potential of such drug delivery systems to be translated into clinical therapies.

Hierarchical Passage of Gold Nanoclusters in Living Bacteria

Gold (Au) nanoclusters chemically synthesized on the cell surface of living Lactobacillus rhamnosus rendered them photoluminescent. Importantly, the bacteria were viable and the clusters were passed down the generations with the loss of luminescence in the first subculture onward. The clusters were agglomerated into spherical structures of 100-200 nm, without being converted to plasmonic Au nanoparticles, on the cell surfaces of the bacteria of all six subcultures studied. The results indicated the role of cell wall remodeling in transforming the Au nanoclusters into larger aggregates down the generations. This may hold important implications for using nanoparticle-studded bacteria in theranostics.

Recent Update on Bacteria as a Delivery Carrier in Cancer Therapy: From Evil to Allies

Cancer is associated with a comprehensive burden that significantly affects patient’s quality of life. Even though patients’ disease condition is improving following conventional therapies, researchers are studying alternative tools that can penetrate solid tumours to deliver the therapeutics due to issues of developing resistance by the cancer cells. Treating cancer is not the only the goal in cancer therapy; it also includes protecting non-cancerous cells from the toxic effects of anti-cancer agents. Thus, various advanced techniques, such as cell-based drug delivery, bacteria-mediated therapy, and nanoparticles, are devised for site-specific delivery of drugs. One of the novel methods that can be targeted to deliver anti-cancer agents is by utilising genetically modified non-pathogenic bacterial species. This is due to the ability of bacterial species to multiply selectively or non-selectively on tumour cells, resulting in biofilms that leads to disruption of metastasis process. In preclinical studies, this technology has shown significant results in terms of efficacy, and some are currently under investigation. Therefore, researchers have conducted studies on bacteria transporting the anti-cancer drug to targeted tumours. Alternatively, bacterial ghosts and bacterial spores are utilised to deliver anti-cancer drugs. Although in vivo studies of bacteria-mediated cancer therapy have shown successful outcome, further research on bacteria, specifically their targeting mechanism, is required to establish a complete clinical approach in cancer treatment. This review has focused on the up-to-date understanding of bacteria as a therapeutic carrier in the treatment of cancer as an emerging field.

Optimized Attenuated Salmonella Typhimurium Suppressed Tumor Growth and Improved Survival in Mice

The gram-negative facultative anaerobic bacteria Salmonella enterica serovar Typhimurium (hereafter S. Typhimurium) has always been considered as one candidate of anti-tumor agents or vectors for delivering drug molecules. In this study, we compared several widely studied S. Typhimurium strains in their anti-tumor properties aiming to screen out the best one for further optimization and use in cancer therapy. In terms of the motility, virulence and anti-tumor efficacy, the three strains 14028, SL1344, and UK-1 were similar and obviously better than LT-2, and UK-1 showed the best phenotypes among them. Therefore, the strain UK-1 (D) was selected for the following studies. Its auxotrophic mutant strain (D1) harboring ∆aroA and ∆purM mutations was further optimized through the modification of lipid A structure, generating a new strain named D2 with stronger immunostimulatory activity. Finally, the ∆asd derivative of D2 was utilized as one live vector to deliver anti-tumor molecules including the angiogenesis inhibitor endostatin and apoptosis inducer TRAIL and the therapeutic and toxic-side effects were evaluated in mouse models of colon carcinoma and melanoma. After intraperitoneal infection, engineered Salmonella bacteria equipped with endostatin and/or TRAIL significantly suppressed the tumor growth and prolonged survival of tumor-bearing mice compared to PBS or bacteria carrying the empty plasmid. Consistently, immunohistochemical studies confirmed the colonization of Salmonella bacteria and the expression of anti-tumor molecules inside tumor tissue, which were accompanied by the increase of cell apoptosis and suppression of tumor angiogenesis. These results demonstrated that the beneficial anti-tumor efficacy of attenuated S. Typhimurium bacteria could be improved through delivery of drug molecules with powerful anti-tumor activities.

Bacterial-Based Cancer Therapy (BBCT): Recent Advances, Current Challenges, and Future Prospects for Cancer Immunotherapy

Currently approximately 10 million people die each year due to cancer, and cancer is the cause of every sixth death worldwide. Tremendous efforts and progress have been made towards finding a cure for cancer. However, numerous challenges have been faced due to adverse effects of chemotherapy, radiotherapy, and alternative cancer therapies, including toxicity to non-cancerous cells, the inability of drugs to reach deep tumor tissue, and the persistent problem of increasing drug resistance in tumor cells. These challenges have increased the demand for the development of alternative approaches with greater selectivity and effectiveness against tumor cells. Cancer immunotherapy has made significant advancements towards eliminating cancer. Our understanding of cancer-directed immune responses and the mechanisms through which immune cells invade tumors have extensively helped us in the development of new therapies. Among immunotherapies, the application of bacteria and bacterial-based products has promising potential to be used as treatments that combat cancer. Bacterial targeting of tumors has been developed as a unique therapeutic option that meets the ongoing challenges of cancer treatment. In comparison with other cancer therapeutics, bacterial-based therapies have capabilities for suppressing cancer. Bacteria are known to accumulate and proliferate in the tumor microenvironment and initiate antitumor immune responses. We are currently well-informed regarding various methods by which bacteria can be manipulated by simple genetic engineering or synthetic bioengineering to induce the production of anti-cancer drugs. Further, bacterial-based cancer therapy (BBCT) can be either used as a monotherapy or in combination with other anticancer therapies for better clinical outcomes. Here, we review recent advances, current challenges, and prospects of bacteria and bacterial products in the development of BBCTs.

Innovative Approaches of Engineering Tumor-Targeting Bacteria with Different Therapeutic Payloads to Fight Cancer: A Smart Strategy of Disease Management

Conventional therapies for cancer eradication like surgery, radiotherapy, and chemotherapy, even though most widely used, still suffer from some disappointing outcomes. The limitations of these therapies during cancer recurrence and metastasis demonstrate the need for better alternatives. Some bacteria preferentially colonize and proliferate inside tumor mass; thus these bacteria can be used as ideal candidates to deliver antitumor therapeutic agents. The bacteria like Bacillus spp., Clostridium spp., E. coli, Listeria spp., and Salmonella spp. can be reprogrammed to produce, transport, and deliver anticancer agents, eg, cytotoxic agents, prodrug converting enzymes, immunomodulators, tumor stroma targeting agents, siRNA, and drug-loaded nanoformulations based on clinical requirements. In addition, these bacteria can be genetically modified to express various functional proteins and targeting ligands that can enhance the targeting approach and controlled drug-delivery. Low tumor-targeting and weak penetration power deep inside the tumor mass limits the use of anticancer drug-nanoformulations. By using anticancer drug nanoformulations and other therapeutic payloads in combination with antitumor bacteria, it makes a synergistic effect against cancer by overcoming the individual limitations. The tumor-targeting bacteria can be either used as a monotherapy or in addition with other anticancer therapies like photothermal therapy, photodynamic therapy, and magnetic field therapy to accomplish better clinical outcomes. The toxicity issues on normal tissues is the main concern regarding the use of engineered antitumor bacteria, which requires deeper research. In this article, the mechanism by which bacteria sense tumor microenvironment, role of some anticancer agents, and the recent advancement of engineering bacteria with different therapeutic payloads to combat cancers has been reviewed. In addition, future prospective and some clinical trials are also discussed.

Current status of intratumour microbiome in cancer and engineered exogenous microbiota as a promising therapeutic strategy

Research on the relationship between microbiome and cancer has made significant progress in the past few decades. It is now known that the gut microbiome has multiple effects on tumour biology. However, the relationship between intratumoral bacteria and cancers remains unclear. Growing evidence suggests that intratumoral bacteria are important components of the microenvironment in several types of cancers. Furthermore, several studies have demonstrated that intratumoral bacteria may directly influence tumorigenesis, progression and responses to treatment. Limited studies have been conducted on intratumoral bacteria, and using intratumoral bacteria to treat tumours remains a challenge. Bacteria have been studied as anticancer therapeutics since the 19th century when William B. Coley successfully treated patients with inoperable sarcomas using Streptococcus pyogenes. With the development of synthetic biological approaches, several bacterial species have been genetically engineered to increase their applicability for cancer treatment. Genetically engineered bacteria for cancer therapy have unique properties compared to other treatment methods. They can specifically accumulate within tumours and inhibit cancer growth. In addition, genetically engineered bacteria may be used as a vector to deliver antitumour agents or combined with radiation and chemotherapy to synergise the effectiveness of cancer treatment. However, various problems in treating tumours with genetically engineered bacteria need to be addressed. In this review, we focus on the role of intratumoral bacteria on tumour initiation, progression and responses to chemotherapy or immunotherapy. Moreover, we summarised the recent progress in the treatment of tumours with genetically engineered bacteria.

The Evolution and Future of Targeted Cancer Therapy: From Nanoparticles, Oncolytic Viruses, and Oncolytic Bacteria to the Treatment of Solid Tumors

While many classes of chemotherapeutic agents exist to treat solid tumors, few can generate a lasting response without substantial off-target toxicity despite significant scientific advancements and investments. In this review, the paths of development for nanoparticles, oncolytic viruses, and oncolytic bacteria over the last 20 years of research towards clinical translation and acceptance as novel cancer therapeutics are compared. Novel nanoparticle, oncolytic virus, and oncolytic bacteria therapies all start with a common goal of accomplishing therapeutic drug activity or delivery to a specific site while avoiding off-target effects, with overlapping methodology between all three modalities. Indeed, the degree of overlap is substantial enough that breakthroughs in one therapeutic could have considerable implications on the progression of the other two. Each oncotherapeutic modality has accomplished clinical translation, successfully overcoming the potential pitfalls promising therapeutics face. However, once studies enter clinical trials, the data all but disappears, leaving pre-clinical researchers largely in the dark. Overall, the creativity, flexibility, and innovation of these modalities for solid tumor treatments are greatly encouraging, and usher in a new age of pharmaceutical development.

Bacterial-based cancer therapy: An emerging toolbox for targeted drug/gene delivery

Precise targeting and high therapeutic efficiency are the major requisites of personalized cancer treatment. However, some unique features of the tumor microenvironment (TME) such as hypoxia, low pH and elevated interstitial fluid pressure cause cancer cells resistant to most therapies. Bacteria are increasingly being considered for targeted tumor therapy owing to their intrinsic tumor tropism, high motility as well as the ability to rapidly colonize in the favorable TME. Compared to other nano-strategies using peptides, aptamers, and other biomolecules, tumor-targeting bacteria are largely unaffected by the tumor cells and microenvironment. On the contrary, the hypoxic TME is highly conducive to the growth of facultative anaerobes and obligate anaerobes. Live bacteria can be further integrated with anti-cancer drugs and nanomaterials to increase the latter's targeted delivery and accumulation in the tumors. Furthermore, anaerobic and facultatively anaerobic bacteria have also been combined with other anti-cancer therapies to enhance therapeutic effects. In this review, we have summarized the applications and advantages of using bacteria for targeted tumor therapy (Scheme 1) in order to aid in the design of novel intelligent drug delivery systems. The current challenges and future prospects of tumor-targeting bacterial nanocarriers have also been discussed.

Use of Salmonella Bacteria in Cancer Therapy: Direct, Drug Delivery and Combination Approaches

Over the years, conventional cancer treatments, such as chemotherapy with only a limited specificity for tumors, have undergone significant improvement. Moreover, newer therapies such as immunotherapy have undergone a revolution to stimulate the innate as well as adaptive immune responses against the tumor. However, it has been found that tumors can be selectively colonized by certain bacteria, where they can proliferate, and exert direct oncolytic effects as well as stimulating the immune system. Bacterial-mediated cancer therapy (BMCT) is now one example of a hot topic in the antitumor field. Salmonella typhimurium is a Gram-negative species that generally causes self-limiting gastroenteritis in humans. This species has been designed and engineered in order to be used in cancer-targeted therapeutics. S. typhimurium can be used in combination with other treatments such as chemotherapy or radiotherapy for synergistic modification of the tumor microenvironment. Considerable benefits have been shown by using engineered attenuated strains for the diagnosis and treatment of tumors. Some of these treatment approaches have received FDA approval for early-phase clinical trials. This review summarizes the use of Salmonella bacteria for cancer therapy, which could pave the way towards routine clinical application. The benefits of this therapy include an automatic self-targeting ability, and the possibility of genetic manipulation to produce newly engineered attenuated strains. Nevertheless, Salmonella-mediated anticancer therapy has not yet been clinically established, and requires more research before its use in cancer treatment.

Tweak to Treat: Reprograming Bacteria for Cancer Treatment

Recent advancements in cancer biology, microbiology, and bioengineering have spurred the development of engineered live biotherapeutics for targeted cancer therapy. In particular, natural tumor-targeting and probiotic bacteria have been engineered for controlled and sustained delivery of anticancer agents into the tumor microenvironment (TME). Here, we review the latest advancements in the development of engineered bacteria for cancer therapy and additional engineering strategies to potentiate the delivery of therapeutic payloads. We also explore the use of combination therapies comprising both engineered bacteria and conventional anticancer therapies for addressing intratumor heterogeneity. Finally, we discuss prospects for the development and clinical translation of engineered bacteria for cancer prevention and treatment.

New technologies in developing recombinant-attenuated bacteria for cancer therapy

Cancer has always been a global problem, with more cases of cancer patients being diagnosed every year. Conventional cancer treatments, including radiotherapy, chemotherapy, and surgery, are still unable to bypass their obvious limitations, and developing effective targeted therapies is still required. More than one century ago, the doctor William B. Coley discovered that cancer patients had tumor regression by injection of Streptococcus bacteria. The studies of cancer therapy using bacterial microorganisms are now very widespread. In particular, the facultative anaerobic bacteria Salmonella typhimurium is widely investigated as it can selectively colonize different types of tumors, locally deliver various antitumor drugs, and inhibit tumor growth. The exciting antitumor efficacy and safety observed in animal tumor models prompted the well-known attenuated Salmonella bacterial strain VNP20009 to be tested in human clinical trials in the early 21st century. Regrettably, no patients showed significant therapeutic effects and even bacterial colonization in tumor tissue was undetectable in most patients. Salmonella bacteria are still considered as a promising agent or vehicle for cancer therapy. Recent efforts have been focused on the generation of attenuated bacterial strains with higher targeting for tumor tissue, and optimization of the delivery of therapeutic antitumor cargoes into the tumor microenvironment. This review will summarize new technologies or approaches that may improve bacteria-mediated cancer therapy.

Therapeutic vaccines for colorectal cancer: The progress and future prospect

Cancer vaccines are usually derived from the patient's tumor cells or the antigens found on their surface, which may help the immune system to identify and kill these malignant cells. Current focus of many researches is designing vaccines with the hope of triggering the immune system to attack cancer cells in a more effective, reliable and safe manner. Although colorectal cancer (CRC) is recognized as the third leading cause of death by cancer, but significant advances in therapy strategies have been made in recent years, including cancer vaccine. In this review, we present various vaccine platforms that have been used in the border battle against CRC, some of which have been approved for clinical use and some are in late-stage clinical trials. Until September 2020 there is approximately 1940 clinical trials of cancer vaccines on patients with different cancer types, and also many more trials are in the planning stages, which makes it the most important period of therapeutic cancer vaccines studies in the history of the immunotherapy. In cancer vaccines clinical trials, there are several considerations that must be taken into account including engineering of antigen-presenting cells, potential toxicity of antigenic areas, pharmacokinetics and pharmacodynamics of vaccines, and monitoring of the patients' immune response. Therefore, the need to overcome immunosuppression mechanisms/immune tolerance is a critical step for the success of introducing therapeutic vaccines into the widely used drugs on market. In this way, better understanding of neoantigens, tumor immune surveillance escape mechanisms and host-tumor interactions are required to develop more effective and safe cancer vaccines.

Role of Extracellular Matrix in Gastrointestinal Cancer-Associated Angiogenesis

Gastrointestinal tumors are responsible for more cancer-related fatalities than any other type of tumors, and colorectal and gastric malignancies account for a large part of these diseases. Thus, there is an urgent need to develop new therapeutic approaches to improve the patients' outcome and the tumor microenvironment is a promising arena for the development of such treatments. In fact, the nature of the microenvironment in the different gastrointestinal tracts may significantly influence not only tumor development but also the therapy response. In particular, an important microenvironmental component and a potential therapeutic target is the vasculature. In this context, the extracellular matrix is a key component exerting an active effect in all the hallmarks of cancer, including angiogenesis. Here, we summarized the current knowledge on the role of extracellular matrix in affecting endothelial cell function and intratumoral vascularization in the context of colorectal and gastric cancer. The extracellular matrix acts both directly on endothelial cells and indirectly through its remodeling and the consequent release of growth factors. We envision that a deeper understanding of the role of extracellular matrix and of its remodeling during cancer progression is of chief importance for the development of new, more efficacious, targeted therapies.

Bacteria-cancer interactions: bacteria-based cancer therapy

Recent advances in cancer therapeutics, such as targeted therapy and immunotherapy, have raised the hope for cures for many cancer types. However, there are still ongoing challenges to the pursuit of novel therapeutic approaches, including high toxicity to normal tissue and cells, difficulties in treating deep tumor tissue, and the possibility of drug resistance in tumor cells. The use of live tumor-targeting bacteria provides a unique therapeutic option that meets these challenges. Compared with most other therapeutics, tumor-targeting bacteria have versatile capabilities for suppressing cancer. Bacteria preferentially accumulate and proliferate within tumors, where they can initiate antitumor immune responses. Bacteria can be further programmed via simple genetic manipulation or sophisticated synthetic bioengineering to produce and deliver anticancer agents based on clinical needs. Therapeutic approaches using live tumor-targeting bacteria can be applied either as a monotherapy or in combination with other anticancer therapies to achieve better clinical outcomes. In this review, we introduce and summarize the potential benefits and challenges of this anticancer approach. We further discuss how live bacteria interact with tumor microenvironments to induce tumor regression. We also provide examples of different methods for engineering bacteria to improve efficacy and safety. Finally, we introduce past and ongoing clinical trials involving tumor-targeting bacteria.

Live tumor-targeting bacteria can selectively induce cancer regression and, with the help of genetic engineering, be made safe and effective vehicles for delivering drugs to tumor cells. In a review article, Jung-Joon Min and colleagues from Chonnam National University Medical School in Hwasun, South Korea, discuss the clinical history of using natural or engineered bacterial strains to suppress cancer growth. Because bacteria such as Salmonella and Listeria preferentially home in on tumors or their surrounding microenvironments, researchers have harnessed these microbial agents to attack cancer cells without causing collateral damage to normal tissues. Bioengineers have also armed bacteria with stronger tumor-sensing and more targeted drug delivery capabilities, and improved control of off-target toxicities. An increasing number of therapeutic bacterial strains are now entering clinical testing, promising to enhance the efficacy of more conventional anticancer treatments.

Genetically engineered Salmonella Typhimurium: Recent advances in cancer therapy

Bacteria have been investigated as anti-tumor therapeutic agents for more than a century, since Coley first observed successful curing of a patient with inoperable cancer by injection of streptococcal organisms. Previous studies have demonstrated that some obligate or facultative anaerobes can selectively accumulate and proliferate within tumors and suppress their growth. Developments in molecular biology as well as the complete genome sequencing of many bacterial species have increased the applicability of bacterial organisms for cancer treatment. In particular, the facultative anaerobe Salmonella Typhimurium has been widely studied and genetically engineered to improve its tumor-targeting ability as well as to reduce bacterial virulence. Moreover, the effectiveness of engineered attenuated S. Typhimurium strains employed as live delivery vectors of various anti-tumor therapeutic agents or combined with other therapies has been evaluated in a large number of animal experiments. The well-known S. Typhimurium mutant VNP20009 and its derivative strain TAPET-CD have even been applied in human clinical trials. However, Salmonella-mediated cancer therapies have not achieved the expected success, except in animal experiments. Many problems remain to be solved to exploit more promising strategies for combatting cancer with Salmonella bacteria. Here, we summarize the promising studies regarding cancer therapy mediated by Salmonella bacteria and highlight the main mechanisms of Salmonella anti-tumor activities.