Recent Progress in the Molecular Imaging of Tumor-Treating Bacteria

Exploiting bacteria for cancer immunotherapy

Immunotherapy has revolutionized the treatment of cancer but continues to be constrained by limited response rates, acquired resistance, toxicities and high costs, which necessitates the development of new, innovative strategies. The discovery of a connection between the human microbiota and cancer dates back 4,000 years, when local infection was observed to result in tumour eradication in some individuals. However, the true oncological relevance of the intratumoural microbiota was not recognized until the turn of the twentieth century. The intratumoural microbiota can have pivotal roles in both the pathogenesis and treatment of cancer. In particular, intratumoural bacteria can either promote or inhibit cancer growth via remodelling of the tumour microenvironment. Over the past two decades, remarkable progress has been made preclinically in engineering bacteria as agents for cancer immunotherapy; some of these bacterial products have successfully reached the clinical stages of development. In this Review, we discuss the characteristics of intratumoural bacteria and their intricate interactions with the tumour microenvironment. We also describe the many strategies used to engineer bacteria for use in the treatment of cancer, summarizing contemporary data from completed and ongoing clinical trials. The work described herein highlights the potential of bacteria to transform the landscape of cancer therapy, bridging ancient wisdom with modern scientific innovation.

Bacterial imaging in tumour diagnosis

Some bacteria, such as Escherichia coli (E. coli) and Salmonella typhimurium (S. typhimurium), have an inherent ability to locate solid tumours, making them a versatile platform that can be combined with other tools to improve the tumour diagnosis and treatment. In anti-cancer therapy, bacteria function by carrying drugs directly or expressing exogenous therapeutic genes. The application of bacterial imaging in tumour diagnosis, a novel and promising research area, can indeed provide dynamic and real-time monitoring in both pre-treatment assessment and post-treatment detection. Different imaging techniques, including optical technology, acoustic imaging, magnetic resonance imaging (MRI) and nuclear medicine imaging, allow us to observe and track tumour-associated bacteria. Optical imaging, including bioluminescence and fluorescence, provides high-sensitivity and high-resolution imaging. Acoustic imaging is a real-time and non-invasive imaging technique with good penetration depth and spatial resolution. MRI provides high spatial resolution and radiation-free imaging. Nuclear medicine imaging, including positron emission tomography (PET) and single photon emission computed tomography (SPECT) can provide information on the distribution and dynamics of bacterial population. Moreover, strategies of synthetic biology modification and nanomaterial engineering modification can improve the viability and localization ability of bacteria while maintaining their autonomy and vitality, thus aiding the visualization of gut bacteria. However, there are some challenges, such as the relatively low bacterial abundance and heterogeneously distribution within the tumour, the high dimensionality of spatial datasets and the limitations of imaging labeling tools. In summary, with the continuous development of imaging technology and nanotechnology, it is expected to further make in-depth study on tumour-associated bacteria and develop new bacterial imaging methods for tumour diagnosis.

Advancements in Positron Emission Tomography Detectors: From Silicon Photomultiplier Technology to Artificial Intelligence Applications

This review article focuses on PET detector technology, which is the most crucial factor in determining PET image quality. The article highlights the desired properties of PET detectors, including high detection efficiency, spatial resolution, energy resolution, and timing resolution. Recent advancements in PET detectors to improve these properties are also discussed, including the use of silicon photomultiplier technology, advancements in depth-of-interaction and time-of-flight PET detectors, and the use of artificial intelligence for detector development. The article provides an overview of PET detector technology and its recent advancements, which can significantly enhance PET image quality.

Engineered live bacteria as disease detection and diagnosis tools

Sensitive and minimally invasive medical diagnostics are essential to the early detection of diseases, monitoring their progression and response to treatment. Engineered bacteria as live sensors are being developed as a new class of biosensors for sensitive, robust, noninvasive, and in situ detection of disease onset at low cost. Akin to microrobotic systems, a combination of simple genetic rules, basic logic gates, and complex synthetic bioengineering principles are used to program bacterial vectors as living machines for detecting biomarkers of diseases, some of which cannot be detected with other sensing technologies. Bacterial whole-cell biosensors (BWCBs) can have wide-ranging functions from detection only, to detection and recording, to closed-loop detection-regulated treatment. In this review article, we first summarize the unique benefits of bacteria as living sensors. We then describe the different bacteria-based diagnosis approaches and provide examples of diagnosing various diseases and disorders. We also discuss the use of bacteria as imaging vectors for disease detection and image-guided surgery. We conclude by highlighting current challenges and opportunities for further exploration toward clinical translation of these bacteria-based systems.

Imaging Tumor-Targeting Bacteria Using 18F-Fluorodeoxysorbitol Positron Emission Tomography

BACKGROUND

Microbial-based cancer treatments are an emerging field, with multiple bacterial species evaluated in animal models and some advancing to clinical trials. Noninvasive bacteria-specific imaging approaches can potentially support the development and clinical translation of bacteria-based cancer treatments by assessing the tumor and off-target bacterial colonization.

METHODS

18F-Fluorodeoxysorbitol (18F-FDS) positron emission tomography (PET), a bacteria-specific imaging approach, was used to visualize an attenuated strain of Yersinia enterocolitica, currently in clinical trials as a microbial-based cancer treatment, in murine models of breast cancer.

RESULTS

Y. enterocolitica demonstrated excellent 18F-FDS uptake in in vitro assays. Whole-body 18F-FDS PET demonstrated a significantly higher PET signal in tumors with Y. enterocolitica colonization compared to those not colonized, in murine models utilizing direct intratumor or intravenous administration of bacteria, which were confirmed using ex vivo gamma counting. Conversely, 18F-fluorodeoxyglucose (18F-FDG) PET signal was not different in Y. enterocolitica colonized versus uncolonized tumors.

CONCLUSIONS

Given that PET is widely used for the management of cancer patients, 18F-FDS PET could be utilized as a complementary approach supporting the development and clinical translation of Y. enterocolitica-based tumor-targeting bacterial therapeutics.

Engineering bacteria as interactive cancer therapies

With increasing evidence that microbes colonize tumors, synthetic biology tools are being leveraged to repurpose bacteria as tumor-specific delivery systems. These engineered systems can modulate the tumor microenvironment using a combination of their inherent immunogenicity and local payload production. Here, we review genetic circuits that enhance spatial and temporal control of therapeutic bacteria to improve their safety and efficacy. We describe the engineering of interactions among bacteria, tumor cells, and immune cells, and the progression from bacteria as single agents toward their rational combination with other modalities. Together, these efforts are building toward an emerging concept of engineering interactions between programmable medicines using synthetic biology.

Progress of engineered bacteria for tumor therapy

Recently, with the rapid development of bioengineering technology and nanotechnology, natural bacteria were modified to change their physiological activities and therapeutic functions for improved therapeutic efficiency of diseases. These engineered bacteria were equipped to achieve directed genetic reprogramming, selective functional reorganization and precise spatio-temporal control. In this review, research progress in the basic modification methodologies of engineered bacteria were summarized, and representative researches about their therapeutic performances for tumor treatment were illustrated. Moreover, the strategies for the construction of engineered colonies based on engineering of individual bacteria were summarized, providing innovative ideas for complex functions and efficient anti-tumor treatment. Finally, current limitation and challenges of tumor therapy utilizing engineered bacteria were discussed.