Emulating the gut-liver axis: Dissecting the microbiome's effect on drug metabolism using multiorgan-on-chip models

The psychostimulant drug, fenethylline (captagon): Health risks, addiction and the global impact of illicit trade

Fenethylline (street name, captagon) is a synthetic amphetamine-type stimulant that is emerging as a significant public health and security concern, particularly in the Middle East. This systematic review synthesizes original research articles, epidemiological studies, systematic reviews, policy analyses, and case reports to provide a comprehensive analysis of fenethylline's health impacts, addiction potential, and dynamics of illicit trade. Initially developed for therapeutic use, fenethylline illicit production and use have escalated, raising concern about its physiological, psychological, and socio-economic impacts. This stimulant profoundly affects the central nervous system, enhancing wakefulness, concentration, and physical stamina while inducing euphoria. These effects come at the cost of serious adverse health outcomes, particularly with prolonged or heavy use, including cardiovascular complications, neurological damage, and addiction. The dependence-forming nature of captagon contributes to escalating substance use disorders, impacting healthcare systems. Beyond its biomedical implications, fenethylline trafficking has become a global issue, with supply chains deeply intertwined with politically unstable regions where illicit economies thrive. The geopolitical dimensions of captagon's trade amplify its global security threat, influencing international relations and regional stability. This paper underscores the urgent need for systematic data collection and coordinated efforts to regulate illicit fenethylline production and distribution. Strategies such as improved surveillance, public health interventions, and international cooperation are essential to mitigate its escalating risks. Addressing this issue requires a multidisciplinary approach, integrating public health, law enforcement, and policy development to curb its impact on global health and security.

Microfluidic organ-on-a-chip models for the gut-liver axis: from structural mimicry to functional insights

The gut-liver axis plays a crucial role in maintaining metabolic balance and overall human health. It orchestrates various processes, such as blood flow, nutrient transfer, metabolite processing, and immune cell communication between the two organs. Traditional methods, such as animal models and two-dimensional (2D) cell cultures, are insufficient in fully replicating the intricate functions of the gut-liver axis. The emergence of microfluidic technology has revolutionized this field, facilitating the development of organ-on-a-chip (OOC) systems. These systems are capable of mimicking the complex structures and dynamic environments of the gut and liver in vitro and incorporating sensors for real-time monitoring. In this article, we review the latest progress in gut-on-a-chip (GOC) and liver-on-a-chip (LOC) systems, as well as the integrated gut-liver-on-a-chip (GLOC) models. Our focus lies in the simulation of physiological parameters, three-dimensional (3D) structural mimicry, microbiome integration, and multicellular co-culture. All these aspects are essential for constructing accurate in vitro models of the gut and liver. Furthermore, we explore the current applications of OOC technology in the study of the gut and liver, including its use in disease modeling, toxicity testing, and drug screening. Finally, we discuss the challenges that remain and outline potential future directions for advancing GOC and LOC development in vitro.

Gut microbiota in health and disease: advances and future prospects

The gut microbiota plays a critical role in maintaining human health, influencing a wide range of physiological processes, including immune regulation, metabolism, and neurological function. Recent studies have shown that imbalances in gut microbiota composition can contribute to the onset and progression of various diseases, such as metabolic disorders (e.g., obesity and diabetes) and neurodegenerative conditions (e.g., Alzheimer's and Parkinson's). These conditions are often accompanied by chronic inflammation and dysregulated immune responses, which are closely linked to specific forms of cell death, including pyroptosis and ferroptosis. Pathogenic bacteria in the gut can trigger these cell death pathways through toxin release, while probiotics have been found to mitigate these effects by modulating immune responses. Despite these insights, the precise mechanisms through which the gut microbiota influences these diseases remain insufficiently understood. This review consolidates recent findings on the impact of gut microbiota in these immune-mediated and inflammation-associated conditions. It also identifies gaps in current research and explores the potential of advanced technologies, such as organ-on-chip models and the microbiome-gut-organ axis, for deepening our understanding. Emerging tools, including single-bacterium omics and spatial metabolomics, are discussed for their promise in elucidating the microbiota's role in disease development.

Recent Advances in Gut- and Gut-Organ-Axis-on-a-Chip Models

The human gut extracts nutrients from the diet while forming the largest barrier against the outer environment. In addition, the gut actively maintains homeostasis through intricate interactions with the gut microbes, the immune system, the enteric nervous system, and other organs. These interactions influence digestive health and, furthermore, play crucial roles in systemic health and disease. Given its primary role in absorbing and metabolizing orally administered drugs, there is significant interest in the development of preclinical in vitro model systems that can accurately emulate the intestine in vivo. A gut-on-a-chip system holds great potential as a testing and screening platform because of its ability to emulate the physiological aspects of in vivo tissues and expandability to incorporate and combine with other organs. This review aims to identify the key physiological features of the human gut that need to be incorporated to build more accurate preclinical models and highlights the recent progress in gut-on-a-chip systems and competing technologies toward building more physiologically relevant preclinical model systems. Furthermore, various efforts to construct multi-organ systems with the gut, called gut-organ-axis-on-a-chip models, are discussed. In vitro gut models with physiological relevance can provide valuable platforms for bridging the gap between preclinical and clinical studies.

An Organ-on-Chip Platform for Simulating Drug Metabolism Along the Gut-Liver Axis

The human microbiome significantly influences drug metabolism through the gut-liver axis, leading to modified drug responses and potential toxicity. Due to the complex nature of the human gut environment, the understanding of microbiome-driven impacts on these processes is limited. To address this, a multiorgan-on-a-chip (MOoC) platform that combines the human microbial-crosstalk (HuMiX) gut-on-chip (GoC) and the Dynamic42 liver-on-chip (LoC), mimicking the bidirectional interconnection between the gut and liver known as the gut-liver axis, is introduced. This platform supports the viability and functionality of intestinal and liver cells. In a proof-of-concept study, the metabolism of irinotecan, a widely used colorectal cancer drug, is imitated within the MOoC. Utilizing liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), irinotecan metabolites are tracked, confirming the platform's ability to represent drug metabolism along the gut-liver axis. Further, using the authors' gut-liver platform, it is shown that the colorectal cancer-associated gut bacterium, Escherichia coli, modifies irinotecan metabolism through the transformation of its inactive metabolite SN-38G into its toxic metabolite SN-38. This platform serves as a robust tool for investigating the intricate interplay between gut microbes and pharmaceuticals, offering a representative alternative to animal models and providing novel drug development strategies.

Liver-on-chips for drug discovery and development

Recent FDA modernization act 2.0 has led to increasing industrial R&D investment in advanced in vitro 3D models such as organoids, spheroids, organ-on-chips, 3D bioprinting, and in silico approaches. Liver-related advanced in vitro models remain the prime area of interest, as liver plays a central role in drug clearance of compounds. Growing evidence indicates the importance of recapitulating the overall liver microenvironment to enhance hepatocyte maturity and culture longevity using liver-on-chips (LoC) in vitro. Hence, pharmaceutical industries have started exploring LoC assays in the two of the most challenging areas: accurate in vitro-in vivo extrapolation (IVIVE) of hepatic drug clearance and drug-induced liver injury. We examine the joint efforts of commercial chip manufacturers and pharmaceutical companies to present an up-to-date overview of the adoption of LoC technology in the drug discovery. Further, several roadblocks are identified to the rapid adoption of LoC assays in the current drug development framework. Finally, we discuss some of the underexplored application areas of LoC models, where conventional 2D hepatic models are deemed unsuitable. These include clearance prediction of metabolically stable compounds, immune-mediated drug-induced liver injury (DILI) predictions, bioavailability prediction with gut-liver systems, hepatic clearance prediction of drugs given during pregnancy, and dose adjustment studies in disease conditions. We conclude the review by discussing the importance of PBPK modeling with LoC, digital twins, and AI/ML integration with LoC.

Integration of multiple flexible electrodes for real-time detection of barrier formation with spatial resolution in a gut-on-chip system

In healthy individuals, the intestinal epithelium forms a tight barrier to prevent gut bacteria from reaching blood circulation. To study the effect of probiotics, dietary compounds and drugs on gut barrier formation and disruption, human gut epithelial and bacterial cells can be cocultured in an in vitro model called the human microbial crosstalk (HuMiX) gut-on-a-chip system. Here, we present the design, fabrication and integration of thin-film electrodes into the HuMiX platform to measure transepithelial electrical resistance (TEER) as a direct readout on barrier tightness in real-time. As various aspects of the HuMiX platform have already been set in their design, such as multiple compressible layers, uneven surfaces and nontransparent materials, a novel fabrication method was developed whereby thin-film metal electrodes were first deposited on flexible substrates and sequentially integrated with the HuMiX system via a transfer-tape approach. Moreover, to measure localized TEER along the cell culture chamber, we integrated multiple electrodes that were connected to an impedance analyzer via a multiplexer. We further developed a dynamic normalization method because the active measurement area depends on the measured TEER levels. The fabrication process and system setup can be applicable to other barrier-on-chip systems. As a proof-of-concept, we measured the barrier formation of a cancerous Caco-2 cell line in real-time, which was mapped at four spatially separated positions along the HuMiX culture area.

Buds and Bugs: A Fascinating Tale of Gut Microbiota and Cannabis in the Fight against Cancer

Emerging research has revealed a complex bidirectional interaction between the gut microbiome and cannabis. Preclinical studies have demonstrated that the gut microbiota can significantly influence the pharmacological effects of cannabinoids. One notable finding is the ability of the gut microbiota to metabolise cannabinoids, including Δ9-tetrahydrocannabinol (THC). This metabolic transformation can alter the potency and duration of cannabinoid effects, potentially impacting their efficacy in cancer treatment. Additionally, the capacity of gut microbiota to activate cannabinoid receptors through the production of secondary bile acids underscores its role in directly influencing the pharmacological activity of cannabinoids. While the literature reveals promising avenues for leveraging the gut microbiome-cannabis axis in cancer therapy, several critical considerations must be accounted for. Firstly, the variability in gut microbiota composition among individuals presents a challenge in developing universal treatment strategies. The diversity in gut microbiota may lead to variations in cannabinoid metabolism and treatment responses, emphasising the need for personalised medicine approaches. The growing interest in understanding how the gut microbiome and cannabis may impact cancer has created a demand for up-to-date, comprehensive reviews to inform researchers and healthcare practitioners. This review provides a timely and invaluable resource by synthesizing the most recent research findings and spotlighting emerging trends. A thorough examination of the literature on the interplay between the gut microbiome and cannabis, specifically focusing on their potential implications for cancer, is presented in this review to devise innovative and effective therapeutic strategies for managing cancer.

Short chain fatty acids: key regulators of the local and systemic immune response in inflammatory diseases and infections

The human intestinal microbiome substantially affects human health and resistance to infections in its dynamic composition and varying release of microbial-derived metabolites. Short-chain fatty acids (SCFA) produced by commensal bacteria through fermentation of indigestible fibres are considered key regulators in orchestrating the host immune response to microbial colonization by regulating phagocytosis, chemokine and central signalling pathways of cell growth and apoptosis, thereby shaping the composition and functionality of the intestinal epithelial barrier. Although research of the last decades provided valuable insight into the pleiotropic functions of SCFAs and their capability to maintain human health, mechanistic details on how SCFAs act across different cell types and other organs are not fully understood. In this review, we provide an overview of the various functions of SCFAs in regulating cellular metabolism, emphasizing the orchestration of the immune response along the gut-brain, the gut-lung and the gut-liver axes. We discuss their potential pharmacological use in inflammatory diseases and infections and highlight new options of relevant human three-dimensional organ models to investigate and validate their biological functions in more detail.

Gut-on-a-chip models for dissecting the gut microbiology and physiology

Microfluidic technologies have been extensively investigated in recent years for developing organ-on-a-chip-devices as robust in vitro models aiming to recapitulate organ 3D topography and its physicochemical cues. Among these attempts, an important research front has focused on simulating the physiology of the gut, an organ with a distinct cellular composition featuring a plethora of microbial and human cells that mutually mediate critical body functions. This research has led to innovative approaches to model fluid flow, mechanical forces, and oxygen gradients, which are all important developmental cues of the gut physiological system. A myriad of studies has demonstrated that gut-on-a-chip models reinforce a prolonged coculture of microbiota and human cells with genotypic and phenotypic responses that closely mimic the in vivo data. Accordingly, the excellent organ mimicry offered by gut-on-a-chips has fueled numerous investigations on the clinical and industrial applications of these devices in recent years. In this review, we outline various gut-on-a-chip designs, particularly focusing on different configurations used to coculture the microbiome and various human intestinal cells. We then elaborate on different approaches that have been adopted to model key physiochemical stimuli and explore how these models have been beneficial to understanding gut pathophysiology and testing therapeutic interventions.

Impact of tuberculosis disease on human gut microbiota: a systematic review

INTRODUCTION

This systematic review evaluates the gut microbiota (GM) status in tuberculosis (TB) patients compared to healthy volunteers due to the disease or its treatment.

AREAS COVERED

We conducted a systematic review of all articles published in PubMed, Web of Science, and Embase that assessed the impact of TB disease and anti-tubercular therapy (ATT) on GM from inception till January 2022 (Protocol registration number in PROSPERO: CRD42021261884). Regarding the microbial diversity indices and taxonomy, we found a significant difference in GM status between the TB and healthy control (HC) groups. We found an overabundance of Phylum Proteobacteria and depletion of some short-chain fatty acid-producing bacteria genera like Bifidobacteria, Roseburia, and Ruminococcus in the TB group. We found that ATT exacerbates the degree of dysbiosis caused by Mycobacteria tuberculosis disease.

EXPERT OPINION

The modulation of GM in TB patients in clinical practice may serve as a promising target to reverse the dysbiosis caused. Moreover, this can optimistically change the TB treatment outcome. We expect that appropriate probiotic supplementation with antimycobacterial treatment during tuberculosis disease will help stabilize the GM throughout the treatment phase and protect the GM from dysbiosis.

Gut-on-a-chip for disease models

The intestinal tract is a vital organ responsible for digestion and absorption in the human body and plays an essential role in pathogen invasion. Compared with other traditional models, gut-on-a-chip has many unique advantages, and thereby, it can be considered as a novel model for studying intestinal functions and diseases. Based on the chip design, we can replicate the in vivo microenvironment of the intestine and study the effects of individual variables on the experiment. In recent years, it has been used to study several diseases. To better mimic the intestinal microenvironment, the structure and function of gut-on-a-chip are constantly optimised and improved. Owing to the complexity of the disease mechanism, gut-on-a-chip can be used in conjunction with other organ chips. In this review, we summarise the human intestinal structure and function as well as the development and improvement of gut-on-a-chip. Finally, we present and discuss gut-on-a-chip applications in inflammatory bowel disease (IBD), viral infections and phenylketonuria. Further improvement of the simulation and high throughput of gut-on-a-chip and realisation of personalised treatments are the problems that should be solved for gut-on-a-chip as a disease model.

Organs-on-Chips Platforms Are Everywhere: A Zoom on Biomedical Investigation

Over the decades, conventional in vitro culture systems and animal models have been used to study physiology, nutrient or drug metabolisms including mechanical and physiopathological aspects. However, there is an urgent need for Integrated Testing Strategies (ITS) and more sophisticated platforms and devices to approach the real complexity of human physiology and provide reliable extrapolations for clinical investigations and personalized medicine. Organ-on-a-chip (OOC), also known as a microphysiological system, is a state-of-the-art microfluidic cell culture technology that sums up cells or tissue-to-tissue interfaces, fluid flows, mechanical cues, and organ-level physiology, and it has been developed to fill the gap between in vitro experimental models and human pathophysiology. The wide range of OOC platforms involves the miniaturization of cell culture systems and enables a variety of novel experimental techniques. These range from modeling the independent effects of biophysical forces on cells to screening novel drugs in multi-organ microphysiological systems, all within microscale devices. As in living biosystems, the development of vascular structure is the salient feature common to almost all organ-on-a-chip platforms. Herein, we provide a snapshot of this fast-evolving sophisticated technology. We will review cutting-edge developments and advances in the OOC realm, discussing current applications in the biomedical field with a detailed description of how this technology has enabled the reconstruction of complex multi-scale and multifunctional matrices and platforms (at the cellular and tissular levels) leading to an acute understanding of the physiopathological features of human ailments and infections in vitro.

Real-time monitoring of immediate drug response and adaptation upon repeated treatment in a microfluidic chip system

Microfluidic tissue culture and organ-on-a-chip models provide efficient tools for drug testing in vivo and are considered to become the basis of in vitro test systems to analyze drug response, drug interactions and toxicity to complement and reduce animal testing. A major limitation is the efficient recording of drug action. Here we present an efficient experimental setup that allows long-term cultivation of cells in a microfluidic system in combination with continuous recording of luciferase reporter gene expression. The system combines a sensitive cooled luminescence camera system in combination with a custom build miniaturized incubation chamber. The setup allows to monitor time-dependent activation, but also the end of drug response. Repeated activation and recovery as well as varying durations of drug treatment periods can be monitored, and different modes of drug activity can be visualized.