Trust Your Gut: Screening Drugs Using Novel Intestinal Models

During drug development, researchers need to perform adequate nonclinical testing to ensure a drug’s safety and efficacy before starting clinical trials. An important aspect they must evaluate is a drug’s pharmacokinetic properties, which explain how the compound will travel through the body to exert its activity including its absorption, distribution, metabolism, and excretion (ADME).

The gastrointestinal (GI) tract is an important system for studying a drug’s ADME characteristics because it is the main absorption site for orally administered drugs.¹ As a consequence, clinicians commonly observe drug-induced adverse gastrointestinal events within patients enrolled in clinical trials.² Although scientists use animal and in vitro models during nonclinical development to assess for adverse events, most GI adverse events are not discovered until human clinical trials.

Difficulties with Current Nonclinical Intestinal Models

Animal models

Researchers mostly rely on animal models including rodents, dogs, pigs, and non-human primates for ADME and toxicity evaluations because they allow scientists to assess a drug’s effect in a complex system that integrates multiple organs and tissue types.³ However, researchers are often unable to accurately extrapolate ADME and toxicity data acquired using animal models to humans due to differences in anatomy, physiology, diet, and microbiome. This major drawback has led scientists to develop better human in vitro models. 

In vitro models

Within the human GI tract, stem cells located in the intestinal crypts replenish the various cell types that make up the epithelial layer every three to four days.⁴ Because differentiated intestinal cells have a short lifespan, it is difficult for researchers to culture mature primary epithelial cells in vitro.⁵ Instead, many use immortalized human cell lines, such as Caco-2, to study barrier integrity and drug transportation across the intestinal epithelia. Caco-2 cells resemble mature enterocytes when grown as a monolayer due to their polarity, absorption capabilities, and expression of some of the tight junctions, transporters, and enzymes found in vivo.⁶ However, scientists derived the Caco-2 cell line from a colon carcinoma tumor and, consequently, the model displays properties not associated with healthy intestinal epithelia. Additionally, Caco-2 monocultures prevent researchers from evaluating the drug’s effect on other intestinal cell types, such as goblet cells or enteroendocrine cells.

When scientists culture intestinal stem cells isolated from native intestinal crypts in the presence of extracellular matrix proteins and growth factors, the cells differentiate and form organoids, where their internal cavity represents the intestinal lumen.⁷ These three-dimensional in vitro models replicate the functionality and structure of intestines in vivo better than Caco-2 monocultures and enable researchers to test a drug’s effect on multiple cell types. However, the organoid’s apical surface faces the internal cavity, which forces scientists to compromise the epithelial layer when injecting drugs into its lumen. This prevents them from testing barrier integrity and from performing high-throughput analyses. Researchers can circumvent these problems by reversing the organoid’s polarity, such that the apical surface faces towards the culture medium. Nevertheless, scientists are unable to easily access the apical and basolateral surfaces simultaneously when using organoids in either confirmation. 

Generating an Advanced In Vitro Intestinal Model 

Despite the use of these available models during nonclinical testing, scientists are often unable to detect drug-induced gastrointestinal toxicity or accurately assess its absorption until they perform clinical trials, likely because the models are not physiologically relevant, predictive of the human reaction, or conducive to high-throughput screening. Accordingly, researchers are still in search of better model systems to avoid late-stage clinical failures.

The RepliGut® Systems from Altis Biosystems are a group of in vitro, nonclinical models that replicate the human intestinal epithelia. Like organoids, these systems employ human donor-extracted intestinal stem cells, which are seeded onto scaffold-coated, semi-permeable membranes to form monolayers. Upon differentiation, researchers obtain the major intestinal cell lineages at the same proportion found within the body. Because of the systems’ experimental set-up, scientists can easily access both the basolateral and apical surfaces of the intestinal epithelium, which is a major limitation of 3D organoid cultures.⁸ Additionally, Altis Biosystems has extracted stem cells from several regions of the GI tract and from multiple donors. This allows researchers to investigate drug toxicity and absorption, as well as the integrity of the epithelial cell barrier, across different regions and donors. These physiologically-relevant in vitro models are simple to use, compatible with high-throughput analysis, and save scientists time and money, thus, accelerating nonclinical research.