Drug discovery is a resource-intensive process riddled with high failure rates. A lack of good preclinical models often results in identified drug candidates showing poor efficacy in human clinical trials. Cells derived from human-induced pluripotent stem cells (hiPSCs) better recapitulate the pathophysiology of human diseases and are quickly becoming the go-to tool for disease modeling and drug testing.
Drug discovery and development are frequently associated with skyrocketing costs, which are often attributed to the high failure rates in the drug discovery process. For instance, 90% of identified drug candidates do not even pass phase I clinical trials. The main reason is the use of preclinical models that do not properly recapitulate human responses to drugs.
Conventionally used preclinical models include animals, such as rats, rabbits, guinea pigs, non-human primates, including monkeys, and chimpanzees, as well as primary human differentiated cells.
“There is a lack of good preclinical models that accurately predict human drug response,” explained Malathi Raman, Senior Product Manager of Stem Cells at Takara Bio Europe.
“Animal models do not recapitulate the pathophysiology of many human diseases. Also, they cannot reliably detect potential damage to the heart, kidney, or liver because they differ from humans in physiology, immune system, and genetics.”
“While primary human differentiated cells better recapitulate human diseases than animal models, they are limited in supply (ethical concerns), show donor to donor variation, and usually cannot be further expanded in vitro,” Raman added.
The discovery of hiPSCs
In 2006, Japanese researcher Shinya Yamanaka and his team introduced four specific transcription factor genes into adult somatic cells, which reprogrammed them back to an embryonic-like stem cell stage. These cells were named induced pluripotent stem cells (iPSCs) and could be differentiated into any cell type of the human body.
Yamanaka’s discovery opened up a whole new array of possibilities in disease modeling, such as the development of hiPSC-derived pancreatic beta cells or dopaminergic neurons with a disease phenotype from patients with diabetes or Parkinson’s disease, respectively. For this work, Yamanaka was awarded the Nobel Prize for Medicine in 2012.
Prior to the discovery of human iPSCs (hiPSCs), researchers were restricted to using pluripotent human embryonic stem cells, which are problematic due to their limited availability.
“These availability issues are laid to rest with the use of hiPSCs, as they can be reprogrammed from easily accessible adult somatic cells from patients or healthy individuals, and then be differentiated towards the cell type of interest,” explained Raman. Due to their ease of usage and versatility, hiPSCs are quickly becoming the go-to solution for researchers.
How are hiPSCs easing drug discovery?
Since they can be derived from patients with specific diseases, hiPSCs can be used to closely mirror human diseases at a cellular and molecular level, a major advantage in disease modeling and drug discovery.
Moreover, they can also be derived from healthy tissues and cells, such as skin or blood, allowing for comparative studies between diseased and healthy cells and tissue, even within the same individual. Indeed, hiPSCs derived from a specific individual can be differentiated into various different cell types, thus providing a more complex scope of a specific disease.
“Unlike animal models, hiPSCs allow researchers the possibility of conducting in vitro “clinical trials” where drugs can be tested for toxicity and efficacy in differentiated hiPSC-derived cells from specific patients,” Raman explained.
“This results in the identification of effective drugs, which can then be tested and correlated in vivo in the same patients. Such in vitro studies can lead to improved patient classification, lower compound attrition rates, and the identification of safer drugs.”
Patient-derived hiPSCs have also enabled the repurposing of already approved drugs for other diseases that lack or have limited treatment options. “We have already seen this happening for amyotrophic lateral sclerosis, spinal muscular atrophy, and Alzheimer’s disease,” Raman added.
Moreover, while conventional primary human differentiated cells have limited growth capacities in culture, hiPSCs proliferate continuously and can provide an unlimited source of downstream hiPSC-derived differentiated cells. This is especially helpful because this limits donor to donor variability, often seen in datasets generated with primary human differentiated cells.
Modifying hiPSCs with CRISPR
For a number of human diseases, the genetic cause is known. For others, genome-editing tools such as CRISPR-Cas9 allow us to investigate the role of specific genotypes in various diseases.
Using CRISPR, one can edit disease-specific genotypes in the patient-derived hiPSCs to study their role in the development and progression of the disease.
For rare diseases, where patient-derived hiPSCs might not be available, CRISPR can be used to introduce mutations and create “diseased” cell lines that can then be used for further research.
“CRISPR genome editing enables the introduction of site-specific genetic changes in hiPSCs, including the introduction of disease-causing mutations and the correction of disease-causing mutations in normal and diseased cell lines, respectively, via gene knockout/knock-in experiments,” said Raman.
This technique also allows the creation of isogenic (genetically-matched) hiPSC lines – where the genetic backgrounds are the same and the lines only differ in the mutation added or removed using CRISPR. “Isogenic lines provide a highly controlled system in which any phenotypic difference is more likely to result from that specific alteration,” added Raman.
However, hiPSCs also come with some limitations. The generation of hiPSCs can be costly, time-consuming, and small variations in the handling of cultures can lead to reproducibility issues between different labs.
Furthermore, hiPSC-derived cells also tend to display more fetal cell-like characteristics and aren’t fully mature. Comparatively, cells in organ tissues are older and accumulate mature features as they age. This makes hiPSC-derived cells more challenging to use to model late-onset diseases such as Alzheimer’s or Parkinson’s disease. To resolve such issues, cellular aging needs to be induced to replicate disease characteristics in the hiPSC-derived cells.
“While hiPSCs are ideal models for studying disorders caused by mutations in a single gene, complex disorders showing low penetrance and involving aberrant pattern formation of target organs are more challenging to model using hiPSCs,” explained Raman.
To help researchers jump-start their hiPSC-based disease modeling and drug development studies, especially those who don’t have the time or in-house expertise required, Takara Bio has developed a robust hiPSC culture system, off-the-shelf hiPSCs as well as hiPSC-derived differentiated cells that can be used to model organs, such as the heart, liver, intestine, and pancreas.
Moreover, Takara Bio also provides a range of custom stem cell services, including CRISPR-Cas9 gene editing, allowing researchers to partially or completely outsource their hiPSC-based disease modeling projects.
The company has also developed protocols for the industrial-scale production of hiPSC-derived liver cells, known as hepatocytes, suitable for the study of liver diseases such as nonalcoholic steatohepatitis (NASH).
In fact, a team of Swedish and Dutch researchers compared Takara Bio’s hiPSC-derived hepatocytes (hiPS-HEP) with primary hepatocytes and found that although there were some differences, Takara Bio’s hiPS-HEP cells were very similar to the adult primary hepatocytes, possessing many of the same features and functions.
The researchers also found that Takara Bio’s hiPS-HEP cells could be co-cultured in 2D and 3D spheroids with specific liver cells called primary hepatic stellate cells, demonstrating the potential for better NASH disease modeling.
The future of preclinical in vitro models
Along with hiPSCs, other preclinical in vitro models are also evolving. For example, researchers are now using organoids to simulate the 3D complexities of organs in vitro.
Advances are also being made in microfluidics and organ-on-chip technologies that facilitate the scaled-down replication of models, saving valuable resources.
For example, a team of French and Japanese researchers used Takara Bio’s hiPSC-derived Beta cells to create a 3D spheroid-based pancreas on-a-chip model that they used to model pancreatic disease and diabetes, and screen anti-diabetic drugs.
“In the near future, ongoing developments in hiPSC-based models will provide drug companies and researchers with better models to test drug safety and efficacy. This will save valuable time and resources by preventing late-stage clinical trial failures,” concluded Raman.
Author: Ameya Paleja, Freelance Science Writer
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