Articles
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Current in vitro cell models—such as animal cells, tumor cell lines, and cadaveric tissue—do not truly reflect human biology and have significant limitations in reproducibility and/or availability. These limitations have resulted in high rates of depreciation of drug candidates as they proceed through development. Often, toxicities emerge when new drug candidates enter clinical trials. In some cases, such as terfenidine and cisapride, toxicities emerged after market launch. Such scenarios result in wasted time and money for drug companies, as well as potentially serious adverse events in patients.
The establishment of human embryonic stem cell (ESC) lines in 19981 and the creation of human induced pluripotent stem cell (iPSC) lines in 20072,3 provided an opportunity to develop novel human cell models for use in disease modeling, drug discovery, and toxicity testing. ESC- and iPSC-derived model systems offer the potential to provide a better understanding of drug behavior in the earlier, less expensive preclinical stages of drug development.
Both ESCs and iPSCs have the capacity to differentiate into any cell type in the body. However, iPSCs have significant advantages over ESCs for research applications. Because iPSCs can be derived from tissue samples from adult donors, these cells avoid the social and political issues associated with embryonic stem cells. Additionally, iPSCs provide researchers access to any cell type from any individual, enabling use of human cell models from a human population whose phenotypes and genotypes can be identified.
Because iPSCs can be derived from any individual, specific populations—either ethnic groups or disease populations—can be studied. An example is the creation of patient group-specific iPSCs, differentiating them into the relevant terminal cell type, and then using the cells in drug screens to identify compounds that alter the disease phenotype of that particular patient group.
Originally, human samples for reprogramming into iPSCs utilized skin fibroblasts as the source of starting material. This reprogramming method remains the most common method today. However, recent work demonstrated that samples of human peripheral blood are a viable alternative.4,5,6 In addition to blood samples being a much more convenient collection method than punch biopsies, there are huge stores of human blood samples that could be used to create iPSCs. It is likely that blood will become the dominant source material in the future.
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Historically, a significant limitation of iPSC technology had been the low efficiency of the reprogramming process as well as process bottlenecks associated with large-scale production of iPSCs and differentiation into terminal cell types. Although research had been performed on the limited quantities of these cells, the quantities were not sufficient for large-scale experiments or for the high-throughput processes employed by pharmaceutical companies in their drug discovery and toxicity testing work. This limitation has been overcome, and iPSCs and their derivatives are now available commercially in the quantity as well as the quality and high level of purity required for these experiments to be executed.
iPSC-derived cell models
The iCell Cardiomyocytes from Cellular Dynamics International (CDI) are a pan population of human heart cells derived from iPSCs for use in disease modeling, drug discovery, and toxicity testing. Validation research completed by CDI and pharmaceutical company scientists have demonstrated that these cells possess the electrical activity expected of human atrial, nodal, and ventricular heart cardiomyocytes. They also showed that these cells respond pharmacologically to a variety of compounds as would be predicted in vivo.
An emerging area of research is using cardiomyocytes to study human disease. For example, the cells can be induced to express a phenotype associated with cardiac hypertrophy. Cells exposed to endothelin-1 show greatly increased induction of mRNA markers, natriuretic peptide precursor A (NPPA), and natriuretic peptide precursor B (NPPB), which are associated with cardiac hypertrophic disease in humans.7 These findings show the potential of human iPSC-derived cells as models for human disease.
iPSC-derived cardiomyocytes could also be developed from patients with family histories of a variety of cardiomyopathies. There are several reports of monogenic cardiac disorders being recapitulated in vitro with iPSC-derived cardiomyocytes.8,9 The indication by genome-wide association studies that left ventricular hypertrophy has a significant genetic component suggests that iPSC-derived cardiomyocytes may also be used to investigate complex, multigenic diseases as well.10 In both instances, cardiomyocytes from both patients and unaffected family members can be used to study the underlying molecular mechanisms of the disease. With iPSC technology, these studies can be conducted with a large number of samples. Other than harvesting cardiac biopsies from these patients, no other technology can enable such research.
In addition to cardiotoxicity, hepatotoxicity is another top-cited cause of drug failure during clinical development.11 Currently, work is in progress to develop human iPSC-derived hepatocytes that exhibit the same phenotypic properties as native hepatocytes.
Other human iPSC types, including neurons and endothelial cells (cells that compose the luminal lining of blood vessels), are in development at CDI and are expected to be available soon.
Conclusion
Pharmaceutical companies are looking for ways to improve their research and development hit rate, lower the cost of research, and reduce pipeline attrition during drug development. The use of iPSC-derived human cell models for in vitro testing will likely have a significant positive impact on these activities. In addition, a reliable and reproducible supply of non-cadaveric and non-immortalized human cells is key not only to drug development, but also to the understanding of human disease.
About the Author
Vanessa Ott earned a doctorate in cellular and molecular biology and conducted postdoctoral training in immunology. She has held faculty research positions at The Ohio State University and the University of Wisconsin-Madison.
Blake Anson received a doctorate in neuroscience and conducted postdoctoral training in molecular genetics. He was an assistant scientist in cardiovascular research under Craig T. January at the University of Wisconsin Medical School.
References
1. Thomson JA, et al. Embryonic stem cell lines derived from human blastocysts. Science. 1998;282(5391):1145-1147.
2. Yu J, et al. Induced pluripotent stem cell lines derived from human somatic cells. Science. 2007;318(5858):1917-1920.
3. Takahashi K, et al. Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell. 2007;131(5):861-872.
4. Brown ME, et al. Derivation of induced pluripotent stem cells from human peripheral blood T lymphocytes. PLoS ONE. 2010;5(6):1-9.
5. Kunisato A, et al. Direct generation of induced pluripotent stem cells from human nonmobilized blood. Stem Cells Development. 2011;20(1):159-168.
6. Rajesh D, et al. Human lymphoblastoid B cell lines reprogrammed to EBV-free induced pluripotent stem cells. Blood. 2011 Jun 27 (epub ahead of print).
7. Foldes G, et al. Modulation of human embryonic stem cell-derived cardiomyocyte growth: a testbed for studying human cardiac hypertrophy? J Mol Cell Cardiol. 2011: 50(2):367-76.
8. Carvajal-Vergara X, et al. Patient-specific induced pluripotent stem-cell-derived models of LEOPARD syndrome. Nature. 2010:465(7299):808-12.
9. Moretti A, et al. Patient-specific induced pluripotent stem-cell models for long-QT syndrome. N Engl J Med. 2010:363(15):1397-409.
10. Arnett DK, et al. Genome-wide association study identifies single-nucleotide polymorphism in KCNB1 associated with left ventricular mass in humans: the HyperGEN Study, BMC Med Genet. 2009;10:43.
11. Schuester D, et al. Why drugs fail – a study on side effects in new chemical entities. Curr Pharm Des. 2005;11(27):3545-3559.
