Stem Cell

These are the basic reasons for using iPSCs for drug discovery.

• iPSCs have led to the advancement of numerous high throughput and combinatorial screening technologies thus supplementing the role of stem cell models in drug discovery.
• iPSCs based methodology is complementarity to drug discovery or prediction of toxicity via animal models. Animals or in-vitro animal derived cells are used as testing systems but are limited by their inability to replicate the “exact” human physiological conditions and related phenotypic attributions.
• Animal models are not good testing models for drug toxicity as a chemical may be toxic to an animal but may not be toxic to a different animal. For carcinogenic agents as well, different agents pose different levels of carcinogenicity in different animals, for example, formaldehyde is more carcinogenic to rats as compared to mice
• A newly discovered drug or therapy must be tested on human cells or human test models itself. These reasons make it more important to be able to use the systems closer to humans. Moreover, these studies need to be done in a system where the results could be directly extrapolated to humans. The use of iPSCs helps to extrapolate and mimic
• Finally, iPSC technology offers the prospect of capturing cells derived from a large number of specific types of pre-diagnosed adult patients, potentially at any age, and a correspondingly large number of controls in a format that can support an industrial level of screening, efficacy, and safety studies.

 

Patient-derived somatic cells (for example, fibroblasts) can be reprogrammed to generate iPSCs carrying a disease-specific genetic aberration. These cells can then be differentiated into the disease-affected cell type (for example, neurons in neurodegenerative diseases). After the establishment of a cellular disorder model with disease-specific phenotypes, three main strategies are commonly used: high-throughput screening (HTS) of drugs, the candidate drug approach or patient-specific therapy. In HTS, a very large number of compounds are tested on the differentiated cells, followed by phenotype re-evaluation. This method is extremely valuable for identifying novel therapies in vitro, by using large libraries of compounds. By contrast, both the candidate drug approach and the patient-specific therapy use a small number of potential drugs to attenuate the disease. These approaches are useful when the disease mechanism is known and potential therapies are available. Drugs found by both the HTS and candidate drug approaches usually require substantial safety assays before being prescribed to patients, whereas drugs already approved by regulatory agencies can be immediately prescribed for treatment.

Use of iPSC-Based Assays in Testing Drugs for Toxicity

Before using drugs on humans, their toxic effects must be properly evaluated for a safe administration of those drugs which is very costly. Only 10% of the drugs that enter clinical trials are able to reach market approval stage. The cost of developing a drug is increasing with the estimated cost of whole process being US $1.2–1.7 billion per drug compound. The development of 30% of the medicines was abandoned because of lack of efficacy and 30% due to concerns associated with safety (cardiotoxicity, hepatotoxicity).

iPSC-derived cells of various tissue types can be cultured in large grids and assayed for toxicity in a manner analogous to high-throughput screens for drug discovery. Using this approach, iPSC technology can be integrated into the current paradigm for drug development as part of safety testing in the early phases of clinical trials.

The benefits of using iPSCs in toxicity testing are:

• This method offers an additional line of safety by testing in nontarget tissues, informing clinical trials before harm to human subjects or patients.
• In addition to the safety of testing therapeutics on iPSC-derived cells, there is also the benefit of cost savings. In recent years, the estimated cost of developing and testing a new drug can potentially introduce savings in the costs of development, possibly generate more candidate compounds in a shorter amount of time, and test for life-threatening toxicity in a multitude of tissues. This body of knowledge may help prevent the costly recall of already-approved drugs and develop a new generation of safer drugs using an alternate, less expensive strategy.

Use in Neurotoxicity

Human neurons derived from iPSCs can be attractive models to study the neurotoxicity. The iPSC-derived neurons exhibit functionality and behavior of mature neurons and are available in large quantities. The neurotoxicity test models will allow for studying on one hand the adverse effect of drug candidates on neuronal cells and on the other hand the general neurotoxicity in assays that are well suited for screening of lead compounds and potentially important for reducing animal experimentation and the cost of preclinical development.
Currently, cell lines such as PC12 are typically used for the analysis of calcium signaling with the purpose of determining the complex cellular changes triggered by environmental and pharmacologic neurotoxicants.

Use in Cardiotoxicity

Cardiotoxicity can lead to the formation of reactive oxygen species (ROS), apoptosis, altered contractibility, change in cardiac rhythm, and altered cardiac gene expression, which can be life threatening or may lead to long-term alterations of cardiovascular functions. Of the 40% of drug failures during the clinical trials[i],19% drug withdrawal has been observed due to cardiotoxicities.
In many cardiotoxicity cases, a direct interaction of drugs with specific ion channels expressed by the cardiomyocytes leads to alteration in ion conduction through these specific channels. Drug effects on potassium currents could lead to QT-prolongation, potentially fatal arrhythmias and sometimes cardiomuscular damage without affecting ion channels.
The ESC- and iPSC-derived cardiomyocytes are considered to be well suited to study the effects of compounds which do not interfere with the ion channel functions but still cause cardiotoxicity, an effect that cannot be revealed by using the conventional cell line and receptor overexpression-based approaches[ii].

• Kola I, Landis J. Can the pharmaceutical industry reduce attrition rates? Nature Reviews Drug Discovery. 2004;3(8):711–715.
• Braam SR, Tertoolen L, van de Stolpe A, Meyer T, Passier R, Mummery CL. Prediction of drug-induced cardiotoxicity using human embryonic stem cell-derived cardiomyocytes. Stem Cell Research. 2010;4(2):107–116.