Drug Discovery
adenovirus vectors to deliver the transcription factors. However, the reprogramming efficiency of these methods (ie, the rate at which cells con- vert to pluripotency) is currently lower than use of retroviral vectors3.
Generation of human and mouse iPS cells have been accomplished using a single, excisable lentivi- ral vector that delivers all four ‘Yamanaka’ tran- scription factors4. Use of a single vector signifi- cantly reduces the number of viral integrations required – in some cases, iPS clones possessing only a single viral integrant can be isolated5.
Tools for discovery and development The ability to revert somatic cells to an embryonic state and subsequently differentiate them into a variety of cell types offers a wealth of opportunities for personalised regenerative medicine and disease research. Because iPS cells can be generated from individuals with different clinical phenotypes and genotypes, they can offer a strategic advantage over embryonic stem cells for use in patient-specif- ic cell replacement therapies and drug discovery and development (Figure 2).
Disease modelling
For decades, researchers have relied on animal models, immortalised cell lines, or short-lived pri- mary cultures to dissect the mechanisms and pathogenesis of diseases. Genetic manipulations including over-expression, knock down, knock out, and knock in strategies are often employed in animal models in an attempt to replicate genetic patterns linked to specific disease phenotypes. Differentiated cells derived from iPS cells have the potential to overcome the inherent limitations of existing disease model systems. Cells derived from patient-specific iPS cells can potentially pro- vide a more relevant model system as their proper- ties more closely resemble those found in the patient’s own system and they do not require genetic manipulation.
Diseases that arise from single base mutations or deletions are well-suited for modelling with iPS technology. iPS cells also offer tremendous oppor- tunity modelling diseases that do not have robust animal models and more complex conditions that involve a number of different cell types such as obesity and metabolic disorders.
For example, while a researcher can easily obtain fat cells from a patient with a metabolic dis- order, these cells cannot be cultured over the long term and may only allow for a one-time endpoint assay. By leveraging iPS cells to generate a sus- tained supply of patient-specific fat cells, the com-
Drug Discovery World Spring 2011
plexities of the metabolic disorder can be examined more effectively. With iPS cells, the researcher can conduct dozens of assays to identify differences in fat cells from a person with a metabolic disorder such as type 2 diabetes versus an unaffected individual. The abil- ity to take a single genotype and potentially make any of the tissues that might be involved in a meta- bolic disorder – such as pancreatic beta cells, hepa- tocytes, or hypothalamic cells – can lead to a pow- erful disease model.
Drug screening
Using iPS derivative cells, potential therapeutics can be screened against a large number of patient- specific cells prior to initiating clinical trials. Variation in the response to drugs by cells of patients with genetic differences can guide more targeted selection of patients for enrolment in clin- ical trials, resulting in trials that are smaller and more likely to be successful.
IPS cells can be particularly useful when pri- mary cells are difficult or impossible to obtain. For example, drug screening applications for cys- tic fibrosis can be developed using lung cells derived from iPS cells6. Obtaining lung cells from a patient with cystic fibrosis is typically only pos- sible if the patient is undergoing a lung transplant. However, these patients have dramatic lung infec- tions so it is difficult to establish primary cell lines. And even if a cell line can be successfully estab- lished, the cells will have a limited ability to be passaged, which greatly limits the ability to gener- ate enough cells for the screening process. Through use of iPS cells, large numbers of cells can be generated from a range of patients, and genomic patterns can be cross-referenced to drug screening results to predict the effectiveness of dif- ferent drugs on different phenotypes.
Investigative toxicity
Drug-induced liver injury has been the most fre- quent single cause of safety-related withdrawals of marketed drugs over the past 50 years7 and is the principal reason clinical trials are suspended. More efficient and predictive toxicity studies can be expected to reduce development costs associated with the late stage failure of drug candidates. Identification of drug candidates with toxicity issues earlier in the discovery process will most likely result in improved safety for clinical trial participants and patients. Investigative in vitro liver toxicity studies are typically conducted using primary human hepato- cytes or an immortalised (genetically transformed)
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