1. Introduction

2. Stem cells

3. The generation of

stem-cell-based models

4. Stem-cell models in drug

discovery

5. Stem-cell models for toxicity

testing

6. Expert opinion

Editorial

Can stem-cell-derived modelsrevolutionize drug discovery?Henrik Landgren & Peter Sartipy†

Cellectis AB, G€oteborg, Sweden

The pharmaceutical industry continues to struggle to bring new and innovative

medicines to the market. Possible reasons for these challenges are the

paradigms currently used during drug discovery and development. Over the

last 15 years, our ability to study the pathology of human disease has increased

tremendously. For example, the advent of human embryonic stem cells, and

later the discovery of induced pluripotent stem cells, now make it possible to

access large quantities of human specialized cells, address the issue of genetic

diversity and to create disease models in a culture dish. Recently, the potential

of pluripotent stem-cell technologies in the pursuit of new medicines has

been demonstrated through the in vitro recreation of many human diseases,

and the subsequent use of these models in proof-of-concept drug screens.

Ultimately, this can, together with the unlimited access to relevant human cells,

aid in reducing both cost and attrition rate of new drug candidates. The field is

now open for large-scale application of stem-cell-derived cells for both drug

screening and safety assessment.

Keywords: disease modeling, drug discovery, iPS cells, stem cells, toxicity testing

Expert Opin. Drug Discov. (2014) 9(1):9-13

1. Introduction

Despite substantial progress in science and technology over the last half-century, thepharmaceutical industry’s output in terms of new medicines has steadily decreased.The cause and possible solutions to this decline have been extensively discussed andreviewed elsewhere [1,2]. It is clear that the pharmaceutical industry has been effec-tive in generating leads, but very inefficient in predicting whether a drug candidatewill make it through the clinical trial process. It seems that an improved paradigmfor de-risking the entire drug discovery and development process is needed. Also,pharma companies are moving their focus away from certain areas, such as neurosci-ence, partly due to the lack of relevant model systems [3]. A possible reason for theindustry’s struggles is the large reliance on target-based screening approaches andthe reductionism that comes with it. Has the hunt for the next blockbuster drugmade us overlook the complexity of biological systems, which despite our increasedknowledge seems higher than ever? Is there a need for a new paradigm in drugdiscovery? As the title reads, ‘Can stem-cell-derived models revolutionize drugdiscovery?’

With their unique properties, human pluripotent stem cells (hPSCs) offer newpossibilities for drug discovery and development. Here, we will highlight in vitrouse of hPSCs as disease models for discovery and for toxicity testing, and makethe point that stem-cell-derived cellular models hold great potential for the industry.Another area of great importance, the prospect of using stem cells for cell therapy orregenerative medicine, will, however, not be discussed, and the interested reader isreferred to other recent insightful reviews on this topic.

10.1517/17460441.2014.867945 © 2014 Informa UK, Ltd. ISSN 1746-0441, e-ISSN 1746-045X 9All rights reserved: reproduction in whole or in part not permitted

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2. Stem cells

The term ‘stem cells’ is broadly used when referring to unspe-cialized cells that can differentiate into functional cell types.The degree of plasticity or potency of the stem cells, as wellas their capacity for long-term proliferation, varies among dif-ferent types of stem cells. Pluripotent stem cells, either origi-nating from in vitro fertilized eggs (i.e., embryonic stem[ES] cells) or reprogrammed from somatic cells (i.e., inducedpluripotent stem cells [iPSCs]), can proliferate indefinitelyand give rise to all cell types of the adult body (Figure 1).

3. The generation of stem-cell-based models

Induced pluripotent stem cells were discovered in 2006 whenTakahashi and Yamanaka showed that introduction of fourtranscription factors into fibroblasts reverted them to a plurip-otent state [4]. iPSCs have been shown to be remarkablysimilar to ES cells in many aspects [5,6]. However, during thereprogramming process, the epigenetic state of the mature ter-minally differentiated cell is only partially reset, and this hasbeen shown to affect the differentiation potential of the result-ing cell lines [7]. If this has significant downstream functionaleffects in an in vitro assay in which the cells are later employedremains to be determined. Further research on both ES andiPS cells is warranted in order to better understand theconsequences of the described, and potentially yet unknown,differences between the cells. There are also examples ofmutations that affect the reprogramming process itself, asseen in Fanconi Anaemia [8], indicating some limitations tothe preconceived idea that it is possible to generate iPSCsfrom any genetic background.Nevertheless, the genetic diversity of ES cells is relatively

hard to anticipate and test, while iPSCs can be selectivelycreated from individuals affected by disease or carrying muta-tions, hence, from starting material of known genotype. Thisopens up novel avenues for in vitro disease modeling withpotentially unlimited access to human cell material.

4. Stem-cell models in drug discovery

The use of stem cells and derived models thereof for drug dis-covery relies on efficient and faithful in vitro reproduction ofdisease phenotypes. Some of the current challenges to thestem-cell field are the availability of differentiation protocolsthat produce a desired cell type, with reasonable efficiency,and where the end point is a cell of sufficient maturity.Human (h) PSCs have been used to recreate many diseasephenotypes although not all lines demonstrate a disease phe-notype in vitro [9]. There is also variation between lines fromthe same individual. Substantial research efforts are currentlyfocused on addressing these issues, and it is expected that wewill witness much progress in the short term. Another signif-icant challenge not directly related to stem-cell differentiation

is the fact that many phenotypes develop late in life and mightnot have time to manifest over the relatively short time periodcells can be cultured. However, there are reports of acceleratedphenotypic presentation in vitro as seen in Down’s syndromeiPSCs where Alzheimer’s disease (AD) pathology actuallydevelops over months in culture rather than years in vivo[10]. Many phenotypes are also non-cell autonomous, thusrequiring interaction with other cell types to manifest, provid-ing another level of complexity. Such co-cultures would meanan even more complicated culturing system, potentiallyreducing throughput and increasing cost. Nevertheless, exam-ples of such setups in screening are starting to emerge, provid-ing support for the feasibility and further development ofsimilar systems [11]. The underlying genetic cause of diseaseis of course also a major factor in phenotypic presentationwhere Mendelian genetic diseases are relatively easy to model,whereas disease with complex genetics might require screeningof several cell lines before satisfactory phenotypes appear.

Monogenic disease models (and models in general) canalso be created with genome engineering tools, such as tran-scription activator-like effector nucleases and zinc-finger nucle-ases [12] where disease-causing mutations are either introducedor corrected.

It can be argued that part of the pharmaceutical industry’sR&D problems come from an over-reliance on a reductionistparadigm in early drug discovery. For instance, over the lastdecade, phenotypic screening has yielded more first-in-classnew medicines than target-based screening [2]. The increasedavailability of stem-cell-derived cells fits well with the resurgentinterest in phenotypic screening and offers several advantagessuch as elimination of pathway bias and the ability to performmulti-target studies all in at least medium-throughput systems.Obvious advantages over artificial target- based screens are a rel-evant cell type, of human origin, expressing genes and proteinsnative to the tissue of interest. The potential to increased reper-toire of potential drug targets (such as hard to study targets asprotein folding or trafficking) provided by stem-cell models isalso needed especially, considering that only 435 unique targetsexist among the FDA-approved drugs [13]. If the clinical predict-ability of stem-cell models proves to be high enough, targetidentification and mode-of-action studies can be performed inparallel to preclinical development, eliminating the need forprior-hand predictions of drug targets made from simplifiedsignaling schemes or incomplete understanding of diseasemechanisms. For studies aimed at identifying novel diseasemechanisms or drug targets, combining stem-cell-derivedmodels with chemical and functional genomic screening wouldcreate powerful platforms.

Most important, screening with stem-cell-derived modelswould offer cells of human origin that, at present, have at leastpartially relevant physiology. This can be considered a techni-cal limitation as developments are made in differentiation pro-tocols and culture systems, but there also likely exist a tradeoffbetween screening speed, size and physiological fidelity forreasons discussed above.

H. Landgren & P. Sartipy

10 Expert Opin. Drug Discov. (2014) 9(1)

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5. Stem-cell models for toxicity testing

One major reason for drug attrition is safety concerns appear-ing late in the development process, sometimes even post-launch. Current screening methods for adverse effects oftenassume previous knowledge of a targets propensity for sucheffects drawn from clinical data or animal studies. Early toxic-ity studies are typically performed in immortalized cell linesexpressing targets of interest [14]. Stem-cell-derived cells offerseveral advantages; they are of human origin and allow studiesof, for instance, protein and compound interactions in a morerelevant physiological setting. In addition, the opportunity touse genome engineering enables the creation of models (e.g.,fluorescent reporters, tagged proteins) suited for specificassays, such as high content analysis or high content screening.

Toxicity testing can be performed using generic hPSC linesto create different cell types of interest, or it can be expandedto include several lines with varying and selected genetic back-ground. Using a panel of human iPSC lines, it was recentlyreported that the susceptibility to drug-induced arrhythmiadiffered significantly between healthy and disease-specificiPSC-derived cardiomyocytes [15]. This clearly illustrates theneed to assess adverse effects in the target patient populationas well as in healthy individuals, and that the iPSC technologynow provides the tools to translate the clinical situation to anin vitro platform. Furthermore, adding different cell typesderived from the same hPSC line would create a model some-times referred to as a ‘Patient-in-a-dish,’ where multiple celltypes can be simultaneously screened for toxic effects and

studied for desired drug effects in the target cell type, thuspushing the industry closer to utilizing the prospects of per-sonalized/stratified medicine. This ‘in vitro clinical trial’would offer very valuable safety and efficacy data, and couldbe employed early in the drug development process. Impor-tantly, such studies can be performed at a fraction of thecost of running a clinical trial.

6. Expert opinion

Stem cells, and derived cells thereof, are able to act both asa drug itself (i.e., in cell therapy), and as a tool for discoveryand development. The focus of the stem-cell field has duringrecent years shifted toward iPSC-based disease modelingand clinical applications. However, for stem-cell-derivedmodels to become an integral part of the drug discovery anddevelopment process, some major hurdles still exist.

First, given the high number of cells required for a typicaldrug screen or toxicology assessment, the availability ofstem-cell-derived cells regarding both supply and qualityneeds to be secured. Presently, the most well-characterizedstem-cell-derived cell types are cardiomyocytes, neurons andhepatocytes, and all of these cell types are commercially avail-able. However, questions regarding, for instance, phenotypicmaturity still exist. This might be considered a technical issuewhere progress in methodology is expected to bridge the gapbut improvements for some cell types are likely needed to pro-vide a more general industry acceptance and broaderclinical predictability.

Human embryonic stem cell Induced pluripotent stem cell (iPS)

Genome engineering

Drug screening Drug safety testing

Self renewal

Figure1. Illustration of the human pluripotent stem cells paradigm. Starting with a fertilized oocyte or terminally

differentiated somatic cells, an hPSC line is created that can self-renew and is amendable to genome engineering. This line

can then be differentiated to specialized cell types (here exemplified with hepatocytes, cardiomyocytes and neurons) that in

turn can be used in drug discovery and toxicity screens.

Can stem-cell-derived models revolutionize drug discovery?

Expert Opin. Drug Discov. (2014) 9(1) 11

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Second, the cost and know-how of creating high-qualitypluripotent stem cell lines are substantial but, recently, severalcompanies have launched iPSC generation and engineeringservices providing commercial alternatives to setting up in-house programs. The quality of the cell lines affects the phe-notype and differentiation characteristics as discussed above.Implementing iPSC-based screening programs also requiresin-house knowledge and expertise. However, in the shortterm, we are likely to see expansion of commercially availableiPSC-based products, and this is also an area where activecollaboration between the pharma industry and specializedbiotech companies might prove fruitful and examples ofsuch partnerships already exist.Third, and perhaps most important, the stem-cell technol-

ogy needs to be properly validated and proven to be superiorto existing paradigms and the clinical translatability must beestablished. If stem-cell models can aid in bridging the gapbetween the prevailing process-centric view and clinicalrelevance existing in the industry today, it will most likely fuelfurther investment in this space. There are concertedinitiatives ongoing to evaluate these models, for instance the‘Consortium for Safety Assessment Using Human iPS Cells’run by the Japanese Pharmaceutical Manufacturing Associa-tion. These efforts will, if successful, open the door for large-scale implementation of stem-cell-based technology and alsoprovide early adopters with advantages versus their competitors.

Fourth, the implementation and integration of stem-celltechnology within the existing work-flow will likely presentwith some challenges. Stem-cell-derived cells might not easilybe adapted to existing high-throughput systems, and initialinvestments to rectify this could be substantial in certain cases.Like with any new technology, knowledge is at a premiumand for successful implementation, the knowledge and exper-tise need to be present throughout the organizations, specifi-cally at decision-making levels. It is clear that many of thefundamental questions regarding stem-cell-derived cells andmodels’ utility have begun to be answered at the researchbench, and they are now starting to appear in the drug discov-ery and development processes in the pharma industry. At themost basic level, human stem cells can provide access to lim-itless quantities of specialized cells, healthy or diseased, whichcan be further modified using, for example, genome engineer-ing. To this end, at least many of the prerequisites for a broaduse of stem cells in drug discovery appear to be in place, and itis reasonable to speculate that the hPSC technologies willbecome an integral part in the development of new drugsthat are both safe and effective, while at the same time reducingattrition rates and development costs.

Declaration of interest

P Sartipy and H Landgren are both employed by Cellectis AB.

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either of interest (�) or of considerable interest(��) to readers.

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AffiliationHenrik Landgren & Peter Sartipy†

†Author for correspondence

Cellectis AB, Arvid Wallgrens backe 20,

SE-413 46 G€oteborg, Sweden

Tel: +46 31 758 09 00;

Fax: +46 31 758 09 10;

E-mail: peter.sartipy@cellectis.com

Can stem-cell-derived models revolutionize drug discovery?

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