Testing time for stem cells

Researchers at Children's Hospital Boston (MA, USA) recently announced another group of stem cells that can produce cardiomyocytes (heart muscle cells). It seems hardly a month goes by without some kind of discovery in the fast-moving world of stem cells. The latest announcement opens up the prospect of treating heart failure with cell therapy. This is important as heart failure, marked by the loss of cardiomyocytes, is on the increase and the only successful treatment is a heart transplant.

Susan Aldridge

Stem cell therapy for heart failure is many years away, but cardiomyocytes, as well as other stem-cell-derived cells, could help patients by providing pharma companies with a powerful new tool for producing safer and more effective small molecule treatments.

Better models

In recent years, companies have come to increasingly rely on cell-based assays to provide them with physiologically relevant information at an earlier stage in the value chain in the hope of decreasing the attrition rate of compounds as they progress from bench to clinic. They are now looking at developments in imaging, molecular biology and stem cell technologies to provide even better cell-based models. For suppliers, meeting pharma's demand for large amounts of cells for assays is an excellent learning experience for the time when they will have to manufacture them for the clinic.

A few years ago, pharma companies began to move towards cell-based screening, starting off with standard cell lines, such as chinese hamster ovary cells or HeLa, a cancer cell line. Today, cellular assays probably account for more than half of all high-throughput screens, and these methods are also being used in secondary screening and lead validation work. "We are seeing requirements for better assays in more relevant cell models," says Ger Brophy, General Manager of Advanced Systems at GE Healthcare in Uppsala (Sweden). GE has devised a service that not only provides cells and related products, but also works with customers on assay development and related issues. There has been an increase in requirements for frozen cells, which are according to Brophy, beginning to be seen more like a reagent than a specialist research product by pharma and biotech customers.

Biochemical pathways

Not only do customers want cells to be similar to chemicals in their consistency and ease of use, they also want them to work harder. The high-throughput concept is well-established and the buzzword now is 'high content', with the maximum amount of information being extracted from each experiment in a cell-based assay — a development that relies on increasingly sophisticated imaging and software systems.

"A cell is basically a bag of biochemical pathways," says Tim Allsopp, Chief Scientific Officer of Stem Cell Sciences (SCS) in Cambridge (UK), which manufactures and delivers physiologically relevant cells derived from stem cells to the pharma industry. "Cells can be genetically modified to tell researchers what is happening to them. For instance, reporter molecules can be engineered into a gene pathway so that they light up if that pathway is activated. There has also been a move to high-content analysis of cells containing a plurality of reporters, labelling a number of distinct biochemical pathways so that researchers can glean a range of biological information."

The author says...

SCS works with tissue-derived stem cells, from which they obtain neural, muscle, fat and bone cells, and both mouse and human embryonic stem cells (ESCs). "Companies are used to having a variety of reagents and now they want a range of cell types," says Allsopp. "Many researchers use mouse cells, but, ultimately, drugs are designed for humans and so should be tested on human cells."

Put simply, stem cells are important for screening because they can be indefinitely propagated. If this process can be properly controlled, then mass production should be possible, along with differentiation into primary cells. Previously, as Allsopp points out, cells used in screening were made from abnormal tissues, including cancer cells, with abnormal biochemical pathways — so how relevant were the data from them?

Fergus McKenzie, Programme Manager of ITI Life Sciences (UK), which is partnered with Cellartis (Sweden), adds: "It is always better to derive an assay in a physiological cell type." Therefore, human ESC-derived hepatocytes (liver cells) are better than those derived from cadavers and cardiomyocytes are now being used as well. "My belief is that you will get better toxicology data from these cells," says McKenzie, explaining that it is not ESCs themselves that companies want, but the cells they can turn into by differentiation. Currently, the progenitor cells of hepatocytes and cardiomyocytes are the most advanced, with pancreatic cells following. There is also interest in getting vascular cells to work in combination with cardiomyocytes in screening models.

Ramping up supply

The challenges on the suppliers are to scale-up and provide a consistent supply of cells, including stem cells, to make them more like chemical reagents. "We are focused on consistency of supply," says Allsopp. "The cells we supply should have reproducible properties as 'drift' in the cell properties during a screening campaign would make data interpretation problematic. Ramping up supply to a customer's needs can be challenging, even for a commercial organization such as ours with automation, but it is a trend for the future so we must optimize our processes."

Brophy adds that at GE Healthcare's facility near Cardiff (UK), a number of rooms of the appropriate size have been established for large-scale production. Cytodex, a bead-based product from GE Healthcare Life Sciences, is proving important as it provides a hugely increased surface area within a bioreactor to which the cells can adhere. The company has also responded to the growing demand for cells that have been transiently transfected rather than exhibiting stable expression. Transient transfection is difficult to scale-up using traditional methods, but GE Healthcare is developing methods for mass transfections in bioreactors rather than in batches. This process should be more efficient and improve consistency.

The other consideration in the large-scale production of cells is the format in which they are supplied and distributed to customers. Ease of handling is important and so is miniaturization where a lot of information can be extracted from a small amount of material, which means cells plated onto a microwell plate are popular. "Customers want to treat cells as a consumable that you use and then dispose of," says Allsopp.

McKenzie adds that stem cells for therapy, as are being used in clinical trials by ES Cell International of Singapore and Geron (CA, USA), must be at GMP standard, but this is not needed when the cells are used as a drug discovery and development tool. "Stem cells are still expensive and a technically advanced product," he says. "There is not a commodity market in stem cells at this time and it will be a few years before their use in drug discovery is widespread."

The future

There is some way to go before all current opportunities in cell-based screening are exploited as it is still too soon to show data proving that such screens do, indeed, lead to reduced attrition rates. "There is resistance to change in Big Pharma and smaller biotechs don't have enough money to invest," McKenzie says. However, Pfizer (NY, USA) has apparently been working with mouse ESCs for some time and turning them into a range of cell types for screening. Other companies are likely working with ESCs 'on the quiet'.

In the future, besides a move to stem cells, McKenzie predicts: "There will also be a move to two-dimensional cellular models with monolayers and more complex structures on scaffolds and maybe even small organs." Brophy adds: "There will be advanced cell models that can bring together validation and toxicity. The complexity of the body will be reflected in the type of cellular assay."