Kevin Mayer Senor Editor Genetic Engineering & Biotechnology News

Going forward, “fit to purpose” metrics promise to be a moving target.

While commercial providers of stem cells and stem cell-derived products have instituted quality-control measures, they are aware that no single, unchanging quality-control regime will suffice for all purposes. Instead, providers aim to apply “fit for purpose” metrics, knowing that these metrics may need to evolve as stem cell applications evolve. Changes may also be called for simply because research indicates that stem cell behavior can be scrutinized more closely.

Which changes should be implemented (and when) may be debated. Moreover, there may be disagreement over the “how.” For example, incremental change may be deemed adequate, or more sweeping change may be advocated.

Such questions were raised soon after a paper appeared March 6 in Stem Cell Reports. The paper, entitled “Quality Metrics for Stem Cell-Derived Cardiac Myocytes,” asserted that “no standardized guidelines currently exist for the evaluation of stem cell-derived myocyte functionality.” It went on to propose the use of a tissue engineering test bed. This test bed, the authors stated, could allow the multifactorial evaluation of stem cell-derived myocyte form and function as well as comparison against a target cardiac phenotype. More to the point, the test bed could help define a “quality-control standard rubric.”

Assessing Functionality

By saying that functionality guidelines didn’t exist but could be developed, the authors, who represented Harvard’s Wyss Institute, might have seemed to advocate urgent, thoroughgoing reform, particularly given the applications at stake—toxicity testing, drug screening, and regenerative medicine. The authors, however, may have had in mind selected research applications, or they may have been thinking of how differently quality control could address future applications.

Whenever one considers the pace and scale of reform—of stem cells or anything else—the words of Thomas Jefferson may come to mind:

“I am not an advocate for frequent changes in laws and constitutions. I think moderate imperfections had better be borne with; because, when once known, we accommodate ourselves to them, and find practical means of correcting their ill effects.”

Jefferson had more to say on the subject, of course. Suffice it to say, the moderation he described was largely absent from a press release that marked the Harvard study’s publication. According to the release, Kevin Kit Parker, Ph.D., the leader of the Harvard research team, said, “We have an entire industry without a single quality control standard.” Also, Dr. Parker, recalling the times he visited commercial stem cell providers, reportedly added, “I’d never seen a dedicated quality control department, never saw a separate effort for quality control.”

To be fair, a press release isn’t necessarily the best place to look for an exhaustively qualified summation. And the Harvard release did distinguish between FDA-mandated safety tests and assessments of functionality. In any case, another article from Parker’s lab, published November 19, 2013 in Stem Cell Reports, was more circumspect. The article, entitled “Functional Differences in Engineered Myocardium from Embryonic Stem Cell-Derived versus Neonatal Cardiomyocyte,” noted that the degree to which stem cell-derived cardiomyocytes compare to mature cardiomyocytes is often debated. Then the article stated that “the standard approach for assessing the differentiated state of these derived myocytes has been to determine mRNA, miRNA, and protein expression, as well as the electrophysiological characteristics of myocytes via patch-clamp or calcium imaging.” While these criteria are important, the article continued, they “offer an incomplete assessment of the myocyte’s physiology.”

Incomplete, the article said, not nonexistent.

Both of the Harvard team’s articles discussed the possibility of expanding quality control to better assess emergent phenomena. For its purposes, the Harvard team wanted to know how differentiated cardiomyocytes would function as a tissue, and they chose to evaluate attributes such as cellular alignment, gap junctional localization, and cytoskeletal architecture. 

Toxicology Applications

Existing quality control metrics may be less advanced with respect to emergent phenomena, but they have been useful. As indicated by Emile Nuwaysir, Ph.D., COO of Cellular Dynamics International (CDI), “Currently marketed human ESC- and iPSC-derived cardiomyocytes’ primary industrial purpose presently lies within the realm of drug discovery screening and toxicity testing.” CDI’s human induced pluripotent stem (iPS) cell-derived cardiomyocytes are marketed under the name iCell Cardiomyocytes.

“We welcome that Dr. Parker’s lab has raised the issue of quality, as we’ve put so much effort into QC standards within our own manufacturing practices. The QC processes at CDI have been in place since the company’s inception to ensure customer safety and fit-for-purpose,” stated Dr. Nuwaysir. “One measurement of successful quality control criteria is continued adoption of the product by the market. In the case of iCell® cardiomyocytes, there have been over two dozen peer-reviewed manuscripts published that detail product characterization and use in basic research, drug discovery, and toxicity testing.”

Dr. Nuwaysir highlighted two particular studies: an article by Guo et al., published June 20, 2011, in Toxicological Sciences (“Estimating the risk of drug-induced proarrhythmia using human induced pluripotent stem cell-derived cardiomyocytes”), and an article by Guo et al., published September 19, 2013, in the same journal (“Refining the human iPSC-cardiomyocyte arrhythmic risk assessment model”).

The first article introduced a high-throughput functional assay that employed a monolayer of beating human induced pluripotent stem cell-derived cardiomyocytes. It described how the assay could be used to detect drug-induced cardiac abnormalities. The follow-up article, noted Dr. Nuwaysir, used a large compound set to “demonstrate that the testing paradigm is more predictive for certain outcomes than current models.” The takeaway, according to Dr. Nuwaysir, is that “the implemented quality control metrics have guaranteed a highly relevant product as well as maintained consistent high-quality production over the years.”

When asked to describe CDI’s quality control measures, Dr. Nuwaysir said that the company’s iCell and MyCell® products are all “human iPSC-derived cells, tested and batch-released based on rigorous quality control standards.” For the iCell Cardiomyocytes product release, he continued, the company “performs an eight-point testing paradigm, including tests for target cell purity (by flow cytometry), cell viability, cell plating efficiency, sterility, mycoplasma contamination, beat rate, field potential duration, and beat regularity (low c.v.).”

Dr. Nuwaysir emphasized the importance of separating quality control from development and manufacturing: “Such separation is essential to assuring customers of the highest quality products. We have a dedicated and separate quality control laboratory, with instrumentation and equipment calibrated and maintained on a dedicated schedule to serve the needs of quality control. Our Quality Management System staff today includes nine full-time positions.”

Future Applications

While noting that stem cell-derived tissues can be made in sufficiently high purity, quality, and quantity to enable safety assessments, another CDI-affiliated scientist, Kyle Kolaja, Ph.D., has written that current stem cell-derived tissues are “generic” genetic backgrounds. In the future, these tissues “will integrate genetic diversity on top of complex three-dimensional models to create truly unique in vitro organoids.”

These comments appeared December 20, 2013, in the Journal of Biological Chemistry, in an article entitled “Stem cells and stem cell-derived tissues and their use in safety assessment.” In this article, Dr. Kolaja mentioned the possibility of creating “man on a chip” models. They could, he said, become the “cell format of choice given their robust, reproducible manufacturing coupled with the ability to make cells from patients with known medical conditions.”

If such models currently seem a bridge too far, it might help to remember that they have benefited from substantial financial support and remain a topic of intense research. An outline of researchers’ attempts to produce organ-like structures for screening and toxicology applications appeared December 20, 2013, in Stem Cell Research and Therapy, in an article entitled “Human induced pluripotent stem cell-based microphysiological tissue models of myocardium and liver for drug development.”

The authors, Mathur et al., emphasized the importance of selecting the right cell source for experiments. Also, they look so far ahead that they consider the possibility of integrating multiple micro-organs on a single chip. “In the human body, organs are interconnected via the circulatory system, and a drug designed for the heart may have off-target toxicity in the liver,” the authors write. “Integrated in vitro microphysiological systems of human cardiac and liver tissue will not only serve as parallelized cardiotoxicity and liver toxicity tests, but also capture off-target toxicity.”

When it comes to defining appropriate quality control measures, it might be helpful to distinguish between future applications and applications for which utility has already been demonstrated. At least, Dr. Nuwaysir seems to think so: “As the uses to which stem cell-derived cell types advance and evolve, the continued implementation and execution of pragmatic quality control metrics will similarly evolve, ensuring that these products will remain a robust, reproducible, and predictive reagent.”

“It is clearly time for placing a flag in the sand,” Dr. Nuwaysir adds. “However, how that flag is observed, and the direction in which it blows, promises for lively debate in the future.” 

This sentiment bears a certain resemblance to the one expressed in the Jefferson quote above. Incidentally, the rest of that quote runs as follows:

“But … laws and institutions must go hand in hand with the progress of the human mind. We might as well require a man to wear still the coat which fitted him when a boy as civilized society to remain ever under the regimen of their barbarous ancestors.”

It is humbling to think that the current understanding of stem cells may eventually be thought barbarous. But at least by the time that happens, our quality control metrics will, presumably, be up to date, too.

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