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The rationale for use of a special type of cardiac stem cell to treat heart failure patients is being questioned by animal research showing that such cells have minimal ability to generate new cardiomyocytes and thus almost no chance of restoring cardiac function—at least by that mechanism.

The preclinical study was published online May 7, 2014, ahead of print in Nature.

One promising branch of stem cell therapy has focused on leveraging the presumed regenerative potential of c-kit-positive cardiac progenitor cells, which in rat models have been reported to differentiate into cardiomyocytes when injected into the infarct region, producing substantial new myocardium. A protein tyrosine kinase receptor, c-kit is found on the surface of certain progenitor cells in both bone marrow and the heart.

Tracking c-kit Cells

In the current study, researchers led by Jeffrey D. Molkentin, PhD, of Cincinnati Children’s Hospital Medical Center (Cincinnati, OH), used genetic engineering to produce a mouse model that enabled fluorescent tagging of cells expressing c-kit. This technique enabled them to track the lineage of and quantify c-kit-positive cells generated in the animals.

When the mice were 4 to 12 weeks old, the investigators identified c-kit-positive fibroblasts, endothelial cells, and various types of immune cells, but tagged cardiomyocytes were extremely rare (0.027%). Because cardiac injury increases cell turnover in the heart, a chemical was also used to simulate MI. When they looked again for c-kit-positive cardiomyocytes, levels had increased slightly. However, the percentage of truly new cardiomyocytes fell about fivefold after accounting for the tendency of c-kit-positive cells from the bone marrow or circulating immune system to fuse with resident parenchymal cells to form cardiomyocytes.

“The percentage of cardiomyocytes emerging from the c-kit-positive lineage was astonishingly low and hence highly unlikely to ever considerably affect cardiac function,” the authors say.

In a press release, Dr. Molkentin cautioned against “further clinical testing until the mechanisms in play here are better defined or we are able to dramatically enhance the potential of these cells to generate cardiomyocytes.”

He added that as a follow-up to the study, his lab is testing genes and protein growth factors that may be able to boost the rate of new cardiomyocyte generation from c-kit stem cells and thus potentially turn them into a truly effective therapy. 

Negative Findings ‘Go Against the Grain’ 

In a telephone interview with TCTMD, Emerson C. Perin, MD, PhD, of the Texas Heart Institute (Houston, TX), noted that in the first phase I clinical trial to test c-kit cells in heart failure patients, SCIPIO, the therapy showed good safety and signals of efficacy, improving LVEF and reducing infarct size.

But importantly, he said, SCIPIO was based on data from multiple preclinical studies all demonstrating that c-kit cells produce meaningful cardiomyogenesis. “So this particular study goes against the current,” Dr. Perin commented, adding that it reaffirms the regenerative capacity of the cells in principle despite detecting much lower numbers of new cardiomyocytes than earlier research.

In addition, he observed, not only is the relevance of the mouse model used in the current study to humans questionable but the c-kit cells delivered in clinical trial were exogenous—before transplantation they were cultured over several months—and may behave differently from endogenous c-kit cells in the body. Moreover, the efficiency of the green fluorescent protein tagging in the study mouse model is problematic, he noted. 

Dr. Perin said that after carefully reviewing the current literature, he and other members of the 7-center Cardiovascular Cell Therapy Research Network (CCTRN) remain “undeterred and very keen on exploring c-kit cells in the only model that really matters, which is patients. It would be crazy to change course or even stop research because of this article.” In fact, he added, CCTRN now has 2 additional c-kit trials in the planning stages.

Furthermore, Dr. Perin acknowledged, even though the weight of evidence favors some impact of c-kit cells on cardiomyogenesis, much of the benefit probably stems from a paracrine effect, since most of the new cells disappear within a month. For example, c-kit cells may stimulate resident cardiac stem cells or encourage other local or direct effects, he observed.

Clinical Translation Takes Priority?

Basic studies can yield discrepant results due to a multitude of technical issues, Dr. Perin commented, and the study authors seem to be overinterpreting their findings. Moreover, once safety has been established, clinical considerations should take priority, he asserted, adding, “With patients dying of heart failure, and studies suggesting this therapy is safe and possibly helpful, why not translate it to the clinic?” Deferring trials until all the mechanisms have been pinned down would mean “waiting till the cows come home,” Dr. Perin said, noting that we still do not understand why most medications work. 

Study coauthor Eduardo Marbán, MD, of Cedars-Sinai Heart Institute (Los Angeles, CA), had a different take on the study and its implications for future of c-kit cell research.

“This work really puts to rest the idea that c-kit [cells] are an important source of beating heart cells under normal situations,” he told TCTMD in a telephone interview. “That doesn’t mean the cells don’t have clinical efficacy, it just means that the scientific basis for having studied them in the first place is pretty shaky.”

As to why the current study results contradict so much previous data, Dr. Marbán suggested one reason may be that earlier research relied heavily on microscopy methods like immunostaining, and some labs may have had problems with its interpretation. In contrast, this is the first study to use genetic fate mapping, which enables irreversible marking of a precursor cell and reliable tracking of its progeny. The results are “as definitive as it’s going to get” in determining c-kit cells’ ability to generate new cardiomyocytes, he added.

Moreover, Dr. Marbán reported, earlier research from his lab showed that c-kit cells fall short compared with other therapeutic cells when it comes to stimulating regeneration of native cardiac cells.

Time to Pack Up the c-kit?

Dr. Marbán “strongly disagreed” with the idea of continuing to test c-kit cells in clinical trials now that “there is direct evidence against their role as cardiomyocyte progenitors as well as allegations of lack of data integrity from the lab that promulgated these theories in the first place and that ran the only clinical trial in which c-kit cells have been used in humans.” The latter point refers in part to investigation of supplemental data contributed to the SCIPIO paper (Lancet. 2014; 383:1279).

As to the possibility that c-kit cell efficacy might be enhanced genetically or pharmacologically, Dr. Marbán was unenthusiastic. “It’s not an area that I think deserves a tremendous amount of attention because there are other viable approaches to cardiac regeneration,” he said, adding that his lab is currently exploring the potential of cardiosphere-derived cells and other researchers are looking at conditioned mesenchymal stem cells.

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• SCIPIO: Cardiac Stem Cell Treatment Looks Promising in Early Results