Treating the Heart with Mesenchymal Stem Cells: Timing and Dosage

Stephen Worthley from the Cardiovascular Investigation Unit at the Royal Adelaide Hospital in Adelaide, Australia and his colleagues have conducted a timely experiment with rodents that examines the effects of dosage and timing on stem cell treatments in the heart after a heart attack.

Mesenchymal stem cells from bone marrow and other sources have been used to treat the heart of laboratory animals and humans after a heart attack. The optimal timing for such a treatment remains uncertain despite a respectable amount of work on this topic. Early intervention (one week) seems offer the best hope for preserving cardiac function, but the heart at this stage is highly inflamed and cell survival is poor. If treatment is delayed (2-3 weeks after the heart attack), the prospects for cell survival are better, but the heart at this time is undergoing remodeling and scar formation. Therefore, stem cell therapy at this time seems unlikely to work. Human clinical trials seem to suggest that mesenchymal stem cell treatment 2-3 weeks after a heart attack does no good (see Traverse JH, et al JAMA 2011;306:2110-9). The efficacy of the delivering mesenchymal stem cells to the heart at these different times has also not been compared.

If that degree of uncertainty is not enough, dosage is also a mystery. Rodent studies have used doses of one million cells, but studies have not established a linear relationship between efficacy and dose, and higher dosages seem to plateau in effectiveness (see Dixon JA, et al Circulation 2009;120(11 Suppl):S220-9). High doses might even be deleterious.

So what is the best time to administer after a heart attack, and how much should be administered? These are not trivial questions. Therefore a systematic study is required and laboratory animals such as rodents are required.

In this study, five groups of rats were given heart attacks by ligation of the left anterior descending artery, and two groups of rats received bone marrow-derived mesenchymal stem cells immediately after the heart attack. The first group received a low dose (one million cells) and the second group received twice as many cells. The three other groups received their treatments one week after the heart attack. The third group received the low dose of stem cells received the low dose of cells (one million cells), and the fourth group received the higher dose (two million cells). The fifth group received no such cell treatment.

All mesenchymal stem cells were conditioned before injection by growing them under low oxygen conditions. Such pretreatments increase the viability of the stem cells in the heart.

The results were interesting to say the least. when assayed four weeks after the heart attacks, the hearts of the control animals showed a left ventricular function that tanked. The ejection fraction fell to 1/3rd the original ejection fraction (~60% to ~20%) and stayed there. The early high dose animals showed the lowest decrease in ejection fraction (-8%). The early low dose group showed a greater decrease in ejection fraction. Clearly dose made a difference in the early-treated animals with a higher dose working better than a lower dose.

In the later-treated animals, dose made little difference and the recovery was better than the early low dose animals. when ejection fraction alone was considered. However, when other measures were considered, the picture becomes much more complex. End diastolic and end systolic volumes were all least increased in the early high dose animals, but all four groups show significantly lower increases than the controls. The mass of the heart, however, was highest in the late high-dose animals as was ventricular wall thickness.

When the movement of the heart walls were considered, the early-treated animals showed the best repair of those territories of the heart near the site of injection, but the later-treated animals showed better repair at a distance from the site of injection. The same held for blood vessel density: higher density in the injected area in the early-treated animals, and higher blood vessel density in those areas further from the site of injection in the later-treated animals.

The size of the heart scar clearly favored the early injected animals, which the lower amount of scarring in the early high dose animals. Finally when migration of the mesenchymal stem cells throughout the heart was determined by using green fluorescent protein-labeled mesenchymal stem cells, the later injected mesenchymal stem cells were much more numerous at remote locations from the site of injection, and the early treated animals only had mesenchymal stem cells at the site of injection and close to it.

These results show that the later doses of mesenchymal stem cells improve the myocardium further from the site of the infarction and the early treatment improve the myocardium at the site of the infraction. Cell dosage is important in the early treatments favoring a higher dose, but not nearly as important in the later treatments, where, if anything, the data favors a lower dose of cells.

Mesenchymal stem cells affect the heart muscle by secreting growth factors and other molecules that aids and abets healing and decreases inflammation. However, research on these cells pretty clearly shows that they modulate their secretions under different environmental conditions (see for example, Thangarajah H et al Stem Cells 2009;27:266-74). Therefore, the cells almost certainly secrete different molecules under these conditions.

In order to confirm these results, similar experiments in larger animals are warranted, since the rodent heart is a relatively poor model for the human heart as it beats much faster than human hearts.

See James Richardson, et al Journal of Cardiac Failure 2013;19(5):342-53.