Timing is everything—including, possibly, when treating cardiovascular disease. By administering a drug-like compound in mice at a certain point during the circadian cycle, researchers were able to slow atherosclerosis while minimizing side effects, according to a recent study in Cell Metabolism. The compound reduced the number of immune cells stuck to the artery walls by inhibiting a molecule called CCR2; timed administration reduced the number of cells at their peak levels of adhesion.
The findings showcase how the carefully timed targeting of a biological process that oscillates during a 24 hour cycle can enhance therapy, an emerging idea that has not been extensively tested, notes Ajay Chawla, professor of physiology and medicine at the University of California San Francisco. “This is a really nice proof of principle,” says Chawla, who was not involved in the study.
An early stage in the formation of atherosclerotic plaques entails the adhesion of myeloid cells, a class of white blood cells, to artery walls. This adhesion is thought to prompt plaque formation, in part because the cells take up cholesterol and then release it, along with cellular toxins, when they die.
In the new study, Oliver Soehnlein, a professor of vascular immunotherapy at Ludwig Maximilian University in Munich, and his colleagues examined myeloid cells in the arteries of mice prone to atherosclerosis due to genetic susceptibility and a high fat diet. The researchers counted cells stuck to the inner wall of the carotid artery, a large artery leading to the brain, using imaging in live animals.
Consistent with studies of normal mice, they found a circadian cycle in myeloid cell stickiness. About three-fold more cells stuck to the artery wall right after the mice began to make the transition to a resting state, compared with 12 hours later. The researchers also found a corresponding circadian cycle in a molecule called CCL2 on the artery wall that helps attach myeloid cells.
Curiously, the researchers observed the opposite circadian pattern of myeloid cell adhesion in very small veins, the microcirculation. Myeloid cell recruitment and attachment to this location helps the immune system defend against disease, and was least active when the animals made the transition to rest.
Previous work had attempted to treat atherosclerosis in mice by blocking the action of CCL2 with an inhibitor of its receptor, CCR2. Those results were disappointing, in part because of side effects such as suppression of host immune defenses, says Soehnlein. Could timed delivery minimize atherosclerosis while keeping the function of the immune system intact?
The answer seems to be yes, at least in mice during the early stages of disease. After starting the high fat diet, the researchers administered the CCR2 inhibitor to the mice once every 24 hours, timed to nudge cells off of the artery walls at their peak. The approach slowed the development of atherosclerosis but did not seem to impair immune processes mediated by CCR2—for instance, the attachment of myeloid cells to the lung microcirculation appeared to be normal in response to treatment with lipopolysaccharide, which mimics the effect of bacterial pneumonia.
“The idea of finding a sweet spot … a time point that is the best time point for therapy is an interesting, provocative idea,” says Filip Swirski, an immunologist and an associate professor at Harvard Medical School in Boston, who was not involved with the study. Chawla adds that many biological processes oscillate during the circadian cycle, as do a large proportion of the biological molecules targeted by currently used drugs—many disease treatments could potentially benefit from a time-targeted approach.
It’s not clear why all these rhythms occur, says Soehnlein, though he suspects the body is optimizing energy expenditure. “Biological processes cost a lot of energy,” he says. “The body reduces some processes for parts of the day but increases others.”
Meanwhile, Soehnlein says his group has preliminary data suggesting that similar circadian oscillations in CCL2 occur in the large arteries of humans prone to heart attacks. His group also plans to test whether timed inhibition of CCR2 can slow the development of already-established plaques in mice, not just the growth of new ones.