Researchers at Johns Hopkins have found that sildenafil citrate (Viagra), a drug used to treat erectile dysfunction (ED) in millions of men, effectively treats enlarged hearts in mice, stopping further muscle growth from occurring and reversing existing growth, including the cellular and functional damage it created.
"A larger-than-normal heart is a serious medical condition, known as hypertrophy, and is a common feature of heart failure that can be fatal," says study senior author and cardiologist David Kass, M.D., a professor at The Johns Hopkins University School of Medicine and its Heart Institute. Kass is also the Abraham and Virginia Weiss Professor of Cardiology at Hopkins.
Sildenafil, Kass says, was the focus of his research because it blocks or stops an enzyme, called phosphodiesterase 5 (PDE5A), involved in the breakdown of a key molecule, cyclic GMP, which serves as a "natural brake" to stresses and overgrowth in the heart. "We thought we could more strongly apply the brake on hypertrophy in the heart if we used sildenafil to prevent the breakdown of cyclic GMP," he says. The makers of the drug had no involvement in the design or support of the research. PDE5A is also the biological pathway blocked in the penis to prevent the relaxation of blood vessels and maintain erections.
The Johns Hopkins findings, to be published in the journal Nature Medicine online Jan. 23, are the first to show that sildenafil is an effective treatment for a chronic heart condition. It is also the first study to reveal that the enzyme pathway blocked by sildenafil (PDE5A), never before known to play a significant role in the heart, is active when the heart is exposed to pressure stress and hypertrophied. The results provide some of the strongest evidence to date that blocking the heart's adaptive response to hypertrophy does not harm its function but, in fact, may improve it, Kass says. Already, plans are under way by the Hopkins researchers for a multicenter trial to test if sildenafil has the same effects on hypertrophy in humans.
In the first of several experiments, each involving groups of 10 to 40 male mice, the Hopkins team stimulated hypertrophy for up to nine weeks, but only by half as much in those that had also consumed sildenafil in their food at 100 milligrams per kilogram per day. In mice, this dose produces blood levels similar to those achieved in humans given standard clinical doses.
The mice fed sildenafil also showed 67 percent less muscle fibrosis, a complication that often occurs with hypertrophy, as compared to mice that were not fed the drug. The treated mice also had smaller hearts and improved heart function, whereas the untreated hearts were dilated with weakened function. For all mice with hypertrophy, the condition was surgically produced by constricting the main artery carrying blood from the heart to create pressure stress.
In a second experiment, the researchers used the same dose of sildenafil and examined its effects on reversing hypertrophy that had already occurred. Initially, these mice were exposed to pressure stress for seven to 10 days, with hearts developing fibrosis and muscle growth by nearly 65 percent. After two weeks of therapy, fibrosis and muscle growth almost completely disappeared. In mice that did not have therapy, hearts continued to get bigger.
In a surprising result, the researchers found that heart function, as measured by pressure-volume analysis of the muscle's ability to contract and pump blood, actually improved after hypertrophy had been stopped and treated. While researchers previously thought that hypertrophy was an adaptive response to pressure stress, the functional gains lasted despite the heart's continued exposure to high blood pressure. Improvements were seen in more than 10 measures of heart function, including heart relaxation, cardiac output and heart contractility, which increased by nearly 40 percent. These improvements were seen even when therapy was deferred and started two weeks after hypertrophy had already developed.
"This study shows that sildenafil can make hypertrophy go away," says Kass. "Its effects can be both stopped in their tracks and reversed. Overall, the results provide a better understanding of the biological pathways involved in hypertrophy and heart dilation, leading contributors to heart failure. They suggest possible therapies in the future, including sildenafil, which has the added benefit of already being studied as safe and effective for another medical condition."
"It also suggests that less but, perhaps, the right kind of hypertrophy can be good for the heart. However, more study is required before we fully understand the benefits of sildenafil on the heart."
Analysis of several enzymes known to play a major role in triggering hypertrophy, including calcineurin and Akt, showed increased activity and amounts in the hypertrophied heart, but after treatment with sildenafil, their levels returned back to normal levels.
Funding for this study, which took three years to complete, was provided entirely by the National Institutes of Health (NIH), American Heart Association, Peter Belfer Laboratory Foundation, American Physiological Society, and the Bernard Family Foundation. The lead Hopkins researchers who took part in this study were Eiki Takimoto, M.D., Ph.D.; Hunter Champion, M.D., Ph.D.; and Maxiang Li, M.D., Ph.D. Other researchers who took part in this study were Diego Belardi, M.D.; Shuxun Ren, M.D.; E. Rene Rodriguez, M.D.; Djahida Bedja, B.S.; Kathleen Gabrielson, D.V.M., Ph.D.; and Yibin Wang, Ph.D.
Cardiac hypertrophy commonly develops from high blood pressure, which forces the heart to pump harder to circulate blood throughout the body. According to the latest statistics from the American Heart Association, in 2002, 65 million Americans have high blood pressure (defined as systolic pressure of 140 millimeters of mercury or greater, and/or a diastolic pressure of 90 millimeters of mercury or greater, taking antihypertensive medication or being told at least twice by a physician or other health professional that they have high blood pressure). Patients that develop hypertrophy have two to three times the risk of suffering cardiovascular disease, including heart failure and sudden cardiac death.
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