A little more than a decade ago, nitric oxide, or NO, was primarily viewed as a gaseous component of air pollution. Since then, however, researchers have discovered that cells in the body produce this simple compound. Nitric oxide serves a variety of purposes, including boosting the immune response and regulating blood pressure (SN: 10/17/98, p. 246). Consequently, physicians have begun to look at nitric oxide’s potential to heal rather than harm.
Because the compound causes blood vessels to dilate, enabling more blood to flow through them, researchers suspected that inhaling nitric oxide might help patients who were having trouble breathing. Drug treatments to widen blood vessels affect the entire body and thus can lead to dangerously low blood pressure.
Inhaled nitric oxide, however, seems to affect blood vessels in the lungs alone, thus avoiding a serious potential side effect. Just last month, the Food and Drug Administration approved the first treatment using inhaled nitric oxide. It targets newborns with respiratory failure.
Physicians are now contemplating roles for inhaled nitric oxide in other diseases, especially those with lung disease. This approach may offer a much-needed treatment for patients suffering from complications of sickle cell disease, an inherited blood disorder characterized by chronic anemia and periodic episodes of pain, says David L. Wessel of Children’s Hospital in Boston. “These folks need many [blood] transfusions over their lifetime and may regularly need to be on ventilators in order to breathe,” he says.
Physicians like Wessel—who tested nitric oxide’s effects in infants in studies leading to the FDA approval—theorized that inhaled nitric oxide might benefit patients with acute chest syndrome, a severe manifestation of sickle cell disease. Abnormally shaped, or sickled, red blood cells get trapped in blood vessels in the lungs of people with the syndrome. By opening up these vessels, nitric oxide would allow more blood cells to move through the lungs and pick up oxygen.
Early reports appear to confirm this idea. Moreover, preliminary and still controversial research suggests that nitric oxide might improve blood flow outside the lungs. Nitric oxide may limit the sickling of blood cells, prevent cells from sticking to vessel walls, or dilate peripheral blood vessels, says Mark T. Gladwin of the National Institutes of Health in Bethesda, Md.
“There are simply so many ways it might work that there’s a growing excitement about using nitric oxide for sickle-cell disease,” Gladwin says. If inhaled-nitric oxide therapy has effects other than dilating blood vessels in the lungs, they must be due to mechanisms that run counter to classical understandings of the compound’s action, Gladwin says.
Sickle-cell disease is a genetic disorder that affects about 72,000 people in the United States, most of them African Americans. A single mutation in the gene that encodes hemoglobin causes the disease. Normal hemoglobin in red blood cells carries oxygen from the lungs to the rest of the body, and picks up carbon dioxide and carries it back to the lungs.
In people with sickle-cell disease, however, hemoglobin doesn’t hold onto oxygen well. Sickle hemoglobin without oxygen bound to it is stickier than normal, so many of these molecules cluster together in long rods. These structures, in turn, cause some red blood cells to become stiff and assume a crescent shape.
The sickled blood cells can’t roll through small vessels as easily as normal, lozenge-shaped red blood cells can. Instead, they pile up into small clots and cause blockages that deprive tissues of oxygen. The outcome is periodic episodes of pain, especially in bones and joints.
People with sickle-cell disease are also at high risk of other complications, especially stroke. There are few effective treatments for the disease, and most have serious side effects. Basic treatment of painful episodes, or crises, relies on painkillers and intravenous fluids. In the past few years, doctors have prescribed a preventive regimen of hydroxyurea to ward off the disease’s painful symptoms.
Acute chest syndrome—characterized by chest pain, fever, high blood pressure in the lungs, and a chest X ray showing clogged or collapsed lungs—is the most life-threatening complication of sickle-cell disease. The syndrome generally strikes children.
In 1997, Wessel and his colleague Andrew M. Atz demonstrated that inhaled nitric oxide dilated blood vessels in the lungs of two young sickle-cell patients suffering from acute chest syndrome.
The treatment led to increased oxygen-carrying capacity of the children’s blood and may have helped them survive the crisis, the researchers reported in the October 1997 Anesthesiology.
Since then, they have treated another 10 patients “with very favorable results,” Wessel says. In just the third published case of nitric oxide use to treat sickle-cell disease, Paulette Mehta of the University of Florida College of Medicine in Gainesville and her colleagues reported similar success in the November 1999 Critical Care Medicine. A child with the disorder had become progressively sicker despite repeated transfusions, the standard treatment for the final stages of the disease. He was on a respirator and appeared to be dying of acute chest syndrome. However, just 3 days after the medical team gave him nitric oxide therapy, he was so improved that he breathed on his own, Mehta says.
“We had nothing else to offer this patient, yet the nitric oxide treatment was extremely successful” in getting the boy off a respirator and out of the hospital, she says. Despite the limited number of patients, these are “dramatic case reports, and [acute chest syndrome is] probably one of the situations in which nitric oxide is most likely to be used,” Gladwin says. Further studies are needed to confirm the benefits, he adds.
There are many possible explanations for nitric oxide’s apparent benefits to people with sickle-cell disease. Some studies have suggested that nitric oxide reduces the number of platelets—the blood’s clotting cells—and thus the number of blockages that deprive tissues of oxygen and lead to pain. Other research has suggested that nitric oxide may make blood cells less likely to adhere to blood-vessel walls.
Finally, some researchers propose that nitric oxide binds directly to hemoglobin. This reaction may prevent the protein from forming long chains, or polymerizing, and deforming cells.
Also, hemoglobin may carry nitric oxide throughout the body, so the compound can have beneficial effects on a variety of tissues.
Nitric oxide shouldn’t act beyond the lungs because it reacts quickly with hemoglobin to produce a toxic compound, says Jack R. Lancaster Jr. of Louisiana State University Medical Center in New Orleans.
Four years ago, however, a group led by Jonathan S. Stamler of the Howard Hughes Medical Institute at Duke University Medical Center in Durham, N.C., suggested that nitric oxide might bind to a special site on hemoglobin and harmlessly hitch a ride to tissues beyond the lungs (SN: 3/23/96, p. 180). Stamler’s team proposed that when the hemoglobin molecules release oxygen in those tissues, they also release nitric oxide. It then dilates the blood vessels and helps the oxygen get to cells that need it.
This work, in part, has prompted some biomedical researchers to examine whether inhaled nitric oxide has effects outside the lungs, and the answer appears to be a qualified yes.
C. Alvin Head of Massachusetts General Hospital in Boston and his colleagues have found that adding nitric oxide to red blood cells in test tubes makes sickle hemoglobin hang on to oxygen better, although it doesn’t appear to affect normal hemoglobin. After nine sickle-cell patients breathed a mixture of air and nitric oxide gas for 45 minutes and then gave blood samples, their abnormal hemoglobin bound to oxygen almost as tightly as normal hemoglobin does, the team first reported in the September 1997 Journal of Clinical Investigation (SN: 9/20/97, p. 188). The oxygen affinity of hemoglobin from three people without sickle-cell disease was unaffected.
“What I’m hoping is that if nitric oxide makes sickle-cell hemoglobin hang on to oxygen more tightly, it will also reduce the number of sickled red blood cells,” Head says. He’s starting a multicenter trial to test whether inhaled nitric oxide can reduce pain or illness in sickle-cell disease.
In mice with a disorder resembling sickle-cell disease, breathing nitric oxide seems to increase the survival of animals stressed by low oxygen, Head reported in the September 1999 Anesthesiology.
“I’m excited by Stamler’s work,” he says. The place on the hemoglobin molecule where Stamler’s experiments suggest that nitric oxide binds is one of the spots where sickle-cell hemoglobin molecules stick together, Head says. “If the nitric oxide does bind there, it should reduce polymerization, which is what we’re seeing,” he says. However, another group that has done a similar study has come up with somewhat conflicting results. Among seven people with sickle-cell disease and five without it, breathing various concentrations of nitric oxide did not change the affinity with which sickle hemoglobin bound to oxygen in blood drawn from the test subjects, Gladwin and his colleagues reported in the October 1999 Journal of Clinical Investigation.
The team found that breathing nitric oxide did increase the hemoglobin bound to nitric oxide. Tests showed slightly more nitric oxide-bound hemoglobin in blood taken from arteries leaving the lungs than in blood in veins returning to the lungs. Like Stamler’s results, these findings suggest that hemoglobin ferries nitric oxide from the lungs, Gladwin says.
“Although, based on our data, we’re skeptical that there is an antisickling effect because of inhaled nitric oxide, we do believe that nitric oxide has effects throughout the body and that it holds promise for the treatment of sickle cell disease,” he says.
Gladwin and his colleagues plan to test their hypothesis that hemoglobin releases nitric oxide in regions where blood flow has been blocked, dilating blood vessels and helping sickled cells move throughout the body.
The study of nitric oxide, its effects on hemoglobin, and its possible value for treatment of sickle-cell disease “is a field in flux,” says Ronald L. Nagel of the Albert Einstein College of Medicine in New York. Nonetheless, he’s cautiously optimistic about the therapy’s value, at least for acute chest syndrome.
“Nitric oxide is a very, very safe drug with virtually no side effects,” agrees Gladwin. No study has yet reported a problem with the toxic compound that nitric oxide and hemoglobin can form.
Wessel cautions that side effects might show up as researchers and physicians use the drug in more patients. If nitric oxide does have effects throughout the body, rather than just in the lungs, low blood pressure is possible, though unlikely, he says.
Wessel, Head, and others hold out the hope that patients might eventually control their sickle-cell disease with a nitric oxide inhaler, much as asthma sufferers today ward off attacks with inhalers delivering medication. However, the best way to use nitric oxide for sickle-cell disease has yet to be determined, researchers say.
“The next step is to organize randomized trials to confirm there’s a benefit,” Wessel says. “Does the drug reduce mortality [or] time spent on a ventilator or lead to fewer transfusions and less pain?” he asks.
“Large-scale clinical trials are certainly not premature, even if we are not exactly sure of the mechanism by which nitric oxide might have an effect,” Nagel says. “After all, humans have been taking aspirin for many years with good effects even though the underlying mechanism was only described recently.”
A breathless account of hemoglobin’s lowly origins
Nitric oxide research may be shedding light on hemoglobin’s evolutionary bloodlines, as well as on its current function in animals’ red blood cells.
The story begins with a common intestinal parasite called Ascaris lumbricoides, which infects about a billion people worldwide. The worm has huge hemoglobin molecules that bind to oxygen about 25,000 times more tightly than human hemoglobin does.
Scientists at Duke University in Durham, N.C., and Washington University in St. Louis have studied how the unusual hemoglobin reacts to nitric oxide and oxygen. The results—reported in the Sept. 30, 1999 Nature—led them to propose that the worm’s hemoglobin seeks out and destroys oxygen by reacting with nitric oxide in a complex, multiple-step reaction. The worm, which lives in low-oxygen environments, probably finds oxygen toxic, explains Jonathan S. Stamler of Duke University.
The discovery places Ascaris hemoglobin at an evolutionary junction between bacteria, whose hemoglobin destroys nitric oxide, and mammals and birds, whose hemoglobin delivers oxygen and possibly nitric oxide to tissues.
“The worm is evolving the first indications of a respiratory function involving oxygen concentrations,” Stamler says. “The worm is controlling oxygen concentrations by using nitric oxide just as we do—only with a different outcome.” Ultimately, evolution loosened the bonds between oxygen and hemoglobin in some creatures and came up with ways of using that oxygen, he believes.
The new findings add up to “an elegant example of molecular evolution,” says Steven S. Gross, a biochemist at Cornell University Medical College in New York City.
Not everyone agrees with Stamler’s theories, however. Takashi Yonetani of the University of Pennsylvania School of Medicine in Philadelphia says that the complicated reaction Stamler proposes is implausible because it requires too much energy. He suggests that the worm’s hemoglobin doesn’t destroy oxygen but binds it tightly to prevent its toxic effects.
If Stamler’s ideas are borne out by further research, however, they may have benefits for human medicine. For example, understanding Ascaris‘ oxygen-hungry hemoglobin may help researchers find novel ways to starve cancerous tumors of oxygen, Stamler says.