In any competition for the pathological research trail-blazer, the winner would doubtless be sickle-cell disease. Consider:
• In 1957, SCD became the first genetic disorder for which a causative mutation was identified at the molecular level.
• It was also the first genetic disease detected by prenatal diagnosis.
• And in 1979, SCD became the first malady for which gene therapy was attempted.
At the molecular level, NIH clinical pulmonary physiologist Mark Gladwin recites the research progression of sickle cell disease: "People have known for more than 50 years that there's hemolysis - destruction of red blood cells - and that the levels of hemoglobin in the plasma of sickle-cell patients are elevated. We have also known for 15 years that nitric oxide [NO] is critically important for blood-flow regulation, and we've known the last 10 years that hemoglobin outside of the red cells - that is, cell-free hemoglobin - destroyed NO 1,000 times faster than hemoglobin safely enclosed within red blood cells."
Gladwin combines research with clinical patient care - pediatric and adult - at NIH's Clinical Center in Bethesda, Md. He is senior author of an article in Nature Medicine published online Nov. 11, 2002. Its title: "Cell-free hemoglobin limits nitric oxide bioavailability in sickle-cell disease."
"Our finding is the first description of a possible mechanism in SCD," Gladwin told BioWorld Today. "We report that substantial hemolysis is part of that disease. Breaking up the red cells and releasing their hemoglobin into the plasma results in that hemoglobin destroying the nitric oxide that's made by blood vessels. And that gas is the most important regulator of blood flow in homeostatic blood vessels. Destroying endogenous NO, we believe, wreaks havoc on the vasculature, and contributes to the pathogenesis of sickle-cell pain crises. Over the last 10 years, people conducting clinical trials of artificial blood substitutes infused cell-free hemoglobin solutions, so there was increased mortality rate in the treated patients."
Putting 1 + 1 Together To Explain NO Loss
"But no one had asked the question, Are elevated levels of cell-free hemoglobin sickle cells destroying NO in patients with SCD?' What we showed was a novel mechanism of disease in patients with hemolysis. Not only is this important to SCD," Gladwin went on, "but may be significant for other hemolytic diseases such as thalassemia, as well as for therapies like transfusion of aged blood or cardiobypass that runs too long. All of these things have been associated with increased mortality, but there wasn't a mechanism to explain it.
"In sickle-cell disease," Gladwin observed, "pain crises begin at birth. In adults they're incredibly painful, probably among the severest pains in the world. Women who've had babies say it's worse than childbirth. Men have compared the pain crises to being hit in the thigh with a sledgehammer, or stabbed repeatedly in the shin with an ice pick. The painful attacks can last as long as four days, recur on average twice a year, with narcotics the only relief.
"The sickling in this process," Gladwin explained, "blocks blood flow to the bone and the periostium, which is the nerve-rich region that surrounds bone. And there's actual lack of blood flow and injury or death to that tissue, which creates exquisite bone pain. It can affect any bone in the body, but most commonly the long bones of the femur, the hips, pelvis and vertebrae.
"In the U.S.," he noted, "one out of 500 African-Americans has the disease, and another one in eight carries the trait - meaning [that carrier] harbors one of the mutant genes, but doesn't suffer from illness. Worldwide, the percentages in Africa are close to 1 percent of the population. SCD is also present in India, Southeast Asia and Saudi Arabia. In Africa, most sickle-cell victims die as babies, so very few adults live with the disease. An estimated 30 million infants are born in sub-Saharan Africa every year with this disease, and 98 percent die of it.
"Relief from pain crisis," Gladwin added, "consists only of fluids and pain-killing compounds There are no effective current therapies, which is why people are interested in the results we present in this paper. Because we suggest a new therapy - inhaled high doses of NO - to try to knock out that hemoglobin circulating loose in the plasma. FDA recently approved hydroxyurea, a drug that reduces the attack rate of pain crises by about half. Other than that -that's it."
Gladwin recounted his clinical experiments: "In 27 patients with SCD we measured the ability of their plasma to destroy NO and found it very high. We showed that that was caused by free hemoglobin in the plasma, which had been released secondary to hemolysis. We then infused an exogenous source of NO called nitroprusside into the patients' forearms. In those with the highest hemoglobin levels in their blood, nitroprusside didn't work, because NO was destroyed by the hemoglobin. Finally, we had those 27 SCD patients inhale exogenous NO gas from cylinders, which knocked out that hemoglobin and restored the NO in their bodies."
Four-Center Clinical Trial To Enroll 200 Kids, Adults
"They breathed it into the alveoli of their lungs where it met the blood going through the lungs. It reacted first with the free hemoglobin outside of the red cells. Then it oxidized the iron in the hemoglobin, thereby knocking out its ability to subsequently scavenge NO in the rest of the body.
"We are now evaluating whether a high level of hemoglobin in the plasma contributes to organ dysfunction, pain and poor blood flow," Gladwin related. "At the same time we're preparing a four-center therapeutic human trial of inhaled NO vs. placebo air. This will take place here at the NIH Clinical Center. Also, at the Johns Hopkins University, probably at Oakland Childrens' and at Harvard Childrens'. Those trials will be enrolling some 200 kids and adults, starting in six to 12 months, and probably running two years.
"One of the challenges in the field of NO therapeutics is attempting to deliver this gas in a way that's practical for patients," Gladwin concluded. "This is an area of active investigation by industry."