U~T San Diego, by Bradley J. Fikes ~ June 16, 2013
A new approach to reducing sickle cell anemia’s painful attacks has entered Phase II clinical trials.
The drug, Lexiscan, is already approved for diagnosing heart disease. Scientists led by Joel Linden, a researcher at the La Jolla Institute for Allergy & Immunology, discovered that the drug might also be useful for relieving the attacks sickle cell patients periodically suffer.
photoNormal red blood cells take a circular shape, while sickle cells take bent, angular shapes that cause them to clump in capillaries, cutting off the flow of blood to tissues. — Betty Pace
Patients are now being recruited for the Lexiscan study, to be conducted in Boston, Baltimore, Detroit, Chicago, Cincinnati, Milwaukee, Chapel Hill, and St. Louis. Dana-Farber Cancer Institute in Boston is the sponsor; the La Jolla Institute is a collaborator. Linden is the study’s co-lead investigator with David G. Nathan, of Dana-Farber (the “dean of hematology,” Linden calls him) and Joshua Field of BloodCenter of Wisconsin.
The trial is funded by a $10.8 million grant from the National Institutes of Health.
Lexiscan has an anti-inflammatory effect, and the trial is to study whether the drug can reduce inflammation and pain during an attack, helping restore blood flow. Research has suggested that sickle cell anemia is not merely the result of deformed red blood cells; inflammation is also involved.
A molecular disease
Separately, San Diego-based HemaQuest said it has finished recruiting sickle-cell patients for a Phase 2b trial of its drug, HQK-1001. That drug, HQK-1001, induces production of fetal hemoglobin, which appears to compensate for the abnormal hemoglobin in sickle cell anemia. It has received orphan drug designation for both sickle cell disease and beta thalassemia in both the United States and Europe.
HemaQuest moved its headquarters from Seattle to San Diego in 2011. It was founded in 2007 in Newton, Mass.
Sickle cell disease is caused by a mutation in hemoglobin, the oxygen-carrying protein in red blood cells. It makes the cells curve in a characteristic sharply curved shape, instead of the normal doughnut shape. The angular lines of these cells cause them to clump in capillaries, cutting off the flow of blood, called ischemia, and starting a cascade of damage.
The disease mainly occurs in people with ancestry from certain parts of sub-Saharan Africa and the Middle East where malaria has been endemic. Those who carry one gene for the mutant hemoglobin are healthy and partially protected from malaria, but those with two genes get the disease. In the United States, the disease is most common in African-Americans.
There are various types of sickle cell disease, some occurring in conjunction with another harmful mutation that causes beta thalassemia. The sickle cell mutation was reported in the November, 1949 issue of Science by researchers including Linus Pauling. It was the first description of a molecular disease.
Linden has studied the ischemic process for years; he specializes in the effects of adenosine, a breakdown element of ATP whose levels are increased by ischemia. (San Diego history note: Gensia, a company once based in San Diego, unsuccessfully tested adenosine-regulating drugs for cardiac uses. It moved to Irvine, changed its name to Gensia Sicor and entered the generics business. The company was later purchased by Israeli pharmaceutical company Teva.)
Heart attack is ischemia in the coronary arteries. It is often treated by installing stents to restore blood flow. But damage can continue even then, Linden said.
“During the period of tissue ischemia, some of those tissues have been damaged,” Linden said. “They’re hypoxic, they’re not getting any nutrients, and when blood starts flowing into those little blood vessels, the white cells become adherent and they literally clog up the microvessels. So you go initially from a vessel that’s clogged with atherosclerosis or a clot, you open that up, and you end up with a secondary clogging. Except now, it’s in the tiny blood vessels that are downstream.”
Cardiologists call this the “no reflow” phenomenon Linden said.
“They’ll balloon open an artery, blood will start to flow, and then it will stop again,” Linden said.
Linden and colleagues studying no-reflow found that drugs activating adenosine receptors on the NKT white blood cells turned off their adherent effect, and could block the reperfusion injury.
“We found that there was an inflammatory cascade initiated by the activation of these cells and then you ended up activating a whole bunch of white cells, including neutrophils, which are abundant, and these would actually clog up the arterioles and venules,” Linden said.
Sickle cell connection
Linden says he made the link between reperfusion injury and sickle cell anemia while reading a journal about the disease.
“There are two things I read: One is that sickle-cell anemia is associated with vaso-occusion, multi-tissue, widely disseminated. The other thing is that patients who have this disease have white-cell activation,” Linden said. “So I thought, geez, this sounds just like ischemia reperfusion injury, in fact it probably is ischemia reperfusion injury. These red cells are basically creating vaso-occlusion, just like you have in coronary artery disease, except that instead of it being caused by a lipid plaque or a clot, it’s caused by red cells that are occluding the micro-vasculature.”
Linden and colleagues experimented on transgenic mice that had been given the mutated human hemoglobin gene, causing the mice to make sickle-shaped red blood cells. The mice were then treated to remove the white blood cells that cause the inflammation. They got much better, although still ill, Linden said.
“What we figured out we were doing is we were blocking the white cell activation,” he said. And that led to a different view of sickle-cell anemia than the classic model.
“Instead of thinking of sickle cell anemia just as a disease of red cells, the new concept was, it’s a combination of red cells and white cells,” Linden said. “Red cells cause the initial injury, then you get ischemia, and then you generate factors that activate these NKT cells and other white cells, and you get a multi-cellular aggregate that clogs up the arteries.”
This change in thinking was exemplified by an Aug. 1, 2000 article in the Journal of Clinical Investigation reporting that reperfusion injury occurred in the transgenic sickle cell mice but not in normal mice. Linden and colleagues then found the mechanism that caused the NKT cells to start the process.
A company called NKT Therapeutics is conducting a Phase I clinical trial of a monoclonal antibody that depletes the NKT cells in patients with stable sickle cell disease.
“The hope is they’ll get long-term protection without getting too much immunosuppression,” Linden said.
However, Linden said it’s better not to deplete white blood cells, to preserve their infection-fighting power. His group’s approach is to temporarily turn off the cells’ ability to aggregate, by administering a drug during a crisis.
“It turns out adenosine receptors on these T cells turn them off,” Linden said. “So we thought all we need to do is treat with adenosine, or synthetic adenosine analogs that activate this particular receptor, and that’s going to turn off these cells.”
Linden and colleagues then looked for such adenosine analogs. They found one in Lexiscan, which was already approved for pharmacological stress-testing for heart disease. Lexiscan opens up coronary arteries, similar to what exercise would do. It’s given for patients who aren’t able to safely exercise for a stress test.
Lexiscan is injected rapidly for stress-testing; for sickle cell patients it is given as an infusion of up to 48 hours. The safety of this approach was successfully tested in a Phase I study. Linden’s role is to analyze the white cells for activation.
“As a secondary endpoint, we took blood from these patients and looked at the NKT cell activation markers,” Linden said. “We found out that in sickle cell disease they were really activated, which is exactly what we saw in the mice. And when we gave them the adenosine compound the activation was inhibited.”
Asked why there were no trial sites in California, Linden said part of the issue is finding sites that not only have large numbers of sickle cell patients, but also research programs suited to the study.