New Blood Substitute Prototypes Engineered Without Blood Pressure Effect

For most people in an emergency room, having blood available - and disease-free - is of utmost concern. To guard against shortages and potential disease transmission, blood substitutes are being developed to deliver oxygen to the brain and heart.

Most of the protein-based blood substitutes now in clinical trials have a side effect that may not be desirable for some patient populations - increases in blood pressure.

Researchers from Baxter Healthcare Corporation's Hemoglobin Therapeutic Division in Boulder, Colo., (formerly Somatogen, Inc.) and Rice biochemist Dr. John Olson have determined why blood pressure increases when hemoglobin-based blood substitutes are given to animals, and they have engineered new prototype molecules without this side effect.

"Researchers have known that hemoglobin, the carrier of oxygen inside a red blood cell, also reacts with nitric oxide, a compound that is required for maintaining normal blood pressure, " Dr. Olson says. "When the two react, the nitric oxide is depleted and blood pressure tends to go up."

Hemoglobins designed to steer clear of nitric oxide may be more suitable for certain blood replacement uses than those which cause significant loss of nitric oxide (NO).

Using genetic engineering methods to alter the hemoglobin reaction with NO, the Baxter team, led by Douglas Lemon, who received a Ph.D. from Rice in 1989, and Dr. Olson found that the rate of reaction with NO determines the magnitude of the blood pressure effect. The faster the cell-free hemoglobin reacts with the NO, depleting the reservoir of NO in the system, the greater the increases in blood pressure.

Their research appears in the July issue of Nature Biotechnology.

Nitric oxide is found in the endothelial and smooth muscle cells lining the outer walls of blood vessels. It is thought that because simple cell-free hemoglobins are small, they slip through the blood vessel walls to react with NO. Researchers first designed interconnected hemoglobins to increase their size and discourage passage through the walls.

Dr. Olson, the Dorothy and Ralph Looney Professor of Biochemistry and Cell Biology and director of the Houston Area Molecular Biophysics Predoctoral Training Program (an NIH-sponsored program which also involves several Texas Medical Center institutions), says he adopted an alternative strategy to this problem: figure out the chemical mechanism of the reaction with nitric oxide and then genetically engineer the hemoglobin to inhibit the process.

"We needed a way to keep the bound oxygen molecule from reacting with nitric oxide and the iron atom from rusting," Dr. Olson says, "so we placed a protective film of large oily amino acids around them."

Says Dr. Lemon: "When the engineered hemoglobin molecules were tested in rats, the blood pressure increase was proportional to the rate of reaction with nitric oxide. At the lowest reaction rate, the blood pressure effect was nearly eliminated."

It is possible that other side effects sometimes seen in clinical trials with blood substitutes, such as gastrointestinal dysfunction, are also based on nitric oxide depletion and inhibition of smooth muscle relaxation, making the new prototype molecules attractive choices for a second generation blood substitute.

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©2006 Texas Medical Center

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