Web Date: August 19, 2013
For Blood Substitutes, Bigger May Be Better
Doctors have dreamed of artificial blood substitutes for when accident victims or wounded soldiers need blood transfusions and donor blood supplies are thin. But many proposed substitutes have failed because they restrict blood flow by causing vessels to narrow. Now researchers report a new oxygen carrier built from animal hemoglobin that could get around this constriction problem (ACS Nano 2013, DOI: 10.1021/nn402073n).
Mimicking blood isn’t easy, as failed experiments on artificial blood in the late 1990s and 2000s disastrously showed. Scientists have based their substitutes on hemoglobin, the protein in blood that delivers oxygen to the body’s tissues by binding and releasing the gas. Unfortunately, the protein is toxic when not contained inside a red blood cell. So researchers started to encase hemoglobin inside biocompatible particles. Previous studies have shown that these particles cause vasoconstriction, a narrowing of blood vessels.
One possible explanation for the side effect is that the particles are so small that they can sneak through gaps between endothelial cells in blood vessels. Once inside these gaps, the hemoglobin starts to bind nitric oxide (NO), a gas produced by endothelial cells to dilate blood vessels. Without NO, the vessels don’t relax properly, and blood can’t flow.
A team led by Hans Bäumler and Yu Xiong, at the Charité Medical University in Berlin, tried to overcome this problem with a strategy used by others: make particles that are between 100 nm and 1 µm in diameter. The upper size limit is to prevent the particles themselves from blocking circulation or being engulfed by immune cells.
The team made these hemoglobin-based oxygen carriers (HBOCs) through a simple novel procedure, Bäumler, says. First they co-precipitated bovine hemoglobin out of cow’s blood with manganese carbonate. The porous manganese carbonate allowed the proteins to assemble into large particles. They then added human serum albumin, a blood plasma protein, to coat the particles and prevent them from clumping together. By adding a small amount of glutaraldehyde, the researchers cross-linked the hemoglobin units, so that they could dissolve away the manganese carbonate and keep the proteins stuck together. The resulting particles were all about 700 nm in diameter.
The team tested their HBOCs by flowing the particles through isolated mouse kidney arterioles. They observed that the blood vessels didn’t constrict significantly. Conversely, hemoglobin alone made the vessels narrow down to just 6.3% of their original diameter.
Through a spectroscopic technique, the researchers also measured the particles’ affinity for oxygen and found that they had significantly higher affinity than free hemoglobin in solution. High affinity for oxygen solves another problem with blood substitutes. Previous carriers released their oxygen cargo too readily, causing dangerous free radicals to form, Bäumler says. “We need carriers that release oxygen only in the regions that need oxygen.” So HBOCs need to have a high affinity for oxygen, holding the gas in place until it reaches a part of the body that needs oxygen.
Although the team’s particles are a promising example of a new HBOC, many tests are needed before they can be ready for clinical trials in humans, says Leif Bülow, a hemoglobin expert from Lund University in Sweden. The researchers should look at a range of questions, including whether hemoglobin can be released from the particles, how the particles are degraded in the body, and whether the process can be scaled up in a cost-effective way.
Bäumler says his group is about to start studies in live animals and hopes to collect enough data to proceed to clinical trials in people.
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