MEDICINE / ARTIFICIAL BLOOD : Genetically Engineered Hemoglobin Tested in Humans
Colorado researchers have used genetic engineering to produce a form of artificial blood, representing a significant step in the search for a solution to the worldwide shortage of blood.
Researchers from Somatogen in Boulder report today in the British journal Nature that they have already begun human trials with the blood, which is produced in bacteria.
The artificial blood is a genetically engineered form of hemoglobin, the complicated protein that--enclosed in red blood cells--carries oxygen from the lungs to tissues throughout the body. The genetically engineered hemoglobin persists in the blood of dogs much longer than the naturally occurring form and causes no adverse side effects, they report. In separate studies not yet published, they have also found that the material effectively carries oxygen to tissues, pulmonologist Steven Shoemaker of Somatogen said.
Many companies have been searching for an artificial blood because of the annual worldwide shortage of about 100 million units of blood and the military’s need for blood replacements that can be stored in field conditions without refrigeration.
An artificial blood also would virtually eliminate the risk of contracting AIDS, hepatitis and other viral diseases through transfusions. Although AIDS is no longer considered a significant risk in transfusions, as many as 250,000 of the 4 million Americans who receive transfusions each year contract hepatitis as a result.
An artificial blood would also eliminate the need to match blood types before a transfusion because only the hemoglobin would be used. Currently people receive transfusions of either whole blood or red blood cells, both of which require matching.
The new research has two important aspects: It provides a potentially large new source of hemoglobin, and it eliminates the need to perform chemical modifications on the hemoglobin to make it perform effectively.
Each red blood cell in the body contains about 5 billion hemoglobin molecules, which are composed of four subunits, two called alpha and two called beta. While it is theoretically possible to extract hemoglobin from cells and inject it into the bloodstream, it rapidly falls apart into the subunits, which are cleared from the blood by the kidneys.
The hemoglobin thus would not last long enough to do the patient much good. Even worse, the fragments can damage the kidneys. Most researchers have attempted to circumvent this problem by chemically linking the subunits into larger molecules, but this process can leave potentially harmful byproducts.
Two other companies, Biopure Corp. of Boston and Northfield Laboratories of Evanston, Ill., have conducted human trials of such chemically modified hemoglobin blood substitutes in the last two years. Both trials were halted in the early stages, apparently because of side effects from impurities in the products.
Because Somatogen’s artificial blood is made by genetic engineering and does not have to be chemically treated, impurities should be less of a problem, Shoemaker said.
Apparently, many companies have shied away from attempting to produce hemoglobin in bacteria because it seemed unlikely that such a large, complex molecule could be made by the bacteria. Instead, geneticists have attempted to produce it in animals, whose cells could process the molecule.
Last summer, DNX Inc. of Princeton, N.J., reported that it could produce human hemoglobin in pigs. But experts predicted that isolating pure human hemoglobin from the pig blood would be very difficult, and it is expected to be at least another four years before the company has a product suitable for testing in humans.
Undaunted, molecular biologist Douglas Looker of Somatogen constructed a modified gene for human hemoglobin in which the subunits were fused together and inserted it into Escherichia coli, a common bacterium that is widely used in the genetic engineering industry. He also modified the gene so that the resultant hemoglobin would more readily release its oxygen to tissues.
He found that the bacteria made the protein and it could be easily separated.
Tests in dogs showed that the modified hemoglobin could carry oxygen to tissues and that it could persist in blood long enough to be of medical value. Perhaps more important, the tests showed that the altered hemoglobin was not excreted in urine and that, unlike normal hemoglobin, it did not damage the kidneys.
Safety tests in humans began in November at an unidentified medical center. Shoemaker said results probably will be reported sometime in the summer.
In addition to the medical value of an artificial blood, the search is also spurred by a strong financial incentive. Most companies involved in the race have estimated a potential selling price of about $250 per unit. A company that acquired only 20% of the U.S. market for blood would thus have annual sales greater than $500 million. Worldwide, the market would be in the billions of dollars.
