Key Molecule Thwarts Effort to Make Artificial Blood : Medicine: Large firms as well as the military have long been on the track of a substitute, but the way the body metabolizes oxygen poses a roadblock.

BOSTON — Attempts to manufacture an artificial blood have run into an unexpected roadblock that may seriously delay commercialization of the products, a UC San Diego physician said here.

Although scientists from the four companies now conducting clinical trials on the artificial bloods have been closemouthed about the preliminary results from their trials, enough information has leaked out to allow observers to deduce the cause of the problems, said Dr. Robert M. Winslow, a professor of medicine at UC San Diego who was formerly in charge of the U.S. Army’s attempts to develop an artificial blood.

Volunteers who have received the artificial bloods in safety testing have reported a broad variety of unexpected side effects, including chest pains, back pains, nausea and hypertension. Physicians had been at a loss to explain how such unlikely symptoms have arisen.

The culprit, it now seems, is the way the artificial blood reacts with a simple molecule called nitric oxide, or NO, Winslow said over the weekend at a meeting of the American Assn. for the Advancement of Science. As recently as two years ago, physiologists were unaware that NO played any important role in human bodies. Since then, however, they have discovered that it is a crucial chemical messenger that participates in a wide variety of functions ranging from penile erections to oxygen intake in the lungs.

Its role in the body is so important that Science magazine last month named it “Molecule of the Year.”

NO was recently recognized as a vasorelaxant, that is, NO in the tissues surrounding blood vessels causes them to expand, thereby allowing freer flow of blood.

It now seems clear, Winslow said, that hemoglobin in the artificial bloods is leaking out of blood vessels and into the surrounding tissues, where it binds NO tightly, blocking its vasorelaxant properties. The blood vessels then constrict, causing the unusual symptoms.

The problem may be very difficult to solve because NO binds with the hemoglobin at the same molecular site as oxygen does. Any attempts to prevent the binding are likely to restrict hemoglobin’s oxygen-carrying capacity.

“I try not to be overly pessimistic,” Winslow said, “but this looks like a very serious problem.”

Many companies, as well as the military, 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.

A blood substitute 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.

Estimates for a potential world market for artificial blood range from a low of $2 billion to more than $10 billion.

The artificial bloods are centered on hemoglobin. Each red blood cell in the human body contains about 5 billion hemoglobin molecules, which bind to oxygen in the lungs, then release it elsewhere in the body.

It is possible to extract hemoglobin from red blood cells and inject it into the bloodstream. But the hemoglobin molecules don’t survive long enough to do the recipient much good. They are so small that the kidneys clear them out of the blood within a few hours. A successful blood substitute would have to last for at least 48 hours, long enough for the body to start regenerating new red blood cells.

One solution is to chemically link individual hemoglobin molecules into a much larger molecule, or polymer, that lasts in the blood for a few days. Several groups have done this with human hemoglobin, as well as hemoglobin obtained from cows or produced by genetic engineering, and shown that the polymer is a safe and effective substitute for blood in a variety of animals, including rodents, dogs and primates.

But it now seems clear that the polymerized hemoglobin is still small enough to escape through the walls of blood vessels into the so-called interstitial spaces between cells, where it scavenges all the free NO.

The solution to this problem is not obvious, Winslow said. If the polymers are made any larger, the blood becomes too viscous, which can cause its own problems.

Another potential approach is to encapsulate the hemoglobin polymers in fat globules called liposomes. This has proved effective in small-scale experiments, but it is difficult to produce the liposomes in large quantities and even more difficult to sterilize it adequately. It will be at least four years, Winslow predicted, until such preparations can be tried in humans.

For the longer term, suggested molecular biologist James M. Manning of the Rockefeller University in New York City, the best solution may be to use genetic engineering to alter hemoglobin so that it doesn’t bind NO so strongly. But because NO is so similar to oxygen, he added, “that will be very difficult.”