There will be blood
The most precious liquid in the world is spilled every day on battlefields and in operating theatres. So creating a synthetic version in the lab is more vital than ever, says Roger Dobson
Monday 02 August 2010
For centuries, scientists have been trying to find a substitute for blood. Almost anything and everything was tried, including milk, red wine, water, and even urine, but with no success. In the summer of 1667, when the Dutch were sending fire ships up the Medway, and John Milton was pondering whether he had been right to take a tenner for Paradise Lost, hopes were briefly raised when a French boy was given an infusion of sheep blood. He survived, probably because he did not get enough of the stuff to cause a reaction, but several subsequent patients died, and the idea was abandoned.
The arrival of human blood transfusion in the early 19th century, brought a temporary halt to the blood substitution quest, but in more recent times, concerns about infections, a drop in donors, an increase in demand and a rise in elective surgery have fuelled a new search for artificial blood.
With more than £1bn spent so far, some scientists believe that a substitute, or artificial blood, may be closer that ever. Over the last two centuries blood transfusions have been hugely successful. Each year more than 75m units of donated blood are given to people for use in hospitals alone, but a global annual shortage of more than 4m units has been forecast.
Public concern about infections like HIV and hepatitis, difficulties with getting enough donors, issues about cultural and religious objections, and problems with storage of blood have helped to intensify the research for a blood replacements. Military needs for instant blood for use on the battlefield have been a key driving force too. But finding a substitute for blood, or creating artificial blood, was never going to be easy, despite the enticing prospect raised in the TV show True Blood, of the development of synthetic blood called so much like the real deal that vampires are able to ingest it.
Blood is a complex material with several key elements, each of which plays a key role in our survival. White blood cells fight disease, and platelets allow the blood to clot when necessary, while red cells carry oxygen and carbon dioxide around the body.
Because of that complexity, all the research so far has concentrated not on a total blood replacement but on products that mimic the action of red blood cells, transporting oxygen into the body and taking carbon dioxide out, but which lack the ability to fight diseases and clot.
To be of any great use, artificial blood needs to be free of the problems associated with donated human blood. As well as being able to work like red blood cells, it needs to be safe and non-toxic, have a long shelf life, be easy to store, not need to be matched for blood group, and be free of contamination.
"What we are looking for is the powdered-milk equivalent for blood," says Professor Chris Cooper, biochemist and leading blood substitute expert at the University of Essex.
"It would be stored in a packet and rehydrated when required. Paramedics could carry it to an accident and start a transfusion on the spot. And as the blood substitute is free of cells, then there would be no problems with blood group matching."
Researchers looking for new blood are split into three main camps: those working on perfluorocarbons; those using artificial haemoglobins; and those harnessing stem cells. Perfluorocarbons, or PFCs, are man-made, milky-coloured liquids or emulsions that can transport both oxygen and carbon dioxide. The first blood substitutes of the modern era – Fluosol-DA was licensed for use in 1989 – they pick up oxygen as they pass through the lungs which is then circulated around the body. PFCs (polymers similar to Teflon) are exhaled as a vapour from the lungs.
Although PFCs tick some of the boxes for a blood substitute, there are disadvantages. One of the main hurdles to overcome is that they are unable to transport as much oxygen as haemoglobin-based materials can. As a result, much more PFC has to be used.
Several groups, in Britain, America, Sweden, France and Australia, are investigating the use of stem cells, with the aim of growing them into red blood cells of the O negative group which can be transfused into anyone without fear of rejection. Such blood would also be free of contamination, but a potential downside is that it may have the limited shelf life and storage restrictions of donor blood. If this is so, its main use would be in hospitals to make up for shortages of blood. It has been predicted that human trials of the stem-cell derived blood could start in 2013.
The third, and burgeoning research area centres on the use of haemoglobin, the protein inside blood cells that gives blood its red colour and which carries oxygen around the body and removes carbon dioxide. Most products in advanced clinical trials are based on haemoglobin. The snag with using haemoglobin is that outside the protective environment of the red cell it can be toxic, producing free radicals that can damage the heart and other organs. In early experiments, where haemo-globin was injected directly into patients, a number of them died. The challenge for scientists has been to take haemoglobin out of the red blood cells and put it into another protective environment where it can go about its business without being potentially toxic. Some researchers have reported successes. In one trial involving 688 patients undergoing elective orthopaedic surgery, the men and women received either one unit of real blood or two units of blood substitute HBOC-201, a haemoglobin-based product from Biopure. Made from a bovine source, the substitute can be kept at room temperature for up to three years and does not need to be matched with blood type.
"We found that we eliminated the need for blood transfusions for 59 per cent of the 350 patients who received the blood substitute," said Dr Colin Mackenzie of the University of Maryland School of Medicine. "The blood substitute worked best in those under 80 years old."
Scientists from the University of Sheffield have been working on an artificial blood designed to be stored as a thick paste that can be dissolved in water before use. It is made from plastic molecules that hold at their core an iron atom, like haemoglobin, that could transport oxygen around the body.
A product that has been in several trials Hemospan from Sangart. It is from haemoglobin taken from outdated human red blood cells which is wrapped in a coating of polyethylene glycol. When the coating is attached to the surface of the protein, a thin layer of water is formed around the haemoglobin, which shields it from the immune system and stops any toxic effects.
Hemospan is produced in powder form, which should allow it to be stored for years. When it is needed, the powder could be mixed into liquid form and transfused immediately, regardless of a patient's blood type, without any risk of transmitting infectious diseases.
"The trick with artificial blood using haemoglobin is to make the protein less toxic but still perform the vital role of carrying oxygen around the body. No one has managed this yet," says Professor Cooper. He and his team are working on genetically modifying haemoglobin to make it less toxic, and then wrapping it in a special long-life plastic cocoon. In such an environment, the protein could work its magic on oxygen supply and carbon-dioxide removal without the risk of toxic effects. Animal studies are likely to start soon, and the patented technology could be used in the first human in five to 10 years, he says: "It is an important time for blood substitution research. A number of drug comp-anies are making artificial haemoglobin molecules, and work on PFCs is continuing.
"Sooner rather than later, we might be faced with the offer of a pint of the red stuff (haemoglobin) or the milky stuff (fluorocarbon) rather than blood donated by an altruistic citizen."
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