午夜福利1000集合

On 23 August 1999, a New York doctor contacted the city鈥檚 health department about two patients with a mysterious form of brain inflammation. Within days several others had come down with similar symptoms. Some of them died. The culprit had never been seen in the city before 鈥� in fact, this was its first appearance in the western hemisphere. Its identity: West Nile virus.

West Nile, spread by mosquitoes, swept the nation. In 2002 there were 4156 cases. As if the threat of catching the virus from mosquitoes weren鈥檛 bad enough, some of the victims were being infected via blood transfusions. The virus, it turned out, had infiltrated the nation鈥檚 blood supply.

In record time, scientists developed a test for West Nile virus, and by the following summer it was being used to screen blood donations, preventing further people from being infected in this way. But the assay isn鈥檛 foolproof. In 2003 two Americans became infected via blood products that had wrongly tested negative.

The virus is a textbook illustration of the threats to the world鈥檚 blood supply. In spite of the battery of tests routinely run on blood and the products that are made from it, scientists feel they are fighting a losing battle. As soon as they develop a test for the latest pathogen, another one pops up. What keeps doctors awake at night is the thought that the next new pathogen might be even deadlier and harder to detect.

But perhaps there is a better way. A number of firms are developing ways of stripping as many pathogens from blood as possible in one fell swoop. The same idea underlies several variations of the technique: to chemically inactivate nucleic acids 鈥� DNA or RNA, the building blocks of genes. By doing this, we should be able to destroy viruses, bacteria, and parasites 鈥� the lot 鈥� without ever having to know their identity. 鈥淚t would sterilise blood before it got into the vein of the patient,鈥� says Mark Popovsky of Haemonetics in Braintree, Massachusetts, a firm working with Baxter 午夜福利1000集合care, one of the companies involved in the research.

The technique, dubbed pathogen inactivation or reduction, is already being used in a limited way. But it is not yet clear whether the technique is safe and effective for most blood products. Blood banks may need persuading that the initial extra cost is worthwhile. Meanwhile the clock is ticking until the next bug emerges. 鈥淎ll it takes is someone getting off the plane and importing a disease,鈥� says Popovsky.

Blood transfusion is a multibillion-dollar business, with more than 40 million pints, or units, of donated blood used every year in the US, Europe and Japan. In the west, blood is split into its various constituents before being given to patients. The most widely used of these are red blood cells, which carry oxygen around the body, platelets, the cell fragments that promote blood clotting, and plasma, the straw-coloured liquid component that contains antibodies and other important molecules.

Blood banks screen their donations for a number of pathogens, including HIV and the hepatitis B and C viruses, which means that blood products are much safer than they were 20 years ago. For example, back in the 1980s the risk of catching HIV or hepatitis C from a blood product was as high as 1 per cent in certain urban areas of the US, while now it is around 1 in 2 million 鈥� no test is perfect.

But what about all the other bugs out there that we don鈥檛 test for? Take Chagas鈥� disease. The culprit is a microscopic parasite called Trypanosoma cruzi, which is transmitted to humans via insects known as kissing bugs. Victims can die from cardiac and digestive problems 20 to 30 years after initial infection. Blood screening tests do exist, but their effectiveness is debatable. Chagas鈥� disease is prevalent in much of Latin America, where blood transfusions are one source of infection, and the disease is slowly making its way to the US.

Malaria is another disease that can be transmitted via transfusions but for which there is no practical way of screening blood for the parasite responsible. And then there are pathogens that are a serious threat to people with weakened immune systems such as the sick, the young and the elderly. An example is cytomegalovirus (CMV), a member of the herpes family that is estimated to infect up to 85 per cent of adults by age 40. The symptoms in healthy adults are usually mild, but if your immune system is compromised, CMV can cause pneumonia and gastrointestinal disease. Tests for CMV exist but they are not performed routinely, and healthcare staff may not always think to request them for blood intended for immunocompromised patients. Nor are tests carried out for another pathogen, called B19 parvovirus, which can cause severe anaemia in people with weakened immune systems.

And the the emergence of HIV, West Nile virus and SARS are ample evidence that the next big threat could be just round the corner. 鈥淚 call it the 鈥淗IV-X,鈥� says Popovsky.

But imagine a scenario in which we didn鈥檛 have to worry about these potential threats. That is what a few companies are hoping to achieve with their pathogen inactivation technologies. All the techniques share certain characteristics: they use very small molecules that can seep through cell membranes and either damage or bind irreversibly to DNA or RNA, so that they cannot be copied. And if an organism鈥檚 nucleic acid cannot be replicated, it cannot reproduce 鈥� it is effectively destroyed. The process may not reach every single bacterium or virus particle, but it certainly seems to reduce them to negligible amounts, slashing the chance of a patient becoming infected.

Risky business

In theory at least, pathogen inactivation should not harm blood itself, as neither platelets nor red blood cells contain DNA. While immune cells in blood do contain DNA they have to be destroyed before blood can be used anyway, to prevent them mounting an immune response to the recipient鈥檚 tissues.

A simple form of pathogen inactivation has been around for years, but it can only be used on plasma, as it would damage the other, cell-based, components of blood. It involves adding a solvent and a detergent to plasma, which destroys all pathogens that have lipid-based outer membranes. But this excludes a subset of viruses that lack such a membrane, which includes hepatitis A, not to mention prions, the protein particles that transmit new variant Creutzfeldt-Jakob disease, the human form of BSE. And because the process of adding solvent and detergent involves pooling thousands of blood units, it actually increases the risk of transmitting these diseases. So the solvent-detergent technique is now mainly confined to the manufacture of blood derivatives such as clotting factors when there is no alternative to blood pooling.

In the past few years, however, major progress has been made in developing pathogen inactivation techniques for the cell-based components of blood. The first product that could be used on platelets was introduced in Europe in 2002. Developed jointly by Cerus based in Concord, California, and the multinational Baxter 午夜福利1000集合care, the technique involves adding a small molecule called amotosalen to platelets, then irradiating them with UV light. This step causes amotosalen to bind to nucleic acids. Importantly, one amotosalen molecule can bind to two nucleic acid strands, so it tends to slip into the double helix of DNA or RNA and cross-link the two strands. With viruses whose genetic material is composed of a single strand of RNA, the strand forms cross-links with itself.

The trouble with this approach, however, is that it cannot be used on red blood cells or whole blood, because the haemoglobin in red blood cells absorbs UV light, preventing it from reaching amotosalen. A potential competitor, which is in early-stage clinical trials, may not have this drawback. Navigant Biotechnologies in Lakewood, Colorado, is developing a pathogen inactivation system using riboflavin, a B vitamin, which binds to nucleic acid. Riboflavin seems to damage nucleic acid by a number of mechanisms, including producing highly reactive free radicals when illuminated with UV or visible light, the latter of which is less absorbed by haemoglobin. And as a natural product that is normally present in our bodies, riboflavin does not need to be removed from the blood product after treatment. 鈥淚t has a wonderful safety profile behind it,鈥� says Ray Goodrich, chief science officer of Navigant.

Other scientists are also trying to crack the challenge of red blood cells by developing pathogen inactivators that don鈥檛 depend on light. Cerus and Baxter are jointly developing a complex molecule called S-303, with three components. S-303, which is prepared and stored at an acidic pH, is activated when added to red blood cells at neutral pH; after the nucleic acid cross-linking, the neutral pH causes the compound to break up. One drawback of S-303 is that it is ineffective against hepatitis A, probably because it cannot penetrate the virus鈥檚 tight protein coat.

Another product called Inactine, made by Vitex, based in Watertown, Massachusetts, does seem to work on hepatitis A, although not to the extent of other viruses. Inactine has a similar three-component structure to S-303, although the molecule is smaller, which may be why it can enter the hepatitis A virus.

Trouble ahead

By 2003, S-303 and Inactine had successfully completed several clinical trials, and Cerus and Vitex had moved on to the final studies needed to get their products approved by drug regulatory authorities. Then came a big surprise. For both products, a small number of patients receiving transfusions for blood disorders such as sickle cell anaemia produced antibodies against the treated red blood cells. The immune reaction did not seem to cause patients any problems but it hinted that something about the red blood cells had changed.

Alarm bells rang, and both firms quickly called a halt to the trials. They initially remained optimistic that the effect was confined to people with blood disorders, who tend to need numerous blood transfusions and so have very sensitive immune systems that would be more reactive to a minor change in the surface of red blood cells. But in February Vitex admitted that a similar antibody reaction had been spotted in someone without a blood disorder, in a trial of people having transfusions because they were undergoing surgery.

Vitex halted that trial and stopped development of Inactine. It is now looking to license out the product to another firm. Cerus stopped a similar study involving surgical patients. 鈥淭here was a sense of disappointment,鈥� says Edward Snyder, a transfusion specialist at Yale New Haven Hospital in Connecticut.

Last December, however, Cerus announced they had figured out why the immune reactions were happening: some S-303 remnants were left stuck on the surface membrane of red blood cells. In research presented at the December meeting of the American Society of Hematology, Cerus showed they could eliminate this by making the solution slightly less acidic and raising the level of a chemical that reduces S-303鈥檚 鈥渟tickiness鈥�. Cerus鈥檚 hopes for S-303 lie with this new formulation. 鈥淚鈥檓 pretty confident,鈥� says Larry Corash, the firm鈥檚 chief medical officer.

Assuming Cerus solves the antibody problem, there are still further obstacles. Preparing a unit of blood product is expensive. In the US, the average price a hospital pays for a unit of screened blood is about $220, which is roughly double the cost in the 1980s. Cerus says S-303 will probably be priced at around $40 to $50 per unit.

On the other hand, if pathogen inactivation becomes widely used and trusted, some of the existing screens and procedures for blood products could be phased out, including gamma irradiation to inactivate immune cells, which costs up to $30 per unit of blood. Blood products can also become contaminated with bacteria during donation or storage, with platelets particularly vulnerable because they are stored at room temperature instead of being refrigerated. Eliminating bacterial testing would save about another $30 per unit. Some of the blood banks using amotosalen to treat platelets are also phasing out CMV testing.

But they may never stop screening blood for potentially fatal diseases such as HIV and hepatitis, especially as it cannot be guaranteed that every single virus particle is destroyed. It would be years, predicts Popovsky, 鈥渂efore anybody would seriously entertain taking the tests away. This is a very conservative field 鈥� blood is a very emotional topic鈥�.

Not everyone is convinced that pathogen inactivation is the answer to all the problems. Theoretical risks aside, blood is one of the safest medical products around, so companies will have to bend over backwards to prove their technologies do not cause any long-term problems, or even cancer. 鈥淭hat鈥檚 the boogeyman,鈥� says Snyder.

In addition, new forms of infectious agents might appear that are resistant to even the most stringent treatments. Only Vitex has published data that their process can remove prions from blood, and that is due to the multiple washing cycles the blood is subjected to and not to the direct effect of the inactivating compound.

So the question is whether countries are ready to take the plunge and adopt a costly process that will essentially buy some peace of mind. 鈥淟ast time I looked,鈥� says Snyder, 鈥渢he number of viruses and bacteria were not going to get any less.鈥�