If medical researchers fail to cook up a cure for the common cold in our lifetime, it won't be for lack of trying. The cold may be a relatively trivial annoyance, but it's not even close to a trivial problem. Despite its name, for instance, the common cold is caused by not one common virus but five different viral families, encompassing a couple of hundred unique viral strains among them. These strains are sufficiently different from one another that even after catching one, we can later be infected and rendered miserable by all the others. This explains why toddlers seem to be in a continuous state of sniffles--being tabulae rasae for every strain that comes along--while adults become ever more immune with each ensuing cold and often go years before encountering a strain they've never had. The regrettable implication is that researchers will have to come up with a cure for every family and strain, or at least most of them. Otherwise any treatment is likely to fail too frequently to fire the public's enthusiasm. Curing the common cold is therefore as much a question of economics and marketing as it is of scientific genius--after all, the average cost of developing a new drug and bringing it to market is $350 million to $500 million.
To make matters worse, the cold is common only in humans. While researchers have lately managed to infect chimps with the cold virus, the animals don't seem actually to get sick. They don't even sniffle, and their mucus weight, an objective measure of symptom severity favored by cold researchers, remains unchanged. This makes it difficult to test potential cures on chimps. Rats and mice, the usual fodder for medical experiments, are blissfully unaffected. Without animal models to work with, a pharmaceutical company will very occasionally take a drug from test tube directly to human--if the condition it's potentially treating is sufficiently dire and life-threatening. To do so for an innocuous little misery like the common cold is a different story.
Nevertheless, there is an outside chance that a cure, or at least a viable treatment, is in the works. If so, the credit belongs as much to serendipity as to any single researcher. Fortunate chance played an important part in the story of a molecule known by the uninspiring name of intercellular adhesion molecule 1, or ICAM-1. As its name implies, ICAM-1 is one of a group of molecules that sit on the surface of cells and adhere to other cells when the need arises. This adhesion is critical to a wide variety of physiological processes--from making our immune systems work to keeping the cells of our bodies sufficiently stuck together that we don't all collapse on the floor in huge cellular puddles. For this reason, the discovery and understanding of intercellular adhesion molecules launched a pharmaceutical revolution that had nothing to do with the common cold and may be where the big money will lie. If any Nobel prizes are given for ICAM-1 and its kin, this will most likely be the reason.
Along the way, however, researchers noticed that ICAM-1 protrudes from cells in your nasal cavity and acts as the entryway--the doorknob, so to speak--by which most members of one particular family of common cold viruses, known as rhinoviruses, penetrates your cells, infecting them. As of last year, a specially designed version of ICAM-1 had been tested as a treatment for the rhinovirus-inspired common cold and had done remarkably well at its job. Whether ICAM-1 or some preparation derived from it will soon appear in your medicine cabinet as a cold cure may be an economic question, but the history of ICAM-1 is a quintessentially scientific tale of collaboration, competition, and luck.
The story starts with Timothy Springer, a biochemist turned immunologist at the Harvard Medical School. In the early 1980s, Springer was studying lymphocytes, white blood cells that do much of the work of fighting off infections and defending against any bacteria or viruses that might invade your personal space. Pus, for example, is made of white blood cells that have crawled through the walls of your blood vessels to get at the site of infection. To do their jobs, lymphocytes must stick to other cells. When Springer began his research, no one knew quite how this stickiness was achieved. In 1981 he and his colleagues identified a single molecule that seemed crucial to the adhesive aspect of a type of lymphocyte known as a killer T cell. Because Springer had only a general idea of what this molecule actually did, he gave it a generic name, lymphocyte function associated antigen (LFA-1), and wrote a paper presenting evidence that it was involved in the adhesion of lymphocytes. He then set out to learn whether his hunch was true, and if so, why.
Serendipity came to Springer's aid. "We were lucky," he says, "to discover an inherited deficiency disease in which patients lacked this molecule." Springer had heard about a child from New York whose immune system could not fight off infection, apparently because the child's white blood cells had lost the capacity to adhere. Preliminary study of the patient's blood, samples of which the child's doctor had sent, suggested that the child's blood lacked several components, including a protein whose weight matched Springer's measurements for LFA-1. When the researchers tried to obtain more blood for further study, however, a new doctor who had taken over the patient's care refused them. "I never got a single drop of blood from that patient again, and I didn't know where to go. I was stymied. So I started calling pediatricians."
One of the people Springer called was a colleague at the University of Pennsylvania who had heard a talk by a pediatrician named Donald Anderson. Anderson was working at the Baylor College of Medicine in the early 1980s, looking for genetic diseases that make children susceptible to infection. "In 1982," says Anderson, "I received a letter from a child's father. The girl was very young, maybe two or three. The letter just said, 'My daughter has all kinds of strange infections. Would anybody in the state of Texas have any interest in taking care of her?' He had sent the letter to all administrators of hospitals in Texas. It was bounced up to my lab. I called him and I did a variety of studies on her. She had infections of the soft tissue. She would have infections on the skin with a cut or any kind of abrasion; she would get sores around her teeth, tongue, mouth, nose, and on her body surfaces. And she did not make pus. If you or I have a thorn in our finger, we will get a big boil, loaded with white blood cells--pus. She would get the opposite. She would get infections on the surface of her skin but she would not have pus. It was as if the white blood cells couldn't get there, even though she had extraordinary numbers in her blood, far higher than most of us. It was unclear what the defect was--presumably some sort of defect of her white blood cells. What we noticed was that her white blood cells lacked the capacity to stick to surfaces. They would slide right off glass or protein surfaces, which is very unusual. White blood cells usually stick avidly to almost everything." By 1985, Anderson had published journal articles on the girl and had located a half dozen similar patients in Texas.
After Anderson's colleague at the University of Pennsylvania told Springer about him, the two began working together. Studying blood samples from Anderson's patients, they verified that the white blood cells would not clump together, as normal white blood cells do, thus strongly suggesting they were missing LFA-1. Further research showed that the protein was indeed missing.
Robert Rothlein, one of Springer's postdoctoral researchers who now works for Boehringer Ingleheim Pharmaceuticals in Ridgefield, Connecticut, set out to identify what molecule LFA-1 was grasping to make cells stick together. He used monoclonal antibodies, a technique for studying proteins by injecting them into a mouse and analyzing the antibodies its immune system produces in response (see box on page 50). In 1986 the team published the result: LFA-1's target was ICAM-1.
The discovery and elucidation of ICAM-1 and LFA-1 launched an entire field of biology dedicated to the study of cellular adhesion in the immune system. (It didn't do Anderson any harm, either. "At Texas," he says, "my grant support went up 1,000 percent in a year.") Researchers went on to discover hundreds of similar adhesion molecules that control the interactions of all cells in the body. By the late 1980s, the pharmaceutical industry had become infatuated with adhesion molecules as well. If the lack of such molecules on white blood cells diminished immune response, thought researchers, then perhaps drugs that blocked adhesion could help tone down immune responses in diseases such as rheumatoid arthritis or asthma, in which an overactive immune system damages the body.
While the cellular adhesion story was playing itself out through the early 1980s, a cadre of biologists and virologists were pursuing the common cold from a variety of angles, none of which were panning out. Some were working on interferon, for example, an agent produced naturally by the body to stimulate immunity. At the time, as one researcher says, interferon was touted as a potential anti-everything, and indeed in clinical trials it showed promise against the common cold. But because of its high cost and questions about the side effects of regularly dosing people with a potent anti-everything, the treatment went nowhere. Other biologists were looking for the cellular receptors that the various cold viruses use to get into cells, where they replicate furiously and wreak havoc. Though the cold investigators didn't realize it at the time, it was this line of research that eventually led to the second incarnation of ICAM-1, as a cold cure rather than a cellular adhesion molecule. The road was a slow and twisted one. First researchers had to establish whether the five families of cold viruses--200 or so strains in all--used the same or similar receptors. Only if they did would it make clinical sense to go after that receptor (or those few receptors). And for years, the researchers scrutinizing the receptors that made cells vulnerable to cold viruses had no idea that cellular adhesion--and ICAM-1 in particular--had any relevance to their work.
In 1982, Richard Colonno, then a molecular biologist at Merck, now a vice president of infectious-disease drug discovery at Bristol-Myers Squibb, began a program to isolate the receptors for the 100 or so strains of rhinovirus, which account for roughly half of all common colds. In the 1960s researchers had suggested that these strains might use several different receptors to enter cells, but Colonno showed definitively that 90 percent of all rhinoviruses used one receptor, and the other 10 percent used another. With that finding, says Colonno, rhinovirus research "all of a sudden took on a life of its own." It meant 90 percent of half of the common cold viruses might be stopped with the same drug, which was a large enough percentage to justify developing a treatment and putting it on the market.
During the mid-eighties, Colonno and his colleagues produced an antibody that attacked the receptor and blocked the virus from getting into the cell and replicating--at least in a test tube. This antibody was of little clinical use, however, because producing enough of it would be prohibitively expensive. "For some diseases these antibodies are priceless," Colonno explains. "But for the common cold, people aren't going to spend a lot of money. We needed something very clean, very safe, and relatively cheap." When Colonno and his colleagues tried to come up with a cheap version of the antibody, they simply failed. With the AIDS epidemic erupting throughout the eighties, Colonno's interest in the common cold began to wane.
Colonno's work, however, had spawned competition, although he didn't know it at the time--from the work of two molecular biologists in particular, Jeffrey Greve and Alan McClelland. The two worked for Molecular Therapeutics, a small New Haven biotech company that was later absorbed into Bayer. Molecular Therapeutics had been founded with the idea of using the new technologies of molecular biology to find drug therapies. One of the obvious targets was the rhinovirus receptor. That the receptor served as a point of entry for viruses that infected so many people, says McClelland, made it both interesting to research and "a potentially commercially significant target as well." And Colonno's discovery made the project practical. "We knew about Colonno's work," McClelland says. "He had sparked a very big competition by showing there was one target, one receptor" for nine out of ten rhinoviruses. "Through that common pathway, there was the potential to inhibit many different rhinoviruses. When we began our effort we were trying to advance science--and to get there first."
Over the next three years, McClelland and Greve tried two approaches to nail down the receptor's identity. One was to take human genes and insert them into mouse cells in a test tube. The mouse cells would express the gene, producing a protein, and the researchers would attempt to infect them with a rhinovirus. Since rhinoviruses can't attach themselves to normal mouse cells, only cells that had been given the human receptor gene and were manufacturing the rhinovirus receptor would allow the viruses to get a purchase on them. To identify the gene, the Molecular Therapeutics researchers just had to sift through large numbers of genes to find which one made rhinoviruses stick to mouse cells, says McClelland.
The second approach was to let monoclonal antibodies find the receptor for them. They injected a mouse with human cells susceptible to rhinovirus infection and watched as the mouse immune system went about its job concocting antibodies against the various proteins sticking out of the human cells. "If you use cells as immunizing agents like this," says McClelland, "you'll get antibodies against many different components of the cell's surface, including, one hopes, the receptor. Then you fish through the collection of antibodies looking for ones that will stick on the receptor protein and stop the virus from getting into the cell."
Both approaches worked, giving McClelland and Greve the gene that coded for the rhinovirus receptor as well as the receptor itself. Greve's laboratory purified the protein and sequenced it, creating a precise map of the string of amino acids that composed the protein. When researchers in Greve's lab compared their sequence with those in a computer database of proteins that had already been sequenced by other researchers, ICAM-1 matched perfectly. "We immediately went to the library and read some papers to find out what ICAM-1 was, and we came up with Springer's group," McClelland says. "That's when we first heard about them."
By "them," he means not just Springer's group but a pair of former researchers with Springer--Rothlein and Steven Marlin--who were working at Boehringer Ingleheim. Rothlein had recruited an immunologist named Vincent Merluzzi, whose program on rhinoviruses, begun in 1987, was cooking along in his laboratory on a back burner. He had test tubes of cells that he could infect with rhinoviruses, and he was waiting for potential treatments to come along that might need such cell cultures for testing and development.
Once Greve and McClelland realized ICAM-1 was the rhinovirus receptor, word began to spread, although at this point no one can quite say how. Certainly one of the places it spread to was BI. Marlin got a call from friends back at Springer's group, asking him if ICAM-1 could be the receptor. Friends at BI also came to him with the same question. "I said, 'Anything is possible. Who knows?'" he recalls. With Merluzzi's cell cultures, they were able to move quickly. "In very short order," says Marlin, "we put together concrete information that ICAM-1 was the receptor to a major group of rhinoviruses." With Springer's group, they quickly wrote up a journal article describing the work, which was published back-to-back with the paper from Molecular Therapeutics, allowing the two teams to officially share credit for the discovery.
During the next decade BI and Bayer, the company that owns Molecular Therapeutics, raced to get a cold cure to market first. "You spend years working really hard trying to identify and clone this molecule," says McClelland, "and all of a sudden, one day, you know exactly what it is, you know the sequence, it opens up all the experiments you've been unable to do over all that time. We started cloning the gene and then manipulating it, expressing it, making mutations, doing all sorts of experiments to try to translate this knowledge as fast as we could into useful information for blocking the virus."
Both teams hit on the same strategy. They produced a form of ICAM-1 that was not rooted in the cell walls but was soluble--free to float in a solution. They could squirt it into noses in the hope that the rhinovirus would snatch up the free-floating ICAM-1 and never get around to grabbing the ICAM-1 receptors that let it into the cells. The strategy is a "decoy effect," says Marlin. "It's a numbers game. If there are ten copies of soluble ICAM around for every one copy of the cellular ICAM, the odds are ten to one the virus will bind to the decoy."
Plenty of problems had to be solved first, however. For instance, the mucus problem: your nose produces mucus as a barrier to foreign particles. Could soluble ICAM-1 survive in this environment, or would the mucus break it down and wash it away before it had the chance to decoy any rhinovirus? At BI the researchers spent considerable time testing the stability of soluble ICAM, which turned out to be what Marlin calls a very hardy protein.
"For want of a polite way to put it," Marlin explains, "we had to incubate it with human snot and see if it held up for a reasonable period of time at nose temperatures." To obtain mucus samples for testing, says Marlin, "you do nasal washes. You put sterile saline into human noses and they blow it out. It's crude, but it wasn't meant to do anything more than say, 'Has this got a snowball's chance of surviving in mucus from a human nose?' " The task of testing the mucus fell to Marlin--"My lab staff didn't want to touch it," he says. "So I put it in a test tube, incubated it at 34 degrees centigrade, which is the average temperature of a nose, as opposed to your body temperature of 37 degrees, and let it sit there overnight. It turned out to be quite stable."
The mucus hurdle was just the beginning. The researchers also had to prove that the soluble ICAM would work on all or most of the 90-plus strains of rhinovirus--antiviral medications are famous for working on one strain and not at all on others. Once they'd succeeded, they had to find out whether the rhinoviruses would quickly mutate, which viruses are wont to do. If so, their promising treatment would only serve to generate a lineage of super-rhinoviruses that in the presence of soluble ICAM would find another way to force entry into cells. Reassuringly, their experiments showed that the virus seemed to remain susceptible to the ICAM decoys, at least in test tubes. "What that really means in living humans," says Marlin, "nobody knows. It gave everybody confidence, however, that we're not going to put this in human noses and very rapidly generate viruses that are no longer affected by the drug."
BI researchers have already conducted two trials of the treatment's effectiveness (putting them ahead of Bayer, which completed safety trials on ICAM just last year). At the University of Virginia School of Medicine, for instance, BI collaborator Frederick Hayden and his colleagues exposed 177 student volunteers to rhinoviruses while spraying soluble ICAM or a placebo up their noses. The researchers applied the treatment six times a day, starting either four hours before dripping rhinovirus into collegiate noses or 12 hours after. They continued the treatment for up to seven days. Then they checked for viral infection, measured symptoms‹chilliness, nasal congestion, cough, headache, malaise, runny nose, sneezing, and sore throat‹and compared the ICAM treatment with the placebo treatment.
The results suggested that ICAM bestowed what Hayden calls a substantial antiviral effect and a moderate clinical benefit. In other words, while the ICAM seemed to have little effect on whether the students actually became infected, it did influence whether they became ill as a result, and how severely. "The symptom burden was cut by a third to a half," says Hayden. For instance, the weight of nasal mucus produced in the group that got ICAM was half of that in the placebo group. "I interpret this as a positive signal," says Hayden. "This provides an impetus to go forward with this particular inhibitor to develop formulations that could be used on a less frequent basis." Ronald Turner, who ran the trials at the Medical University of South Carolina, says that since 1982 he has tested perhaps ten different types of common cold medications, both agents that combat viruses and treatments that target specific symptoms. Of the antiviral agents, soluble ICAM was by far the best. "To be honest," he says, "I think it's better than anything in the literature too."
Whether ICAM is good enough to make a marketable cold treatment remains to be seen. After all, it will do nothing for many of the colds you're likely to get. It could still come in handy, however, in certain situations, says Springer. For instance, if somebody in your household or office gets a cold, you might start squirting ICAM up your nose as a preventive measure. And it's worth noting that existing common cold treatments such as zinc lozenges and echinacea, which appear to have absolutely no effect whatsoever on the progression of a cold, nevertheless do extremely brisk business, so a treatment that actually does something might be profitable as well.
Most treatments that fail to make it to market, says Marlin, fail in precisely the early trials that ICAM has already successfully completed. But that is no guarantee for the future. Soluble ICAM still faces a long progression to market, perhaps five years. Assuming it continues to work according to expectations, researchers will have to establish in trials the correct dosage, for instance, and how often it can be taken. Most important, they'll have to prove that it works on natural colds, not just on colds induced by white-coated researchers dripping the rhinovirus into the nasal cavities of college students lying on their backs on gurneys. McClelland, for one, calls it a good shot, but that's as far as he'll commit himself. "I think it's the kind of approach that, even if it doesn't work, will certainly have advanced our understanding quite a bit. And--who knows?--maybe it will work. The common cold has caused enough problems for us all. It would be nice to have something you could just take and be done with it."
