Nanomedicine Nears the Clinic
The death last fall of a 17-year-old patient at the University of Pennsylvania Medical Center shocked the medical research community. The young man, who suffered from a rare genetic defect that stopped his body from metabolizing ammonia, was undergoing an experimental form of treatment called gene therapy. In theory at least, it should be possible to treat a number of devastating diseases by patching in bits of DNA to repair defective or missing genes.
In practice, gene therapy remains difficult and the methods crude. One problem is in sneaking the new genetic material into the cell and getting it incorporated into existing DNA without the individual’s immune system going berserk. Most approaches rely on nature’s masterful Trojan Horse, the virus. In the case of the patient at Penn, researchers injected very high doses of a modified form of the common cold virus directly into his liver. Once inside, the viral particles were supposed to insert replacement DNA into tissue cells so his liver could start to process ammonia normally. Four days later the patient was dead from multiple organ failure, and speculation centered on the possibility that the immense and sudden injection of the virus caused an overwhelming immune response.
Medical researchers have been terrified of just this scenario since human gene therapy experiments began a decade ago. Now that it’s happened, their motivation to find safer alternatives for delivering genes to human cells has been redoubled. Enter nanotechnology. The fabrication of objects and devices on the scale of nanometers has been making rapid progress in the physical sciences. But you wouldn’t necessarily think of it in connection with medicine. Yet nanotechniques might offer a solution to current problems in gene therapy-and some remarkable advantages in treating stubborn diseases such as cancer and diabetes.
A small vanguard of medical explorers is exploiting the tools of nanotechnology to manipulate biomolecules that regulate life and death, illness and health. The key to these efforts is that researchers are learning how to tailor devices and materials on the scale of billionths of a meter, thereby acquiring the ability to engineer structures and machines no bigger than biomolecules such as DNA. They’re finally playing on the size scale of biology itself. And that means they may be able to design tiny tools to safely and effectively fix the nanoscopic machinery of illness, just as a mechanic works on a car’s engine using tools that are on the same scale as the engine. This may sound like science fiction-and until recently it was-but it’s reaching the verge of possibility because teams of doctors and scientists are combining advances from biology and chemistry with the synthesis and fabrication tools from chemical engineering, even the microchip industry.
One believer in “nanomedicine” is James Baker, chief of the allergy and immunology department at the University of Michigan’s Medical School. He might seem an unlikely champion of nanotech in medicine, a field that has more often been associated with sci-fi notions of tiny machines cruising the human body than with clinically feasible treatments. But Baker is convinced that the tools of nanotechnology will eventually provide a far safer and more effective way to repair genes. So convinced, in fact, that last year Baker founded the University of Michigan’s Center for Biologic Nanotechnology, bringing doctors and medical researchers together with chemists and engineers to turn nanomedicine from a futurist dream into clinical reality.
For Baker, the attraction to nanostructures is that these nonbiological substances can be constructed so that they will not trigger an immune response. Years of immunology research has convinced Baker that viral methods are fraught with trouble, because of the severe immune reaction they trigger. “That started me thinking about synthetic systems,” says the Michigan immunologist. In particular, Baker began to wonder if a novel type of polymer called dendrimers-tree-shaped synthetic molecules that can be engineered on a nanometer scale-could be used to slip DNA covertly through immune defenses into target cells. Dendrimers were invented two decades ago by Baker’s colleague and scientific director of the new center, polymer chemist Donald Tomalia. Since then, dendrimer research has exploded, with researchers pursuing applications from drug delivery to medical imaging.
What sets dendrimers apart from other polymers is their precise nanostructure. Dendrimers form nanometer by nanometer, so the number of synthetic steps dictates their exact size. Their surfaces can be made to form a dense field of molecular groups that serve as hooks for attaching other useful molecules. Dendrimers can also carry internal molecular baggage. These properties could make dendrimers excellent transporters for sneaking DNA into cells. Scientists decorate the dendrimer molecule with the DNA, which scrunches down on the polymer’s surface. The dendrimer-DNA bundles are injected into the tissue; dendrimers of just the right size trigger a process called endocytosis in which the cell deforms to let the DNA-dendrimer package in. Once inside, the DNA is released and migrates to the nucleus where it becomes part of the cell’s genome.
While the research is still in its early stages, initial results suggest that these synthetic nanomaterials just might be a safer alternative to viral transporters for gene therapy. So far, building on work done at the University of California, San Francisco, in the early 1990s, Baker and his Michigan colleagues have shown in lab experiments that they can use dendrimers to efficiently transfer DNA into the cell’s genes. They are now conducting animal trials with rats and mice to demonstrate that the dendrimers don’t cause toxic side effects and to see exactly how efficient dendrimers can be. The next step will be to carry out similar studies in humans to assess the promise of dendrimers for fixing genes. Then the arduous process of setting up actual experiments must begin, involving approval by a special advisory committee at the National Institutes of Health and the Food and Drug Administration. With perseverance, Baker hopes to see clinical trials of dendrimer gene therapy in two years.
The Real Action
Baker’s efforts may or may not pay off in a breakthrough for gene therapy (the field is littered with promising strategies that never panned out). But whether this specific approach is successful or not, it reflects an extremely important development in medicine on the very smallest scale. A typical human cell, like the red blood cells that course through your veins, is five micrometers in diameter. But much of the real action in biology occurs at a considerably smaller level. DNA, for example, is less than three nanometers in diameter-about 100 times smaller than the cell. Many common proteins are only a few nanometers across.
Biomedical researchers have never been able to play effectively on a field this small. To do so now, they are importing techniques and expertise from other areas. Semiconductor manufacturers have been making features on silicon chips only a few hundred nanometers across for several years. Mauro Ferrari, among others, is taking these fabrication techniques and applying them to medicine. A professor of internal medicine and mechanical engineering at Ohio State and director of the university’s new biomedical engineering center, Ferrari has made tiny silicon capsules that can hold healthy cells to replace ones that are not functioning; if, say, the pancreatic cells of a diabetes patient are not working, capsules containing replacement cells can be implanted beneath the patient’s skin. Supplying new cells to the body could be a very valuable way to treat certain diseases such as those caused by enzyme or hormone deficiencies, and various medical researchers have been wrestling with the strategy for years. But as in gene therapy, immune reactions are a major problem. Replacement cells are foreign to the body and are therefore attacked by the body’s immune system, with disastrous results.
But Ferrari has come up with a scheme to swindle the immune system using the tools of nanotechnology. When the immune system sees something foreign, it dispatches antibodies to attack it. If, reasoned Ferrari, you could block the antibodies using an artificial barrier, the immune system wouldn’t be able to see the transplanted cells. Ferrari fabricated his silicon capsules to include membranes with pores small enough to screen out antibodies-but large enough to let desirable molecules flow in and out. “The biological recognition molecules don’t know what the hell is inside,” says Ferrari.
The concept is elegant. But in practice, it’s not easy to make nanoholes small enough to keep antibodies out. It turns out that antibodies can get through anything larger than about 18 nanometers (the exact size is still uncertain). Photolithography tools for making state-of-the-art integrated circuits are good for making features only as small as a few hundred nanometers. By adapting these methods used in the semiconductor industry, however, Ferrari managed to create holes only a few nanometers wide.
Technology in hand, Ferrari is launching his attack on a pressing medical problem: diabetes. In one form of the disease, the cells in the pancreas that normally produce insulin do not function properly. The most attractive cure would be to implant fresh copies of the body’s tiny glandular insulin factories (called islets of Langerhans) into the body. These would replace the broken pancreatic machinery and restore the body’s delicate feedback loop. Such new tissue, however, must be harvested from a compatible nonhuman species. “You would want to use pig islet cells,” says Ferrari, “but then your immune system would go crazy and destroy them because they are foreign.” Previous attempts to transplant foreign cells required that the patient take drugs to suppress the immune response. That strategy, however, can leave a patient dangerously susceptible to infections.
Ferrari’s solution is to house replacement cells in a container made with his nanoporous membrane material. Small glucose molecules could stream freely through the nanoholes into the capsule to activate the cells, and the insulin could trickle out to control the blood chemistry. Ferrari says that he has the technology ready to go. “We’ve had success in small animals,” he says, “but we need to do it in larger animals like dogs. That would be the slam dunk to allow us to go into human trials.” What’s holding Ferrari back is funding, and he hopes to get enough money to do large animal studies in a year, and possibly human studies in two years.
Ferrari isn’t the only one who thinks that cells can be smuggled into the body to restore normal function. Tejal Desai, who is a bioengineer at the University of Illinois and former student of Ferrari when they were both at the University of California, Berkeley (Desai was recently named to TR100’s list of young innovators), is investigating the approach for use in the brain-bringing in normal cells that would secrete neurotransmitters. Those neurotransmitters might be able to replace the ones lost when cells are damaged in diseases such as Alzheimer’s. Desai is utilizing the same nanopore fabrication technology used by Ferrari to make microcapsules for implanting neurons in the brain. Once the capsules are implanted, the neurons can be electrically stimulated to release neurotransmitters. Eventually, says Desai, this technology “could be used for such applications as treating Alzheimer’s or Parkinson’s-basically any disorder where the basic neurosecretory-cells are missing or damaged.”
Making use of materials engineered on the nanoscale is an intriguing approach to medicine. But it’s by no means the end of how nanotechnology might ultimately change medical care. Farther out on the nanomedicine horizon, Carlo Montemagno of Cornell University is working on mechanical devices-motors, pumps, all the equipment for a chemical factory-smaller than a living cell. Nanomotors, for example, might ultimately power small mixers to whip up tiny batches of drugs, then pump out the freshly made pharmaceuticals directly to tissues that need them.
The idea of incorporating motors into your body might seem wild-and it is. But it does have the advantage of being inspired, at least in part, by biology itself. Some bacteria, for instance, move by whipping around a tiny tail, or flagellum. The business end of the flagellum is essentially a motor, and, if you took it apart, you’d see a protein rotor nestled in a pocket formed by six proteins in a ring. Each protein is an enzyme called ATPase, which converts the cellular fuel ATP (adenosine triphosphate) into ADP (adenosine diphosphate); the chemical energy released by this reaction is what powers the machine. When the motor is running, the rotor ratchets around this ring of proteins. Beyond that, biologists actually know little of how the thing functions.
But they do know it works. And to fabricate his nanomotor, Montemagno stole from nature, grafting the moving parts from a bacterial motor onto a metal nanostructure. The Cornell team found a way to attach the nanomotors to an array of tiny pedestals on a micromachined nickel surface. The technique works well enough that Montemagno and his co-workers have demonstrated one of these hybrid motors spinning away: Montemagno’s team is measuring things like horsepower and motor efficiency, tests that would feel right at home to any mechanical engineer scrutinizing a car engine.
Montemagno envisions that tiny chemical factories could one day operate within a cell. He speculates that these nanofactories could be targeted to specific cells, such as those of tumors, where they would synthesize and deliver chemotherapy agents. This selective targeting and local delivery would reduce toxicity to other tissues and pack a much bigger punch than current therapies. One neat trick the group has achieved is to combine the light-harvesting mechanisms from photosynthesis with the biomotor to make a solar-powered nanomachine. Light energy creates ATP, which in turn fuels the nanoengine. It’s the first step to creating autonomous nanodevices that don’t need external fuel sources.
Nanopharmacies like Montemagno’s are at the outer limits of medical technology. It will be years before scientists even know if these nanodevices are practical. But long before that, researchers like Baker hope nanotechnology will be making an impact.
One nearer-term project on Baker’s agenda is smart bombs for treating cancer. These dendrimer-based devices are designed to infiltrate living cells and detect pre-malignant and cancerous changes. If the dendrimer bomb senses such threatening changes, it will release a substance to kill the cell (in one version, laser light is used to trigger the release of chemical agents from the polymer). Just for good measure, when its work is done, the dendrimer device will be able to verify that the cancerous cell is dead.
That may sound just as far out as the nanomotor, but in the eyes of the nation’s preeminent medical researchers, it isn’t. Indeed, last fall, the National Cancer Institute gave Baker’s center $4.4 million to rig up some smart bombs against cancer. Baker hopes to demonstrate the proof of concept in three years. He predicts that in a decade these microscopic SWAT teams will be in the pharmacy. “Being able to engineer things on the scale of biomolecules is very powerful,” he says. So powerful, in fact, that the engineering of the very small could soon pave the way for an entirely different kind of medicine: nanomedicine.
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