A High-Voltage Fight Against Cancer
Researchers are trying to kill tumors by zapping them with high-voltage, nanosecond electric pulses
10 June 2004--In the relentless battle against cancer, researchers are now experimenting with a shocking new treatment--literally. They discovered that by zapping cells with extremely brief, high-voltage electric pulses, they could trigger the self-destruct mechanism in the cells' biochemical machinery. This mechanism, called apoptosis or programmed cell death, occurs naturally in the body, as tissues continually eliminate cells that are old, damaged, or simply no longer necessary.
The researchers are trying to find types of electric pulses that can trigger the suicide mechanism in cancer cells without affecting healthy ones. They hope the method will one-day serve as a tumor treatment that is less invasive than surgical removal and has fewer harmful side effects than chemotherapy. But critics caution that the technology is clinically unproven and may not make it out of the lab.
The technique's co-discoverer, Karl H. Schoenbach, an IEEE fellow and electrical engineering professor at Old Dominion University in Norfolk, Va., is expected to report his latest findings on 21 June at the Bioelectromagnetics Society's annual conference in Washington, D.C. Research groups in England, France, Germany, and the United States are currently conducting experiments with the technique.
Programmed To Die
Human leukemia cells on a microscope slide were zapped with a single, extremely brief, high-voltage electric field pulse [far left]. The cells had been dyed with a fluorescent compound that reveals the presence of enzymes associated with the activation of the cells' self-destruct mechanism. As time goes on, progressively more cells [left to right] develop the enzyme.
Schoenbach and Stephen Beebe, a professor of pediatrics at Eastern Virginia Medical School, also in Norfolk, first reported inducing apoptosis in cancer cells with electric pulses in 2001. They took mice and injected cancer cells in both flanks--one for treatment and the other to serve as the control--and let the tumors develop. After some time, using needle electrodes, they zapped one of the tumors with a series of electric pulses 300 nanoseconds long and 60 kilovolts per centimeter in magnitude. They found that the treated tumor grew only 50 to 60 percent as big as the untreated tumor, with many cells dying by apoptosis. Since then, the pair has been working to eliminate tumors completely and to do it with a single pulse.
While the result seems promising, it is far too early to celebrate. "This is very interesting science and new technology, but it is far too early to even hint that the method may have a clinical application and, if so, what that might be," says Kenneth R. Foster, a bioengineering professor at the University of Pennsylvania, in Philadelphia, and an expert on the effects of electromagnetic fields on living tissue.
Researchers have yet to perform the many animal and human trials needed to get the technique approved for use by doctors, and those experiments are probably years in the future, Foster says. "There is a big difference between inducing apoptosis in some cells in suspension or in some cells in a tumor and in destroying a tumor in any clinically meaningful way," he adds.
Schoenbach and Beebe's technology is far from the only way to induce cell suicide. Dianne E. Godar, a research biochemist at the U.S. Food and Drug Administration's Center for Devices and Radiological Health in Rockville, Md., who works with cancer causes and treatments, says she "can list about a thousand biological, chemical, and physical agents that can induce apoptosis" in cancer cells and in solid tumors, or that can inhibit tumor growth. Additional experiments, she says, have to be done if we want to compare the electric-pulse treatment with existing treatments.
One goal of future experiments is to identify what it is in cells that first senses the electric pulses and triggers apoptosis. That information can then be used to find a way to target cancer cells specifically. "There's something that occurs in the cell that it cannot resolve, it cannot fix; so it commits suicide," says Beebe. He says cellular structures known to regulate programmed cell death, including the energy-producing mitochondria and the DNA-storing nucleus, might be involved.
Understanding apoptosis is among the hottest topics in medicine and molecular biology nowadays. The 2002 Nobel Prize in Physiology or Medicine went to three researchers--Sydney Brenner, John Sulston, and Robert Horvitz--for their seminal work on apoptosis and the genetic regulation of organ development.
Problems with apoptosis are implicated in many diseases, including cancer--when cells fail to undergo apoptosis and multiply wildly--and neurodegenerative disorders such as Alzheimer's disease--when too many cells die. So, for cancer, scientists want to find ways to induce apoptosis; for Alzheimer's, they want to block it.
Decades before the focus on apoptosis , scientists used electric pulses of a lower voltage and longer duration to create temporary pores in the outer membranes of cells. This technique, called electroporation, is now widely used in laboratories to inject cells with DNA, drugs, and other kinds of molecules.
A cell in an electric field behaves essentially as a tiny spherical capacitor: its 5-nanometer-thick membrane is a good insulator and is surrounded, inside and out, mostly by salty water. When a field is applied, ions and other charged molecules in the water accumulate outside the cell's membrane. The same process happens inside the cell, and as a result, a voltage builds up across the membrane.
" When the voltage across the membrane gets up to between 0.5 and 1 volt, something dramatic happens," says James C. Weaver, a senior research scientist at the Harvard-Massachusetts Institute of Technology's Division of Health Sciences and Technology, in Cambridge. The voltage causes a breakdown in the membrane's insulating properties, opening a large number of temporary pores all over it. Ions and other large molecules can pass through the pores in the membrane, which temporarily changes from being an insulator to being a conductor.
Today, electroporation is used experimentally to enhance the efficacy of chemotherapy, Weaver says that the procedure opens large pores in cancer cells, forcing them to absorb more of an anticancer drug such as bleomycin, which damages DNA, killing the cell. Researchers working on this combination of electroporation and chemotherapy include Lluis Mir at the CNRS-Institute Gustave-Roussy, in Villejuif, in France; Richard Heller at the University of South Florida, Tampa; and researchers at the Genetronics Biomedical in San Diego.
The electric field pulses used in such experiments have intensities on the order of kilovolts per centimeter, which last from microseconds to milliseconds. But Schoenbach realized that a pulse with a shorter duration, on the order of nanoseconds, would not last long enough to bring ions to the cell membrane and build a voltage high enough to break it down. Instead, the faster pulse seems to bypass the cell membrane, affecting structures inside cells such as the nucleus. Like the cell itself, these structures have membranes that are good insulators, and therefore they also act as tiny capacitors that can charge up.
Schoenbach and Beebe discovered that these short, high intensity pulses could jolt the guts of the cells in a way that activates their self-destruction machinery. So, in principle, a treatment based on these short pulses would not require a drug like bleomycin: the pulses themselves would be the killing agents.
Delivering such a tremendous voltage in just a few billionths of a second is akin to accelerating a car from 0 to 100 kilometers per hour and then decelerating it back to 0, all within 1 second. The system Schoenbach's group built to perform the task consists of a commercially available high-voltage source used in powerful lasers and X-ray devices connected to a so-called pulse form network, a circuit that creates the nanoseconds-long powerful electric field between two electrodes.
The pulse form network, Schoenbach says, is a complex arrangement of interconnected cables and electronic components, but it works essentially as a transmission line. The 40 000-kV source delivers a burst of electrical energy to one end of the line through a spark-gap switch and it travels toward the electrodes. During the brief time the stream of energy flows along the line--a few billionths of a second--an electric field of hundreds of kilovolts per centimeter appears between the tiny electrodes.
The electrodes, usually a few millimeters apart, don't necessarily need to touch the cells. It is the electric field between them that does the work. So far, the group has used needles as electrodes, but researchers are studying other sophisticated ways of applying the field, including antennas that could zap a tumor inside the body at a distance. "That's a daunting task," Schoenbach says. "We need extremely high electric fields and high power broadband antennas. That's really futuristic at this point."
Regardless of how the pulses are delivered, the high voltages involved make the setup sounds more like a new cooking technique than a cancer treatment. But the actual energy delivered is quite low, less than a joule, not even enough to heat the cells single degree Celsius. This is because, even though the amount of power involved is enormous--16 megawatts or 16 megajoules per second--it is applied for only a few nanoseconds.
" Usually when people think about electricity, they think about a brutal way of killing--electrocution, burning, this kind of thing," says Schoenbach. "Our method is focusing on extremely short pulses, so that there's no thermal effect, no heating involved. It's purely an electrical effect."
Ultimately, the success of ultrafast, high-voltage pulses as a cancer treatment depends on whether a pulse of particular duration or voltage will preferentially kill tumor cells rather than normal cells. "The trick with all cancer therapies is to find a therapeutic window where the therapy kills the tumor cells without too great a collateral effect on sensitive normal tissues," says Gerard I. Evan, a professor of cancer biology at the University of California at San Francisco.
Evan says the results obtained by Schoenbach, Beebe, and their colleagues are intriguing, but more experiments are necessary to determine whether such electric fields would exhibit the necessary specificity. It's particularly important, he says, that researchers identify the molecular mechanism by which the pulses trigger apoptosis. Only then will it be possible to get some idea as to whether the pulses might be effective as a treatment some day.
And researchers have yet to figure out what exactly is happening within zapped cells. "We know the start--we have to have this series of extremely short pulses. And we know the end--we create apoptosis," Schoenbach says. "What we're looking for now is what is in between."