RIDE a wave on the beach and the distance you go will tell you something about your mass. But get a living cell to surf a laser beam, and you鈥檒l learn something altogether different: how healthy the cell is.
The scientists who have discovered this bizarre effect don鈥檛 yet know why it works 鈥� just that it can distinguish with considerable accuracy between cancer cells, virally infected cells and healthy cells. The technique, dubbed optophoresis, has already won backing from the US government. The company behind the idea, Genoptix of San Diego in California, has been given $750,000 in funding by the Pentagon鈥檚 research arm, DARPA, which wants the firm to explore a possible application as a bioweapons detector.
The technique uses a near-infrared laser beam, which is harmless to living tissue. In cross-section the beam is most intense at its centre, so as the cell moves into the vertical beam from right to left, say, it experiences a rising 鈥渨ave鈥� of intensity, strongest on its left side. Light striking the cell on the left is refracted to the right, and vice versa, so the net effect is that more light is refracted to the right. The law of conservation of momentum dictates that the cell feels a force in the opposite direction 鈥� to the left. This pings it through the 鈥渨ave鈥� and out the other side, and it is the size of this jump that is all-important.
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How much the cell shifts appears to depend on many factors, including its size, shape, and the proteins dotting its surface. For some reason, cancer cells tend to move more than their healthy counterparts. However, each cell鈥檚 shift is impossible to predict, says Jeff Hall, vice-president of cell biology at Genoptix, so the technique will have to be carefully calibrated using known cell types.
Nonetheless, the shifts of certain classes of cells reveal much about them, says Hall. If a cancer cell succumbs to a certain drug, for example, its shift usually decreases, while other drugs might actually increase it. Optophoresis appears to be reliable even with samples of only a few hundred cells, and it doesn鈥檛 rely on chemicals such as dyes or biochemicals that might alter the cells鈥� physiology. And each cell can be tested many times.
Hall鈥檚 team is now trying to predict how patients will respond to different treatments, based on how their cancer cells surf the laser before and after treatment with certain drugs. Genoptix chief executive Tina Nova says the company plans to offer a service to test drug efficacy by the beginning of next year.
鈥淭his has enormous potential for detecting events in cancer,鈥� says Terry Phillips, an immunochemist at the US National Institutes of Health in Bethesda, Maryland. But he points out that in data the company presented at the 2003 American Society of Clinical Oncology meeting in Chicago, Illinois, samples containing 20 per cent cancer cells were read as normal in the assay. 鈥淯nfortunately, you鈥檇 want better sensitivity than that if you want to catch cancer in its early stages.鈥�
Hall agrees that sensitivity is a key issue and says that new instruments and software have already improved it.
