The first goal--targeting tumors--has not been easy. "Specificity in drug delivery has been historically a very elusive goal," says Mauro Ferrari, chair of the biomedical-engineering department at the University of Texas Health Science Center, in Houston. Because the new particles have dendrimers on which the researchers can attach different targeting molecules, the technique might work. But the real test will be doing that inside the body. "Targeting cancer cells can be done in a million different ways in the lab," Ferrari says. "But translating the technique into animals and humans has proven to be very difficult."

Baker believes that the polymer dendrimers should do the trick. In a 2005 study, his research team showed that dendrimer molecules--without gold inside--that were loaded with folic acid and a cancer drug specifically targeted human tumors in mice, and slowed or killed the tumors more efficiently than the drug alone. The researchers are now testing the new gold-dendrimer hybrid particles in mice and expect the particles to be just as effective as the plain dendrimers.

The small size of the new particles should ensure that they get eliminated from the body. The particles are smaller than most other nanoparticle systems designed for cancer therapy, according to Baker, so they shouldn't accumulate in vital organs such as the kidney, liver, or lungs. But their small size might raise other safety issues. Inside animals or humans, the nanoparticles could get into other cells, says Raoul Kopelman, a professor at the University of Michigan's Center for Biological Nanotechnology, who was not involved in the new work. "If you deal with animals or humans, there are many kinds of cells," Kopelman says. "Will they get into other cells like immune cells? It needs to be tested."

The most important problem to solve, says Ferrari, is how to make nanoparticles that can stealthily avoid the body's natural defense mechanisms and get to tumors. "The body has so many booby traps that keep drugs and nanoparticles and everything that is foreign [from getting] into anything of significance in the body," he says. "If you can build on top of the dendrimer platform the ability to make it across biological barriers with great efficiency, then we have a great breakthrough."