Swarms of microscopic "pills" may someday deliver large doses of anticancer drugs to tumors. As described in tomorrow's issue of Nature, the capsules are actually tiny polymer beads coated with artificial membranes. A bead releases its cargo only after its membrane ruptures--something doctors might be able to trigger with bursts of ultrasound trained directly on a tumor.
The inspiration for the beads comes from tiny jellylike bubbles called secretory granules. When certain cells in the lungs and airways detect allergens, their granules fuse with the cell membrane and eject chemicals that alert the immune system. Colloid chemist David Needham of Duke University in Durham, North Carolina, and his graduate student Patrick Kiser set out to mimic these secretory granules.
The duo fabricated tiny beads from hydrogel, a highly absorbent polymer used in disposable diapers and contact lenses. Hydrogel latches onto positively charged molecules, including the anticancer drug doxorubicin. Because the gel sops up the drug so well, beads made from it can carry 10 to 100 times more drug by volume of the bead than other delivery systems, says Needham. To keep the drug from leaking out, Needham and Kiser wrapped it in an artificial membrane. In their test tube experiment, they zapped the 5-micrometer-wide beads with an electric pulse that ripped open the membrane. Saline solution flooded into the bead, expelling the drug.
Experts like the idea. "You can think of it as a storage device that has a quick release," says Ronald Siegel, a pharmaceutical scientist at the University of California, San Francisco. But, he adds, this new drug-delivery concept "is going to require a lot of [further work]." For example, Needham's group next plans to make 0.05-micrometer-wide beads small enough to travel through the bloodstream and into tumors. They are also working on ways to disrupt the membrane once the beads have been injected, possibly by using ultrasound or by making the coating degradable.