Stopping cancer cells in their tracks.

Most cancer drugs work by killing tumor cells as they grow and divide. But cancer's lethality stems from the ability of malignant cells to break off from the initial tumor and spread to distant tissues, seeding new tumors. Scientists are now investigating a drug that works by halting the spreading cancer cells in their tracks.

The compound, called CAI, "constitutes a new approach to cancer therapy," asserts Lance A. Liotta, head of the pathology laboratory at the National Cancer Institute (NCI) in Bethesda, Md. He says his group expects to begin testing it in humans within the next six months.

Although CAI (carboxamide-amino-imidazole) was originally developed as an antifungal drug by the Merck Institute for Therapeutic Research, Liotta's team discovered that it also trips up traveling tumor cells.

Now, Liotta and Elise C. Kohn of NCI have uncovered the mechanism behind CAI's anticancer effect. Normally, the spread of cancer cells through the bloodstream is triggered by signals received from other cancer cells. Liotta reports that CAI shuts down this line of communication by blocking a chemical pathway involving arachidonic acid, a cell-membrane protein known to enable cells to receive signals from each other.

Last year, Kohn and Liotta found that CAI prevented malignant melanoma cells from moving around in a culture dish. They also showed that mice injected with human ovarian cancer cells and treated with CAI lived more than two times as long as mice receiving the cancer-cell injections but no CAI.

Liotta says cell-culture experiments by his group have shown that CAI acts against 20 different types of cancer. He adds that the compound offers several advantages over conventional cancer drugs: Patients can take it orally, a single dose stays in the bloodstream for as long as three days, and the treatment causes few side effects.