Birch bark has an anticancer bite.

Tree-loving Joyce Kilmer didn't foresee this. Last June, the drug taxol, derived from the Pacific yew tree, made headlines as a potent anticancer agent. Now betulinic acid, extracted from the bark of the common white birch, has emerged as the next potential sylvan pharmaceutical.

Unlike taxol, betulinic acid specifically affects melanoma cells. In mice, it blocks the growth of these skin cancers, which often spread to other organs. It also seems to leave normal cells unscathed, say researchers at the University of Illinois at Chicago.

Melanoma, which afflicts 1 in 90 white people over a lifetime, has the most rapidly rising incidence of any cancer in the United States. The drug most often used to treat it, DTIC, helps only a quarter of patients, and its effect soon fades. "We clearly need something else," says pharmaceutical biologist John M. Pezzuto, whose team reports on its studies in the October Nature Medicine.

The researchers extracted the raw material, betulin from the bark of birches culled from a Chicago parking lot.

From betulin they synthesized betulinic acid, and they tested it, along with other drugs, on human cancer cell cultures. These included lymph, lung, liver, and skin melanomas, plus some nonmelanoma cancers. Betulinic acid wasn't the deadliest agent, but it stood out in its dogged focus on the melanoma cells. "We don't know why," Pezzuto says.

That focus may explain the compound's apparent kindness to normal cells, he speculates. Most anticancer drugs attack a variety of cancers but harm some body cells as well. Betulinic acid, however, may interact with something only in melanomas.

The researchers gave betulinic acid to a strain of mice whose weakened immune systems allow introduced cancers to grow readily. When they injected human melanoma cells into these mice, the compound "completely inhibited the growth of tumors," Pezzuto says. In mice with existing melanomas, betulinic acid stalled tumor growth.

Even at high doses, moreover, these effects came with none of the typical side effects of anticancer drugs, such as weight change, diarrhea, or organ damage. "Having such results," Pezzuto says, "is really unusual."

But Antonio Buzaid, a melanoma specialist at the University of Texas M.D. Anderson Cancer Center in Houston, warns against extrapolating from mice to humans. "In most instances, such effective drugs don't pan out in people," he says. Even so, he adds, betulinic acid's specificity may help finger what makes melanoma unique.