Arsenic helps tumors, blood vessels grow.

In a blow against oncologists' hopes that arsenic might serve as a versatile antitumor agent, researchers have found that the 33rd element of the periodic table may actually speed the growth of tumors.

High doses of arsenic are toxic to the heart, but lesser amounts have been shown to work therapeutically against leukemia. Some researchers therefore have considered periodic arsenic infusions for treating other forms of cancer.

The first published experiments on how arsenic affects blood vessel growth in animals could quell that optimism.

Administering small amounts of the arsenic ion arsenite to cancerfree mice and to chicken embryos causes extra blood vessels to develop, says Aaron Barchowsky of the University of Pittsburgh. In cancer, such vessel development typically supports the growth of tumors.

In tests on mice with skin cancer, the researchers found that twice-weekly injections of 0.5 milligram of arsenite per kilogram of body weight helped tumors grow larger and spread more often than they did in animals not receiving the ion. The detrimental effects lessened but persisted at arsenite doses up to 5.0 mg/kg, beyond which arsenic's toxicity would be too high for therapeutic purposes.

Because it's difficult to precisely regulate blood levels of arsenic in patients, these results present daunting obstacles to expanding medical uses of the substance, says Barchowsky. He and his colleagues at Dartmouth Medical School in Hanover, N.H., and the University of Oklahoma in Oklahoma City report their findings in the December 2003 Toxicological Sciences.--B.H.