Dog shark

NEW YORK (CNN) -- Sharks are generally thought of as killers, but one type may be a lifesaver.

The relatively gentle dog shark possesses squalamine, a naturally occurring cancer-fighting chemical that gives hope for cancer patients, said researchers from Johns Hopkins.

The hormone-like substance that can be isolated from the shark's liver and synthesized in a lab, can dramatically slow tumor growth without damaging healthy cells by stopping blood vessels from growing, said Dr. Henry Brem of Johns Hopkins Hospital.

Experiments in rats showed squalamine shrinks the deadliest form of brain cancer, glioblastoma.

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All cancerous tumors require blood vessels to feed their cells, and squalamine may be able to starve them to death.

Like the research on angiogenesis inhibitors, the much-talked about treatment that blocks the growth of new blood vessels to tumors in mice, the investigation of squalamine is in the beginning stages.

"We're optimistic that the principles we found in the laboratory are applicable to people," Brem said. "It's being tested now in people, and so we'll first learn if it's safe in people and then if it's effective in people."

Some health food proponents claim shark cartilage, a dietary supplement found in health food stores, has the same effect as squalamine. But health experts said they are skeptical.

"Most of the compounds that are sold over-the-counter are not regulated so we don't know how much of any of these active ingredients they would contain," said Dawn Willis of the American Cancer Society.

CNN Medical Correspondent Dr. Steve Salvatore contributed to this report.