Natural Substances In Orange, Tangerine Inhibit Cancer

SAN FRANCISCO, March 30 — Naturally occurring substances in citrus juices, called flavonoids, show promise against prostate cancer, lung cancer and melanoma in laboratory studies, according to a joint Canadian-United States study. The findings were presented here today at the 219th national meeting of the American Chemical Society, the world’s largest scientific society. The weeklong meeting is expected to draw nearly 20,000 attendees from around the world.

Twenty-two flavonoids were examined during the study; this is the first time they have been tested against melanoma and prostate and lung cancer, says one of the lead researchers, Najla Guthrie, Ph.D., president of KGK Synergize, Inc., a research and development company in London, Ontario, Canada. The research was conducted in collaboration with the U.S. Department of Agriculture (USDA).

Orange and tangerine juice, in particular, produced “very encouraging results,” according to Guthrie.

Of the 22 flavonoids studied, two that occur naturally in tangerine juice — tangeretin and nobiletin —“were the most effective inhibitors of human prostate cancer cells,” according to Guthrie and colleague John Manthey, Ph.D., of the USDA’s Agricultural Research Station in Winter Haven, Fla. The compounds also inhibited the growth of melanoma cells, notes Guthrie. Her lab previously reported that the tangerine compounds were effective against breast cancer cells.

A flavonoid found in both tangerines and oranges, 5-desmethyl sinensetin, inhibited human lung cancer cells most effectively, according to Guthrie.

In comparison with other compounds the researchers have studied, the tangerine and orange flavonoids are “very effective.” Small amounts inhibit proliferation of cancer cells, according to Guthrie.

A synthetic flavonoid, quercetrin tetramethyl ether, also inhibited the growth of colon cancer cells during the studies, says Guthrie. The synthetic flavonoid was made by modifying quercetrin extracts. Quercetrin is found in many plants other than citrus, notes Guthrie.

“We looked at 22 flavonoids — some naturally occurring, some synthetic — and these are the most effective out of them,” Guthrie points out. Some of the citrus compounds being studied are inhibiting the cells in the same concentration range as the breast cancer drug tamoxifen, she adds.

“The next step is to look at them in animals.” Among the questions still to be answered, she said, are: “How are they metabolized? Do they get to the cells? Do they have the same effect?”

Animal studies have begun, says Guthrie, but it is too early to report preliminary data. She expects some results within about a year.

Dr. Guthrie is President, KGK Synergize, Inc. in London, Ontario, Canada.

Dr. Manthey is Research Chemist, U.S. Citrus and Subtropical Products Research Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Winter Haven, Fla.