University of Minnesota researchers have identified the compound (S)-NNN as a strong oral carcinogen in smokeless tobacco.

For years, evidence has been mounting that people who use smokeless tobacco products have an increased risk of oral and esophageal cancers. Research has shown that these products prompt the formation of DNA adducts, which can promote carcinogenesis. However, until now, no specific substance in smokeless tobacco had been clearly implicated as a cause of mouth cancer.

Researchers in the medicinal chemistry department and Masonic Cancer Center at the University of Minnesota in Minneapolis recently identified (S)-NNN, a form of N′-nitrosonornicotine (NNN), one of hundreds of compounds called nitrosamines, as a culprit. The discovery was presented in August at the 244th National Meeting & Exposition of the American Chemical Society in Philadelphia.

“There are significant amounts of (S)-NNN in all smokeless tobacco products—every single one of them,” says Stephen Hecht, PhD, the study's principal investigator.

In earlier studies, Hecht's team demonstrated that (S)-NNN and its relative (R)-NNN produced DNA adducts in the oral cavity and esophagus, raising the possibility these compounds might lead to tumor formation.

To find out, the researchers gave a group of 12 rats water containing racemic NNN, which includes both (S)-NNN and (R)-NNN. Two more groups, each including 20 rats, received water laced with (S)-NNN or (R)-NNN. The rats consumed the water for 17 months, leading to total doses roughly equivalent to a person consuming half a tin of smokeless tobacco every day for 30 years. Rats in a control group drank tap water.

All of the rats treated with (S)-NNN and racemic NNN developed multiple oral tumors, including malignant squamous cell carcinomas, in addition to esophageal tumors. (R)-NNN induced tumors in only a few rats.

A series of chemical reactions during the curing process leads to the formation of NNN in tobacco. Levels of NNN in smokeless tobacco products range from 1 to 10 parts per million; about two thirds of NNN is (S)-NNN. Hecht suggests implementing regulations that limit NNN levels to less than 10 parts per billion, which would be consistent with nitrosamine levels in other products, such as bacon and beer.

“That's completely doable,” says Hecht, noting that the U.S. Food and Drug Administration (FDA) has the authority to regulate tobacco products.

“The main message here,” says Hecht, “is that we need to get this material out of tobacco products—and we need to get the FDA to do it now.”

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