Most people probably would not associate car exhaust fumes with cancer prevention. Those fumes, however, contain nitric oxide (NO), gas molecules also produced by human cells that are essential to the regulation of a host of important biological functions, from the immune response to blood pressure.

A great deal of research these days is focused on taking advantage of some of this air pollutant's remarkable regulatory talents. Human clinical trials are now testing, for example, "NO-donating" compounds to treat diseases and conditions as diverse as asthma and Alzheimer's. Last week the first human clinical trial was initiated testing an NO-donating aspirin as a chemopreventive agent against colorectal cancer.

The research into this compound, a derivative of aspirin-releasing nitric oxide dubbed NCX4016, builds on data from epidemiologic studies and clinical trials showing that regular use of traditional aspirin can significantly reduce colon polyp formation in those at high risk of developing them, including those already treated for colorectal cancer. According to Dr. Basil Rigas, chief of the Division of Cancer Prevention at the State University of New York at Stony Brook, in laboratory and animal model studies he has led, NCX4016 has proven hundreds of times more potent than traditional aspirin in inhibiting growth of colon cancer cells in cell cultures. And in a mouse model of colon cancer, mice given NCX4016 daily for 3 weeks had a 59 percent tumor reduction on average. In a similar study using rats, tumor growth was reduced by 75 percent, and new tumors failed to grow. In both cases, the drug was effectively free from toxicity.

The gastrointestinal (GI) toxicity often seen with regular aspirin use and other nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen fueled the development of NO-donating NSAIDs in the mid-1990s. "NO," says Dr. James Crowell, a program director in the NCI Division of Cancer Prevention, "stimulates vasodilation and mucous secretion by the cells that line the GI tract." So an NSAID that releases NO may effectively nullify the NSAID's ability to cause serious, sometimes life-threatening problems such as bleeding ulcers.

What initially brought NO to the research forefront, however, was something altogether different: the discovery in the late 1980s of its role as a signaling molecule in the cardiovascular system - the first time a gas molecule was found to participate in the communication network within cells that regulate functions such as growth, division, and death. The discovery, for which a trio of scientists was awarded the 1998 Nobel Prize, spurred researchers from across the globe to see if NO played a similar role in other organ systems.

What they found is that "nitric oxide regulates nearly every tissue in your body," says Dr. David Wink, a principal investigator in NCI's Center for Cancer Research Radiation Biology Branch who has been studying the ubiquitous molecule for nearly 15 years. His lab's work has led to some intriguing discoveries. "We have found that changes in the doses of our NO donor compounds by very small amounts cause profound changes in tumor cells and signal transduction," Dr. Wink says.

Following on their studies showing that, in cell culture, tumors can use NO to promote angiogenesis, Dr. Wink's laboratory is now in the early stages of investigating how inhibiting NO affects standard cancer therapy. "We've found that if we inhibit NO after radiation or chemotherapy treatments, we see tremendous increases in the treatments' efficacy," he says.

As for chemoprevention, NCX4016's promise, Dr. Rigas notes, is not limited to colorectal cancer. In an animal model system of pancreatic cancer, treatment or pre-treatment with NCX4016 prevented 90 percent of pancreatic cancers. His laboratory has received several grants to study NO's mechanism of action, work primarily focused on elucidating what intracellular signaling pathways it affects. So far, several pathways have stood out, including NF-κB and Wnt, both of which are thought to be involved in carcinogenesis.

The molecular biology of NO in cancer is still not well understood, stresses Dr. Crowell. Additional research will provide insight into the potential long-term impact of NCX4016's use and help guide its potential use in combination with other therapies.

The trial initiated last week - supported by NCI and conducted in conjunction with NicOx, the French company that is developing a number of NO-donating agents for a wide variety of indications - will include a pharmacokinetic component aimed at answering some of those questions. It will recruit 240 patients at high risk of colorectal cancer and test whether, after 6 months of treatment, the drug can prevent or arrest the growth of microscopic lesions found in the colon lining, called aberrant crypt foci, which are thought to be polyp precursors.

Although the potency NCX4016 has displayed in laboratory and animal model studies is enticing, says Dr. Rigas, in the relatively new area of chemoprevention, establishing safety is paramount. "With chemoprevention, you're administering an agent to an otherwise healthy individual at risk for developing a cancer," he says. "And that person is committed to receiving that agent for a very long time."

By Carmen Phillips