A vaccine helped block the high felt by cocaine users in 38 percent of people who took it, U.S. researchers said on Monday, offering promise of a new approach to treating those addicted to the drug.

The aim is to prevent cocaine's rewarding effects -- the high -- in order to reduce cravings that trigger drug relapses.

The concept works, Dr. Thomas Kosten of Baylor College of Medicine in Houston, Texas, whose study appears in the Archives of General Psychiatry, said in a statement.

Cocaine molecules on their own are too small to draw the attention of the immune system. To get the body to recognize cocaine, the researchers designed a vaccine that uses a harmless version of the cholera toxin with a few attached cocaine molecules.

When the immune system reacts to the toxin, it makes both cholera and cocaine antibodies.

These antibodies bind to the cocaine, preventing it from leaving the bloodstream, Kosten said. An enzyme called cholinesterase breaks down the cocaine and flushes it out of the body.

For the study, Kosten and colleagues studied 94 volunteers -- mostly users of crack cocaine, which is a solid, smokable form of the drug -- who were on methadone treatment at the Veterans Affairs Connecticut Healthcare System.

Over three months, the participants either got five shots of the vaccine or a placebo injection.

Those who had the highest antibody response were better able to stay cocaine-free. Although the vaccine does not wipe out cocaine cravings, it may help prevent relapse in some people, Kosten said.

The problem is that it does not create antibodies in everybody. Twenty-five percent of the people who get the vaccine do not make much antibody response, he said.

Even so, the vaccine represents a promising step toward an effective medical treatment, Dr. Nora Volkow, director of the National Institute on Drug Abuse, which is part of the National Institutes of Health, said in a statement.

STUDY PLANS

Plans are under way to study the vaccine in many sites.

According to a 2007 National Survey on Drug Use and Health, 35.9 million Americans aged 12 and older reported having used cocaine, and 8.6 million reported having used crack. In 2006, cocaine accounted for about 14 percent of all admissions to drug abuse treatment programs.

Kosten said he plans to tinker with the vaccine to make it more effective. He has already tried a different carrier -- a modified version of a meningitis bacterium -- supplied by drug firm Merck & Co.

Animal studies showed it produced five times the antibody response as the cholera carrier.

He has used a similar approach on a nicotine vaccine called TA-NIC now being tested in Europe. Both the cocaine and nicotine vaccines are being developed through private equity firm Celtic Pharma.

Swiss drugmaker Novartis and Nabi Biopharmaceuticals of Rockville, Maryland, are also developing nicotine vaccines based on the same principle. Last week, Nabi won a $10 million grant from the NIDA to develop its NicVax vaccine.

Kosten said he shared the results of the animal studies using Merck's vaccine carrier with the company. They are sufficiently impressed that they will probably make a nicotine product out of it, he said.

Medicine