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Researchers announced this week the early trial results of a new vaccine to fight cocaine addiction — a compound that, by vaccine standards, was only mildly effective but served as an important proof of concept that vaccination against addiction could work.

"Vaccines are one of our top priorities," says Dr. Nora Volkow, director of the National Institute on Drug Abuse (NIDA), which helped fund the study and will finance a larger study of the cocaine vaccine next year. In September, NIDA also granted $10 million for a clinical trial to the makers of NicVAX, a nicotine vaccine that works similarly to the cocaine vaccine — by stimulating the immune system to create antibodies that bind to drug molecules and prevent them from entering the brain. (Because people don't generally make natural antibodies to cocaine, the cocaine vaccine combines a cocaine molecule with an inactive cholera toxin; incidentally, the vaccine protects against cholera as well).

In the new study, published in the October issue of the Archives of General Psychiatry, researchers recruited 115 cocaine-dependent people who were also addicted to heroin and enrolled in a methadone-treatment program, which made it easier for investigators to keep people in the study for a full 12 weeks. Half of the participants were given five injections of the vaccine over the course of the study, while the other half received a placebo.

The vaccine produced a strong antibody response in 38% of the 55 people who received all five doses. When antibody levels were highest — during weeks 9 through 16 of the trial — these participants used significantly less cocaine (as measured by urine samples taken three times a week) than either the placebo group or those who did not produce sufficient antibodies. Overall, cocaine use was reduced by at least half in 53% of the people who produced a strong antibody response, compared with 23% of those who had a weaker antibody response. That's far from abstinence, although reduction in drug use still has benefits, Volkow notes — like reducing people's risk of overdose and heart attack, for example.

About 25% of the study participants did not produce cocaine antibodies when vaccinated. In about half of these cases, researchers think smoking crack was the problem. Crack causes intensely hot cocaine molecules to attach to lung cells. The body begins to produce antibodies against this damaged tissue, but ultimately, rather than deciding that it is alien, the body views the cell clusters as "self" and doesn't fight them. (Similarly, allergists scratch the skin and expose it to allergens like grass seed to produce this kind of desensitization.) "They're a body constituent as far as [the immune system] is concerned," says Dr. Tom Kosten, professor of psychiatry at Baylor College of Medicine and the lead author of the study.

Another issue is that the vaccine would require booster shots to maintain a consistent level of antibodies, so Volkow suggests that a successful vaccine would in theory be most useful for relapse prevention, rather than to initiate abstinence. "If you give it to someone who has gone through rehab and is trying to stay clean and relapses, the vaccine will be able to interfere with that relapse and that will be incredibly important," she says. When former drug abusers attempt to use "just once," it often leads rapidly back to full addiction, a cycle that could perhaps be curbed if the drug's high were blunted by a vaccine.

But the vaccine is far from ready for public consumption, and researchers think that even if future trials confirm its utility, it may never reach the market. That's due in part to legal hurdles — drugmakers fear that patients who take an addiction drug, then later overdose or develop another ailment, like cancer, may lay blame on the vaccine. Addiction experts also caution that no drug-based addiction treatment is a panacea, and that behavior-based quit programs must play a role. "It's good that they're doing this research, but we need to temper our enthusiasm," says Carl Hart, associate professor of psychology at Columbia University, who studies cocaine and other drugs. He notes that a vaccine cannot fight craving itself. "It might be helpful for select people," he says, "but we need to go into this with our eyes open."

• Maia Szalavitz
• An early trial suggests that addiction vaccines could theoretically work, but researchers have doubts that they will ever get to market