Studies suggest that high levels of anticocaine antibodies can help addicts to abstain
A vaccine to treat cocaine use helps some addicts to halve their dependency on the drug, researchers say.
Doctors at Yale University School of Medicine gave the vaccine to 55 cocaine addicts and found that 38% were able to achieve the necessary antibody levels.
Animal and human studies have suggested that high levels of anti-cocaine antibodies in the blood can stop addicts experiencing a high.
But the researchers say the addicts would need repeat injections.
The study published in the journal of the American Medical Association says the Yale doctors conducted a 24 week trial of an experimental vaccine.
A total of 115 cocaine and opiate-dependent individuals were enrolled and randomly assigned to receive five vaccinations of the active vaccine or five vaccinations of a dummy treatment over 12 weeks.
Cocaine is flushed out the body in about three days so the doctors tested all the addicts urine for metabolised cocaine three times a week for 24 weeks.
Of the 55 people who completed the course 21 (38%) attained antibody levels of 43 micrograms per millilitre or higher.
Those with that level of antibodies had significantly more cocaine-free urine samples (45%) between week nine and 16 of the study than individuals who had lower antibodies (35%) and those who received dummy treatments.
The proportion of participants who reduced their cocaine use by half was significantly greater among those treated with the active vaccine - 53% compared to 23% in the placebo group.
The researchers said about 40% of the participants achieved antibody levels of 20 micrograms per millilitre.
They said this was enough to combat one to two doses of cocaine which should be enough to prevent relapses in many patients.
Dr Thomas Kosten of Baylor College of Medicine, who started the study while he was at Yale, said: "While these antibodies are in the blood targeting cocaine the drug does not have an effect.
"They don't destroy, they neutralize the cocaine and make it vulnerable to a cholinesterase enzyme which will then break it down.
"The vaccine binds the cocaine so that it can't affect the brain, the heart or any other organ.
"This is the first successful placebo controlled test of a vaccine for cocaine."
Two years of treatment
The adverse events associated with the vaccine were mild to moderate with the most frequent complaint being hardening and tenderness at the injection site.
Dr Kosten said: "We think most people will need two years of treatment with the vaccine - bearing in mind there is an average six to eight years of abuse before they come for treatment.
"It's only at week eight that we get full levels of blocking antibodies.
"We have already tried the vaccine with a different carrier supplied by Merck Pharmaceuticals - this is based on the meningitis virus and it is provoking much better levels of antibodies.
"We are excited because of all the other drugs we can design antibody vaccines for.
"All the other drugs of addiction should work except alcohol because that molecule is too small to make an antibody to it."