Vaccines usually protect us from invaders, priming our immune systems to combat nasty pathogens if they should ever find their way inside us. But scientists at The Scripps Research Institute have been working on a different kind of vaccine — one that would inoculate people against substances they willingly put into their own bodies.

Vaccines usually protect us from invaders. But scientists at The Scripps Research Institute have been making progress on a different kind of vaccine — one that would inoculate people against substances they willingly put into their own bodies.

"It works under the same principle," said Joel E. Schlosburg, describing the heroin vaccine he and his colleagues describe in a study published yesterday in The Proceedings of the National Academy of Sciences. "Your body ends up mounting an immune response against the heroin itself. You're training your body to treat heroin as a foreign pathogen and create antibodies against it."

Addiction researchers have been trying for decades to concoct a vaccine that neutralizes the effects of heroin, with varying degrees of success. But the Scripps team thinks its experiments prove that this one really works.

They started off by getting a bunch of rats hooked on heroin. For 12 hours a day, the rodents were allowed to gorge on all the heroin they wanted. Once they started exhibiting withdrawal symptoms, the scientists slowly weaned them off the drug. During this four-week detox stage, some rats were administered the vaccine while others got a placebo.

When the researchers again turned the rats loose on an unlimited supply of heroin, they saw the control group relapse, compulsively consuming the drug like they had before. But they saw the vaccinated rats lose interest in heroin.

This showed that the vaccine had succeeded in binding to heroin in the rats' bloodstreams before it could be absorbed in their brains. The rewards system involved in getting a fix had vanished for these formerly addicted rats.

"This kind of experiment has never been done with vaccines before," said George Koob, a co-author of the study. "It's a pretty rigorous test of the hypothesis that vaccines will be useful in the human condition."

Heroin has been a tough target in vaccine research, because the drug quickly breaks down into other molecules once it enters the bloodstream. This new heroin vaccine improves upon previous ones because it's what they call a "dynamic vaccine," meaning that it binds to all the metabolites that heroin turns into once it gets into the body.

For the last three years, Schlosburg has been working on this vaccine in Kim Janda's lab, a hotbed for research on addiction. Janda's first paper on a vaccine for cocaine was published by Nature in 1995. But so far, none of his vaccines have FDA approval.

Even if this vaccine were approved, some still question whether it would be enough to truly break an addict's behavior. Critics of such vaccines point out that addiction is as much a social problem as a biological one.

And, because this vaccine wouldn't block the uptake of other opiates, there's a risk that vaccinated addicts could simply switch to a new drug of choice, perhaps to the increasingly popular prescription painkiller oxycodone.

"My view is that this is going to work best in individuals who are motivated to quit," said Koob, clarifying that if the vaccine were approved for people, it should be coupled with existing addiction treatment programs.

The Scripps researchers say their vaccine is ready to be tested in human clinical trials. But finding the money to run clinical trials will be difficult. "That's going to depend on someone opening their purse strings," said Koob.

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