A vaccine against cocaine was effective in nearly 40 percent of users of the drug, researchers said.
In a randomized, placebo-controlled study, volunteers who reached a high level of antibodies after vaccination used less cocaine than those whose response was lower, according to Dr. Thomas Kosten of Baylor College of Medicine in Houston and colleagues.
But as the effect of the vaccine wore off, antibody levels fell and many volunteers returned to abusing the drug, Kosten and colleagues said in the October issue of Archives of General Psychiatry.
"We didn't expect to cure them in six months," Kosten said, "and we didn't."
But he said that other studies have shown that a booster shot every two months can keep antibody levels high enough to block the euphoric effects of cocaine.
"In clinical practice, you'd probably give people a booster shot every two months," he said, which, compared with some long-term treatments, is "pretty easy to provide."
That, he said, might allow enough time off the drug to let psychosocial forms of treatment have an effect.
Kosten and colleagues tested a vaccine designed to tie up cocaine molecules in the blood where they can be degraded. That delay ensures that less of the drug reaches the brain and the euphoric effect is muted or nonexistent.
Indeed, Kosten said, some vaccinated volunteers said they tried larger than normal doses of cocaine during the study and reported no effect.
Cocaine-using volunteers, all enrolled in a methadone program for opiate abuse, either received five shots of vaccine or a placebo over a 12-week period. They were observed for an additional 12 weeks.
The researchers performed urine tests for evidence of cocaine use three times a week. What they found was thatstudy subjects who were administered the vaccine had cocaine-free urine tests 45 percent of the time. In contrast, those with lower levels of antibodies and those getting placebo were cocaine-free only 35 percent of the time.
All volunteers reduced cocaine use during the study, but those with high levels of antibody -- indicating they had received the vaccine -- had a significantly greater decrease during weeks nine through 16 of the study.
Adverse effects of the vaccine were limited to redness and soreness at the injection site.
A major hurdle, Kosten said, is to find a pharmaceutical company that's willing to develop a cocaine vaccine.
In the meantime, there are no approved pharmacologic approaches to cocaine addiction, he said, and even a treatment that would work for 38 percent of addicts would be "damn good."
"Overall, this vaccine -- even as it is -- would probably do pretty well with people who have already stopped using cocaine and just want to keep themselves from relapsing back," he said.