Those results come on the heels of last week's announcement that the federal government would fund a large clinical trial of a nicotine vaccine based on earlier promising studies.
Instead the products are designed for use with other treatments, such as behavioral therapy, to help people kick the habit.
They work by stimulating the body's immune system to create antibodies that bind to the drug and prevent it from reaching the brain and producing a chemical high.
"Fifteen years ago, people thought this was completely stupid," said Dr. Thomas R. Kosten, a psychiatrist at Baylor College of Medicine in Houston and senior author of the cocaine study.
But a series of technological advances has changed that, he said.
"This is trying to come out and attack drug abuse in a different manner," said Kim D. Janda, a professor of chemistry and immunology who works on vaccines at the Scripps Research Institute in La Jolla. "The National Institutes of Health has spent hundreds of millions of dollars to develop drug treatments, and they haven't gone anywhere. They have to see this is a better way."
But even as recent scientific developments highlight the promise of such vaccines, the hurdles to getting such addiction-battling drugs to consumers are revealed. Legal and financial barriers may prevent all but a nicotine vaccine from reaching the market in the near future.
"It's very worthy science, and we have to move it forward," Janda said. "Addiction tears at the fabric of society. Families are ruined. But that doesn't translate into dollars in pharmaceutical companies' pockets."
Drugs like cocaine and nicotine are comprised of tiny molecules that are too small for the immune system to recognize, allowing them to slip through the body's blood-brain barrier. Researchers realized they could circumvent that problem by tethering drug molecules to larger proteins that the body recognizes as foreign. Then the immune system would mount a defense against both.
"You have to trick the body into making an antibody response to this small molecule," said Berma M. Kinsey, a research chemist at Baylor who works on cocaine vaccines.
Vaccines against nicotine and cocaine are the most advanced, but versions to block the effects of methamphetaimine, heroin and phencyclidine, or PCP, are also in development.
"It's like a sponge for that specific drug," Janda said. "But a problem is some people respond very well and some don't."
Further, such vaccines address only the chemical addiction of drug abuse, not the behavioral aspects.
For example, NicVAX, the nicotine vaccine that got a $10-million grant last week from the National Institute on Drug Abuse, can block the release of dopamine in a smoker's brain.
"But unless you are interested in stopping smoking, the habit continues," said biochemist Raafat Fahim, chief executive of Nabi Biopharmaceuticals in Rockville, Md., which makes NicVax.
In the new cocaine study, researchers enrolled 115 cocaine-dependent people in a 24-week trial. The participants were also addicted to opioids and were enrolled in a methadone treatment program, which made it easier for researchers to retain the subjects long enough to test the vaccine.
Half of the individuals received five injections of the vaccine over 12 weeks, and the other half received placebo injections. The participants' urine was tested three times per week.
Of the 55 people who received all five vaccine injections, 38% attained cocaine antibody levels of 43 micrograms per milliliter or higher, which previous research indicates is necessary to blunt the cocaine response. Those individuals had significantly more cocaine-free urine samples between weeks nine and 16 than individuals who did not achieve sufficient antibody levels or who received placebo injections.
Among high-antibody producers, 53% were abstinent from cocaine more than half the time, compared with 23% of people who made lower levels of antibodies.
No serious side effects were reported. The study was published in the Archives of General Psychiatry.
The antibodies eventually subside over a period of weeks, so booster shots would be required, and more work is needed to improve the response rate, Kosten said.
"This is a first-generation vaccine, and we wanted something simple and safe to show it works," he said.
The National Institute on Drug Abuse will fund a larger study of the cocaine vaccine beginning in January.
People who want to quit smoking already have several behavioral and pharmaceutical treatments to choose from, but there are no medical treatments for cocaine use, methamphetamine or PCP dependence. Methadone is only modestly effective in treating heroin addiction.
However, need and desire may not be enough to move vaccine technology to the market.
"Drug manufacturers shy away from addiction treatments due to liability," Kosten said. "They are afraid that if someone has an overdose later, they would blame it on what the vaccine did to their body. We have people lining up and down the street to take the vaccine. It's almost a no-brainer. The problem is we have to get by this huge legal barrier."
The vaccine approach won't work to treat alcohol addiction because alcohol molecules are simply too small and shapeless to tether to larger proteins, said Baylor's Kinsey.
It's also not likely to work for marijuana because the active chemical -- THC -- is extremely sensitive to light, air, heat and acid, all of which makes it difficult to manipulate in the laboratory, she said.