New Haven, Conn. – Dosage appears to be a critical factor in the effectiveness of a cocaine vaccine being tested by Yale researchers that is designed to block the euphoria drug abusers experience.
Of eight patients in the second phase of clinical trials of the vaccine, one received one dose and the others received three to four doses. Six of the eight patients reported only one or two uses of cocaine during six months of follow up treatment and the two other patients used cocaine on a regular basis during the six months.
"These are very good results for people who abuse cocaine," said Thomas Kosten, M.D., professor of psychiatry at Yale School of Medicine and lead researcher on the project. There are an estimated three million cocaine abusers, making cocaine the second most commonly abused illicit drug after marijuana.
Kosten hopes to enroll a total of 150 patients in the second phase of the trial. The goal in this next stage, which is being conducted on an out patient basis, is to determine the dosage and the degree of addiction.
The first phase of the clinical trial included 34 subjects and was intended to test how the vaccine worked, whether it was safe, and if there were any side effects. Kosten published the results of the Phase One trial in the January issue of the journal Vaccine.
"We found that the vaccine, TA-CD, is safe, made antibodies, and there is no significant toxicity," he said. "The vaccine was well tolerated and had no serious drug-related adverse events, although three subjects at the highest dose experienced brief post injection twitching at the injection site. We’re now trying to get some sense of whether we can increase the frequency of dosing to perhaps five times to see if we can get the level of antibodies even higher."
The 34 subjects enrolled in the Phase One trial were abstinent cocaine abusers who were confined in a residential drug treatment program. Of that number, 27 completed the full course of three monthly injections. Of these, 24 returned for the final scheduled visit after three months. Fifteen of those inoculated were followed for one year.
"Anti-cocaine antibodies were detected after the second injection, peaked at three months, and declined to baseline by one year," Kosten said.
The vaccine works by binding the cocaine to antibodies on entering the bloodstream, preventing uptake of cocaine across the blood-brain barrier and dulling or even obliterating the euphoric rush.
Kosten said the vaccine probably only will be effective for those drug abusers who are motivated to stop using cocaine because someone intent on getting high can override the vaccine’s binding action by taking more cocaine.
Since just a small amount of cocaine stimulates intense craving for more of the substance, leading to a binge, he said the vaccine may be most effective at reducing this priming effect. "The vaccine may be able to prevent a cocaine slip from turning into a full scale binge and relapse to dependence," Kosten said.
The research is funded by the National Institute on Drug Abuse. The vaccine is being developed by Xenova Pharmaceuticals plc, a biopharmaceutical company based in Cambridge, England.
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