The experimental vaccine could enter human clinical trials in six months, Kim D. Janda, whose laboratory at Scripps developed the vaccine and has tested it in rodents, said yesterday.

Vaccines to fight some forms of drug addiction are being designed to trigger the immune system to clear the drug from the bloodstream before it reaches the brain, where it becomes addictive.

Janda's lab and others have worked on vaccines to treat cocaine addiction, and have moved to those that marshal the body's immune system to neutralize nicotine.

Nicotine vaccines created by Nabi Biopharmaceuticals, based in Florida, and Xenova, headquartered in Britain, are undergoing clinical trials in humans.

Scripps' vaccine is different from those under development, Janda said, because it is designed to prompt an immune response against two common forms of the nicotine molecule. The structure and design of the vaccine will be described in an upcoming issue of the Journal of the American Chemical Society.

Nicotine molecules are flexible and can adopt many shapes, making it difficult for antibodies to attach to all of them and launch an immune response.

Janda, with colleague Michael M. Meijler, the study's lead author, chemically altered nicotine molecules to fix them in two specific shapes.

The Nabi and Xenova vaccines are not made this way, so the nicotine molecules remain flexible, Janda said.

In the Scripps study, the rigid nicotine molecules in the vaccine triggered the animals' immune systems to produce antibodies.

Those antibodies then fought effectively against nicotine later introduced into the animals' bloodstream, suggesting that a human form of the vaccine might clear most nicotine from the bloodstream before it reaches the brain, Janda said.

"These induced antibodies make a much more focused immune response" than antibodies triggered by vaccines that are not made with the engineered nicotine molecules, said Meijler.

Scripps has formulated a version of the vaccine for humans, but it still must conduct tests to make sure it is safe, Janda said. Once those are completed, in about six months, clinical trials could begin. The Skaggs Institute for Research at Scripps funded the study.