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LA JOLLA, Calif.—“Tobacco use disorder is responsible for over 400,000 deaths annually in the United States. It is critical to develop novel approaches to reduce the psychoactive effects of nicotine, decrease craving, and prevent relapse,” wrote scientists at Scripps Research, who have successfully tested a potential new smoking-cessation treatment in rodents in a study (“An enzymatic approach reverses nicotine dependence, decreases compulsive-like intake, and prevents relapse”) published online in Science Advances in October.
While tobacco use disorder is a leading cause of disease and preventable death worldwide, currently available medications based on pharmacodynamics have low efficacy, the Scripps scientists said, adding, “Novel pharmacokinetic approaches to prevent nicotine from reaching the brain have been tested using vaccines, but these efforts have failed because antibody affinity and concentration are not sufficient to completely prevent nicotine from reaching the brain.”
With funding provided by the National Institute on Drug Abuse, the researchers gave nicotine-dependent rats an engineered enzyme—NicA2-J1, an engineered nicotine-degrading enzyme that was originally isolated from Pseudomonas putida strain S16—to break down nicotine in the bloodstream before it can reach the brain. The treatment quickly reduced the animals’ motivation to take nicotine, reversed their signs of nicotine dependence and kept them from relapsing when they were given access to nicotine again.
The enzyme “completely prevented nicotine from reaching the brain and reversed somatic signs of withdrawal, hyperalgesia and irritability-like behavior in nicotine-dependent rats with a history of escalation of nicotine self-administration. NicA2-J1 also decreased compulsive-like nicotine intake, reflected by responding despite the adverse consequences of contingent footshocks, and prevented nicotine- and stress (yohimbine)-induced relapse,” the researchers said.
“This is a very exciting approach because it can reduce nicotine dependence without inducing cravings and other severe withdrawal symptoms, and it works in the bloodstream, not the brain, so its side effects should be minimal,” says principal investigator Dr. Olivier George, an associate professor at Scripps Research. “These results demonstrate the efficacy of enzymatic therapy in treating nicotine addiction in advanced animal models and provide a strong foundation for the development of biological therapies for smoking cessation in humans.”
While most people are aware of the harm smoking can do, nicotine dependence keeps them smoking. About 60 percent of the people who try cigarettes end up as daily smokers, and about 75 percent of daily smokers relapse after quitting. While reversing nicotine dependence by preventing the nicotine in tobacco smoke from reaching the brain has been considered a promising strategy, previous efforts have not yielded drugs that reduce blood levels of nicotine enough to be effective.
NicA2-J1, a version of a natural enzyme produced by the bacterium Pseudomonas putida, was modified to optimize its potency, its staying time in the blood and other pharmacological properties by the laboratory of Dr. Kim Janda, the Ely R. Callaway, Jr., Professor of Chemistry at Scripps Research. Previous studies have shown that NicA2-J1 strongly reduces blood levels of nicotine in rodents and provides efficacy in a simple rat model of nicotine dependency. The new study tested the enzyme in more sophisticated animal models, developed in George’s laboratory, which better mimic human smokers’ addiction to nicotine.
In one set of experiments, lab rats learned to self-administer nicotine and became dependent on it. Then they were given much reduced access to nicotine, leading them to experience withdrawal symptoms between access periods, and to increase their intake, thus deepening their addiction, when they regained access. The animals treated with the highest dose of NicA2-J1 continued to self-administer nicotine when available but showed very low blood levels of the molecule compared to controls. Signs of nicotine withdrawal were correspondingly reduced during the no-access periods, compared with untreated controls.
“It’s as if they were smoking 20 cigarettes but receiving the nicotine dose of only one or two, so that made their withdrawal process much less severe,” explained study first author Dr. Marsida Kallupi, a postdoctoral research associate in the George Laboratory.
Usually, when a drug blocks nicotine activity, there will be immediate withdrawal symptoms. According to George, “What’s unique about this enzyme is that it removes enough nicotine to reduce the level of dependence but leaves enough to keep the animals from going into severe withdrawal.”
The researchers also showed that NicA2-J1 could reduce this compulsive motivation for nicotine in the addicted rats. When each lever-press for nicotine also brought a 30 percent chance of receiving an electric shock to the feet, the NicA2-J1-treated rats—unlike untreated controls—quickly reduced their lever presses.
To determine susceptibility to relapse after abstinence, the researchers removed the rats from nicotine for 10 days, then gave them an injection of nicotine to reawaken their desire for the drug and restored access via the lever-presses. Untreated rats increased their lever presses by a large amount, but NicA2-J1 treated rats did it much less.
The Scripps Research team hopes to take NicA2-J1 into clinical trials in humans. They will attempt to optimize NicA2-J1’s properties as a drug. They also plan to test it in rats against varenicline (Chantix), a compound that blocks nicotine activity in the brain and is currently viewed as the most effective smoking-cessation drug.