Reduced CYP2B6 metabolism influences smoking initiation and treatment response: i

Centre For Addiction And Mental Health, Toronto ON

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Abstract

DESCRIPTION (provided by applicant): About 21% of US adults are current smokers and smoking rates have not declined in recent years. Smoking causes about 3 million deaths a year world-wide, and while roughly 80% of smokers desire to quit smoking, most fail. Because of continued smoking acquisition and low long term abstinence rates, smoking remains a significant health problem. This project aims to gain a better understanding of the mechanisms behind the variation between people in their risk for acquisition of smoking and their success in smoking cessation and to develop better prevention strategies and treatments. Family and twin studies illustrate an underlying genetic component to many aspects of smoking, and people with genetic variants of the drug metabolizing enzyme CYP2B6, that result in reduced enzyme (slow CYP2B6 metabolizers), are at greater risk for becoming nicotine-dependent, and upon stopping smoking, experience more craving and withdrawal;they have lower quit rates but respond better to the cessation drugs bupropion and varenicline. CYP2B6 is unlikely to contribute to overall nicotine metabolism, but is present in the brain where it can contribute to local metabolism of endogenous compounds, and to the brain levels of nicotine and its metabolites, such as nornicotine. This proposal will investigate the role of reduced brain CYP2B6 in the differences in smoking- related behaviors between CYP2B6 slow and normal metabolizers. A novel procedure will be used in rats to model reduced brain CYP2B6 metabolism in human CYP2B6 slow metabolizers, wherein a specific mechanism-based chemical inhibitor will be injected directly into rat brain cerebral ventricles. This procedure will be used in combination with a rat nicotine self-administration paradigm to investigate the role of reduced brain CYP2B6 metabolism in the increased rate and risk of becoming nicotine dependent. The rate of acquisition of nicotine self-administration by rats should be higher with inhibition of brain CYP2B, as there should be higher nicotine levels in the brain than without inhibition, increasing nicotine's reinforcement. This brain CYP inhibition procedure will also be used in combination with a rat model of nicotine withdrawal to investigate the role of reduced brain CYP2B6 metabolism in severity of withdrawal from smoking, and in the response to bupropion and varenicline. With brain CYP2B inhibition, the rats should experience increased withdrawal symptoms upon cessation of nicotine treatment, due to increased brain exposure to nicotine and reduced nornicotine levels while receiving nicotine. Similarly, these rats should experience greater attenuation of withdrawal symptoms with bupropion and varenicline. The results of this project may lead to further investigation of the role of brain CYPs in drug abuse and addiction, and to novel approaches for development of better treatment strategies for smoking cessation. The field of brain CYP-mediated enzyme metabolism is highly unique;examining manipulation of brain CYP-mediated metabolism is novel and represents an exciting new avenue of research. PUBLIC HEALTH RELEVANCE: About 21% of US adults are smokers and smoking causes about 3 million deaths a year world-wide. Smoking rates have not declined in recent years in the US indicating many people are still becoming smokers and only some are able to stop smoking. This project takes a novel approach to understanding the mechanisms whereby some people are more vulnerable to both becoming and remaining smokers, and whereby some smokers respond better to cessation drugs than others. Ultimately these studies may be useful in developing better smoking prevention strategies and treatments.