Smoking is a significant public health problem, and there is a great need for research to improve smoking cessation treatment outcomes. The goal of the Pharmacogenetics of Nicotine Addiction Treatment (PNAT) research program is to generate the evidence base to optimize treatment decisions for Individuals who want to quit smoking. During the past 4 years of PNAT1, we have characterized genetic variants altering nicotine pharmacokinetics as well as pharmacodynamic genetic variants influencing response to pharmacotherapies for smoking cessation treatment. We have shown that the CYP2A6 enzyme is critical in the metabolic inactivation of nicotine, and inherited variation in nicotine clearance influences smoking behavior and cessation. With a vision toward translation of our research to practice, we have characterized a genetically-informed biomarker of CYP2A6 activity, specifically the nicotine metabolite ratio (NMR;3'hydroxycotinine/cotinine), which reflects both genetic and environmental influences on CYP2A6 activity and nicotine clearance. The NMR is measured noninvasively in smokers with established reliability, stability, analytic validity, and efficacy as a predictor of therapeutic response in multiple independent (retrospective) clinical trials. Translation of these findings to clinical practice, the ultimate goal of the PGRN, requires validation In a prospective stratified clinical trial comparing alternative therapies for smoking cessation. In this competing renewal, we propose to conduct a prospective placebo-controlled multi-center pharmacogenetic (PGx) clinical trial of alternative therapies for smoking cessation treatment in 1,350 smokers. Randomization to placebo, transdermal nicotine, or varenicline will be stratified prospectively based on the NMR, the most robust genetically-informed biomarker for smoking cessation identified to date. Further, to facilitate translation to practice, we will determine the cost-effectiveness of our proposed PGx approach using both primary data and simulation models. In addition to these goals, we propose within this UOI to: identify additional sources of genetic variation in nicotine clearance and the NMR;investigate additional pharmacokinetic and pharmacodynamic gene associations with therapeutic response biomarker;and elucidate the mechanisms involved in identified PGx effects on smoking cessation. The proposed research provides the next critical step to validate a genetically-informed diagnostic tool, the NMR, which clinicians can use in the future to optimize treatment decisions for their patients who wish to quit smoking. As outlined recently by NIDA, due to the devastating health consequences of smoking and the urgent demand for better treatments, the validation of biomarker strategies to improve the outcomes of treatments a major public health priority.