Cigarette smoking accelerates the metabolism of certain drugs, particularly those primarily metabolized by cytochrome P450 1A2 (CYP1A2) and, to a lesser extent, CYP2E1 and some UDP-glucuronosyltransferases [1,2]. The induction of CYP1A2 is mediated by binding of polycyclic aromatic hydrocarbons of the tobacco smoke to the aryl hydrocarbon receptor (AHR) with consequent transcriptional activation of the CYP1A2 gene. Furthermore, CYP1A1 and CYP1B1 enzymes are induced by tobacco smoking via AHR in various human tissues such as lung and placenta [3]. As CYP1A1 and CYP1B1 are mostly expressed in extrahepatic tissues, their induction by smoking is not known to affect the pharmacokinetics of any medication. There is evidence of the role of nicotine in the induction of CYP1A1 and CYP1A2 enzymes in vitro in rat lung [4], and in vivo in rat lung, kidney and liver [5][6][7], liver and placenta of pregnant rats [8] and in brains of mice and rats [9,10], probably through mechanisms not involving AHR. Some evidence exists for the induction of CYP1A1 by nicotine in human pulmonary explant culture [11].We recently published a study on the effects of 10 day dosing of nicotine on human CYP2A6 and CYP2E1 activities [12]. An additional aim of that study was to determine the effects of high dose nicotine on the pharmacokinetics of oral caffeine and to test the hypothesis that nicotine induces CYP1A2-mediated metabolism of caffeine to paraxanthine, a well-established probe reaction of CYP1A2 activity [13]. No previous study has studied the effects of nicotine on CYP1A2 activity in humans in vivo.The details of the experimental protocol and the subject characteristics are described in a prior report [12]. Briefly, 12 healthy smokers were given two 21 mg transdermal patches delivering a total of 42 mg nicotine day -1 or placebo patches, each for 10 days in a randomized and crossover design. Subjects were not allowed to smoke or to use any tobacco products during hospitalization. At noon on the eighth hospital day, 200 mg of oral caffeine was given. Blood samples were collected for measurement of caffeine and metabolites at 0, 30 and 60 min, and then 2, 3, 4, 6, 8, 12, 20, 32, 44 and 52 h after ingestion of caffeine. In addition, deuterium-labelled nicotine-D2 and cotinine-D4 phenotyping for CYP2A6, bupropion phenotyping for CYP2B6 and chlorzoxazone phenotyping for CYP2E1 were performed on the seventh and eighth hospital days as previously described [12].Concentrations of caffeine, paraxanthine, theobromine and theophylline in plasma were determined using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Stable isotope-labelled analogues of paraxanthine and caffeine were used as internal standards. Following protein precipitation, samples (0.2 ml) were treated with phosphate buffer and extracted with a mixture of methylene chloride, ethyl acetate and isopropyl alcohol. The extracts were evaporated, reconstituted in the LC mobile phase, and injected into the LC-MS/MS system. The mass spectrometer was operated using ...