Nicotine Pharmacokinetics in Rat Brain and Blood by Simultaneous Microdialysis, Stable-Isotope Labeling, and UHPLC–HRMS: Determination of Nicotine Metabolites

The disposition and metabolism of nicotine in the brain is an important determinant of its exposure. We have developed a novel method for the dynamic determination of nicotine and its metabolites in rat brain and blood by simultaneous microdialysis sampling, stable-isotope labeling, and ultra high-performance liquid chromatography–high-resolution mass spectrometry (UHPLC–HRMS) assaying. Microdialysis probes were inserted into both the right striatum and jugular vein of Sprague–Dawley rats. The collections of dialystes after nicotine intraperitoneal injection were analyzed by the optimized UHPLC–HRMS. Nicotine-pyridyl-d4 was used as a metabolic tracer, and several stably labeled isotopes were applied to calibrate the in vivo recoveries of retrodialysis. The quadrupole-Orbitrap HRMS provided reliable characterization of the nicotine derivatives with less than 3.5 ppm mass measurement accuracy. Good precision and accuracy were obtained for different analytes within the predefined limits of acceptability and the range of the standard curve. Nicotine and its 11 metabolites were identified in most microdialysis samples from the blood and brain tissue samples. Besides cotinine as the main metabolic product of nicotine, trans-3′-hydroxy-cotinine, nicotine-N-oxide, and norcotinine were proven to be the second most abundant metabolites. The other seven nicotine products, including 4-oxo-4-(3-pyridyl)-butanoic acid, 4-hydroxy-4-(3-pyridyl)-butanoic acid, cotinine-N-oxide, nicotine-N-glucuronide, cotinine-N-glucuronide, and trans-3′- hydroxy-cotinine-O-glucuronide, which have not been determined previously in animal brain, were present in minor amounts. The pharmacokinetic profile of nicotine metabolites indicated that the metabolic characteristic of nicotine in the brain was relatively different from that in the blood. The present work would provide comprehensive evidence for clarifying the differences between nicotine metabolism in the brain and peripheral system.