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Quantification of Methylphenidate, Dexamphetamine, and Atomoxetine in Human Serum and Oral Fluid by HPLC With Fluorescence Detection

Background: For psychostimulants, a marked individual variability in the dose–response relationship and large differences in plasma concentrations after similar doses are known. Therefore, optimizing the efficacy of these drugs is at present the most promising way to exploit their full pharmacological potential. Moreover, it seems important to examine oral fluid as less invasive biological matrix for its benefit in therapeutic drug monitoring for patients with hyperkinetic disorder. Methods: A high-performance liquid chromatography method for quantification of methylphenidate (MPH), dexamphetamine (DXA), and atomoxetine in serum and oral fluid has been developed and validated. The analytical procedure involves liquid–liquid extraction, derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl)benzoyl chloride as a label and chromatographic separation on a Phenomenex Gemini-NX C18 analytical column using gradient elution with water–acetonitrile. The derivatized analytes were detected at 330 nm (excitation wavelength) and 440 nm (emission wavelength). To examine the oral fluid/serum ratios, oral fluid samples were collected simultaneously to blood samples from patients with hyperkinetic disorder. Results: The method allows quantification of all analytes in serum and oral fluid within 16 minutes under the same or similar conditions. Oral fluid/serum ratios for MPH and DXA were highly variable and showed an accumulation of these drugs in oral fluid. Conclusions: The developed method covers the determination of MPH, DXA, and atomoxetine concentrations in serum and oral fluid after the intake of therapeutic doses. Oral fluid samples are useful for the qualitative detection of MPH and DXA.