Organic cation transporters are responsible for the disposition of various endogenous and therapeutic agents in humans; thus, there is a great need for the development of a simple assay for simultaneous assessment of the activities of multiple transporters. Using liquid chromatography-mass spectrometry (LC/MS), we developed an assay that allows for simultaneous quantitation of plasma and urinary levels of N(1)-methylnicotinamide (a substrate of hOCT2/hMATEs), L-carnitine (a substrate of hOCTN2), and creatinine (an indicator of glomerular filtration). Samples were diluted with ultrapure water, deproteinized with trichloroacetic acid, filtered, and then injected on a cation exchange column. The analytes were separated with a gradient LC technique and detected by MS. The total assay time was less than 8 min. The lower detection limits for N(1)-methylnicotinamide, L-carnitine, and creatinine were 2, 10, and 24 ng/mL, respectively. Recovery of the analytes was almost complete. A preliminary clinical study conducted in 25 healthy subjects revealed that the mean±SD for the renal clearance (CLR) of N(1)-methylnicotinamide (272.7±81.0 mL/min) far exceeded the glomerular filtration rate (116.3±19.6 mL/min), indicating the involvement of active tubular secretion, while the mean CLR of clearance of L-carnitine was close to nil (1.5±1.4 mL/min), indicating almost complete tubular reabsorption. The present method is potentially useful for clinical studies on the genetic control of cationic transporter activities and the transporter-mediated drug interactions.
Keywords: Creatinine; Humans; LC/MS; N(1)-Methylnicotinamide; Plasma; Urine; l-carnitine.
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