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Influence of Amino Acids on the Kinetics of Metoprolol Metabolism Using the Isolated, Perfused Rat Liver: A Study Related to the Food Effect




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Metoprolol, 144-(2-methoxyethyl)phenoxy]-3-isopropylamino-2-propanol, is a relatively selective 131-adrenergic blocking drug. Metoprolol is among several lipophilic, basic drugs which are highly extracted on the first passage through the liver. Its AUCoral is increased by an average of 40% when concomitantly administered with a protein-rich meal. Research during the past two decades has shown that a reduction in the first-pass metabolism of certain lipophilic, highly extracted drugs could be the main factor that accounts for the substantial increase in the AUCoral. By ruling out changes in the splanchnic-hepatic blood flow and the fraction of unbound drug in plasma after the meal, it was speculated that a transient inhibition in hepatic intrinsic clearance may contribute to the food effect. In this research, it was hypothesized that amino acids as released into the portal vein, after ingestion of food, could cause a transient inhibition of metoprolol hepatic metabolism and changes to the pooled Vmax and Km values of Michaelis-Menten kinetics of metoprolol metabolism as well as each metabolic pathway. This research project consisted of two parts. The initial task was to develop an assay method. By using a C 18 column, a mobile phase consisting of water-acetonitrile-triethylamine (91: 9: 0.3, vol/vol/vol, adjusted to pH 3 with H3PO4) and a fluorescence detector with an excitation wavelength of 224 nm and no emission filter, chromatographic resolution was achieved for a-ydroxymetoprolol, O-demethylmetoprolol, metoprolol acid, nadolol (used as internal standard) and metoprolol. The quantitization of metoprolol and its three metabolites in aqueous samples was accomplished based on the ratio of peak area or height of analyte to that of nadolol. The calibration ranges for the quantitization of ivmetoprolol, a-hydroxymetoprolol, O-demethylmetoprolol, and metoprolol acid were 0.04 to 20 µg/mL, 0.06 to 4.05 µg/mL, 0.06 to 2.6 µg/mL, and 0.1 to 5 µg/mL, respectively. The analytical method was validated in that the intraday and interday coefficients of variation (CV) for the assay of each analyte at each of three concentration levels were less than 5%. The accuracy for all analytes concerned were in the range of 95 to 105%. The HPLC method developed provided a simple, quick and accurate method for the quantitization of the four analytes.





Master of Science (M.Sc.)


Pharmacy and Nutrition


Pharmacy and Nutrition



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