Investigation of the metabolism of14C/13C-practolol in rat using directly coupled radio-HPLC-NMR-MS

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Abstract
1. The metabolic fate of 14C 13C-practolol was investigated using on-line HPLCNMR-MS following oral administration to rat. The major route of elimination for the radiolabel was via the urine with the principal biotransformation products confirmed as the 2-hydroxy- and 2-hydroxyglucronide metabolites. 2. In addition, futile deacetylation, determined by the replacement of 13C-labelled acetyl groups with endogenous 12C-acetyls accounted for ~7-10% of the urinary metabolites, corresponding to ~5% of the dose undergoing N-deacetylation. 3. Evidence for chiral metabolism was sought via NMR of isolated metabolites using beta cyclodextrin as a chiral shift agent. Practolol was excreted as a racemate. However, some enantioselective metabolism excretion had occurred as the hydroxy- and hydroxyglucuronide were not excreted as racemic mixtures. 4. Directly coupled radio-HPLC-NMR-MS is extremely effective for the identification of the metabolites of radiolabelled xenobiotics in urine samples.