Inhibition of metoprolol metabolism by amino acids in perfused rat livers. Insights into the food effect?

A mixture of amino acids inhibits propranolol metabolism in perfused rat livers. To obtain mechanistic information about the interaction, a related but less tissue-bound drug, metoprolol, was used to determine Vmax and K(M) for parent drug and two metabolites in the presence and absence of amino acids. Six groups of 4 livers from 24 male Sprague-Dawley rats were perfused in the single-pass mode at 3 ml/min/g liver for 130 min with oxygenated buffer containing 3.74, 4.49, 5.61, 7.48, 18.7, or 44.9 microM metoprolol. From 50 to 90 min, a balanced amino acid mixture was included in the buffer. Samples of liver effluent taken every 5 min were analyzed by HPLC for metoprolol and two metabolites, alpha-hydroxymetoprolol and O-demethylmetoprolol. Steady-state concentrations of drug determined before, during, and after amino acids were used to determine Vmax and apparent K(M) values by nonlinear curve-fitting under each condition. Amino acids reversibly reduced the Vmax values of metoprolol and both metabolites by approximately 50% without significantly affecting apparent K(M) values. As a result, large increases in availability occurred, especially at low metoprolol inlet concentrations (> 90%). Amino acids also increased oxygen consumption until the effluent buffer was almost depleted. Possible mechanisms influencing Vmax include direct inhibition of metabolic enzymes by amino acids or cosubstrate (NADPH or oxygen) limitation. Amino acid-mediated pericentral oxygen depletion in the hepatic sinusoids could result in inhibition of drug-metabolizing enzymes, and is consistent with a reduction of Vmax and oxygen depletion in the effluent buffer during amino acid coinfusion. We postulate that one or more of these mechanisms could contribute to the interaction between food and high first-pass drugs observed in humans.