GLC assay of verapamil in plasma: identification of fluorescent metabolites after oral drug administration.

The fluorometric assay for verapamil in plasma is not useful after oral drug administration because of interference by inactive, but fluorescent, metabolites extracted along with the parent drug. A GLC method using a flame-ionization detector after silylation allows the separation of unchanged active verapamil from the metabolites and quantitation to a sensitivity of 0.025 microgram/ml. After a single oral dose of drug in dogs, up to 80% of "fluorescent verapamil" represented inactive metabolites.