Quantification of lidocaine and several metabolites utilizing chemical-ionization mass spectrometry and stable isotope labeling.

Quantification of the suspected metabolites of lidocaine in humans was carried out using the direct insertion probe and chemicalionization mass spectrometry. Deuterated analogs of the metabolites of lidocaine were added to serial human plasma and urine samples and were used as internal standards following oral administration of 250 mg of lidocaine hydrochloride monohydrate to two male subjects and 202 mg of lidocaine free base to one male subject. The average results after analysis of the 0-24 hr urine samples, before beta-glucuronidase-sulfatase treatment, indicated the presence of seven of the possible metabolites in the following amounts (percent of administered dose based on the free base): lidocaine, 1.95; omega-ethylamino-2,6-dimethylacetanilide, 4.90; omega-amino-2,6-dimethylacetanilide, 0.88; m- and/or p-hydroxylidocaine, 0.73; m- and/or p-hydroxy-omega-ethylamino-2,6-dimethylacetanilide, 0.56; 2,6-dimethylaniline, 0.97; and 4-hydroxy-2,6-dimethylaniline, 63.5. Both N-ethyl- and N,N-diethylglycine were detected in human and Rhesus monkey urine, although quantification was not achieved.