Abstract

The convulsant potencies of lidocaine, monoethylglycinexylidide (MEGX) and glycinexylidide (GX) were compared by standard dose-response studies in rats. The median convulsant dose (CD50) was 52 mg/kg for lidocaine and 67 mg/kg for MEGX. convulsions were not observed even when lethal doses of GX were given alone, but in combination GX appeared to potentiate lidocaine and MEGX seizures. The extent of this potentiation suggested that GX might have 1/10 the convulsant activity of lidocaine and MEGX, which places the hypothetical CD50 for this metabolite well above its LD50 of 183 mg/kg. Blood concentrations of lidocaine, MEGX and GX were determined at the onset of convulsions by quadrupole mass fragmentography. The blood level studies showed that lidocaine and MEGX were equally potent and suggested that metabolism of MEGX to less active GX caused the dose-response studies to underestimate MEGX potency. The sum of plasma lidocaine and MEGX levels ranged from 18.7 to 21.7 µg/ml in agreement with threshold lidocaine levels found when seizures have resulted from the rapid administration of this drug to monkeys or man. These findings may be directly applicable to the interpretation of the results of blood level measurements in patients treated with lidocaine.