ENANTIOSPECIFIC QUANTIFICATION OF HEXOBARBITAL AND ITS METABOLITES IN BIOLOGICAL-FLUIDS BY GAS-CHROMATOGRAPHY ELECTRON-CAPTURE NEGATIVE-ION CHEMICAL IONIZATION MASS-SPECTROMETRY

A highly sensitive and specific assay based on gas chromatography/electron capture negative ion chemical ionization mass spectrometry has been developed for the analysis of the enantiomers of hexobarbital and its major metabolites in human urine and plasma. S-(+)-(5-H-2(3))hexobarbital and R-(-)-(5-H-2(3))hexobarbital were synthesized for clinical studies along with (+/-)-(1,5-H-2(6))hexobarbital and the deuterated major metabolites for use as internal and reference standards. Hexobarbital enantiomers and their metabolites were analyzed after pentafluorobenzyl and trimethylsilyl derivatization, following solid-phase extraction from plasma and urine. Intense negative ion spectra were observed for all of the derivatives. The base peak in the spectra corresponded to the M - pentafluorobenzyl anion [M - PFB]- except for 1,5-dimethylbarbituric acid, where M-. was the most abundant ion. The applicability of the method was demonstrated by following the plasma concentration-time profiles and urinary excretion in a male extensive metabolizer of mephenytoin who was given a pseudoracemic oral dose of hexobarbital containing equal 50 mg amounts of S-(+)-2(H0)hexobarbital and R-(-)-(H-2(3))hexobarbital. Marked stereoselective disposition was observed, with the R-(-)-enantiomer being more efficiently metabolized, primarily by alicyclic oxidation and ring cleavage.