Quantitative mapping of metabolites of imipramine and desipramine in plasma samples by gas chromatographic-mass spectrometry.

Imipramine and desipramine are known to metabolize primarily by demethylation and hydroxylation. Hydroxy metabolites, 2-hydroxyimipramine and 2-hydroxydesipramine, are reported to be pharmacologically active by in vitro studies. We now report a simple gas chromatographic-mass spectrometric selected ion-monitoring (SIM) method for hydroxy metabolites of imipramine and desipramine from plasma samples using deuterated analogues as internal standards. d4-Internal standards for imipramine, desipramine, and their 2-hydroxy derivatives, were prepared in our laboratory and added to plasma samples, extracted at pH 9 with ethyl acetate, then at pH greater than 11 with hexane-isopropanol. The extracts were combined and evaporated under nitrogen. The trifluoroacetyl derivatives were prepared using N-methyl-bis-trifluoroacetamide and analyses were performed using GC-MS selected ion-monitoring in the electron ionization mode. Hydroxylation to 2-hydroxy metabolites varied widely between subjects. We also noticed in vivo methylation of desipramine to imipramine as a pathway of its metabolism in 15% of the subjects. It is suggested that plasma levels of all active metabolites must be considered in the assessment of the relationship of plasma levels of the drug to clinical response.