SIMULTANEOUS ANALYSIS OF DIPHENHYDRAMINE AND A STABLE-ISOTOPE ANALOG (H-2(10) DIPHENHYDRAMINE USING CAPILLARY GAS-CHROMATOGRAPHY WITH MASS-SELECTIVE DETECTION IN BIOLOGICAL-FLUIDS FROM CHRONICALLY INSTRUMENTED PREGNANT EWES

This report describes both the synthesis of a stable isotope analog of the H-1 receptor antagonist diphenhydramine (DPHM), and the simultaneous quantitation of DPHM and a deuterated stable isotope analog of DPHM, viz. (H-2(10))DPHM in biological fluids from the chronically instrumented pregnant ewe. (H-2(10))DPHM was synthesized and purified, and both its structure and purity were verified. Biological samples were prepared for analysis using liquid-liquid extraction prior to capillary gas chromatography/mass spectrometry. The method employed electron impact ionization with selective ion monitoring of ions with m/z 165 for DPHM and m/z 173 for (H-2(10))DPHM. The minimal quantifiable concentration of DPHM and (H-2(10))DPHM from a 1.0 ml sample was 2.0 ng ml-1 in fetal and maternal plasma, fetal tracheal fluid and amniotic fluid. The method was validated from 2.0 ng ml-1 to 200.0 ng ml-1 for both DPHM and (H-2(10))DPHM in plasma, fetal tracheal fluid and amniotic fluid. Differences in the disposition between DPHM and (H-2(10))DPHM were not apparent during a control experiment in which both labeled and unlabeled DPHM were administered to a chronically instrumented fetal lamb. This method provides the required sensitivity and selectivity for the simultaneous quantitation of unlabeled and labeled DPHM during pharmacokinetic experiments conducted in near-term pregnant sheep.