Simultaneous measurement of plasma levels of d-Propoxyphene and l-Propoxyphene using stable isotope labels and mass fragmentography.

When racemic propoxyphene was administered orally to dogs, plasma levels of d-propoxyphene were higher than those of l-propoxyphene. The half-life of d-propoxyphene was also longer than that of the l-isomer. Measurements were performed by labeling one isomer of a racemic mixture with deuterium and then measuring d- vs. l- ratios by mass fragmentography. An intravenous experiment was also performed and confirmed the preferential uptake of the levo isomer. To do these experiments successfully propoxyphene isomers were labeled on the benzyl methylene with deuterium atoms. In vivo studies demonstrated that this labeling position was sufficiently far removed from the point of metabolic attack in the molecule so that no interfering isotope effect existed. The methodology outlined in this paper will be of great value in the study of the disposition of the individual isomers of racemic drugs.