Simultaneous estimation of propoxyphene-do and propoxyphene-d2 levels by quantitative mass fragmentography. Potential utility in multidose investigations.

A quantitative mass fragmentographic method for the simultaneous determination of labeled and unlabeled propoxyphene in plasma is described. Dogs treated daily with propoxyphene-do were treated with a pulse dose of propoxyphene-d2 at day 20. It was found that following an initial rapid equilibrium phase levels of propoxyphene-d2 fell more rapidly than those of propoxyphene-do. This result suggests that 'deep' pools of tissue bound propoxyphene exist which exchange very slowly with drug present in the central compartment. Experimental evidence is presented which demonstrates that the difference in behavior of propoxyphene-do and -d2 is not due to unanticipated secondary isotope effects.