Specimen handling in an HPLC determination of phenylbutazone and its major metabolites in plasma, avoiding degradation of the compounds.

A problem usually not taken into account when a quantitative HPLC method for phenylbutazone is developed is the degradation of this drug and its metabolites not only upon storage, but also on extraction under acidic conditions, especially when the temperature is raised. Moreover, the degradation products in the chromatograms may interfere with the determination of gammahydroxyphenylbutazone. In our newly developed HPLC method, using feprazone as an internal standard, extreme care is taken to avoid degradation of the compounds during the extraction procedure. In view of the present results it is concluded that previously published data on phenylbutazone, oxyphenbutazone and gammahydroxyphenylbutazone levels should be considered with reserve.