A selective HPLC/RIA for the determination of budesonide

A combined HPLC/RIA procedure is described for the selective determination of budesonide (BUD) in plasma. The assay involves the extraction of plasma or serum samples with ethylacetate, consequent HPLC separation of intact budesonide from cross-reacting metabolites on a C8 reversed phase column, collection of the budesonide containing fraction and determination of budesonide immunoreactivity with the budesonide antiserum. The method was accurate, sensitive (IC50 value of 0.9 ng ml-1) and reproducible (intra- and inter-day less than 15%) with a limit of quantification of 0.133 ng ml-1 (RSD < 25%). The evaluation of a limited number of clinical samples after oral administration of budesonide by both the HPLC/RIA procedure and a direct RIA using the same antiserum differed in average by a factor of 2, with the ratio of HPLC/RIA-RIA results declining as a function of time. Thus, this ratio might be a suitable indicator for probing for the ratio of budesonide and overall metabolites on a semi-quantitative level.