A selective LC/RIA for dexamethasone and its prodrug dexamethasone-21-isonicotinate in biological fluids

A combined LC/RIA procedure is described for the selective determination of dexamethasone (DEX) and its prodrug dexamethasone-21-isonicotinate (DIN) in plasma. The low affinity of the employed dexamethasone antiserum for DIN (cross-reactivity less than 0.5%) allowed the direct determination of DEX in plasma extracts. For the determination of DIN, both substances of interest were separated by LC, the DIN containing fraction was collected, hydrolysed and the generated DEX was consequently assayed by radioimmunoassay. The assay detection limits were 0.1 ng ml-1 for DEX and 0.75 ng ml-1 for DIN. For both substances, inter- and intra-day variabilities (RSDs) were 6 and 12%, respectively.