Development of radioimmunoassays for prednisone and prednisolone. Application to studies of hepatic metabolism of prednisone.

Radioimmunoassays for measuring prednisone and prednisolone (delta1 corticosteroids) in serum have been developed. By using 6,7-3H-delta1 corticosteroids as tracer, rabbit antibodies against delta1 corticosteroid-21-hemisuccinate in bovine serum albumin, and ammonium sulfate precipitation, the assays detected less than 0.8 ng/ml of delta1 corticosteroid and interassay coefficients of variation were less than 8.5 %. The specificity of antibodies, tested against all drug metabolites and major endogenous steroids, showed that only 21-glucuronic acid esters of delta1 corticosteroids had cross-reactivity of possible clinical importance. Assays were validated by measuring levels in samples of fastingstate sera with or without adding known amounts of prednisolone. Measurements of serum concentrations of both prednisone and prednisolone in normal dogs and in those with hepatic vascular exclusion were made after intravenous administration of prednisone. Although high levels of prednisolone appeared rapidly in normal dogs, only slight amounts were measured in dogs with hepatic vascular exclusion, which emphasized the importance of the liver in the conversion of prednisone to prednisolone, its active metabolite.