6α‐methylprednisolone and 6α‐methylprednisone plasma protein binding in humans and rabbits

The binding of 6α‐methylprednisolone and 6α‐methylprednisone to proteins of rabbit and human plasma was studied in vitro by equilibrium dialysis. Steroid binding was determined using radiolabeled compounds and HPLC analysis methods. Both methods produced equivalent results. Plasma protein binding of 6α‐methylprednisolone and 6α‐methylprednisone averaged 75–82% and was independent of steroid concentration, suggestive of low affinity, nonspecific protein binding. A positive linear correlation of the log octanol‐water partition coefficient with the nonspecific binding affinities of a homologous series of steroids, including 6α‐methylprednisolone and 6α‐methylprednisone, was demonstrated. This correlation suggests that hydrophobic binding is a major determinant of nonspecific steroid‐protein interactions.