PATHWAY AND KINETICS OF PREDNISOLONE METABOLISM IN THE HUMAN PLACENTA

Prednisolone is metabolized in the perfused human placental lobule to prednisone, 20alpha-dihydroprednisone, 20beta-dihydroprednisone and 20beta-dihydroprednisolone. The pathway of metabolite formation was defined in perfusions of placental lobules using prednisone and 20beta-hydroprednisone separately as substrates and with prednisolone co-perfused with glycyrrhetinic acid, a potent inhibitor of the 11-oxidase component of the 11beta-hydroxysteroid dehydrogenase enzyme system. The pattern of metabolites identified from 6 h samples indicated a reversible formation of prednisone from prednisolone, the production of the 20alpha- and 20beta-hydro metabolites of prednisone from prednisone, the formation of 20beta-dihydroprednisolone from 20beta-dihydroprednisone only and no direct formation of 20beta-dihydroprednisolone from prednisolone. Kinetic analysis at two substrate concentrations confirmed that the formation of three of the four steroid metabolites followed first order kinetics. In perfusions with an initial prednisolone concentration of 1 mug/ml (n = 4) or 100 ng/ml (n = 3), the rate constants obtained were (mean + SD, maternal compartment, h-1): prednisone, 1.97 +/- 0.49 and 2.25 +/- 0.15, P > 0.1; 20alpha-dihydroprednisone, 0.0006 +/- 0.0004 and 0.0017 +/- 0.0006, P < 0.1; 20beta-dihydroprednisone, 0.15 +/- 0.022 and 0.15 +/- 0.0077, P > 0.1. In contrast, the rate constant for formation of 20beta-dihydroprednisolone at an initial prednisolone concentration of 100 ng/ml (0.083 +/- 0.0095 h-1) was significantly (P < 0.01) greater than the corresponding rate constant at the higher initial prednisolone concentration (0.039 +/- 0.015 h-1). A significant increase (P < 0.05) was observed for the formation of 20beta-dihydroprednisolone at the end of 6 h perfusions at the lower initial substrate concentration (11.2 +/- 1.9%) compared with the 1 mug/ml concentration (6.0 +/- 2.5%).