The macromolecular binding of prednisone in plasma of healthy volunteers including pregnant women and oral contraceptive users.

The macromolecular binding of prednisone has been studied in the plasma of eight healthy human volunteers. The subjects included two control males, two control females, two females taking estrogen-containing oral contraceptives, and two females in the third trimester of pregnancy. All volunteers exhibited the expected nonlinear plasma binding of prednisolone with free fraction increasing as the total prednisolone concentration was increased. Both the oral contraceptive and pregnant subjects had increased transcortin binding capacity over the control volunteers as evidenced by their increased binding of prednisolone. Prednisone macromolecular binding, however, was not altered by either changing total prednisone concentration or transcortin binding capacity. The mean prednisone free fraction was 0.250 +/- 0.027 in the eight subjects over the concentration range 0-2500 ng/ml. The addition of prednisolone in up to a 25-fold excess did not alter prednisone's free fraction. Prednisone apparently does not bind to transcortin with the same strong affinity that characterizes prednisolone's binding, and due to albumin's extensive binding capacity, prednisone macromolecular binding is not saturable over the pharmacological drug concentration range.