Melphalan concentration dependent plasma protein binding in healthy humans and rats

The binding of melphalan to plasma proteins from four healthy humans and from rats was measured by centrifugal ultrafiltration. Melphalan concentrations were determined by HPLC and by measuring 14C-melphalan activity. In whole blood, melphalan was distributed preferentially in plasma. However, a constant fraction, 37%, which was independent of the total melphalan concentration in whole blood, was present within the red blood cells. The binding of melphalan to plasma proteins from humans was less than that from rats. In both, however, the fraction bound was constant throughout the concentration range (0.1 to 9.0 microM) that is achieved during standard-dose melphalan therapy. Albumin was the primary binding protein. At concentrations equal to or in excess of 33 microM, which have been achieved during high-dose melphalan therapy, free plasma melphalan concentrations were no longer linearly related to total drug concentrations, and the plasma protein binding of melphalan in the human became concentration dependent. This occurred at concentrations of 70 microM in the rat. Scatchard analysis of the data indicated the presence of 2 groups of binding sites. Class I sites had 0.03 and 0.4 binding sites per albumin molecule in humans and rats, with respective association constants of 4.43 X 10(4) M-1 and 1.92 X 10(4) M-1. Class II sites had 5.18 and 2.60 binding sites per molecule, with respective association constants of 3.82 X 10(2) M-1 and 2.01 X 10(2) M-1.