In vitro binding of MX2 (KRN8602) and epirubicin to human plasma protein.

This study compares the human plasma protein binding characteristics of MX2 and epirubicin. The binding characteristics were determined by equilibrium dialysis at various concentrations of the drugs. The binding dissociation constant (Kd), binding capacity (Bmax) and partitioning constant (Kp) were obtained by Scatchard analysis of the free and bound drugs in the dialysis compartments. Our results have demonstrated that plasma protein binds epirubicin or MX2 in an unsaturable appearance over the concentration up to 150 mumol/l. At the same concentrations, plasma protein binds more epirubicin than MX2. The nature of the interaction may consist of two classes of specific binding, and a partitioning. The binding dissociation constants were 18 and 17.5 mumol/l for the higher binding class (Kd1) and 315.8 and 316.9 mumol/l for the lower binding class (Kd2), respectively, for epirubicin and MX2. The respective maximum binding capacities (Bmax) of plasma protein for epirubicin and MX2 were significantly different, 0.045 and 0.029 mumol/g protein for the higher binding class (Bmax1), and 0.39 and 0.29 mumol/g protein for the lower binding class (Bmax2). The partitioning constants (Kp) were 21.5 x 10(-5) and 20 x 10(-5) litres/g protein for epirubicin and MX2, respectively. The results suggest that plasma protein binds epirubicin or MX2 with a similar affinity, but has less binding sites for MX2. One contributing mechanism to the difference in activity noted between epirubicin and MX2 may be changes in free drug fractions.