Factors Affecting In Vitro Protein Binding of Etoposide in Humans

Abstract

Clinical studies have demonstrated that the plasma protein binding of etoposide, a widely used anticancer drug, is extensive (~94%), highly variable among patients (10-fold range), and significantly related to serum albumin and total bilirubin concentration. The present study was designed to more thoroughly evaluate factors likely to affect etoposide protein binding under controlled in vitro conditions where single variables could be changed. Protein binding was determined using an equilibrium dialysis method with tritiated etoposide. The binding of etoposide was similar in serum or plasma, and heparin had no effect on binding. Etoposide binding decreased with increased pH, but no clinically significant difference was noted within the range of physiologic pH. Etoposide binding evaluated in single-source donor plasma was concentration-dependent over a concentration range of 1 to 250 μg/mL Etoposide binding parameters determined in normal human plasma were characterized by a single class of binding sites of moderate affinity (K = 2.88 ± 0.47 × 104) and high capacity (nP = 5.07 ± 0.5 × 10−4; where n is the number of binding sites). The etoposide binding ratio was significantly correlated with albumin concentration (r2 = 99%, p <0.05). The characteristics of etoposide binding in a 4.0-g/dL solution of human serum albumin (K = 3.56 ± 1.22 ± 104 and nP = 5.58 ± 0.16 × 10−4) suggest that the single class of binding sites is on albumin. Bilirubin caused a significant decrease in K, consistent with competitive binding, but only at higher bilirubin concentrations. Collectively, these in vitro data provide insights into the clinical variables most likely to alter the plasma protein binding of etoposide and corroborate previous clinical studies demonstrating a correlation between etoposide binding and serum albumin or bilirubin concentrations.