High affinity binding of paclitaxel to human serum albumin

Paclitaxel, a very potent antitumor agent is a hydrophobic molecule with low aqueous solubility. Its currently used formula (Taxol(R)) contains the drug in a 1 : 1 (v/v) mixture of ethanol and Cremophor EL. To minimize vehicle-related toxicity, we developed a novel, water-soluble formulation in which paclitaxel is bound noncovalently to human serum albumin. For this purpose, studies of the paclitaxel-albumin binding equilibrium were performed. Paclitaxel dissolved in ethanol was added to the aqueous solution of human serum albumin. Precipitated paclitaxel was removed and unbound drug was separated by ultrafiltration. Paclitaxel concentration was measured by RP-HPLC. Binding data were evaluated based both on the Scatchard plot and the general binding equation describing binding equilibria with the stepwise: stoichiometric binding constants. The Scatchard plot was found to be curvilinear with a slight positive slope of the final part. Parameters of high affinity specific binding were determined from the initial part of the curve (n(sp) = 1.3 and K-sp = 1.7 x 10(6) M-1). Stoichiometric binding constants were estimated by fitting the general binding equation to the experimental data (K-1 = 2.4 x 10(6) M-1 and K-2 = 1.0 x 10(5) M-1). Saturation of the protein with paclitaxel, similarly to other ligands of albumin, could not be reached. The greatest observed value of r (number of paclitaxel molecules bound to one albumin molecule) was 6.6.