High affinity binding of paclitaxel to human serum albumin

Paclitaxel, a very potent antitumor agent is a hydrophobic molecule with lo w aqueous solubility. Its currently used formula (Taxol(R)) contains the dr ug in a 1 : 1 (v/v) mixture of ethanol and Cremophor EL. To minimize vehicl e-related toxicity, we developed a novel, water-soluble formulation in whic h paclitaxel is bound noncovalently to human serum albumin. For this purpos e, studies of the paclitaxel-albumin binding equilibrium were performed. Pa clitaxel dissolved in ethanol was added to the aqueous solution of human se rum albumin. Precipitated paclitaxel was removed and unbound drug was separ ated by ultrafiltration. Paclitaxel concentration was measured by RP-HPLC. Binding data were evaluated based both on the Scatchard plot and the genera l binding equation describing binding equilibria with the stepwise: stoichi ometric binding constants. The Scatchard plot was found to be curvilinear w ith 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 es timated 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 reache d. The greatest observed value of r (number of paclitaxel molecules bound t o one albumin molecule) was 6.6.