Embryotoxicity of free and liposome-encapsulated taxol in the chick

Taxol, an inhibitor of microtubule disaggregation, is used in the therapy of breast, ovarian, and other human malignancies. The toxicity of taxol administration is due in part to the polyoxyethylated castor oil (Cremaphor) vehicle in which it is administered; taxol embryotoxicity appears also to be partially attributable to vehicle toxicity. Liposome encapsulation is a novel vehicle for drug administration. The administration of taxol encapsulated in liposomes was evaluated in the chick embryo. Albumen injections of taxol doses up to 30 micrograms/egg were used to characterize dose-response curves for free and liposome-encapsulated taxol, compared to liposome-only and saline-injected control eggs. Sixty percent embryotoxicity (death or malformation) occurred with taxol doses of 1.5 micrograms/egg. A 20-fold higher dose was necessary to produce the same degree of toxicity with liposome-encapsulated taxol. Curve-fitting equations were used to estimate median effective doses (ED50s) and slope functions of the dose response curves. The ED50 for taxol was more than an order of magnitude lower than that for liposome-encapsulated taxol. Estimated slope functions for the two dosage forms of taxol were the same, suggesting similar mechanisms of toxicity. The toxicity of liposomes alone was low.