Measurements of the film pressure-area (∏-A) isotherms for paclitaxel/L-α-distearoyl phosphatidylcholine (DSPC) mixed monolayers were conducted at 24.5 and 37°C. Thermodynamic properties of paclitaxel and DSPC in a monolayer at the air/water interface were studied from data obtained. The ∏-A isotherms showed that paclitaxel and DSPC were miscible at 24.5°C and at higher temperature, desorption of paclitaxel from the interface into subphase was observed. Incorporation of paclitaxel had a profound influence on the shape of isotherms and the compressibility of a mixed monolayer. These observations were directly dependent on the concentration of paclitaxel in the monolayer. Comparison with the data of compressibility for mixed monolayers suggests that a monolayer containing 5% paclitaxel possessed the strongest attractive interaction, and also the most stable state one according to the estimation of the free energy and the excess free energy of mixing. Additionally, the relevance of these observations to the behavior of paclitaxel-lipid liposomes was discussed.
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