Effect of bovine serum on the phase transition temperature of cholesterol-containing liposomes

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Abstract
The phase transition temperature of liposomes composed of dipalmitoylphos-phatidylcholine (DPPC)/hydrogenated soy phosphatidylcholine (HSPC) at a 2:1 molar ratio was estimated in buffer, 30% and 50%) bovine serum by monitoring the leakage of encapsulated self quenched doxorubicin (Dox) from the vesicles when exposed to a temperature increasing from 30–52°C. The results showed that bovine serum caused a slight decrease in the phase transition temperature from 44 to 41 °C in 50% serum. Addition of 50% cholesterol to this liposomal composition resulted in the disappearance of transition temperature in buffer and 30% serum, whereas 50% bovine serum resulted in the reappearance of the transition temperature at 46 °C. The data suggest that bovine serum affects the transition temperature of liposomes in a concentration-dependent manner, and this effect is more pronounced in cholesterol-rich liposomes. The time course for the release of Dox from both kinds of liposomes (cholesterol rich and cholesterol free) during incubation in 50% bovine serum, at temperatures close to the transition temperature (42°, 45 °C), was followed. The results showed an increased leakage of Dox from both kinds of liposomes, at both temperatures. However, liposomes with high cholesterol content released more drug at 42° than at 45 °C. The size of these liposomes was monitored in 10% bovine serum at 25 °C for a period of 1 h using photon correlation spectroscopy. The data showed no variation in the size of both cholesterol-rich and cholesterol-free liposomes for this period, which indicates that bovine serum does not affect the size of either cholesterol-rich or cholesterol-free liposomes.