Determination of the amount of micelles in alkylphospholipid liposome formulations with electron paramagnetic resonance method.

Alkylphospholipids (APLs) are physiologically active derivatives of lipids with high efficiency in the therapy of experimental human breast cancer. Therefore, they are usually used as liposome formulations, since the application of free, micellar APLs results in serious side effects, such as hemolysis. Since only rough upper estimates of micelle proportions were obtained until now, we developed a new method, based on the spectral decomposition of electron paramagnetic resonance (EPR) spectra. The EPR spectra of different spin-labeled mixed liposome-micelle samples were fitted with a fast, automatic fitting procedure. In this way, we achieved more accurate estimates of the relative proportion of micelles, in comparison to the previously used methods. We confirmed findings of previous studies, which showed that the amount of micelles in OPP liposome formulations increases with a decreasing amount of cholesterol. We analyzed liposome formulations with 29, 39, and 46 mol% of cholesterol and determined the relative proportion of micelles to be 20 ± 9, 11 ± 4, and 5 ± 2, respectively. This study confirms that the efficiency of OPP liposome formulations in experimental breast cancer therapy is not solely a consequence of the amount of micelles present in a liposome formulation, but stems from a propensity of liposomes to interact with cells.