**4. Interaction of free Perifosine (OPP) with breast cancer cells**

In this chapter the investigation of the interaction of Perifosine (octadecyl(1,1-di-methyl-4 piperidinium-4-yl)phosphate – OPP), with ER positive (ER+) and ER negative (ER-) breast cancer cell lines is emphasized. Perifosine was chosen since it is one of the most cancerostatically active lipids, with strong antitumor effect on xenotransplanted human breast cancer. We summarize results obtained mostly by the electron paramagnetic resonance (EPR) method in order to measure the influence of Perifosine on cell membrane fluidity and to measure transport of free and liposome incorporated Perifosine into breast cancer cells.
