PGP and breast cancer: Lack of cytocidal activity of alkylating agents may not come as a surprise as MDA-MB-231 cells show the highest level of resistance to an array of chemotherapeutic drugs by comparison to other BC cancer lines (e.g. MDA-MB-435, MDA-MB-468, MDA-MB-453S, and MCF-7), which is most likely due to MDR and P-gp overexpression and function [44].