PGP and breast cancer: Cytotoxic activities (ED50 values μg/mL) of compounds 41–44b against the epidermoid carcinoma of the nasopharynx KB, P-gp-expressing epidermoid carcinoma of the masopharynx KB-VIN, osteosarcoma Hos, melanoma SKMEL-2, ileocecal carcinoma HCT-8, breast cancer MCF-7, lung carcinoma A-549, glioblastoma U87-MG, and ovarian cancer 1A9 cell lines.