PGP and neoplasm: SAR studies of a series of novel N-substituted derivatives of 45 on breast P-gp-overexpressing tumor cell line (LCC6MDR) showed that analogs containing one methoxy group and one benzyloxy group on ring C are the most potent P-gp modulators (1 μM desensitized cell line by 42.7 folds) without showing cytotoxicity IC50 for L929 fibroblast >100 μM) [117,119,120].