In this study, a bioguided fractionation of Plectranthus mutabilis extract was performed by chromatographic methods. It yielded one new nor-abietane diterpene, mutabilol (1), and three known abietanes, coleon-U-quinone (2), 8α,9α-epoxycoleon-U-quinone (3), and coleon U (4). The abietane diterpenoid 5 was also tentatively identified using HPLC-MS/MS. Moreover, the extract profile and quantification of each isolated compound were determined by HPLC-DAD. Compound 4 was the major compound in the extract. Compounds 2-4 were found to be selective toward cancer cell lines and were able to inhibit P-glycoprotein (P-gp) activity in NCI-H460/R cells at longer exposure of 72 h and consequently revert doxorubicin (DOX) resistance in subsequent combined treatment. None of the compounds influenced the P-gp expression in NCI-H460/R cells, while the extract significantly increased it. acceptedVersion