P-gp modulation and biosynthetic relationship of isolated compounds from Plectranthus mutabilis Codd.

The development of multidrug resistance (MDR) is one of the major challenges in the successful treatment of cancer. MDR is often associated with the P-glycoprotein efflux pump. Natural products are a source of promising lead compounds to overcome MDR and, among them, diterpenoids from Plectranthus spp. are known as P-gp modulators. Bioguided fractionation of P. mutabilis acetone extract led to the isolation of one new nor-abietane diterpene, mutabilol (1), and three coleon compounds (coleon-U-quinone (2), 8α,9α-epoxycoleon-U-quinone (3), and coleon U (4)). Moreover, an additional acetoxy derivative of an abietane diterpenoid was tentatively identified using HPLC-MS/MS. The compounds were quantified using HPLC-DAD and coleon U was found to be the major compound in the extract. Using computational studies, a biosynthetic relationship between compounds 2 - 4 revealed that both compounds 2 and 3 were formed directly from compound 4. Compounds 2 - 4 were found to be selective towards the cancer cell lines and their anticancer effect was not compromised by the P-gp activity in resistant NCI-H460/R cells. Importantly 2, 3, and 4 were able to inhibit P-gp activity in NCI-H460/R cells at longer exposure (72 h) and revert doxorubicin (DOX) resistance in combined treatment. None of the compounds influenced the P-gp expression in NCI-H460/R cells, while the extract significantly increased it. Our study identified abietane diterpenoids from P. mutabilis that can evade MDR in cancer cells and inhibit the P-gp activity in prolonged treatment.