C20- nor-Abietane and Three Abietane Diterpenoids from Plectranthus mutabilis Leaves as P-Glycoprotein Modulators

Abstract

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.

Figure 1

Structures of compounds 1–4 from P. mutabilis.

Figure 2

Key ¹H–¹H COSY (bold lines) and HMBC (blue arrows) correlations for compound 1.

Figure 3

Electrophilic (f_k^–), nucleophilic (f_k⁺), and radical (f_k⁰) Fukui functions of 2–4. The higher condensed Fukui indexes are indicated as green circles, red triangles, and blue arrows, respectively, representing the sites in the molecules that are most susceptible to a radical attack, most nucleophilic, and most electrophilic.

Figure 4

Electrophilic Fukui functions (f_k^–) and Gibbs free energies of formation for anions derived from deprotonation of coleon U (4) at hydroxyl positions on (a) C11 and (b) C14.

Scheme 1

Figure 5

Effects of compounds and the extract on ABCB1 expression. The mean fluorescence intensity (MFI) of untreated NCI-H460/R cells was set to 100 and used for the comparison of other MFI values (for untreated NCI-H460 cells and treated NCI-H460/R cells) in GraphPad Prism 6 software (unpaired t test). The MFI was calculated from three independent experiments. A difference from untreated NCI-H460/R cells was considered to be significant if p < 0.05 (*) or p < 0.001 (***).