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Polyporenic acids N-R (1–5), five novel 24-methylene lanostane triterpenes along with seven known polyporenic acids (6–12), were identified from the fruiting bodies of Buglossoporus quercinus. The isolation of compounds 1–12 was performed by a combination of multistep flash chromatography and reversed-phase high-performance liquid chromatography (HPLC). The structure determination was carried out by extensive spectroscopic analysis, including 1D and 2D nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) experiments. The isolated fungal metabolites were investigated for their antiproliferative activity in vitro by 3-(4,5-dimethylthiazol2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay on the resistant Colo 320 human colon adenocarcinoma cell line expressing P-glycoprotein (ABCB1). The lanostane triterpenes exerted moderate antiproliferative activity with IC50 values in the range of 20.7–106.2 μM. A P-glycoprotein efflux pump modulatory test on resistant Colo 320 cells highlighted that fungal metabolites 3, 5, 8, and 10–12 have the ability to inhibit the efflux pump activity of cancer cells. Moreover, the drug interactions of triterpenes with doxorubicin were studied by the checkerboard method. Compounds 3–4, and 7–12 interacted in a synergistic manner, while an outstanding potency was detected for compound 9, which was defined as strong synergism (CI = 0.276). The current study reveals that B. quercinus is a remarkable source of fungal steroids with considerable chemosensitizing activity on cancer cells.