*3.6. SRB Cytotoxicity Assay and Reversal Fold Calculation*

Briefly, after 72 h of treatment with series concentrations of chemotherapeutic drugs with or without the test compounds, 50% trichloroacetic acid (TCA) was added to fix the cells for 30 min. After air-drying, the cells were stained with 0.04% sulforhodamine B (SRB) for 30 min and washed with 1% acetic acid. The bound stain was solubilized in 10 mM Tris base, and the absorbance was measured by a Synergy HT Multi-Mode Microplate Reader (BioTek, Winooski, VT, USA) at 515 nm. Reversal folds were calculated by dividing the IC50 of each drug by the IC50 of the compound–drug combination treatment.

#### **4. Conclusions**

Although numerous anti-cancer drugs are marketed, resistance to cancer treatments is still the top reason for cancer death. Multidrug resistance is largely due to the high expression of efflux pumps. *T. camphoratus*, a medicinal fungus, was reported to exhibit anti-cancer properties, but its effects toward cancer multidrug resistance are unknown. In this study, four new chemical constituents, camphoratins K (**1**) and N (**2**) and benzocamphorins G (**3**) and I (**4**), were reported for the first time, and the main constituents of *T. camphoratus*, zhankuic acids A–C, were found to have P-gp inhibitory effects in a dose dependent manner. In addition, zhankuic acid A (20 μM), the most potent P-gp inhibitor, could effectively reverse MDR in KBvin cells, leading to an IC50 of 78.5 nM for doxorubicin, of 48.5 nM for paclitaxel, and of 321.5 nM for vincristine, corresponding to reversal folds of 48, 38, and 45, respectively. **Supplementary Materials:** The following are available online. S01: References of the known compounds; Figures S1–S36: NMR spectra of compounds **1**–**4**.

**Author Contributions:** Conceptualization, P.-C.K. and T.-S.W.; investigation, H.-Y.H., C.-C.H., J.-W.L.; resources, P.-C.S., P.-C.K., and T.-S.W.; data curation, C.-F.C., H.-Y.C.; writing—original draft preparation, H.-Y.H., C.-C.H.; writing—review and editing, P.-C.K. and T.-S.W. All authors read and approved the final manuscript.

**Funding:** This research was sponsored by the Ministry of Science and Technology (MOST), Taiwan, granted to T.-S.W.

**Acknowledgments:** Thanks TWHERB Biomedical Co., LTD for providing plant materials and thanks Taiwan Forestry Research Institute for helping identifying the fungus.

**Conflicts of Interest:** The authors declare no conflict of interest.
