**1. Introduction**

Treatment failure or metastasis are still the leading causes of death for cancer patients [1]. One of the important factors causing treatment failure is drug resistance, which can be intrinsic or acquired [2,3]. Cancer stem cells [4], tumor microenvironment, and host effects are the main reasons of intrinsic resistance, while efflux pumps, alteration of drug targets, degradation of anticancer drugs, and DNA self-repair cause acquired resistance. In particular, overexpression of efflux pumps leads to multidrug resistance (MDR) [2,3]. In order to increase the efficacy of anticancer drugs, strategies to reverse multidrug resistance are extensively studied, and P-glycoprotein (P-gp) inhibitors have evolved to the fourth generation [5–8]. However, because of their inability to of improve drug efficacy in patients as well as their toxicity, P-gp inhibitors are not yet available in the clinic. Therefore, research is now focusing on natural products, hoping to find safe and effective P-gp inhibitors [5–8].

*Taiwanofungus camphoratus*, previously named *Ganoderma camphoratum*, *Antrodia cinnamomea*, *or Antrodia camphorate* (Polyporaceae, Aphyllophorales), whose Chinese name is Zhan-Ku or Niu-Chang-Chih, is a kind of fungus parasitic to *Cinnamomum kanehirai* Hay (Lauraceae), found in the inner part of old hollow trunks. Traditionally, it has been used as a kind of medicinal food against intoxication from food, alcohol, and drugs and for its anti-diarrhea, anti-hypertensive, anti-inflammatory, and hepatoprotective effects [9]. Recent studies have revealed that Niu-Chang-Chih exerts immunomodulatory effects [10,11], anti-lung cancer effects [12–14], and tumor-suppressive effects in metastatic patients unresponsive to or unwilling to use chemotherapy [15]. A recent published review summarized the pharmacological effects of this mushroom [16] reporting its anticancer activity against a large variety of cancers, including breast, cervical, ovarian, prostate, bladder, colorectal, pancreatic, liver, and lung cancers, melanoma, leukemia, lymphoma, neuroblastoma, and glioblastoma. Other biological activities include anti-inflammatory, anti-atopic dermatitis, anti-cachexia, immunoregulatory, anti-obesity, anti-diabetic, anti-hyperlipidemic, anti-atherosclerotic, anti-hypertensive, anti-platelet, anti-oxidative, anti-photodamaging, hepatoprotective, renoprotective, neuroprotective, testis protecting, anti-asthmatic, osteogenic, osteoprotective, antiviral, antibacterial, and wound healing properties [16]. The major chemical constituents of this fungus are triterpenoids [17–19] and benzenoids [20,21]. Other components are steroids [22], diterpenoids [23], terpenoids [24], lignans [22], maleic and succinic acid derivatives [25], etc. Although the extract of *T. camphoratus* has been used as a nutritional supplement for treating cancers, the P-gp inhibitory effects of its main constituents are still unknown. Therefore, in addition to reporting new chemical constituents of *T. camphoratus*, this study reveals the P-gp inhibitory effects of zhankuic acids A–C.

#### **2. Results and Discussion**
