Targeting multidrug resistance in cancer by natural chemosensitizers

Statistics on cancer incidence and mortalities indicate that this disease still has a fatal outcome for a majority of patients due to non-sufficient treatment. The options available for cancer treatment include chemotherapy, which still commands a leading position in clinical oncology. A major obstacle to successful chemotherapy is the development of cellular resistance to multiple structurally unrelated anticancer drugs. This phenomenon has been termed multidrug resistance (MDR), which occurs in a majority of cancer patients. MDR is mainly due to the overexpression of ABC transporters which extrude chemotherapeutic drugs outside of cancer cells. A plethora of synthetic chemosensitizers have been described during the past decades that block ABC transporter function to reverse their MDR. However, none of them reached clinical routine application as of yet. In this review, we highlight the potential of natural products derived from plants, marine organisms, fungi, and other sources as chemosensitizers to the targeted major ABC transporters (ABCB1, ABCC1, and ABCG2). Natural compounds may serve as lead compounds for the development of novel ABC transporter inhibitors with improved pharmacological features that can be used as adjuvant therapy to enhance the efficacy of chemotherapeutic drugs against MDR.