Therefore, there have been many efforts to improve tumor drug delivery by using competitive or non-competitive inhibitors (Figure 4B(b,c)), and many P-gp and BCRP inhibitors have been clinically evaluated for their use as adjuvants on chemotherapy to treat non-brain tumors, including valspodar, dexverapamil, tariquidar, biricodar, and elacridar; as well as indirect inhibition by anti-P-gp monoclonal antibodies [14,41,106]. Here, ABCG2 is linked to brain neoplasm.