In the experiments to reverse a mitoxantrone-resistant phenotype and inhibit the cellular BCRP-dependent mitoxantrone accumulation using the displayed BCRP-mediated MDR cells EPG85-257RNOV (human gastric carcinoma cell line) and MCF-7/AdrVp (human breast cancer cells), only tryprostatin A (Figure 7) showed activity in a concentration range of 10–50 μM and did not exhibit cytotoxicity at evaluated concentrations [69]. The gene discussed is ABCG2; the disease is breast carcinoma.