Resistance of cancer cells towards chemotherapy is the major cause of therapy failure. Hence, the evaluation of cellular defense mechanisms is essential in the establishment of new chemotherapeutics. Archazolid B, a novel vacuolar H+-ATPase inhibitor, displayed cytotoxicity in the low nanomolar range on a panel of different tumor cell lines. First, we investigated tumor-specific cytotoxicity of archazolid B by comparing cancer to non-cancer cells. Breast, liver and colon cancer cells displayed higher drug sensitivity than corresponding non-tumorous cells, whereas leukemia cell lines were as sensitive as peripheral mononuclear blood cells. Investigating classical drug resistance mechanisms, archazolid B was identified as a possible substrate of the ABC transporters ABCB1 (P-glycoprotein) and ABCG2 (BCRP), whereas collateral sensitivity was observed in ABCB5-expressing cells. Our results pointed to a possible binding competition of archazolid B with verapamil on P-glycoprotein. However, archazolid B did not reverse resistance towards doxorubicin indicating that it might be a substrate but not an inhibitor of P-glycoprotein mediated transport. Furthermore, the cytotoxicity of archazolid B was independent of the p53 status of the cell. Mechanisms of aquired resistance were investigated establishing an archazolid B-resistant MCF-7 cell line. Interestingly, drug resistance was not conferred by aberrant expression or DNA mutations of the gene encoding vacuolar H+-ATPase subunit c, the direct target of archazolids. Instead, long-term treatment with archazolid B led to a slight overexpression of ABCB1 and a significant overexpression of the epidermal growth factor receptor and reduced cell growth, all of which can be assumed to contribute to archazolid B resistance.
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