Abstract
We have selected an etoposide-resistant variant (CCRF-CEM/VP-16) of the human T-lymphoblastic CCRF-CEM leukemia for study. Resistance to the topoisomerase II (topo II) inhibitor was about 11-fold and stable. Other data revealed that the new cell line had acquired an atypical, non-P-glycoprotein overexpressing multidrug resistant (MDR) phenotype with cross-resistance to other topo II inhibitors (amsacrine, doxorubicin, and mitoxantrone) and to glucocorticoids, but not to novobiocin, ICRF-187, vincristine or cisplatin. In a first instance, we assumed that altered drug-topo II interactions, based on quantitative and/or qualitative modifications of the enzyme, are a cause of resistance in the cell line. We tried to modify the drug sensitivity of the cells by means of various agents and cytokines. Positive results were obtained with verapamil and, to a lesser extent, cyclosporin A, but they were not specific for the drug resistant variant and occurred in the parental CCRF-CEM as well. Other attempts with buthionine sulfoximine, novobiocin, pentoxifylline, interleukin-1, interferon-alpha, retinoic acid, TNF-alpha, bryostatin 1 or phorbol myristate acetate were substantially unsuccessful, thus confirming the difficulty of pharmacologically overcoming atypical MDR. More encouragingly, however, CCRF-CEM/VP-16 cells exhibited hypersensitivity to other agents, including actinomycin D and taxol.
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