BACKGROUND: Exposure of cancer cells to chemotherapeutic agents may result in 
reduced sensitivity to structurally unrelated agents, a phenomenon known as 
multidrug resistance, MDR. The purpose of this study is to investigate cell 
growth inhibition of wild type and the corresponding MDR cells by Tumor Treating 
Fields--TTFields, a new cancer treatment modality that is free of systemic 
toxicity. The TTFields were applied alone and in combination with paclitaxel and 
doxorubicin.
METHODS: Three pairs of wild type/MDR cell lines, having resistivity resulting 
from over-expression of ABC transporters, were studied: a clonal derivative 
(C11) of parental Chinese hamster ovary AA8 cells and their emetine-resistant 
sub-line EmtR1; human breast cancer cells MCF-7 and their mitoxantrone-resistant 
sub lines MCF-7/Mx and human breast cancer cells MDA-MB-231 and their 
doxorubicin resistant MDA-MB-231/Dox cells. TTFields were applied for 72 hours 
with and without the chemotherapeutic agents. The numbers of viable cells in the 
treated cultures and the untreated control groups were determined using the XTT 
assay. Student t-test was applied to asses the significance of the differences 
between results obtained for each of the three cell pairs.
RESULTS: TTFields caused a similar reduction in the number of viable cells of 
wild type and MDR cells. Treatments by TTFields/drug combinations resulted in a 
similar increased reduction in cell survival of wild type and MDR cells. 
TTFields had no effect on intracellular doxorubicin accumulation in both wild 
type and MDR cells.
CONCLUSIONS: The results indicate that TTFields alone and in combination with 
paclitaxel and doxorubicin effectively reduce the viability of both wild type 
and MDR cell sub-lines and thus can potentially be used as an effective 
treatment of drug resistant tumors.