Pamidronate resistance and associated low ras levels in breast cancer cells: A role for combinatorial therapy

To identify markers sensitive to inhibitors of the farnesylation pathway, we used 3 breast cancer cell lines (SKBR-3, MDA-175, and MDA-231) to evaluate the in vitro effects of pamidronate, an inhibitor of farnesyl diphosphate synthase. In response to pamidronate, there was significant inhibition of cell proliferation in MDA-231 and SKBR-3 cells, compared to MDA-175 cells. This correlated with their respective basal levels of N-ras and H-ras. N-ras and H-ras protein levels were both reduced in MDA-231 cells, and to lesser extent in SKBR-3 cells, following exposure to pamidronate, whereas these markers were not altered in MDA-175 cells. Combinatorial therapy with pamidronate and Gleevec, an inhibitor of several tyrosine kinases; Velcade, a proteasome inhibitor; or rapamycin, an inhibitor of the mammalian target of rapamycin (m-TOR) all showed additive effects in causing proliferative inhibition in MDA-175 cells. In summary, resistance to pamidronate may result from low levels of GTPase-activating proteins, such as N-ras and H-ras, in tumor cells. Combinatorial therapies directed against other signaling pathways, not dependent upon ras, may be required to overcome such resistance.