The distribution of IC50 values across the panel led us to classify 18 cell lines as relatively paclitaxel sensitive and four cell lines as rela tively paclitaxel resistant. We determined if the four resistant cell lines could be sensitized to paclitaxel using the novel drug combinations presented above and assayed the two lines used in our RNAi screening, MDA MB 231 and MDA MB 468 for either comparison. A four day cell viability assay after combination treat ments was used to Inhibitors,Modulators,Libraries assess drug synergy, defined as the combination of two agents that have a greater therapeutic effect than would be expected by the addition of individ ual effects of each drug. Inhibitors,Modulators,Libraries The well established Chou and Talalay method was used to determine drug synergy, as described in Materials and Methods.

Combination index values were derived from the median effect plots of single agents alone or in combination and statisti cal tests were used to determine whether the CI values at multiple dose effect levels were statisti cally significantly different from 1. CI values significantly 1 indicate synergy, not significantly differ ent from 1 indicates additive, Inhibitors,Modulators,Libraries and a CI value significantly 1 indicates antagonism. CCT007093 was synergistic with paclitaxel in two paclitaxel sensi tive cell lines, MDA MB 468 and MDA MB 231, average CI value of 0. 56 and 0. 38, respectively, and in two of the four paclitaxel resistant cell lines CAL120 and HDQP1. CCT007093 was additive with paclitaxel in the two other paclitaxel resistant cell lines SW527 and MT3. Mithramycin was synergistic with paclitaxel in the two paclitaxel sensitive lines Inhibitors,Modulators,Libraries MDA MB 468 and MDA MB 231, average CI value of 0.

66 and 0. 54, respectively, and the paclitaxel resistant cell line Inhibitors,Modulators,Libraries HDQP1 average CI value 0. 87. However, mithramycin and paclitaxel were antago nistic, average CI values significantly 1, in reducing cell viability at high effective drug doses in the paclitaxel resistant lines CAL120, SW527 and MT3. Collectively these data indicate that novel drug combinations with paclitaxel can effectively reduce cell viability of select paclitaxel sensitive and importantly, paclitaxel resistant TNBC cell lines. Discussion Our RNAi screen represents a directed approach selleck chemicals to iden tifying breast cancer relevant, druggable targets to enhance drug sensitivity. The screen was validated by our finding that several of the positive hits are genes that are known targets of paclitaxel sensitivity and have been clin ically targeted in combination with taxanes. We identified additional novel gene tar gets and respective agents that were not previously iden tified by drug sensitivity RNAi screens or whose inhibitors were not previously combined with paclitaxel.