In vitro activity: Tariquidar displays high-affinity binding to P-gp with Bmax of 275 pmol/mg. Tariquidar shows non-competitive interaction with the P-gp substrates vinblastine and paclitaxel. Tariquidar increases the steady-state accumulation of these cytotoxics in CHr<>/supB30 cells to levels observed in non-P-gp-expressing AuxB1 cells with EC50 of 487 nM. Tariquidar is able to inhibit the vanadate-sensitive ATPase activity of P-gp by 60-70%, with potent IC50 values of 43 NM. Tariquidar may inhibit other resistance mechanisms at higher concentrations. 1 μM Tariquidar abrogates ABCG2 (BCRP)-mediated resistance to camptothecins in vitro. Tariquidar potentiates the cyto-toxicity of several drugs including doxorubicin, paclitaxel, etoposide, and vincristine; complete reversal of resistance is achieved in the presence of 25- 80 nM Tariquidar. In MC26, a murine colon carcinoma cell line with intrinsic chemoresistance, the doxorubicin IC50 is fivefold lower in the presence of 0.1 μM Tariquidar (36 vs 7 nM). In murine mammary carcinoma, human small-cell lung carcinoma and human ovarian carcinoma cell lines with acquired chemotherapeutic resistance (EMT6/AR1.0, H69/LX4 and 2780 AD), the in vitro doxorubicin IC50 is 22-150-fold lower in the presence of 0.1 μM Tariquidar. P-gp inhibition persists for 23 h after removal of Tariquidar from the culture system. Tariquidar restored the cyto-toxicity of doxorubicin and vinblastine in the National Cancer Institute (NCI)/ADRRES multicellular tumor spheroid model derived from the MCF7WT breast cancer cell line.

Kinase Assay: AuxB1 and CHrB30 cells are grown to confluency in 12-well (24 mm) tissue culture dishes and the steady-state accumulation of [3H]-vinblastine is measured. Accumulation is initiated by the addition of 0.1 μ Ci [3H]-vinblastine and unlabelled vinblastine to a final concentration of 100 nM . The accumulation of [3H]-paclitaxel is measured using 0.1 μ Ci [3H]-paclitaxel and unlabelled drug to a final concentration of 1 μM . Cells are incubated in a reaction volume of 1 mL for 60 min at 37 ℃ under 5% CO2 in order to reach steady-state. The effect of the modulators XR9576 on [3H]-ligand accumulation is investigated in the concentration range 10-9 - 10-6 M. Modulators are added from a DMSO stock giving a final solvent concentration of 0.2 % (v/v). Following cell harvesting, accumulated drug is measured by liquid scintillation counting and normalized for cell protein content. Plots of amount accumulated as a function of modulator concentration are fitted with the general dose-response equation: Y={(a-b)/(1+(X/c)d)}+b Where: Y=response; a=initial response; b=final response; c=EC50 concentration; d=slope value; X=drug concentration.

Cell Assay: Cells are seeded into 96-well plates at 800/well, in 100 μL of medium and incubated for 4 h at 37 ℃. Varying concentrations of modulator or solvent control (50 μL/well) are subsequently added and incubated for an additional 1 h before the addition of the cytotoxic drug. The cytotoxic drug (50 μL) is added to give a range of final concentrations in quadruplicate wells. After incubation for an additional 4 days, cell proliferation of adherent cells is assessed using the sulforhodamine B assay.