Rapid development of multiple drug resistance against current therapies is a major barrier in the treatment of cancer. Therefore, anticancer agents that can overcome acquired drug resistance in cancer cells are of great importance. Previously, we have demonstrated that ethyl 2-amino-4-(2-ethoxy-2-oxoethyl)-6-phenyl-4H-chromene-3-carboxylate (5a, sHA 14-1), a stable analogue of ethyl 2-amino-6-bromo-4-(1-cyano-2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (6, HA 14-1), mitigates drug resistance and synergizes with a variety of cancer therapies in leukemia cells. Structure-activity relationship (SAR) studies of 5a guided the development of ethyl 2-amino-6-(3',5'-dimethoxyphenyl)-4-(2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (5q, CXL017), a compound with low micromolar cytotoxicity against a wide-range of hematologic and solid tumor cells. More excitingly, our studies of 5q in camptothecin (CCRF-CEM/C2) and mitoxantrone (HL-60/MX2) resistant cancer cells highlight its ability to selectively kill drug-resistant cells over parent cancer cells. 5q inhibits tumor cell growth through the induction of apoptosis, with detailed mechanism of its selectivity toward drug-resistant cancer cells under investigation. These results suggest that 5q is a promising candidate for treatment of cancers with multiple drug resistance.

HA 14-1, a small-molecule antagonist against anti-apoptotic Bcl-2 proteins, was demonstrated to induce selective cytotoxicity toward malignant cells and to overcome drug resistance. Due to its poor stability and the reactive oxygen species (ROS) generated by its decomposition, chemical modification of HA 14-1 is needed for its future development. We have synthesized a stabilized analog of HA 14-1 -sHA 14-1, which did not induce the formation of ROS. As expected for a putative antagonist against anti-apoptotic Bcl-2 proteins like HA 14-1, sHA 14-1 disrupted the binding interaction of a Bak BH3 peptide with Bcl-2 or Bcl-X L protein, inhibited the growth of tumor cells through the induction of apoptosis, and circumvented the drug resistance induced by the overexpression of anti-apoptotic Bcl-2 and Bcl-X L proteins. Interestingly, the impairment of extrinsic apoptotic pathway induced moderate resistance to sHA 14-1. The moderate resistance suggested that sHA 14-1 generated part of its apoptotic stress through the intrinsic pathway, possibly through its antagonism against anti-apoptotic Bcl-2 proteins. The resistance indicated that sHA 14-1 generated apoptotic stress through the extrinsic apoptotic pathway as well. The ability of sHA 14-1 to induce apoptotic stress through both pathways was further supported by the synergism of sHA 14-1 towards the cytotoxicities of Fas ligand and dexamethasone in Jurkat cells. Taken together, these findings suggest that sHA 14-1 may represent a promising candidate for the treatment of drug-resistant cancers either as a monotherapy or in combination with current cancer therapies.

Multidrug resistance (MDR) in cancer is a phenomenon in which administration of a single chemotherapeutic agent causes cross-resistance of cancer cells to a variety of therapies even with different mechanisms of action. Development of MDR against standard therapies is a major challenge in the treatment of cancer. Previously we have demonstrated a unique ability of CXL017 (5) to selectively target MDR cancer cells and synergize with mitoxantrone (MX) in HL60/MX2 MDR cells. Here we expand its scope and demonstrate that 5 can synergize with both vincristine and paclitaxel in three different MDR cell lines (HL60/DNR, K562/HHT300, and CCRF-CEM/VLB100). We also demonstrate that 5 has potent cytotoxicity in the NCI-60 panel of cell lines with an average IC(50) of 1.04 μM. In addition, 5 has a unique mechanism of action in comparison with standard agents in the NCI database based on COMPARE analysis. Further structure-activity relationship study led to the development of a more potent analogue, compound 7d, with an IC(50) of 640 nM in HL60/MX2. Additionally, one enantiomer of 5 is 13-fold more active than the less active enantiomer. Taken together, our study has led to the discovery of a series of analogues that selectively target drug-resistant cancer cells with the potential for the treatment of drug-resistant cancers.

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BackgroundADAM8 (a disintegrin and metalloproteinase 8) protein promotes the invasive and metastatic phenotype of triple-negative breast cancer (TNBC) cells. High ADAM8 expression in breast cancer patients is an independent predictor of poor prognosis. Here, we investigated whether ADAM8 regulates specific miRNAs, their roles in aggressive phenotype, and potential use as biomarkers of disease.MethodsMicroarray analysis was performed on RNA from MDA-MB-231 cells after transient ADAM8 knockdown using TaqMan miRNA cards. Changes in miRNA levels were confirmed using two ADAM8 siRNAs in TNBC cell lines. Kinase inhibitors, β1-integrin antagonist antibody, and different forms of ADAM8 were employed to elucidate the signaling pathway required for miR-720 expression. miR-720 levels were modulated using a specific antagomiR or a mimic, and effects on aggressive phenotype of TNBC cells were determined using Boyden chamber and 3D-Matrigel outgrowth assays. Plasma was isolated from mice before and after implantation of MDA-MB-231 cells and analyzed for miR-720 levels. Serum samples of TNBC patients were evaluated for their ADAM8 and miR-720 levels.ResultsWe identified 68 miRNAs differentially regulated upon ADAM8 knockdown, including decreased levels of secreted miR-720. Ectopic overexpression of wild-type ADAM8 or forms that lack metalloproteinase activity similarly induced miR-720 levels. The disintegrin and cysteine-rich domains of ADAM8 were shown to induce miR-720 via activation of a β1-integrin to ERK signaling cascade. Knockdown of miR-720 led to a significant decrease in migratory and invasive abilities of TNBC cells. Conversely, miR-720 overexpression rescued these properties. A profound increase in plasma levels of miR-720 was detected 7 days after TNBC cell inoculation into mouse mammary fat pads when tumors were barely palpable. Concordantly, miR-720 levels were found to be significantly higher in serum samples of TNBC patients with high ADAM8 expression.ConclusionsWe have shown for the first time that miR-720 is induced by ADAM8 signaling via ERK and plays an essential role in promoting the aggressive phenotype of TNBCs. miR-720 is elevated in serum of patients with ADAM8-high TNBC and, in a group with other miRNAs downstream of ADAM8, holds promise as a biomarker for early detection of or treatment response of ADAM8-positive TNBCs.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-016-0699-z) contains supplementary material, which is available to authorized users.