abt-199 and Breast-Neoplasms

The addition of a CDK4/6 inhibitor to endocrine therapy improves progression-free and overall survival in women with metastatic estrogen receptor-positive breast cancer. In that setting, CDK4/6 inhibitors induce a potent cell-cycle arrest (which may be accompanied by tumor senescence) but fail to induce apoptotic cell death. Venetoclax is a potent inhibitor of BCL2, a pro-survival protein overexpressed in the majority of estrogen receptor-positive cancers. Pre-clinical findings indicate that venetoclax augments tumor response to the CDK4/6 inhibitor palbociclib by triggering apoptosis, including in senescent cells. The PALVEN phase Ib trial will further examine this finding. The primary objective is to identify the maximum tolerated dose and determine the recommended phase II dose for palbociclib, letrozole and venetoclax combination therapy.. The current ‘gold standard' treatment for estrogen receptor-positive, HER2-negative metastatic breast cancer is endocrine therapy with a CDK4/6 inhibitor. This combination improves tumor response and patient outcomes, primarily by reducing tumor cell growth. Paradoxically, less killing of tumor cells is observed in the presence of a CDK4/6 inhibitor. The authors hypothesize that co-treatment with venetoclax, an inhibitor of the BCL2 survival protein, will help trigger tumor death, thereby further improving tumor responses and patient outcomes. As a first step, combination therapy comprising letrozole, palbociclib and venetoclax will be tested in a phase I trial to identify the recommended doses for subsequent studies.

Topics: Antineoplastic Combined Chemotherapy Protocols; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Clinical Trials, Phase I as Topic; Female; Humans; Letrozole; Piperazines; Protein Kinase Inhibitors; Proto-Oncogene Proteins c-bcl-2; Pyridines; Receptor, ErbB-2; Receptors, Estrogen; Sulfonamides

Topics: Aged; Allografts; Anemia, Refractory, with Excess of Blasts; Aniline Compounds; Antineoplastic Combined Chemotherapy Protocols; Azacitidine; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Clinical Trials, Phase III as Topic; Cytarabine; Daunorubicin; Fatal Outcome; Female; fms-Like Tyrosine Kinase 3; Humans; Leukemia, Myeloid, Acute; Liposomes; Male; Middle Aged; Myelodysplastic Syndromes; Neoplasm, Residual; Neoplasms, Radiation-Induced; Oncogene Proteins, Fusion; Peripheral Blood Stem Cell Transplantation; Point Mutation; Protein Kinase Inhibitors; Pyrazines; Remission Induction; Salvage Therapy; Staurosporine; Sulfonamides

The last three decades have seen significant progress in our understanding of the role of the pro-survival protein BCL-2 and its family members in apoptosis and cancer. BCL-2 and other pro-survival family members including Mcl-1 and BCL-XL have been shown to have a key role in keeping pro-apoptotic 'effector' proteins BAK and BAX in check. They also neutralize a group of 'sensor' proteins (such as BIM), which are triggered by cytotoxic stimuli such as chemotherapy. BCL-2 proteins therefore have a central role as guardians against apoptosis, helping cancer cells to evade cell death. More recently, an increasing number of BH3 mimetics, which bind and neutralize BCL-2 and/or its pro-survival relatives, have been developed. The utility of targeting BCL-2 in hematological malignancies has become evident in early-phase studies, with remarkable clinical responses seen in heavily pretreated patients. As BCL-2 is overexpressed in ~75% of breast cancer, there has been growing interest in determining whether this new class of drug could show similar promise in breast cancer. This review summarizes our current understanding of the role of BCL-2 and its family members in mammary gland development and breast cancer, recent progress in the development of new BH3 mimetics as well as their potential for targeting estrogen receptor-positive breast cancer.

Topics: Aniline Compounds; Animals; Antineoplastic Agents; Apoptosis; Apoptosis Regulatory Proteins; Biomarkers, Tumor; Breast; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Carcinoma; Drug Design; Drug Synergism; Estrogens; Female; Gene Expression Regulation, Neoplastic; Genes, bcl-2; Humans; Mice; Mitochondria; Molecular Targeted Therapy; Multigene Family; Neoplasm Proteins; Neoplasms, Hormone-Dependent; Peptide Fragments; Prognosis; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-bcl-2; Receptors, Estrogen; Signal Transduction; Sulfonamides; TOR Serine-Threonine Kinases; Xenograft Model Antitumor Assays

Despite promising activity in hematopoietic malignancies, efficacy of the B-cell lymphoma 2 (BCL2) inhibitor venetoclax in solid tumors is unknown. We report the prespecified VERONICA primary results, a randomized phase II clinical trial evaluating venetoclax and fulvestrant in estrogen receptor (ER)-positive, HER2-negative metastatic breast cancer, post-cyclin-dependent kinase (CDK) 4/6 inhibitor progression.. Pre-/postmenopausal females ≥18 years were randomized 1:1 to venetoclax (800 mg orally daily) plus fulvestrant (500 mg intramuscular; cycle 1: days 1 and 15; subsequent 28-day cycles: day 1) or fulvestrant alone. The primary endpoint was clinical benefit rate (CBR); secondary endpoints were progression-free survival (PFS), overall survival, and safety. Exploratory biomarker analyses included BCL2 and BCL extra-large (BCLXL) tumor expression, and PIK3CA circulating tumor DNA mutational status.. At primary analysis (cutoff: August 5, 2020; n = 103), venetoclax did not significantly improve CBR [venetoclax plus fulvestrant: 11.8% (n = 6/51; 95% confidence interval (CI), 4.44-23.87); fulvestrant: 13.7% (7/51; 5.70-26.26); risk difference -1.96% (95% CI, -16.86 to 12.94)]. Median PFS was 2.69 months (95% CI, 1.94-3.71) with venetoclax plus fulvestrant versus 1.94 months (1.84-3.55) with fulvestrant (stratified HR, 0.94; 95% CI, 0.61-1.45; P = 0.7853). Overall survival data were not mature. A nonsignificant improvement of CBR and PFS was observed in patients whose tumors had strong BCL2 expression (IHC 3+), a BCL2/BCLXL Histoscore ratio ≥1, or PIK3CA-wild-type status.. Our findings do not indicate clinical utility for venetoclax plus fulvestrant in endocrine therapy-resistant, CDK4/6 inhibitor-refractory metastatic breast tumors, but suggest possible increased dependence on BCLXL in this setting.

Topics: Antineoplastic Combined Chemotherapy Protocols; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Class I Phosphatidylinositol 3-Kinases; Cyclin-Dependent Kinase 4; Cyclin-Dependent Kinase 6; Female; Fulvestrant; Humans; Protein Kinase Inhibitors; Proto-Oncogene Proteins c-bcl-2; Receptor, ErbB-2; Receptors, Estrogen; Sulfonamides

Venetoclax, a potent and selective BCL2 inhibitor, synergizes with endocrine therapy in preclinical models of ER-positive breast cancer. Using a phase Ib 3 + 3 dose-escalation and expansion study design, 33 patients with ER and BCL2-positive metastatic disease (mean prior regimens, 2; range, 0-8) were treated with daily tamoxifen (20 mg) and venetoclax (200-800 mg). Apart from uncomplicated "on-target" lymphopenia, no dose-limiting toxicities or high-grade adverse events were observed in the escalation phase (15 patients), and 800 mg was selected as the recommended phase II dose (RP2D). In the expansion phase (18 patients), few high-grade treatment-related adverse events were observed. For 24 patients treated at the RP2D, the confirmed radiologic response rate was 54% and the clinical benefit rate was 75%. Treatment responses were preempted by metabolic responses (FDG-PET) at 4 weeks and correlated with serial changes in circulating tumor DNA. Radiologic responses (40%) and clinical benefit (70%) were observed in 10 patients with plasma-detected

Topics: Adult; Aged; Antineoplastic Combined Chemotherapy Protocols; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Circulating Tumor DNA; Disease-Free Survival; Dose-Response Relationship, Drug; Drug Administration Schedule; Estrogen Receptor alpha; Female; Humans; Middle Aged; Neoplasm Metastasis; Proto-Oncogene Proteins c-bcl-2; Sulfonamides; Tamoxifen; Tissue Distribution

Breast cancer is recognized globally as one of the leading causes of malignant morbidity. It is a heterogeneous disease that accounts for 30 percent of all women diagnosed with cancer. To bring an anti-cancer drug from the bench to the bedside is an expensive and time-consuming process. The drug repurposing approach targets new enemies (new diseases) with old weapons (known drugs). The present study identified an FDA-approved drug targeting the γ-secretase complex involved in the Notch signaling pathway in breast cancer stem cells (BCSCs). A literature survey and in-silico study identified Venetoclax as a γ-secretase inhibitor (GSI) from 1615 FDA-approved drug compounds. In-silico docking potential of Venetoclax was better than the standard γ-secretase inhibitor RO4929097. Also, the molecular dynamics simulations of 200 ns confirmed the stability of the Venetoclax-γ-secretase complex. These findings suggest that the use of Venetoclax represents a potential γ-secretase inhibitor in breast cancer stem cells. Eventually, the in vitro and clinical evaluation will be needed to confirm the potential chemopreventive and treatment effects of Venetoclax against breast cancer and breast cancer stem cells. Venetoclax appeared as the most promising drug of the 1615 FDA-approved drugs. Our in-silico findings suggest that Venetoclax may act as a γ-secretase inhibitor against the Notch signaling pathway in breast cancer stem cells.

Topics: Amyloid Precursor Protein Secretases; Breast Neoplasms; Drug Repositioning; Female; Humans; Neoplastic Stem Cells; Receptors, Notch; Signal Transduction

Breast cancer resistance protein (ABCG2) is a human ATP-binding cassette (ABC) that plays a paramount role in multidrug resistance (MDR) in cancer therapy. The discovery of ABCG2 inhibitors could assist in designing unprecedented therapeutic strategies for cancer treatment. There is as yet no approved drug targeting ABCG2, although a large number of drug candidates have been clinically investigated. In this work, binding affinities of 181 drug candidates in clinical-trial or investigational stages as ABCG2 inhibitors were inspected using in silico techniques. Based on available experimental data, the performance of AutoDock4.2.6 software was first validated to predict the inhibitor-ABCG2 binding mode and affinity. Combined molecular docking calculations and molecular dynamics (MD) simulations, followed by molecular mechanics-generalized Born surface area (MM-GBSA) binding energy calculations, were then performed to filter out the studied drug candidates. From the estimated docking scores and MM-GBSA binding energies, six auspicious drug candidates-namely, pibrentasvir, venetoclax, ledipasvir, avatrombopag, cobicistat, and revefenacin-exhibited auspicious binding energies with value < -70.0 kcal/mol. Interestingly, pibrentasvir, venetoclax, and ledipasvir were observed to show even higher binding affinities with the ABCG2 transporter with binding energies of < -80.0 kcal/mol over long MD simulations of 100 ns. The stabilities of these three promising candidates in complex with ABCG2 transporter were demonstrated by their energetics and structural analyses throughout the 100 ns MD simulations. The current study throws new light on pibrentasvir, venetoclax, and ledipasvir as curative options for multidrug resistant cancers by inhibiting ABCG2 transporter.

Topics: ATP Binding Cassette Transporter, Subfamily G, Member 2; Benzimidazoles; Binding Sites; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Databases, Chemical; Drug Discovery; Drug Resistance, Neoplasm; Female; Fluorenes; Humans; Hydrogen Bonding; Molecular Docking Simulation; Molecular Dynamics Simulation; Neoplasm Proteins; Protein Binding; Pyrrolidines; Sulfonamides; Thermodynamics

Antiapoptotic and proapoptotic BCL-2 protein family members regulate mitochondrial apoptotic pathway. Small molecule inhibitors of antiapoptotic BCL-2 proteins including BCL-2-specific inhibitor ABT-199 (Venetoclax) are in clinical development. However, the efficiency of ABT-199 as a single agent in solid tumors is limited. We performed a high-throughput RNAi kinome screen targeting 691 kinases to identify potentially targetable kinases to enhance ABT-199 response in breast cancer cells. Our studies identified Wee1 as the primary target kinase to overcome resistance to ABT-199. Depletion of Wee1 by siRNA-mediated knockdown or inhibition of Wee1 by the small molecule Wee1 inhibitor AZD1775 sensitized SKBR3, MDA-MB-468, T47D and CAMA-1 breast cancer cells to ABT-199 along with decreased MCL1. BH3-only proteins PUMA and BIM functionally contribute to apoptosis signaling following co-targeting BCL-2 and Wee1. Suppression of Wee1 function increased mitochondrial cell death priming. Furthermore, we found that Wee1 inhibition altered MCL1 phosphorylation and protein stability, which led to HUWE1-mediated MCL1 degradation. Our findings suggest that Wee1 inhibition can overcome resistance to ABT-199 and provide a rationale for further translational investigation of BCL-2 inhibitor/Wee1 inhibitor combination in breast cancer.

Topics: Antineoplastic Agents; Apoptosis; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Cell Cycle Proteins; Cell Proliferation; Female; Gene Expression Regulation, Enzymologic; Gene Expression Regulation, Neoplastic; High-Throughput Screening Assays; Humans; Protein Kinase Inhibitors; Protein Kinases; Protein-Tyrosine Kinases; RNA Interference; Sulfonamides; Tumor Cells, Cultured

Breast cancer is the most frequent type of cancer and the major cause of mortality in women. The rapid development of various therapeutic options has led to the improvement of treatment outcomes; nevertheless, one-third of estrogen receptor (ER)-positive patients relapse due to cancer cell acquired resistance. Here, we use dynamic BH3 profiling (DBP), a functional predictive assay that measures net changes in apoptotic priming, to find new effective treatments for ER+ breast cancer. We observed anti-apoptotic adaptations upon treatment that pointed to metronomic therapeutic combinations to enhance cytotoxicity and avoid resistance. Indeed, we found that the anti-apoptotic proteins BCL-xL and MCL-1 are crucial for ER+ breast cancer cells resistance to therapy, as they exert a dual inhibition of the pro-apoptotic protein BIM and compensate for each other. In addition, we identified the AKT inhibitor ipatasertib and two BH3 mimetics targeting these anti-apoptotic proteins, S63845 and A-1331852, as new potential therapies for this type of cancer. Therefore, we postulate the sequential inhibition of both proteins using BH3 mimetics as a new treatment option for refractory and relapsed ER+ breast cancer tumors.

Topics: Antineoplastic Agents; Antineoplastic Combined Chemotherapy Protocols; Apoptosis; bcl-X Protein; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Cell Line, Tumor; Drug Resistance, Neoplasm; Drug Synergism; Estrogen Receptor alpha; Everolimus; Female; Fulvestrant; Gene Expression Regulation, Neoplastic; Humans; MCF-7 Cells; Myeloid Cell Leukemia Sequence 1 Protein; Piperazines; Protein Isoforms; Proto-Oncogene Proteins c-akt; Pyridines; Pyrimidines; Signal Transduction; Sulfonamides; Thiazoles; Thiophenes

As a proapoptotic death effect domain (DED)-containing protein, DED-containing DNA-binding protein (DEDD) has been demonstrated to inhibit tumor growth, invasion and metastasis in our previous studies. Here, we demonstrated that knockdown of DEDD in MCF-7 cells resulted in characteristic drug resistance to doxorubicin and paclitaxel, and overexpression of DEDD in MDA-MB-231 cells increased their sensitivity to doxorubicin and paclitaxel. The expression levels of DEDD were positively correlated with Bcl-2 in breast cancer cell lines as well as in human breast cancer tissue. Knockdown of DEDD downregulated the transcriptional activity of the bcl-2 gene and shortened the time for Bcl-2 degradation. DEDD interacts with and stabilizes Bcl-2, and breast cancer cells with low DEDD expression were more sensitive to treatment with a BH3 mimetic, ABT-199, than were those with high DEDD expression. In total, our findings highlight a new strategy for treating breast cancer with no/low DEDD expression by targeting Bcl-2 with the BH3 mimetic ABT-199.

Topics: Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Death Domain Receptor Signaling Adaptor Proteins; DNA-Binding Proteins; Doxorubicin; Drug Resistance, Neoplasm; Female; Gene Expression Regulation, Neoplastic; Gene Knockdown Techniques; Humans; MCF-7 Cells; Paclitaxel; Proto-Oncogene Proteins c-bcl-2; RNA, Messenger; Sulfonamides; Transcription, Genetic

Standard treatment for estrogen receptor-positive metastatic breast cancer involves antiestrogen therapy used alone or in combination with inhibitors of CDK4/6 or mTOR; this approach works mechanistically by eliciting and reinforcing cell-cycle arrest. In this issue, Lok and colleagues diverge from this paradigm by combining the BCL2 inhibitor venetoclax with tamoxifen in a phase Ib clinical trial, building on preclinical work to demonstrate that targeting apoptosis could represent a promising new strategy in the treatment of breast cancer.

Topics: Apoptosis; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Humans; Proto-Oncogene Proteins c-bcl-2; Receptors, Estrogen; Sulfonamides; Tamoxifen

Breast cancer cells with stem cell properties play an important role in tumor progression and thus are key targets for therapy. Here, we show that combined Bcl-2/Src inhibition synergize to deplete stem-like cells. While Src inhibition increases pro-apoptotic PUMA, we find that a significant amount interacts with Bcl-2 and Bcl-xL, promoting resistance to cell death. Consistent with this, the clinically-approved Bcl-2 selective drug venetoclax was sufficient to overcome resistance by preventing PUMA/Bcl-2 binding, enhancing apoptosis. This effect was specific to stem-like breast cancer cells as there was no effect on luminal or basal-like cell types. In contrast, the Mcl-1 inhibitor S63845 potently targeted basal-like, but not stem-like cells, highlighting dependency on distinct sentinel Bcl-2 family members. Our findings reveal Bcl-2/Src inhibition as a superior therapy to target stemness, providing a foundation for a potential personalized strategy to reduce breast cancer progression.

Topics: Antineoplastic Combined Chemotherapy Protocols; Apoptosis; Apoptosis Regulatory Proteins; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Dasatinib; Female; Humans; MCF-7 Cells; Neoplastic Stem Cells; Protein Kinase Inhibitors; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-bcl-2; Signal Transduction; Spheroids, Cellular; src-Family Kinases; Sulfonamides

The irreversible ERBB1/2/4 inhibitor, neratinib, down-regulates the expression of ERBB1/2/4 as well as the levels of MCL-1 and BCL-XL. Venetoclax (ABT199) is a BCL-2 inhibitor. At physiologic concentrations neratinib interacted in a synergistic fashion with venetoclax to kill HER2 + and TNBC mammary carcinoma cells. This was associated with the drug-combination: reducing the expression and phosphorylation of ERBB1/2/3; in an eIF2α-dependent fashion reducing the expression of MCL-1 and BCL-XL and increasing the expression of Beclin1 and ATG5; and increasing the activity of the ATM-AMPKα-ULK1 S317 pathway which was causal in the formation of toxic autophagosomes. Although knock down of BAX or BAK reduced drug combination lethality, knock down of BAX and BAK did not prevent the drug combination from increasing autophagosome and autolysosome formation. Knock down of ATM, AMPKα, Beclin1 or over-expression of activated mTOR prevented the induction of autophagy and in parallel suppressed tumor cell killing. Knock down of ATM, AMPKα, Beclin1 or cathepsin B prevented the drug-induced activation of BAX and BAK whereas knock down of BID was only partially inhibitory. A 3-day transient exposure of established estrogen-independent HER2 + BT474 mammary tumors to neratinib or venetoclax did not significantly alter tumor growth whereas exposure to [neratinib + venetoclax] caused a significant 7-day suppression of growth by day 19. The drug combination neither altered animal body mass nor behavior. We conclude that venetoclax enhances neratinib lethality by facilitating toxic BH3 domain protein activation via autophagy which enhances the efficacy of neratinib to promote greater levels of cell killing.

Topics: Animals; Antineoplastic Combined Chemotherapy Protocols; Autophagy; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Cell Line, Tumor; Drug Synergism; ErbB Receptors; Female; Humans; Mice; Mice, Nude; Proto-Oncogene Proteins c-bcl-2; Quinolines; Receptor, ErbB-2; Receptor, ErbB-4; Receptors, Estrogen; Sulfonamides; Treatment Outcome; Xenograft Model Antitumor Assays

Despite the development of novel targeted therapies, de novo or acquired chemoresistance remains a significant factor for treatment failure in breast cancer therapeutics. Neratinib and dacomitinib are irreversible panHER inhibitors, which block their autophosphorylation and downstream signaling. Moreover, neratinib and dacomitinib have been shown to activate cell death in HER2-overexpressing cell lines. Here we showed that increased MCL1 and decreased BIM and PUMA mediated resistance to neratinib in ZR-75-30 and SKBR3 cells while increased BCL-XL and BCL-2 and decreased BIM and PUMA promoted neratinib resistance in BT474 cells. Cells were also cross-resistant to dacomitinib. BH3 profiles of HER2+ breast cancer cells efficiently predicted antiapoptotic protein dependence and development of resistance to panHER inhibitors. Reactivation of ERK1/2 was primarily responsible for acquired resistance in SKBR3 and ZR-75-30 cells. Adding specific ERK1/2 inhibitor SCH772984 to neratinib or dacomitinib led to increased apoptotic response in neratinib-resistant SKBR3 and ZR-75-30 cells, but we did not detect a similar response in neratinib-resistant BT474 cells. Accordingly, suppression of BCL-2/BCL-XL by ABT-737 was required in addition to ERK1/2 inhibition for neratinib- or dacomitinib-induced apoptosis in neratinib-resistant BT474 cells. Our results showed that different mitochondrial apoptotic blocks mediated acquired panHER inhibitor resistance in HER2+ breast cancer cell lines as well as highlighted the potential of BH3 profiling assay in prediction of panHER inhibitor resistance in breast cancer cells.

Topics: bcl-X Protein; Benzothiazoles; Biphenyl Compounds; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Cell Line, Tumor; Cell Proliferation; Cell Survival; Drug Resistance, Neoplasm; Female; Gene Expression Regulation, Neoplastic; Humans; Indazoles; Isoquinolines; Nitrophenols; Piperazines; Protein Kinase Inhibitors; Proto-Oncogene Proteins c-bcl-2; Quinazolinones; Quinolines; Receptor, ErbB-2; Sulfonamides

Venetoclax (ABT-199) and idasanutlin (RG7388) are efficient anticancer drugs targeting two essential apoptosis markers, Bcl-2 and MDM2, respectively. Recent studies have shown that the combination of these two drugs leads to remarkable enhancement of anticancer efficacy, both in vitro and in vivo. In an attempt to disclose the relationships of their protein targets, competitive affinity-based proteome profiling coupled with bioimaging was employed to characterize their protein targets in the same cancer cell line and tumor tissue. A series of protein hits, including ITPR1, GSR, RER1, PDIA3, Apoa1, and Tnfrsf17 were simultaneously identified by pull-down/LC-MS/MS with the two sets of affinity-based probes. Dual imaging was successfully carried out, with the simultaneous detection of Bcl-2 and MDM2 expression in various cancer cells. This could facilitate the novel diagnostic and therapeutic strategies of dual targeting of Bcl-2/MDM2.

Topics: Antineoplastic Agents; Apoptosis; Biomarkers, Tumor; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Cell Proliferation; Dose-Response Relationship, Drug; Drug Screening Assays, Antitumor; Fluorescent Dyes; Humans; MCF-7 Cells; Models, Molecular; Molecular Structure; Optical Imaging; para-Aminobenzoates; Proteome; Proto-Oncogene Proteins c-bcl-2; Proto-Oncogene Proteins c-mdm2; Pyrrolidines; Structure-Activity Relationship; Sulfonamides

Our previous studies showed that MYB is required for proliferation of, and confers protection against apoptosis on, estrogen receptor-positive (ER(+ve)) breast cancer cells, which are almost invariably also MYB(+ve). We have also shown that MYB expression in ER(+ve) breast cancer cells is regulated at the level of transcriptional elongation and as such, is suppressed by CDK9i. Here we examined the effects of CDK9i on breast cancer cells and the involvement of MYB in these effects. ER(+ve) breast cancer cell lines including MCF-7 were much more sensitive (> 10 times) to killing by CDK9i than ER(-ve)/MYB(-ve) cells. Moreover, surviving cells showed a block at the G2/M phase of the cell cycle. Importantly, ectopic MYB expression conferred resistance to apoptosis induction, cell killing and G2/M accumulation. Expression of relevant MYB target genes including BCL2 and CCNB1 was suppressed by CDK9 inhibition, and this too was reversed by ectopic MYB expression. Nevertheless, inhibition of BCL2 alone either by MYB knockdown or by ABT-199 treatment was insufficient for significant induction of apoptosis. Further studies implied that suppression of MCL-1, a well-documented target of CDK9 inhibition, was additionally required for apoptosis induction, while maximal levels of apoptosis induced by CDK9i are likely to also involve inhibition of BCL2L1 expression. Taken together these data suggest that MYB regulation of BCL2 underlies the heightened sensitivity of ER(+ve) compared to ER(-ve) breast cancer cells to CDK9 inhibition, and that these compounds represent a potential therapeutic for ER(+ve) breast cancers and possibly other MYB-dependent cancers.

Topics: Antineoplastic Agents; Apoptosis; bcl-X Protein; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Cell Line, Tumor; Cell Proliferation; Cell Survival; Cyclin B1; Cyclin E; Cyclin-Dependent Kinase 9; Female; G2 Phase Cell Cycle Checkpoints; Humans; MCF-7 Cells; Myeloid Cell Leukemia Sequence 1 Protein; Oncogene Proteins; Proto-Oncogene Proteins c-bcl-2; Proto-Oncogene Proteins c-myb; Receptors, Estrogen; RNA Interference; RNA, Small Interfering; Sulfonamides

The prosurvival protein BCL-2 is frequently overexpressed in estrogen receptor (ER)-positive breast cancer. We have generated ER-positive primary breast tumor xenografts that recapitulate the primary tumors and demonstrate that the BH3 mimetic ABT-737 markedly improves tumor response to the antiestrogen tamoxifen. Despite abundant BCL-XL expression, similar efficacy was observed with the BCL-2 selective inhibitor ABT-199, revealing that BCL-2 is a crucial target. Unexpectedly, BH3 mimetics were found to counteract the side effect of tamoxifen-induced endometrial hyperplasia. Moreover, BH3 mimetics synergized with phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitors in eliciting apoptosis. Importantly, these two classes of inhibitor further enhanced tumor response in combination therapy with tamoxifen. Collectively, our findings provide a rationale for the clinical evaluation of BH3 mimetics in therapy for breast cancer.

Topics: Animals; Biphenyl Compounds; Breast Neoplasms; Bridged Bicyclo Compounds, Heterocyclic; Female; Humans; Mice; Mice, SCID; Nitrophenols; Phosphoinositide-3 Kinase Inhibitors; Piperazines; Proto-Oncogene Proteins c-bcl-2; Receptors, Estrogen; Sulfonamides; TOR Serine-Threonine Kinases; Xenograft Model Antitumor Assays