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Multiple myeloma (MM) is marked by several genetic abnormalities, including chromosome translocation t(11;14). Overexpression of anti-apoptotic BCL-2 in t(11;14) MM promotes disease progression, prompting clinical use of the BH3 mimetic and BCL-2 inhibitor venetoclax in combination with proteasome inhibitor therapy. Despite high initial response rates and prolonged progression-free survival, patients commonly relapse. To delineate mechanisms contributing to acquired drug resistance we modeled responses to venetoclax in two highly sensitive MM cell lines (KMS27 and KMS-12PE). Colonies generated from a surviving cell were cultured in high-dose venetoclax to generate monoclonal drug-tolerant expanded persister (DTEP) clones. To determine whether venetoclax resistance in DTEP clones is mediated by transcriptional adaptation via genomic or epigenomic regulation and transcriptional reprogramming, we conducted whole-genome sequencing (WGS) and RNA-seq of the clones. WGS analysis did not show significant differences between parental and resistant clones, but transcriptomic analysis showed shared and unique transcriptome signatures in DTEP clones. Gene set enrichment analysis of the common significantly modulated genes in resistant clones revealed that PKA-ERK-CREB and K-Ras pathway genes were significantly upregulated, whereas apoptotic genes were downregulated in resistant clones compared to parental cells. Importantly, ectopically expressed ERK in venetoclax-sensitive cells conferred a resistant phenotype that was rescued using two specific ERK inhibitors in DTEP clones. These data confirm a key role for ERK activation in acquired venetoclax resistance. Resistant clones were further characterized by reduced mitochondrial priming assessed by dynamic BH3 profiling, with altered expression of anti-apoptotic regulators including MCL-1, BCL-xL, and BCL-W and the replaced BCL-2: BIM complex by both MCL-1 and BCL-xL. Because these data suggested a functional substitution between anti-apoptotic BCL-2 family members in cells with acquired resistance to venetoclax, we next evaluated if MCL-1 or BCL-xL are codependent in MM cells that are insensitive or resistant to venetoclax. Simultaneous inhibition of MCL-1 (via S63845) or BCL-xL (via A155463) and BCL-2 (via venetoclax) increased BIM release and enhanced cell death in resistant clones (vs single agents), with combination index values < 0.3 in all doses. Upregulation of BCL-xL or MCL-1 in MM cells also mediated primary venetoclax resistance independent of genetic hallmarks (e.g. t [11;14]-translocated cells). Thus, simultaneous inhibition of MCL-1 or BCL-xL and BCL-2 triggered synergistic cytotoxicity in MM cell lines intrinsically resistant to venetoclax. These data suggest that combined inhibition of BCL-2 and BCL-xL may overcome venetoclax resistance. However, the dependence of BCL-xL in mature platelets had triggered thrombocytopenia for patients under therapy using BCL-xL inhibitor. To further explore the potential clinical application of targeting BCL-xL, we employed novel BCl-2/BCL-xL dual inhibitor, BH3 mimetic pelcitoclax (APG-1252). Using pro-drug strategy for design, pelcitoclax has limited cell permeability during circulation, and was converted to a more potent metabolite APG-1252-M1 in tumors/tissues. APG-1252-M1 was thus used for in vitro cell based assays. We discovered that APG-1252-M1 induced cytotoxicity in MM cell lines intrinsically resistant to venetoclax (regardless of genetic background or BCL-2:BCL-xL ratio) and also significantly reduced MM cell viability in clones with acquired venetoclax resistance, overcoming ERK activation and decreasing BIM sequestration by BCL-xL. In vivo study using pelcitoclax is ongoing and will be presented at the meeting. In conclusion, we report that venetoclax resistance in MM evolves from outgrowth of persister clones displaying activation of the ERK pathway and a shift in mitochondrial dependency towards BCL-xL, which can potentially be effectively targeted via the novel BCL-2/BCL-xL inhibitor pelcitoclax (APG-1252), which is currently in clinical investigation for solid tumors (NCT03080311). Disclosures Deng: Ascentage Pharma Group: Current Employment. Zhai: Ascentage Pharma Group Inc.: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests, Patents & Royalties, Research Funding; Ascentage Pharma (Suzhou) Co., Ltd.: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests, Patents & Royalties, Research Funding. Anderson: Sanofi-Aventis: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Millenium-Takeda: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Scientific Founder of Oncopep and C4 Therapeutics: Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company; AstraZeneca: Membership on an entity's Board of Directors or advisory committees; Mana Therapeutics: Membership on an entity's Board of Directors or advisory committees. Munshi: Novartis: Consultancy; Janssen: Consultancy; Adaptive Biotechnology: Consultancy; Takeda: Consultancy; Celgene: Consultancy; Bristol-Myers Squibb: Consultancy; Karyopharm: Consultancy; Oncopep: Consultancy, Current equity holder in publicly-traded company, Other: scientific founder, Patents & Royalties; Amgen: Consultancy; Abbvie: Consultancy; Legend: Consultancy; Pfizer: Consultancy.

Natural killer/T-cell lymphoma (NK/TCL) is one of the most common subtypes (10.4%) of peripheral T-cell lymphoma, which in turn accounts for 10% to 15% of all cases of non-Hodgkin lymphoma. NK/TCL occurs more frequently in Asia and Latin America than other regions, and although associated with Epstein-Barr virus (EBV) infection, NK/TCL has an unclear pathogenesis of genetic and molecular alterations. Asparaginase-based chemotherapy regimens are frequently used, typically with unsatisfactory outcomes. Novel targeted therapies, such as histone deacetylase (HDAC) inhibitors and programmed death-1 antibodies, are reported effective either as single agents or in combination with other agents. Studies have also shown that EBV-positive NK/TCL cell lines are B-cell lymphoma-extra large (BCL-xL) dependent. Other preclinical experiments have shown that BH3-mimetic drugs targeting BCL-xL-induced cell death in NK/TCL cell lines were effective in NK/TCL xenograft models. BCL-xL inhibitors have shown narrow therapeutic windows in clinical trials because of dose-limiting on-target thrombocytopenia. Pelcitoclax (APG-1252) is a novel dual BCL-2/BCL-xL inhibitor under clinical development for solid tumors. As a result of its unique prodrug design, APG-1252 can overcome the undesired platelet toxicity. This study evaluated the potential antitumor effect of APT-1252 in preclinical models of NK/TCL. Cell-based antiproliferation studies showed activity of APG-1252 and its more potent metabolite APG-1252-M1 toward NK/TCL cell lines that overexpressed BCL-xL. Half-maximal inhibitory concentrations (IC 50) for APG-1252 in SNK-1, SNK-6, and SNK-8 (EBV-positive NK/TCL) cell lines were 2.652 ± 2.606 µM, 1.568 ± 1.109 µM, and 0.557 ± 0.383 µM, respectively. Corresponding values for APG-1252-M1 were 0.133 ± 0.056 µM, 0.064 ± 0.014 µM, and 0.020 ± 0.008 µM, respectively. Mechanistic studies demonstrated that APG-1252 and APG-1252-M1 disrupted the complex of BCL-xL/BCL-2-associated X protein (Bax) and BCL-xL/BCL-2 homologous antagonist killer protein (Bak) in SNK-6 cells, thereby liberating these proapoptotic proteins and further activating downstream apoptosis pathways by cleaving poly-ADP ribose polymerase-1 (PARP-1) and caspase-3. In an SNK-6 xenograft model, administration of APG-1252 at 65 mg/kg and 100 mg/kg either twice or once weekly resulted in significant antitumor effects, with tumor growth rate (T/C%) values ranging from 13.7% to 30.7%. Furthermore, the combination of APG-1252 with HDAC inhibitor chidamide or DDGP (dexamethasone, cisplatin, gemcitabine, and pegaspargase) chemotherapy demonstrated synergistic effects. Pharmacokinetic assessment in mice showed that APG-1252 had a long half-life in plasma (127 hours) and tumor tissues (25.2 hours), justifying intermittent dosing schedules used in vivo. Importantly, the transformation of APG-1252 to APG-1252-M1 was 16 times higher in tumor tissues compared to plasma (22% vs. 1.3%) after administration of APG-1252, thereby suggesting that APG-1252 can reduce platelet toxicity caused by APG-1252-M1 in plasma. In conclusion, APG-1252 has promising antitumor effects in NK/TCL, either as a single agent or in combination with an HDAC inhibitor or chemotherapy. These findings provide evidence to further evaluate APG-1252 as a potential treatment for NK/TCL. Disclosures Wang: Ascentage Pharma Co., Ltd., Shanghai: Current Employment, Current equity holder in publicly-traded company. Liang: Ascentage Pharma Group Inc.: Current Employment, Current equity holder in publicly-traded company. Ming: Ascentage Pharma Co., Ltd., Shanghai: Current Employment, Current equity holder in publicly-traded company. Rui: Ascentage Pharma Co., Ltd., Taizhou: Current Employment, Current equity holder in publicly-traded company. Tang: Ascentage Pharma Co., Ltd., Taizhou: Current Employment, Current equity holder in publicly-traded company. LV: Ascentage Pharma Co., Ltd., Shanghai: Current Employment, Current equity holder in publicly-traded company. Ge: Ascentage Pharma Co., Ltd., Shanghai: Current Employment, Current equity holder in publicly-traded company. Zhang: Ascentage Pharma Co., Ltd., Taizhou: Current Employment, Current equity holder in publicly-traded company. Wang: Ascentage Pharma Co., Ltd., Taizhou: Current Employment, Current equity holder in publicly-traded company. Shang: Ascentage Pharma Co., Ltd., Shanghai: Current Employment, Current equity holder in publicly-traded company. Yang: Ascentage Pharma (Suzhou) Co., Ltd.: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests, Patents & Royalties, Research Funding. Zhai: Ascentage Pharma Group Inc.: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests, Patents & Royalties, Research Funding; Ascentage Pharma (Suzhou) Co., Ltd.: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests, Patents & Royalties, Research Funding.

3509 Background: Targeting Bcl-2/Bcl-xL proteins is considered as an important approach for anticancer drug development. Palcitoclax (APG-1252) was being developed to reduce on-target platelet toxicity without diminishing antitumor potency. Methods: The phase 1 study was to evaluate the safety/tolerability, pharmacokinetics (PK), and preliminary efficacy (assessed per RECIST 1.1) of palcitoclax in US patients with metastatic small-cell lung cancer (SCLC) or other solid tumors (NCT03080311). A standard “3+3” design was applied to the dose-escalation stage. Palcitoclax was administered IV infusion for 30 minutes, twice a week (BIW) or once a week (QW) for 3 weeks in a 28-day cycle. Once the maximum tolerated dose / recommended phase 2 dose (MTD/RP2D) was determined, additional patients were treated in a dose-expansion stage. Results: The dose-escalation phase has been completed with 42 patients (31 on BIW and 11 on QW) who received palcitoclax at 8 dose cohorts ranging 10 mg - 400 mg. Most adverse events (AEs) were grade 1 or 2 (G1 or G2), and 26.2% patients had ≥ G3 TRAEs. The most common TRAEs were platelet count decreased (14.3%), aspartate aminotransferase increased (9.5%), and alanine aminotransferase increased (7.1%). Rapid platelet drop was observed in patients treated at 320 mg and 400 mg, which was transient and resolved rapidly within 2-6 days. Palcitoclax at 240 mg once weekly was determined to be MTD/RP2D. Of 36 efficacy-evaluable patients, 3 patients with SCLC, neuroendocrine prostate cancer, and ovarian cancer respectively achieved partial response (PR) and 8 patients had stable disease (SD) as their best overall response. One patient with SCLC had a PR that lasted over 21 cycles. Preliminary PK analyses showed that Cmax and AUC were approximately dose proportional over the range of 10 mg to 320 mg following the IV infusion on Day 1, with a mean T1/2 of 3.0-13.0 hours. Conclusions: Palcitoclax is safe and well tolerated, with a favorable platelet toxicity profile. Its promising antitumor effect supports its further development in combination therapies for treatment of patients with SCLC and other solid tumors. Clinical trial information: NCT03080311 .

Our previously reported Bcl-2/Bcl-xL inhibitor, 4, effectively inhibited tumor growth but failed to achieve complete regression in vivo. We have now performed extensive modifications on its pyrrole core structure, which has culminated in the discovery of 32 (BM-1074). Compound 32 binds to Bcl-2 and Bcl-xL proteins with Ki values of < 1 nM and inhibits cancer cell growth with IC50 values of 1-2 nM in four small-cell lung cancer cell lines sensitive to potent and specific Bcl-2/Bcl-xL inhibitors. Compound 32 is capable of achieving rapid, complete and durable tumor regression in vivo at a well-tolerated dose-schedule. Compound 32 is the most potent and efficacious Bcl-2/Bcl-xL inhibitor reported to date.

OBJECTIVE: Repurposing represents a promising approach to safely accelerate the clinical application of therapeutics with anti-cancer activity. In this study, we examined whether inhibition of the anti-apoptotic Bcl-2 family proteins Bcl-2 and Bcl-xL enhances the biological effects of the repurposed CUSP9 regimen in an in vitro setting of glioblastoma. METHODS: We applied MTT assays to assess cellular proliferation. Annexin V/PI and TMRE staining were used to examine apoptosis. Western blotting, RT-PCR and specific knockdown experiments using siRNA were employed to examine molecular mechanisms of action. RESULTS: Bcl-2/Bcl-xL inhibition by the BH3 mimetic ABT263, yielded synergistic anti-proliferative effects across a wide panel of established and primary cultured glioblastoma cells when combined with CUSP9 which had been reduced to only one tenth of its original concentration (CUSP9 1/10). The combination treatment also led to enhanced apoptosis with loss of mitochondrial membrane potential and activation of caspases. On the molecular level, CUSP9 1/10 counteracted ABT263-mediated upregulation of Mcl-1. Silencing of Mcl-1 enhanced ABT263-mediated apoptosis, indicating that Mcl-1 is crucial for the induction of cell death conveyed by the combination treatment. Levels of Mcl-1 mRNA were not decreased following combination therapy, and co-treatment with cycloheximide showed reduced protein stability, pointing towards a post-translational mechanism of action. CONCLUSION: These data suggest that Bcl-2/Bcl-xL inhibition enhances the susceptibility of glioblastoma cells towards CUSP9, allowing dramatic dose reduction and potentially decreased toxicity when applied clinically. A clinical trial involving the original CUSP doses (CUSP9v3) is currently ongoing in our institution (NCT02770378). The Bcl-2/Bcl-xL inhibitor ABT263 is in clinical trials and might represent a valuable adjunct to the original CUSP.

Background: Intrinsic resistance to agents targeting the PI3K/AKT/mTOR pathway has been commonly encountered in clinical trials of patients with PIK3CA mutant colorectal cancer (CRC). Upregulation of antiapoptotic signaling has been proposed as a mechanism of resistance to these therapies, including upregulation of BCL-2 and BCL-xL. To investigate if inhibition of BCL-2 family members would sensitize Pik3ca mutant cancers to MTORC1/2 inhibition, treatment studies were performed with TAK-228 (MTORC1/2 inhibitor), BEZ235 (dual PI3K/mTOR inhibitor), navitoclax (ABT-263; BCL-2, BCL-xL and BCL-w inhibitor) and the combination of navitoclax with either TAK-228 or BEZ235. Methods: Therapeutic investigations with 200 nM TAK-228 or 200 nM BEZ235 and 250 nM navitoclax were performed in murine CRC spheroids with loss of APC and a constitutively active PI3K. Images were taken both pre- and post-treatment and changes in spheroid diameter were measured. Parallel treatment studies were performed on patient-derived organotypic CRC spheroids. Additionally, treatment studies were performed in vivo using a novel transgenic mouse model carrying Apc and Pik3ca mutations. The mice were treated with the combination of BEZ235 and navitoclax or with a single agent alone for 7 consecutive days. Results: Treatment of CRC spheroids with TAK-228 resulted in a reduction of sphere size by 16% while control treated spheres increased by 77% of their size at day 0. No response was seen with navitoclax treatment alone. A profound synergistic treatment response was observed with the combination of TAK-228 and navitoclax (reduction of 26%, p Conclusion: Synergistic activity was seen with the combination of TAK-228 or BEZ235 and navitoclax. This combination deserves further study in future clinical trials. Citation Format: Stephanie L. Fricke, Susan N. Payne, Cheri A. Pasch, Demetra P. Korkos, Gioia Sha, Alex E. Yueh, Christopher Babiarz, Linda Clipson, Kristina A. Matkowskyj, Michael A. Newton, Dustin A. Deming. MTORC1/2 inhibition in combination with BCL-2/BCL-xL inhibition in APC and PIK3CA mutant colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2936.

2594Background: We have developed a unique strategy to tactically reduce on-target platelet toxicity with APG-1252, a novel dual Bcl-2/Bcl-xL inhibitor, while maintaining strong in vivo antitumor a...

PurposeUveal melanoma (UM) is associated with a high risk of metastases and lack of efficient therapies. Reduced capacity for apoptosis induction by chemotherapies is one obstacle to efficient treatments. Human UM is characterized by high expression of the anti-apoptotic protein Bcl-2. Consequently, regulators of apoptosis such as Bcl-2 family inhibitors may constitute an attractive approach to UM therapeutics. In this aim, we have investigated the efficacy of the Bcl-2/Bcl-XL inhibitor S44563 on 4 UM Patient-Derived Xenografts (PDXs) and derived-cell lines.Experimental designFour well characterized UM PDXs were used for in vivo experiments. S44563 was administered alone or combined with fotemustine either concomitantly or after the alkylating agent. Bcl-2, Bcl-XL, and Mcl-1 expressions after S44563 administration were evaluated by immunohistochemistry (IHC).ResultsS44563 administered alone by at 50 and 100 mg/kg i.p. induced a significant tumour growth inhibition in only one xenograft model with a clear dose effect. However, when S44563 was concomitantly administered with fotemustine, we observed a synergistic activity in 3 out of the 4 tested models. In addition, S44563 administered after fotemustine induced a tumour growth delay in 2 out of 3 tested xenografts. Finally, IHC analyses showed that Bcl-2, Bcl-XL, and Mcl-1 expression were not modified after S44563 administration.ConclusionThe novel anti-apoptotic experimental compound S44563, despite a relative low efficacy when administered alone, increased the efficacy of fotemustine in either concomitant or sequential combinations or indeed subsequent to fotemustine. These data support further exploration of potential therapeutic effect of Bcl-2/Bcl-xl inhibition in human UM.

Purpose Nearly half of primary Uveal melanoma (UM) metastasizes in liver, but there are currently no effective therapies. Human UM are characterized by a high expression of Bcl-2, ranging between 50% and 100%. This observation has been confirmed in our panel of 16 human UM xenografts obtained from patient’s tumors (Nemati et al, CCR 2010). We have investigated the efficacy of the new Bcl-2/Bcl-XL inhibitor S44563 on 4 primary human UM xenografts. Methods Four well characterized primary human UM xenografts were used. S44563 (50 or 100 mg/kg, days 1-5/8-12/22-26/29-33) was administered IP alone or combined with fotemustine, concomitantly (15 or 30 mg/kg days 1 and 22), or after chemotherapy (100 mg/kg, days 43-47/50-54/64-68/71-75). Tumor Growth Inhibition (TGI) was calculated to measure the efficiency of drugs. Bcl-2, Bcl-XL, and Mcl-1 expressions were determined by immunohistochemistry (IHC). Results S44563 administered alone induced a moderate TGI of about 50% in 1 model (MP41). When combined S44563 to fotemustine, we observed a synergistic activity in 2 models (MP77 and MM66), without impact on the proportion of complete remission. Finally, when S44563 was concomitantly and/or administered after fotemustine, we found a delay of tumor growth in 2 among the 3 tested xenografts (MP77, and MM26). IHC analyses showed that Bcl-2, Bcl-XL, and Mcl-1 expressions were not modified after S44563 administration. Conclusion We have shown that S44563 increased the efficacy of chemotherapy in concomitant combination or after fotemustine. Such preliminary results underline the therapeutic potential of this new Bcl-2/Bcl-xl inhibitor in human UM.

Commentary to:Bcl2/bcl-xl Inhibitor Engenders Apoptosis and Increases Chemo-sensitivity In MesotheliomaXiaobo Cao, Charles Rodarts, Lidong Zhang, Clinton D. Morgan, James Littlejohn and W. Roy SmytheCancer Biology & Therapy Volume: 6 | Issue: 2 | Pages: 246 - 252

BACKGROUND. Autophagy is a mechanism of cellular destruction whereby cytoplasmic proteins and organelles are sequestered in vacuoles and delivered to lysosomes for degradation and recycling. Autophagy is not always pro-death but can also be pro-survival under conditions of cellular stress. Autophagy and apoptosis can be regulated by Bcl-2 family proteins. We determined whether the anti-neoplastic agent celecoxib can induce autophagy and/or apoptosis, and if a small molecule Bcl-2/Bcl-xL antagonist, e.g., ABT-737, can enhance either/both processes. Furthermore, we determined whether autophagy inhibition can drive colon cancer cells into apoptosis. EXPERIMENTAL METHODS. Human colon cancer cell lines (HCT116, SW480 ± Bcl-2, HT-29 ± Bcl-xL lentiviral shRNA) were incubated with celecoxib (0-120 µM) alone and in combination with ABT-737 (0- 5 µM). Autophagy was studied by LC3I/II expression (Western blotting) and using a GFP-LC3B chimeric plasmid. Additionally, autophagy was imaged by monodansylcadaverine (MDC) staining of autophagic vacuoles and acridine orange staining of acidic vesicles by fluorescent confocal microscopy. Apoptosis was analyzed by caspase cleavage (Western blotting) by annexin V labeling and FACS. RESULTS. Celecoxib triggered conversion of the autophagosome-associated protein light chain 3 (LC3) from a cytosolic (LC3I) to a membrane-bound (LC3II) form as shown by Western blotting, and a change in the localization of the fluorescence pattern of an ectopic GFP-LC3B protein. Celecoxib-induced conversion of LC3 was due to autophagy induction, as shown using the lysosome inhibitor, Bafilomycin A1 that produced an accumulation of LC3II. Celecoxib-induced autophagy and apoptosis were attenuated by ectopic Bcl-2, but were augmented by Bcl-xL knockdown. ABT-737 was shown to enhance celecoxib-induced autophagy as shown by LC3 conversion, acridine orange staining of acidic vesicles, and MDC staining of autophagic vacuoles. Celecoxib also induced apoptosis, as shown by caspase cleavage and annexin V labeling, that was synergistically enhanced by ABT-737. Given that autophagy can be pro-survival under conditions of cellular stress, we determined whether inhibition of autophagy can drive cells into apoptosis. Use of the autophagy inhibitor 3-methyladenine (3-MA) was shown to block LC3 conversion and to enhance celecoxib-induced apoptosis that was further augmented by ABT-737. Similarly, Vps34 or Atg8/LC3 siRNA were shown to enhance caspase cleavage in cells treated with celecoxib plus ABT-737. CONCLUSION. Celecoxib induces autophagy and apoptosis that can both be potentiated by a Bcl-2/ Bcl-xL antagonist. Furthermore, inhibition of autophagy was shown to drive cells into apoptosis, suggesting a novel therapeutic strategy against colon cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4858.

e13578 Background: Paclitaxel, a microtubule stabilizing agent, has been a standard of care for breast cancer. AT-101, is an -/- enantiomer of gossypol, inhibits the Bcl-2 family proteins which contain BH3 domain. We reported previously that Paclitaxel in combination with AT-101 showed strong synergistic cytotoxic and apoptotic effects in human breast cancer cells. In this study, to elucidate the molecular mechanisms involved in the apoptotic effect of AT-101/Paclitaxel combination treatment in breast cancer cells, we investigated the possible roles of anti-apoptotic Bcl-2, Bcl-XL and Mcl-1 proteins which contain BH3 domain. Involvement of caspase 3 and 7 activation was also investigated. Methods: Human breast cancer cells were treated with increasing concentrations of drugs alone or with the synergistic combination doses of AT-101 and Paclitaxel. Cell Death Detection Elisa Plus Kit (Roche) was used to detect apoptosis. Caspase 3/7 activity was evaluated by Caspase-Glo 3/7 (Promega, Madison, WI) kit. Changes in the mRNA levels of Bcl-2, Bcl-XL and Mcl-1 genes were evaluated by qRT-PCR. Expression levels of these proteins were also investigated by Western blot analysis. Results: Combined treatment was shown to have strong synergistic apoptotic effects in MCF-7 and MDA- MB-231 human breast cancer cells. mRNA levels of Bcl-2, Bcl-XL and Mcl-1 molecules were reduced by the combination treatment in both cell lines. In parallel with mRNA levels, Bcl-2, Bcl-XL and Mcl-1 protein levels were significantly reduced after this novel drug combination. Combined treatment also induced caspase 3/7 activation in breast cancer cells. Conclusions: These preliminary data suggest that anti-apoptotic proteins such as Bcl-2, Bcl-XL and Mcl-1 may play important role in the underlying mechanistic rationale of apoptotic effect of AT-101/paclitaxel combination, while pro-apoptotic Bcl-2 related genes (Caspase-3 and Caspase-7) also seem to regulate this synergistic interaction.