Binding of Released Bim to Mcl-1 is a Mechanism of Intrinsic Resistance to ABT-199 which can be Overcome by Combination with Daunorubicin or Cytarabine in AML Cells

Inhibition of CHK1 enhances cell death induced by the Bcl-2-selective inhibitor ABT-199 in acute myeloid leukemia cells

Resistance to standard chemotherapy agents remains a major obstacle for improving treatment outcomes for acute myeloid leukemia (AML). The Bcl-2-selective inhibitor ABT-199 has demonstrated encouraging preclinical results, drug resistance remains a concern. Mcl-1 has been demonstrated to contribute to ABT-199 resistance, thus combining with therapies that target Mcl-1 could overcome such resistance. In this study, we utilized a CHK1 inhibitor, LY2603618, to decrease Mcl-1 and enhance ABT-199 efficacy. We found that LY2603618 treatment resulted in abolishment of the G2/M cell cycle checkpoint and increased DNA damage, which was partially dependent on CDK activity. LY2603618 treatment resulted in decrease of Mcl-1, which coincided with the initiation of apoptosis. Overexpression of Mcl-1 in AML cells significantly attenuated apoptosis induced by LY2603618, confirming the critical role of Mcl-1 in apoptosis induced by the agent. Simultaneous treatment with LY2603618 and ABT-199 resulted in synergistic induction of apoptosis in both AML cell lines and primary patient samples. Our findings provide new insights into overcoming a mechanism of intrinsic ABT-199 resistance in AML cells and support the clinical development of combined ABT-199 and CHK1 inhibition.

Targeting PI3K, mTOR, ERK, and Bcl-2 signaling network shows superior antileukemic activity against AML ex vivo

Acute myeloid leukemia (AML) remains challenging to treat and needs more effective treatments. The PI3K/mTOR pathway is involved in cell survival and has been shown to be constitutively active in 50-80% of AML patients. However, targeting the PI3K/mTOR pathway results in activation of the ERK pathway, which also plays an important role in cell survival. In addition, AML cells often overexpress antiapoptotic Bcl-2 family proteins (e.g., Bcl-2), preventing cell death. Thus, our strategy here is to target the PI3K, mTOR (by VS-5584, a PI3K and mTOR dual inhibitor), ERK (by SCH772984, an ERK-selective inhibitor), and Bcl-2 (by ABT-199, a Bcl-2-selective inhibitor) signaling network to kill AML cells. In this study, we show that while inhibition of PI3K, mTOR, and ERK showed superior induction of cell death compared to inhibition of PI3K and mTOR, the levels of cell death were modest in some AML cell lines and primary patient samples tested. Although simultaneous inhibition of PI3K, mTOR, and ERK caused downregulation of Mcl-1 and upregulation of Bim, immunoprecipitation of Bcl-2 revealed increased binding of Bim to Bcl-2, which was abolished by the addition of ABT-199, suggesting that Bim was bound to Bcl-2 which prevented cell death. Treatment with combined VS-5584, SCH772984, and ABT-199 showed significant increase in cell death in AML cell lines and primary patient samples and significant reduction in AML colony formation in primary patient samples, while there was no significant effect on colony formation of normal human CD34+ hematopoietic progenitor cells. Taken together, our findings show that inhibition of PI3K, mTOR, and ERK synergistically induces cell death in AML cells, and addition of ABT-199 enhances cell death further. Thus, our data support targeting the PI3K, mTOR, ERK, and Bcl-2 signaling network for the treatment of AML.

Found in Translation: How Preclinical Research Is Guiding the Clinical Development of the BCL2-Selective Inhibitor Venetoclax

Since the discovery of apoptosis as a form of programmed cell death, targeting the apoptosis pathway to induce cancer cell death has been a high-priority goal for cancer therapy. After decades of effort, drug-discovery scientists have succeeded in generating small-molecule inhibitors of antiapoptotic BCL2 family proteins. Innovative medicinal chemistry and structure-based drug design, coupled with a strong fundamental understanding of BCL2 biology, were essential to the development of BH3 mimetics such as the BCL2-selective inhibitor venetoclax. We review a number of preclinical studies that have deepened our understanding of BCL2 biology and facilitated the clinical development of venetoclax.Significance: Basic research into the pathways governing programmed cell death have paved the way for the discovery of apoptosis-inducing agents such as venetoclax, a BCL2-selective inhibitor that was recently approved by the FDA and the European Medicines Agency. Preclinical studies aimed at identifying BCL2-dependent tumor types have translated well into the clinic thus far and will likely continue to inform the clinical development of venetoclax and other BCL2 family inhibitors. Cancer Discov; 7(12); 1376-93. ©2017 AACR.

Combined venetoclax and alvocidib in acute myeloid leukemia

More effective treatment options for elderly acute myeloid leukemia (AML) patients are needed as only 25-50% of patients respond to standard-of-care therapies, response duration is typically short, and disease progression is inevitable even with some novel therapies and ongoing clinical trials. Anti-apoptotic BCL-2 family inhibitors, such as venetoclax, are promising therapies for AML. Nonetheless, resistance is emerging. We demonstrate that venetoclax combined with cyclin-dependent kinase (CDK) inhibitor alvocidib is potently synergistic in venetoclax-sensitive and -resistant AML models in vitro, ex vivo and in vivo. Alvocidib decreased MCL-1, and/or increased pro-apoptotic proteins such as BIM or NOXA, often synergistically with venetoclax. Over-expression of BCL-XL diminished synergy, while knock-down of BIM almost entirely abrogated synergy, demonstrating that the synergistic interaction between alvocidib and venetoclax is primarily dependent on intrinsic apoptosis. CDK9 inhibition predominantly mediated venetoclax sensitization, while CDK4/6 inhibition with palbociclib did not potentiate venetoclax activity. Combined, venetoclax and alvocidib modulate the balance of BCL-2 family proteins through complementary, yet variable mechanisms favoring apoptosis, highlighting this combination as a promising therapy for AML or high-risk MDS with the capacity to overcome intrinsic apoptosis mechanisms of resistance. These results support clinical testing of combined venetoclax and alvocidib for the treatment of AML and advanced MDS.

Cucurbitacin B and SCH772984 exhibit synergistic anti-pancreatic cancer activities by suppressing EGFR, PI3K/Akt/mTOR, STAT3 and ERK signaling

Cucurbitacin B (CuB) is a natural tetracyclic triterpene product and displays antitumor activity across a wide array of cancers. In this study, we explored the anti-pancreatic cancer activity of CuB alone and in combination with SCH772984, an ERK inhibitor, in vitro and in vivo. CuB inhibited proliferation of pancreatic cancer cells by arresting them in the G2/M cell cycle phase. This was associated with inhibition of EGFR expression and activity and downstream signaling, including PI3K/Akt/mTOR and STAT3. Interestingly, ERK activity was markedly enhanced by activating AMPK signaling after 12 h of CuB treatment. SCH772984 potentiates the cytotoxic effect of CuB on pancreatic cancer cells through complementary inhibition of EGFR, PI3K/Akt/mTOR, STAT3 and ERK signaling, followed by an increase in the pro-apoptotic protein Bim and a decrease in the anti-apoptotic proteins Mcl-1, Bcl-2, Bcl-xl and survivin. Furthermore, combined therapy with CuB and SCH772984 resulted in highly significant growth inhibition of pancreatic cancer xenografts. These results may provide a basis for further development of combining CuB and ERK inhibitors to treat pancreatic cancer.

Targeting apoptosis in acute myeloid leukaemia

Acute myeloid leukaemia (AML) is a molecularly and clinically heterogeneous disease, and its incidence is increasing as the populations in Western countries age. Despite major advances in understanding the genetic landscape of AML and its impact on the biology of the disease, standard therapy has not changed significantly in the last three decades. Allogeneic haematopoietic stem cell transplantation remains the best chance for cure, but can only be offered to a minority of younger fit patients. Molecularly targeted drugs aiming at restoring apoptosis in leukaemic cells have shown encouraging activity in early clinical trials and some of these drugs are currently being evaluated in randomised controlled trials. In this review, we discuss the current development of drugs designed to trigger cell death in AML.

Enhancing venetoclax activity in acute myeloid leukemia by co-targeting MCL1

Targeted therapies are frequently combined with standard cytotoxic drugs to enhance clinical response. Targeting the B-cell lymphoma 2 (BCL-2) family of proteins is an attractive option to combat chemoresistance in leukemia. Preclinical and clinical studies indicate modest single-agent activity with selective BCL-2 inhibitors (for example, venetoclax). We show that venetoclax synergizes with cytarabine and idarubicin to increase antileukemic efficacy in a TP53-dependent manner. Although TP53 deficiency impaired sensitivity to combined venetoclax and chemotherapy, higher-dose idarubicin was able to suppress MCL1 and induce cell death independently of TP53. Consistent with an MCL1-specific effect, cell death from high-dose idarubicin was dependent on pro-apoptotic Bak. Combining higher-dose idarubicin with venetoclax was able to partially overcome resistance in Bak-deficient cells. Using inducible vectors and venetoclax to differentially target anti-apoptotic BCL-2 family members, BCL-2 and MCL1 emerged as critical and complementary proteins regulating cell survival in acute myeloid leukemia. Dual targeting of BCL-2 and MCL1, but not either alone, prolonged survival of leukemia-bearing mice. In conclusion, our findings support the further investigation of venetoclax in combination with standard chemotherapy, including intensified doses of idarubicin. Venetoclax should also be investigated in combination with direct inhibitors of MCL1 as a chemotherapy-free approach in the future.

Inhibition of Mcl-1 enhances cell death induced by the Bcl-2-selective inhibitor ABT-199 in acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a serious disease. The 5-year survival rates remain frustratingly low (65% for children and 26% for adults). Resistance to frontline chemotherapy (usually cytarabine) often develops; therefore a new treatment modality is needed. Bcl-2 family proteins play an important role in balancing cell survival and apoptosis. The antiapoptotic Bcl-2 family proteins have been found to be dysregulated in AML. ABT-199, a BH3 mimetic, was developed to target antiapoptotic protein Bcl-2. Although ABT-199 has demonstrated promising results, resistance occurs. Previous studies in AML show that ABT-199 alone decreases the association of proapoptotic protein Bim with Bcl-2, but this is compensated by increased association of Bim with prosurvival protein Mcl-1, stabilizing Mcl-1, resulting in resistance to ABT-199. In this study, we investigated the antileukemic activity of the Mcl-1-selective inhibitor A-1210477 in combination with ABT-199 in AML cells. We found that A-1210477 synergistically induced apoptosis with ABT-199 in AML cell lines and primary patient samples. The synergistic induction of apoptosis was decreased upon Bak, Bax and Bim knockdown. While A-1210477 treatment alone also increased Mcl-1 protein levels, combination with ABT-199 reduced binding of Bim to Mcl-1. Our results demonstrate that sequestration of Bim by Mcl-1, a mechanism of ABT-199 resistance, can be abrogated by combined treatment with the Mcl-1 inhibitor A-1201477.

'Did He Who Made the Lamb Make Thee?' New Developments in Treating the 'Fearful Symmetry' of Acute Myeloid Leukemia

Malignant cells must circumvent endogenous cell death pathways to survive and develop into cancers. Acquired cell death resistance also sets up malignant cells to survive anticancer therapies. Acute Myeloid Leukemia (AML) is an aggressive blood cancer characterized by high relapse rate and resistance to cytotoxic therapies. Recent collaborative profiling projects have led to a greater understanding of the 'fearful symmetry' of the genomic landscape of AML, and point to the development of novel potential therapies that can overcome factors linked to chemoresistance. We review here the most recent research in the genetics of AML and how these discoveries have led, or might lead, to therapies that specifically activate cell death pathways to substantially challenge this 'fearful' disease.

Mechanisms responsible for the synergistic antileukemic interactions between ATR inhibition and cytarabine in acute myeloid leukemia cells

Acute myeloid leukemia (AML) continues to be a challenging disease to treat, thus new treatment strategies are needed. In this study, we investigated the antileukemic effects of ATR inhibition alone or combined with cytarabine in AML cells. Treatment with the ATR-selective inhibitor AZ20 caused proliferation inhibition in AML cell lines and primary patient samples. It partially abolished the G2 cell cycle checkpoint and caused DNA replication stress and damage, accompanied by CDK1-independent apoptosis and downregulation of RRM1 and RRM2. AZ20 synergistically enhanced cytarabine-induced proliferation inhibition and apoptosis, abolished cytarabine-induced S and G2/M cell cycle arrest, and cooperated with cytarabine in inducing DNA replication stress and damage in AML cell lines. These key findings were confirmed with another ATR-selective inhibitor AZD6738. Therefore, the cooperative induction of DNA replication stress and damage by ATR inhibition and cytarabine, and the ability of ATR inhibition to abrogate the G2 cell cycle checkpoint both contributed to the synergistic induction of apoptosis and proliferation inhibition in AML cell lines. Synergistic antileukemic interactions between AZ20 and cytarabine were confirmed in primary AML patient samples. Our findings provide insight into the mechanism of action underlying the synergistic antileukemic activity of ATR inhibition in combination with cytarabine in AML.

Synergistic anti-leukemic interactions between ABT-199 and panobinostat in acute myeloid leukemia ex vivo

Cure rates for acute myeloid leukemia (AML) remain suboptimal; thus new treatment strategies are needed for this deadly disease. Poor clinical outcomes have been associated with overexpression of the anti-apoptotic Bcl-2 family proteins Bcl-2, Bcl-xL, and Mcl-1, which have garnered great interest as therapeutic targets. While the Bcl-2-selective inhibitor ABT-199 has demonstrated promising preclinical anti-leukemic activities, intrinsic drug resistance remains a problem. In our most recent study, we identified Mcl-1 sequestration of Bim as a mechanism of intrinsic resistance to ABT-199 in AML cells, thus upregulating Bim could overcome such resistance. Histone deacetylase (HDAC) inhibitors (HDACI) are a class of agents that have been confirmed to upregulate Bim. This prompted our hypothesis that combining an HDACI with ABT-199 would overcome intrinsic resistance to ABT-199 and result in synergistic anti-leukemic activity against AML. In this study, we investigated the anti-leukemic activity of panobinostat, a pan-HDACI, in combination with ABT-199 in AML cell lines and primary patient samples. We found that the combined drug treatment resulted in synergistic induction of cell death in both AML cell lines and primary patient samples. Panobinostat treatment resulted in upregulation of Bim, which remained elevated in the presence of ABT-199. In addition, shRNA knockdown of Bim in AML cell lines significantly attenuated apoptosis induced by combined panobinostat and ABT-199. Our results provide compelling evidence that Bim plays a key role in the combined anti-leukemic activity of panobinostat and ABT-199 against AML, and support clinical evaluation of combined panobinostat and ABT-199 in the treatment of AML.

Synthesis and Antileukemic Activities of Piperlongumine and HDAC Inhibitor Hybrids against Acute Myeloid Leukemia Cells

Synergistic-to-additive antileukemic interactions of piperlongumine (PL) and HDAC inhibitor (HDACi) SAHA (Vorinostat) provide a compelling rationale to construct PL-HDACi hybrids, such as 1-58, which recapitulated the synergism between the parental compounds in high-risk and chemoresistant AML cells. Both PL and HDACi components, either in combination or in hybrid molecules, are essential for inducing significant DNA damage and apoptosis. Introducing C2-chloro substituent to 1-58 yielded 3-35 with increased cytotoxicity but decreased selectivity in noncancerous MCF-10A cells; eliminating C7-C8 olefin of PL obtained 3-31/3-98 scaffolds which were still more active than PL or SAHA in AML and were well-tolerated by MCF-10A cells. The HDACi function was crucial for modulating expression of DNA repair and apoptosis-related proteins. Collectively, PL and SAHA hybrids are potent, multifunctional anti-AML agents, acting in part, by interfering cellular GSH defense, suppressing expression of DNA repair and pro-survival proteins, and inducing expression of pro-apoptotic proteins.