Directly targeting the mitochondrial pathway of apoptosis for cancer therapy using BH3 mimetics - recent successes, current challenges and future promise

Apoptosis within cancer cells is controlled by the BCL-2 family of proteins, making them powerful arbiters of cell fate in response to stress induced by neoplastic transformation as well as exposure to anti-cancer therapies. Many cancers evade pro-apoptotic stress signals by up-regulating anti-apoptotic proteins such as BCL-2, BCL-X_L or MCL-1 to maintain their survival. However, this may come at a cost, as these cancers may also become dependent on these anti-apoptotic proteins for survival. The development and deployment of BCL-2 family inhibitors (drugs that mimic the activity of pro-apoptotic BH3-only proteins or 'BH3 mimetics') is based on this paradigm, and the first potent and specific molecules are now being evaluated in clinical trials. We review the recent successes in this field, the challenges currently being faced, and the promising future ahead.

Inhibition of Bcl-2 or IAP proteins does not provoke mutations in surviving cells

Chemotherapy and radiotherapy can cause permanent damage to the genomes of surviving cells, provoking severe side effects such as second malignancies in some cancer survivors. Drugs that mimic the activity of death ligands, or antagonise pro-survival proteins of the Bcl-2 or IAP families have yielded encouraging results in animal experiments and early phase clinical trials. Because these agents directly engage apoptosis pathways, rather than damaging DNA to indirectly provoke tumour cell death, we reasoned that they may offer another important advantage over conventional therapies: minimisation or elimination of side effects such as second cancers that result from mutation of surviving normal cells. Disappointingly, however, we previously found that concentrations of death receptor agonists like TRAIL that would be present in vivo in clinical settings provoked DNA damage in surviving cells. In this study, we used cell line model systems to investigate the mutagenic capacity of drugs from two other classes of direct apoptosis-inducing agents: the BH3-mimetic ABT-737 and the IAP antagonists LCL161 and AT-406. Encouragingly, our data suggest that IAP antagonists possess negligible genotoxic activity. Doses of ABT-737 that were required to damage DNA stimulated Bax/Bak-independent signalling and exceeded concentrations detected in the plasma of animals treated with this drug. These findings provide hope that cancer patients treated by BH3-mimetics or IAP antagonists may avoid mutation-related illnesses that afflict some cancer survivors treated with conventional DNA-damaging anti-cancer therapies.

Caspase-dependent Mcl-1 cleavage and effect of Mcl-1 phosphorylation in ABT-737-induced apoptosis in human acute lymphoblastic leukemia cell lines

ABT-737 is a BH3-mimetic that has a wide spectrum of single-agent activity against acute lymphoblastic leukemia (ALL) cell lines and xenografts. Previously, we reported that in response to ABT-737, ABT-737-resistant ALL cell lines showed an apparent increase in Mcl-1 (an anti-apoptotic Bcl-2 family protein that is not effectively inhibited by ABT-737) while ABT-737-sensitive ALL cell lines showed decreased Mcl-1 levels. Here we explored the mechanism of Mcl-1 cleavage by ABT-737 and the effect of adjacent phosphorylation sites on Mcl-1 cleavage and apoptosis induced by ABT-737 in a human B-lineage ALL cell line. Caspase cleavage sites in Mcl-1 and the effect of mutation in Mcl-1 phosphorylation sites were determined by transducing Mcl-1 variants tagged with the V5 epitope into human ALL cells. Cytotoxicity was by fluorescence-based DIMSCAN, and changes in protein by immunoblotting. ABT-737 induced a caspase-dependent cleavage of Mcl-1. Of the two Mcl-1 caspase cleavage sites (D127 and D157), D157 was the site of ABT-737-induced cleavage in ALL cells. Cells with exogenously expressed Mcl-1 Δ157 fragment showed greater caspase-3 and caspase-9 activation when they were treated with ABT-737 compared with cells expressing wild-type or D157A mutant Mcl-1. Cells with mutated phosphorylation sites on Mcl-1 (S159A and T163A) were less susceptible to Mcl-1 cleavage and apoptosis induced by ABT-737. Our data showed that Mcl-1 is post-translationally regulated in response to ABT-737 treatment, primarily via a caspase-dependent cleavage that generates a pro-apoptotic Mcl-1 fragment.

ABT-737 is highly effective against molecular subgroups of multiple myeloma

Multiple myeloma is a plasma cell malignancy that is heterogeneous with respect to its causative molecular abnormalities and the treatment response of patients. The Bcl-2 protein family is critical for myeloma cell survival. ABT-737 is a cell-permeant compound that binds to Bcl-2 and Bcl-xL but not to Mcl-1. Using a myeloma cell line collection (n = 25) representative of different molecular translocations, we showed that ABT-737 effectively kills a subset of cell lines (n = 6), with a median lethal dose ranging from 7 ± 0.4nM to 150 ± 7.5nM. Of interest, all sensitive cell lines harbored a t(11;14). We demonstrated that ABT-737–sensitive and ABT-737–resistant cell lines could be differentiated by the BCL2/MCL1 expression ratio. A screen of a public expression database of myeloma patients indicates that the BCL2/MCL1 ratio of t(11;14) and hyperdiploid patients was significantly higher than in all other groups (P < .001). ABT-737 first induced the disruption of Bcl-2/Bax, Bcl-2/Bik, or Bcl-2/Puma complexes, followed by the disruption of Bcl-2 heterodimers with Bak and Bim. Altogether, the identification of a subset of cell lines and primary cells effectively killed by ABT-737 alone supported the evaluation of ABT-263, an orally active counterpart to ABT-737, for the treatment of t(11;14) and hyperdiploid groups of myeloma harboring a Bcl-2high/Mcl-1low profile.

Targeted therapy in haematological malignancies

The recent and rapid development of molecularly targeted therapy is best illustrated by advances in the management of haematological malignancy. In myeloid diseases we have seen dramatic improvements in the overall survival and quality of life for patients with chronic myeloid leukaemia treated with ABL and Src/ABL kinase inhibitors and we are poised to discover whether JAK2 inhibitors may offer similar benefit in myeloproliferative diseases. For acute myeloid leukaemia, the introduction of ATRA and myelotarg have had major impacts on the design of therapy regimens and many novel targeted agents, including farnesyl transferase, FLT3 and histone deacetylase inhibitors, are now in clinical trial. In lymphoid malignancies the highlight has been the introduction of rituximab, with significant improvements in the management of non-Hodgkin lymphoma and chronic lymphocytic leukaemia. The last 10 years has experienced a rapidly expanding interest and acceptance that leukaemic stem cells, including an improved ability to target them, may hold the key to improved response and reduced relapse rates across both myeloid and lymphoid disease. We now eagerly anticipate an era in which a wealth of preclinical discoveries are progressed to the clinic.

Mimicking the BH3 domain to kill cancer cells

Cancer cells show deviant behavior that induces apoptotic signaling. To survive, cancer cells typically acquire changes enabling evasion of death signals. One way they do this is by increasing the expression of anti-apoptotic BCL-2 proteins. Anti-apoptotic BCL-2 family proteins antagonize death signaling by forming heterodimers with pro-death proteins. Heterodimer formation occurs through binding of the pro-apoptotic protein's BH3 domain into the hydrophobic cleft of anti-apoptotic proteins. The BH3 mimetics are small molecule antagonists of the anti-apoptotic BCL-2 members that function as competitive inhibitors by binding to the hydrophobic cleft. Under certain conditions, antagonism of anti-apoptotic BCL-2 family proteins can unleash pro-death molecules in cancer cells. Thus, the BH3 mimetics are a new class of cancer drugs that specifically target a mechanism of cancer cell survival to selectively kill cancer cells.

Rational combinations using HDAC inhibitors

In addition to well-characterized genetic abnormalities that lead to cancer onset and progression, it is now recognized that alterations to the epigenome may also play a significant role in oncogenesis. As a result, epigenetic-modulating agents such as histone deacetylase inhibitors (HDACi) have attracted enormous attention as anticancer drugs. In numerous in vitro and preclinical settings, these compounds have shown their vast potential as single agent anticancer therapies, but unfortunately equivalent responses have not always been observed in patients. Given the pleiotropic effects HDACi have on malignant cells, their true therapeutic potential most likely lies in combination with other anticancer drugs. In this review we will focus on the anticancer effects of HDACi when combined with other cancer therapeutics with an emphasis on those combinations based on a strong molecular rationale.

Molecular and cellular mechanisms of CLL: novel therapeutic approaches

The mainstay of therapy of chronic lymphocytic leukemia (CLL) is cytotoxic chemotherapy; however, CLL is still an incurable disease with resistance to therapy developing in the majority of patients. In recent years, our understanding of the biological basis of CLL pathogenesis has substantially improved and novel treatment strategies are emerging. Tailoring and individualizing therapy according to the molecular and cellular biology of the disease is on the horizon, and advances with targeted agents such as monoclonal antibodies combined with traditional chemotherapy have lead to improved remission rates. The proposed key role of the B-cell receptor (BCR) in CLL pathogenesis has led to a number of possible opportunities for therapeutic exploitation. We are beginning to understand that the microenvironment is of utmost importance in CLL because certain T-cell subsets and stromal cells support the outgrowth and development of the malignant clone. Furthermore, an increase in our understanding of the deregulated cell-death machinery in CLL is a prerequisite to developing new targeted strategies that might be more effective in engaging with the cell-death machinery. This Review summarizes the progress made in understanding these features of CLL biology and describes novel treatment strategies that have also been exploited in current clinical trials.

Different forms of cell death induced by putative BCL2 inhibitors

Several inhibitors of BCL2 proteins have been identified that induce apoptosis in a variety of tumor cells, indicating their potential in cancer therapy. We investigated the specificity of six putative BCL2 inhibitors (obatoclax, gossypol, apogossypol, EM20-25, chelerythrine and ABT-737). Using cells deficient either for Bax/Bak or caspase-9, we found that only ABT-737 specifically targeted BCL2 proteins and induced apoptosis by activation of caspase-9, as only ABT-737 induced apoptosis was completely inhibited in cells deficient for Bax/Bak or caspase-9. Our data show that only ABT-737 is a specific BCL2 inhibitor and all other compounds investigated were not specific for BCL2 proteins. Furthermore, investigations of the effects of these compounds in primary chronic lymphocytic leukemic cells showed that all compounds induced certain biochemical hallmarks of apoptosis, such as release of cytochrome c and caspase cleavage. However, they all caused strikingly different ultrastructural changes. ABT-737 induced all the characteristic ultrastructural changes of apoptosis together with early rupture of the outer mitochondrial membrane, whereas obatoclax, chlelerythrine and gossypol induced pronounced mitochondrial swelling with formation of phospholipid inclusions. Therefore, we conclude that biochemical measurements used earlier to define apoptosis like mitochondrial release of cytochrome c and caspase cleavage, are insufficient to distinguish between classic apoptosis and other forms of cell death.

Bcl-2 inhibitors: targeting mitochondrial apoptotic pathways in cancer therapy

Defects in apoptotic pathways can promote cancer cell survival and also confer resistance to antineoplastic drugs. One pathway being targeted for antineoplastic therapy is the anti-apoptotic B-cell lymphoma-2 (Bcl-2) family of proteins (Bcl-2, Bcl-XL, Bcl-w, Mcl-1, Bfl1/A-1, and Bcl-B) that bind to and inactivate BH3-domain pro-apoptotic proteins. Signals transmitted by cellular damage (including antineoplastic drugs) or cytokine deprivation can initiate apoptosis via the intrinsic apoptotic pathway. It is controversial whether some BH3-domain proteins (Bim or tBid) directly activate multidomain pro-apoptotic proteins (e.g., Bax and Bak) or act via inhibition of those anti-apoptotic Bcl-2 proteins (Bcl-2, Bcl-XL, Bcl-w, Mcl-1, Bfl1/A-1, and Bcl-B) that stabilize pro-apoptotic proteins. Overexpression of anti-apoptotic Bcl-2 family members has been associated with chemotherapy resistance in various human cancers, and preclinical studies have shown that agents targeting anti-apoptotic Bcl-2 family members have preclinical activity as single agents and in combination with other antineoplastic agents. Clinical trials of several investigational drugs targeting the Bcl-2 family (oblimersen sodium, AT-101, ABT-263, GX15-070) are ongoing. Here, we review the role of the Bcl-2 family in apoptotic pathways and those agents that are known and/or designed to inhibit the anti-apoptotic Bcl-2 family of proteins.

Chronic lymphocytic leukemia: new concepts and emerging therapies

OPINION STATEMENT

Remarkable progress in elucidating the biology of chronic lymphocytic leukemia (CLL) has been made over the last two decades. Improved understanding of CLL has lead to new prognostic tools and therapeutic options, and holds promise for eventually finding a cure for this disease. Challenges lie in incorporating the various treatment modalities, including chemotherapy, monoclonal antibodies, immunotherapeutic strategies and novel small molecules, into a comprehensive treatment strategy guided by the biological complexity of CLL.

Targeting the Bcl-2-regulated apoptosis pathway by BH3 mimetics: a breakthrough in anticancer therapy?

Induction of apoptosis in tumor cells by direct activation of the Bcl-2-regulated apoptosis pathway by small molecule drugs carries high hopes to overcome the shortcomings of current anticancer therapies. This novel therapy concept builds on emerging insights into how Bcl-2-like molecules maintain mitochondrial integrity and how pro-apoptotic BH3-only proteins lead to its disruption. Means to unleash the pro-apoptotic potential of BH3-only proteins in tumor cells, or to bypass the need for BH3-only proteins by directly blocking possible interactions of Bcl-2-like pro-survival molecules with Bax and/or Bak, constitute interesting options for the design of novel anticancer therapies. For the optimization and clinical implementation of these novel anticancer strategies, a detailed understanding of the role of individual BH3-only proteins in cell death signaling in healthy cells and during tumor suppression is required. In this review, we will touch on the latest findings on BH3-only protein function and attempts to define the molecular properties of the so-called 'BH3 mimetics,' a novel class of anticancer agents, able to prompt apoptosis in tumor cells, regardless of their p53 or Bcl-2 status.