Navitoclax, a targeted high-affinity inhibitor of BCL-2, in lymphoid malignancies: a phase 1 dose-escalation study of safety, pharmacokinetics, pharmacodynamics, and antitumour activity

Targeting Bcl-2 family proteins in adult T-cell leukemia/lymphoma: in vitro and in vivo effects of the novel Bcl-2 family inhibitor ABT-737

Adult T-cell leukemia/lymphoma (ATLL) is a peripheral T-cell malignancy caused by human T-lymphotropic virus type I (HTLV-1). ABT-737, a small molecule inhibitor of Bcl-2, Bcl-X(L), and Bcl-w, significantly induced apoptosis in HTLV-1 infected T-cell lines as well as in fresh ATLL cells, and synergistically enhanced the cytotoxicity and apoptosis induced by conventional cytotoxic drugs. Moreover, ABT-737 significantly inhibited the in vivo tumor growth of an ATLL mouse model. These results suggest that the use of an agent targeting anti-apoptotic bcl-2 family proteins, either alone or in combination with other conventional drugs, represents a novel promising approach for ATLL.

New molecular targets in mantle cell lymphoma

Mantle cell lymphoma (MCL) is a malignancy of mature B cells characterized by aberrant expression of cyclin D1 due to the translocation t(11;14). Epigenomic and genomic lesions in pathways regulating B-cell activation, cell cycle progression, protein homeostasis, DNA damage response, cell proliferation and apoptosis contribute to its pathogenesis. While patients typically respond to first-line chemotherapy, relapse is the rule resulting in a median survival of 5-7 years. The PI3K/AKT/mTOR appears as a key pathway in the pathogenesis and can be targeted with small molecules. Most experience is with mTOR inhibitors of the rapamycin class. Second-generation mTOR inhibitors and the PI3K inhibitor CAL-101 are novel options to more effectively target this pathway. Bruton's tyrosine kinase inhibition by PCI-32765 has promising activity and indicates immunoreceptor signaling as a novel therapeutic target. Up to 50% of relapsed patients respond to the proteasome inhibitor bortezomib suggesting that MCL may be particularly sensitive to disruption of protein homeostasis and/or induction of oxidative stress. Recent work has focused on elucidating the mechanism of bortezomib-induced cytotoxicity and the development of second-generation proteasome inhibitors. DNA hypomethylating agents and histone deacetylase inhibitors effect epigenetic de-repression of aberrantly silenced genes. These epigenetic pharmaceuticals and HSP90 inhibitors can synergize with proteasome inhibitors. Finally, BH3 mimetics are emerging as tools to sensitize tumor cells to chemotherapy. Participation in clinical trials offers patients a chance to benefit from these advances and is essential to maintain the momentum of progress. Innovative trial designs may be needed to expedite the clinical development of these targeted agents.

New therapeutic targets and drugs in non-Hodgkin's lymphoma

PURPOSE OF REVIEW

Although enormous progress has been made in treating non-Hodgkin's lymphoma (NHL), and some patients can be cured with combination immunochemotherapy, patients with relapsed and refractory lymphoma often succumb to their disease. Advances in our understanding of lymphoma biology and molecular pathogenesis are yielding new therapeutic targets.

RECENT FINDINGS

This article reviews NHL biology and describes how our understanding of molecular pathogenesis is leading to the discovery of many therapeutic targets, including the cell signaling and cell cycle regulatory proteins, pro-apoptotic family members, the B-cell antigen receptor (BCR), and histone deacetylase. Recent preclinical and clinical data with inhibitors of phosphatidylinositol 3-kinase, AKT, mammalian target of rapamycin, histone deacetylase, bcl-2, and the Bruton's tyrosine kinase, a pivotal enzyme in the BCR pathway, are discussed.

SUMMARY

Understanding these novel targets in the context of NHL biology will bring new therapies and allow us to develop new therapeutic platforms for the treatment of relapsed and refractory NHL, and will hopefully improve the clinical outcome for these patients.

BAX unleashed: the biochemical transformation of an inactive cytosolic monomer into a toxic mitochondrial pore

BAX, the BCL-2-associated X protein, is a cardinal proapoptotic member of the BCL-2 family, which regulates the critical balance between cellular life and death. Because so many medical conditions can be categorized as diseases of either too many or too few cells, dissecting the biochemistry of BCL-2 family proteins and developing pharmacological strategies to target them have become high priority scientific objectives. Here, we focus on BAX, a latent, cytosolic and monomeric protein that transforms into a lethal mitochondrial oligomer in response to cellular stress. New insights into the structural location of BAX's 'on switch', and the multi-step conformational changes that ensue upon BAX activation, are providing fresh opportunities to modulate BAX for potential benefit in human diseases characterized by pathologic cell survival or unwanted cellular demise.

Synergistic activity of fenretinide and the Bcl-2 family protein inhibitor ABT-737 against human neuroblastoma

PURPOSE

Fenretinide (4-HPR) is a cytotoxic retinoid with minimal systemic toxicity that has shown clinical activity against recurrent high-risk neuroblastoma. To identify possible synergistic drug combinations for future clinical trials, we determined whether ABT-737, a small-molecule BH3-mimetic that inhibits most proteins of the antiapoptotic Bcl-2 family, could enhance 4-HPR activity in neuroblastoma.

EXPERIMENTAL DESIGN

Eleven neuroblastoma cell lines were tested for the cytotoxic activity of 4-HPR and ABT-737 as single agents and in combination using the DIMSCAN fluorescence digital imaging cytotoxicity assay. The effect of these agents alone and in combination on mitochondrial membrane depolarization and apoptosis (by flow cytometry), cytochrome c release, caspases, Bax-α, t-Bid, and Bak activation, and subcutaneous xenografts in nu/nu mice was also determined.

RESULTS

Multilog synergistic cytotoxicity was observed for the drug combination in all of the 11 neuroblastoma cell lines tested, including MDR lines and those insensitive to either drug as single agents. 4-HPR + ABT-737 induced greater mitochondrial membrane depolarization and mitochondrial cytochrome c release, greater activation of caspases, Bax-α, t-Bid, and Bak, and a higher level of apoptosis than either drug alone. In vivo, 4-HPR + ABT-737 increased the event-free survival of the MDR human neuroblastoma line CHLA-119 implanted subcutaneously in nu/nu mice (194.5 days for the combination vs. 68 days for ABT-737 and 99 days for 4-HPR).

CONCLUSION

Thus, the combination of 4-HPR with a BH3-mimetic drug warrants clinical trials in recurrent neuroblastoma.

Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets

Deletion of Bak and Bax, the effectors of mitochondrial apoptosis, does not affect platelet production, however, loss of prosurvival Bcl-x_L results in megakaryocyte apoptosis and failure of platelet shedding.

It is believed that megakaryocytes undergo a specialized form of apoptosis to shed platelets. Conversely, a range of pathophysiological insults, including chemotherapy, are thought to cause thrombocytopenia by inducing the apoptotic death of megakaryocytes and their progenitors. To resolve this paradox, we generated mice with hematopoietic- or megakaryocyte-specific deletions of the essential mediators of apoptosis, Bak and Bax. We found that platelet production was unperturbed. In stark contrast, deletion of the prosurvival protein Bcl-x_L resulted in megakaryocyte apoptosis and a failure of platelet shedding. This could be rescued by deletion of Bak and Bax. We examined the effect on megakaryocytes of three agents that activate the intrinsic apoptosis pathway in other cell types: etoposide, staurosporine, and the BH3 mimetic ABT-737. All three triggered mitochondrial damage, caspase activation, and cell death. Deletion of Bak and Bax rendered megakaryocytes resistant to etoposide and ABT-737. In vivo, mice with a Bak^−/− Bax^−/− hematopoietic system were protected against thrombocytopenia induced by the chemotherapeutic agent carboplatin. Thus, megakaryocytes do not activate the intrinsic pathway to generate platelets; rather, the opposite is true: they must restrain it to survive and progress safely through proplatelet formation and platelet shedding.

ABT-737 induces apoptosis in mantle cell lymphoma cells with a Bcl-2high/Mcl-1low profile and synergizes with other antineoplastic agents

PURPOSE

Mantle cell lymphoma (MCL) is considered to be incurable. ABT-737 is a BH3 mimetic that targets Bcl-2, which is overexpressed in MCL and implicated in drug resistance. The present work investigated the antitumor effect of ABT-737.

EXPERIMENTAL DESIGN

Six MCL cell lines and primary MCL cells (n = 13) were used. Sensitivity to ABT-737 was assessed, and expression levels of Bcl-2 and Mcl-1 were analyzed. Finally, ABT-737 was combined with other cytotoxic agents to promote tailored therapy.

RESULTS

MINO and GRANTA-519 cell lines were highly sensitive to ABT-737 [the median lethal dose (LD₅₀) = 20 and 80 nmol/L, respectively], whereas other cell lines were resistant. In primary MCL cells, 46% of patients' samples were sensitive to ABT-737. The analysis of protein expression levels revealed that both sensitive cell lines and primary MCL cells could be characterized by a Bcl-2(high)/Mcl-1(low) profile, whereas resistant MCL cells contained high levels of Mcl-1. ABT-737 induced a rapid disruption of both Bcl-2/Bax and Bcl-2/Bik complexes. In addition, silencing of Mcl-1 by siRNA sensitized MCL cell lines to ABT-737. Similarly, flavopiridol, which induces Mcl-1 downregulation, in combination with ABT-737 led to a synergistic anti-MCL effect in ABT-737-resistant cell lines. This synergy was also observed when ABT-737 was combined with either bortezomib or cytarabine.

CONCLUSIONS

The present work shows that ABT-737 induces strong apoptosis in MCL cells expressing a Bcl-2(high)/Mcl-1(low) profile. In ABT-737-resistant MCL cells, downregulation of Mcl-1 overcomes Mcl-1-induced resistance and synergizes ABT-737 effects. Our results strongly support the use of ABT-737 according to the Bcl-2/Mcl-1 tumor cell profiles in the treatment of MCL.

The use of microarray technologies in mantle cell lymphoma

Mantle cell lymphoma (MCL) is characterized by adverse prognosis and the development of novel therapeutic approaches is essential to improve outcome. The introduction of gene expression profiling using DNA microarrays has significantly enhanced our understanding of the molecular pathogenesis of MCL, which is a prerequisite to the development of novel treatment strategies. Gene expression profiling can furthermore be applied to predict treatment response at diagnosis and thus can be used to assess a patient's individual risk profile. This review summarizes our current knowledge on the use of microarray technology in MCL.

An antiapoptotic BCL-2 family expression index predicts the response of chronic lymphocytic leukemia to ABT-737

The antiapoptotic BCL-2 proteins regulate lymphocyte survival and are over-expressed in lymphoid malignancies, including chronic lymphocytic leukemia. The small molecule inhibitor ABT-737 binds with high affinity to BCL-2, BCL-XL, and BCL-W but with low affinity to MCL-1, BFL-1, and BCL-B. The active analog of ABT-737, navitoclax, has shown a high therapeutic index in lymphoid malignancies; developing a predictive marker for it would be clinically valuable for patient selection or choice of drug combinations. Here we used RT-PCR as a highly sensitive and quantitative assay to compare expression of antiapoptotic BCL-2 genes that are known to be targeted by ABT-737. Our findings reveal that the relative ratio of MCL-1 and BFL-1 to BCL-2 expression provides a highly significant linear correlation with ABT-737 sensitivity (r = 0.6, P < .001). In contrast, antiapoptotic transcript levels, used individually or in combination for high or low affinity ABT-737-binding proteins, could not predict ABT-737 sensitivity. The (MCL-1 + BFL-1)/BCL-2 ratio was validated in a panel of leukemic cell lines subjected to genetic and pharmacologic manipulations. Changes after ABT-737 treatment included increased expression of BFL-1 and BCL-B that may contribute to treatment resistance. This study defines a highly significant BCL-2 expression index for predicting the response of CLL to ABT-737.

BCL2/BCL-X(L) inhibition induces apoptosis, disrupts cellular calcium homeostasis, and prevents platelet activation

Apoptosis in megakaryocytes results in the formation of platelets. The role of apoptotic pathways in platelet turnover and in the apoptotic-like changes seen after platelet activation is poorly understood. ABT-263 (Navitoclax), a specific inhibitor of antiapoptotic BCL2 proteins, which is currently being evaluated in clinical trials for the treatment of leukemia and other malignancies, induces a dose-limiting thrombocytopenia. In this study, the relationship between BCL2/BCL-X(L) inhibition, apoptosis, and platelet activation was investigated. Exposure to ABT-263 induced apoptosis but repressed platelet activation by physiologic agonists. Notably, ABT-263 induced an immediate calcium response in platelets and the depletion of intracellular calcium stores, indicating that on BCL2/BCL-X(L) inhibition platelet activation is abrogated because of a diminished calcium signaling. By comparing the effects of ABT-263 and its analog ABT-737 on platelets and leukemia cells from the same donor, we show, for the first time, that these BCL2/BCL-X(L) inhibitors do not offer any selective toxicity but induce apoptosis at similar concentrations in leukemia cells and platelets. However, reticulated platelets are less sensitive to apoptosis, supporting the hypothesis that treatment with ABT-263 induces a selective loss of older platelets and providing an explanation for the transient thrombocytopenia observed on ABT-263 treatment.

Deregulated apoptosis signaling in core-binding factor leukemia differentiates clinically relevant, molecular marker-independent subgroups

Core-binding factor (CBF) leukemias, characterized by translocations t(8;21) or inv(16)/t(16;16) targeting the CBF, constitute acute myeloid leukemia (AML) subgroups with favorable prognosis. However, about 40% of patients relapse and the current classification system does not fully reflect this clinical heterogeneity. Previously, gene expression profiling (GEP) revealed two distinct CBF leukemia subgroups displaying significant outcome differences and identified apoptotic signaling, MAPKinase signaling and chemotherapy-resistance mechanisms among the most significant differentially regulated pathways. We now tested different inhibitors of the respective pathways in a cell line model (six cell lines reflecting the CBF subgroup-specific gene expression alterations), and found apoptotic signaling to be differentiating between the CBF subgroup models. In accordance, primary samples from newly diagnosed CBF AML patients (n=23) also showed differential sensitivity to in vitro treatment with a Smac mimetic such as BV6, an antagonist of inhibitor of apoptosis (IAP) proteins, and ABT-737, a BCL2 inhibitor. Furthermore, GEP revealed the BV6-resistant cases to resemble the previously identified unfavorable CBF subgroup. Thus, our current findings show deregulated IAP expression and apoptotic signaling to differentiate clinically relevant CBF subgroups, which were independent of known molecular markers, thereby providing a starting point for novel therapeutic approaches.

Small molecule inhibition of the TNF family cytokine CD40 ligand through a subunit fracture mechanism

BIO8898 is one of several synthetic organic molecules that have recently been reported to inhibit receptor binding and function of the constitutively trimeric tumor necrosis factor (TNF) family cytokine CD40 ligand (CD40L, aka CD154). Small molecule inhibitors of protein-protein interfaces are relatively rare, and their discovery is often very challenging. Therefore, to understand how BIO8898 achieves this feat, we characterized its mechanism of action using biochemical assays and X-ray crystallography. BIO8898 inhibited soluble CD40L binding to CD40-Ig with a potency of IC(50) = 25 μM and inhibited CD40L-dependent apoptosis in a cellular assay. A co-crystal structure of BIO8898 with CD40L revealed that one inhibitor molecule binds per protein trimer. Surprisingly, the compound binds not at the surface of the protein but by intercalating deeply between two subunits of the homotrimeric cytokine, disrupting a constitutive protein-protein interface and breaking the protein's 3-fold symmetry. The compound forms several hydrogen bonds with the protein, within an otherwise hydrophobic binding pocket. In addition to the translational splitting of the trimer, binding of BIO8898 was accompanied by additional local and longer-range conformational perturbations of the protein, both in the core and in a surface loop. Binding of BIO8898 is reversible, and the resulting complex is stable and does not lead to detectable dissociation of the protein trimer. Our results suggest that a set of core aromatic residues that are conserved across a subset of TNF family cytokines might represent a generic hot-spot for the induced-fit binding of trimer-disrupting small molecules.

Bcl-xL-inhibitory BH3 mimetics can induce a transient thrombocytopathy that undermines the hemostatic function of platelets

BH3 mimetics are a new class of proapo-ptotic anticancer agents that have shown considerable promise in preclinical animal models and early-stage human trials. These agents act by inhibiting the pro-survival function of one or more Bcl-2-related proteins. Agents that inhibit Bcl-x(L) induce rapid platelet death that leads to thrombocytopenia; however, their impact on the function of residual circulating platelets remains unclear. In this study, we demonstrate that the BH3 mimetics, ABT-737 or ABT-263, induce a time- and dose-dependent decrease in platelet adhesive function that correlates with ectodomain shedding of the major platelet adhesion receptors, glycoprotein Ibα and glycoprotein VI, and functional down-regulation of integrin α(IIb)β(3). Analysis of platelets from mice treated with higher doses of BH3 mimetics revealed the presence of a subpopulation of circulating platelets undergoing cell death that have impaired activation responses to soluble agonists. Functional analysis of platelets by intravital microscopy revealed a time-dependent defect in platelet aggregation at sites of vascular injury that correlated with an increase in tail bleeding time. Overall, these studies demonstrate that Bcl-x(L)-inhibitory BH3 mimetics not only induce thrombocytopenia but also a transient thrombocytopathy that can undermine the hemostatic function of platelets.

Targeting oncogenic protein-protein interactions by diversity oriented synthesis and combinatorial chemistry approaches

We are currently witnessing a decline in the development of efficient new anticancer drugs, despite the salient efforts made on all fronts of cancer drug discovery. This trend presumably relates to the substantial heterogeneity and the inherent biological complexity of cancer, which hinder drug development success. Protein-protein interactions (PPIs) are key players in numerous cellular processes and aberrant interruption of this complex network provides a basis for various disease states, including cancer. Thus, it is now believed that cancer drug discovery, in addition to the design of single-targeted bioactive compounds, should also incorporate diversity-oriented synthesis (DOS) and other combinatorial strategies in order to exploit the ability of multi-functional scaffolds to modulate multiple protein-protein interactions (biological hubs). Throughout the review, we highlight the chemistry driven approaches to access diversity space for the discovery of small molecules that disrupt oncogenic PPIs, namely the p53-Mdm2, Bcl-2/Bcl-xL-BH3, Myc-Max, and p53-Mdmx/Mdm2 interactions.

Bcl-2 is a better therapeutic target than c-Myc, but attacking both could be a more effective treatment strategy for B-cell lymphoma with concurrent Bcl-2 and c-Myc overexpression

OBJECTIVE

The prognosis for diffuse large B-cell lymphomas with concomitant overexpression of c-Myc and Bcl-2 remains dismal; there is an urgent need to clarify the significance of these two oncogenes as therapeutic targets for a more effective treatment strategy.

MATERIALS AND METHODS

We established two novel cell lines, KPUM-MS3 and KPUM-UH1, from two chemoresistant patients with diffuse large B-cell lymphomas with concomitant overexpression of c-Myc and Bcl-2, and investigated the significance of c-Myc and Bcl-2 as therapeutic targets.

RESULTS

KPUM-MS3 possesses t(14;18)(q32;q21) chromosomal translocation and KPUM-UH1 bcl-2 gene amplification, both of which account for Bcl-2 overexpression. Chromosomal translocation t(8;14)(q24;q34) was found to coexist only in KPUM-UH1, overexpression of pvt-1 messenger RNA was detected only in KPUM-MS3, and reduced expression of miR-143 and miR-145 was identified in both. Working together, these abnormalities can contribute to c-Myc overexpression. Using ABT-263, an inhibitor for Bcl-2, and 10058-F4, an inhibitor for c-Myc, we found that both cell lines were more highly sensitive to cell death as a result of Bcl-2 inhibition than of c-Myc inhibition. When combined with genotoxic agents, ABT-263 exerted additive and/or synergistic cell-killing effects, while 10058-F4 showed, at most, a modest combinatory effect. Importantly, the combination of ABT-263 and 10058-F4 had a synergistic cell-killing effect on both cell lines.

CONCLUSIONS

Our data suggest that Bcl-2 is a better therapeutic target than c-Myc, but attacking both Bcl-2 and c-Myc would be an even more effective treatment strategy for diffuse large B-cell lymphomas with concurrent Bcl-2 and c-Myc overexpression.

Should there be a standard therapy for mantle cell lymphoma?

Mantle cell lymphoma (MCL) is an uncommon subtype of B-cell lymphomathat is characterized by monoclonal B cells that express CD5 on their surface, but not CD23, and harbor the t(11:14) chromosomal translocation that leads to dysregulated expression of cyclin D1. MCL is a biologically and clinically heterogeneous disease. It has the unfavorable characteristics of both aggressive and indolent lymphoma in that MCL is not curable with current standard therapy, yet patients have a shorter survival compared with other indolent histology. MCL is incurable, yet more intensive therapy does lead to longer disease-free intervals; therefore, treatment must be designed to optimize survival while maintaining quality of life. Thus, therapy should be individualized based on both the clinical behavior of the lymphoma and the patient’s status. While there is no clear standard therapy that can be recommended for all patients, there may be an optimal choice for each patient. Knowledge of the expected clinical benefits and toxicities of various approaches will allow the physician and patient to appropriately select the therapy.

Navitoclax enhances the efficacy of taxanes in non-small cell lung cancer models

PURPOSE

To explore the potential of navitoclax in combination with taxane-based chemotherapy in the treatment of non-small cell lung cancer (NSCLC) by defining mechanism of synergy and identifying correlative biomarkers.

EXPERIMENTAL DESIGN

We treated a panel of NSCLC lines with a dose matrix of paclitaxel and navitoclax (formerly ABT-263), an inhibitor of Bcl-2, Bcl-x(L), and Bcl-w (1), and evaluated synergy. We next used time-lapse microscopy to explore mechanism of synergy. Finally, we developed an immunohistochemical assay and assessed prevalence of Bcl-x(L) in NSCLC tumor tissues.

RESULTS

All cell lines exhibit greater than additive response to the combination of navitoclax and a taxane. These results were extended to mouse xenograft tumor models, in which the combination is more efficacious than either single-agent docetaxel or navitoclax. Addition of navitoclax to paclitaxel decreases the time from mitotic entry to cell death and changes cell fate from mitotic slippage to death during mitotic arrest. The relative levels of Bcl-x(L) and Mcl-1 correlate with the extent of synergy, suggesting that cancers with elevated levels of Bcl-x(L) will be relatively resistant to taxane-based therapy but could benefit from the addition of navitoclax to taxane treatment. Finally, a significant percentage of NSCLC patient samples exhibit relatively high Bcl-x(L) levels.

CONCLUSIONS

The addition of navitoclax to taxane-based chemotherapy in NSCLC has the potential to increase efficacy, particularly in patients whose tumors express high levels of Bcl-x(L).

Relapsed/refractory diffuse large B-cell lymphoma

Despite overall improvements in outcomes of diffuse large B-cell lymphoma (DLBCL), approximately one-third of patients will develop relapsed/refractory disease that remains a major cause of morbidity and mortality. Novel insights from gene-expression analyses have increased our understanding of chemotherapy resistance and yielded rational targets for therapeutic intervention to both prevent and treat relapsed/refractory DLBCL. The clinical approach to relapsed/refractory DLBCL should include high-dose therapy and autologous stem cell transplantation (HD-ASCT) with curative intent in patients without comorbidities. Results from the recently reported CORAL study suggest that patients refractory to rituximab-containing regimens have inferior outcomes with HD-ASCT. Ongoing efforts to improve ASCT include novel conditioning regimens and evaluation of maintenance approaches after ASCT. Unfortunately, because the majority of patients are not eligible for ASCT due to refractory disease or age/comorbidities, these approaches have limited impact. The large group of patients not eligible for ASCT have incurable disease and should be referred for clinical trials of rationally targeted agents.