Bryostatin 1 and UCN-01 potentiate 1-beta-D-arabinofuranosylcytosine-induced apoptosis in human myeloid leukemia cells through disparate mechanisms

Randomized phase II trial of cytosine arabinoside with and without the CHK1 inhibitor MK-8776 in relapsed and refractory acute myeloid leukemia

PURPOSE

Cytosine arabinoside (AraC) remains the backbone of most treatment regimens for acute myeloid leukemia (AML). Incorporation of AraC into DNA activates checkpoint kinase 1 (Chk1), leading to cell-cycle arrest and diminished AraC cytotoxicity, which can be reversed by the selective Chk1 inhibitor MK-8776. Building on a Phase I trial, we conducted a phase II trial comparing timed sequential AraC with or without MK-8776.

METHODS

Patients with relapsed or primary refractory AML were randomized 1:1 to receive either AraC with MK-8776 (Arm A); or AraC alone (Arm B).

RESULTS

32 patients were treated: 14 assigned to Arm A and 18 to Arm B. There were 5 (36%) complete responses (CR/CRi) and 1 (7%) partial response (PR) in Arm A, and 8 (44%) CR/CRis and 1 (6%) PR in Arm B. Median survival did not differ significantly between the two groups (5.9months in Arm A vs. 4.5 months in Arm B). MK-8776 led to a robust increase in DNA damage in circulating leukemic blasts as measured by increased γ-H2AX (16.9%±6.1% prior and 36.4%±6.8% at one hour after MK-8776 infusion, p=0.016).

CONCLUSION

Response rates and survival were similar between the two groups in spite of evidence that MK-8776 augmented DNA damage in circulating leukemic blasts. Better than expected results in the control arm using timed sequential AraC and truncated patient enrollment may have limited the ability to detect clinical benefit from the combination.

The impact of Tegillarca granosa extract haishengsu on HL-60 cell

Haishengsu (Hss) is a purified protein from Tegillarca granosa that has been used as a traditional Chinese medicine to treat cancer for more than a century. In this study, we observed the impact of Haishengsu (Hss) on the proliferation and differentiation of HL-60 cells in the leukemic cell line by taking tretinoin and AS₂O₃ as a positive control and making a comparative analysis between the effect of Hss and tretinoin and AS₂O₃. We found that Hss could significantly inhibit the proliferation of HL-60 cells and caused most of the cells to stay in the G0/G1 phase. Its effect was much stronger than that of tretinoin and AS₂O₃, and the ability of Hss to induce differentiation was close to tretinoin. Hss functions probably by inhibiting the expression of the Bcl-2 and MPO genes and further promoting the expression of the Bax gene. Hss has a significant effect on both inhibiting the proliferation and inducing the differentiation of HL-60 cells. It is possible that Hss may be a new kind of clinical differentiation inducer.

Profiling of the Tox21 chemical collection for mitochondrial function to identify compounds that acutely decrease mitochondrial membrane potential

BACKGROUND

Mitochondrial dysfunction has been implicated in the pathogenesis of a variety of disorders including cancer, diabetes, and neurodegenerative and cardiovascular diseases. Understanding whether different environmental chemicals and druglike molecules impact mitochondrial function represents an initial step in predicting exposure-related toxicity and defining a possible role for such compounds in the onset of various diseases.

OBJECTIVES

We sought to identify individual chemicals and general structural features associated with changes in mitochondrial membrane potential (MMP).

METHODS

We used a multiplexed [two end points in one screen; MMP and adenosine triphosphate (ATP) content] quantitative high throughput screening (qHTS) approach combined with informatics tools to screen the Tox21 library of 10,000 compounds (~ 8,300 unique chemicals) at 15 concentrations each in triplicate to identify chemicals and structural features that are associated with changes in MMP in HepG2 cells.

RESULTS

Approximately 11% of the compounds (913 unique compounds) decreased MMP after 1 hr of treatment without affecting cell viability (ATP content). In addition, 309 compounds decreased MMP over a concentration range that also produced measurable cytotoxicity [half maximal inhibitory concentration (IC50) in MMP assay/IC50 in viability assay ≤ 3; p < 0.05]. More than 11% of the structural clusters that constitute the Tox21 library (76 of 651 clusters) were significantly enriched for compounds that decreased the MMP.

CONCLUSIONS

Our multiplexed qHTS approach allowed us to generate a robust and reliable data set to evaluate the ability of thousands of drugs and environmental compounds to decrease MMP. The use of structure-based clustering analysis allowed us to identify molecular features that are likely responsible for the observed activity.

New opportunities in chemosensitization and radiosensitization: modulating the DNA-damage response

Many current cancer treatments, including certain classes of chemotherapeutics and radiation, induce cytotoxicity by damaging DNA. However, many cancers are resistant to these therapies, which represents a significant challenge in the clinic. Thus, modulating DNA-damage responses to selectively enhance the sensitivity of cancer cells to these therapies is highly desirable. When DNA damage is detected, DNA checkpoint mechanisms are activated to halt cells at various phases of the cell cycle. Simultaneously, DNA-damage sensors transduce signals to activate DNA-repair mechanisms via de novo expression or post-translational modification of enzymes required for DNA repair. p53 is the major player in a checkpoint that arrests cells at the G1/S boundary, while checkpoint kinase (Chk)1 is critical for the G2/M checkpoint and also the S checkpoint that prevents cell cycle progression after replication defects (intra-S-phase checkpoint) or S/M uncoupling (S/M checkpoint). Poly(ADP-ribose) polymerase is involved in sensing DNA single-strand breaks and inducing DNA repair via poly(ADP-ribosyl)ating various DNA-binding and DNA-repair proteins. In this review, strategies for implementing small-molecule inhibitors of poly(ADP-ribose) polymerase and Chk1, which are emerging as potential adjuncts to current therapies, are discussed.

Biological profile of the less lipophilic and synthetically more accessible bryostatin 7 closely resembles that of bryostatin 1

The bryostatins are a group of 20 macrolides isolated by Pettit and co-workers from the marine organism Bugula neritina. Bryostatin 1, the flagship member of the family, has been the subject of intense chemical and biological investigations due to its remarkably diverse biological activities, including promising indications as therapy for cancer, Alzheimer's disease, and HIV. Other bryostatins, however, have attracted far less attention, most probably due to their relatively low natural abundance and associated scarcity of supply. Among all macrolides in this family, bryostatin 7 is biologically the most potent protein kinase C (PKC) ligand (in terms of binding affinity) and also the first bryostatin to be synthesized in the laboratory. Nonetheless, almost no biological studies have been carried out on this agent. We describe herein the total synthesis of bryostatin 7 based on our pyran annulation technology, which allows for the first detailed biological characterizations of bryostatin 7 with side-by-side comparisons to bryostatin 1. The results suggest that the more easily synthesized and less lipophilic bryostatin 7 may be an effective surrogate for bryostatin 1.

Phase I and pharmacologic trial of cytosine arabinoside with the selective checkpoint 1 inhibitor Sch 900776 in refractory acute leukemias

PURPOSE

Incorporation of cytarabine into DNA activates checkpoint kinase 1 (Chk1), which stabilizes stalled replication forks, induces S-phase slowing, and diminishes cytarabine cytotoxicity. The selective Chk1 inhibitor SCH 900776 abrogates cytarabine-induced S-phase arrest and enhances cytarabine cytotoxicity in acute leukemia cell lines and leukemic blasts in vitro. To extend these findings to the clinical setting, we have conducted a phase I study of cytarabine and SCH 900776.

EXPERIMENTAL DESIGN

Twenty-four adults with relapsed and refractory acute leukemias received timed sequential, continuous infusion cytarabine 2 g/m(2) over 72 hours (667 mg/m(2)/24 hours) beginning on day 1 and again on day 10. SCH 900776 was administered as a 15- to 30-minute infusion on days 2, 3, 11, and 12. The starting dose of SCH 900776 was 10 mg/m(2)/dose.

RESULTS

Dose-limiting toxicities consisting of corrected QT interval prolongation and grade 3 palmar-plantar erythrodysesthesia occurred at 140 mg flat dosing (dose level 5, equivalent to 80 mg/m(2)). Complete remissions occurred in 8 of 24 (33%) patients, with 7 of 8 at 40 mg/m(2) or higher. SCH 900776 did not accumulate at any dose level. Marrow blasts obtained pretreatment and during therapy showed increased phosphorylation of H2Ax after SCH 900776 beginning at 40 mg/m(2), consistent with unrepaired DNA damage.

CONCLUSIONS

These data support a randomized phase II trial of cytarabine +/- SCH 900776 at a recommended flat dose of 100 mg (equivalent to 56 mg/m(2)) for adults with poor-risk leukemias. The trial (SP P05247) was registered at www.clinicaltrials.gov as NCT00907517.

Effects of selective checkpoint kinase 1 inhibition on cytarabine cytotoxicity in acute myelogenous leukemia cells in vitro

PURPOSE

Previous studies have shown that the replication checkpoint, which involves the kinases ataxia telangiectasia mutated and Rad3 related (ATR) and Chk1, contributes to cytarabine resistance in cell lines. In the present study, we examined whether this checkpoint is activated in clinical acute myelogenous leukemia (AML) during cytarabine infusion in vivo and then assessed the impact of combining cytarabine with the recently described Chk1 inhibitor SCH 900776 in vitro.

EXPERIMENTAL DESIGN

AML marrow aspirates harvested before and during cytarabine infusion were examined by immunoblotting. Human AML lines treated with cytarabine in the absence or presence of SCH 900776 were assayed for checkpoint activation by immunoblotting, nucleotide incorporation into DNA, and flow cytometry. Long-term effects in AML lines, clinical AML isolates, and normal myeloid progenitors were assayed using clonogenic assays.

RESULTS

Immunoblotting revealed increased Chk1 phosphorylation, a marker of checkpoint activation, in more than half of Chk1-containing AMLs after 48 hours of cytarabine infusion. In human AML lines, SCH 900776 not only disrupted cytarabine-induced Chk1 activation and S-phase arrest but also markedly increased cytarabine-induced apoptosis. Clonogenic assays demonstrated that SCH 900776 enhanced the antiproliferative effects of cytarabine in AML cell lines and clinical AML samples at concentrations that had negligible impact on normal myeloid progenitors.

CONCLUSIONS

These results not only provide evidence for cytarabine-induced S-phase checkpoint activation in AML in the clinical setting, but also show that a selective Chk1 inhibitor can overcome the S-phase checkpoint and enhance the cytotoxicity of cytarabine. Accordingly, further investigation of the cytarabine/SCH 900776 combination in AML appears warranted.

Constitutive activation of the DNA damage signaling pathway in acute myeloid leukemia with complex karyotype: potential importance for checkpoint targeting therapy

Genomic instability in solid tumors participates in the oncogenetic process and is associated with the activation of the DNA damage response pathway. Here, we report the activation of the constitutive DNA damage and checkpoint pathway associated with complex karyotypes in samples from patients with acute myeloid leukemia (AML). We show that antagonizing CHK1 kinase with a small inhibitory compound or by RNA interference strongly reduces the clonogenic properties of high-DNA damage level AML samples, particularly those with complex karyotypes. Moreover, we observe a beneficial effect of CHK1 inhibition in high-DNA damage level AML samples treated with 1-beta-d-arabinofuranosylcytosine. In contrast, CHK1 inhibition has no effect on the clonogenic properties of normal hematopoietic progenitors. All together, our results indicate that CHK1 inhibition may represent an attractive therapeutic opportunity in AML with complex karyotype.

Substitution on the A-ring confers to bryopyran analogues the unique biological activity characteristic of bryostatins and distinct from that of the phorbol esters

A close structural analogue of bryostatin 1, which differs from bryostatin 1 only by the absence of the C(30) carbomethoxy group (on the C(13) enoate of the B-ring), has been prepared by total synthesis. Biological assays reveal a crucial role for substitution in the bryostatin 1 A-ring in conferring those responses which are characteristic of bryostatin 1 and distinct from those observed with PMA.

Synergism between etoposide and 17-AAG in leukemia cells: critical roles for Hsp90, FLT3, topoisomerase II, Chk1, and Rad51

PURPOSE

DNA-damaging agents, such as etoposide, while clinically useful in leukemia therapy, are limited by DNA repair pathways that are not well understood. 17-(Allylamino)-17-demethoxygeldanamycin (17-AAG), an inhibitor of the molecular chaperone heat shock protein 90 (Hsp90), inhibits growth and induces apoptosis in FLT3(+) leukemia cells. In this study, we evaluated the effects of etoposide and 17-AAG in leukemia cells and the roles of Hsp90, FMS-like tyrosine kinase 3 (FLT3), checkpoint kinase 1 (Chk1), Rad51, and topoisomerase II in this inhibition.

EXPERIMENTAL DESIGN

The single and combined effects of 17-AAG and etoposide and the mechanism of these effects were evaluated. FLT3 and the DNA repair-related proteins, Chk1 and Rad51, were studied in small interfering RNA (siRNA)-induced cell growth inhibition experiments in human leukemia cells with wild-type or mutated FLT3.

RESULTS

We found that etoposide and the Hsp90/FLT3 inhibitor 17-AAG, had synergistic inhibitory effects on FLT3(+) MLL-fusion gene leukemia cells. Cells with an internal tandem duplication (ITD) FLT3 (Molm13 and MV4;11) were more sensitive to etoposide/17-AAG than leukemias with wild-type FLT3 (HPB-Null and RS4;11). A critical role for FLT3 was shown in experiments with FLT3 ligand and siRNA targeted to FLT3. An important role for topoisomerase II and the DNA repair-related proteins, Chk1 and Rad51, in the synergistic effects was suggested from the results.

CONCLUSIONS

The repair of potentially lethal DNA damage by etoposide in leukemia cells is dependent on intact and functioning FLT3 especially leukemias with ITD-FLT3. These data suggest a rational therapeutic strategy for FLT3(+) leukemias that combines etoposide or other DNA-damaging agents with Hsp90/FLT3 inhibitors such as 17-AAG.

Marine pharmacology in 2003–2004: Anti-tumour and cytotoxic compounds

During 2003 and 2004, marine pharmacology research directed towards the discovery and development of novel anti-tumour agents was published in 163 peer-reviewed articles. The purpose of this review is to present a structured assessment of the anti-tumour and cytotoxic properties of 150 marine natural products, many of which are novel compounds that belong to diverse structural classes, including polyketides, terpenes, steroids and peptides. The organisms yielding these bioactive marine compounds include invertebrate animals, algae, fungi and bacteria. Anti-tumour pharmacological studies were conducted with 31 structurally defined marine natural products in a number of experimental and clinical models that further defined their mechanisms of action. Particularly potent in vitro cytotoxicity data generated with murine and human tumour cell lines was reported for 119 novel marine chemicals with as yet undetermined mechanisms of action. Noteworthy is the fact that marine anti-cancer research was sustained by a global collaborative effort, involving researchers from Australia, Austria, Canada, China, Egypt, France, Germany, Italy, Japan, Mexico, the Netherlands, New Zealand, Papua New Guinea, the Philippines, South Africa, South Korea, Spain, Switzerland, Taiwan, Thailand and the United States of America (USA). Finally, this 2003-2004 overview of the marine pharmacology literature highlights the fact that the discovery of novel marine anti-tumour agents continued at the same pace as during 1998-2002.

Pharmacodynamics of cytarabine alone and in combination with 7-hydroxystaurosporine (UCN-01) in AML blasts in vitro and during a clinical trial

Chk1 and Akt signaling facilitate survival of cells treated with nucleoside analogues. Activation of Chk1 in response to cytarabine (ara-C) induced an S-phase checkpoint characterized by the inhibition of Cdk2, cell cycle arrest, no change in constitutively active Akt, or low-stress kinase signaling in ML-1 cells. However, inhibition of Chk1 by UCN-01 in S-phase-arrested cells resulted in an abrogation of the checkpoint, inhibition of Akt, activation of JNK, and a rapid induction of apoptosis. Similarly, primary acute myelogenous leukemia (AML) blasts exposed to ara-C and UCN-01 demonstrated a selective loss in cloning potential when compared with normal progenitors. Therefore, we evaluated a pilot clinical trial of ara-C in combination with UCN-01 in patients with relapsed AML. Blasts from some patients demonstrated a previously activated Chk1-Cdk2 DNA damage response pathway that decreased during therapy. Constitutively phosphorylated Akt kinase declined on addition of UCN-01 to the ara-C infusion, an action accompanied by an activation of JNK and reduction in absolute AML blast counts. Thus, use of UCN-01 in combination with ara-C decreases Chk1 phosphorylation, inhibits the Akt survival pathway, and activates JNK during the course of therapy, offering a rationale for the cytotoxic action of this combination during AML treatment.

Heat shock protein 90 inhibition sensitizes acute myelogenous leukemia cells to cytarabine

Previous studies demonstrated that ataxia telangiectasia mutated- and Rad3-related (ATR) kinase and its downstream target checkpoint kinase 1 (Chk1) facilitate survival of cells treated with nucleoside analogs and other replication inhibitors. Recent results also demonstrated that Chk1 is depleted when cells are treated with heat shock protein 90 (Hsp90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG). The present study examined the effects of 17-AAG and its major metabolite, 17-aminogeldanamycin (17-AG), on Chk1 levels and cellular responses to cytarabine in human acute myelogenous leukemia (AML) cell lines and clinical isolates. Cytarabine, at concentrations as low as 30 nM, caused activating phosphorylation of Chk1, loss of the phosphatase Cdc25A, and S-phase slowing. Conversely, treatment with 100 to 300 nM 17-AAG for 24 hours caused Chk1 depletion that was accompanied by diminished cytarabine-induced S-phase accumulation, decreased Cdc25A degradation, and enhanced cytotoxicity as measured by inhibition of colony formation and induction of apoptosis. Additional studies demonstrated that small inhibitory RNA (siRNA) depletion of Chk1 also sensitized cells to cytarabine, whereas disruption of the phosphatidylinositol 3-kinase (PI3k) signaling pathway, which is also blocked by Hsp90 inhibition, did not. Collectively, these results suggest that treatment with 17-AAG might represent a means of reversing checkpoint-mediated cytarabine resistance in AML.

Potent antileukemic interactions between flavopiridol and TRAIL/Apo2L involve flavopiridol-mediated XIAP downregulation

Interactions between the cyclin-dependent kinase inhibitor flavopiridol (FP) and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/Apo2L), were examined in human leukemia cells (U937 and Jurkat). Coexposure of cells to marginally toxic concentrations of TRAIL and FP (24 h) synergistically increased mitochondrial injury (eg, cytochrome c, AIF, Smac/DIABLO release), cytoplasmic depletion of Bax, activation of Bid as well as caspase-8 and -3, PARP cleavage, and apoptosis. Coadministration of TRAIL markedly increased FP-induced apoptosis in leukemic cells ectopically expressing Bcl-2, Bcl-x(L), or a phosphorylation loop-deleted form of Bcl-2 (DeltaBcl-2), whereas lethality was substantially attenuated in cells ectopically expressing CrmA, dominant-negative-FADD, or dominant-negative-caspase-8. TRAIL/FP induced no discernible changes in FLIP, DR4, DR5, Mcl-1, or survivin expression, modest declines in levels of DcR2 and c-IAP, but resulted in the marked transcriptional downregulation of XIAP. Moreover, cells stably expressing an XIAP-antisense construct exhibited a pronounced increase in TRAIL sensitivity comparable to degrees of apoptosis achieved with TRAIL/FP. Conversely, enforced XIAP expression significantly attenuated caspase activation and TRAIL/FP lethality. Together, these findings suggest that simultaneous activation of the intrinsic and extrinsic apoptotic pathways by TRAIL and FP synergistically induces apoptosis in human leukemia cells through a mechanism that involves FP-mediated XIAP downregulation.

Mechanism targeted discovery of antitumor marine natural products

Antitumor drug discovery programs aim to identify chemical entities for use in the treatment of cancer. Many strategies have been used to achieve this objective. Natural products have always played a major role in anticancer medicine and the unique metabolites produced by marine organisms have increasingly become major players in antitumor drug discovery. Rapid advances have occurred in the understanding of tumor biology and molecular medicine. New insights into mechanisms responsible for neoplastic disease are significantly changing the general philosophical approach towards cancer treatment. Recently identified molecular targets have created exciting new means for disrupting tumor-specific cell signaling, cell division, energy metabolism, gene expression, drug resistance and blood supply. Such tumor-specific treatments could someday decrease our reliance on traditional cytotoxicity-based chemotherapy and provide new less toxic treatment options with significantly fewer side effects. Novel molecular targets and state-of-the-art, molecular mechanism-based screening methods have revitalized antitumor research and these changes are becoming an ever-increasing component of modern antitumor marine natural products research. This review describes marine natural products identified using tumor-specific mechanism-based assays for regulators of angiogenesis, apoptosis, cell cycle, macromolecule synthesis, mitochondrial respiration, mitosis, multidrug efflux and signal transduction. Special emphasis is placed on natural products directly discovered using molecular mechanism-based screening.

Interruption of the NF-κB pathway by Bay 11-7082 promotes UCN-01-mediated mitochondrial dysfunction and apoptosis in human multiple myeloma cells

Interactions between pharmacologic NF-κB inhibitors (eg, Bay 11-7082, SN-50) and the checkpoint abrogator UCN-01 have been examined in human multiple myeloma (MM) cells. Exposure of U266 cells to Bay 11-7082 (Bay) in combination with UCN-01 resulted in the abrogation of NF-κB/DNA binding activity and the synergistic induction of apoptosis. Comparable synergism was observed in other MM cell lines and patient-derived CD138+ cells and between an inhibitory peptide of NF-κB (SN50) and UCN-01. Bay/UCN-01-mediated lethality involved mitochondrial dysfunction, caspase cleavage, and poly adenosine diphosphate-ribose polymerase (PARP) degradation. Although Bay modestly blocked UCN-01-induced extracellular signal-regulated kinase (ERK) phosphorylation, coadministration activated c-Jun N-terminal kinase (JNK) and cdc2/cdk1 and down-regulated Mcl-1, XIAP, and Bcl-xL. Transfection with a constitutively activated mitogen-activated protein kinase kinase (MEK1)/green fluorescent protein (GFP) construct failed to block apoptosis induced by Bay/UCN-01 but significantly attenuated MEK inhibitor (U0126)/UCN-01-induced lethality. Inhibiting JNK activation with SP600125 or D-JNKI1 peptide markedly reduced Bay/UCN-01-mediated mitochondrial dysfunction and apoptosis and the down-regulation of Mcl-1, XIAP, and Bcl-xL but not of cdc2/cdk1 activation. Stable transfection of cells with dominant-negative caspase-9 dramatically diminished Bay/UCN-01 lethality without altering JNK or cdc2/cdk1 activation. Neither interleukin-6 (IL-6)- nor fibronectin-mediated adherence conferred resistance to Bay/UCN-01-induced apoptosis. Together, these findings suggest that a strategy combining UCN-01 with disruption of the IκB kinase (IKK)/IκB/NF-κB pathway warrants attention in MM. (Blood. 2004;103:2761-2770)

Issues and progress with protein kinase inhibitors for cancer treatment

Identification of the key roles of protein kinases in cancer has led to extensive efforts to develop kinase inhibitors for the treatment of a wide range of cancers, and more than 30 such agents are now in clinical trials. Here, we consider the crucial issues in the development of kinase inhibitors for cancer, and discuss strategies to address the challenges raised by these issues in the light of preclinical and clinical experiences so far.