apoptosis in the treatment of cancer: a promise kept?

Apoptosis in the treatment of cancer: a promise kept?Xue Wei Meng, Sun-Hee Lee and Scott H Kaufmann

A common feature of cancer cells is their ability to evade

apoptosis as a result of alterations that block cell death

signaling pathways. The extensive research efforts that

elucidated these signaling pathways over the past decade have

set the stage for the development of therapeutic agents that

either kill cancer cells selectively or reset their apoptotic

threshold. Over the past two years a number of these agents

have been evaluated in preclinical and clinical trials. The results

of these studies suggest that it might soon be possible to

modulate apoptosis in cancer cells for therapeutic benefit.

Addresses

Division of Oncology Research (X.W.M., S.H.K.) and Department of

Molecular Pharmacology (S-H.L., S.H.K.), Mayo Clinic College of

Medicine, Rochester, MN 55905

Corresponding author: Kaufmann, Scott H ([email protected])

Xue Wei Meng and Sun-Hee Lee contributed equally to this manuscript.

Current Opinion in Cell Biology 2006, 18:668–676

This review comes from a themed issue on

Cell division, and growth and death

Edited by Bill Earnshaw and Yuri Lazebnik

Available online 16th October 2006

0955-0674/$ – see front matter

# 2006 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.ceb.2006.10.008

IntroductionThe promise of improved cancer therapy has been one of

the motivations driving cell death research over the past

decade. This hope of improved treatment has sprung

from two sets of observations. First, even though cancer

was originally viewed as a disorder involving increased

proliferation, there has been a growing awareness that

many of the changes contributing to cancer development

also diminish the ability of cells to undergo apoptosis

[1,2]. When this morphologically and biochemically dis-

tinct cell-autonomous death process is inhibited,

damaged or defective cells that ordinarily would be

eliminated instead accumulate and wreak havoc. Second,

a variety of studies have demonstrated that apoptosis is a

frequent outcome of effective therapy [3]. Although

chemotherapy can produce other outcomes, including

transient cell cycle arrest, senescence and autophagy

[2,4,5], one current view is that permanent elimination

of neoplastic cells through a process such as apoptosis is

required for cancer eradication. Given these observations,


it has been suggested that an important goal of cancer

drug development should be to facilitate apoptosis in

neoplastic cells.

In the paragraphs that follow, we review recent preclinical

and clinical studies of potential anticancer agents that

directly affect components of the core apoptotic machin-

ery. These studies demonstrate significant progress but

also indicate that much work remains to be done before

apoptosis can be successfully modulated in the clinic.

Tickling cancer cells to death: triggeringapoptosis through the death receptorpathwayOne of the two canonical cell death pathways begins with

ligation of a distinct group of cell surface receptors,

recruitment of adaptor molecules, and activation of an

intracellular protease cascade that leads to cellular dis-

assembly (see legend to Figure 1). The components of

this ‘death receptor (DR)’ or ‘extrinsic’ pathway have

been extensively described in recent reviews [6–9]. As

the broad outlines of this pathway have been elucidated,

several approaches for triggering this pathway in cancer

cells have been studied.

Administration of death ligandThe possibility of directly activating this pathway by

administering death ligands has been examined. Tumor

necrosis factor-a (TNFa) has demonstrable activity

against soft tissue sarcomas when infused locally but is

too toxic for systemic administration. Because of hepato-

toxicity, Fas ligand is likewise too toxic for systemic

dosing. In contrast, TRAIL, a major component of the

arsenal used by natural killer cells to keep nascent neo-

plasms in check, is toxic to human tumor cells in vitro and

in vivo but has limited effects on normal cells [10,11].

On the basis of these observations, several TRAIL recep-

tor ligands recently entered the clinic. Agonistic antibody

to DR4 (TRAIL receptor 1) was evaluated in a phase I

(dose escalation/toxicity) trial involving 22 patients with

various solid tumors [12]. At doses up to 10 mg/kg, no

dose-limiting toxicities were observed. A phase II (effi-

cacy) trial of this antibody in 40 patients with relapsed or

refractory non-Hodgkins lymphoma yielded a response

rate of 8%, including one complete remission (CR) and

two partial responses (PRs), as well as prolonged stabili-

zation in 30% of patients [13]. Agonistic anti-DR5 anti-

body also underwent phase I testing in solid tumor

patients [14]. Doses of up to 10 mg/kg resulted in serum

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Apoptosis in the treatment of cancer Meng, Lee and Kaufmann 669

Figure 1

Strategies to induce or enhance death receptor (DR) signaling. Several members of the tumor necrosis factor-a (TNFa) receptor superfamily,

a group of type 1 membrane polypeptides that are activated by members of the TNFa family of ligands, signal cell death under certain

circumstances [6–9]. In particular, Fas, TNFa receptor 1, DR3, TRAIL receptors 1 and 2 (DR4 and DR5) and DR6, which all contain related

intracellular motifs called death domains (DDs), can initiate signaling that results in cell death. In each case, a poorly understood biochemical

change induced by ligation of the receptor results in binding of a cytoplasmic adaptor molecule typified by the FADD (Fas-associated death

domain) polypeptide to the DD. Upon receptor-mediated oligomerization, FADD in turn serves as a cofactor for the oligomerization and

activation of caspase 8 and/or caspase 10, two members of a cysteine protease family that cleave substrates on the carboxyl terminal side of

aspartate residues [74]. Caspases 8 and/or 10 then cleave a limited number of substrates, including procaspase 3 and Bid, to initiate the

cell death process. As a BH3-only polypeptide that is activated by caspase 8-mediated cleavage, Bid represents an important means of

crosstalk between this pathway and the mitochondrial pathway (see Figure 2) in some cells.Anti-cancer strategies undergoing preclinical or

clinical testing can affect this pathway in the following five ways (numbers refer to steps shown in the figure). 1. Ligation of DR4 and/or DR5.

Examples of death ligands in clinical trials include recombinant human TRAIL (AMG 951) and agonistic antibody to DR4 (HGS-ETR1) or

DR5 (HGS-ETR2). 2. Up-regulation of DR5 and enhanced DR clustering. These alterations are observed after treatment with the proteasome

inhibitors bortezimib and MG-132, Cox-2 inhibitors, or casein kinase inhibitors. 3. Dephosphorylation of FADD. Drugs that act in this manner

include MEK inhibitors CI-1040 and PD098059. 4. Enhanced procaspase 8 recruitment to FADD. This alteration is observed after treatment

with the proteasome inhibitor MG-132 or casein kinase 2 inhibitors. 5. Down-regulation of c-FLIP. This alteration has been reported after

exposure to the proteasome inhibitor bortezimib or the mTOR inhibitor rapamycin.

levels of agonistic antibody that are associated with

activity in preclinical models and produced disease

stabilization in 7 of 22 patients with few side effects.

Finally, a soluble form of recombinant human TRAIL


itself underwent phase I testing (http://www.amgen.com/

science/pipe_AMG951.html). In contrast to agonistic anti-

DR5 antibody, which exhibits a serum half-life of 15 days

[14], TRAIL was rapidly cleared from the circulation [10],


http://www.amgen.com/science/pipe_AMG951.html

http://www.amgen.com/science/pipe_AMG951.html

670 Cell division, and growth and death

raising the possibility that a prolonged infusion or depot

form of this agent might be required to provide the

sustained exposure required to trigger cell death. Further

results of these trials are awaited with interest.

Enhancing the activity of TRAIL receptorligandsBecause almost half of all human tumor cell lines are

TRAIL-resistant, there has been considerable interest in

augmenting TRAIL action. One strategy involves design-

ing TRAIL derivatives that elicit increased cytotoxicity

[15�]. A second approach involves administration of

TRAIL or agonistic antibodies in combination with other

agents that enhance activity of the death ligand

(Figure 1). For example, it has been reported that the

proteasome inhibitor bortezomib sensitizes various neo-

plastic cells to TRAIL [16–18] but not to DNA damaging

agents [19]. In some cells, this selective sensitization

reportedly reflects increased caspase 8 recruitment to

FADD [17]. In other cells, synergy between the experi-

mental proteasome inhibitor MG-132 and TRAIL reflects

MG-132-induced stabilization of CCAAT/enhancer-

binding protein homologous protein, a regulator of

DR5 transcription, leading to DR5 upregulation

[20,21]. In a similar vein, histone deacetylase inhibitors

enhance the cytotoxicity of TRAIL in some cells by

increasing expression of DR5 or TRAIL (reviewed in [9]).

A number of other agents also sensitize cells to death

ligands. Cyclooxygenase-2 inhibitors, which are being

developed as potential chemopreventative and therapeu-

tic agents for colorectal cancer [22], sensitize cells to both

Fas ligand and TRAIL [23,24], either by upregulating

death receptors [25] or by enhancing death receptor

clustering and sequestration into caveolae [24]. Casein

kinase 2 inhibitors, which are likewise undergoing pre-

clinical testing as possible anticancer agents [26], enhance

TRAIL sensitivity by increasing the recruitment of

FADD and caspase 8 [27,28].

Another strategy for enhancing death ligand signaling

involves modulation of c-FLIP (Figure 1). While the role

of c-FLIPL in death receptor signaling has been contro-

versial, several groups have reported that c-FLIPS

diminishes TRAIL-induced apoptosis in a variety of

model systems [29,30]. More recently, it has been

reported that c-FLIPS in glioblastoma cells is selectively

downregulated by treatment with rapamycin [31�], an

immunosuppressant that is undergoing clinical evaluation

as a potential antineoplastic agent [32]. If similar results

are confirmed in additional cell types and in vivo models,

rapamycin might represent an important means of mod-

ulating TRAIL sensitivity.

The possibility of triggering the extrinsic pathway down-

stream of the receptors has received less attention. Lim-

ited studies of this approach have indicated that FADD is


a phosphoprotein [33] and that drug-induced FADD

dephosphorylation is associated with receptor-indepen-

dent adaptor aggregation and caspase 8 activation [34].

This approach might be particularly useful in neoplasms

harboring death receptor mutations or expressing high

levels of decoy receptors. Curiously, while high concen-

trations of the MEK1/2 inhibitor CI-1040 could induce

this effect in multiple lymphoid lines, similar results were

not observed in myeloid leukemia cell lines, raising the

possibility of cell-type-specific induction of apoptosis

through this strategy.

Finally, it has been reported that silencing of the Caspase8gene by methylation in medulloblastoma or neuroblas-

toma cells results in TRAIL resistance and enhanced

metastasis [35–37]. Building on these findings, several

groups have reported that DNA methyltransferase inhi-

bitors such as 5-aza-20-deoxycytidine restore TRAIL-

induced apoptosis in these tumor types [35,38,39].

Poking holes in the backdoor screen:facilitating activation of the mitochondrialpathwayThe other canonical caspase activation pathway involves

mitochondrial outer membrane permeabilization leading

to release of cytochrome c and other mitochondrial inter-

membrane polypeptides, which then facilitate cytoplas-

mic activation of caspases 9 and 3. As illustrated in

Figure 2, activation of this pathway is regulated by

Bcl-2 family members. Although the roles of the various

Bcl-2 homologs are still far from settled [40,41], efforts

have nonetheless focused on modulating the activity of

these polypeptides as a way of enhancing cell death.

Bcl-2 antisense: a failed strategyIn response to reports that Bcl-2 is upregulated in a

variety of cancers [42], oblimersen sodium, an antisense

oligonucleotide targeting the first six codons of the Bcl-2

message, was developed for clinical testing. Preclinical

studies demonstrated that oblimersen sensitized neoplas-

tic cells to a variety of agents, including antimetabolites,

DNA crosslinking agents and glucocorticoids. Phase III

clinical trials comparing the efficacy of otherwise identical

therapies in the absence and presence of oblimersen,

however, were disappointing [43]. When combined with

dacarbazine for the treatment of melanoma, oblimersen

increased survival by only 1.2 months, an improvement

that was not statistically significant. When combined with

fludarabine and cyclophosphamide for chronic lympho-

cytic leukemia, oblimersen increased the frequency of

major responses but decreased the median time to disease

progression. Likewise, oblimersen failed to increase pro-

gression-free survival when combined with dexametha-

sone in patients with multiple myeloma.

Several factors might have contributed to these disap-

pointing results. First, oblimersen, like other antisense



Figure 2

Strategies to facilitate activation of the mitochondrial pathway. The mitochondrial or intrinsic death pathway involves release of mitochondrial

intermembrane proteins to the cytoplasm [62,75]. Once released, cytochrome c facilitates the binding of procaspase 9 to Apaf-1, a cytoplasmic

scaffolding protein that serves as an activation subunit for the procaspase 9 zymogen [8]. Importantly, this pathway is regulated both upstream

and downstream of cytochrome c release. Members of the Bcl-2 family of polypeptides regulate this pathway upstream of mitochondrial outer

membrane permeabilization [40,75]. The proapoptotic family members Bax and Bak are hypothesized to form channels that allow release of

cytochrome c and other mitochondrial intermembrane proteins. Insertion of Bax and Bak into the outer mitochondrial membrane is modulated

by two other groups of Bcl-2 family members. Antiapoptotic family members such as Bcl-2, Bcl-xL and Mcl-1 bind and neutralize Bax and/or

Bak. Conversely, proapoptotic BH3-only family members, typified by Bid, Bad, Bim, Puma, Noxa and Bmf, neutralize antiapoptotic Bcl-2 family

members [40,76��] and possibly, in some cases, directly induce Bax activation [41].IAP proteins regulate this pathway downstream of

mitochondrial outer membrane permeabilization. XIAP binds and inhibits caspases 3, 7 and 9 as described in the text [8,60]. Other IAP proteins

regulate XIAP by binding and inhibiting the XIAP antagonist Smac [77], another polypeptide that is released to the cytoplasm when the

mitochondrial outer membrane is breeched, or by downregulating XIAP when cleaved [72�]. Experimental strategies that are undergoing

preclinical or early clinical testing affect this pathway in the following three ways (numbers refer to steps shown in the figure). 1. Downregulation

of Bcl-2 or Bcl-XL as a consequence of mRNA targeting by antisense oligonucleotides. 2. Inhibition of antiapoptotic Bcl-2 family member

function. Molecules thought to act in this manner include BH3 peptidomimetics as well as the BH3 agonist ABT-737 and possibly (�)-gossypol,

apogossypol and EGCG. 3. Inhibition of XIAP function. Agents that act in this manner include Smac peptidomimetics, embelin and polyphenylureas.

oligonucleotides, has off-target effects that result in

thrombocytopenia and preclude further dose escalation

[44]. Second, it does not appear that elevated Bcl-2 levels

were required for entry of patients on the melanoma trial,

and the decision to perform a multiple myeloma trial can

be questioned in view of the strong dependence of multi-


ple myeloma cells on Bcl-xL rather than Bcl-2 for survival

[45]. Finally, even though Bcl-2 is downregulated after

oblimersen treatment in some neoplasms in situ [44], the

ability of this antisense oligonucleotide to sensitize Bcl-2-

depleted cells [46�] has raised serious questions about the

mechanism of this agent [47�].



Alternative inhibitors of antiapoptotic Bcl-2family membersBecause of these concerns about the development of

oblimersen, it is important not to abandon Bcl-2 as a

potential therapeutic target. An alternative approach for

inhibiting Bcl-2 involves treatment with peptides mod-

eled after the proapoptotic domains of BH3-only poly-

peptides. These ‘BH3 mimetics’ are thought to bind to

the BH3 receptor, a hydrophobic groove formed by the

BH1, BH2 and BH3 domains of antiapoptotic Bcl-2

family members, thereby preventing the antiapoptotic

Bcl-2 homologs from neutralizing the active conformation

of Bax and/or Bak (Figure 2). For example, recent results

indicate that the Bad or Bid BH3 domain peptide fused to

arginine homopolymer (to facilitate cell penetration)

induces apoptosis in neuroblastoma cell lines in vitroand diminishes growth of neuroblastoma xenografts invivo [48]. Although important as a test of concept, this

approach is limited by the requirement for frequent

intratumoral injections and the high (10–50 mM) concen-

trations of the peptides necessitated by their suscept-

ibility to proteolysis and/or their limited ability to assume

an active tertiary structure inside cells.

To overcome some of these disadvantages, Walensky et al.designed a novel Bid BH3 derivative containing substi-

tuted amino acids that can be crosslinked to stabilize the

a-helical conformation of the peptide [49]. Subsequent

analysis demonstrated that this peptide bound to Bcl-xL

with a KD of 40 nM and exhibited a prolonged serum half-

life in vitro and in vivo. Despite its ability to induce

apoptosis in human leukemia cell lines in vitro, however,

this agent displayed disappointing antileukemic activity invivo, with seven days of treatment yielding only a six-day

increase in median survival [49], a result that is vastly

inferior to the effects of active therapeutic agents in similar

models [50,51]. Nonetheless, the hydrocarbon-stapled Bid

peptide represents a possible starting point for the devel-

opment of additional Bcl-2 family antagonists.

An alternative approach involves the identification or

design of non-peptidic molecules that bind the BH3

receptors of Bcl-2 homologs. In silico screening, a process

involving computer-based docking of a library of small

molecules to an established protein structure, predicted

that the male contraceptive gossypol and some of its

derivatives would bind Bcl-2 and Bcl-xL. Wet bench

studies have reportedly verified these predictions

[52,53] and also indicated that (�)-gossypol induces cyto-

chrome c release from isolated mitochondria [54],

enhances the cytotoxic effects of pro-apoptotic treat-

ments in tissue culture and increases the efficacy of

CHOP chemotherapy in a diffuse large cell lymphoma

xenograft model [55]. In view of this evidence for selec-

tivity in vivo, the results of currently ongoing phase I trials

of (�)-gossypol (AT-101) and its derivatives are awaited

with interest.


Even though protein–protein interactions have pre-

viously been difficult for medicinal chemists to disrupt,

Oltersdorf et al. recently described the structure-based

design and synthesis of ABT-737, a small molecule that

binds with subnanomolar affinity to the BH3 receptor of

Bcl-xL, Bcl-2 and Bcl-w [56��]. Further analysis demon-

strated that ABT-737 diminishes survival of neoplastic

cells that exhibit prominent Bcl-2 overexpression, includ-

ing follicular lymphoma and chronic lymphocytic leuke-

mia clinical isolates exposed ex vivo as well as human

small cell lung cancer cell lines treated in vitro and in vivo.

These promising results presumably reflect the presence

of constitutively expressed BH3-only polypeptides that

prime certain cancer cells to die upon BH3 receptor

occupation [57��]. Additional experiments showed that

ABT-737, in contrast to other putative BH3 mimetics,

kills cells in a Bax- or Bak-dependent manner, but only if

Mcl-1 is also disabled [58��], providing confirmation of

the proposed mechanism of cytotoxicity of ABT-737 as

well as insight into an additional critical parameter that

will affect sensitivity to this agent. Because current

understanding suggests that Bcl-2 antagonists might be

most effective when combined with a proapoptotic sti-

mulus that acts through the mitochondrial pathway, com-

binations of ABT-737 with a variety of apoptotic stimuli

need further investigation. Results of additional precli-

nical and ultimately clinical studies of ABT-737 and other

small molecules that disrupt Bcl-2/binding partner inter-

actions [59] are awaited with interest.

XIAP antagonists and the release of caspaseinhibition: rousing the horse after it is alreadyout of the barnA final group of agents targets the regulation of apoptosis

downstream of cytochrome c release (Figure 2). Studies

performed over the past decade have demonstrated that

mammalian cells contain endogenous caspase inhibitors,

the best studied of which is the X-chromosome-linked

inhibitor of apoptosis (XIAP) protein [60]. This polypep-

tide contains three zinc finger-like bacculovirus inhibitor

repeat (BIR) domains and a C-terminal RING domain

that exhibits E3 ubiquitin ligase activity. Caspases 3 and 7

are potently inhibited by the second BIR domain and its

flanking sequences, whereas caspase 9 is inhibited by

sequences encompassing BIR3 [8,61]. Consistent with

this inhibition profile, XIAP reportedly inhibits caspase

activation triggered by stimuli that act on either the death

receptor or mitochondrial pathways. Although the lack of

a phenotype in XIAP knockout cells has been difficult to

reconcile with this model [62], XIAP has nonetheless

become a fashionable anticancer target [60].

Strategies for antagonizing XIAP are based on the action

of Smac/Diablo (second mitochondrial activator of cas-

pases/direct IAP binder with low pI). Like cytochrome c,this polypeptide is released to the cytoplasm by mito-

chondrial permeabilization. Once Smac is released, its



N-terminal tetrapeptide H2N-AVPI interacts with BIR2

and BIR3 of XIAP [8]. Earlier studies not only demon-

strated that Smac release from mitochondria is deficient in

many tumor cell lines, but also showed that peptides

containing an N-terminal AVPI sequence can enhance

the effects of drugs that trigger the mitochondrial pathway

[63].

Building on these observations, several groups have

synthesized peptidomimetics patterned after the N-term-

inal Smac tetrapeptide [60]. Some of these peptidomi-

metics not only inhibit binding of XIAP to caspase

fragments, but also induce apoptosis directly [64] or

enhance the effects of proapoptotic chemotherapy [65].

The currently available peptidomimetics, however,

exhibit limited potency, possibly because of poor cell

penetration.

Small organic molecules that disrupt the XIAP/caspase

interaction have also been described recently. In silicoscreening identified embelin as a nonpeptidic inhibitor of

XIAP/caspase interactions that selectively facilitates

drug-induced apoptosis in cells overexpressing XIAP

but not their parental counterparts [66]. Structure-based

synthesis led Oost et al. to a series of small BIR3-binding

molecules that induce caspase-dependent killing of

human tumor cell lines in vitro and in xenograft models

[67]. Using a similar structure-based approach, Li et al.[68��] identified a small molecule that disrupts the BIR3/

caspase interaction at subnanomolar concentrations and

enhances death-ligand-induced caspase activation in

tissue culture at nanomolar concentrations. Further

preclinical and ultimately clinical studies of these

BIR3-directed molecules are awaited with interest.

As an alternative approach, Schimmer and coworkers [69�]screened a series of small molecule libraries for agents that

disrupt the XIAP/BIR2 interaction. Polyphenylureas iden-

tified in this screen and their derivatives were reported to

induce apoptosis in cancer cell lines, chronic lymphocytic

leukemia samples and acute myelogenous leukemia speci-

mens in vitro [69�,70,71]. Further experiments demon-

strated that these BIR2 inhibitors sensitize cancer cells

to death ligands and chemotherapeutic agents in vitro and

diminish xenograft growth in vivo.

A totally different approach is suggested by recent work

of Silke and coworkers [72�], who showed that overex-

pression of the RING domain of the cIAP1 causes ubi-

quitylation and proteasomal degradation of XIAP along

with concomitant sensitization of melanoma cells to cis-

platin. It will be interesting to see whether a small

molecule that activates endogenous cIAP1 and facilitates

the same changes can be identified in future studies.

These recently reported effects of XIAP modulators

have both scientific and therapeutic implications. The


observation that XIAP downregulation or XIAP antago-

nists can sometimes induce apoptosis without additional

stimuli suggests that XIAP might be playing an important

anti-apoptotic role in cancer cells, although the nature of

the processes driving this apoptosis remains to be eluci-

dated. Moreover, the efficacy of the XIAP antagonists

suggests that XIAP might be a practical drug target even

though it is far downstream in the apoptotic cascade

(Figure 2).

ConclusionsAs indicated at the beginning of this review, the hope of

improved cancer therapy has provided strong motivation

for the detailed elucidation of apoptotic pathways.

Although results published to date have not improved

the clinical treatment of cancer, it is important to empha-

size that the process of drug development is long and

complicated [73]. The failure of Bcl-2 antisense oligonu-

cleotides to significantly enhance anticancer therapy

should not be a cause for undue pessimism. On the

contrary, an exciting group of small molecules that

directly and specifically act on various components of

apoptotic pathways has been identified over the past few

years. Recently published preclinical studies of these

agents provide new hope that previous suggestions of

improved therapy might soon be a promise kept.

AcknowledgementsSupported in part by R01 CA69008 and a predoctoral fellowship toS-H.L. from the Mayo Foundation for Education and Research

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This paper provides a mechanism-based rationale for combining TRAILreceptor ligands with an already licensed signal transduction inhibitor.

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35. Grotzer MA, Eggert A, Zuzak TJ, Janss AJ, Marwaha S,Wiewrodt BR, Ikegaki N, Brodeur GM, Phillips PC: Resistance toTRAIL-induced apoptosis in primitive neuroectodermal braintumor cells correlates with a loss of Caspase-8 expression.Oncogene 2000, 19:4604-4610.

36. Teitz T, Wei T, Valentine MB, Vanin EF, Grenet J, Valentine VA,Behm FG, Look AT, Lahti JM, Kidd VJ: Caspase 8 is deleted orsilenced preferentially in childhood neuroblastomas withamplification of MYCN. Nat Med 2000, 6:529-535.

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Raffo A, Lai JC, Stein CA, Miller P, Scaringe S, Khvorova A,Benimetskaya L: Antisense RNA down-regulation of bcl-2expression in DU145 prostate cancer cells does not diminishthe cytostatic effects of G3139 (Oblimersen). Clin Cancer Res2004, 10:3195-3206.

See annotation to [47�].

47.�

Benimetskaya L, Wittenberger T, Stein CA, Hofmann HP, Weller C,Lai JC, Miller P, Gekeler V: Changes in gene expression inducedby phosphorothioate oligodeoxynucleotides (including G3139)in PC3 prostate carcinoma cells are recapitulated at least inpart by treatment with interferon-b and -g. Clin Cancer Res2004, 10:3678-3688.

[46�] and [47�] provide a nice demonstration of how off-target effects ofoligonucleotide-based therapies can mislead drug development.

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56.��

Oltersdorf T, Elmore SW, Shoemaker AR, Armstrong RC,Augeri DJ, Belli BA, Bruncko M, Deckwerth TL, Dinges J,Hajduk PJ et al.: An inhibitor of Bcl-2 family proteins inducesregression of solid tumours. Nature 2005, 435:677-681.

This paper, which represents roughly a decade of research, illustrateshow structural biology and modern medicinal chemistry approaches canbe combined to devise potent inhibitors of protein–protein interactions.

57.��

Certo M, Moore Vdel G, Nishino M, Wei G, Korsmeyer S,Armstrong SA, Letai A: Mitochondria primed by death signalsdetermine cellular addiction to antiapoptotic BCL-2 familymembers. Cancer Cell 2006, 9:351-365.

See annotation to [58��].

58.��

van Delft MF, Wei AH, Mason KD, Vandenberg CJ, Chen LB,Czabotar PE, Willis SN, Scott CL, Day CL, Cory S, et al.: The BH3mimetic ABT-737 targets selective Bcl-2 proteins andefficiently idnuces apoptosis via Bak/Bax if Mcl-1 isneutralized. Cancer Cell 2006, in press.

Collectively, [57��] and [58��] identify important determinants of ABT-737action.


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68.��

Li L, Thomas RM, Suzuki H, De Brabander JK, Wang X, Harran PG:A small molecule Smac mimic potentiates TRAIL- andTNFa-mediated cell death. Science 2004, 305:1471-1474.

This paper demonstrates that nanomolar antagonists of XIAP can bedevised and used to enhance apoptosis in tumor cells.

69.�

Schimmer AD, Welsh K, Pinilla C, Wang Z, Krajewska M,Bonneau MJ, Pedersen IM, Kitada S, Scott FL, Bailly-Maitre Bet al.: Small-molecule antagonists of apoptosis suppressorXIAP exhibit broad antitumor activity. Cancer Cell 2004,5:25-35.

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Silke J, Kratina T, Chu D, Ekert PG, Day CL, Pakusch M,Huang DC, Vaul DL: Determination of cell survival byRING-mediated regulation of inhibitior of apoptosis (IAP)protein abundance. Proc Nat Acad Sci USA 2005,102:16182-16187.

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Chen L, Willis SN, Wei A, Smith BJ, Fletcher JI, Hinds MG,Colman PM, Day CL, Adams JM, Huang DC: Differentialtargeting of prosurvival Bcl-2 proteins by their BH3-onlyligands allows complementary apoptotic function. Mol Cell2005, 17:393-403.


This paper contains quantitative analysis of BH3 peptide/Bcl-2 familymember interactions that provides tremendous insight into the actions ofBcl-2 family members.

77. Vucic D, Franklin MC, Wallweber HJA, Das K, Eckelman BP,Shin H, Elliott LO, Kadkhodayan S, Deshayes K, Salvesen GS et al.:Engineering ML-IAP to produce an extraordinarily potentcaspase 9 inhibitor: implications for Smac-dependentanti-apoptotic activity of ML-IAP. Biochemical Journal 2005,385:11-20.


apoptosis in the treatment of cancer: a promise kept?

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