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Malignant rhabdoid tumors (MRT) are highly aggressive pediatric cancers that respond poorly to current therapies. In this study, we screened several MRT cell lines with large-scale RNAi, CRISPR-Cas9, and small-molecule libraries to identify potential drug targets specific for these cancers. We discovered MDM2 and MDM4, the canonical negative regulators of p53, as significant vulnerabilities. Using two compounds currently in clinical development, idasanutlin (MDM2-specific) and ATSP-7041 (MDM2/4-dual), we show that MRT cells were more sensitive than other p53 wild-type cancer cell lines to inhibition of MDM2 alone as well as dual inhibition of MDM2/4. These compounds caused significant upregulation of the p53 pathway in MRT cells, and sensitivity was ablated by CRISPR-Cas9–mediated inactivation of TP53. We show that loss of SMARCB1, a subunit of the SWI/SNF (BAF) complex mutated in nearly all MRTs, sensitized cells to MDM2 and MDM2/4 inhibition by enhancing p53-mediated apoptosis. Both MDM2 and MDM2/4 inhibition slowed MRT xenograft growth in vivo, with a 5-day idasanutlin pulse causing marked regression of all xenografts, including durable complete responses in 50% of mice. Together, these studies identify a genetic connection between mutations in the SWI/SNF chromatin-remodeling complex and the tumor suppressor gene TP53 and provide preclinical evidence to support the targeting of MDM2 and MDM4 in this often-fatal pediatric cancer.Significance:This study identifies two targets, MDM2 and MDM4, as vulnerabilities in a deadly pediatric cancer and provides preclinical evidence that compounds inhibiting these proteins have therapeutic potential.

Expanded methodological detail on large-scale data analysis (RNAi, CRISPR-Cas9, p53 status annotations, RNA-seq, and primary tumor expression).

S1: Sources, media, and other information on all cell lines. S2: p53 status annotations for all cell lines. S3: sgRNA sequences for TP53 inactivation. S4: Primers for TP53 allele screening and RT-qPCR. S5: Sources, catalog numbers, and dilutions of all antibodies. S6: RNA-seq data for p53 target genes. S7: GSEA from RNA-seq data.

S1: RNAi data and CRISPR-Cas9 data divided by p53 status. S2: Expression of MDM2 and MDM4 in MRT and control cell lines. S3: CRISPR-Cas9 mediated inactivation of TP53 in MRT cell lines. S4: Immunoblots for p53 pathway activation upon MDM2 and MDM2/4 inhibition in MRT and control cell lines. S5: Cell counts, flow cytometry, and senescence assays in MRT cells following MDM2 and MDM2/4 inhibition. S6: Characterization of MRT cells expressing SMARCB1 or p16. S7: MRT xenograft growth characteristics and pharmacodynamic responses.

Purpose: Sensitivity of tumor cells toward chemotherapy mainly determines the prognosis of patients suffering from acute lymphoblastic leukemia (ALL); nevertheless, underlying mechanisms regulating chemosensitivity remain poorly understood. Here, we aimed at characterizing the role of caspase-8 for chemosensitivity of B- and T-ALL cells.Experimental Design: Primary tumor cells from children with ALL were evaluated for expression levels of the caspase-8 protein, were amplified in nonobese diabetic/severe combined immunodeficient mice, transfected with siRNA, and evaluated for their chemosensitivity in vitro.Results: Effective cell death in B- and T-ALL cells depended on the presence of caspase-8 for the majority of cytotoxic drugs routinely used in antileukemia treatment. Caspase-8 was activated independently from extrinsic apoptosis signaling. Accordingly in primary ALL cells, the expression level of caspase-8 protein correlated with cell death sensitivity toward defined cytotoxic drugs in vitro. In the subgroup of primary ALL cells, with low expression of caspase-8, methotrexate (MTX) upregulated the expression of caspase-8 mediated by the transcription factor p53, suggesting epigenetic silencing of caspase-8. RNA interference in patient-derived B- and T-ALL cells revealed that effective cell death induction by most routine drug combinations involving MTX depended on the presence of caspase-8.Conclusion: Our results indicate that caspase-8 is crucial for the high antileukemic efficiency of numerous routine cytotoxic drugs. Reexpression of epigenetically downregulated caspase-8 represents a promising approach to increase efficiency of antileukemic therapy. Clin Cancer Res; 17(24); 7605–13. ©2011 AACR.

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Inflammatory myofibroblastic tumor (IMT) is a rare, mesenchymal tumor that has an increased incidence in childhood. Tumors are usually isolated to the chest, abdomen, and retroperitoneum, but metastatic presentations can be seen. Presenting symptoms are nonspecific and include fever, weight loss, pain, shortness of breath, and cough. Approximately 85% of IMTs harbor actionable kinase fusions. The diagnosis can be delayed because of overlapping features with inflammatory disorders, such as elevated inflammatory markers, increased immunoglobin G levels, fever, weight loss, and morphologic similarity with nonmalignant conditions. We present a girl aged 11 years with a TFG-ROS1 fusion–positive tumor of the lung that was initially diagnosed as an immunoglobin G4–related inflammatory pseudotumor. She underwent complete left-sided pneumonectomy and later recurred with widely metastatic disease. We then report the case of a boy aged 9 years with widely metastatic TFG-ROS1 fusion–positive IMT with rapid molecular diagnosis. In both children, there was an excellent response to oral targeted therapy. These cases reveal that rapid molecular testing of inflammatory tumors is not only important for diagnosis but also reveals therapeutic opportunities. Targeted inhibitors produce significant radiologic responses, enabling potentially curative treatment approaches for metastatic ROS1 fusion IMT with previously limited treatment options. Primary care pediatricians and pediatric subspecialists have a crucial role in the early consultation of a pediatric oncology center experienced in molecular diagnostics to facilitate a comprehensive evaluation for children with inflammatory tumors.

Malignant rhabdoid tumors (MRT) are highly aggressive pediatric cancers that respond poorly to current therapies. In this study, we screened several MRT cell lines with large-scale RNAi, CRISPR-Cas9, and small-molecule libraries to identify potential drug targets specific for these cancers. We discovered MDM2 and MDM4, the canonical negative regulators of p53, as significant vulnerabilities. Using two compounds currently in clinical development, idasanutlin (MDM2-specific) and ATSP-7041 (MDM2/4-dual), we show that MRT cells were more sensitive than other p53 wild-type cancer cell lines to inhibition of MDM2 alone as well as dual inhibition of MDM2/4. These compounds caused significant upregulation of the p53 pathway in MRT cells, and sensitivity was ablated by CRISPR-Cas9–mediated inactivation of TP53. We show that loss of SMARCB1, a subunit of the SWI/SNF (BAF) complex mutated in nearly all MRTs, sensitized cells to MDM2 and MDM2/4 inhibition by enhancing p53-mediated apoptosis. Both MDM2 and MDM2/4 inhibition slowed MRT xenograft growth in vivo, with a 5-day idasanutlin pulse causing marked regression of all xenografts, including durable complete responses in 50% of mice. Together, these studies identify a genetic connection between mutations in the SWI/SNF chromatin-remodeling complex and the tumor suppressor gene TP53 and provide preclinical evidence to support the targeting of MDM2 and MDM4 in this often-fatal pediatric cancer. Significance: This study identifies two targets, MDM2 and MDM4, as vulnerabilities in a deadly pediatric cancer and provides preclinical evidence that compounds inhibiting these proteins have therapeutic potential.

Malignant rhabdoid tumors (MRT) are aggressive cancers of early childhood that are largely resistant to traditional therapies. MRT exhibit a remarkably low mutation rate, with no recurrent mutations beyond the defining biallelic inactivating mutation in SMARCB1, a core subunit of the SWI/SNF (BAF) chromatin-remodeling complex. Thus, MRT do not display traditional oncogenic mutations that are amenable to targeted therapies, limiting their use for this disease. In order to nominate new drug targets for MRT, we screened MRT cell lines with large-scale RNAi, CRISPR-Cas9, and small-molecule libraries. The most significant vulnerabilities consistent across all three screens were MDM2 and MDM4, the major negative regulators of p53. We found that MRT cell lines are more sensitive than other p53 wild-type cancer cell lines to both idasanutlin (MDM2-specific) and ATSP-7041 (MDM2/4-dual) in vitro. Both inhibitors induced substantial activation of the p53 pathway in MRT cell lines, which responded with permanent apoptotic or senescent cell fate decisions. CRISPR-Cas9-mediated inactivation of TP53 caused a significant resistance to these compounds, confirming that on-target mechanisms were responsible for MRT sensitivity. We found that loss of SMARCB1 sensitizes MRT cells to idasanutlin by shifting the p53 response towards apoptosis. In MRT xenograft studies, both idasanutlin and ATSP-7041 slowed tumor growth. Most strikingly, an idasanutlin pulse of only five days was sufficient to induce sizable regression of all tumors, which remained complete and durable in 50% of mice. Finally, gene expression analysis of primary MRT predicts that, like cell lines and xenografts, MRT in patients are likely to be sensitive to MDM2 inhibition. Collectively, these studies describe a genetic link between SWI/SNF complex mutations and p53, while providing evidence to support the use of MDM2 and MDM2/4 inhibitors that have already entered clinical trials for the treatment of this devastating pediatric cancer. Citation Format: Thomas P. Howard, Taylor E. Arnoff, Melinda R. Song, Andrew O. Giacomelli, Xiaofeng Wang, Andrew L. Hong, Neekesh V. Dharia, Su Wang, Francisca Vazquez, Minh-Tam Pham, Ann M. Morgan, Franziska Wachter, Gregory H. Bird, Guillaume Kugener, Elaine M. Oberlick, Matthew G. Rees, Hong Tiv, Justin H. Hwang, Katherine H. Walsh, April Cook, John M. Krill-Burger, Aviad Tsherniak, Prafulla C. Gokhale, Peter J. Park, Kimberly Stegmaier, Loren D. Walensky, William C. Hahn, Charles W. Roberts. MDM2 and MDM4 are therapeutic vulnerabilities in malignant rhabdoid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2867.

Background and Purpose In polychemotherapy protocols, that is for treatment of neuroblastoma and Ewing sarcoma, Vinca alkaloids and cell cycle-arresting drugs are usually administered on the same day. Here we studied whether this combination enables the optimal antitumour effects of Vinca alkaloids to be manifested. Experimental Approach Vinca alkaloids were tested in a preclinical mouse model in vivo and in vitro in combination with cell cycle-arresting drugs. Signalling pathways were characterized using RNA interference. Key Results In vitro, knockdown of cyclins significantly inhibited vincristine-induced cell death indicating, in accordance with previous findings, Vinca alkaloids require active cell cycling and M-phase transition for induction of cell death. In contrast, anthracyclines, irradiation and dexamethasone arrested the cell cycle and acted like cytostatic drugs. The combination of Vinca alkaloids with cytostatic therapeutics resulted in diminished cell death in 31 of 36 (86%) tumour cell lines. In a preclinical tumour model, anthracyclines significantly inhibited the antitumour effect of Vinca alkaloids in vivo. Antitumour effects of Vinca alkaloids in the presence of cytostatic drugs were restored by caffeine, which maintained active cell cycling, or by knockdown of p53, which prevented drug-induced cell cycle arrest. Therapeutically most important, optimal antitumour effects were obtained in vivo upon separating the application of Vinca alkaloids from cytostatic therapeutics. Conclusion and Implications Clinical trials are required to prove whether Vinca alkaloids act more efficiently in cancer patients if they are applied uncoupled from cytostatic therapies. On a conceptual level, our data suggest the implementation of polychemotherapy protocols based on molecular mechanisms of drug–drug interactions. Linked Article This article is commented on by Solary, pp 1555–1557 of this issue. To view this commentary visit http://dx.doi.org/10.1111/bph.12101

BCL-2-associated X protein (BAX) is a critical apoptotic regulator that can be transformed from a cytosolic monomer into a lethal mitochondrial oligomer, yet drug strategies to modulate it are underdeveloped due to longstanding difficulties in conducting screens on this aggregation-prone protein. Here, we overcame prior challenges and performed an NMR-based fragment screen of full-length human BAX. We identified a compound that sensitizes BAX activation by binding to a pocket formed by the junction of the α3-α4 and α5-α6 hairpins. Biochemical and structural analyses revealed that the molecule sensitizes BAX by allosterically mobilizing the α1-α2 loop and BAX BH3 helix, two motifs implicated in the activation and oligomerization of BAX, respectively. By engaging a region of core hydrophobic interactions that otherwise preserve the BAX inactive state, the identified compound reveals fundamental mechanisms for conformational regulation of BAX and provides a new opportunity to reduce the apoptotic threshold for potential therapeutic benefit.

Chemotherapy induced peripheral neuropathy (CIPN) is a debilitating side effect of many cancer treatments. The hallmark of CIPN is degeneration of long axons required for transmission of sensory information; axonal degeneration causes impaired tactile sensation and persistent pain. Currently the molecular mechanisms of CIPN are not understood, and there are no available treatments. Here we show that the chemotherapeutic agent paclitaxel triggers CIPN by altering IP3 receptor phosphorylation and intracellular calcium flux, and activating calcium-dependent calpain proteases. Concomitantly paclitaxel impairs axonal trafficking of RNA-granules and reduces synthesis of Bclw (bcl2l2), a Bcl2 family member that binds IP3R1 and restrains axon degeneration. Surprisingly, Bclw or a stapled peptide corresponding to the Bclw BH4 domain interact with axonal IP3R1 and prevent paclitaxel-induced degeneration, while Bcl2 and BclxL cannot do so. Together these data identify a Bclw-IP3R1-dependent cascade that causes axon degeneration, and suggest Bclw-mimetics could provide effective therapy to prevent CIPN.

Hydrocarbon-stapled peptides that display key residues of the p53 transactivation domain have emerged as bona fide clinical candidates for reactivating the tumor suppression function of p53 in cancer by dual targeting of the negative regulators HDM2 and HDMX. A recent study questioned the mechanistic specificity of such stapled peptides based on interrogating their capacity to disrupt p53/HDM2 and p53/HDMX complexes in living cells using a new recombinase enhanced bimolecular luciferase complementation platform (ReBiL). Here, we directly evaluate the cellular uptake, intracellular targeting selectivity, and p53-dependent cytotoxicity of the clinical prototype ATSP-7041. We find that under standard serum-containing tissue culture conditions, ATSP-7041 achieves intracellular access without membrane disruption, dose-dependently dissociates both p53/HDM2 and p53/HDMX complexes but not an unrelated protein complex in long-term ReBiL experiments, and is selectively cytotoxic to cancer cells bearing wild-type p53 by inducing a surge in p53 protein level. These studies underscore the importance of a thorough step-wise approach, including consideration of the time-dependence of cellular uptake and intracellular distribution, in evaluating and advancing stapled peptides for clinical translation.

Hydrocarbon stapling has been applied to restore and stabilize the α-helical structure of bioactive peptides for biochemical, structural, cellular, and in vivo studies. The peptide sequence, in addition to the composition and location of the installed staple, can dramatically influence the properties of stapled peptides. As a result, constructs that appear similar can have distinct functions and utilities. Here, we perform a side-by-side comparison of stapled peptides modeled after the pro-apoptotic BIM BH3 helix to highlight these principles. We confirm that replacing a salt-bridge with an i, i + 4 hydrocarbon staple does not impair target binding affinity and instead can yield a biologically and pharmacologically enhanced α-helical peptide ligand. Importantly, we demonstrate by electron microscopy that the pro-apoptotic activity of a stapled BIM BH3 helix correlates with its capacity to achieve cellular uptake without membrane disruption and accumulate at the organellar site of mechanistic activity.

Summary Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating side effect of many cancer treatments. The hallmark of CIPN is degeneration of long axons required for transmission of sensory information; axonal degeneration causes impaired tactile sensation and persistent pain. Currently the molecular mechanisms of CIPN are not understood, and there are no available treatments. Here we show that the chemotherapeutic agent paclitaxel triggers CIPN by altering IP 3 receptor phosphorylation and intracellular calcium flux, and activating calcium-dependent calpain proteases. Concomitantly paclitaxel impairs axonal trafficking of RNA-granules and reduces synthesis of Bclw ( bcl2l2 ), a Bcl2 family member that binds IP 3 R1 and restrains axon degeneration. Surprisingly, Bclw or a stapled peptide corresponding to the Bclw BH4 domain interact with axonal IP 3 R1 and prevent paclitaxel-induced degeneration, while Bcl2 and Bclx L cannot do so. Together these data identify a Bclw-IP 3 R1-dependent cascade that causes axon degeneration and suggest that Bclw-mimetics could provide effective therapy to prevent CIPN.

The first anti-cancer stapled peptide drug has now been advanced to clinical testing in relapsed human cancers that retain the expression of wild-type p53. A common mechanism for cancer cell suppression of wild-type p53 is overexpression of the negative regulators HDM2 and HDMX, which neutralize p53 through protein interaction. Selective HDM2 inhibitor molecules can effectively reactivate p53 in certain cancers, but the co-expression of HDMX can cause resistance, highlighting the need for dual HDM2/HDMX targeting. We previously developed stapled peptides modeled after the p53 transactivation domain to harness the natural propensity of this alpha-helical motif to engage both targets with high affinity and selectivity. Here, we demonstrate that ALRN-7041, a next-generation, clinical-grade stapled peptide, achieves time-dependent cellular uptake and nuclear localization without membrane perturbation, dose-dependently dissociates p53/HDM2 and p53/HDMX complexes as assessed by real-time protein interaction monitoring in live cells, and impairs the viability of cancer cells bearing wild-type p53 by inducing a surge in p53 protein level. Applying an unbiased statistical approach to determine which biophysical parameters dictate the cellular uptake of stapled peptides, we elucidated the design features of ALRN-7041 that confer intracellular access, providing a roadmap for generating cell-permeable stapled peptides with on mechanism cellular activity for clinical translation. We find that cancer cells exhibiting a signature of HDM2, HDMX, and wild-type p53 co-expression are strikingly susceptible to ALRN-7041, highlighting the therapeutic potential of dual HDM2/HDMX inhibition by a cell-penetrant stapled peptide in human cancer. Citation Format: Loren D. Walensky, Franziska Wachter, Ann Morgan, Marina Godes, Gregory Bird. Targeted intracellular disruption of oncogenic p53/HDM2 and p53/HDMX complexes underlies the therapeutic activity of a stapled peptide drug [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2158. doi:10.1158/1538-7445.AM2017-2158

BCL-2 is a negative regulator of apoptosis implicated in homeostatic and pathologic cell survival. The canonical anti-apoptotic mechanism involves entrapment of activated BAX by a groove on BCL-2, preventing BAX homo-oligomerization and mitochondrial membrane poration. The BCL-2 BH4 domain also confers anti-apoptotic functionality, but the mechanism is unknown. We find that a synthetic α-helical BH4 domain binds to BAX with nanomolar affinity and independently inhibits the conformational activation of BAX. Hydrogen-deuterium exchange mass spectrometry demonstrated that the N-terminal conformational changes in BAX induced by a triggering BIM BH3 helix were suppressed by the BCL-2 BH4 helix. Structural analyses localized the BH4 interaction site to a groove formed by residues of α1, α1-α2 loop, and α2-α3 and α5-α6 hairpins on the BAX surface. These data reveal a previously unappreciated binding site for targeted inhibition of BAX and suggest that the BCL-2 BH4 domain may participate in apoptosis blockade by a noncanonical interaction mechanism.

Background The p53 protein is the best studied target in human cancer. For decades, p53 has been believed to act mainly as a tumor suppressor and by transcriptional regulation. Only recently, the complex and diverse function of p53 has attracted more attention. Using several molecular approaches, we studied the impact of different p53 variants on extrinsic and intrinsic apoptosis signaling. Results We reproduced the previously published results within intrinsic apoptosis induction: while wild-type p53 promoted cell death, different p53 mutations reduced apoptosis sensitivity. The prediction of the impact of the p53 status on the extrinsic cell death induction was much more complex. The presence of p53 in tumor cell lines and primary xenograft tumor cells resulted in either augmented, unchanged or reduced cell death. The substitution of wild-type p53 by mutant p53 did not affect the extrinsic apoptosis inducing capacity. Conclusions In summary, we have identified a non-expected impact of p53 on extrinsic cell death induction. We suggest that the impact of the p53 status of tumor cells on extrinsic apoptosis signaling should be studied in detail especially in the context of therapeutic approaches that aim to restore p53 function to facilitate cell death via the extrinsic apoptosis pathway.

Resting tumor cells represent a huge challenge during anticancer therapy due to their increased treatment resistance. TNF-related apoptosis-inducing ligand (TRAIL) is a putative future anticancer drug, currently in phases I and II clinical studies. We recently showed that TRAIL is able to target leukemia stem cell surrogates. Here, we tested the ability of TRAIL to target cell cycle-arrested tumor cells. Cell cycle arrest was induced in tumor cell lines and xenografted tumor cells in G0, G1 or G2 using cytotoxic drugs, phase-specific inhibitors or RNA interference against cyclinB and E. Biochemical or molecular arrest at any point of the cell cycle increased TRAIL-induced apoptosis. Accordingly, when cell cycle arrest was disabled by addition of caffeine, the antitumor activity of TRAIL was reduced. Most important for clinical translation, tumor cells from three children with B precursor or T cell acute lymphoblastic leukemia showed increased TRAIL-induced apoptosis upon knockdown of either cyclinB or cyclinE, arresting the cell cycle in G2 or G1, respectively. Taken together and in contrast to most conventional cytotoxic drugs, TRAIL exerts enhanced antitumor activity against cell cycle-arrested tumor cells. Therefore, TRAIL might represent an interesting drug to treat static-tumor disease, for example, during minimal residual disease.

Purpose: Sensitivity of tumor cells toward chemotherapy mainly determines the prognosis of patients suffering from acute lymphoblastic leukemia (ALL); nevertheless, underlying mechanisms regulating chemosensitivity remain poorly understood. Here, we aimed at characterizing the role of caspase-8 for chemosensitivity of B- and T-ALL cells. Experimental Design: Primary tumor cells from children with ALL were evaluated for expression levels of the caspase-8 protein, were amplified in nonobese diabetic/severe combined immunodeficient mice, transfected with siRNA, and evaluated for their chemosensitivity in vitro. Results: Effective cell death in B- and T-ALL cells depended on the presence of caspase-8 for the majority of cytotoxic drugs routinely used in antileukemia treatment. Caspase-8 was activated independently from extrinsic apoptosis signaling. Accordingly in primary ALL cells, the expression level of caspase-8 protein correlated with cell death sensitivity toward defined cytotoxic drugs in vitro. In the subgroup of primary ALL cells, with low expression of caspase-8, methotrexate (MTX) upregulated the expression of caspase-8 mediated by the transcription factor p53, suggesting epigenetic silencing of caspase-8. RNA interference in patient-derived B- and T-ALL cells revealed that effective cell death induction by most routine drug combinations involving MTX depended on the presence of caspase-8. Conclusion: Our results indicate that caspase-8 is crucial for the high antileukemic efficiency of numerous routine cytotoxic drugs. Reexpression of epigenetically downregulated caspase-8 represents a promising approach to increase efficiency of antileukemic therapy. Clin Cancer Res; 17(24); 7605–13. ©2011 AACR.

Application of anthracyclines and Vinca alkaloids on the same day represents a hallmark of polychemotherapy protocols for hematopoietic malignancies. Here we show, for the first time, that both drugs might act most efficiently if they are applied on different days. Proof-of-concept studies in 18 cell lines revealed that anthracyclines inhibited cell death by Vinca alkaloids in 83% of cell lines. Importantly, in a preclinical mouse model, doxorubicin reduced the anti–tumor effect of vincristine. Both drugs acted in a sequence-dependent manner and the strongest anti–tumor effect was obtained if both drugs were applied on different days. Most notably for clinical relevance, in 34% of 35 fresh primary childhood leukemia cells tested in vitro, doxorubicin reduced the anti–tumor effect of vincristine. As underlying mechanism, doxorubicin activated p53, p53 induced cell-cycle arrest, and cell-cycle arrest disabled inactivation of antiapoptotic Bcl-2 family members by vincristine; therefore, vincristine was unable to activate downstream apoptosis signaling. As molecular proof, antagonism was rescued by knockdown of p53, whereas knockdown of cyclin A inhibited vincristine-induced apoptosis. Our data suggest evaluating anthracyclines and Vinca alkaloids on different days in future trials. Selecting drug combinations based on mechanistic understanding represents a novel conceptional strategy for potent polychemotherapy protocols.

Resting tumor cells represent a huge challenge during anti-cancer therapy due to their increased treatment resistance. Example given, during minimal residual disease remaining tumor cells might be non-cycling. TNF-related apoptosis-inducing ligand (TRAIL) is a putative future anti-cancer drug currently in phase I and II clinical studies. We recently showed that TRAIL is able to target leukemia stem cell surrogates (Blood 2012,119,4224). Here we tested the ability of TRAIL to target cell cycle arrested tumor cells. Primary tumor cells from children with acute lymphoblastic leukemia were amplified in immuno-compromised mice and used for in vitro knockdown studies, besides tumor cell lines. Cell cycle arrest was induced in G0, G1 or G2 using cytotoxic drugs, phase-specific inhibitors or RNA interference against cyclinB and E. Cell cycle was visualized by flowcytometry analysis including phosphorylation of Serine 10 in Histone H3 to discriminate G2 from M phase. Biochemical or molecular arrest at any point of the cell cycle increased TRAIL-induced apoptosis, independently from the tumor cell type, the cell cycle inhibitor used or the check point addressed. In contrast, when cell cycle arrest was disabled by addition of caffeine, the anti-tumor activity of TRAIL was reduced. Most importantly for clinical translation, tumor cells from three children with B-precursor or T-cell acute lymphoblastic leukemia showed increased TRAIL-induced apoptosis upon knockdown of either cyclinB or cyclinE arresting the cell cycle in G2 or G1, respectively. Taken together and in contrast to most conventional cytotoxic drugs, TRAIL exerts enhanced anti-tumor activity against cell cycle arrested tumor cells. Therefore, TRAIL might represent an interesting drug to treat static tumor disease, e.g., during minimal residual disease. Citation Format: Harald Ehrhardt, Franziska Wachter, Michaela Grunert, Irmela Jeremias. TRAIL induces apoptosis preferentially in cell cycle arrested tumor cells, e.g., in tumor cells from children with acute lymphoblastic leukemia upon knockdown of cyclinE. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3326. doi:10.1158/1538-7445.AM2013-3326 Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.

Abstract 1879 Leukemic stem- and progenitor cells exhibit low cycling activity which might represent a major cause for their increased treatment resistance. TRAIL (TNF-related apoptosis inducing ligand) is a novel putative anticancer drug currently in phase I and II clinical testing. We recently showed that TRAIL is able to address stem- and progenitor cells from patients with acute lymphoblastic leukemia (ALL) in xenotransplantation assays (Alves et al., Blood 2012,119,4224). As stem- and progenitor cells are often non-cycling, we asked here, whether TRAIL is able to address resting leukemia cells. We used cell lines and primary tumor cells from children with ALL which were amplified in severely immuno-compromised mice (NSG mice). Cell cycle arrest was induced (i) by addition of conventional cytotoxic drugs which are known to act as cytostatic drugs such as doxorubicine; (ii) by biochemical inhibitors known to induce cell cycle arrest at different defined points of the cell cycle such as mimosine; (iii) by molecular approaches and knockdown of cyclinB arresting cell cycle in G2 or knockdown of cyclinE arresting cell cycle in G1. Unexpectedly, TRAIL-induced apoptosis was enhanced, whenever cell cycle was arrested. Cell cycle arrest sensitized towards TRAIL-induced apoptosis independently from the point or phase of cell cycle which was arrested (G0, G1 or G2) and independently from the agent used to arrest the cell cycle. Similarly, knockdown of cyclinB or cyclinE both clearly sensitized cell line cells towards TRAIL-induced apoptosis. Cytotoxic drugs and cell cycle inhibitors might arrest the cell cycle by activation of p53. Accordingly, when caffeine was added which inhibited p53 activity and drug-induced cell cycle arrest, sensitization towards TRAIL-induced apoptosis was blocked. We have recently established a novel method which enables performing knockdown experiments in tumor cells derived from ALL patients (Höfig et al., Cell Comm. Signal. 2012,10,8). Using this method and most important for clinical translation, we could show that knockdown of either cyclinB or cyclinE clearly sensitized patient-derived ALL cells towards TRAIL-induced apoptosis. Taken together and in contrast to most conventional cytotoxic drugs, TRAIL exerts anti-tumor activity preferentially against tumor cells in cell cycle arrest and less against actively cycling tumor cells. This special feature of TRAIL might explain its anti-tumor activity against stem- and progenitor cells in patients with ALL. Thus, TRAIL might represent an interesting drug to treat disease stages with accumulation of stem- and progenitor cells and static tumor disease, e.g., during minimal residual disease. Disclosures: No relevant conflicts of interest to declare.