Synergistic Increase In Apoptosis Research Articles

MS-553, a Selective PKC-ß Inhibitor Inhibits BCR Signaling and Synergizes with Venetoclax for the Treatment of High Risk CLL

Inhibitors of Bruton's tyrosine kinase (BTK) have been transformative in treating patients with chronic lymphocytic leukemia (CLL) due to their ability to inhibit B-Cell Receptor (BCR) signaling. However, despite durable remissions in many patients, complete responses are uncommon and acquired mutations in BTK and its downstream target phospholipase Cγ2 (PLCγ2) lead to relapses and poor survival. In parallel, genomic high-risk features such as aberrations in TP53 represent an independent risk factor that favor progression on BTK inhibitors (BTKi). TP53 aberrations comprise of deletions in chromosome 17 (del17p) which result in the loss of one allele of TP53 and point mutations in the TP53 gene. CLL patients with del17p acquire clonal mutations that affect the other p53 allele in 82% of cases and largely express only mutant TP53 protein. Therefore, there is an urgent need to identify therapies for patients who progress on BTKi. We have observed over-expression of protein kinase C-β (PKC-β) in primary CLL patient samples including in those that have mutations in TP53. PKC-β is a downstream of the BCR and functions downstream of BTK. PKC-β is required for CLL cell survival and proliferation and therefore can be a potential target for patients with mutations in p53 that do not respond to other targeted therapies. We hypothesized that targeting PKC-β will inhibit BCR signaling downstream of BTK and may offer an effective therapeutic strategy for patients who relapse on ibrutinib. Mechanistically, we found that CLL cells (n=6 patients) in contact with stromal support upregulated their levels of PKC-β. Pre-exposure of stromal cells to MS-553, a selective PKC-β inhibitor prevented the stromal cell contact-induced increase in PKC-β within the CLL cells establishing that stromal contact was an important source of PKC-β in CLL cells. We have shown in primary CLL patient samples that exposure to MS-553 decreases activation of PKC-β and its downstream effectors GSK3-β and ERK in a dose-dependent manner regardless of mutations in p53 both in the presence and absence of stromal support. In parallel, MS-553 also decreased the levels of BCL-xL, a key pro-survival BCL2 family protein via downregulation of β-catenin. Using dynamic BH3 profiling we show that CLL samples (both WT and mutant TP53) exposed to MS-553 (5µM, 24h) increase their dependence on BCL-2 and BCL-xL for survival. This finding suggested that MS-553 could synergize with the BCL-2 inhibitor venetoclax to eliminate CLL cells. Correspondingly, cells exposed to a combination of MS-553 and venetoclax released more cytochrome c (Cyt c), used as a measure of mitochondrial dysfunction, compared to each single agent alone and led to a synergistic increase in apoptosis as measured by Annexin V positivity. These data provide strong rationale for an ongoing Phase I/II trial that evaluates MS-553 as a single agent and in combination with targeted agents such as venetoclax. Early interim data indicate that MS-553 is safe, tolerable and effective both as a single agent and in combination with venetoclax.

Targeting of the Hypoxia-Induced Acid Microenvironment of Multiple Myeloma Cells Increases Hypoxia-Mediated Apoptosis

Multiple myeloma (MM) is an incurable disease in which malignant plasma cells engraft within the bone marrow (BM). It is postulated that components of the BM microenvironment provide pro-survival and pro-growth signals to MM cells thereby facilitating their survival and proliferation. Because the BM is known to be hypoxic, we hypothesized that low pO2 activates an adaptive response mediated by hypoxia inducible transcriptional factors (HIFs) that protect MM cells from hypoxia-mediated apoptosis. We recently showed that targeting HIF activity with a polyamide compound (HIF-PA) that targets the hypoxic response element and blocks the ability of HIF to bind to its cognate DNA sequence both inhibits HIF-mediated gene expression and makes MM cells in culture MM xenografts engrafted in the BM of NOG mice sensitive to hypoxia-mediated killing in vitro and. In other tumors, hypoxia leads to acidification of the microenvironment that is a critical factor in their survival and spread. To examine if this is also the case for MM, we exposed a panel of cell lines to severe hypoxia (down to 0.2%) for 48 hrs. This produced a marked acidification of the microenvironment (decrease in pH from ~7.4 to ~6.4) as well as increase in the expression of carbonic anhydrase IX (CA9), a HIF-dependent enzyme important to pH regulation. Moreover, while MM cells are usually resistant to hypoxia-mediated apoptosis, exposure of 8226 MM cells to a specific CA9 inhibitor (acetazolamide) or Na+/H+ transporter inhibitor (amiloride) increased hypoxia-mediated apoptosis by 3-4 fold. Tumor cells are able to sense changes in pO2 levels through the activity of the Pryol-Hydroxylase (PHD1-3) sensing pathways. In particular, PHD3 is a known tumor suppressor gene that is frequently silenced in MM cells (including 8226), and confers resistance to hypoxia by constitutively activating HIF2a. We exogenously re-expressed PHD3 in MM cells and this concomitantly downregulated HIF2a expression in an oxygen-dependent manner and sensitized these cells to hypoxia-mediated apoptosis. Apoptosis was further increased with an acid pH. In addition, HIF-PA treatment of cells cultured under hypoxic and acidic pH (6.8) conditions showed a synergistic increase in apoptosis compared to control cells which was dependent on the PHD3 expression. Re-expression of PHD3 also resulted in an increased sensitivity to both acetazolamide and amiloride, suggesting that inhibiting HIF2a expression is critical for sensitizing MM cells to a low pH. Finally, exposure of MM cells to a low pO2 and acidic pH (6.8) significantly enhanced MM migration as detected with the transwell migration assay; whereas the movement of the MM cells was almost totally inhibited (>80%) by treatment with low concentrations of acetazolamide. The results of these studies support our hypothesis that hypoxic conditions within the BM results in MM-mediated acidification of the tumor microenvironment that facilitates the survival and migration of malignant myeloma cells. Targeting the pathways that lead to acidification of the microenvironment might be a novel and effective primary or ancillary strategy for the treatment of MM. DisclosuresNo relevant conflicts of interest to declare.

Abstract 1774: Targeting aurora A kinase (AAK) in platinum-resistant high grade serous ovarian cancer (HGSOC)

Abstract Resistance to platinum-based chemotherapy is a major problem in the clinical management of recurrent HGSOC. While the use of poly ADP-ribose polymerase inhibitors (PARPi) have made an impact in BRCA1/2 mutated and homologous recombinant-deficient HGSOC, resistance and limited response-rates to PARPi persist. Alisertib (AL) an inhibitor of AAK can inhibit entry and progression of cells through mitosis as well as the interaction with N-myc and its degradation (Front. Oncol. doi:10.3389/fonc.2015.00189). In models of HGSOC from patients with recurrent platinum-resistant disease, cisplatin (CP) treatment does not lead to cell cycle traverse perturbations and significant accumulation of cells in G2-M. Since a defect in the G2-M DNA damage checkpoint is a potential mechanism of resistance to cisplatin, we sought to test the hypothesis that inhibition of AAK that is involved in centrosome maturation and entry into mitosis would enhance the anti-tumor activity of cisplatin in platinum-resistant HGSOC. To test this hypothesis, we treated HGSOC cell lines from patients with recurrent platinum-refractory disease with increasing doses of CP (1- 2.5 μM), of AL (0.05 μM) or the combination and evaluated effects on cell proliferation, apoptosis and generation of reactive oxygen species (ROS). The sequential treatment of four different HGSOC cell lines with CP for 3 hours followed by AL for 48 hours and recovery in drug-free medium for 96-120 hours led to a synergistic increase in apoptosis (p<0.01) and a synergistic decrease in cell survival (p<0.001) compared to treatment with either drug alone. The enhanced apoptosis and decreased cell survival with (CP→AL) treatment was accompanied by a significant increase (p<0.05) in ROS. In contrast, sequential treatment with AL followed by CP (AL→CP) led to an antagonistic response. The synergistic effect of (CP→AL) was accompanied by a modest increase in mRNA and protein levels of AAK. However, protein levels of N-myc protein, which interacts with AAK and is degraded, were not altered. Interestingly, (CP→AL) treatment did not elicit the synergistic cytotoxic response or increased ROS in immortalized normal human ovarian surface epithelium or normal human fallopian tube secretory epithelium cells. In platinum-refractory HGSOC cells that are wild-type for BRCA1/2 and resistant to olaparib, treatment with the combination of 0.01 μM AL and 1 μM olaparib for 48 hours resulted in an additive cytotoxic response. Based on the mutation landscape of the HGSOC cell panel evaluated, the synergistic cytotoxicity with the CP→AL treatment was independent of BRCA1/2 or TP53 mutation status. In summary, CP resistance in HGSOC due to a defective G2-M DNA damage response, can be potentially circumvented by sequential treatment with CP followed by AL. Citation Format: Ram N. Ganapathi, Eric J. Norris, Ashley P. Sutker, Hala Soliman, Mahrukh K. Ganapathi. Targeting aurora A kinase (AAK) in platinum-resistant high grade serous ovarian cancer (HGSOC) [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 1774.

Synergistic anticancer effects of ruxolitinib and calcitriol in estrogen receptor‑positive, human epidermal growth factor receptor 2‑positive breast cancer cells.

The Janus kinase (JAK)1 and JAK2 inhibitor, ruxolitinib, and the active form of vitamin D (calcitriol) were previously reported to possess anticancer effects in breast cancer. The present study investigated the combined effects of ruxolitinib and calcitriol on an estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-positive, breast cancer cell line. The ER and HER2-positive MCF7-HER18 breast cancer cell line was used to investigate the combination effect of ruxolitinib and calcitriol. A bromodeoxyuridine (BrdU) assay was used to investigate cell growth inhibition. The synergism of this combination therapy was examined using the Chou-Talalay method. Cell cycle analysis was performed by flow cytometry, and apoptosis was evaluated by flow cytometry following Annexin V-fluorescein isothiocyanate (FITC) and propidium iodide (PI) staining. Alterations in protein expression levels were analyzed by western blotting. The BrdU assay indicated that combination treatment using ruxolitinib and calcitriol produced a synergistic anti-proliferative effect in MCF7-HER18 breast cancer cells. Annexin V-FITC/PI staining and cell cycle analysis identified a synergistic increase in apoptosis and sub-G1 arrest in the presence of ruxolitinib and calcitriol. Western blot analysis revealed that these synergistic effects of ruxolitinib and calcitriol were associated with reduced protein levels of JAK2, phosphorylated JAK2, c-Myc proto oncogene protein, cyclin-D1, apoptosis regulator Bcl-2 and Bcl-2-like protein 1, and with increased levels of caspase-3 and Bcl-2-associated agonist of cell death proteins. The results of the present study demonstrated the synergistic anticancer effects of ruxolitinib and calcitriol in ER and HER2-positive MCF7-HER18 breast cancer cells. Based on these findings, ruxolitinib and calcitriol may have potential as a combination therapy for patients with ER and HER2-positive breast cancer.

Combination of Eribulin and Aurora A Inhibitor MLN8237 Prevents Metastatic Colonization and Induces Cytotoxic Autophagy in Breast Cancer.

Recent findings suggest that the inhibition of Aurora A (AURKA) kinase may offer a novel treatment strategy against metastatic cancers. In the current study, we determined the effects of AURKA inhibition by the small molecule inhibitor MLN8237 both as a monotherapy and in combination with the microtubule-targeting drug eribulin on different stages of metastasis in triple-negative breast cancer (TNBC) and defined the potential mechanism of its action. MLN8237 as a single agent and in combination with eribulin affected multiple steps in the metastatic process, including migration, attachment, and proliferation in distant organs, resulting in suppression of metastatic colonization and recurrence of cancer. Eribulin application induces accumulation of active AURKA in TNBC cells, providing foundation for the combination therapy. Mechanistically, AURKA inhibition induces cytotoxic autophagy via activation of the LC3B/p62 axis and inhibition of pAKT, leading to eradication of metastases, but has no effect on growth of mammary tumor. Combination of MLN8237 with eribulin leads to a synergistic increase in apoptosis in mammary tumors, as well as cytotoxic autophagy in metastases. These preclinical data provide a new understanding of the mechanisms by which MLN8237 mediates its antimetastatic effects and advocates for its combination with eribulin in future clinical trials for metastatic breast cancer and early-stage solid tumors. Mol Cancer Ther; 15(8); 1809-22. ©2016 AACR.

Abstract LB-016: Nutlin and oxaliplatin induce p53-dependent addiction to FLIP

Abstract Background: The tumour suppressive functions of the p53 transcription factor are inactivated through mutations or suppressed through non-mutational mechanisms in almost all cancer cells. This work focusses on identifying mechanisms through which, tumours retaining wild-type p53 evade apoptosis in response to chemotherapies and p53 agonists such as Nutlin-3A. Methods: A panel of matched p53 wild-type and deficient colorectal cancer cell line models were studied, using Nutlin-3A and oxaliplatin as direct and indirect p53 activating agents. A number of molecular (Western blots, RT-PCR), phenotypic (cell death) and genomic analyses were used to investigate the importance of p53 and its downstream transcriptional programs. Results: We demonstrate the importance of p53 in priming cancer cell sensitivity to apoptotic cell death following treatment with Nutlin-3A or oxaliplatin through activation of pro-apoptotic p53 targets, such as TRAIL-R2, Fas/CD95, PUMA and NOXA. Surprisingly, our analysis further reveals that this priming is accompanied by a parallel early p53-dependent pro-survival response that imposes a critical dependence on the caspase-8 inhibitor FLIP. Preventing FLIP up-regulation with siRNA resulted in synergistic increases in apoptosis in response to Nutlin-3A and oxaliplatin in p53 wild-type, but not p53 null cells. p53-mediated FLIP up-regulation was blocked by pharmacological inhibition of HDACs1-3 using Entinostat, which caused hyper-acetylation of the p53 C-terminus and altered transactivation of a number of other p53 apoptotic target genes. Notably, the synergy between Nutlin-3A or oxaliplatin and Entinostat was abrogated in cells over-expressing FLIP trans-genes or in cells in which caspase-8 was depleted using RNAi. Conclusion: These results uncover a p53-regulated apoptosis priming event, in which a number of key pro-apoptotic genes are upregulated in response to direct p53 agonism (Nutlin-3A) or indirect activation downstream of DNA damage (oxaliplatin). However, immediate commitment to apoptosis is prevented by the concomitant p53-mediated up-regulation of FLIP. This may be a mechanism that has evolved to allow DNA damage repair time to take place, while priming the cell to undergo apoptosis if the damage is irreparable. From a cancer therapeutics point-of-view, this work identifies FLIP upregulation in p53 wild-type cancers as a key target for overcoming resistance to direct and indirect p53 activating therapies. Citation Format: Alexander McIntyre, Fiammetta Falcone, Patrick G. Johnston, Daniel B. Longley, Simon S. McDade. Nutlin and oxaliplatin induce p53-dependent addiction to FLIP. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-016.

Tamoxifen synergizes with 4-(E)-{(4-hydroxyphenylimino)-methylbenzene, 1,2-diol} and 4-(E)-{(p-tolylimino)-methylbenzene-1,2-diol}, novel azaresveratrol analogs, in inhibiting the proliferation of breast cancer cells.

We have recently shown that 4-(E)-{(4-hydroxyphenylimino)-methylbenzene, 1,2-diol} (HPIMBD) and 4-(E)-{(p-tolylimino)-methylbenzene-1,2-diol} (TIMBD), novel analogs of resveratrol (Res), selectively inhibited the proliferation of breast cancer cells. In the current study, we tested HPIMBD and TIMBD individually in combination with tamoxifen (Tam) for inhibition of growth of breast cancer cells. Tamoxifen was first tested on non-neoplastic breast epithelial cell lines and its dose that does not inhibit their growth was determined. A combination of this low dose of Tam with either of the Res analogs HPIMBD or TIMBD, resulted in synergistic inhibition of proliferation of breast cancer cells. Both estrogen receptor (ER)-positive and negative breast cancer cell lines responded to the combination. The combination resulted in a substantial decrease in IC50 values of Res analogs in all breast cancer cell lines tested. Mechanistic studies showed a synergistic increase in apoptosis and autophagy genes (beclin-1 and LC3BII/I) with the combination in ER-negative MDA-MB-231 cells. In ER-positive MCF-7 and T47D cells, the mechanism of synergy was found to be inhibition of expression of ERα and oncogene c-Myc. The combination treatment had a synergistic effect in inhibiting the colony forming and spheroid forming ability of cancer cells. Taken together, our findings indicate that a combination of Tam and Res analogs HPIMBD or TIMBD represents a novel approach to enhancing the use of Tam in therapy for breast cancers. Considering the urgent need for novel therapeutic strategies to treat ER-negative breast cancers and overcoming resistance in ER-positive cancers, this combinatorial approach is worthy of continued investigation.

Abstract A84: Regulation of 6-phosphofructo-2-kinase (PFKFB3) by estradiol and implications for the treatment of ER+ metastatic breast cancer

Abstract Estradiol (E2) administered to estrogen receptor positive (ER+) breast cancer patients stimulates glucose uptake by tumors. This E2-induced metabolic flare is predictive of the clinical effectiveness of anti-estrogens and downstream metabolic regulators of E2 are expected to have utility as targets for the development of anti-breast cancer agents. While the stimulation of glucose metabolism by E2 has been demonstrated, relatively little is known about the precise downstream effectors required for E2 to stimulate glucose metabolism in breast cancer. The family of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB1-4) control glycolytic flux via their product, fructose-2,6-bisphosphate (F26BP), which activates 6-phosphofructo-1-kinase (PFK-1). We recently demonstrated that PFKFB3 expression is elevated in breast cancer lymph node metastases and that exposure of human MCF-7 and T-47D breast cancer cells to E2 causes a rapid increase in 14C-glucose uptake and glycolysis that is coincident with an induction of PFKFB3 mRNA (via ER binding to its promoter), protein expression and the intracellular concentration of its product, F26BP. Importantly, we also found that selective inhibition of PFKFB3 expression and activity using siRNA or a PFKFB3 inhibitor, PFK158, markedly reduces the E2-mediated increase in F26BP, 14C-glucose uptake and glycolysis. In the current study, we sought to determine if co-administration of PFK158, with ICI 182,780 (fulvestrant), would offer a greater anti-tumor effect. In unpublished results, we demonstrated that the combination of the anti-estrogen, fulvestrant, with PFK158 results in greater regressions of MCF-7 xenografts than either drug alone in athymic BALB/C mice. In addition to regulating glucose metabolism, PFKFB3 has been found to be a regulator of the cell cycle via cyclin dependent kinases. We postulated that the combination of PFK158 with the newly FDA-approved CDK4/6 inhibitor palbociclib may result in a synergistic increase in apoptosis. We found that PFK158 markedly increased apoptosis caused by palbociclib in MCF-7 breast cancer cells but not in normal human mammary epithelial cells. Taken together, these data indicate that PFKFB3 inhibitors such as PFK158 may have clinical utility for the treatment of ER+ breast cancers when combined with anti-estrogen agents and/or CDK4/6 inhibitors. Citation Format: Yoannis Imbert-Fernandez, Brian Clem, Gilles Tapolsky, Jason Chesney. Regulation of 6-phosphofructo-2-kinase (PFKFB3) by estradiol and implications for the treatment of ER+ metastatic breast cancer. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr A84.

ACY-241, a Novel, HDAC6 Selective Inhibitor: Synergy with Immunomodulatory (IMiD®) Drugs in Multiple Myeloma (MM) Cells and Early Clinical Results (ACE-MM-200 Study)

Histone deacetylase (HDAC) enzymes are attractive therapeutic targets in oncology, but non-selective HDAC inhibitors have led to dose-limiting toxicities in patients, particularly in combination with other therapeutic agents. Ricolinostat (ACY-1215), a first-in-class orally available HDAC inhibitor that is 11-fold selective for HDAC6, synergizes in vitro and in vivo in models of MM and lymphoma with bortezomib (Santo, Blood, 2012; Amengual, Clin Cancer Res, 2015) or carfilzomib (Mishima, Br J Haematol, 2015; Dasmahapatra, Mol Cancer Ther, 2014). Furthermore, ricolinostat has demonstrated an excellent safety and tolerability profile in phase I trials as an oral liquid formulation (Raje, Haematologica, 2014, Suppl 1). We have now identified ACY-241 as a structurally related and orally available selective inhibitor of HDAC6 that is undergoing clinical evaluation in tablet form.In combination with ricolinostat, the immunomodulatory (IMiD®) class of drugs, including lenalidomide (Len) and pomalidomide (Pom), exhibit striking anti-myeloma properties in a variety of MM models (Quayle, AACR, 2014) and have demonstrated clinical activity in MM patients (Yee, ASH, 2014). In support of our ongoing development of ACY-241, we show here that combination with either Len or Pom leads to synergistic decrease in MM cell viability in vitro. Time course studies demonstrated cell cycle arrest followed by progressive induction of apoptosis after prolonged exposure to Len or Pom. Notably, the addition of ACY-241 to either Len or Pom resulted in synergistic increases in apoptosis of MM cells. At the molecular level, treatment with IMiDs reduced expression of the critical transcription factors MYC and IRF4, which was further reduced by combination treatment with ACY-241. Current studies are exploring the molecular mechanism underlying this effect, which may be a consequence of low level inhibition of HDAC1, 2, and 3 by ACY-241. Prolonged treatment with ACY-241 plus Pom was well tolerated in vivo with no evidence of toxicity, and the combination resulted in a significant extension of survival in a xenograft model of MM. Given the comparable tolerability profiles of ricolinostat and ACY-241 and the similar preclinical activity in combination with IMiDs, a clinical trial (NCT02400242) is currently evaluating ACY-241 in combination with Pom and low-dose dexamethasone in MM patients.Predicated upon the clinical experience with ricolinostat and the non-clinical pharmacokinetics of ACY-241, we designed an expedited first-in-human phase 1a/1b clinical trial of a single cycle of ACY-241 monotherapy followed by ACY-241 in combination with Pom and dexamethasone in MM patients. A merged monotherapy/combination trial design was chosen to grant patients access to combination therapy with an established regimen while enabling insight into the safety, pharmacokinetics, and pharmacodynamics of ACY-241 monotherapy. Patients with relapsed or relapsed-and-refractory MM previously treated with at least two cycles of Len and a proteasome inhibitor were eligible for this trial. The first patient was enrolled in June 2015. This patient tolerated monotherapy well and pharmacokinetics showed maximal plasma levels of ACY-241 in the micromolar range, consistent with predictions. An update on enrollment, pharmacokinetic and pharmacodynamic profiles as well as safety of monotherapy and combination therapy will be provided. DisclosuresNiesvizky:Celgene: Consultancy, Speakers Bureau. Richardson:Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Millennium Takeda: Membership on an entity's Board of Directors or advisory committees; Gentium S.p.A.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees. Gabrail:Onyx: Honoraria, Speakers Bureau; BI: Honoraria, Speakers Bureau; Janssen: Speakers Bureau; Sanofi: Honoraria, Speakers Bureau. Madan:Onyx: Speakers Bureau; Celgene: Speakers Bureau. Quayle:Acetylon Pharmaceuticals, Inc.: Employment, Equity Ownership. Almeciga-Pinto:Acetylon Pharmaceuticals, Inc: Employment. Jones:Acetylon Pharmaceuticals, Inc.: Employment, Equity Ownership. Houston:Acetylon Pharmaceuticals, Inc: Employment. Hayes:Acetylon Pharmaceuticals, Inc: Employment. Van Duzer:Acetylon Pharmaceuticals, Inc: Employment. Wheeler:Acetylon Pharmaceuticals, INC: Employment. Trede:Acetylon Pharmaceuticals, Inc: Employment. Raje:Acetylon: Research Funding; Celgene Corporation: Consultancy; BMS: Consultancy; Amgen: Consultancy; Millenium: Consultancy; AstraZeneca: Research Funding; Novartis: Consultancy; Onyx: Consultancy; Eli Lilly: Research Funding; Takeda: Consultancy.

Combination Effects of B Cell Receptor Pathway Inhibitors (Entospletinib, ONO/GS-4059, and Idelalisib) and a BCL-2 Inhibitor in Primary CLL Cells

BackgroundCompared to single-agent therapy, combination therapy in chronic lymphocytic leukemia (CLL) has the goal to achieve complete responses with greater durability and ultimately to achieve minimal residual disease negativity and cure with finite therapy. The investigational agents entospletinib, ONO/GS-4059, and ABT-199 are selective, oral, small molecule inhibitors of SYK, BTK, and BCL-2, respectively, and currently are each being developed for the treatment of CLL. Idelalisib (Zydelig™), a first-in-class, selective, oral inhibitor of PI3Kδ, is approved for the treatment of relapsed chronic lymphocytic leukemia (CLL) in combination with rituximab. Though significant clinical efficacy has been achieved in CLL with currently approved agents, improvement in outcome could be enhanced with combinations of novel agents. We investigated the activity of combinations of the B cell receptor pathway inhibitors, entospletinib, ONO/GS-4059 and idelalisib, and the BCL-2 inhibitor, ABT-199, in vitro in primary CLL cells to define the potential utility of these combinations for clinical investigation in patients.MethodsPeripheral blood mononuclear cells (PBMCs) prepared from CLL patients were stimulated with αIgM/αIgG and αCD40 with or without co-culture with the human bone marrow stromal cell line, HS-5. Entospletinib, ONO/GS-4059, idelalisib, or ABT-199 were combined and incubated with cells for 66-72 hours at clinically achievable concentrations of each. Apoptosis was assessed in CD5+ CD19+ CLL cells by Annexin V/Live-Deadassay (Promega). Combinations of the compounds were assessed using a range of concentrations in an 8 × 3 or 3 × 3 matrix format and assayed for apoptosis using a minimum of 8 individual CLL donors after 66-72 hours. Synergy was evaluated using the Bliss model of independence (Meletiadis, J., et al., Med Mycol, 2005).ResultsStimulation of CLL cells by αIgM/αIgG/αCD40 led to a variable, but statistically significant, protection of primary CLL cells in vitro (p=0001), and co-culture with the stromal cell line, HS-5, in conjunction with αIgM/αIgG/αCD40 stimulation further reduced the observed level of apoptosis (p=0.0051). Entospletinib, ONO/GS-4059, idelalisib, and ABT-199, were able to induce apoptosis in primary αIgM/αIgG/αCD40 stimulated CLL cells with an average maximal proportion of apoptotic cells of 50.2%, 26.1%, 23%, and 91.4%, respectively. Addition of ONO/GS-4059 + entospletinib or idelalisib, had an additive effect on induction of apoptosis in primary CLL cells from 5/9 and 6/8 patients, respectively (Figure 1). The addition of ABT-199 to entospletinib, ONO/GS-4059, or idelalisib showed additive to synergistic effects on induction of apoptosis in primary CLL cells from all patients tested, and increased the maximal level of apoptosis (Figure 1).Primary CLL cells were exposed to pairwise combinations of ONO/GS-4059 with entospletinib or idelalisib (A) or ONO/GS-4059 entospletinib, or idelalisib with ABT-199 (B) using a range of concentrations in an 8 x 3 or 3 x 3 matrix format for 66-72 h. Each point represents the mean result for each pairwise combination for each compound at each concentration tested. The percent apoptosis was assessed by AnnexinV/Live-Dead cell staining. The plots show the measured apoptosis versus the calculated, Bliss predicted score for each combination at each concentration tested. Observed apoptosis greater than the bliss predicted score indicates a synergistic relationship. Area within the dashed lines, indicate additive responses.SummaryCombination treatment with the B cell receptor pathway inhibitors resulted in additive increases in apoptosis in primary CLL cells compared to treatment with the individual agents. The combination of the BCL2 inhibitor ABT-199 with entospletinib, ONO/GS-4059 or idelalisib resulted in an additive to synergistic increase in apoptosis in these primary CLL cells and achieved the greatest maximal levels of apoptosis. These in vitro data support clinical investigation of these combinations in patients with CLL. [Display omitted] [Display omitted] DisclosuresJones:Gilead Sciences: Employment, Equity Ownership. Axelrod:Gilead Sciences: Employment, Equity Ownership. Tumas:Gilead Sciences: Employment, Equity Ownership. Quéva:Gilead Sciences: Employment, Equity Ownership. Di Paolo:Gilead Sciences: Employment, Equity Ownership.

Abstract 3632: Synergistic interaction of auranofin with PARP inhibitors in ATM-proficient mantle cell lymphoma

Abstract Auranofin (AF) (Ridaura®) is an oral, FDA-approved, lipophilic, gold-containing compound that was used for treating arthritis. Its primary mechanism of action is the inhibition of thioredoxin reductase (TRR) activity. It consequently induces oxidative/ER stress and also inhibits Heat shock protein (Hsp) 90 chaperone function. Ataxia-telangiectasia mutated and Rad3-related (ATR), Brca1, Chk1 and DNA-PK are Hsp90 client proteins that participate in DNA repair pathways. Based on our observation that AF induces DNA damage, we evaluated whether treatment with AF also impairs Hsp90-dependent DNA damage repair pathways in Mantle Cell Lymphoma (MCL) cells. It has been reported that ATM mutations are found in about 50% cases of MCL. Earlier studies have shown that ATM mutant MCL cells are more sensitive to poly-ADP ribose polymerase (PARP) inhibitors in vitro and in vivo. We hypothesized that by inhibiting Hsp90-dependent DNA repair pathways (such as the ATR pathway), AF would sensitize ATM-proficient MCL cells to PARP inhibition by ABT-888. We demonstrate that treatment of ATM-proficient MCL cell lines, JeKo-1, MO2058 and Z138C with AF induced reactive oxygen species (ROS) levels and resulted in a concomitant decrease in TRR activity. Microarray analysis of AF-treated primary MCL cells revealed that AF significantly altered heat shock response genes, ER and oxidative stress-inducible genes and a subset of genes that were regulated by ATM or Brca1. Treatment with AF also induced Hsp90 hyperacetylation and resulted in the depletion of Hsp90 client proteins including ATR, AKT and Cyclin D1 in MCL cells. Exposure to AF induced significantly more apoptosis in primary MCL cells, as compared to CD19+ normal B cells, CD34+ human cord blood and bone marrow progenitor cells (< 15% apoptosis) (p < 0.01). Co-treatment of MCL cells with AF and the PARP inhibitor ABT-888 resulted in synergistic increase in apoptosis of ATM-proficient MCL cells, with an increase in the induction of γH2AX foci and depletion of p-Chk1 (a downstream target of ATR signaling). This was accompanied with a concomitant increase in the levels of the ER stress-induced transcription factor CHOP (C/EBP homologous protein), cleaved Caspase 3 and PARP. Collectively, our data suggests the synthetic lethal effects observed by the simultaneous inhibition of DNA repair pathway(s) and PARP activity can be extended to ATM-proficient MCL cells. These pre-clinical studies create a strong rationale to determine the in vivo activity of the combination ABT-888 with AF in MCL. Citation Format: Siddhartha Ganguly, Trisha Home, Sumedha Gunewardena, Scott Weir, Joseph McGuirk, Rekha M. Rao. Synergistic interaction of auranofin with PARP inhibitors in ATM-proficient mantle cell lymphoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3632. doi:10.1158/1538-7445.AM2015-3632

Abstract 4478: 6-Phosphofructo-2-Kinase (PFKFB3): At the crossroads of resistance to targeted cancer therapies

Abstract Targeted cancer therapies that block the growth and spread of cancer by interfering with specific onco-proteins are limited by signaling mechanisms that drive intrinsic and acquired resistance. In particular, the MAPK and the PI3K/AKT pathways have been found to both reduce response rates to these agents and to mediate the acquisition of resistance via mutations that activate both pathways. For example, two recently approved BRAFV600E inhibitors, vemurafenib and dabrafenib, cause dramatic clinical responses in ∼50% of BRAFV600E + melanoma patients but universally become ineffective within 5-6 months as a result of genetic alterations that activate the MAPK and PI3K-AKT pathways, including amplifications in BRAFV600E, loss of PTEN, and activating mutations in NRAS and KRAS, MEK, AKT1/3, PIK3CA and PIK3CG. Several of these mutated signaling proteins stimulate glucose metabolism, required for survival and proliferation, in part by increasing the expression and activity of 6-phosphofructo-2-kinase (PFKFB3). For example, PFKFB3 transcription is induced by HIF-1α (which is increased by BRAFV600E, RAS and MEK), loss of PTEN, and PFKFB3 activity is stimulated by AKT via phosphorylation of serine 461. We postulated that PFKFB3 is an essential downstream target of targeted cancer therapies and that the multitude of mutations and amplifications in signaling pathways that cause resistance to these agents activate PFKFB3. In new studies, we demonstrate that BRAFV600E, estradiol and epidermal growth factor each regulate PFKFB3 expression in melanoma cells, breast cancer cells and NSCLC cells, respectively. Moreover, we find that simultaneous inhibition of these oncoproteins or their co-ligands (i.e. the estrogen and EGF receptors) and PFKFB3 using a novel PFKFB3 inhibitor currently in a phase 1 trial, PFK-158, causes a synergistic increase in apoptosis and cytotoxicity in vitro, suggesting that PFK-158 can overcome the intrinsic resistance to these agents. We then decided to determine if these significant synergies would translate in vivo by investigating them in the A375 and A2058 melanoma preclinical models, and in the MCF-7 ER-dependent breast model. For instance, we compared the anti-tumor effects in A375 melanoma xenograft-bearing mice of: (i) the BRAFV600E inhibitor vemurafenib; (ii) PFK-158; or (iii) the combination of vemurafenib and PFK-158. Although we observed significant tumor growth inhibition with both monotherapies (>80%), we only observed tumor regression with the combination therapy (>50%). Results in these different models will be presented. Taken together, these data demonstrate that the PFKFB3 inhibitor PFK-158 may be able to universally overcome resistance to targeted cancer therapies. Furthermore, we predict that phase 1/2 trials of PFK-158 in combination with targeted cancer agents will yield improvements in objective response rates as well as improvements in progression-free survival. Citation Format: Sucheta Telang, Julie O'Neal, Yoannis Imbert-Fernandez, Brian Clem, Nadiia Lypova, Gilles H. Tapolsky, John Trent, Jason Chesney. 6-Phosphofructo-2-Kinase (PFKFB3): At the crossroads of resistance to targeted cancer therapies. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4478. doi:10.1158/1538-7445.AM2015-4478

Combination AZD5363 with Enzalutamide Significantly Delays Enzalutamide-resistant Prostate Cancer in Preclinical Models

The phosphatidylinositol-4,5-bisphosphate 3-kinase/Akt (PI3K/Akt) pathway is a key pathway activated in castrate-resistant prostate cancer (CRPC). This preclinical study evaluates targeting of Akt with AZD5363 alone and in combination with enzalutamide (ENZ) to prevent and delay resistance. Our results demonstrate AZD5363 has significant proapoptotic, antiproliferative activity as monotherapy in ENZ-resistant cell lines in vitro and significantly decreased tumour growth in ENZ-resistant xenograft. The combination of AZD5363 and ENZ showed synergistic decreases in cell proliferation and induced cell-cycle arrest and apoptosis in prostate cancer cell lines LNCaP and C4-2. Notably, the combination of AZD5363 and ENZ resulted in an impressive regression of castrate-resistant LNCaP xenograft tumours without any recurrence demonstrated, whereas progression occurred with both monotherapies. Serum prostate-specific antigen (PSA) levels were also continuously suppressed, and nadir PSA levels were lower in the combination arm compared to ENZ alone. Combination AZD5363 and ENZ at time of castration similarly resulted in significant regression of tumours, with greater relative suppression of PSA compared to when administered to castrate-resistant xenografts. In summary, combination AZD5363 and ENZ significantly delays the development of ENZ resistance in preclinical models through synergistic increases in apoptosis and cell cycle arrest. Our results also suggest greater efficacy may be seen with earlier combination treatment. This study provides preclinical data to support evaluation of combination targeting of the PI3K/Akt pathway and the androgen-receptor axis in the clinic using AZD5363 and ENZ, respectively. Patient summaryTargeting of the Akt and androgen receptor pathways with AZD5363 and enzalutamide, respectively, significantly delayed the development of enzalutamide-resistant prostate cancer through increased apoptosis and cell cycle arrest. This preclinical synergy provides a strong rationale for clinical evaluation of this combination.

Protein kinase C-activating tumor promoters modulate the DNA damage response in UVC-irradiated TK6 cells

12-O-Tetradecanoylphorbol-13-acetate (TPA) is a non-genotoxic tumor promoter that dysregulates the protein kinase C (PKC) pathway and causes variable cellular responses to DNA damage in different experimental models. In the present study, we pretreated human lymphoblastoid TK6 cells (wild-type p53) for 72h with TPA, and five other PKC-activating tumor promoters, to determine how sustained exposure to these chemicals modulates key DNA damage response (DDR) endpoints induced by UVC-irradiation. Here we show that pre-treatment with PKC-activating tumor promoters augmented the sensitivity of TK6 cells to UVC-irradiation characterized by a synergistic increase in apoptosis compared to that induced by either stress alone. In addition, high residual levels of the DNA damage repair signal γH2AX was observed in tumor promoter treated cells indicating a delayed DDR recovery. NH32 (p53-null, isogenic to TK6) cells were resistant to the synergistic effects on apoptosis implicating p53 as a central mediator of the DDR modulating effects. In addition, analysis of p53 target genes in TPA-pre-treated TK6 cells revealed a significant modulation of UVC-induced gene expression that supported a shift toward a pro-apoptotic phenotype. Therefore, sustained exposure to tumor promoting agents modulates the UVC-induced DDR in TK6 cells, which may represent important synergistic interactions that occur during tumor promotion.

Valproic acid, an anti-epileptic drug and a histone deacetylase inhibitor, in combination with proteasome inhibitors exerts antiproliferative, pro-apoptotic and chemosensitizing effects in human colorectal cancer cells: underlying molecular mechanisms.

Although the therapeutic efficacy of valproic acid (VPA) has been observed in patients with solid tumors, the very high concentration required to induce antitumor activity limits its clinical utility. The present study focused on the development of combined molecular targeted therapies using VPA and proteasome inhibitors (PIs: MG132, PI-1 and PR-39) to determine whether this combination of treatments has synergistic anticancer and chemosensitizing effects against colorectal cancer. Furthermore, the potential molecular mechanisms of action of the VPA/PI combinations were evaluated. The effects of VPA in combination with PIs on the growth of colorectal cancer cells were assessed with regard to proliferation, cell cycle, apoptosis, reactive oxygen species (ROS) generation and the expression of genes that control the cell cycle, apoptosis and pro-survival/stress-related pathways. Treatment with combinations of VPA and PIs resulted in an additive/synergistic decrease in colorectal cancer cell proliferation compared to treatment with VPA or PIs alone. The combination treatment was associated with a synergistic increase in apoptosis and in the number of cells arrested in the S phase of the cell cycle. These events were associated with increased ROS generation, pro-apoptotic gene expression and stress-related gene expression. These events were also associated with the decreased expression of anti-apoptotic genes and pro-survival genes. The combination of VPA with MG132 or PI-1 enhanced the chemosensitivity of the SW1116 (29-185‑fold) and SW837 (50-620-fold) colorectal cancer cells. By contrast, the combination of VPA/PR-39 induced a pronounced increase in the chemosensitivity of the SW837 (16-54-fold) colorectal cancer cells. These data provide a rational basis for the clinical use of this combination therapy for the treatment of colorectal cancer.

Combining PARP-1 Inhibition and Radiation in Ewing Sarcoma Results in Lethal DNA Damage

Ewing sarcomas (ES) harbor a chromosomal translocation that fuses the EWS gene to an ETS transcription factor, most commonly Friend leukemia integration 1 (FLI1). The EWS-FLI1 fusion protein acts in a positive feedback loop to maintain the expression of PARP-1, which is involved in repair of DNA damage. Here, we examine the effects of PARP-1 inhibition and radiation therapy on Ewing sarcomas. In proliferation assays, the Ewing sarcoma cell lines RD-ES and SK-N-MC were much more sensitive than non-Ewing sarcoma cell lines to the PARP-1 inhibitor olaparib (Ola; IC50 0.5-1 μmol/L vs. >5 μmol/L) and to radiation (IC50 2-4 Gy vs. >6 Gy). PARP-1 inhibition with short hairpin RNA (shRNA) or Ola sensitized Ewing sarcoma cells, but not non-Ewing sarcoma cells, to radiation therapy in both proliferation and colony formation assays. Using the Comet assay, radiation of Ewing sarcoma cells with Ola, compared to without Ola, resulted in more DNA damage at 1 hour (mean tail moment 36-54 vs. 26-28) and sustained DNA damage at 24 hours (24-29 vs. 6-8). This DNA damage led to a 2.9- to 4.0-fold increase in apoptosis and a 1.6- to 2.4-fold increase in cell death. The effect of PARP-1 inhibition and radiation therapy on Ewing sarcoma cells was lost when EWS-FLI1 was silenced by shRNA. A small dose of radiation therapy (4 Gy), when combined with PARP-1 inhibition, stopped the growth of SK-N-MC flank tumors xenografts. In conclusion, PARP-1 inhibition in Ewing sarcomas amplifies the level and duration of DNA damage caused by radiation therapy, leading to synergistic increases in apoptosis and cell death in a EWS-FLI1-dependent manner.

Ellagic Acid and Embelin Affect Key Cellular Components of Pancreatic Adenocarcinoma, Cancer, and Stellate Cells

Ellagic acid is a polyphenolic phytochemical present in many fruits and nuts with anticancer properties demonstrated in experimental tumor studies. Embelin is a benzoquinone phytochemical isolated from the Japanese herb Ardisiae Japonicae and has been shown to induce apoptosis in cancer cells. We found that ellagic acid and embelin each dose-dependently increased apoptosis and inhibited proliferation in human pancreatic cancer cells, MIA PaCa-2 and HPAF-II cells, and in pancreatic stellate cells, which are progenitors of pancreatic cancer desmoplasia. In each of these cell types, combinations of ellagic acid and embelin at low micromolar concentrations (0.5–3 μM) induced synergistic increases in apoptosis and decreases in proliferation. Ellagic acid decreased NF-κB transcriptional activity, whereas embelin decreased STAT-3 phosphorylation and protein expression of its downstream target survivin in cancer cells. In vivo dietary ellagic acid alone or in combination with embelin decreased tumor size and tumor cellularity in a subcutaneous xenograft mouse model of pancreatic cancer. These results show that ellagic acid and embelin interact with divergent intracellular signaling pathways resulting in augmentation of apoptosis and inhibition of proliferation at low micromolar concentrations for the key cellular components of pancreatic adenocarcinoma.

Profilin 1 Potentiates Apoptosis Induced by Staurosporine in Cancer Cells

The correlation between the loss of Profilin 1 (Pfn1) with tumor progression indicated that Pfn1 is a tumor suppressor in human carcinoma. The molecular mechanisms underlying Pfn1 tumor suppression has yet to be elucidated. In this study, we showed that Pfn1 overexpression sensitizes cancer cells to apoptosis through the typical intrinsic apoptotic pathway. Mechanistically, the increased Pfn1 expression mediated the upregulation of p53R273H, one of the most common tumor-associated hotspot mutations of p53, with transactivation deletion in tumorigenesis and increased localization of p53R273H in cytoplasm. Further studies showed that mutant p53R273H was involved in apoptosis induced by Staurosporine (STS) via transcription-independent mitochondrial functions. We observed (i) the increased cytosolic localization of p53R273H, (ii) the activation of phosphorylation at Ser15, (iii) its mitochondrial localization; Pfn1 acted as a positive regulator of these processes. We also found that Pfn1 interacted with p53R273H and thus facilitated its exertion over the transcription-independent activity in the cytoplasm during drug action. Our results define a new function and mechanism of Pfn1 demonstrating that the combined effect with apoptotic agents led to a synergistic increase in apoptosis. In addition, p53R273H abrogating DNA binding was found to play a major role in the Pfn1- sensitized apoptosis through a transactivation-independent and cytosolic activity.

Rational Design of a Therapeutic Program for Multiple Myeloma Using a Strategy of Drug Repurposing and Combination Therapy

AbstractAbstract 5022 Background:The extremely long development time and low success rate for new drug development programs has translated into limited clinical options in the treatment of cancer. The challenge is further compounded by drug resistance seen in clinical settings for approved standard of care therapeutic options like chemotherapy and targeted drugs like Bortezomib in Multiple Myeloma (MM). This has accelerated the need for initiatives and strategies which promote innovation but drastically reduce drug development failures through prediction of clinical outcomes. We present here a methodology and rationally designed therapeutic program for MM. This program has novel first-in-class mechanism of action and has been developed using a strategy of re-purposing and combinations based on National Center for Advanced Translational Sciences (NCATS) library of industry-provided drugs. Methods:The therapy design and development was completed using validated proprietary technology from Cellworks which enables simulation of cancer disease physiology computationally for predicting clinical outcomes. The comprehensive integrated representation of signaling and metabolic networks across all disease phenotypes and functional proteomics abstraction facilitated a large number of predictive studies in weeks with quantitative transparency into the network. The predictive cancer technology was customized to four MM profiles: OPM2, RPM1, SKMM2 and U266. Ten of the molecularly targeted drugs from the NCATS library were digitally screened alone and in combinations of two across the four MM profiles at four concentrations (C, C/2, C/4, and C/8). Based on screening of over six thousand predictive studies, three designed therapeutics hits were intelligently shortlisted based on synergistic impact on viability and proliferation. Results:The focused therapeutic candidate hit was selected based on synergy using viability endpoints and apoptosis markers such as caspase 3, PARP1 at sub-therapeutic doses. The proposed therapeutic regimen is a sub-therapeutic combination of AT101 and AVE0847. AT101 is a BCL2 inhibitor and AVE0847 is a PPAR alpha and gamma agonist agent. The concentration of each drug used in the designed therapy was close to IC30. The predictive results showed a synergistic increase in apoptotic markers caspase 3 and PARP1 cleaved form. This hit candidate is being pre-clinically validated experimentally across four MM profiles and results will be presented.Table 1:Showing the impact on viability phenotype and apoptotic biomarker caspase 3 with the IC30 combination of the two drugs.PhenotypesU266OPM2SKMMRPMIViability−58.96−62.71−55.39−69.64Caspase 3+90.26+132.67+69.31+202.91 Conclusion:The design of first in class mechanism of action based therapeutic strategies of drug rescue/repurposing has been used to identify AT101 and AVE0847 as a promising drug combination for use in MM which predicatively results in a synergistic increase in apoptosis. These results are currently being validated in vitro. The inherent multi target mechanism of action predicted by this combination could potentially eliminate the drug resistance challenges of single target therapeutics. Disclosures:No relevant conflicts of interest to declare.

Hsp90 inhibitors sensitise human colon cancer cells to topoisomerase I poisons by depletion of key anti-apoptotic and cell cycle checkpoint proteins

Hsp90 and topoisomerase I are both targets for chemotherapeutic agents. Topoisomerase I poisons are standard clinical treatments, whilst Hsp90 inhibitors are progressing through clinical trials. We have demonstrated that when an Hsp90 inhibitor and topoisomerase I poison are combined they produce a synergistic increase in apoptosis in both p53 +/+ and p53 −/− HCT116 human colon cancer cells. Lack of p53 is associated with an increase in sensitivity to the combination treatment; p53 +/+ cells treated with the topoisomerase I poison topotecan (TPT) arrest at G2, whereas in p53 −/− cells the additional presence of the Hsp90 inhibitor geldanamycin (GA) selectively abrogates the G2M checkpoint. More importantly we report that there is a common underlying p53-independent mechanism behind the observed synergistic combined drug effect. We show that concurrent treatment with GA and TPT is able to reverse TPT induced up-regulation of the anti-apoptotic protein Bcl2 in both p53 +/+ and p53 −/− HCT116 cells. The data suggests that inhibition of Hsp90 mediates down-regulation of Bcl2 following the combination treatment and cause a synergistic increase in apoptosis in both p53 +/+ and p53 −/− HCT116 cells; p53 −/− HCT116 cells are more sensitive to the treatment because they also fail to arrest at G2 in the cell cycle.