LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential.

Cancer Biology & Therapy

ISSN: 1538-4047 (Print) 1555-8576 (Online) Journal homepage: http://www.tandfonline.com/loi/kcbt20

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential Christopher S Hong, Winson Ho, Chao Zhang, Chunzhang Yang, J Bradley Elder & Zhengping Zhuang To cite this article: Christopher S Hong, Winson Ho, Chao Zhang, Chunzhang Yang, J Bradley Elder & Zhengping Zhuang (2015) LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential, Cancer Biology & Therapy, 16:6, 821-833, DOI: 10.1080/15384047.2015.1040961 To link to this article: http://dx.doi.org/10.1080/15384047.2015.1040961

Accepted online: 21 Apr 2015.Published online: 21 Apr 2015.

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Date: 11 September 2015, At: 14:21

REVIEW Cancer Biology & Therapy 16:6, 821--833; June 2015; ©2015 Taylor & Francis Group, LLC

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential Christopher S Hong1,2, Winson Ho2, Chao Zhang2, Chunzhang Yang2, J Bradley Elder1, and Zhengping Zhuang2,* 1

The Ohio State University Wexner Medical Center; Department of Neurological Surgery; Columbus, OH USA; 2National Institute of Neurological Disorders and Stroke; National Institutes of Health; Surgical Neurology Branch; Bethesda, MD USA

Downloaded by [Ryerson University Library] at 14:21 11 September 2015

Keywords: cell cycle, chemosensitization, mitotic catastrophe, protein phosphatase 2A, small molecule inhibitor, radiosensitizationreview Abbreviations: PP2A, protein phosphatase 2A; APC, adenomatous polyposis coli; DVL, dishevelled; ATM, ataxia-telangiectasia mutated; MDM2, mouse double minute 2 homolog; ENSA, a-endosulphine; ARPP19, cyclic AMP-regulated phosphoprotein 19; Plk1, polo-like kinase 1; CIP2A, cancerous inhibitor of PP2A; GBM, glioblastoma; HDACs, histone deacetylase complexes; CNTF, ciliary neurotrophic factor; GFAP, glial fibrillary acidic protein; TMZ, temozolomide; TCTP, translationally-controlled tumor protein; MRI, magnetic resonance imaging; TRAIL, TNF-related apoptosis-inducing ligand; DISC, death-inducing signaling complex; NPC, nasopharyngeal carcinoma; HCC, hepatocellular carcinoma; VEGF, vascular endothelial growth factor; HIF-1a, hypoxia-inducible factor-1a; HRR, homologous recombination repair; ABC, ATP-binding cassette.

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase that plays a significant role in mitotic progression and cellular responses to DNA damage. While traditionally viewed as a tumor suppressor, inhibition of PP2A has recently come to attention as a novel therapeutic means of driving senescent cancer cells into mitosis and promoting cell death via mitotic catastrophe. These findings have been corroborated in numerous studies utilizing naturally produced compounds that selectively inhibit PP2A. To overcome the known human toxicities associated with these compounds, a water-soluble small molecule inhibitor, LB100, was recently developed to competitively inhibit the PP2A protein. This review summarizes the pre-clinical studies to date that have demonstrated the anti-cancer activity of LB100 via its chemo- and radio-sensitizing properties. These studies demonstrate the tremendous therapeutic potential of LB100 in a variety of cancer types. The results of an ongoing phase 1 trial are eagerly anticipated.

Introduction Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase with a broad array of regulatory functions. PP2A is comprised of 3 subunits. Subunits A and C, with a or b isoforms, serve structural and catalytic functions respectively. Subunit B, with many isoforms, is the regulatory subunit that determines PP2A localization and enzymatic specificity.1,2 PP2A function is implicated in a diverse set of cellular processes including those governing cell survival, apoptosis, mitosis, DNA damage response, and tau dephosphorylation. Initially, PP2A was regarded as a tumor suppressor as its inhibition was associated with tumorigenesis, it is now clear that the role of PP2A in tumor *Correspondence to: Zhengping Zhuang; Email: [email protected] Submitted: 01/20/2015; Revised: 03/16/2015; Accepted: 04/09/2015 http://dx.doi.org/10.1080/15384047.2015.1040961

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formation is much more complex as PP2A suppression results in potent tumoricidal effects.

Inhibition of Wnt/beta-catenin Signaling The Wnt pathway is a highly conserved signaling pathway with key involvement in embryogenesis, including developmental axis polarity, cell fate, proliferation, and migration.3 Wnt ligand binding to cell surface receptors leads to intracellular accumulation and subsequent nuclear translocation of b-catenin, a transcriptional co-activator of the Tcf transcriptional factor family.4 Together with Tcf, b-catenin transcribes target genes, which when activated constitutively have been implicated in early oncogenesis.5,6 PP2A plays a critical role in the baseline degradation of cytoplasmic b-catenin in the absence of Wnt signaling. PP2A complexes with adenomatous polyposis coli (APC), axin, glycogen synthase kinase-3-b, and dishevelled (DVL) to target b-catenin for proteasomal degradation.7,8 Conversely, the PP2A-C catalytic subunit, has also been shown to positively reinforce Wnt signaling as a downstream signal transducer of the Wnt/b-catenin pathway.9 This has been recapitulated in aspirin-treated colorectal cancer cell lines, in which phosphorylative deactivation of PP2A, induced by aspirin treatment, mediated dose-dependent decreases in Wnt/b-catenin pathway activation.10 Recent studies in pancreatic cancer and colorectal cancer cell lines have suggested that the positive feedback of PP2A on Wnt/b-catenin signaling may be specific to transformed tumor cells and be essential for maintenance of active Wnt signaling.11,12

Induction of Apoptosis PP2A regulates apoptosis through modulation of PI3K/Akt pathway signaling and activation of pro- and anti-apoptotic factors. Dephosphorylation of the anti-apoptotic factor, Bcl2, by


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PP2A is essential for apoptosis in cells with functional Bcl2.13 PP2A also dephosphorylatively activates the pro-apoptotic factor, Bad, in a 14–3–3 protein-dissociation dependent manner in the context of cytokine-induced cell stress signaling.14 On the other hand, PP2A confers resistance to apoptosis in highly metabolic cells through dephosphorylatively activating CaMKII, an inhibitory kinase of caspase-2, highlighting the dynamic processes in which PP2A activity promotes or inhibits apoptosis.15 In addition to its effects on apoptotic factors, PP2A regulates p53 signaling, which when activated drives apoptosis via Bax, Noxa, and Puma activation and Bcl2 inactivation. In response to DNA damage, ataxia telangiectasia-mutated (ATM) signaling directly activates and stabilizes PP2A via phosphorylating a site targeted by the E3 ubiquitin ligase, mouse double minute 2 (MDM2), for inducing proteasomal degradation.16,17 Subsequently, PP2A suppresses protein kinase B (AKT1) signaling which diminishes levels of activated phosphorylated MDM2 (pMDM2) and prevents MDM2-mediated proteasomal degradation of p53.17,18 PP2A also directly dephosphorylates sites on p53, promoting p53 stability, cell cycle arrest, expression of Bax, and apoptosis in the context of irreparable DNA damage.19,20

such, PP2A has been shown to inhibit various known oncogenic signaling pathways. For example, aberrant activation of the JAK/ STAT pathway is associated with numerous human malignancies,38 and PP2A negatively regulates the activating phosphorylation of JAK3 and STAT5 in the context of IL-2 stimulation.39 Likewise, PP2A regulates the major signaling pathway, Ras/Raf/ MEK/ERK, by directly associating with ERK2 and MEK1 and indirectly with Ras and Raf.40-43 Constitutive activation of the Ras/Raf/MEK/ERK pathway has been well characterized in malignant transformation of susceptible cells.44 PP2A also targets c-Myc for proteasomal degradation by dephosphorylation and constitutive c-Myc expression is regarded as a significant event in oncogenic transformation.45 The endogenous protein, cancerous inhibitor of PP2A (CIP2A), indirectly inhibits PP2A through binding c-Myc and preventing its ubiquitation by PP2A-mediated dephosphorylation at key residues.46,47 As such, overexpression of CIP2A has been demonstrated in numerous human malignancies, consistent with a role for PP2A as a tumor suppressor.41,48,49

PP2A as an Oncogene Mitotic Cell Cycle Progression PP2A activity plays a key role in the regulation of normal mitotic progression. In particular, PP2A is regulated by Greatwall kinase (called microtubule-associated serine/threonine kinase-like [Mastl] in mammals), which when depleted is associated with severe mitotic defects.21,22 Greatwall has been shown to inhibit PP2A via the small proteins, a-endosulphine (ENSA) and cyclic AMP-regulated phosphoprotein 19 (ARPP19),23,24 relieving PP2A-mediated dephosphorylation of various Cdk1 substrates and promoting mitotic progression.25,26 PP2A also negatively regulates Cdk1 through activating wee1 and myt1 and inhibiting cdc25 through dephosphorylation.24 Furthermore, PP2A silencing has been shown to upregulate downstream Cdk1 transcriptional targets and promote mitosis.27,28 As such, the GreatwallPP2A network has been proposed as a key signaling axis that promotes normal Cdk1-driven entry through mitosis.29 PP2A also acts on other mitotic mediators, including the key mitosis-specific kinase, Polo-like kinase 1 (PLK1), which localizes to centrosomes during mitosis and when inactivated by PP2A is an important hallmark of G2/M arrest and activation of the DNA damage response.30 PP2A also participates in a negative feedback loop, antagonizing Aurora B and Plk1 kinases, thus maintaining the spindle assembly checkpoint until microtubules are properly attached to chromosome kinetichores.31,32

PP2A As a Tumor Suppressor PP2A has been classically characterized as a tumor suppressor gene. Both reduced expression and loss of function in PP2A have been implicated in numerous cancers, including those of the lung, colon, skin, breast, ovary, endometrium, and brain.33-37 As

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Although restoration of PP2A activity continues to be a focus of combating cancer, inhibition of PP2A has intriguingly become an area of interest for targeting tumors resistant to conventional treatments. In particular, tumor cell senescence is a major contributor to treatment resistance, particularly with radiotherapy and conventional DNA-damaging chemo-agents that preferentially affect actively dividing cells.50,51 Essential to cell senescence is proper functioning of PP2A during the G2 phase, during which Ras signaling is modulated to achieve stable quiescence.28,52 Inhibition of PP2A leads to hyperactivation of Ras signaling and stabilization of c-Myc during G2, which drives cells into mitosis through accumulation of cyclin E: Cdk1 complexes.28,53 The efficacy of PP2A inhibition in overcoming cellular senescence has been corroborated in myelodysplastic syndromes and colon cancer, among others.12,54,55 Pharmacological inhibition of PP2A has been primarily studied using naturally produced, albeit toxic compounds. These include okadaic acid, a cytotoxin produced by certain algae with potent apoptotic effects in human cancer cell lines as a single agent or in combination with standard chemotherapeutic agents.56-58 Another natural compound, cantharidin, a toxin produced by the Mylabris beetle, also exhibits selective PP2A inhibition and tumor suppressive properties in cancer cells.59-61 Inhibition of PP2A with either of these compounds results in abnormal mitotic figures characteristic of mitotic catastrophe, a form of cell death distinct from apoptosis.62-64 To overcome the toxicities of okadaic acid and cantharidins, a competitive small molecule inhibitor of PP2A called LB100 (Lixte Biotechnology, East Setauket, NY) was recently developed and was studied in a variety of pre-clinical cancer models. These pre-clinical studies demonstrated that LB100 exhibits potent chemo- and radio-sensitizing properties, which has prompted interest in evaluation of LB100 in human clinical trials. This review summarizes the


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literature as of November 2014, describing the tumor suppressive properties of LB100 both in vitro and in vivo.


LB100: A Small Molecule Inhibitor of PP2A Glioblastoma Glioblastoma (GBM) is a highly aggressive tumor arising from astrocytes and is the most common primary malignant brain tumor. Despite intense, ongoing research efforts, this disease remains highly lethal with a median survival of 12–15 months from time of diagnosis.65 It is thought that a major reason for treatment resistance stems from to a small subset population of highly undifferentiated cells capable of self-renewal, termed cancer stem cells. Given their quiescent slow-growth, cancer stem cells are largely spared by traditional chemotherapies that target rapidly dividing tumor cells. Recently, there has been great interest in the N-CoR complex as a major component in maintaining the “stem-ness” of cancer stem cells in GBM. Indeed, GBM has been shown to exhibit increased expression of this epigenetic regulator of differentation-inducing genes compared to normal brain tissue. Briefly, the N-CoR complex comprises of N-CoR, histone deacetylase complexes (HDACs), steroid hormone receptors and transcriptional factors, and functions to repress transcription of genes that promote astroglial differentiation.66-68 Stimulation of astroglial precursor cells with the differentiation-inducing cytokine ciliary neurotrophic factor (CNTF) precludes N-CoR complex activity via Akt/PI3K-mediated phosphorylation of N-CoR, which leads to degradation of N-CoR in the cytoplasm. Given that PP2A suppresses Akt1, a PP2A inhibitor like LB100 would in theory enhance Akt1 phosphorylation of N-CoR, precluding N-CoR complex formation and thus promoting cell differentiation. Lu et al. studied the efficacy of LB100 on N-CoR-mediated maintenance of undifferentiated Nestin-positive tumor cells in the U87 GBM cell line.69 Exposure to 2.5 mM LB100 for one hour significantly increased levels of phosphorylated Akt (pAkt)., Consistent with inhibition of N-CoR complex formation by pAkt, LB100 treatment resulted in nuclear exportation of N-CoR documented by immunofluorescence and diminished levels of nuclear N-CoR confirmed by Western blot. There was concurrent increased expression in glial fibrillary acidic protein (GFAP) in Nestin-positive U87 cancer stem cells, suggesting astroglial differentiation. The phenotype of LB100-treated cells largely resembled that of CNTF-treated (0.2 mg/ml) counterparts with similar patterns of increased pAkt1 activity, disruption of N-CoR complex, and cellular differentiation. Similar findings on immunofluorescence and Western blotting were found in U87 mouse xenografts excised from mice after treatment with 21 consecutive daily doses of LB100 (1.5 mg/kg). Histology of organs (heart, lung, kidney, brain, spleen, pancreas, liver) procured from sacrificed animals were examined and showed no toxic injury. As such, this provided in vitro and in vivo evidence that LB100 treatment in GBM may target the cancer stem cell population through inducing cytoplasmic

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degradation of the N-CoR complex and promoting cell differentiation. In a separate study of GBM conducted by the same group, the authors studied LB102, a lipid-soluble homolog of LB100, and demonstrated its efficiency as a chemo-sensitizer to the DNAalkylating agent temozolomide (TMZ).70 Although currently a standard first line therapy after surgery for patients with newly diagnosed GBM, TMZ universally fails to prevent tumor recurrence, particularly in MGMT-intact tumors.65,71 Using U87 GBM cell lines, Lu et al. demonstrated dose-dependent inhibition by LB102 of GBM cell growth (IC50 D 5 mM). Furthermore, unlike okadaic acid, which inhibits both PP2A and PP1, LB102 had potent and selective inhibitory activity against PP2A (IC50 D 0.4 mM), compared to PP1 (IC50 D 80 mM). Treatment of cultured cells with LB102 (2.5 mM) led to downstream phosphorylated-suppression of Plk1 and Akt1 as well as diminished p53 activation. Furthermore, drug treatment potently diminished levels of translationally-controlled tumor protein (TCTP), a ubiquitous protein that contributes to microtubule stabilization during mitosis. TCTP overexpression has previously been associated with numerous malignancies.72-74 Immunofluorescent histologic analysis of cells exposed to LB102 exhibited abnormal mitotic figures and distorted microtubules, characteristics of mitotic catastrophe. These in vitro experiments were replicated in U373 GBM cell line with a p53 gain-of-function mutation. Similar downstream effects on protein expression and mitotic catastrophe were observed.75 These findings suggest that chemo-sensitization to TMZ with LB102 is independent of p53 status in GBM cells. In a murine flank xenograft model using U87 cell, animals were treated with LB102 (1.5 mg/kg), administered daily for 3 d (one cycle) for a total of 3 cycles (one day between cycles). LB102 treated mice exhibited a modestly improved survival of 4–5 weeks compared to controls with survival of 3 weeks. Treatment with TMZ alone (80 mg/kg; administered every 4 d for 3 cycles) led to complete tumor regression by week 5, but rapid and aggressive recurrence occur shortly thereafter, requiring sacrifice of all treated animals by week 7–9. However, combined treatment of TMZ and LB102, with TMZ given between LB102 cycles, resulted in complete regression of tumor formation by 5 weeks in all 5 treated animals. However, 3 of these mice developed tumor recurrence at weeks 7, 11, and 13, requiring sacrifice of one at week 11 and 2 at week 15. Animals were also treated with doxorubicin, which is a DNA intercalating agent in comparison to TMZ, which is an alkylating agent. Combined LB102 and doxorubicin (2.0 mg/kg) treatment led to tumor regression in all animals whereas treatment with doxorubicin alone only slowed progression. Average tumor volume measured 2 weeks after start of treatment was approximately 10 mm3, compared to nearly 300 mm3, 100 mm3, and 100 mm3 in controls, LB102-treated, and doxorubicin-treated mice, respectively. These data suggests that the chemo-sensitizing effects of LB100 were independent of the mechanism of action of the chemotherapeutic agent. Mice did not exhibit any signs of systemic toxicity such as weight loss or change in body conditioning scoring.


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Notably, the authors performed PP2A activity assays in excised tissues from the xenografted tumors and normal brains. They found that at 2 hours after intraperitoneal injection of LB102, PP2A activity was diminished by 20% in the flank tumors but was unchanged in brain tissue compared to controls. However, at 4 hours after treatment, PP2A activity was markedly suppressed in both tumor and brain tissues, by approximately 40%. This inhibition was incompletely reversed at 8 and 16 hours after treatment before normalizing by 24 hours. Importantly, these experiments demonstrated that LB102 could penetrate into the CNS and alter intracranial PP2A activity after intraperitoneal administration. Unlike LB100, LB102 is lipidsoluble, a property that is required for diffusion of small molecules across the blood brain barrier.76 As such, in future studies of intracranial tumors, it may be more appropriate to utilize LB102 over LB100 in the targeting of PP2A.

Sarcoma Sarcomas are aggressive, malignant tumors of mesenchymal stem cell origin that arise predominantly from soft tissues. At the time of diagnosis, a large percentage of patients have distant metastases, most commonly to the lungs via hematogenous spread.77 Despite optimal treatment, approximately one quarter of patients go on to develop progressive metastatic disease.78,79 The failure of these systemic treatments has been attributed to the resistance of presumably quiescent mesenchymal cancer stem cells to standard chemotherapeutic agents.80-82 To study these tumors in-vivo, Zhang et al. established a cell line of fibrosarcoma derived from transformed murine mesenchymal stem cells.83 These cells stained positively for the vimentin, S-100, and a-SMA, consistent with a mesenchymal origin. Furthermore, the authors demonstrated that these cells could differentiate into adipocytes, osteocytes, and myocytes when exposed to proper inducing media. Similar to human sarcomas, this fibrosarcoma line demonstrated potent resistance to standard chemotherapy agents, including doxorubicin. On histopathological review, the xenografted fibrosarcoma cell line resembled human fibrosarcomas. Tumors also tended to metastasize to the lungs, mirroring the metastatic pattern in human. The chemo-sensitizing potential of LB100 was tested in this novel fibrosarcoma cell line in conjunction with doxorubicin treatment. In vitro treatment with LB100 demonstrated a dose-dependent inhibition of cell viability with IC50 of 4.36 mM. Treatment with LB100 (5 mM) or doxorubicin (2 mg/ml) alone resulted in modest growth inhibition. When the 2 drugs were combined, the inhibitory effect almost doubled. Immunocytochemistry staining for a-tubulin showed abnormal mitotic figures and disorganized microtubule formation, suggesting that cells underwent mitotic catastrophe. Cell cycle analyses showed an increased accumulation of cells in G2/M after combined exposure to LB100 and doxorubicin. Compared to doxorubin only-treated cells, the addition of LB100 significantly increased protein levels of pMDM2, presumably via Akt1 activation and reduction in phosphorylated p53, indicating suppression of p53 pathwaymediated DNA damage response.

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In vivo experiments were carried out in flank xenografted mice model. Mice were treated with LB100 (1.5 mg/kg; administered intraperitoneally every other day, beginning day 4 after tumor injection for 6 total treatments), doxorubicin (2.5 mg/kg; same schedule except treatments beginning day 5 after tumor injection), or combined therapy. All animals were sacrificed 16 d after tumor injection for evaluation of tumor mass and volume. LB100 treatment alone did not affect growth of the fibrosarcoma xenografts and tumor size was similar to controls. Treatment with doxorubicin modestly slowed tumor growth with about 33% tumor reduction compared to controls. Combined treatment, however, led to significant regression of tumors in all animals. Grossly, these tumors measured less than 5 mm in diameter compared to 1–1.5 cm-sized tumors in the control and LB100-treated groups. Strikingly, combination treatment also prevented development of pulmonary metastases in all treated animals compared to an approximately 30% rate of metastasis formation in control and single treatment groups. No animal exhibited treatment-related toxicities during clinical monitoring or upon histological examination of tissues. Taken together, these findings demonstrated potent chemo-sensitizing potential for LB100 in this fibrosarcoma model. Given that LB100 was effective against a mesenchymal stem cell-derived cell line, it is likely that the stem cell-targeting properties of LB100 demonstrated in GBM is also pertinent in fibrosarcoma. Furthermore, the 100% success rate of metastasis prevention in animals treated with LB100 and doxorubicin was striking. The mechanism of metastasis prevention was unclear. Whether it was due to overall reduction of tumor burden or was preferential toxicity to cells predisposing to metastasis.

Pheochromocytoma Pheochromocytoma is a neuroendocrine tumor arising from chromaffin cells within the adrenal medulla, leading to symptoms of excessive catecholamine synthesis and release. Pheochromocytoma are often associated with familial tumor syndrome including multiple endocrine neoplasia type 2, neurofibromatosis type 1, and von Hippel-Lindau syndrome.84-87 While these tumors are highly differentiated and slow growing, approximately 10% of patients develop metastatic disease and 5 year-survival ranges between 34–60%.88-90 Because of its low growth fraction, metastatic pheochromocytoma has remained a challenge to treat with conventional chemotherapeutic modalities. Martiniova et al. investigated the effects of LB100 treatment in mouse pheochromocytoma (MPC 4/30PRR) cells in vitro and in a metastatic model in vivo.91 Experiments were designed to test the efficacy of LB100 alone and combined with TMZ, a standard chemotherapy used for disseminated pheochromocytoma. In vitro, LB100 failed to achieve greater than 50% inhibition despite treatment with concentrations up to 20 mM. Likewise, MPC cells exhibited significant resistance to TMZ, requiring a concentration of 600 mM before 50% inhibition was reached. However, the addition of LB100 (5–7.5 mM) lowered the TMZ concentrations (100–300 mM) needed to achieve the same or greater inhibition as with 600 mM TMZ alone.


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The authors proceeded to test LB100 efficacy in an in vivo metastatic pheochromocytoma mouse model, in which hepatic tumor nodules were formed after intravenous tail vein injection of MPC cells. Hepatic masses were evident on magnetic resonance imaging (MRI) within 4 weeks of injection. LB100 (1.5 mg/kg/day) was infused continuously for 14 days, beginning 5 d after tumor injection. TMZ (80 mg/kg) was given orally every 3 days, starting 10 d after LB100 was first given for a total of 3 doses. Interestingly, treatment with LB100 alone resulted in increased rate of growth of hepatic lesions, compared to control. Tumor formation was delayed one week in TMZ-treated mice (5 weeks after tumor injection) versus LB100-treated (4 weeks) and control animals (4 weeks). Combination treatment delayed tumor formation to 9 weeks after tumor injection and, notably, 2 of 10 animals failed to develop hepatic tumors upon sacrifice at 12 weeks. TMZ in conjunction with LB100 also prolonged median survival to 15 weeks compared to 10 and 7 weeks for TMZ-treated and control animals, respectively. The authors also tested the efficacy of increasing the number of TMZ treatments from 3 to 14 doses. The increase in cumulative dose extended time to tumor formation by 3 weeks, from 5 weeks to 8 weeks. However, the addition of LB100 to this intensified TMZ regimen further delayed tumor formation from 9 weeks to 12 weeks. Three of 5 animals treated with combination therapy were grossly and microscopically free of hepatic tumors upon autopsy examination. An additional experiment was conducted utilizing a TMZ dosing schedule of 14 total treatments beginning concurrently with LB100 administration. Similar chemo-sensitizing effect of LB100 was observed. There was no reported drug toxicity observed in the animals in any of the dosing scheduled tested. The authors corroborated their findings by examining protein expression in treated tumors compared to control; there was an increased level of pAkt, pPlk1, and pMDM2 with combination treatment. Interestingly, mice that were treated with LB100 alone grew hepatic tumors at a faster rate than the control group, and cell cycle analyses confirmed there were higher numbers of tumor cells in S and M phases. This finding suggests that, at least in pheochromocytoma cells, LB100 alone may enhance tumor cell growth by inhibiting PP2A, possibly resulting in enhanced activation of the growth-stimulatory factor Akt1. In this model of metastatic pheochromocytoma, however, LB100 exhibited a strong chemo-sensitizing effect with TMZ. Breast cancer Breast cancer represents the leading cause of cancer-related death in women worldwide.92 Despite significant progress in screening and early detection, many patients experience local or regional recurrence, metastasis, and resistance to conventional chemotherapies.93,94 Numerous studies have elucidated some of the many mechanisms of drug resistance in breast cancer.95 TNF-related apoptosis-inducing ligand (TRAIL), a member of the tumor necrosis family, is an integral initiator of the extrinsic apoptosis pathway.96,97 TRAIL has gained particular interest recently due to the finding that TRAIL-mediated apoptosis has been shown to preferentially affect cancer cells over normal

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cells.98 Normally, TRAIL binds death receptors 4 and 5 and initiates the death-inducing signaling complex (DISC) that phosphorylates caspase-8 and in turn triggers the extrinsic apoptosis pathway. Many cancers, including breast cancer, exhibit disruption of TRAIL-mediated signaling, contributing to chemotherapeutic resistance and tumor metastasis.99-101 However, the mechanism of TRAIL-resistance in cancers remains unclear. PP2A has been shown to dephosphorylate and therefore inhibit Src, a non-receptor tyrosine kinase that has been shown to mediate TRAIL resistance in breast cancer cells via activation of Akt pathway survival signaling.102,103 Using the BT549 breast cancer cell line, Xu et al. demonstrated that TRAIL signaling activated Src, which in turn phosphorylated caspase-8, initiating apoptosis.104 Using immunoprecipitation, they found that TRAIL stimulation led to increased PP2A dephosphorylation of Src, abrogating Src-mediated activation of caspase-8. Subsequently, the authors studied the effects of PP2A inhibition with LB100 in TRAIL-sensitive and TRAIL-resistant cell lines, MDA231 and MCF7, respectively. They found that combined treatment with 30 ng/ml TRAIL and 3 mM LB100 significantly sensitized MCF7 cells to TRAIL-mediated apoptosis, on par with MDA231 cells. Thus, targeting PP2A with LB100 sensitized these cell lines to apoptosis via TRAIL signaling and may also be a mechanism of cell death in other cancers. As interest for therapeutic development of TRAIL agonists continues to rise, LB100 may warrant further study as an adjuvant therapy, particularly in TRAIL-resistant cancers.105-107 Nasopharyngeal carcinoma Nasopharyngeal carcinoma (NPC) affects approximately one in every 100,000 individuals per year worldwide. However, in certain regions including Southeast Asia, North Africa and Middle East the incidence is nearly 25 cases per 100,000 people per year.108 Standard treatment includes maximal surgical resection followed by adjuvant chemotherapy and radiation, which together confer a high rate of local disease control.109 However, considerable co-morbidities are associated with chemo-radiation treatment. In particular, severe complications include mucosal necrosis, temporal lobe necrosis, and cranial neuropathies, affecting approximately 20–30% of patients after radiotherapy.110-112 As such, radiosensitization of nasopharyngeal carcinoma may potentially allow for lower radiation dosing in order to minimalize treatment-related adverse events. Lv et al. studied the radiosensitizing potential of LB100 in 2 NPC cell lines, including the radioresistant CNE1 cell line.113 Radiation treatment in both in vitro and in vivo experiments showed an approximately 200% increases in PP2A activity, while LB100 treatment attenuated these effects to 80–90% of baseline. Furthermore, LB100 exhibited dose enhancement factors of 1.83 and 1.97 in both treated cell lines, strongly suggesting radiosensitizing properties of the drug. Mitotic catastrophe was observed in radiated cells treated with LB100, evidenced by accumulation of cells in the G2/M phase and increased levels of pPlk1, pAkt1, and Cdk1. Additionally, as previously demonstrated by Lu et al., expression of TCTP was diminished.69 LB100 exposure also increased pMDM2 and subsequent downstream deactivation of


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the p53 pathway response to radiation-induced DNA damage. The radio-sensitizing properties of LB100 was further studied in vivo in CNE1 and CNE2 subcutaneous xenograft mouse models, utilizing single high doses of radiation rather than fractionated dosing, consistent with common practices in human patients. Single intraperitoneal administration of LB100 (1.5 mg/kg) resulted in reduction of PP2A activity to 77% of control animals in both cell line xenografts. The addition of 20 Gy radiation, administered 3 hours after LB100 treatment, did not alter PP2A activity significantly, demonstrating persistent PP2A inhibition by LB100 in the face of radiation. Per protocol, mice were sacrificed when tumor volume reached 3000 mm3. Mean tumor volume doubling time was not significantly affected by treatment with only LB100. Radiation treatment modestly prolonged the mean doubling time from 6.0 and 6.8 d for control to 9.2 and 10.6 d (CNE1 and CNE2, respectively). The addition of radiation, however, extended tumor volume doubling time to 27.7 (CNE1) and 26.7 (CNE2) days, resulting in dose enhancement factors of 2.98 (CNE1) and 2.27 (CNE2). Interestingly, these data indicated that combination therapy was slightly more efficacious in the CNE1 xenograft model, which has been shown to be radioresistant. Taken together, this study demonstrated that LB100 could significantly enhance the efficacy of radiation in the treatment of NPC. As such, LB100 holds considerable promise as an adjuvant therapy for radiosensitization and to decrease treatment related adverse effects. Hepatocellular carcinoma Hepatocellular carcinoma (HCC) remains among the deadliest of cancers with a median survival following diagnosis between 6 and 20 months.114 Systemic treatments have failed to significantly improve outcome and have been attributed to genetic alterations that confer chemoresistance.115-117 HCC associated with hepatitis C infection has been found to express high levels of PP2A, and one study has demonstrated significant cytotoxicity of cantharidin treatment in HCC cells.115,118 Bai et al. demonstrated dose-dependent cytotoxicity in 4 different HCC cell lines after treatment with LB100 and established that doses up to 5 mM in vitro did not affect cell viability but did reduce PP2A levels to 70%.119 As such, the authors utilized 5 mM as the treatment concentration of LB100 in conjunction with standard-of-care HCC chemotherapeutic agents, doxorubicin (0.2 mg/) and cisplatin (2 mg/mL). Inclusion of LB100 increased levels of pAkt1 in treated cell lines and also attenuated the activation of p53 in p53-intact HCC cell lines. In agreement with other tumor models, treatment with LB100 led to abnormal mitotic figures on immunocytochemistry indicative of mitotic catastrophe. Interestingly, the inclusion of LB100 to doxorubicin treatment did not augment cytotoxicity in normal liver cells. In PP2A-overexpressing Huh-7 mouse xenografts, combination treatment of LB100 (2 mg/kg) and doxorubicin (1.5 mg/kg), administered on alternate days for a total of 16 days, resulted in an approximate tumor volume of 400 mm3 at completion of therapy, compared to an approximate tumor volume of 1250 mm3, 1000 mm3, and 800 mm3 in controls, LB100-only treated, and doxorubicin-only treated groups respectively.

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Similarly, the addition of LB100 (2.5 mg/kg; administered every other day) to cisplatin (2 mg/kg, administered every 4 days) demonstrated significant reductions in tumor volume, measured 16 d after start of treatment, approximately 1000 mm3 with combination treatment compared to 4500 mm3 in control. Notably, the authors initially found on immunohistochemistry that specimens excised from LB100- and doxorubicin-treated xenografts expressed significantly higher levels of CD31 and lower levels of Ki-67 proliferation index, suggesting increased microvascular proliferation with LB100 treatment. Indeed, using Doppler technology, the authors found significantly increased blood flow in LB100-treated tumor compared to untreated tumors. ELISA assay also showed increased vascular endothelial growth factor (VEGF) levels in LB100 treated tumors. Interestingly, while translation of VEGF was increased, transcription was not, suggesting that LB100 was promoting secretion of VEGF. Injection of fluorescent doxorubicin demonstrated significantly higher fluorescent intensities in combined doxorubicin and LB100 treated animals than in doxorubicin-only treated animals. Also, LB100-treated tumors had significantly higher Evans Blue dye uptake, indicative of increased microvascular permeability. Through an elegant flow control system with HUVEC cells, the authors further showed that LB100 treatment led to higher permeability of doxorubicin through the model vessel barrier without inducing histologic evidence of cytotoxicity in treated endothelial cells. Taken together, these findings suggested that an additional potential mechanism of chemosensitization by LB100 was through increased tumor angiogenesis and perfusion, leading to improved penetration of chemotherapeutic agents into the bulk of the tumor. While PP2A has previously been implicated in regulation of angiogenesis, Bai et al. was the first group to confirm these findings in vivo.120-123 These results were recently corroborated in a pancreatic cancer model, in which increased intratumoral doxorubicin levels was found in tumors with LB100 pre-treatment.124 Notably, this study also demonstrated that LB100 induced upregulation of hypoxia-inducible factor-1a (HIF-1a) expression, which correlated with elevated VEGF levels. This enhancement was abolished with HIF-1a siRNA, suggesting that PP2A inhibition by LB100 upregulated VEGF-mediated angiogenesis in a HIF-1a dependent manner. Taken together, these recent findings represent a novel mechanism to target cancer-related angiogenesis. Given the mixed results of using traditional angiogenesis inhibitors, LB100 holds promise in chemosensitizing poorly perfused tumors by increasing vascular permeability of chemotheraputic agents. Pancreatic cancer Pancreatic cancer has one of the most dismal prognoses of all solid tumors with 5-year survival ranging between 10– 30%.125-128 Despite the combination of chemotherapy and radiation, disease progression occurs in almost all patients and median survival after diagnosis ranges between 8 to 12 months.129,130 Pancreatic cancer exhibits significant radioresistance, evidenced by the low response rates after external beam radiotherapy.133,134


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Inhibition of PP2A with cantharidins has been previously shown to suppress growth of pancreatic cancer cells in vitro through sustained activation of the NF-kB pathway, hyperactivation of the c-Jun/JNK pathway, and inhibition of the Wnt/B-catenin pathway.11,135,136 Based on these findings, PP2A inhibition was studied in pancreatic cancer cells for its potential as a radiosensitizing agent. Via high-throughput siRNA library screen in human pancreatic cancer cells, Wei et al. identified depletion of the subunit PPP2R1A as a significant sensitizer to gemcitabine and radiation.137 This was validated in 2 separate pancreatic cell lines, Mia-PaCa-2 and Panc-1. Twenty four hours after 7.5 Gy irradiation of PPP2R1A-depleted cells, there was diminished levels of phosphorylated (activated) Cdk1 (pCdk1), increased percentage of cells in mitosis, and significantly elevated number of gamma-H2AX positive cells, indicative of double strand DNA breaks. In comparison, treatment with radiation alone activated the DNA damage response, evidenced by increased levels of pCdk1 levels and arrest of cells in the G2 phase. Subsequently, LB100 was studied as a drug candidate for sensitizing pancreatic cancer cells to radiation. Wei et al. demonstrated that LB100 treatment at concentrations of 1 or 3 mM did not radiosensitize normal, small intestinal epithelial cells (CCL¡241), which has clinical significance given that the proximity of normal duodenal tissue is the dose-limiting factor in the radiotherapy of pancreatic tumors.138 However, treatment of both pancreatic cancer cell lines exhibited radiation dose enhancement factors ranging from 1.2 to 1.4. The authors also found that LB100 treatment with and without radiation led to increased levels of pPlk1, phosphorylated cdc25c, and subsequent cell cycle-promoting activity of dephosphorylated Cdk1. They also discovered diminished levels of Wee1, which normally functions to target Cdk1 for ubiquitinmediated proteasomal degradation, suggesting a potential feedback loop between Plk1 and Cdk1 with subsequent proteasomal degradation of phosphorylated Wee1.139 There is ongoing interest in the role of homologous recombination repair (HRR) in promoting survival of cancer cells after radiation-induced DNA damage. As such, inhibitors of HRR signaling have been studied as another class of small molecular drugs that may radiosensitize tumors, including pancreatic cancer.140,141 Recently, PP2A was demonstrated to regulate HRR pathway activity via increased ATM signaling and act as a potential predictor of response to PARP inhibition.142 Wei et al. showed in vitro that LB100 treatment in pancreatic cells inhibited HRR in concurrently radiated cells and also has decreased Rad51 foci and increased gamma-H2AX foci, indicative of increased double strand DNA damage with decreased HRR repair. As such, inhibition of PP2A with LB100 may not only drive cancer cells through the mitotic cycle, but also decease HRR repair of radiation-induced DNA double stranded breaks. The radiosensitizing potential of LB100 was also confirmed in mouse xenografts of pancreatic tumors derived from Mia-PaCa-2 cells.137 Mice in the combination therapy group were given LB100 (1.5 mg/kg) 2 hours before radiation (1.2 Gy/fraction) as this time point had been shown in vitro to exhibit the greatest inhibition of PP2A. Mice were subjected to a radiation fractionation schedule for 5 consecutive days per week for 2 weeks.

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Similar to previous studies in other cancers, LB100 enhanced the efficacy of radiation, average tumor doubling times were 8.5, 12.5, 24 and 33.5 d in control, LB100 only, radiation only and combined treatment groups respectively. No side effects were reported in the animals. Consistent with in vitro studies, analysis of tumor tissue extracted from xenografts after combined treatment demonstrated increased levels of phosphorylated cdc25c levels, decreased pCdk1, and increased gamma-H2AX, consistent with cell cycle progression and accumulation of DNA damage. Bai and colleagues also studied the ability of LB100 to chemosensitize pancreatic cell lines, Panc-1 and BxPc-3, to doxorubicin.124 Although treatment with LB100 in vitro exhibited potent IC50 values of 3.98 and 0.85 for these cell lines, respectively, combination treatment with doxorubicin was only enhanced in the BxPc-3 cell line (IC50 D 1.09 vs. 2.33 for doxorubicintreated alone). Addition of LB100 in Panc-1 cells was antagonistic (IC50 D 1.91 vs. 1.73 for doxorubicin-treated alone). Interestingly, in Panc-1 cell line xenografted mice, treatment with 1.5 and 2 mg/kg of doxorubin and LB100, respectively decreased tumor sizes in combination-treated animals by more than half compared to controls. As such, these discrepancies suggested that chemosensitization to doxorubicin by LB100 in Panc-1-derived tumors relied on factors only found in vivo. As such, Bai et al. further explored vascular permeabilization as the mechanism of chemosensitization in Panc-1 xenografted animals. In agreement with the study of LB100 in HCC,119 treatment with LB100 alone in Panc-1 xenografts yielded approximately 2-fold increases in VEGF secretion compared to controls. These findings were confirmed in vitro and elevated VEGF level was accompanied by increased HIF-1a expression. Immunohistochemical staining of CD31 and Doppler-detected blood flow were both elevated, suggesting microvascular proliferation. Notably, fluorescent-tagged doxorubicin was significantly more intense in tumors excised from combination therapytreated animals compared to doxorubicin alone-treated animals. Likewise, in an in vitro experiment of a flow control system with HUVEC cells, the addition of LB100 led to an approximately 2.5-fold increased accumulation of doxorubicin within tumor cells, supporting the notion that LB100 permeabilized tumor vasculature to doxorubicin penetration. Taken together, these findings in conjunction with those from the LB100-treated HCC model, suggested the tumor permeabilizing effects of LB100 may be non-organ specific and as such warrants further exploration in future pre-clinical studies. Ovarian cancer Ovarian cancer is the second most common gynecologic malignancy and a leading cause of cancer-related death in women.143 One of the reasons for a poor prognosis is that most patients are diagnosed with advanced disease. First-line therapy constitutes optimal surgical debulking followed by administration of a platinum-based regimen.144 Unfortunately, approximately 60–80% of patients relapse after initial treatment, and develop cisplatin resistance.145,146 As such, Chang et al. studied the ability of LB100 to sensitize ovarian cancer to cisplatin treatment using known cisplatin-resistant, p53-null (SKOV-3,


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Table 1. Summary of pre-clinical studies utilizing LB100. Studies reflect a review of the literature as of November 2014

Authors

Tumor Type

IC50 in vitro or Dose Enhancement Factor (DEF) in vitro

Lu et al., 200970; 201069

Glioblastoma

IC50 D 5 mM

Temozolomide

Sarcoma

IC50 D 4.36 mM

Doxorubicin

Pheochromocytoma

IC50 D >20 mM

Temozolomide

Xu et al., 2013103

Breast Cancer

not reported

Recombinant TRAIL

Lv et al., 2014112

Nasopharyngeal Carcinoma

DEF D 1.83–1.97

Radiation

Bai et al., 2014118

Hepatocellluar Carcinoma

IC50 D >10 mM

Doxorubicin

Wei et al., 2013136 Bai et al., 2014123

Pancreatic Cancer

DEF D 1.2–1.4; IC50 D 3.98 and 0.85

Radiation; Doxorubicin

Chang et al., 2014146

Ovarian Cancer

IC50 D 5 – 10.1 mM

Cisplatin

Zhang et al., 201082


Martiniova et al., 201190

OVCAR-8) cell lines as well as patient-derived cell lines harvested both prior to and after clinical development of cisplatin resistance.147 IC50 values after LB100 treatment ranged between 5 mM to 10.1 mM. LB100 administration combined with cisplatin resulted in increased levels of cleaved caspase-3 and cleaved PARP in both cisplatin-resistant, p53-null cell lines, indicative of apoptotic cell death rather than mitotic catastrophe. In contrast, of the studies discussed previously in this review, the cell lines studied were either p53 wild type or p53 mutated. Apoptosis, rather than mitotic catastrophe, may be the mechanism of cell death in p53-null cells after the addition of LB100 treatment to chemotherapy. Chang and colleagues also demonstrated that the addition of LB100 to cisplatin treatment impaired cell cycle arrest. Chk1 is a well-established player in the normal cellular response to DNA damage by activating cell cycle arrest in S or G2/M phases.148-150 The addition of LB100 led to persistence of the inactivated, phosphorylated form of Chk1, indicative of impaired activation

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Adjuvant Therapy

Significant findings in addition to chemo/ radio-sensiziation by LB100 LB100 disrupted formation of N-CoR complex, inducing cellular differentiation. Efficacy of LB100 was independent of p53 mutation status. LB100 effectively chemosensitized a stem cell-derived cell line.Adjuvant LB100 treatment prevented development of metastases in in vivo experiments. Addition of LB100 to TMZ in vivo completely inhibited tumor formation in majority of animals. Inhibition of PP2A with LB100 overcame TRAIL-mediated resistance to apoptosis in breast cancer cells. LB100 radio-sensitized known radioresistant NPC cells, in vitro and in vivo. LB100 was effective using single high dose radiation without significant treatmentrelated toxicity. Chemosensitization by LB100 was equally effective in p53-null versus p53 intact cells.LB100 increased VEGF secretion, promoting tumor angiogenesis and drug penetration into tumor parenchyma, in vitro. Therapeutic LB100 and radiation doses did not harm peri-pancreatic normal duodenal cells in vitro. Radio-sensitization by LB100 was associated with inhibition of HRR pathway response to DNA damage, in vitro.LB100 promoted penetration of doxorubicin into tumor parenchyma, in vivo. LB100 sensitized known cisplatin-resistant ovarian cancer cells to cisplatin treatment, in vitro and in vivo. LB100 was not subjected to drug efflux by ABC transporters, in vitro.

of Chk1 in the setting of cisplatin-induced DNA damage. Similarly, there was continued suppression of key DNA damage response mediators, BRCA1, JNK, and Chk2 in response to LB100 and cisplatin treatment. To further demonstrate aberrant cell cycle progression, the authors studied expression of dephosphorylated Wee1 and activated cdc25c, an antagonist and agonist of complex formation between cdk1 and cyclin B, respectively, promoting G2/M cell cycle progression. After combined LB100 and cisplatin administration, there was persistent phosphorylated Wee1 and cdc25c expression, indicative of a bias toward cdc25cmediated progression of the cell cycle, which was confirmed with phosphorylated-histone H3 assays. These findings were confirmed in vivo in mouse xenografts injected intraperitoneally with SKOV-3 cells to mimic the predilection of ovarian cancer to metastasize to the peritoneum. Similar to previous studies, there were no reported treatment-related systemic toxicities and the combination of LB100 plus cisplatin delayed tumor progression almost 5-fold compared to cisplatin


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Figure 1. Downstream molecular events after addition of LB100 to chemotherapy and radiation. The schematic diagram summarizes the major downstream molecular events that occur after LB100 is administered concurrently with chemotherapy and/or radiation. Increased and decreased expression of the target proteins is indicated by a green and red arrow, respectively. The end consequences of LB100 treatment include increased tumor permeability to drug exposure, induction of cell differentiation, and tumor cell death resulting from mitotic catastrophe or apoptosis.

treatment alone. Ex vivo analysis of excised specimens demonstrated hyperphosphorylation of BRCA1 and Chk1, consistent with in vitro findings of aberrant cell cycle progression and enhanced cellular apoptosis. Interestingly, Chang et al. also demonstrated, in vitro, the ability of LB100 to penetrate the tumor parenchyma without being subjected to drug transporter efflux. ATP-binding cassette (ABC) efflux transporters are well characterized as key players in suboptimal drug penetration into the tumor mass.151 LB100 treatment in cells from the HEK293 human embryonic kidney cell line overexpressing ABC efflux transporters, Pgp, MRP1, and ABCG2, did not result in significantly different IC50 values from treatment in ovarian cancer cells. Comparatively, treatment with paclitaxel, etoposide, and mitoxantrone all demonstrated significantly elevated IC50 values, suggesting active drug efflux. As such, Chang et al. established that LB100 was not subjected to vascular exportation by ABC transporters and therefore, may effectively penetrate the bulk of the tumor mass.

Conclusions In summary, mounting data indicates that LB100 holds promise as a novel anti-cancer agent in human malignancies. The aforementioned studies demonstrated that LB100 acts as both a chemo- and radio-sensitizer, without significant systemic toxicity. These studies and their major findings are highlighted in Table 1. The reviewed data have shown that LB100 overcomes

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cancer cell senescence, induces cellular differentiation in progenitor cells, increases drug penetration, and promotes mitotic catastrophe and apoptosis (Fig. 1). As such, a phase 1 study (NCTO1837667) was recently initiated to analyze the efficacy of LB100 administration in conjunction with docetaxel in patients with advanced solid tumors with a tentative completion date of early 2016. Notably, patients with CNS tumors, whether primary or metastatic, have been excluded from the phase 1 trial. Aside from Lu et al. who importantly showed reduced PP2A activity in normal brains of mice intraperitoneally injected with LB102,70 further work investigating blood brain barrier penetration by LB100 or LB102 is lacking. None of the aforementioned studies utilized models of intracranial xenografted tumors, nor studied levels of LB100 in CSF after systemic treatment. However, the molecular weight of 268 Da of LB100 is within the often-cited figure of 400 Da as the cut-off for free diffusion of lipid-soluble small molecules into the CNS.76 Furthermore, as demonstrated by Chang et al.,152 LB100 is not subjected to ABC efflux transporters, which are often responsible for multidrug resistance in the delivery of therapeutic compounds to the brain.153 As such, it is clear that additional study of LB100 against intracranial tumors is needed. Future plans by the authors (CSH and JBE) involve establishment of a phase 0/1 clinical trial to study the efficacy of LB100 in sensitizing newly diagnosed patients with GBM to TMZ treatment and patients with metastatic CNS tumors to radiation. These and other clinical studies will hopefully validate the observations in pre-clinical studies and help determine the potential role for LB100 in human cancer therapy.


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Disclosure of Potential Conflicts of Interest

Funding

This work was supported by the Intramural Research Program of the National Institute of Neurological Disorders and Stroke at the National Institutes of Health (NIH).

No potential conflicts of interest were disclosed. Acknowledgments

The authors thank John S Kovach, MD, for his expertise and helpful review of this manuscript.


References 1. Bononi A, Agnoletto C, De Marchi E, Marchi S, Patergnani S, Bonora M, Giorgi C, Missiroli S, Poletti F, Rimessi A, et al. Protein kinases and phosphatases in the control of cell fate. Enzyme Res 2011; 2011:329098; PMID:21904669; http://dx.doi.org/ 10.4061/2011/329098 2. Eichhorn PJ, Creyghton MP, Bernards R. Protein phosphatase 2A regulatory subunits and cancer. Biochimica Biophys Acta 2009; 1795:1-15; PMID:18588945 3. Niehrs C. The complex world of WNT receptor signalling. Nat Rev Mol Cell Biol 2012; 13:767-79; PMID:23151663; http://dx.doi.org/10.1038/ nrm3470 4. Behrens J, von Kries JP, Kuhl M, Bruhn L, Wedlich D, Grosschedl R, Birchmeier W. Functional interaction of beta-catenin with the transcription factor LEF1. Nature 1996; 382:638-42; PMID:8757136; http:// dx.doi.org/10.1038/382638a0 5. Peters U, Jiao S, Schumacher FR, Hutter CM, Aragaki AK, Baron JA, Berndt SI, Bezieau S, Brenner H, Butterbach K, et al. Identification of Genetic Susceptibility Loci for Colorectal Tumors in a GenomeWide Meta-analysis. Gastroenterology 2013; 144:799-807 e24; PMID:23266556; http://dx.doi. org/10.1053/j.gastro.2012.12.020 6. Kandoth C, McLellan MD, Vandin F, Ye K, Niu B, Lu C, Xie M, Zhang Q, McMichael JF, Wyczalkowski MA, et al. Mutational landscape and significance across 12 major cancer types. Nature 2013; 502:3339; PMID:24132290; http://dx.doi.org/10.1038/ nature12634 7. Yamamoto H, Hinoi T, Michiue T, Fukui A, Usui H, Janssens V, Van Hoof C, Goris J, Asashima M, Kikuchi A. Inhibition of the Wnt signaling pathway by the PR61 subunit of protein phosphatase 2A. J Biol Chem 2001; 276:26875-82; PMID:11297546; http://dx.doi.org/10.1074/jbc.M100443200 8. Seeling JM, Miller JR, Gil R, Moon RT, White R, Virshup DM. Regulation of beta-catenin signaling by the B56 subunit of protein phosphatase 2A. Science 1999; 283:2089-91; PMID:10092233; http://dx.doi. org/10.1126/science.283.5410.2089 9. Ratcliffe MJ, Itoh K, Sokol SY. A positive role for the PP2A catalytic subunit in Wnt signal transduction. J Biol Chem 2000; 275:35680-3; PMID:11007767; http://dx.doi.org/10.1074/jbc.C000639200 10. Bos CL, Kodach LL, van den Brink GR, Diks SH, van Santen MM, Richel DJ, Peppelenbosch MP, Hardwick JC. Effect of aspirin on the Wnt/beta-catenin pathway is mediated via protein phosphatase 2A. Oncogene 2006; 25:6447-56; PMID:16878161; http://dx.doi.org/10.1038/sj.onc.1209658 11. Wu MY, Xie X, Xu ZK, Xie L, Chen Z, Shou LM, Gong FR, Xie YF, Li W, Tao M. PP2A inhibitors suppress migration and growth of PANC-1 pancreatic cancer cells through inhibition on the Wnt/beta-catenin pathway by phosphorylation and degradation of beta-catenin. Oncol Rep 2014; 32:513-22; PMID:24926961 12. Carmen Figueroa-Aldariz M, Castaneda-Patlan MC, Santoyo-Ramos P, Zentella A, Robles-Flores M. Protein phosphatase 2A is essential to maintain active Wnt signaling and its Ab tumor suppressor subunit is not expressed in colon cancer cells. Mol Carcinog. 2014 Sep 22. Doi: 10.1002/mc.22217. PMID:25252130

830

13. Ruvolo PP, Clark W, Mumby M, Gao F, May WS. A functional role for the B56 alpha-subunit of protein phosphatase 2A in ceramide-mediated regulation of Bcl2 phosphorylation status and function. J Biol Chem 2002; 277:22847-52; PMID:11929874; http://dx.doi.org/10.1074/jbc.M201830200 14. Chiang CW, Kanies C, Kim KW, Fang WB, Parkhurst C, Xie M, Henry T, Yang E. Protein phosphatase 2A dephosphorylation of phosphoserine 112 plays the gatekeeper role for BAD-mediated apoptosis. Mol Cell Biol 2003; 23:6350-62; PMID:12944463; http://dx.doi.org/10.1128/MCB.23.18.6350-6362. 2003 15. Huang B, Yang CS, Wojton J, Huang NJ, Chen C, Soderblom EJ, Zhang L, Kornbluth S. Metabolic Control of Ca2C/Calmodulin-dependent Protein Kinase II (CaMKII)-mediated Caspase-2 Suppression by the B55beta/Protein Phosphatase 2A (PP2A). J Biol Chem 2014; 289:35882-90; PMID:25378403; http://dx.doi.org/10.1074/jbc.M114.585844 16. Shouse GP, Cai X, Liu X. Serine 15 phosphorylation of p53 directs its interaction with B56gamma and the tumor suppressor activity of B56gamma-specific protein phosphatase 2A. Mol Cell Biol 2008; 28:448-56; PMID:17967874; http://dx.doi.org/10.1128/ MCB.00983-07 17. Shouse GP, Nobumori Y, Panowicz MJ, Liu X. ATM-mediated phosphorylation activates the tumorsuppressive function of B56gamma-PP2A. Oncogene 2011; 30:3755-65; PMID:21460856; http://dx.doi. org/10.1038/onc.2011.95 18. Okamoto K, Kamibayashi C, Serrano M, Prives C, Mumby MC, Beach D. p53-dependent association between cyclin G and the B’ subunit of protein phosphatase 2A. Mol Cell Biol 1996; 16:6593-602; PMID:8887688 19. Li HH, Cai X, Shouse GP, Piluso LG, Liu X. A specific PP2A regulatory subunit, B56gamma, mediates DNA damage-induced dephosphorylation of p53 at Thr55. EMBO J 2007; 26:402-11; PMID:17245430; http://dx.doi.org/10.1038/sj.emboj.7601519 20. Jin Z, Wallace L, Harper SQ, Yang J. PP2A:B56{epsilon}, a substrate of caspase-3, regulates p53-dependent and p53-independent apoptosis during development. J Biol Chem 2010; 285:34493-502; PMID:20807766; http://dx.doi.org/10.1074/jbc. M110.169581 21. Voets E, Wolthuis RM. MASTL is the human orthologue of Greatwall kinase that facilitates mitotic entry, anaphase and cytokinesis. Cell Cycle 2010; 9:3591601; PMID:20818157; http://dx.doi.org/10.4161/ cc.9.17.12832 22. Yu J, Fleming SL, Williams B, Williams EV, Li Z, Somma P, Rieder CL, Goldberg ML. Greatwall kinase: a nuclear protein required for proper chromosome condensation and mitotic progression in Drosophila. J Cell Biol 2004; 164:487-92; PMID:14970188; http://dx.doi.org/10.1083/ jcb.200310059 23. Gharbi-Ayachi A, Labbe JC, Burgess A, Vigneron S, Strub JM, Brioudes E, Van-Dorsselaer A, Castro A, Lorca T. The substrate of Greatwall kinase, Arpp19, controls mitosis by inhibiting protein phosphatase 2A. Science 2010; 330:1673-7; PMID:21164014; http://dx.doi.org/10.1126/ science.1197048


24. Mochida S, Maslen SL, Skehel M, Hunt T. Greatwall phosphorylates an inhibitor of protein phosphatase 2A that is essential for mitosis. Science 2010; 330:1670-3; PMID:21164013; http://dx.doi.org/ 10.1126/science.1195689 25. Adhikari D, Diril MK, Busayavalasa K, Risal S, Nakagawa S, Lindkvist R, Shen Y, Coppola V, Tessarollo L, Kudo NR, et al. Mastl is required for timely activation of APC/C in meiosis I and Cdk1 reactivation in meiosis II. J Cell Biol 2014; 206:843-53; PMID:25246615; http://dx.doi.org/10.1083/ jcb.201406033 26. Vigneron S, Brioudes E, Burgess A, Labbe JC, Lorca T, Castro A. Greatwall maintains mitosis through regulation of PP2A. EMBO J 2009; 28:2786-93; PMID:19680222; http://dx.doi.org/10.1038/ emboj.2009.228 27. Mochida S, Ikeo S, Gannon J, Hunt T. Regulated activity of PP2A-B55 delta is crucial for controlling entry into and exit from mitosis in Xenopus egg extracts. EMBO J 2009; 28:2777-85; PMID:19696736; http://dx.doi.org/10.1038/ emboj.2009.238 28. Naetar N, Soundarapandian V, Litovchick L, Goguen KL, Sablina AA, Bowman-Colin C, Sicinski P, Hahn WC, DeCaprio JA, Livingston DM, et al. PP2Amediated regulation of Ras signaling in G2 is essential for stable quiescence and normal G1 length. Mol Cell 2014; 54:932-45; PMID:24857551; http://dx.doi. org/10.1016/j.molcel.2014.04.023 29. Wang P, Malumbres M, Archambault V. The Greatwall-PP2A axis in cell cycle control. Methods Mol Biol 2014; 1170:99-111; PMID:24906311; http://dx. doi.org/10.1007/978-1-4939-0888-2_6 30. Wang L, Guo Q, Fisher LA, Liu D, Peng A. Regulation of polo-like kinase 1 by DNA damage and PP2A/ B55a. Cell Cycle 2015; 14(1):157-66. doi:10.4161/ 15384101.2014.986392. 31. Nijenhuis W, Vallardi G, Teixeira A, Kops GJ, Saurin AT. Negative feedback at kinetochores underlies a responsive spindle checkpoint signal. Nat Cell Biol 2014; 16:1257-64; PMID:25402682; http://dx.doi. org/10.1038/ncb3065 32. Foley EA, Maldonado M, Kapoor TM. Formation of stable attachments between kinetochores and microtubules depends on the B56-PP2A phosphatase. Nat Cell Biol 2011; 13:1265-71; PMID:21874008; http://dx.doi.org/10.1038/ncb2327 33. Ruediger R, Pham HT, Walter G. Disruption of protein phosphatase 2A subunit interaction in human cancers with mutations in the A alpha subunit gene. Oncogene 2001; 20:10-5; PMID:11244497; http:// dx.doi.org/10.1038/sj.onc.1204059 34. Wang SS, Esplin ED, Li JL, Huang L, Gazdar A, Minna J, Evans GA. Alterations of the PPP2R1B gene in human lung and colon cancer. Science 1998; 282:284-7; PMID:9765152; http://dx.doi.org/ 10.1126/science.282.5387.284 35. Jones S, Wang TL, Shih Ie M, Mao TL, Nakayama K, Roden R, Glas R, Slamon D, Diaz LA Jr, Vogelstein B, et al. Frequent mutations of chromatin remodeling gene ARID1A in ovarian clear cell carcinoma. Science 2010; 330:228-31; PMID:20826764; http://dx.doi. org/10.1126/science.1196333 36. Colella S, Ohgaki H, Ruediger R, Yang F, Nakamura M, Fujisawa H, Kleihues P, Walter G. Reduced expression of the Aalpha subunit of protein

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37.

38.

39.


40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

50.

phosphatase 2A in human gliomas in the absence of mutations in the Aalpha and Abeta subunit genes. Intl J Cancer 2001; 93:798-804; PMID:11519040; http://dx.doi.org/10.1002/ijc.1423 Nagendra DC, Burke J, 3rd, Maxwell GL, Risinger JI. PPP2R1A mutations are common in the serous type of endometrial cancer. Mol Carcinog 2012; 51:826-31; PMID:21882256; http://dx.doi.org/10.1002/mc.20850 Quintas-Cardama A, Verstovsek S. Molecular pathways: Jak/STAT pathway: mutations, inhibitors, and resistance. Clin Cancer Res 2013; 19:1933-40; PMID:23406773; http://dx.doi.org/10.1158/10780432.CCR-12-0284 Ross JA, Cheng H, Nagy ZS, Frost JA, Kirken RA. Protein phosphatase 2A regulates interleukin-2 receptor complex formation and JAK3/STAT5 activation. J Biol Chem 2010; 285:3582-91; PMID:19923221; http://dx.doi.org/10.1074/jbc.M109.053843 Ugi S, Imamura T, Ricketts W, Olefsky JM. Protein phosphatase 2A forms a molecular complex with Shc and regulates Shc tyrosine phosphorylation and downstream mitogenic signaling. Mol Cell Biol 2002; 22:2375-87; PMID:11884620; http://dx.doi.org/ 10.1128/MCB.22.7.2375-2387.2002 Zhou B, Wang ZX, Zhao Y, Brautigan DL, Zhang ZY. The specificity of extracellular signal-regulated kinase 2 dephosphorylation by protein phosphatases. J Biol Chem 2002; 277:31818-25; PMID:12082107; http://dx.doi.org/10.1074/jbc.M203969200 Abraham D, Podar K, Pacher M, Kubicek M, Welzel N, Hemmings BA, Dilworth SM, Mischak H, Kolch W, Baccarini M. Raf-1-associated protein phosphatase 2A as a positive regulator of kinase activation. J Biol Chem 2000; 275:22300-4; PMID:10801873; http:// dx.doi.org/10.1074/jbc.M003259200 Ory S, Zhou M, Conrads TP, Veenstra TD, Morrison DK. Protein phosphatase 2A positively regulates Ras signaling by dephosphorylating KSR1 and Raf-1 on critical 14-3-3 binding sites. Curr Biol 2003; 13:1356-64; PMID:12932319; http://dx.doi.org/ 10.1016/S0960-9822(03)00535-9 De Luca A, Maiello MR, D’Alessio A, Pergameno M, Normanno N. The RAS/RAF/MEK/ERK and the PI3K/AKT signalling pathways: role in cancer pathogenesis and implications for therapeutic approaches. Expert Opin Ther Targets 2012; 16 Suppl 2:S17-27; PMID:22443084; http://dx.doi.org/10.1517/ 14728222.2011.639361 Wolf E, Lin CY, Eilers M, Levens DL. Taming of the beast: shaping Myc-dependent amplification. Trends Cell Biol 2014; PMID:25475704 Yeh E, Cunningham M, Arnold H, Chasse D, Monteith T, Ivaldi G, Hahn WC, Stukenberg PT, Shenolikar S, Uchida T, et al. A signalling pathway controlling c-Myc degradation that impacts oncogenic transformation of human cells. Nat Cell Biol 2004; 6:308-18; PMID:15048125; http://dx.doi.org/ 10.1038/ncb1110 Junttila MR, Puustinen P, Niemela M, Ahola R, Arnold H, Bottzauw T, Ala-aho R, Nielsen C, Ivaska J, Taya Y, et al. CIP2A inhibits PP2A in human malignancies. Cell 2007; 130:51-62; PMID:17632056; http://dx.doi.org/10.1016/j. cell.2007.04.044 Khanna A, Bockelman C, Hemmes A, Junttila MR, Wiksten JP, Lundin M, Junnila S, Murphy DJ, Evan GI, Haglund C, et al. MYC-dependent regulation and prognostic role of CIP2A in gastric cancer. J Natl Cancer Inst 2009; 101:793-805; PMID:19470954; http://dx.doi.org/10.1093/jnci/djp103 Come C, Laine A, Chanrion M, Edgren H, Mattila E, Liu X, Jonkers J, Ivaska J, Isola J, Darbon JM, et al. CIP2A is associated with human breast cancer aggressivity. Clin Cancer Res 2009; 15:5092-100; PMID:19671842; http://dx.doi.org/10.1158/10780432.CCR-08-3283 Lee M, Lee JS. Exploiting tumor cell senescence in anticancer therapy. BMB Rep 2014; 47:51-9;

www.tandfonline.com

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

63.

64.

PMID:24411464; http://dx.doi.org/10.5483/ BMBRep.2014.47.2.005 Perez-Mancera PA, Young AR, Narita M. Inside and out: the activities of senescence in cancer. Nat Rev Cancer 2014; 14:547-58; PMID:25030953; http:// dx.doi.org/10.1038/nrc3773 Campbell PM. Oncogenic Ras pushes (and pulls) cell cycle progression through ERK activation. Methods Mol Biol 2014; 1170:155-63; PMID:24906314; http://dx.doi.org/10.1007/978-1-4939-0888-2_9 Mannava S, Omilian AR, Wawrzyniak JA, Fink EE, Zhuang D, Miecznikowski JC, Marshall JR, Soengas MS, Sears RC, Morrison CD, et al. PP2A-B56alpha controls oncogene-induced senescence in normal and tumor human melanocytic cells. Oncogene 2012; 31:1484-92; PMID:21822300; http://dx.doi.org/ 10.1038/onc.2011.339 Sallman DA, Wei S, List A. PP2A: The Achilles Heal in MDS with 5q Deletion. Front Oncol 2014; 4:264; PMID:25295231; http://dx.doi.org/10.3389/ fonc.2014.00264 Pores Fernando AT, Andrabi S, Cizmecioglu O, Zhu C, Livingston DM, Higgins JM, Higgins JM, Schaffhausen BS, Roberts TM. Polyoma small T antigen triggers cell death via mitotic catastrophe. Oncogene 2014; PMID:24998850 Wang R, Lv L, Zhao Y, Yang N. Okadaic acid inhibits cell multiplication and induces apoptosis in a549 cells, a human lung adenocarcinoma cell line. Int J Clin Exp Med 2014; 7:2025-30; PMID:25232383 Liu CY, Hung MH, Wang DS, Chu PY, Su JC, Teng TH, Huang CT, Chao TT, Wang CY, Shiau CW, et al. Tamoxifen induces apoptosis through cancerous inhibitor of protein phosphatase 2A dependent phospho-Akt inactivation in estrogen-receptor negative human breast cancer cells. Breast Cancer Res 2014; 16:431; PMID:25228280; http://dx.doi.org/ 10.1186/s13058-014-0431-9 Ferron PJ, Hogeveen K, Fessard V, Le Hegarat L. Comparative analysis of the cytotoxic effects of okadaic acid-group toxins on human intestinal cell lines. Marine Drugs 2014; 12:4616-34; PMID:25196936; http://dx.doi.org/10.3390/md12084616 Li W, Xie L, Chen Z, Zhu Y, Sun Y, Miao Y, Xu Z, Han X. Cantharidin, a potent and selective PP2A inhibitor, induces an oxidative stress-independent growth inhibition of pancreatic cancer cells through G2/M cell-cycle arrest and apoptosis. Cancer Sci 2010; 101:1226-33; PMID:20331621; http://dx.doi. org/10.1111/j.1349-7006.2010.01523.x Peng F, Wei YQ, Tian L, Yang L, Zhao X, Lu Y, Mao YQ, Kan B, Lei S, Wang GS, et al. Induction of apoptosis by norcantharidin in human colorectal carcinoma cell lines: involvement of the CD95 receptor/ ligand. J Cancer Res Clin Oncol 2002; 128:223-30; PMID:11935314; http://dx.doi.org/10.1007/s00432002-0326-5 Kok SH, Chui CH, Lam WS, Chen J, Lau FY, Cheng GY, Wong RS, Lai PP, Leung TW, Tang JC, et al. Apoptotic activity of a novel synthetic cantharidin analogue on hepatoma cell lines. Int J Mol Med 2006; 17:945-9; PMID:16596285 Gliksman NR, Parsons SF, Salmon ED. Okadaic acid induces interphase to mitotic-like microtubule dynamic instability by inactivating rescue. J Cell Biol 1992; 119:1271-6; PMID:1447301; http://dx.doi. org/10.1083/jcb.119.5.1271 Chen B, Cheng M, Hong DJ, Sun FY, Zhu CQ. Okadaic acid induced cyclin B1 expression and mitotic catastrophe in rat cortex. Neurosci Lett 2006; 406:178-82; PMID:16919876; http://dx.doi.org/ 10.1016/j.neulet.2006.06.074 Bonness K, Aragon IV, Rutland B, Ofori-Acquah S, Dean NM, Honkanen RE. Cantharidin-induced mitotic arrest is associated with the formation of aberrant mitotic spindles and lagging chromosomes resulting, in part, from the suppression of PP2Aalpha. Mol


65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

Cancer Ther 2006; 5:2727-36; PMID:17121919; http://dx.doi.org/10.1158/1535-7163.MCT-06-0273 Stupp R, Mason WP, van den Bent MJ, Weller M, Fisher B, Taphoorn MJ, Belanger K, Brandes AA, Marosi C, Bogdahn U, et al. Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N Engl J Med 2005; 352:987-96; PMID:15758009; http://dx.doi.org/10.1056/ NEJMoa043330 Horlein AJ, Naar AM, Heinzel T, Torchia J, Gloss B, Kurokawa R, Ryan A, Kamei Y, S€oderstr€om M, Glass CK, et al. Ligand-independent repression by the thyroid hormone receptor mediated by a nuclear receptor corepressor. Nature 1995; 377:397-404; PMID:7566114; http://dx.doi.org/10.1038/377397a0 Li J, Wang J, Wang J, Nawaz Z, Liu JM, Qin J, Wong J. Both corepressor proteins SMRT and N-CoR exist in large protein complexes containing HDAC3. EMBO J 2000; 19:4342-50; PMID:10944117; http://dx.doi.org/10.1093/emboj/19.16.4342 Park DM, Li J, Okamoto H, Akeju O, Kim SH, Lubensky I, Vortmeyer A, Dambrosia J, Weil RJ, Oldfield EH, et al. N-CoR pathway targeting induces glioblastoma derived cancer stem cell differentiation. Cell Cycle 2007; 6:467-70; PMID:17312396; http:// dx.doi.org/10.4161/cc.6.4.3856 Lu J, Zhuang Z, Song DK, Mehta GU, Ikejiri B, Mushlin H, Park DM, Lonser RR. The effect of a PP2A inhibitor on the nuclear receptor corepressor pathway in glioma. J Neurosurg 2010; 113:225-33; PMID:20001590; http://dx.doi.org/10.3171/ 2009.11.JNS091272 Lu J, Kovach JS, Johnson F, Chiang J, Hodes R, Lonser R, Lonser R, Zhuang Z. Inhibition of serine/ threonine phosphatase PP2A enhances cancer chemotherapy by blocking DNA damage induced defense mechanisms. Proc Natl Acad Sci U S A 2009; 106:11697-702; PMID:19564615; http://dx.doi.org/ 10.1073/pnas.0905930106 Hegi ME, Diserens AC, Gorlia T, Hamou MF, de Tribolet N, Weller M, Kros JM, Hainfellner JA, Mason W, Mariani L, et al. MGMT gene silencing and benefit from temozolomide in glioblastoma. N Engl J Med 2005; 352:997-1003; PMID:15758010; http://dx.doi.org/10.1056/NEJMoa043331 Arcuri F, Papa S, Carducci A, Romagnoli R, Liberatori S, Riparbelli MG, Sanchez JC, Tosi P, del Vecchio MT. Translationally controlled tumor protein (TCTP) in the human prostate and prostate cancer cells: expression, distribution, and calcium binding activity. Prostate 2004; 60:130-40; PMID:15162379; http://dx.doi.org/10.1002/pros.20054 Tuynder M, Susini L, Prieur S, Besse S, Fiucci G, Amson R, Telerman A. Biological models and genes of tumor reversion: cellular reprogramming through tpt1/TCTP and SIAH-1. Proc Natl Acad Sci U S A 2002; 99:14976-81; PMID:12399545; http://dx.doi. org/10.1073/pnas.222470799 Kim JE, Koo KH, Kim YH, Sohn J, Park YG. Identification of potential lung cancer biomarkers using an in vitro carcinogenesis model. Exp Mol Med 2008; 40:709-20; PMID:19116456; http://dx.doi.org/ 10.3858/emm.2008.40.6.709 Short SC, Martindale C, Bourne S, Brand G, Woodcock M, Johnston P. DNA repair after irradiation in glioma cells and normal human astrocytes. Neuro Oncol 2007; 9:404-11; PMID:17704360; http://dx. doi.org/10.1215/15228517-2007-030 Fischer H, Gottschlich R, Seelig A. Blood-brain barrier permeation: molecular parameters governing passive diffusion. J Membr biol 1998; 165:201-11; PMID:9767674; http://dx.doi.org/10.1007/ s002329900434 Christie-Large M, James SL, Tiessen L, Davies AM, Grimer RJ. Imaging strategy for detecting lung metastases at presentation in patients with soft tissue sarcomas. Eur J Cancer 2008; 44:1841-5;

831

78.

79.

80.


81.

82.

83.

84.

85.

86.

87.

88.

89.

90.

91.

832

PMID:18640829; http://dx.doi.org/10.1016/j. ejca.2008.06.004 Weitz J, Antonescu CR, Brennan MF. Localized extremity soft tissue sarcoma: improved knowledge with unchanged survival over time. J Clin O 2003; 21:2719-25; PMID:12860950; http://dx.doi.org/ 10.1200/JCO.2003.02.026 Lindberg RD, Martin RG, Romsdahl MM, Barkley HT, Jr. Conservative surgery and postoperative radiotherapy in 300 adults with soft-tissue sarcomas. Cancer 1981; 47:2391-7; PMID:7272893; http://dx.doi. org/10.1002/1097-0142(19810515)47:10%3c2391:: AID-CNCR2820471012%3e3.0.CO;2-B Krikelis D, Judson I. Role of chemotherapy in the management of soft tissue sarcomas. Exp Rev Anticancer Ther 2010; 10:249-60; PMID:20132000; http:// dx.doi.org/10.1586/era.09.176 Mendenhall WM, Indelicato DJ, Scarborough MT, Zlotecki RA, Gibbs CP, Mendenhall NP, Mendenhall CM, Enneking WF. The management of adult soft tissue sarcomas. Am J Clin Oncol 2009; 32:436-42; PMID:19657238; http://dx.doi.org/10.1097/ COC.0b013e318173a54f Schmitt T, Kasper B. New medical treatment options and strategies to assess clinical outcome in soft-tissue sarcoma. Expert Rev Anticancer Ther 2009; 9:115967; PMID:19671035; http://dx.doi.org/10.1586/ era.09.64 Zhang C, Peng Y, Wang F, Tan X, Liu N, Fan S, Wang D, Zhang L, Liu D, Wang T, et al. A synthetic cantharidin analog for the enhancement of doxorubicin suppression of stem cell-derived aggressive sarcoma. Biomaterials 2010; 31:9535-43; PMID:20875681; http://dx.doi.org/10.1016/j. biomaterials.2010.08.059 Brouwers FM, Eisenhofer G, Tao JJ, Kant JA, Adams KT, Linehan WM, Pacak K. High frequency of SDHB germline mutations in patients with malignant catecholamine-producing paragangliomas: implications for genetic testing. J Clin Endocrinol Metab 2006; 91:4505-9; PMID:16912137; http://dx.doi. org/10.1210/jc.2006-0423 Baysal BE. Clinical and molecular progress in hereditary paraganglioma. J Med Genet 2008; 45:689-94; PMID:18978332; http://dx.doi.org/10.1136/ jmg.2008.058560 Amar L, Bertherat J, Baudin E, Ajzenberg C, Bressacde Paillerets B, Chabre O, Chamontin B, Delemer B, Giraud S, Murat A, et al. Genetic testing in pheochromocytoma or functional paraganglioma. J Clin Oncol 2005; 23:8812-8; PMID:16314641; http://dx.doi. org/10.1200/JCO.2005.03.1484 Neumann HP, Bausch B, McWhinney SR, Bender BU, Gimm O, Franke G, Schipper J, Klisch J, Altehoefer C, Zerres K, et al. Germ-line mutations in nonsyndromic pheochromocytoma. N Engl J Med 2002; 346:1459-66; PMID:12000816; http://dx.doi. org/10.1056/NEJMoa020152 Bravo EL. Pheochromocytoma: new concepts and future trends. Kidney Int 1991; 40:544-56; PMID:1787652; http://dx.doi.org/10.1038/ ki.1991.244 Plouin PF, Chatellier G, Fofol I, Corvol P. Tumor recurrence and hypertension persistence after successful pheochromocytoma operation. Hypertension 1997; 29:1133-9; PMID:9149678; http://dx.doi.org/ 10.1161/01.HYP.29.5.1133 Goldstein RE, O’Neill JA, Jr., Holcomb GW, 3rd, Morgan WM, 3rd, Neblett WW, 3rd, Oates JA, Brown N, Nadeau J, Smith B, Page DL, et al. Clinical experience over 48 years with pheochromocytoma. Annals Surgery 1999; 229:755-64; discussion 64–6; PMID:10363888; http://dx.doi.org/10.1097/ 00000658-199906000-00001 Martiniova L, Lu J, Chiang J, Bernardo M, Lonser R, Zhuang Z, Pacak K. Pharmacologic modulation of serine/threonine phosphorylation highly sensitizes PHEO in a MPC cell and mouse model to

92.

93.

94.

95.

96.

97.

98.

99.

100.

101.

102.

103.

104.

conventional chemotherapy. PloS One 2011; 6: e14678; PMID:21339823; http://dx.doi.org/ 10.1371/journal.pone.0014678 DeSantis CE, Lin CC, Mariotto AB, Siegel RL, Stein KD, Kramer JL, Alteri R, Robbins AS, Jemal A. Cancer treatment and survivorship statistics, 2014. CA Cancer J Clin 2014; 64:252-71; PMID:24890451 Beslija S, Bonneterre J, Burstein HJ, Cocquyt V, Gnant M, Heinemann V, Jassem J, K€ostler WJ, Krainer M, Menard S, et al. Third consensus on medical treatment of metastatic breast cancer. Ann Oncol 2009; 20:1771-85; PMID:19608616; http://dx.doi. org/10.1093/annonc/mdp261 Pagani O, Senkus E, Wood W, Colleoni M, Cufer T, Kyriakides S, Costa A, Winer EP, Cardoso F; ESOMBC Task Force. International guidelines for management of metastatic breast cancer: can metastatic breast cancer be cured? J Natl Cancer Inst 2010; 102:456-63; PMID:20220104; http://dx.doi.org/ 10.1093/jnci/djq029 Groenendijk FH, Bernards R. Drug resistance to targeted therapies: deja vu all over again. Mol Oncol 2014; 8:1067-83; PMID:24910388; http://dx.doi. org/10.1016/j.molonc.2014.05.004 Wiley SR, Schooley K, Smolak PJ, Din WS, Huang CP, Nicholl JK, Sutherland GR, Smith TD, Rauch C, Smith CA, et al. Identification and characterization of a new member of the TNF family that induces apoptosis. Immunity 1995; 3:673-82; PMID:8777713; http://dx.doi.org/10.1016/1074-7613(95)90057-8 Pitti RM, Marsters SA, Ruppert S, Donahue CJ, Moore A, Ashkenazi A. Induction of apoptosis by Apo-2 ligand, a new member of the tumor necrosis factor cytokine family. J Biol Chem 1996; 271:12687-90; PMID:8663110; http://dx.doi.org/ 10.1074/jbc.271.22.12687 Rowinsky EK. Targeted induction of apoptosis in cancer management: the emerging role of tumor necrosis factor-related apoptosis-inducing ligand receptor activating agents. J Clin Oncol 2005; 23:9394-407; PMID:16361639; http://dx.doi.org/10.1200/ JCO.2005.02.2889 Zang F, Wei X, Leng X, Yu M, Sun B. C-FLIP(L) contributes to TRAIL resistance in HER2-positive breast cancer. Biochem Biophys Res Commun 2014; 450:267-73; PMID:24909691; http://dx.doi.org/ 10.1016/j.bbrc.2014.05.106 Wang H, Xu C, Kong X, Li X, Kong X, Wang Y, Ding X, Yang Q. Trail resistance induces epithelialmesenchymal transition and enhances invasiveness by suppressing PTEN via miR-221 in breast cancer. PloS One 2014; 9:e99067; PMID:24905916; http://dx. doi.org/10.1371/journal.pone.0099067 Srivastava RK, Kurzrock R, Shankar S. MS-275 sensitizes TRAIL-resistant breast cancer cells, inhibits angiogenesis and metastasis, and reverses epithelialmesenchymal transition in vivo. Mol Cancer Ther 2010; 9:3254-66; PMID:21041383; http://dx.doi. org/10.1158/1535-7163.MCT-10-0582 Zhang XH, Wang Q, Gerald W, Hudis CA, Norton L, Smid M, Foekens JA, Massague J. Latent bone metastasis in breast cancer tied to Src-dependent survival signals. Cancer Cell 2009; 16:67-78; PMID:19573813; http://dx.doi.org/10.1016/j. ccr.2009.05.017 Eichhorn PJ, Creyghton MP, Wilhelmsen K, van Dam H, Bernards R. A RNA interference screen identifies the protein phosphatase 2A subunit PR55gamma as a stress-sensitive inhibitor of c-SRC. PLoS Genet 2007; 3:e218; PMID:18069897; http:// dx.doi.org/10.1371/journal.pgen.0030218 Xu J, Xu Z, Zhou JY, Zhuang Z, Wang E, Boerner J, Wu GS. Regulation of the Src-PP2A interaction in tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. J Biol Chem 2013; 288:33263-71; PMID:24100030; http://dx. doi.org/10.1074/jbc.M113.508093


105. von Pawel J, Harvey JH, Spigel DR, Dediu M, Reck M, Cebotaru CL, Humphreys RC, Gribbin MJ, Fox NL, Camidge DR. Phase II trial of mapatumumab, a fully human agonist monoclonal antibody to tumor necrosis factor-related apoptosis-inducing ligand receptor 1 (TRAIL-R1), in combination with paclitaxel and carboplatin in patients with advanced nonsmall-cell lung cancer. Clin Lung Cancer 2014; 15:188-96 e2; PMID:24560012; http://dx.doi.org/ 10.1016/j.cllc.2013.12.005 106. Fuchs CS, Fakih M, Schwartzberg L, Cohn AL, Yee L, Dreisbach L, Humphreys RC, Gribbin MJ, Fox NL, Camidge DR, et al. TRAIL receptor agonist conatumumab with modified FOLFOX6 plus bevacizumab for first-line treatment of metastatic colorectal cancer: A randomized phase 1b/2 trial. Cancer 2013; 119:4290-8; PMID:24122767; http://dx.doi.org/ 10.1002/cncr.28353 107. Herbst RS, Eckhardt SG, Kurzrock R, Ebbinghaus S, O’Dwyer PJ, Gordon MS, Novotny W, Goldwasser MA, Tohnya TM, Lum BL, et al. Phase I dose-escalation study of recombinant human Apo2L/TRAIL, a dual proapoptotic receptor agonist, in patients with advanced cancer. J Clin Oncol 2010; 28:2839-46; PMID:20458040; http://dx.doi.org/10.1200/ JCO.2009.25.1991 108. Chang ET, Adami HO. The enigmatic epidemiology of nasopharyngeal carcinoma. Cancer Epidemiol Biomarkers Prev 2006; 15:1765-77; PMID:17035381; http://dx.doi.org/10.1158/1055-9965.EPI-06-0353 109. Lin S, Pan J, Han L, Zhang X, Liao X, Lu JJ. Nasopharyngeal carcinoma treated with reduced-volume intensity-modulated radiation therapy: report on the 3-year outcome of a prospective series. Int J Rad Oncol Biol Phys 2009; 75:1071-8; PMID:19362784; http://dx.doi.org/10.1016/j.ijrobp.2008.12.015 110. Tian YM, Guan Y, Xiao WW, Zeng L, Liu S, Lu TX, Zhao C, Han F. Long-term survival and late complications in intensity-modulated radiotherapy of locally recurrent T1-T2 nasopharyngeal carcinoma. Head Neck 2014; PMID:25244494 111. Tian YM, Zhao C, Guo Y, Huang Y, Huang SM, Deng XW, Lin CG, Lu TX, Han F. Effect of total dose and fraction size on survival of patients with locally recurrent nasopharyngeal carcinoma treated with intensity-modulated radiotherapy: A phase 2, single-center, randomized controlled trial. Cancer 2014; 120(22):3502-9; PMID:25056602 112. Chen HY, Ma XM, Ye M, Hou YL, Xie HY, Bai YR. Effectiveness and toxicities of intensity-modulated radiotherapy for patients with locally recurrent nasopharyngeal carcinoma. PloS One 2013; 8:e73918; PMID:24040115; http://dx.doi.org/10.1371/journal. pone.0073918 113. Lv P, Wang Y, Ma J, Wang Z, Li JL, Hong CS, Zhuang Z, Zeng YX. Inhibition of protein phosphatase 2A with a small molecule LB100 radiosensitizes nasopharyngeal carcinoma xenografts by inducing mitotic catastrophe and blocking DNA damage repair. Oncotarget 2014; 5:7512-24; PMID:25245035 114. A new prognostic system for heptatocellular carcinoma: a retrospective study of 435 patients: the Cancer of the Liver Italian Program (CLIP) investigators. Hepatology. 1998 Sep; 28(3):7510-5; PMID:9731568 115. Soini Y, Virkajarvi N, Raunio H, Paakko P. Expression of P-glycoprotein in hepatocellular carcinoma: a potential marker of prognosis. J Clin Pathol 1996; 49:470-3; PMID:8763260; http://dx.doi.org/ 10.1136/jcp.49.6.470 116. Huang CC, Wu MC, Xu GW, Li DZ, Cheng H, Tu ZX, Jiang HQ, Gu JR. Overexpression of the MDR1 gene and P-glycoprotein in human hepatocellular carcinoma. J Natl Cancer Inst 1992; 84:262-4; PMID:1346405; http://dx.doi.org/10.1093/jnci/ 84.4.262 117. Caruso ML, Valentini AM. Overexpression of p53 in a large series of patients with hepatocellular

Volume 16 Issue 6

118.

119.


120.

121.

122.

123.

124.

125.

126.

127.

128.

129.

130.

carcinoma: a clinicopathological correlation. Anticancer Res 1999; 19:3853-6; PMID:10628323 Wang CC, Wu CH, Hsieh KJ, Yen KY, Yang LL. Cytotoxic effects of cantharidin on the growth of normal and carcinoma cells. Toxicology 2000; 147:7787; PMID:10874155; http://dx.doi.org/10.1016/ S0300-483X(00)00185-2 Bai XL, Zhang Q, Ye LY, Hu QD, Fu QH, Zhi X, Su W, Su RG, Ma T, Chen W, et al. Inhibition of protein phosphatase 2A enhances cytotoxicity and accessibility of chemotherapeutic drugs to hepatocellular carcinomas. Mol Cancer Ther 2014; 13:2062-72; PMID:24867249; http://dx.doi.org/10.1158/15357163.MCT-13-0800 Martin M, Potente M, Janssens V, Vertommen D, Twizere JC, Rider MH, Goris J, Dimmeler S, Kettmann R, Dequiedt F. Protein phosphatase 2A controls the activity of histone deacetylase 7 during T cell apoptosis and angiogenesis. Proc Natl Acad Sci U S A 2008; 105:4727-32; PMID:18339811; http://dx.doi. org/10.1073/pnas.0708455105 Le Guelte A, Galan-Moya EM, Dwyer J, Treps L, Kettler G, Hebda JK, Dubois S, Auffray C, Chneiweiss H, Bidere N, et al. Semaphorin 3A elevates endothelial cell permeability through PP2A inactivation. J Cell Sci 2012; 125:4137-46; PMID:22685328; http://dx.doi.org/10.1242/ jcs.108282 Chao CY, Lii CK, Ye SY, Li CC, Lu CY, Lin AH, Liu KL, Chen HW. Docosahexaenoic Acid Inhibits Vascular Endothelial Growth Factor (VEGF)-Induced Cell Migration via the GPR120/PP2A/ERK1/2/ eNOS Signaling Pathway in Human Umbilical Vein Endothelial Cells. J Agricul Food Chem 2014; 62 (18):4152-8; PMID:24734983 Mehra VC, Jackson E, Zhang XM, Jiang XC, Dobrucki LW, Yu J, Bernatchez P, Sinusas AJ, Shulman GI, Sessa WC, et al. Ceramide-activated phosphatase mediates fatty acid-induced endothelial VEGF resistance and impaired angiogenesis. Am J Pathol 2014; 184:1562-76; PMID:24606881; http:// dx.doi.org/10.1016/j.ajpath.2014.01.009 Bai X, Zhi X, Zhang Q, Liang F, Chen W, Liang C, Hu Q, Sun X, Zhuang Z, Liang T. Inhibition of protein phosphatase 2A sensitizes pancreatic cancer to chemotherapy by increasing drug perfusion via HIF1alpha-VEGF mediated angiogenesis. Cancer Lett 2014; 355(2):281-7; PMID:25304380 Siegel R, Ma J, Zou Z, Jemal A. Cancer statistics, 2014. CA Cancer J Clin 2014; 64:9-29; PMID:24399786 Cameron JL, Riall TS, Coleman J, Belcher KA. One thousand consecutive pancreaticoduodenectomies. Ann Surg 2006; 244:10-5; PMID:16794383; http:// dx.doi.org/10.1097/01.sla.0000217673.04165.ea Geer RJ, Brennan MF. Prognostic indicators for survival after resection of pancreatic adenocarcinoma. Am J Surg 1993; 165:68-72; discussion -3; PMID:8380315; http://dx.doi.org/10.1016/S00029610(05)80406-4 Bakkevold KE, Arnesjo B, Dahl O, Kambestad B. Adjuvant combination chemotherapy (AMF) following radical resection of carcinoma of the pancreas and papilla of Vater–results of a controlled, prospective, randomised multicentre study. Eur J Cancer 1993; 29A:698-703; PMID:8471327; http://dx.doi.org/ 10.1016/S0959-8049(05)80349-1 Bond A. Where nowhere can lead you. Hastings Center Rep 2006; 36:22-4; PMID:17278869; http://dx. doi.org/10.1353/hcr.2006.0089 Gourgou-Bourgade S, Bascoul-Mollevi C, Desseigne F, Ychou M, Bouche O, Guimbaud R, Becouarn Y, Adenis A, Raoul JL, Boige V, et al. Impact of FOLFIRINOX compared with gemcitabine on quality of

www.tandfonline.com

131.

132.

133.

134.

135.

136.

137.

138.

139.

140.

life in patients with metastatic pancreatic cancer: results from the PRODIGE 4/ACCORD 11 randomized trial. J Clin Oncol 2013; 31:23-9; PMID:23213101; http://dx.doi.org/10.1200/ JCO.2012.44.4869 A multi-institutional comparative trial of radiation therapy alone and in combination with 5-fluorouracil for locally unresectable pancreatic carcinoma. The Gastrointestinal Tumor Study Group. Ann Surg. 1979; 189(2):205-8; PMID:426553 Moertel CG, Frytak S, Hahn RG, O’Connell MJ, Reitemeier RJ, Rubin J, Schutt AJ, Weiland LH, Childs DS, Holbrook MA, et al. Therapy of locally unresectable pancreatic carcinoma: a randomized comparison of high dose (6000 rads) radiation alone, moderate dose radiation (4000 rads C 5-fluorouracil), and high dose radiation C 5-fluorouracil: The Gastrointestinal Tumor Study Group. Cancer 1981; 48:1705-10; PMID:7284971; http://dx.doi.org/ 10.1002/1097-0142(19811015)48:8%3c1705::AIDCNCR2820480803%3e3.0.CO;2-4 Roldan GE, Gunderson LL, Nagorney DM, Martin JK, Ilstrup DM, Holbrook MA, Kvols LK, McIlrath DC. External beam versus intraoperative and external beam irradiation for locally advanced pancreatic cancer. Cancer 1988; 61:1110-6; PMID:3342371; http://dx.doi. org/10.1002/1097-0142(19880315)61:6%3c1110:: AID-CNCR2820610610%3e3.0.CO;2-6 Castro JR, Saunders WM, Quivey JM, Chen GT, Collier JM, Woodruff KH, Lyman JT, Twomey P, Frey C, Phillips TL. Clinical problems in radiotherapy of carcinoma of the pancreas. AmJ Clin Oncol 1982; 5:579-87; PMID:6762086; http://dx.doi.org/ 10.1097/00000421-198212000-00004 Li W, Chen Z, Zong Y, Gong F, Zhu Y, Zhu Y, Lv J, Zhang J, Xie L, Sun Y, et al. PP2A inhibitors induce apoptosis in pancreatic cancer cell line PANC-1 through persistent phosphorylation of IKKalpha and sustained activation of the NF-kappaB pathway. Cancer Lett 2011; 304:117-27; PMID:21376459; http:// dx.doi.org/10.1016/j.canlet.2011.02.009 Li W, Chen Z, Gong FR, Zong Y, Chen K, Li DM, Yin H, Duan WM, Miao Y, Tao M, et al. Growth of the pancreatic cancer cell line PANC-1 is inhibited by protein phosphatase 2A inhibitors through overactivation of the c-Jun N-terminal kinase pathway. Eur J Cancer 2011; 47:2654-64; PMID:21958460; http:// dx.doi.org/10.1016/j.ejca.2011.08.014 Wei D, Parsels LA, Karnak D, Davis MA, Parsels JD, Marsh AC, Zhao L, Maybaum J, Lawrence TS, Sun Y, et al. Inhibition of protein phosphatase 2A radiosensitizes pancreatic cancers by modulating CDC25C/CDK1 and homologous recombination repair. Clin Cancer Res 2013; 19:4422-32; PMID:23780887; http://dx.doi.org/10.1158/10780432.CCR-13-0788 Murphy JD, Christman-Skieller C, Kim J, Dieterich S, Chang DT, Koong AC. A dosimetric model of duodenal toxicity after stereotactic body radiotherapy for pancreatic cancer. Int J Radiat Oncol Biol Phys 2010; 78:1420-6; PMID:20399033; http://dx.doi. org/10.1016/j.ijrobp.2009.09.075 Watanabe N, Arai H, Nishihara Y, Taniguchi M, Watanabe N, Hunter T, Osada H. M-phase kinases induce phospho-dependent ubiquitination of somatic Wee1 by SCFbeta-TrCP. Proc Natl Acad Sci U S A 2004; 101:4419-24; PMID:15070733; http://dx.doi. org/10.1073/pnas.0307700101 Morgan MA, Parsels LA, Zhao L, Parsels JD, Davis MA, Hassan MC, Arumugarajah S, Hylander-Gans L, Morosini D, Simeone DM, et al. Mechanism of radiosensitization by the Chk1/2 inhibitor AZD7762 involves abrogation of the G2 checkpoint and inhibition of homologous recombinational DNA repair.


141.

142.

143.

144.

145.

146.

147.

148.

149.

150.

151.

152.

153.

Cancer Res 2010; 70:4972-81; PMID:20501833; http://dx.doi.org/10.1158/0008-5472.CAN-09-3573 Engelke CG, Parsels LA, Qian Y, Zhang Q, Karnak D, Robertson JR, Tanska DM, Wei D, Davis MA, Parsels JD, et al. Sensitization of pancreatic cancer to chemoradiation by the Chk1 inhibitor MK8776. Clin Cancer Res 2013; 19:4412-21; PMID:23804422; http://dx.doi.org/10.1158/1078-0432.CCR-12-3748 Kalev P, Simicek M, Vazquez I, Munck S, Chen L, Soin T, Danda N, Chen W, Sablina A. Loss of PPP2R2A inhibits homologous recombination DNA repair and predicts tumor sensitivity to PARP inhibition. Cancer Res 2012; 72:6414-24; PMID:23087057; http://dx.doi.org/10.1158/00085472.CAN-12-1667 Siegel R, Ward E, Brawley O, Jemal A. Cancer statistics, 2011: the impact of eliminating socioeconomic and racial disparities on premature cancer deaths. CA Cancer J Clin 2011; 61:212-36; PMID:21685461 Aabo K, Adams M, Adnitt P, Alberts DS, Athanazziou A, Barley V, Bell DR, Bianchi U, Bolis G, Brady MF, et al. Chemotherapy in advanced ovarian cancer: four systematic meta-analyses of individual patient data from 37 randomized trials. Advanced Ovarian Cancer Trialists’ Group. Br J Cancer 1998; 78:1479-87; PMID:9836481; http://dx.doi.org/10.1038/ bjc.1998.710 Davis A, Tinker AV, Friedlander M. “Platinum resistant” ovarian cancer: what is it, who to treat and how to measure benefit? Gynecol Oncol 2014; 133:624-31; PMID:24607285; http://dx.doi.org/ 10.1016/j.ygyno.2014.02.038 McKeage MJ. New-generation platinum drugs in the treatment of cisplatin-resistant cancers. Expert Opin Invest Drugs 2005; 14:1033-46; PMID:16050795; http://dx.doi.org/10.1517/13543784.14.8.1033 Chang KE, Wei BR, Madigan JP, Hall MD, Simpson RM, Zhuang Z, Gottesman MM. The protein phosphatase 2A inhibitor LB100 sensitizes ovarian carcinoma cells to cisplatin-mediated cytotoxicity. Mol Cancer Ther 2014; 14(1):90-100; PMID:25376608 Sorensen CS, Syljuasen RG, Falck J, Schroeder T, Ronnstrand L, Khanna KK, Zhou BB, Bartek J, Lukas J. Chk1 regulates the S phase checkpoint by coupling the physiological turnover and ionizing radiationinduced accelerated proteolysis of Cdc25A. Cancer Cell 2003; 3:247-58; PMID:12676583; http://dx.doi. org/10.1016/S1535-6108(03)00048-5 Zhao H, Watkins JL, Piwnica-Worms H. Disruption of the checkpoint kinase 1/cell division cycle 25A pathway abrogates ionizing radiation-induced S and G2 checkpoints. Proc Natl Acad Sci U S A 2002; 99:14795-800; PMID:12399544; http://dx.doi.org/ 10.1073/pnas.182557299 Walworth N, Davey S, Beach D. Fission yeast chk1 protein kinase links the rad checkpoint pathway to cdc2. Nature 1993; 363:368-71; PMID:8497322; http://dx.doi.org/10.1038/363368a0 Noguchi K, Katayama K, Sugimoto Y. Human ABC transporter ABCG2/BCRP expression in chemoresistance: basic and clinical perspectives for molecular cancer therapeutics. Pharmacogenomics Pers Med 2014; 7:53-64; PMID:24523596; http://dx.doi.org/ 10.2147/PGPM.S38295 Chang KE, Wei BR, Madigan JP, Hall MD, Simpson RM, Zhuang Z, Gottesman MM. The protein phosphatase 2A inhibitor LB100 sensitizes ovarian carcinoma cells to cisplatin-mediated cytotoxicity. Mol Cancer Ther 2015; 14:90-100; PMID:25376608; http://dx.doi.org/10.1158/1535-7163.MCT-14-0496 Begley DJ. ABC transporters and the blood-brain barrier. Curr Pharm Des 2004; 10:1295-312; PMID:15134482; http://dx.doi.org/10.2174/ 1381612043384844

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Identification of verrucarin a as a potent and selective steroid receptor coactivator-3 small molecule inhibitor.

Ani9, A Novel Potent Small-Molecule ANO1 Inhibitor with Negligible Effect on ANO2.

A Novel, Potent, Small Molecule AKT Inhibitor Exhibits Efficacy against Lung Cancer Cells In Vitro.

b with Therapeutic Potential for Prostate Cancer and Chronic Myeloid Leukemia.

PP2A inhibition with LB100 enhances cisplatin cytotoxicity and overcomes cisplatin resistance in medulloblastoma cells.

Potential radiosensitizing agents. Dinitroimidazoles.

A potent and selective small molecule inhibitor of sirtuin 1 promotes differentiation of pluripotent P19 cells into functional neurons.

Bufalin is a potent small-molecule inhibitor of the steroid receptor coactivators SRC-3 and SRC-1.

Molecular Dynamics simulations of Inhibitor of Apoptosis Proteins and identification of potential small molecule inhibitors.

Selective and potent small-molecule inhibitors of PI3Ks.

A phage-displayed peptide recognizing porcine aminopeptidase N is a potent small molecule inhibitor of PEDV entry.

Targeting of the breast cancer microenvironment with a potent and linkable oxindole based antiangiogenic small molecule.

Development of EHop-016: a small molecule inhibitor of Rac.

A novel small-molecule inhibitor of HIV-1 entry.

A small-molecule inhibitor of hepatitis C virus infectivity.

Anticancer activity of a novel small molecule tubulin inhibitor STK899704.

A small-molecule allosteric inhibitor of Mycobacterium tuberculosis tryptophan synthase.

Ebselen, a Small-Molecule Capsid Inhibitor of HIV-1 Replication.

Structure-activity exploration of a small-molecule Lipid II inhibitor.

A novel small-molecule inhibitor of 3-phosphoglycerate dehydrogenase.

Inhibition of protein phosphatase 2A with the small molecule LB100 overcomes cell cycle arrest in osteosarcoma after cisplatin treatment.

Small-molecule Hedgehog inhibitor attenuates the leukemia-initiation potential of acute myeloid leukemia cells.

A novel small-molecule MRCK inhibitor blocks cancer cell invasion.

Inflammation. Potent small molecule extinguishes the NLRP3 inflammasome.