Topoisomerase levels determine chemotherapy response in vitro and in vivo

Replication-dependent and transcription-dependent mechanisms of DNA double-strand break induction by the topoisomerase 2-targeting drug etoposide

Etoposide is a DNA topoisomerase 2-targeting drug widely used for the treatment of cancer. The cytoxicity of etoposide correlates with the generation of DNA double-strand breaks (DSBs), but the mechanism of how it induces DSBs in cells is still poorly understood. Catalytically, etoposide inhibits the re-ligation reaction of Top2 after it nicks the two strands of DNA, trapping it in a cleavable complex consisting of two Top2 subunits covalently linked to the 5' ends of DNA (Top2cc). Top2cc is not directly recognized as a true DSB by cells because the two subunits interact strongly with each other to hold the two ends of DNA together. In this study we have investigated the cellular mechanisms that convert Top2ccs into true DSBs. Our data suggest that there are two mechanisms, one dependent on active replication and the other dependent on proteolysis and transcription. The relative contribution of each mechanism is affected by the concentration of etoposide. We also find that Top2α is the major isoform mediating the replication-dependent mechanism and both Top2α and Top2 mediate the transcription-dependent mechanism. These findings are potentially of great significance to the improvement of etoposide's efficacy in cancer therapy.

Next-generation NAMPT inhibitors identified by sequential high-throughput phenotypic chemical and functional genomic screens

Phenotypic high-throughput chemical screens allow for discovery of small molecules that modulate complex phenotypes and provide lead compounds for novel therapies; however, identification of the mechanistically relevant targets remains a major experimental challenge. We report the application of sequential unbiased high-throughput chemical and ultracomplex small hairpin RNA (shRNA) screens to identify a distinctive class of inhibitors that target nicotinamide phosphoribosyl transferase (NAMPT), a rate-limiting enzyme in the biosynthesis of nicotinamide adenine dinucleotide, a crucial cofactor in many biochemical processes. The lead compound STF-118804 is a highly specific NAMPT inhibitor, improves survival in an orthotopic xenotransplant model of high-risk acute lymphoblastic leukemia, and targets leukemia stem cells. Tandem high-throughput screening using chemical and ultracomplex shRNA libraries, therefore, provides a rapid chemical genetics approach for seamless progression from small-molecule lead identification to target discovery and validation.

MiR-23a-mediated inhibition of topoisomerase 1 expression potentiates cell response to etoposide in human hepatocellular carcinoma

Background

microRNAs have been shown to regulate the chemosensitivity of cancer cells. The aim of this study is to investigate the role and mechanism of mir-23a in enhancing the anti-tumor effect of topoisomerase 2A (TOP2A) poison etoposide in human hepatocellular carcinoma (HCC).

Methods

The anti-tumor effect of chemotherapeutic agents in HCC cells were examined in vitro and in vivo xenograft model. Expression of mRNA and miRNAs were determined by quantitative real-time PCR. Protein expression was analyzed by immunoblotting.

Results

Overexpression of mir-23a could significantly potentiate the in vitro and in vivo anti-tumor effect of etoposide; however, ectopic expression of miR-23a fails to sensitize HCC cells to 5-fluorouracil treatment, indicating the miR-23a-induced cancer cell hypersensitivity in chemotherapy is TOP2A-specific though miR-23a overexpression could not directly up-regulate TOP2A expression. Topoisomerase 1(TOP1) is down-regulated in miR-23a-overexpressed HCC cells. MiR-23a could directly bind to 3′untranslated region of TOP1 mRNA, and suppress the corresponding protein expression and inhibition of miR-23a further arguments the expression of TOP1. MiR-23a was up-regulated during DNA damage in cancer cells in line with the p53 expression. Up-regulation of p53 induces mir-23a expression, while suppression of p53 inhibits miR-23a in HCC cells.

Conclusion

Our study sheds light on the role of miR-23a as a potential target in regulating chemosensitivity of HCC cells.

TOP2A protein by quantitative immunofluorescence as a predictor of response to epirubicin in the neoadjuvant treatment of breast cancer

Aim: Anthracyclines are commonly used in breast cancer, although they lack validated predictive biomarkers. We explored the interaction between TOP2A protein by quantitative immunofluorescence (QIF) and anthracycline sensitivity. Patients & methods: Patients with estrogen receptor-negative breast cancer received neoadjuvant epirubicin. Pretreatment biopsies were analyzed using AQUA®. Total, cytoplasmic (C) and nuclear (N) TOP2A protein concentrations were expressed as QIF scores and compared with pathologic complete response (pCR), TOP2A by immunohistochemistry, TOP2A mRNA, TOP2A and HER2 gene status, and Ki-67 level. Results: In total, 76 cases were assessable. C, N, and total scores did not correlate with pCR, or other markers. The N:C ratio differed significantly by HER2 status. No pCRs occurred in patients in the lowest N:C quartile. Conclusion: Although no relevant correlation between TOP2A QIF scores and pCR was found, N:C ratio may have a negative predictive role, and may merit further exploration in a multifactorial predictive model that includes tumor and host factors.

Effectiveness and complications of anthracycline and taxane in the therapy of breast cancer: a meta-analysis

OBJECTIVE

To compare the efficacy and safety between anthracycline & taxane and anthracycline in the treatment of breast cancer.

METHODS

Computer-assisted literature search was performed with PubMed, MEDLINE, EMBASE and Cochrane Controlled Trials Register (CCTR) to identify pertinent literatures. Software RevMan 5.0 was used for statistical analysis. The measurement of interest outcomes included severe neurotoxicity, death without breast cancer recurrences, leukemia, venous thrombus and severe cardiotoxicity.

RESULTS

A total of 10 randomized controlled trial studies (RCTs) containing 18,198 cases were selected in this meta-analysis. Of which, 9,902 cases were treated with anthracycline & taxane and 8,296 cases treated with anthracycline alone as control. Anthracycline & taxane showed lower risks of incident leukemia (RR = 0.40; 95% CI: 0.18, 0.90), venous thrombus (RR = 0.49; 95% CI: 0.29, 0.84) and severe cardiotoxicity (RR = 0.41, 95%CI: 0.26, 0.66), but higher risks of incident severe neurotoxicity (RR = 5.97; 95% CI: 1.72, 20.65) and non-recurrent death (RR = 1.79; 95% CI: 1.06, 3.04), compared to anthracycline alone.

CONCLUSION

Clinically important differences exist for general safety in favour of the anthracycline & taxane rather than anthracycline alone. However, as a result of tumor recurrent rate, anthracycline might be superior to anthracycline & taxane. A longer duration of follow-up and a larger number of cases are required to better assess the efficacy and safety profile of the treatment of breast cancer.

A new mathematical pharmacodynamic model of clonogenic cancer cell death by doxorubicin

Previous models for predicting tumor cell growth are mostly based on measurements of total cell numbers. The purpose of this paper is to provide a new simple mathematical model for calculating tumor cell growth focusing on the fraction of cells that is clonogenic. The non-clonogenic cells are considered to be relatively harmless. We performed a number of different types of experiments: a long-term drug "treatment", several concentrations/fixed time experiments and time-series experiments, in which human MCF-7 breast cancer cells were exposed to doxorubicin and the total number of cells were counted. In the latter two types, at every measurement point a plating efficiency experiment was started. The final number of colonies formed is equal to the number of clonogenic cells at the onset of the experiment. Based on the intracellular drug concentration, our model predicts cell culture effects taking clonogenic ability and growth inhibition by neighboring cells into account. The model fitted well to the experimental data. The estimated damage parameter which represents the chance of an MCF-7 cell to become non-clonogenic per unit time and per unit intracellular doxorubicin concentration was found to be 0.0025 ± 0.0008 (mean ± SD) nM(-1) h(-1). The model could be used to calculate the effect of every doxorubicin concentration versus time (C-t) profile. Although in vivo parameters may well be different from those found in vitro, the model can be used to predict trends, e.g. by comparing effects of different in vivo C-t profiles.

Increased expression of biological markers as potential therapeutic targets in Saudi women with triple-negative breast cancer

Aims and background

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that lacks the expression of hormone receptors and human epidermal growth factor receptor 2 (HER2). Although TNBC represents only 15% of all types of breast cancer, it accounts for a large number of metastatic cases and deaths. Because of the high metastatic rate and both local and systemic recurrence associated with TNBC, extensive research efforts are actively looking for target therapies to effectively treat this aggressive disease. Accordingly, this study has been initiated to investigate the differential expression of biological markers in TNBC and non-TNBC Saudi women that might be utilized as potential targeted therapy and/or predict the sensitivity to currently available therapeutic regimens.

Methods and study design

Two hundred formalin-fixed, paraffin-embedded (FFPE) breast cancer tissues were selected and divided into 3 groups: benign breast tissues (20), TNBC tissues (80) and non-TNBC tissues (100). Expression of mRNA in FFPE tissues was analyzed using real-time polymerase chain reaction (RT-PCR) for the following genes: poly (ADP-ribose) polymerase 1 (PARP-1), topoisomerase 2A (TOPO-2A), vascular endothelial growth factor (VEGF), C-MYC, basic fibroblast growth factor (bFGF), matrix metalloproteinases (MMP-2 and MMP-9), human epidermal growth factor 1 (HER1) and multidrug resistance (MDR) genes.

Results

In the TNBC group, expression of PARP-1, TOPO-2A, HER1, C-MYC, VEGF, bFGF and MMP-2 showed a highly significant increase compared to the non-TNBC group.

Conclusions

The results of this study suggest that (1) TNBC patients will benefit more from TOPO-2A inhibitors as well as antiangiogenic and antimetastatic therapies; (2) inhibition of these target genes is emerging as one of the most exciting and promising targeted therapeutic strategies to treat TNBC in which the intended targets are DNA repair, tumor angiogenesis and metastasis.

A rapid and sensitive assay for DNA-protein covalent complexes in living cells

A number of proteins form covalent bonds with DNA as obligatory transient intermediates in normal nuclear transactions. Drugs that trap these complexes have proven to be potent therapeutics in both cancer and infectious disease. Nonetheless, current assays for DNA–protein adducts are cumbersome, limiting both mechanistic studies and translational applications. We have developed a rapid and sensitive assay that enables quantitative immunodetection of protein–DNA adducts. This new ‘RADAR’ (rapid approach to DNA adduct recovery) assay accelerates processing time 4-fold, increases sample throughput 20-fold and requires 50-fold less starting material than the current standard. It can be used to detect topoisomerase 1-DNA adducts in as little as 60 ng of DNA, corresponding to 10 000 human cells. We apply the RADAR assay to demonstrate that expression of SLFN11 does not increase camptothecin sensitivity by promoting accumulation of topoisomerase 1-DNA adducts. The RADAR assay will be useful for analysis of the mechanisms of formation and resolution of DNA–protein adducts in living cells, and identification and characterization of reactions in which covalent DNA adducts are transient intermediates. The assay also has potential application to drug discovery and individualized medicine.

Multidrug resistance in primary tumors and metastases in patients with esophageal squamous cell carcinoma

Studies have demonstrated that radical esophagectomy can significantly prolong disease-free survival and improve the survival rate of patients with T3 or T4 esophageal cancer and lymph node metastasis. Multidrug resistant cancer cells have active efflux mechanisms that prevent the accumulation of chemotherapeutic drugs in the cells. The purpose of this study was to compare the expression of five MDR related proteins between primary tumors in patients with thoracic esophageal squamous cell carcinoma (ESCC) and metastatic cancer in lymph nodes to explore the clinical significance of heterogeneity in MDR metastatic cancer cells. Fifty-four patients with ESCC and lymph node metastasis were included. All patients underwent subtotal esophagectomy and D2/D3 lymph node resection. The expression of lung resistance-related protein (LRP), P-glycoprotein, topoisomerase-II, thymidylate synthase, and glutathione S-transferase P1-1 (GST-π) were determined in the primary tumors and lymph nodes via immunohistochemistry. The expression of LRP was significantly different between the primary tumors and lymph nodes (P = 0.026). No significant differences were found for the other four proteins, and protein expression was not associated with either degree of differentiation or disease stage. It was also found that GST-π was expressed in all patients in both the primary tumors and lymph nodes, suggesting that the design and application of chemotherapeutic protocols capable of reducing GST-π expression may be beneficial for patients with ESCC. Additional research regarding the clinical utility of MDR protein expression in ESCC is warranted to design effective chemotherapeutic protocols.

Iron chelators with topoisomerase-inhibitory activity and their anticancer applications

SIGNIFICANCE

Iron and topoisomerases are abundant and essential cellular components. Iron is required for several key processes such as DNA synthesis, mitochondrial electron transport, synthesis of heme, and as a co-factor for many redox enzymes. Topoisomerases serve as critical enzymes that resolve topological problems during DNA synthesis, transcription, and repair. Neoplastic cells have higher uptake and utilization of iron, as well as elevated levels of topoisomerase family members. Separately, the chelation of iron and the cytotoxic inhibition of topoisomerase have yielded potent anticancer agents.

RECENT ADVANCES

The chemotherapeutic drugs doxorubicin and dexrazoxane both chelate iron and target topoisomerase 2 alpha (top2α). Newer chelators such as di-2-pyridylketone-4,4,-dimethyl-3-thiosemicarbazone and thiosemicarbazone -24 have recently been identified as top2α inhibitors. The growing list of agents that appear to chelate iron and inhibit topoisomerases prompts the question of whether and how these two distinct mechanisms might interplay for a cytotoxic chemotherapeutic outcome.

CRITICAL ISSUES

While iron chelation and topoisomerase inhibition each represent mechanistically advantageous anticancer therapeutic strategies, dual targeting agents present an attractive multi-modal opportunity for enhanced anticancer tumor killing and overcoming drug resistance. The commonalities and caveats of dual inhibition are presented in this review.

FUTURE DIRECTIONS

Gaps in knowledge, relevant biomarkers, and strategies for future in vivo studies with dual inhibitors are discussed.

Using functional genomics to overcome therapeutic resistance in hematological malignancies

Despite great advances in our understanding of the driving events involved in malignant transformation, only a small number of oncogenic drivers have been targeted and translated into tangible clinical benefit. Moreover, even when a targeted therapy can be shown to effectively inhibit an oncogenic driver, leading to cancer remission, disease persistence and/or relapse is typically inevitable. Reemergence of the cancer can result from either intrinsic or acquired resistance mechanisms that result in failure to eliminate all cancer cells. Intrinsic mechanisms of resistance include tumor heterogeneity and pathways that can compensate for the inhibition of the oncogenic driver. Acquired resistance mechanisms include mutation of the oncogenic driver to directly prevent drug-mediated inhibition and the activation of compensatory survival pathways. RNA interference (RNAi)-based screening provides a powerful approach for the interrogation of both intrinsic and acquired resistance mechanisms. The availability of short interfering (si)RNA libraries targeting all human and mouse genes has made it possible to perform large-scale unbiased screens to identify pathways that are specifically required in cancer cells of particular genotypes or following particular treatments, facilitating the design of potential new therapeutic strategies that may limit resistance mechanisms. In this review, we will discuss how RNAi screens can be used to uncover critical growth and survival pathways and aid in the identification of novel therapeutic targets for improved treatment of hematological malignancies.

Target identification for small bioactive molecules: finding the needle in the haystack

Identification and confirmation of bioactive small-molecule targets is a crucial, often decisive step both in academic and pharmaceutical research. Through the development and availability of several new experimental techniques, target identification is, in principle, feasible, and the number of successful examples steadily grows. However, a generic methodology that can successfully be applied in the majority of the cases has not yet been established. Herein we summarize current methods for target identification of small molecules, primarily for a chemistry audience but also the biological community, for example, the chemist or biologist attempting to identify the target of a given bioactive compound. We describe the most frequently employed experimental approaches for target identification and provide several representative examples illustrating the state-of-the-art. Among the techniques currently available, protein affinity isolation using suitable small-molecule probes (pulldown) and subsequent mass spectrometric analysis of the isolated proteins appears to be most powerful and most frequently applied. To provide guidance for rapid entry into the field and based on our own experience we propose a typical workflow for target identification, which centers on the application of chemical proteomics as the key step to generate hypotheses for potential target proteins.

Drugging topoisomerases: lessons and challenges

Topoisomerases are ubiquitous enzymes that control DNA supercoiling and entanglements. They are essential during transcription and replication, and topoisomerase inhibitors are among the most effective and most commonly used anticancer and antibacterial drugs. This review consists of two parts. In the first part ("Lessons"), it gives background information on the catalytic mechanisms of the different enzyme families (6 different genes in humans and 4 in most bacteria), describes the "interfacial inhibition" by which topoisomerase-targeted drugs act as topoisomerase poisons, and describes clinically relevant topoisomerase inhibitors. It generalizes the interfacial inhibition principle, which was discovered from the mechanism of action of topoisomerase inhibitors, and discusses how topoisomerase inhibitors kill cells by trapping topoisomerases on DNA rather than by classical enzymatic inhibition. Trapping protein-DNA complexes extends to a novel mechanism of action of PARP inhibitors and could be applied to the targeting of transcription factors. The second part of the review focuses on the challenges for discovery and precise use of topoisomerase inhibitors, including targeting topoisomerase inhibitors using chemical coupling and encapsulation for selective tumor delivery, use of pharmacodynamic biomarkers to follow drug activity, complexity of the response determinants for anticancer activity and patient selection, prospects of rational combinations with DNA repair inhibitors targeting tyrosyl-DNA-phosphodiesterases 1 and 2 (TDP1 and TDP2) and PARP, and the unmet need to develop inhibitors for type IA enzymes.

Pooled shRNA screenings: computational analysis

Genome-wide RNA interference screening has emerged as a powerful tool for functional genomic studies of disease-related phenotypes and the discovery of molecular therapeutic targets for human diseases. Commercial short hairpin RNA (shRNA) libraries are commonly used in this area, and state-of-the-art technologies including microarray and next-generation sequencing have emerged as powerful methods to analyze shRNA-triggered phenotypes. However, computational analysis of this complex data remains challenging due to noise and small sample size from such large-scaled experiments. In this chapter we discuss the pipelines and statistical methods of processing, quality assessment, and post-analysis for both microarray- and sequencing-based screening data.

Development and validation of an immunoassay for quantification of topoisomerase I in solid tumor tissues

BACKGROUND

Topoisomerase I (Top1) is a proven target for cancer therapeutics. Recent data from the Fluorouracil, Oxaliplatin, CPT-11: Use and Sequencing (FOCUS) trial demonstrated that nuclear staining of Top1 correlates with chemotherapeutic efficacy. Such a correlation may help identify patients likely to respond to Top1 inhibitors and illuminate their mechanism of action. Cellular response to Top1 inhibitors is complex, but Top1 target engagement is a necessary first step in this process. This paper reports the development and validation of a quantitative immunoassay for Top1 in tumors.

METHODOLOGY/PRINCIPAL FINDINGS

We have developed and validated a two-site enzyme chemiluminescent immunoassay for quantifying Top1 levels in tumor biopsies. Analytical validation of the assay established the inter-day coefficient of variation at 9.3%±3.4% and a 96.5%±7.3% assay accuracy. Preclinical fit-for-purpose modeling of topotecan time- and dose-effects was performed using topotecan-responsive and -nonresponsive xenografts in athymic nude mice. Higher baseline levels of Top1 were observed in topotecan-responsive than -nonresponsive tumors. Top1 levels reached a maximal decrease 4 to 7 hours following treatment of engrafted mice with topotecan and the indenoisoquinoline NSC 724998.

CONCLUSIONS/SIGNIFICANCE

Our analysis of Top1 levels in control and treated tumors supports the previously proposed mechanism of action for Top1 inhibitor efficacy, wherein higher baseline Top1 levels lead to formation of more covalent-complex-dependent double-strand break damage and, ultimately, cell death. In contrast, xenografts with lower baseline Top1 levels accumulate fewer double-stand breaks, and may be more resistant to Top1 inhibitors. Our results support further investigation into the use of Top1 levels in tumors as a potential predictive biomarker. The Top1 immunoassay described in this paper has been incorporated into a Phase I clinical trial at the National Cancer Institute to assess pharmacodynamic response in tumor biopsies and determine whether baseline Top1 levels are predictive of response to indenoisoquinoline Top1 inhibitors.

Defining principles of combination drug mechanisms of action

Combination chemotherapies have been a mainstay in the treatment of disseminated malignancies for almost 60 y, yet even successful regimens fail to cure many patients. Although their single-drug components are well studied, the mechanisms by which drugs work together in clinical combination regimens are poorly understood. Here, we combine RNAi-based functional signatures with complementary informatics tools to examine drug combinations. This approach seeks to bring to combination therapy what the knowledge of biochemical targets has brought to single-drug therapy and creates a statistical and experimental definition of "combination drug mechanisms of action." We show that certain synergistic drug combinations may act as a more potent version of a single drug. Conversely, unlike these highly synergistic combinations, most drugs average extant single-drug variations in therapeutic response. When combined to form multidrug regimens, averaging combinations form averaging regimens that homogenize genetic variation in mouse models of cancer and in clinical genomics datasets. We suggest surprisingly simple and predictable combination mechanisms of action that are independent of biochemical mechanism and have implications for biomarker discovery as well as for the development of regimens with defined genetic dependencies.

Dual functions of the homeoprotein DLX4 in modulating responsiveness of tumor cells to topoisomerase II-targeting drugs

Topoisomerase II (TOP2)-targeting poisons such as anthracyclines and etoposide are commonly used for cancer chemotherapy and kill tumor cells by causing accumulation of DNA double-strand breaks (DSB). Several lines of evidence indicate that overexpression of TOP2A, the gene encoding topoisomerase IIα, increases sensitivity of tumor cells to TOP2 poisons, but it is not clear why some TOP2A-overexpressing (TOP2A-High) tumors respond poorly to these drugs. In this study, we identified that TOP2A expression is induced by DLX4, a homeoprotein that is overexpressed in breast and ovarian cancers. Analysis of breast cancer datasets revealed that TOP2A-high cases that also highly expressed DLX4 responded more poorly to anthracycline-based chemotherapy than TOP2A-high cases that expressed DLX4 at low levels. Overexpression of TOP2A alone in tumor cells increased the level of DSBs induced by TOP2 poisons. In contrast, DLX4 reduced the level of TOP2 poison-induced DSBs irrespective of its induction of TOP2A. DLX4 did not stimulate homologous recombination-mediated repair of DSBs. However, DLX4 interacted with Ku proteins, stimulated DNA-dependent protein kinase activity, and increased erroneous end-joining repair of DSBs. Whereas DLX4 did not reduce levels of TOP2 poison-induced DSBs in Ku-deficient cells, DLX4 stimulated DSB repair and reduced the level of TOP2 poison-induced DSBs when Ku was reconstituted in these cells. Our findings indicate that DLX4 induces TOP2A expression but reduces sensitivity of tumor cells to TOP2 poisons by stimulating Ku-dependent repair of DSBs. These opposing activities of DLX4 could explain why some TOP2A-overexpressing tumors are not highly sensitive to TOP2 poisons.

Gene expression alterations in doxorubicin resistant MCF7 breast cancer cell line

Many molecular mechanisms contribute to the development of doxorubicin resistance and different cancers can express wide and diverse arrays of drug-resistance genes. The aim of this study was to identify the changes in gene expression associated with the development of doxorubicin resistance in MCF7 breast cancer cell line. The doxorubicin resistant MCF7 cell line was developed by stepwise selection of MCF7 cells and was tested using the MTT assay. The alterations in gene expression were examined using the real-time based PCR array. The findings showed an up-regulation of many phase I/II metabolizing genes, specifically, the CYP1A1 and the CYP1A2 that were up-regulated by 206- and 96-fold respectively. Drug efflux pump genes were also up-regulated profoundly. TOP2A was strongly down-regulated by 202-fold. Many other changes were observed in genes crucial for cell cycle, apoptosis and DNA repair. The findings of this project imply that the development of doxorubicin resistance is a multi-factorial process.

The DNA topoisomerase II catalytic inhibitor merbarone is genotoxic and induces endoreduplication

In the last years a number of reports have shown that the so-called topoisomerase II (topo II) catalytic inhibitors are able to induce DNA and chromosome damage, an unexpected result taking into account that they do not stabilize topo II-DNA cleavable complexes, a feature of topo II poisons such as etoposide and amsacrine. Merbarone inhibits the catalytic activity of topo II by blocking DNA cleavage by the enzyme. While it was first reported that merbarone does not induce genotoxic effects in mammalian cells, this has been challenged by reports showing that the topo II inhibitor induces efficiently chromosome and DNA damage, and the question as to a possible behavior as a topo II poison has been put forward. Given these contradictory results, and the as yet incomplete knowledge of the molecular mechanism of action of merbarone, in the present study we have tried to further characterize the mechanism of action of merbarone on cell proliferation, cell cycle, as well as chromosome and DNA damage in cultured CHO cells. Merbarone was cytotoxic as well as genotoxic, inhibited topo II catalytic activity, and induced endoreduplication. We have also shown that merbarone-induced DNA damage depends upon ongoing DNA synthesis. Supporting this, inhibition of DNA synthesis causes reduction of DNA damage and increased cell survival.

Treatment with the PARP-inhibitor PJ34 causes enhanced doxorubicin-mediated cell death in HeLa cells

Adjuvant therapies can incorporate a number of different drugs to minimize the cardiotoxicity of cancer chemotherapy, decrease the development of drug resistance and increase the overall efficacy of the treatment regime. Topoisomerase IIα is a major target of many commonly used anticancer drugs, where cell death is brought about by an accumulation of double-strand DNA breaks. Poly (ADP-ribose) polymerase (PARP)-1 has been extensively studied for its role in the repair of double-strand DNA breaks, but its ability to add highly negative biopolymers (ribosylation) to target proteins provides a vast number of pathways where it can also be important in mediating cell death. In this study, we combine the classical topoisomerase IIα poison doxorubicin with the PARP inhibitor PJ34 to investigate the potentiation of chemotherapeutic efficiency in HeLa cells. We demonstrate that PJ34 treatment has the capacity to increase endogenous topoisomerase IIα protein by about 20%, and by combining doxorubicin treatment with PJ34, we observed a 50% improvement in doxorubicin-mediated cell death in HeLa cells. These results were correlated with the ribosylation of transcription factor specificity factor 1 after doxorubicin treatment, thereby altering its affinity for binding to known regulatory elements within the human topoisomerase IIα promoter. Taken together, these results highlight the synergistic potential of combining PARP inhibitors with classical topoisomerase IIα-targeting drugs.

Topoisomerase degradation, DSB repair, p53 and IAPs in cancer cell resistance to camptothecin-like topoisomerase I inhibitors

Topoisomerase I (TOP1) inhibitors applied in cancer therapy such as topotecan and irinotecan are derivatives of the natural alkaloid camptothecin (CPT). The mechanism of CPT poisoning of TOP1 rests on inhibition of the re-ligation function of the enzyme resulting in the stabilization of the TOP1-cleavable complex. In the presence of CPTs this enzyme-DNA complex impairs transcription and DNA replication, resulting in fork stalling and the formation of DNA double-strand breaks (DSB) in proliferating cells. As with most chemotherapeutics, intrinsic and acquired drug resistance represents a hurdle that limits the success of CPT therapy. Preclinical data indicate that resistance to CPT-based drugs might be caused by factors such as (a) poor drug accumulation in the tumor, (b) high rate of drug efflux, (c) mutations in TOP1 leading to failure in CPT docking, or (d) altered signaling triggered by the drug-TOP1-DNA complex, (e) expression of DNA repair proteins, and (f) failure to activate cell death pathways. This review will focus on the issues (d-f). We discuss degradation of TOP1 as part of the repair pathway in the processing of TOP1 associated DNA damage, give a summary of proteins involved in repair of CPT-induced replication mediated DSB, and highlight the role of p53 and inhibitors of apoptosis proteins (IAPs), particularly XIAP and survivin, in cancer cell resistance to CPT-like chemotherapeutics.

Gene expression signature predicting high-grade prostate cancer responses to oxaliplatin

Prostate cancer is one of the leading causes of cancer-related deaths among men. Several prognostic factors allow differentiation of low-grade tumors from high-grade tumors with high metastatic potential. High-grade tumors are currently treated with hormone therapy, to which taxanes are added when the tumors become resistant to castration. Clinical trials with other anticancer agents did not take into account the genetic backgrounds of the tumors, and most trials demonstrated low response rates. Here we used an in silico approach to screen for drug candidates that might be used as alternatives to taxanes, on the basis of a published expression signature involving 86 genes that could distinguish high-grade and low-grade tumors (Proc Natl Acad Sci USA 103:10991-10996, 2006). We explored the National Cancer Institute databases, which include data on the gene expression profiles of 60 human tumor cell lines and the in vitro sensitivities of the cell lines to anticancer drugs, and we identified several genes in the signature for which expression levels were correlated with chemosensitivity. As an example of the validation of this in silico approach, we identified a set of six genes for which expression levels could predict cell sensitivity to oxaliplatin but not cisplatin. This signature was validated in vitro through silencing of the genes in DU145, LNCaP, and C4-2B prostate cancer cells, which was accompanied by changes in oxaliplatin but not cisplatin cytotoxicity. These results demonstrate the relevance of our approach for the identification of both alternative treatments for high-grade prostate cancers and new biomarkers to predict clinical tumor responses.

The effects of deregulated DNA damage signalling on cancer chemotherapy response and resistance

Tumours with specific DNA repair defects can be completely dependent on back-up DNA repair pathways for their survival. This dependence can be exploited therapeutically to induce synthetic lethality in tumour cells. For instance, homologous recombination (HR)-deficient tumours can be effectively targeted by DNA double-strand break-inducing agents. However, not all HR-defective tumours respond equally well to this type of therapy. Tumour cells may acquire resistance by invoking biochemical mechanisms that reduce drug action or by acquiring additional alterations in DNA damage response pathways. A thorough understanding of these processes is important for predicting treatment response and for the development of novel treatment strategies that prevent the emergence of therapy-resistant tumours.

Sensitivity to TOP2 targeting chemotherapeutics is regulated by Oct1 and FILIP1L

Topoisomerase II (TOP2) targeting drugs like doxorubicin and etoposide are frontline chemotherapeutics for a wide variety of solid and hematological malignancies, including breast and ovarian adenocarcinomas, lung cancers, soft tissue sarcomas, leukemias and lymphomas. These agents cause a block in DNA replication leading to a pronounced DNA damage response and initiation of apoptotic programs. Resistance to these agents is common, however, and elucidation of the mechanisms causing resistance to therapy could shed light on strategies to reduce the frequency of ineffective treatments. To explore these mechanisms, we utilized an unbiased shRNA screen to identify genes that regulate cell death in response to doxorubicin treatment. We identified the Filamin A interacting protein 1-like (FILIP1L) gene as a crucial mediator of apoptosis triggered by doxorubicin. FILIP1L shares significant similarity with bacterial SbcC, an ATPase involved in DNA repair. FILIP1L was originally described as DOC1, or “down-regulated in ovarian cancer” and has since been shown to be downregulated in a wide variety of human tumors. FILIP1L levels increase markedly through transcriptional mechanisms following treatment with doxorubicin and other TOP2 poisons, including etoposide and mitoxantrone, but not by the TOP2 catalytic inhibitors merbarone or dexrazoxane (ICRF187), or by UV irradiation. This induction requires the action of the OCT1 transcription factor, which relocalizes to the FILIP1L promoter and facilitates its expression following doxorubicin treatment. Our findings suggest that the FILIP1L expression status in tumors may influence the response to anti-TOP2 chemotherapeutics.

Optimized PCR conditions and increased shRNA fold representation improve reproducibility of pooled shRNA screens

RNAi screening using pooled shRNA libraries is a valuable tool for identifying genetic regulators of biological processes. However, for a successful pooled shRNA screen, it is imperative to thoroughly optimize experimental conditions to obtain reproducible data. Here we performed viability screens with a library of ∼10,000 shRNAs at two different fold representations (100- and 500-fold at transduction) and report the reproducibility of shRNA abundance changes between screening replicates determined by microarray and next generation sequencing analyses. We show that the technical reproducibility between PCR replicates from a pooled screen can be drastically improved by ensuring that PCR amplification steps are kept within the exponential phase and by using an amount of genomic DNA input in the reaction that maintains the average template copies per shRNA used during library transduction. Using these optimized PCR conditions, we then show that higher reproducibility of biological replicates is obtained by both microarray and next generation sequencing when screening with higher average shRNA fold representation. shRNAs that change abundance reproducibly in biological replicates (primary hits) are identified from screens performed with both 100- and 500-fold shRNA representation, however a higher percentage of primary hit overlap between screening replicates is obtained from 500-fold shRNA representation screens. While strong hits with larger changes in relative abundance were generally identified in both screens, hits with smaller changes were identified only in the screens performed with the higher shRNA fold representation at transduction.

A target based approach identifies genomic predictors of breast cancer patient response to chemotherapy

Background

The efficacy of chemotherapy regimens in breast cancer patients is variable and unpredictable. Whether individual patients either achieve long-term remission or suffer recurrence after therapy may be dictated by intrinsic properties of their breast tumors including genetic lesions and consequent aberrant transcriptional programs. Global gene expression profiling provides a powerful tool to identify such tumor-intrinsic transcriptional programs, whose analyses provide insight into the underlying biology of individual patient tumors. For example, multi-gene expression signatures have been identified that can predict the likelihood of disease reccurrence, and thus guide patient prognosis. Whereas such prognostic signatures are being introduced in the clinical setting, similar signatures that predict sensitivity or resistance to chemotherapy are not currently clinically available.

Methods

We used gene expression profiling to identify genes that were co-expressed with genes whose transcripts encode the protein targets of commonly used chemotherapeutic agents.

Results

Here, we present target based expression indices that predict breast tumor response to anthracycline and taxane based chemotherapy. Indeed, these signatures were independently predictive of chemotherapy response after adjusting for standard clinic-pathological variables such as age, grade, and estrogen receptor status in a cohort of 488 breast cancer patients treated with adriamycin and taxotere/taxol.

Conclusions

Importantly, our findings suggest the practicality of developing target based indices that predict response to therapeutics, as well as highlight the possibility of using gene signatures to guide the use of chemotherapy during treatment of breast cancer patients.

Cancer drug pan-resistance: pumps, cancer stem cells, quiescence, epithelial to mesenchymal transition, blocked cell death pathways, persisters or what?

Although chemotherapy of tumours has scored successes, drug resistance remains the major cause of death of cancer patients. Initial treatment often leaves residual disease, from which the tumour regrows. Eventually, most tumours become resistant to all available chemotherapy. I call this pan-resistance to distinguish it from multi-drug resistance, usually describing resistance caused by upregulation of drug transporters, such as P-glycoprotein. In this review, I discuss mechanisms proposed to explain both residual disease and pan-resistance. Although plausible explanations are at hand for residual disease, pan-resistance is still a mystery. My conclusion is that it is time for a major effort to solve this mystery using the new genetically modified mouse tumour models that produce real tumours resembling cancer in human patients.

Multiplication of Chromosome 17 Centromere Is Associated with Prognosis in Patients with Invasive Breast Cancers Exhibiting Normal HER2 and TOP2A Status

PURPOSE

This study aimed to investigate the clinical significance of chromosome 17 centromere (CEP17) multiplication (increased copy number of CEP17) related to human epidermal growth factor receptor 2 (HER2) and topoisomerase II alpha (TOP2A) status in patients with invasive breast cancer.

METHODS

We constructed tissue microarrays using 594 invasive breast cancer samples and performed single-color silver-enhanced in situ hybridization (SISH) assay for HER2, TOP2A, and CEP17 to assess for copy number aberrations. The association of CEP17 multiplication with patient survival was analyzed according to HER2 and TOP2A status.

RESULTS

Among 567 informative cases, HER2 amplification was noted in 22.8%, TOP2A amplification in 8.3% and TOP2A deletion in 11.1%. CEP17 multiplication was identified in 33.2% and was significantly associated with worse overall survival (OS) (p=0.02) and disease-free survival (DFS) (p=0.02). CEP17 multiplication correlated with patient survival in patients with normal TOP2A or non-amplified HER2 status, but the prognostic significance was lost in those with altered TOP2A or amplified HER2. On multivariate analyses, CEP17 multiplication was an independent prognostic factor for poorer OS (p=0.02) and DFS (p=0.01) in patients with normal TOP2A and non-amplified HER2.

CONCLUSION

CEP17 multiplication was identified as a promising prognostic marker in patients with invasive breast cancer exhibiting either non-amplified HER2 or normal TOP2A status.

Downregulation of HuR as a new mechanism of doxorubicin resistance in breast cancer cells

Background

HuR, an RNA binding protein involved in the post-transcriptional regulation of a wide spectrum of mRNAs, has been demonstrated to be a determinant of carcinogenesis and tumor aggressiveness in several cancer types. In this study, we investigated the role of HuR in the apoptosis and in the chemoresistance induced by the widely used anticancer drug doxorubicin in human breast cancer cells (MCF-7).

Results

We showed that HuR acts in the early phase of cell response to doxorubicin, being induced to translocate into the cytoplasm upon phosphorylation. Reducing HuR levels diminished the apoptotic response to doxorubicin. Doxorubicin-induced apoptosis was also correlated with the presence of HuR in the cytoplasm. Rottlerin, which was able to block HuR nuclear export, had correspondingly antagonistic effects with doxorubicin on cell toxicity. The proapoptotic activity of HuR was not due to cleavage to an active form, as was previously reported. In in vitro selected doxorubicin resistant MCF-7 cells (MCF-7/doxoR) overexpressing the multidrug resistance (MDR) related ABCG2 transporter, we observed a significant HuR downregulation that was paralleled by a corresponding downregulation of HuR targets and by loss of rottlerin toxicity. Restoration of HuR expression in these cells resensitized MCF-7/doxoR cells to doxorubicin, reactivating the apoptotic response.

Conclusions

The present study shows that HuR is necessary to elicit the apoptotic cell response to doxorubicin and that restoration of HuR expression in resistant cells resensitizes them to the action of this drug, thereby identifying HuR as a key protein in doxorubicin pharmacology.

Predictive biomarkers with potential of converting conventional chemotherapy to targeted therapy in patients with metastatic colorectal cancer

The availability of systemic chemotherapy regimens for the treatment of patients with metastatic colorectal cancer (mCRC) is based on the results from large prospective, randomized studies. The main chemotherapeutic drugs used in treatment of mCRC are the fluoropyrimidines (5-fluorouracil (5-FU); capecitabine) in combination with either oxaliplatin (FOLFOX) or irinotecan (FOLFIRI). The objective response rate to either combination is approximately 50%, where no significant differences with regard to progression free survival or overall survival have been observed. Interestingly, a number of preclinical and clinical studies have indicated lack of full cross resistance between oxaliplatin based and irinotecan based treatment. Therefore, it is possible that certain mCRC patient subpopulations would benefit more from one drug combination rather than the other. To address this clinical problem there has been much focus on development and validation of predictive biomarkers for these three drugs. Here, we present a thorough review on the current status of predictive biomarkers for 5-FU, oxaliplatin and irinotecan treatment of mCRC patients. The overall conclusions were as follows: Several promising biomarker candidates were identified, notably thymidylate synthase for 5-FU, topoisomerase I for irinotecan and ERCC1 for oxaliplatin. However, these candidates warrant further analysis, where assay performance and clinical trial design should be in focus.

Circumventing cancer drug resistance in the era of personalized medicine

All successful cancer therapies are limited by the development of drug resistance. The increase in the understanding of the molecular and biochemical bases of drug efficacy has also facilitated studies elucidating the mechanism(s) of drug resistance. Experimental approaches that can help predict the eventual clinical drug resistance, coupled with the evolution of systematic genomic and proteomic technologies, are rapidly identifying novel resistance mechanisms. In this review, we provide a historical background on drug resistance and a framework for understanding the common ways by which cancers develop resistance to targeted therapies. We further discuss advantages and disadvantages of experimental strategies that can be used to identify drug resistance mechanism(s).

SIGNIFICANCE

Increased knowledge of drug resistance mechanisms will aid in the development of effective therapies for patients with cancer. We provide a summary of current knowledge on drug resistance mechanisms and experimental strategies to identify and study additional drug resistance pathways.

[Molecular determinants of response to topoisomerase II inhibitors]

Human nuclear topoisomerases II (Top2) are involved in the relaxation of DNA supercoiling during transcription and replication but also play a pivotal role in the segregation of newly replicated chromosomes and in chromatin remodelling. Top2 have been used as targets for the development of anticancer drugs. These inhibitors include anthracyclines (doxorubcin, daunorubicin, epirubicin) and epipodophyllotoxins (etoposide), which are widely used in the clinic. These drugs poison Top2 by trapping the enzyme on its DNA cleavage sites, which results in irreversible double-strand breaks that are responsible for cell death. They also include Top2 catalytic inhibitors such as bisdioxopiperazines (ICRF-187 and merbarone), which inhibit Top2 binding to its substrate. Efficacy of Top2 inhibitors is still limited by the problem of resistance, which involves various mechanisms from drug transport and/or metabolism to the signalling and/or repair of Top2-mediated DNA lesions. Secondary malignancies induced by the poisoning of Top2β are also a major clinical issue. A better understanding of these mechanisms is critical for the future development of new Top2 inhibitors and the identification of biomarkers that could be used to predict tumour response to these drugs in the clinic and to adapt the treatment to each patient.

Integrating Breast Cancer Genetics into Clinical Practice

Breast cancer prognosis and treatment is guided by traditional clinicopathological parameters and individual molecular markers. Despite the remarkable advances in our scientific understanding of breast cancer genetics, the impact of such information on medical care has, to date, been modest. Although the use of simple genetics is already in vogue in clinical practice, the concept of molecular profiling and multiparameter gene classifiers was raised after the introduction of the high-throughput gene expression microarrays. This technology, in addition to highlighting the molecular heterogeneity of breast cancer, has led to the development of prognostic and predictive gene signatures. Studies are underway to assess the clinical validity and clinical utility of these multigene assays and their incorporation into clinical practice. This article reviews the current status and projected future use of genetics and genomics in breast cancer management and their impact on the refinement of risk stratification to permit individualized and patient-tailored therapy. Limitations based on our current scientific understanding and realistic expectations are also explored.

Genetic and proteomic approaches to identify cancer drug targets

While target-based small-molecule discovery has taken centre-stage in the pharmaceutical industry, there are many cancer-promoting proteins not easily addressed with a traditional target-based screening approach. In order to address this problem, as well as to identify modulators of biological states in the absence of knowing the protein target of the state switch, alternative phenotypic screening approaches, such as gene expression-based and high-content imaging, have been developed. With this renewed interest in phenotypic screening, however, comes the challenge of identifying the binding protein target(s) of small-molecule hits. Emerging technologies have the potential to improve the process of target identification. In this review, we discuss the application of genomic (gene expression-based), genetic (short hairpin RNA and open reading frame screening), and proteomic approaches to protein target identification.

Sex and dosing-time dependencies in irinotecan-induced circadian disruption

Circadian disruption accelerates malignant growth; thus, it should be avoided in anticancer therapy. The circadian disruptive effects of irinotecan, a topoisomerase I inhibitor, was investigated according to dosing time and sex. In previous work, irinotecan achieved best tolerability following dosing at zeitgeber time (ZT) 11 in male and ZT15 in female mice, whereas worst toxicity corresponded to treatment at ZT23 and ZT3 in male and female mice, respectively. Here, irinotecan (50 mg/kg intravenous [i.v.]) was delivered at the sex-specific optimal or worst circadian timing in male and female B6D2F1 mice. Circadian disruption was assessed with rest-activity, body temperature, plasma corticosterone, and liver mRNA expressions of clock genes Rev-erbα, Per2, and Bmal1. Baseline circadian rhythms in rest-activity, body temperature, and plasma corticosterone were more prominent in females as compared to males. Severe circadian disruption was documented for all physiology and molecular clock endpoints in female mice treated at the ZT of worst tolerability. Conversely, irinotecan administration at the ZT of best tolerability induced slight alteration of circadian physiology and clock-gene expression patterns in female mice. In male mice, irinotecan produced moderate alterations of circadian physiology and clock-gene expression patterns, irrespective of treatment ZT. However, the average expression of Rev-erbα, Per2, and Bmal1 were down-regulated 2- to 10-fold with irinotecan at the worst ZT, while being minimally or unaffected at the best ZT, irrespective of sex. Corticosterone secretion increased acutely within 2 h with a sex-specific response pattern, resulting in a ZT-dependent phase-advance or -delay in both sex. The mRNA expressions of irinotecan clock-controlled metabolism genes Ce2, Ugt1a1, and Top1 were unchanged or down-regulated according to irinotecan timing and sex. This study shows that the circadian timing system represents an important toxicity target of irinotecan in female mice, where circadian disruption persists after wrongly timed treatment. As a result, the mechanisms underling cancer chronotherapeutics are expectedly more susceptible to disruption in females as compared to males. Thus, the optimal circadian timing of chemotherapy requires precise determination according to sex, and should involve the noninvasive monitoring of circadian biomarkers.

High-throughput RNA interference screening using pooled shRNA libraries and next generation sequencing

RNA interference (RNAi) screening is a state-of-the-art technology that enables the dissection of biological processes and disease-related phenotypes. The commercial availability of genome-wide, short hairpin RNA (shRNA) libraries has fueled interest in this area but the generation and analysis of these complex data remain a challenge. Here, we describe complete experimental protocols and novel open source computational methodologies, shALIGN and shRNAseq, that allow RNAi screens to be rapidly deconvoluted using next generation sequencing. Our computational pipeline offers efficient screen analysis and the flexibility and scalability to quickly incorporate future developments in shRNA library technology.

Translational control of TOP2A influences doxorubicin efficacy

The cellular abundance of topoisomerase IIα (TOP2A) critically maintains DNA topology after replication and determines the efficacy of TOP2 inhibitors in chemotherapy. Here, we report that the RNA-binding protein HuR, commonly overexpressed in cancers, binds to the TOP2A 3'-untranslated region (3'UTR) and increases TOP2A translation. Reducing HuR levels triggered the recruitment of TOP2A transcripts to RNA-induced silencing complex (RISC) components and to cytoplasmic processing bodies. Using a novel MS2-tagged RNA precipitation method, we identified microRNA miR-548c-3p as a mediator of these effects and further uncovered that the interaction of miR-548c-3p with the TOP2A 3'UTR repressed TOP2A translation by antagonizing the action of HuR. Lowering TOP2A by silencing HuR or by overexpressing miR-548c-3p selectively decreased DNA damage after treatment with the chemotherapeutic agent doxorubicin. In sum, HuR enhances TOP2A translation by competing with miR-548c-3p; their combined actions control TOP2A expression levels and determine the effectiveness of doxorubicin.

Bcl-2 family genetic profiling reveals microenvironment-specific determinants of chemotherapeutic response

The Bcl-2 family encompasses a diverse set of apoptotic regulators that are dynamically activated in response to various cell-intrinsic and -extrinsic stimuli. An extensive variety of cell culture experiments have identified effects of growth factors, cytokines, and drugs on Bcl-2 family functions, but in vivo studies have tended to focus on the role of one or two particular members in development and organ homeostasis. Thus, the ability of physiologically relevant contexts to modulate canonical dependencies that are likely to be more complex has yet to be investigated systematically. In this study, we report findings derived from a pool-based shRNA assay that systematically and comprehensively interrogated the functional dependence of leukemia and lymphoma cells upon various Bcl-2 family members across many diverse in vitro and in vivo settings. This approach permitted us to report the first in vivo loss of function screen for modifiers of the response to a front-line chemotherapeutic agent. Notably, our results reveal an unexpected role for the extrinsic death pathway as a tissue-specific modifier of therapeutic response. In particular, our findings show that particular tissue sites of tumor dissemination play critical roles in demarcating the nature and extent of cancer cell vulnerabilities and mechanisms of chemoresistance.

Metastatic triple-negative breast cancer

The triple-negative class (oestrogen receptor-negative, progesterone receptor-negative and human epidermal growth factor receptor 2 [HER2]-negative) comprises about 15% of breast cancer. It is associated with a poor prognosis compared with tumours that are positive for hormone receptors or HER2. Despite being sensitive to chemotherapy, many women with metastatic triple-negative breast cancer (TNBC) relapse quickly, and commonly develop visceral metastasis, including lung, liver and brain metastasis. TNBC has molecular features that overlap with breast cancer in BRCA1 germline mutation carriers and with those of the basal-like molecular class of tumours. Furthermore, tumours with the triple-negative phenotype have specific features and express markers that are potential therapeutic targets, for example an impaired DNA repair mechanism and increased expression of proliferation and basal-associated markers. The presence of these features has important implications for clinical practice and for the design of clinical trials looking at novel therapies. Targeted agents that are currently being investigated include poly (ADP-ribose) polymerase inhibitors, epidermal growth factor receptor inhibitors and anti-angiogenic compounds. Here we discuss the epidemiology, morphological and molecular spectrum of TNBC, the clinical significance of this important class of breast cancer and the current treatment options.

Flt3-ITD alters chemotherapy response in vitro and in vivo in a p53-dependent manner

OBJECTIVE

The FLT3 internal tandem duplication (Flt3-ITD) confers a worse prognosis for patients with acute myeloid leukemia (AML); however, the mechanisms involved are unknown. As AML is treated with cytarabine (Ara-C) and an anthracycline, we sought to determine the effects of the Flt3-ITD on response to these agents.

MATERIALS AND METHODS

A genetically defined mouse model of AML was used to examine the effects of the Flt3-ITD on response to cytarabine and doxorubicin in vitro and in vivo.

RESULTS

In vitro, the Flt3-ITD conferred resistance to doxorubicin and doxorubicin plus Ara-C, but sensitivity to Ara-C alone. This resistance was reversible by the Flt3-ITD inhibitor sorafenib. The Flt3-ITD did not affect DNA damage levels after treatment, but was associated with increased levels of p53. The p53 response was critical to the observed changes as the Flt3-ITD had no effect on chemotherapy response in the setting of p53 null AML. In vivo, the Flt3-ITD accelerated engraftment that was partially reversible by Ara-C but not doxorubicin. Additionally, Ara-C provided a significant reduction in disease burden and a survival advantage that was not increased by the addition of doxorubicin. Doxorubicin alone led to only minimal disease reduction and no survival benefit.

CONCLUSIONS

These data demonstrate that the Flt3-ITD confers sensitivity to Ara-C, but resistance to doxorubicin in a manner that depends on p53. Thus, patients with Flt3-ITD positive AML may not benefit from treatment with an anthracycline.