Is resistance useless? Multidrug resistance and collateral sensitivity

Mechanisms and insights into drug resistance in cancer

Cancer drug resistance continues to be a major impediment in medical oncology. Clinically, resistance can arise prior to or as a result of cancer therapy. In this review, we discuss different mechanisms adapted by cancerous cells to resist treatment, including alteration in drug transport and metabolism, mutation and amplification of drug targets, as well as genetic rewiring which can lead to impaired apoptosis. Tumor heterogeneity may also contribute to resistance, where small subpopulations of cells may acquire or stochastically already possess some of the features enabling them to emerge under selective drug pressure. Making the problem even more challenging, some of these resistance pathways lead to multidrug resistance, generating an even more difficult clinical problem to overcome. We provide examples of these mechanisms and some insights into how understanding these processes can influence the next generation of cancer therapies.

Ethyl 2-amino-6-(3,5-dimethoxyphenyl)-4-(2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (CXL017): a novel scaffold that resensitizes multidrug resistant leukemia cells to chemotherapy

Multidrug resistance (MDR) is a major hurdle in the treatment of cancer, and there is a pressing need for new therapies. We have recently developed ethyl 2-amino-6-(3,5-dimethoxyphenyl)-4-(2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (CXL017), derived from a dual inhibitor of Bcl-2 and SERCA proteins, sHA 14-1, with selective cytotoxicity toward MDR cancer cell lines in vitro. In this study, we present new evidence for its therapeutic potential in treatment of MDR cancers and offer mechanistic insights toward its preferential targeting of drug-resistant cancer. CXL017 selectively suppressed the growth of tumors derived from the MDR cancer cell line, HL60/MX2, in vivo. In addition, even after chronic exposure to CXL017, HL60/MX2 failed to develop stable resistance to CXL017, whereas it acquired >2000-fold resistance to cytarabine (Ara-C), the major first-line chemotherapy for the treatment of acute myeloid leukemia (AML). Remarkably, instead of acquiring further cross-resistance, HL60/MX2 cells exposed to CXL017 were resensitized to standard therapies (10- to 100-fold). Western blotting analyses revealed that CXL017 exposure significantly down-regulated Mcl-1 and Bax and up-regulated Noxa, Bim, Bcl-X(L), SERCA2, and SERCA3 proteins, along with a reduction in endoplasmic reticulum (ER) calcium content. Given the well-established functions of Bcl-2 family proteins and ER calcium in drug resistance, our results suggest that the down-regulation of Mcl-1 and the up-regulation of Noxa and Bim along with the decrease in ER calcium content are likely responsible for CXL017-induced resensitization of MDR cancer cells. These data also demonstrate the unique potential of CXL017 to overcome MDR in cancer treatment.

Contributions of microRNA dysregulation to cisplatin resistance in adenocarcinoma cells

Cisplatin resistance in cancer cells is due to a pleiotropic phenotype transition that allows cells to resist cell death. miRNAs have been shown to be reliable markers of phenotype, critical in cell differentiation, and dysregulated in cancer and other pathologies. Here we investigate the influence of miRNA on cisplatin resistance in KB adenocarcinoma cells. Silencing both DICER and TRBP2 in the miRNA biosynthesis pathway in KB-3-1 (sensitive parental), KB-CP.5 (cisplatin-resistant), and KB-CP20 (highly cisplatin-resistant) cells resulted in the reversal of cisplatin resistance, with no effect on cell viability in the absence of cisplatin. We found miR-181 expression differences in the cell lines using RT-PCR, with several members of the miR-181 family overexpressed in two KB cisplatin-resistant lines and in two cisplatin-resistant lung cancer lines, compared to their respective parental cells. Functional assays showed minimal effects of miR-181 on cisplatin resistance. We conclude that the miRNA biosynthesis pathway is critical for maintaining the cisplatin-resistant phenotype, but that it is difficult to determine the precise miRNAs involved in cisplatin resistance simply using expression profiles of individual miRNA species. Functional assays are needed to determine the influence of a specific miRNA and different members of the same miRNA family may have opposite effects.

New insight into hyperthermic intraperitoneal chemotherapy: induction of oxidative stress dramatically enhanced tumor killing in in vitro and in vivo models

BACKGROUND

The aim of hyperthermic intraperitoneal chemotherapy (HIPEC) is to eradicate microscopic residual tumor after radical surgical tumor excision in patients with peritoneal carcinomatosis. The common use of antineoplastic agents such as mitomycin C, doxorubicin, or oxaliplatin with hyperthermia fails to eradicate tumors in a significant subset of patients, and alternative approaches to target chemoresistant cells are needed. The induction of reactive oxygen species (ROS) by inhibiting the critical detoxification enzyme superoxide dismutase (SOD) during hyperthermia is an appealing approach to induce death of residual cancer cells.

METHODS

Human and murine colon cancer cell lines were subjected to mild hyperthermia (40-42°C), and treated with chemotherapy, similar to clinical protocols. ROS were induced by the SOD inhibitor diethyldithiocarbamate (DDC), a metabolite of the drug disulfiram. In mice, peritoneal carcinomatosis use C57Bl/6 was induced in C57Bl/6 by intraperitoneal injection of syngenic tumor cells (MC38).

RESULTS

Hyperthermia alone failed to kill cells but induced intracellular ROS and activated protective mechanisms. Chemotherapy conferred inconsistent cytotoxicity depending on the cell line and dose. In contrast, induction of ROS by DDC consistently activated apoptotic pathways, with increased cell death in combination with mild hyperthermia. In vivo, combined treatment with DDC and hyperthermia significantly delayed tumor progression in tumor-bearing mice. In addition, hyperthermic combined treatment with chemotherapy and DDC significantly improved animal survival compared with chemotherapy alone.

CONCLUSIONS

Addition of DDC improves the efficacy of existing HIPEC protocols in a safe way and may open the door to a more effective, multimodal HIPEC.

Jatrophane diterpenes from Euphorbia mellifera and their activity as P-glycoprotein modulators on multidrug-resistant mouse lymphoma and human colon adenocarcinoma cells

Three new macrocyclic jatrophane diterpenes, named euphomelliferine (1) and euphomelliferenes A (2) and B (3), and one new tetracyclic triterpene, 19(10→9)-abeo-8α,9β,10α-tirucalla-5,25-diene-3β,24-diol (6, C-24 epimers), were isolated from the methanolic extract of Euphorbia mellifera. A known ingenane (7) and two jatrophane diterpenes (4 and 5) were also isolated. Their structures were elucidated by extensive spectroscopic methods, including 1D and 2D homo- and heteronuclear NMR experiments. Jatrophane diterpenes 1-3 and 5 were evaluated for their effects on the reversion of multidrug resistance (MDR) mediated by P-glycoprotein, by using the rhodamine-123 exclusion test, on human MDR1 gene-transfected mouse lymphoma cells (L5178Y MDR) and on human colon adenocarcinoma cells (COLO 320). The apoptosis-inducing activity of these compounds was also tested on COLO 320 cells, using the annexin-V/propidium iodide assay. Diterpenes 1 and 2 displayed significant MDR reversing activity, in a dose-dependent manner, on both cancer cell models. The tested compounds did not induce apoptosis in the COLO 320 cells.

Oleanolic acid derivative methyl 3,11-dioxoolean-12-en-28-olate targets multidrug resistance related to ABCB1

Multidrug resistance (MDR) in leukemia patients is a great incentive to the development of new drugs. In a search for potential multidrug resistance modulators we tested a group of oleanolic acid (OA) analogues modified at C-3, C-11, C-12 and C-28 using an experimental model consisting of three human acute lymphoblastic leukemia cell lines (CCRF-CEM and the multidrug resistant sublines CCRF-VCR1000 and CCRF-ADR5000). The most effective compound, methyl 3,11-dioxoolean-12-en-28-olate (DIOXOL) was more potent in cell viability inhibition than its precursor - OA, and showed similar or even higher activity in the drug resistant than in the wild-type cells. Resistance factor (RF) values obtained for CCRF-VCR1000 and CCRF-ADR-5000 cells using MTT assay were 0.7 and 0.8 (24 h of treatment) and after 72 h of treatment 0.9 and 1.1, respectively. Moreover, 5 μM DIOXOL significantly reduced the expression of the ABCB1 gene in MDR cells by around 30%, and also decreased the level of P-gp protein. Compared to untreated control cells, DIOXOL treatment resulted in a significant P-gp decrease (30% in CCRF-ADR5000 and 50% in CCRF-VCR1000), that was detected by western blot and confirmed by flow cytometry analysis. Moreover, DIOXOL (at 10 μM) significantly inhibited P-gp transport function by more than twofold comparing to control, untreated cells that was demonstrated using rhodamine 123-based functional test. The compound exhibited synergistic activity with ABCB1 substrate - adriamycin in CCRF-VCR1000 cells, indicating partial but significant MDR reversing ability.

Acquisition of MDR phenotype by leukemic cells is associated with increased caspase-3 activity and a collateral sensitivity to cold stress

The acquisition of a multidrug-resistant (MDR) phenotype by tumor cells that renders them unsusceptible to anti-neoplasic agents is one of the main causes of chemotherapy failure in human malignancies. The increased expression of P-glycoprotein (MDR1, P-gp, ABCB1) in tumor cells contributes to drug resistance by extruding chemotherapeutic agents or by regulating programmed cell death. In a study of MDR cell survival under cold stress conditions, it was found that resistant leukemic cells with P-gp over-expression, but not their sensitive counterparts, are hypersensitive to cold-induced cell death when exposed to temperatures below 4 °C. The transfection of parental cells with a P-gp-expressing plasmid makes these cells sensitive to cold stress, demonstrating an association between P-gp expression and cell death at low temperatures. Furthermore, we observed increased basal expression and activity of effector caspase-3 at physiological temperature (37 °C) in MDR cells compared with their parental cell line. Treatment with a caspase-3 inhibitor partially rescues MDR leukemic cells from cold-induced apoptosis, which suggests that the cell death mechanism may require caspase-3 activity. Taken together, these findings demonstrate that P-gp expression plays a role in MDR cell survival, and is accompanied by a collateral sensitivity to death induced by cold stress. These findings may assist in the design of specific therapeutic strategies to complement current chemotherapy treatment against cancer.

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.

A new anticancer compound, oblongifolin C, inhibits tumor growth and promotes apoptosis in HeLa cells through Bax activation

Oblongifolin C (OC) was identified as a potent apoptosis inducer from an herbal plant, Garcinia yunnanensis, during our previous bioassay-guided drug screening. In this study, we investigated the signaling pathways through which OC activated apoptosis in HeLa cells. We also compared the IC(50) values of OC with that of etoposide, paclitaxel and vinblastine in multiple cancer cell lines including HER2 and P-glycoprotein overexpressing cells. In addition, the in vivo antitumor effect of OC was studied in nude mice model. Our results showed that OC induced a caspase-dependent apoptosis by triggering a series of events in HeLa cells including Bax translocation, cytochrome c release, caspase-3 activation, chromosome fragmentation followed by caspase-8 activation, Bid cleavage and eventually cell death. Addition of a pan-caspase inhibitor or overexpression of an anti-apoptotic protein, Bcl-xL, prevented OC-induced cell death. Moreover, OC exhibited a wide anticancer spectrum in multiple cancer cell lines with comparable IC(50) values, regardless of the expression levels of HER2 and P-glycoprotein. In contrast, the IC(50) values of three clinical anticancer drugs, etoposide, paclitaxel and vinblastine were significantly elevated in HER2 and/or P-glycoprotein overexpressing cells. Furthermore, OC showed a similar antitumor effect but lower general toxicity than etoposide against xenografted human tumors in nude mice model. All these data suggested that OC is a promising apoptosis inducer with the potential to be developed into a clinical anticancer drug.

Cisplatin sensitivity mediated by WEE1 and CHK1 is mediated by miR-155 and the miR-15 family

Resistance to platinum-based therapies arises by multiple mechanisms, including by alterations to cell-cycle kinases that mediate G(2)-M phase arrest. In this study, we conducted parallel high-throughput screens for microRNAs (miRNA) that could restore sensitivity to cisplatin-resistant cells, and we screened for kinases targeted by miRNAs that mediated cisplatin resistance. Overexpression of the cell-cycle kinases WEE1 and CHK1 occurred commonly in cisplatin-resistant cells. miRNAs in the miR-15/16/195/424/497 family were found to sensitize cisplatin-resistant cells to apoptosis by targeting WEE1 and CHK1. Loss-of-function and gain-of-function studies showed that miR-15 family members controlled the expression of WEE1 and CHK1. Supporting these results, we found that in the presence of cisplatin altering expression of miR-16 or related genes altered cell cycle distribution. Our findings reveal critical regulation of miRNAs and their cell-cycle-associated kinase targets in mediating resistance to cisplatin.

Investigational ABC transporter inhibitors

INTRODUCTION

Multidrug resistance (MDR) is the main cause of failure in cancer therapy. One mechanism responsible for MDR is the active efflux of drugs by ATP-binding cassette (ABC) transporters. Several agents have been developed to block transporter-mediated drug efflux and some of these compounds have entered Phase II/III clinical testing. Evidence is also emerging of the role played by ABC transporters in cancer cell signalling that is likely to be important in disease progression and which is distinct from MDR.

AREAS COVERED

This article reviews current literature to analyse the rationale for targeting ABC transporters in cancer. Preclinical and clinical results of ABC transporter inhibitors in early clinical trials, as single agents or in combination with other drugs, are described. The development of new strategies to target MDR and the emerging roles of ABC transporters in cancer signalling are discussed.

EXPERT OPINION

The intense active search for safe and effective inhibitors of ABC transporters has led to some success in MDR reversal in preclinical studies. However, there has been little impact on clinical outcome. The discovery of novel, potent and nontoxic inhibitors as well as new treatment strategies is therefore needed.

New artesunic acid homodimers: potent reversal agents of multidrug resistance in leukemia cells

To evade the problem of multidrug resistance, hybridization of natural products in dimers is considered as an effective method. After the successful synthesis of three artesunic acid homodimers connected by different types of chemical linkers, we analyzed their activity against human CCRF-CEM and multidrug-resistant P-glycoprotein-overexpressing CEM/ADR 5000 leukemia cells and observed, that multidrug resistant cells were not cross-resistant to the new compounds. Collateral sensitivity was observed for artesunic acid homodimer 2. The obtained results deliver valuable information about the linker's structure which is required for homodimers to be highly cytotoxic.

P-glycoprotein and its inhibition in tumors by phytochemicals derived from Chinese herbs

P-glycoprotein belongs to the family of ATP-binding cassette (ABC) transporters. It functions in cellular detoxification, pumping a wide range of xenobiotic compounds, including anticancer drugs out of the cell. In cancerous cells, P-glycoprotein confers resistance to a broad spectrum of anticancer agents, a phenomenon termed multidrug resistance. An attractive strategy for overcoming multidrug resistance is to block the transport function of P-glycoprotein and thus increase intracellular concentrations of anticancer drugs to lethal levels. Efforts to identify P-glycoprotein inhibitors have led to numerous candidates, none of which have passed clinical trials with cancer patients due to their high toxicity. The search for naturally inhibitory products from traditional Chinese medicine may be more promising because natural products are frequently less toxic than chemically synthesized substances. In this review, we give an overview of molecular and clinical aspects of P-glycoprotein and multidrug resistance in the context of cancer as well as Chinese herbs and phytochemicals showing inhibitory activity towards P-glycoprotein.

Structure-activity relationship (SAR) study of ethyl 2-amino-6-(3,5-dimethoxyphenyl)-4-(2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate (CXL017) and the potential of the lead against multidrug resistance in cancer treatment

Multidrug resistance (MDR) against standard therapies poses a serious challenge in cancer treatment, and there is a clinical need for new anticancer agents that would selectively target MDR malignancies. Our previous studies have identified a 4H-chromene system, CXL017 (4) as an example, that can preferentially kill MDR cancer cells. To further improve its potency, we have performed detailed structure-activity relationship (SAR) studies at the 3, 4, and 6 positions of the 4H-chromene system. The results reveal that the 3 and 4 positions prefer rigid and hydrophobic functional groups while the 6 position prefers a meta or para-substituted aryl functional group and the substituent should be small and hydrophilic. We have also identified and characterized nine MDR cancer cells that acquire MDR through different mechanisms and demonstrated the scope of our new lead, 9g, to selectively target different MDR cancers, which holds promise to help manage MDR in cancer treatment.

Multidrug-resistant cells overexpressing P-glycoprotein are susceptible to DNA crosslinking agents due to attenuated Src/nuclear EGFR cascade-activated DNA repair activity

We synthesized several novel bifunctional alkylating derivatives of 3a-aza-cyclopenta[a]indene (BO-1012, BO-1005, BO-1099 and BO-1101) that are potent DNA interstrand crosslinking agents. In in vitro cytotoxicity assay, these compounds were more cytotoxic to multidrug-resistant (MDR) cells, such as KBvin10, KBtax50 and CEM/VBL, than their parental cells. Using a xenograft model, BO-1012, at a dose of 5 mg/kg, partially suppressed the growth of parental KB cells but completely suppressed the growth of KBvin10 cells in nude mice. In exploring the possible mechanism, we found that DNA double-strand break (DSB) repair activity in MDR cells, KBvin10 and CEM/VBL, was significantly reduced compared with their parental cells, KB and CEM. Reduced DSB repair activity in KBvin10 cells was likely due to a defect in nuclear translocation of DNA-dependent protein kinase (DNA-PK), a component of the non-homologous end-joining repair machinery. Furthermore, BO-1012-induced DNA-PK translocation from the cytosol into the nucleus in KB cells is associated with the activation of the Src/nuclear epidermal growth factor receptor (EGFR) cascade, which is defective in MDR cells. As knockdown of P-glycoprotein (P-gp) by siRNA reactivated the Src/nuclear EGFR cascade, DNA-PK translocation and DNA repair activity in MDR cells, overexpression of P-gp attenuates the activity of DNA DSB repair through suppression of Src/nuclear EGFR cascade. Therefore, DNA interstrand crosslinking agents may have potential therapeutic use against P-gp-overexpressing MDR cells.

Enhanced brain disposition and effects of Δ9-tetrahydrocannabinol in P-glycoprotein and breast cancer resistance protein knockout mice

The ABC transporters P-glycoprotein (P-gp, Abcb1) and breast cancer resistance protein (Bcrp, Abcg2) regulate the CNS disposition of many drugs. The main psychoactive constituent of cannabis Δ(9)-tetrahydrocannabinol (THC) has affinity for P-gp and Bcrp, however it is unknown whether these transporters modulate the brain accumulation of THC and its functional effects on the CNS. Here we aim to show that mice devoid of Abcb1 and Abcg2 retain higher brain THC levels and are more sensitive to cannabinoid-induced hypothermia than wild-type (WT) mice. Abcb1a/b (-/-), Abcg2 (-/-) and wild-type (WT) mice were injected with THC before brain and blood were collected and THC concentrations determined. Another cohort of mice was examined for THC-induced hypothermia by measuring rectal body temperature. Brain THC concentrations were higher in both Abcb1a/b (-/-) and Abcg2 (-/-) mice than WT mice. ABC transporter knockout mice exhibited delayed elimination of THC from the brain with the effect being more prominent in Abcg2 (-/-) mice. ABC transporter knockout mice were more sensitive to THC-induced hypothermia compared to WT mice. These results show P-gp and Bcrp prolong the brain disposition and hypothermic effects of THC and offer a novel mechanism for both genetic vulnerability to the psychoactive effects of cannabis and drug interactions between CNS therapies and cannabis.

Collateral sensitivity as a strategy against cancer multidrug resistance

While chemotherapy remains the most effective treatment for disseminated tumors, acquired or intrinsic drug resistance accounts for approximately 90% of treatment failure. Multidrug resistance (MDR), the simultaneous resistance to drugs that differ both structurally and mechanistically, often results from drug efflux pumps in the cell membrane that reduce intracellular drug levels to less than therapeutic concentrations. Expression of the MDR transporter P-glycoprotein (P-gp, MDR1, ABCB1) has been shown to correlate with overall poor chemotherapy response and prognosis. This review will focus on collateral sensitivity (CS), the ability of compounds to kill MDR cells selectively over the parental cells from which they were derived. Insights into CS may offer an alternative strategy for the clinical resolution of MDR, as highly selective and potent CS agents may lead to drugs that are effective at MDR cell killing and tumor resensitization. Four main mechanistic hypotheses for CS will be reviewed, followed by a discussion on quantitative and experimental evaluation of CS.

Inhibition of P-glycoprotein by two artemisinin derivatives

P-Glycoprotein/MDR1 represents an important component of the blood brain barrier and contributes to multidrug resistance. We investigated two derivatives of the anti-malarial artemisinin, SM616 and GHP-AJM-3/23, concerning their ability to interact with P-glycoprotein. The ability of the two compounds to inhibit P-glycoprotein (P-gp) activity was examined in sensitive CCRF-CEM and P-gp over-expressing and multidrug-resistant CEM/ADR5000 cells as well as in porcine brain capillary endothelial cells (PBCEC) by means of calcein-AM assays. Verapamil as well-known P-gp inhibitor was used as control drug. CEM/ADR5000 cells exhibited cross-resistance to GHP-AJM-3/23, but slight collateral sensitivity to SM616. Furthermore, SM616 inhibited calcein efflux both in CEM/ADR5000 and PBCEC, whereas GHP-AJM-3/23 did only increase calcein fluorescence in PBCEC, but not CEM/ADR5000. This may be explained by the fact that CEM/ADR5000 only express P-gp but not other ATP-binding cassette transporters, whereas PBCEC are known to express several ABC transporters and calcein is transported by more than one ABC transporter. Hence, SM616 may be the more specific P-gp inhibitor. In conclusion, the collateral sensitivity of SM616 as well as the inhibition of calcein efflux in both CEM/ADR5000 cells and PBCEC indicate that this compound may be a promising P-gp inhibitor to treat cancer therapy and to overcome the blood brain barrier.

Starvation, detoxification, and multidrug resistance in cancer therapy

The selection of chemotherapy drugs is based on the cytotoxicity to specific tumor cell types and the relatively low toxicity to normal cells and tissues. However, the toxicity to normal cells poses a major clinical challenge, particularly when malignant cells have acquired resistance to chemotherapy. This drug resistance of cancer cells results from multiple factors including individual variation, genetic heterogeneity within a tumor, and cellular evolution. Much progress in the understanding of tumor cell resistance has been made in the past 35 years, owing to milestone discoveries such as the identification and characterization of ABC transporters. Nonetheless, the complexity of the genetic and epigenetic rewiring of cancer cells makes drug resistance an equally complex phenomenon that is difficult to overcome. In this review, we discuss how the remarkable changes in the levels of glucose, IGF-I, IGFBP-1 and in other proteins caused by fasting have the potential to improve the efficacy of chemotherapy against tumors by protecting normal cells and tissues and possibly by diminishing multidrug resistance in malignant cells.

Cancer: beyond speciation

A good account of the nature of cancer should provide not only a description of its consistent features, but also how they arise, how they are maintained, why conventional chemotherapy succeeds, and fails, and where to look for better targets. Cancer was once regarded as enigmatic and inexplicable; more recently, the "mutation theory," based on random alterations in a relatively small set of proto-oncogenes and tumor suppressor genes, has enjoyed widespread acceptance. The "mutation theory," however, is noticeable for its failure to explain the basis of differential chemosensitivity, for providing a paucity of targets, especially druggable ones, and for justifying the development of targeted therapies with, in general, disappointingly abbreviated clinical benefit. Furthermore, this theory has mistakenly predicted a widespread commonality of consistent genetic abnormalities across the range of cancers, whereas the opposite, that is, roiling macrogenomic instability, is generally the rule. In contrast, concerning what actually is consistent, that is, the suite of metabolic derangements common to virtually all, especially aggressive, cancers, the "Mutation Theory" has nothing to say. Other hypotheses merit serious consideration "aneuploidy theories" posit whole-genome instability and imbalance as causally responsible for the propagation of the tumor. Another approach, that is, "derepression atavism," suggests cancer results from the release of an ancient survival program, characterized by the emergence of remarkably primitive features such as unicellularity, fermentation, and immortality; existential goals are served by heuristic genomic instability coupled with host-to-tumor biomass interconversion, mediated by the Warburg effect, a major component of the program. Carcinogenesis is here seen as a process of de-speciation; however, genomic nonrestabilization raises issues as to where on the tree of life cancers belong, as a genuinely alternative modus vivendi. Philosophical considerations aside, genomic instability offers the prospect of subtle new therapies based on loss of information rather than gain; and the consistent, specific, and broad-spectrum perfidy of the Warburg effect highlights a supplemental target of the highest priority.

Polo Like Kinase 2 Tumour Suppressor and cancer biomarker: new perspectives on drug sensitivity/resistance in ovarian cancer

The polo-like kinase PLK2 has recently been identified as a potential theranostic marker in the management of chemotherapy sensitive cancers. The methylation status of the PLK2 CpG island varies with sensitivity to paclitaxel and platinum in ovarian cancer cell lines. Importantly, extrapolation of these in vitro data to the clinical setting confirms that the methylation status of the PLK2 CpG island predicts outcomes in patients treated with carboplatin and paclitaxel chemotherapy. A second cell cycle regulator, p57Kip2, is also subject to epigenetic silencing in carboplatin resistance in vitro and in vivo, emphasising that cell cycle regulators are important determinants of sensitivity to chemotherapeutic agents and providing insights into the phenomenon of collateral drug sensitivity in oncology. Understanding the mechanistic basis and identification of robust biomarkers to predict collateral sensitivity may inform optimal use of chemotherapy in patients receiving multiple lines of treatment.

MVP and vaults: a role in the radiation response

Vaults are evolutionary highly conserved ribonucleoproteins particles with a hollow barrel-like structure. The main component of vaults represents the 110 kDa major vault protein (MVP), whereas two minor vaults proteins comprise the 193 kDa vault poly(ADP-ribose) polymerase (vPARP) and the 240 kDa telomerase-associated protein-1 (TEP-1). Additionally, at least one small and untranslated RNA is found as a constitutive component. MVP seems to play an important role in the development of multidrug resistance. This particle has also been implicated in the regulation of several cellular processes including transport mechanisms, signal transmission and immune responses. Vaults are considered a prognostic marker for different cancer types. The level of MVP expression predicts the clinical outcome after chemotherapy in different tumour types. Recently, new roles have been assigned to MVP and vaults including the association with the insulin-like growth factor-1, hypoxia-inducible factor-1alpha, and the two major DNA double-strand break repair machineries: non-homologous endjoining and homologous recombination. Furthermore, MVP has been proposed as a useful prognostic factor associated with radiotherapy resistance. Here, we review these novel actions of vaults and discuss a putative role of MVP and vaults in the response to radiotherapy.

Synthesis and structure-activity evaluation of isatin-β-thiosemicarbazones with improved selective activity toward multidrug-resistant cells expressing P-glycoprotein

Cancer multidrug resistance (MDR) mediated by ATP-binding cassette (ABC) transporters presents a significant unresolved clinical challenge. One strategy to resolve MDR is to develop compounds that selectively kill cells overexpressing the efflux transporter P-glycoprotein (MDR1, P-gp, ABCB1). We have previously reported structure-activity studies based around the lead compound NSC73306 (1, 1-isatin-4-(4'-methoxyphenyl)-3-thiosemicarbazone, 4.3-fold selective). Here we sought to extend this work on MDR1-selective analogues by establishing whether 1 showed "robust" activity against a range of cell lines expressing P-gp. We further aimed to synthesize and test analogues with varied substitution at the N4-position, and substitution around the N4-phenyl ring of isatin-β-thiosemicarbazones (IBTs), to identify compounds with increased MDR1-selectivity. Compound 1 demonstrated MDR1-selectivity against all P-gp-expressing cell lines examined. This selectivity was reversed by inhibitors of P-gp ATPase activity. Structural variation at the 4'-phenyl position of 1 yielded compounds of greater MDR1-selectivity. Two of these analogues, 1-isatin-4-(4'-nitrophenyl)-3-thiosemicarbazone (22, 8.3-fold selective) and 1-isatin-4-(4'-tert-butyl phenyl)-3-thiosemicarbazone (32, 14.8-fold selective), were selected for further testing and were found to retain the activity profile of 1. These compounds are the most active IBTs identified to date.

A novel screen using the Reck tumor suppressor gene promoter detects both conventional and metastasis-suppressing anticancer drugs

The membrane-anchored matrix metalloproteinase-regulator RECK is often downregulated in various types of cancers; the levels of residual RECK in resected tumors often correlate with better prognosis. Forced expression of RECK in cancer cells suppresses tumor angiogenesis, invasion, and metastasis in xenograft models. RECK is therefore a promising marker for benignancy and a potential effector in cancer therapy. We established a cell line containing two transgene systems: (1) the secreted alkaline phosphatase (SEAP) gene fused to Reck promoter and (2) the HRAS(12V) oncogene driven by the Tet-off promoter system. This cell line exhibits transformed phenotype in regular medium and flat morphology with increased SEAP activity in the presence of doxycycline, allowing the assessment of RECK-inducing activity of chemicals in the contexts of both transformed and untransformed cells. Our pilot experiments with 880 known bioactive compounds detected 34 compounds that activate RECK promoter; among these, 10 were authentic anticancer drugs. Four selected compounds up-regulated endogenous RECK protein in several human cancer cell lines. The top-ranking compound, disulfiram, strongly suppressed spontaneous lung-metastasis of human fibrosarcoma cells in nude mice. Our data demonstrate the value of this screen in discovering effective cancer therapeutics.

Interactions of dietary phytochemicals with ABC transporters: possible implications for drug disposition and multidrug resistance in cancer

Common foods, such as fruits and vegetables, contain a large variety of secondary metabolites known as phytochemicals, many of which have been associated with health benefits. However, there is a limited knowledge of the processes by which these, mainly charged, phytochemicals (and/or their metabolites) are absorbed into the body, reach their biological target, and how they are eliminated. Recent studies have indicated that some of these phytochemicals are substrates and modulators of specific members of the superfamily of ABC transporting proteins. In this review, we present the reported interactions between the different classes of phytochemicals and ABC transporters and the mechanism by which they modulate the activity of these transporters. We also discuss the implications that such interactions may have on the pharmacokinetics of xenobiotics and the possible role of phytochemicals in the reversal of multidrug resistance in cancer chemotherapy.

Collateral sensitivity of multidrug-resistant cells to the orphan drug tiopronin

A major challenge in the treatment of cancer is multidrug resistance (MDR) that develops during chemotherapy. Here we demonstrate that tiopronin (1), a thiol-substituted N-propanoylglycine derivative, was selectively toxic to a series of cell lines expressing the drug efflux pump P-glycoprotein (P-gp, ABCB1) and MRP1 (ABCC1). Treatment of MDR cells with 1 led to instability of the ABCB1 mRNA and consequently a reduction in P-gp protein, despite functional assays demonstrating that tiopronin does not interact with P-gp. Long-term exposure of P-gp-expressing cells to 1 sensitized them to doxorubicin and paclitaxel, both P-gp substrates. Treatment of MRP1-overexpressing cells with tiopronin led to a significant reduction in MRP1 protein. Synthesis and screening of analogues of tiopronin demonstrated that the thiol functional group was essential for collateral sensitivity while substitution of the amino acid backbone altered but did not destroy specificity, pointing to future development of targeted analogues.

P-glycoprotein antagonists confer synergistic sensitivity to short-chain ceramide in human multidrug-resistant cancer cells

P-glycoprotein (P-gp) antagonists inhibit ceramide metabolism at the juncture of glycosylation. The purpose of this study was to test whether targeting P-gp would be a viable alternative to targeting glucosylceramide synthase (GCS) for enhancing ceramide cytotoxicity. A2780 wild-type, and multidrug-resistant 2780AD and NCI/ADR-RES human ovarian cancer cell lines and the cell-permeable ceramide analog, C6-ceramide (C6-cer), were employed. Compared to P-gp-poor A2780 cells, P-gp-rich 2780AD cells converted 3.7-fold more C6-cer to nontoxic C6-glucosylceramide (C6-GC), whereas cell-free GCS activities were equal. 2780AD cells displayed resistance to C6-cer (10 μM) that was reversed by inclusion of the P-gp antagonist tamoxifen (5 μM) but not by inclusion of a GCS inhibitor. Co-administration of C6-cer and P-gp antagonists was also effective in NCI/ADR-RES cells. For example, C6-cer, VX-710 (Biricodar), and cyclosporin A (cyc A) exposure resulted in viabilities of ~90% of control; however, C6-cer/VX-710 and C6-cer/cyc A additions were synergistic and resulted in viabilities of 22% and 17%, respectively. Further, whereas C6-ceramide and cyc A imparted 1.5- and 0-fold increases in caspase 3/7 activity, the combination produced a 3.5-fold increase. Although the upstream elements of cell death have not been elucidated, the novel C6-ceramide/P-gp antagonist combination merits further study and assessment of clinical translational potential.

Overcoming tumor multidrug resistance using drugs able to evade P-glycoprotein or to exploit its expression

Multidrug resistance (MDR) is a major obstacle to the effective treatment of cancer. Cellular overproduction of P-glycoprotein (P-gp), which acts as an efflux pump for various anticancer drugs (e.g. anthracyclines, Vinca alkaloids, taxanes, epipodophyllotoxins, and some of the newer antitumor drugs) is one of the more relevant mechanisms underlying MDR. P-gp belongs to the superfamily of ATP-binding cassette transporters and is encoded by the ABCB1 gene. Its overexpression in cancer cells has become a therapeutic target for circumventing MDR. As an alternative to the classical pharmacological strategy of the coadministration of pump inhibitors and cytotoxic substrates of P-gp and to other approaches applied in experimental tumor models (e.g. P-gp-targeting antibodies, ABCB1 gene silencing strategies, and transcriptional modulators) and in the clinical setting (e.g. incapsulation of P-gp substrate anticancer drugs into liposomes or nanoparticles), a more intriguing strategy for circumventing MDR is represented by the development of new anticancer drugs which are not substrates of P-gp (e.g. epothilones, second- and third-generation taxanes and other microtubule modulators, topoisomerase inhibitors). Some of these drugs have already been tested in clinical trials and, in most of cases, show relevant activity in patients previously treated with anticancer agents which are substrates of P-gp. Of these drugs, ixabepilone, an epothilone, was approved in the United States for the treatment of breast cancer patients pretreated with an anthracycline and a taxane. Another innovative approach is the use of molecules whose activity takes advantage of the overexpression of P-gp. The possibility of overcoming MDR using the latter two approaches is reviewed herein. 

A unique P-glycoprotein interacting agent displays anticancer activity against hepatocellular carcinoma through inhibition of GRP78 and mTOR pathways

P-glycoprotein (P-gp) overexpression has been demonstrated in many malignancies being a predominant mechanism by which cancer cells develop multidrug resistance. Several categories of P-gp inhibitors have been demonstrated to potentiate anticancer effect induced by cancer chemotherapeutic drugs through competitive inhibition of P-gp pumping activity. Few studies show the agent that selectively acts on P-gp and, by itself, causes cell apoptosis while remain P-gp-deficient cells unaffected. KNG-I-322, a desmosdumotin B derivative, displayed a direct interaction with P-gp and demonstrated selective anti-proliferative and apoptotic activities in P-gp overexpressed Hep3B/VIN other than P-gp-deficient Hep3B cells. KNG-I-322 induced an inhibitory effect on the phosphorylation of mTOR(Ser2448), p70S6K(Thr389) and 4E-BP(Thr37/46) in Hep3B/VIN but not Hep3B cells. The inhibition was fully blocked by the knockdown of P-gp using siRNA techniques. Notably, the P-gp inhibitor, verapamil, also directly interacted with P-gp but significantly diminished KNG-I-322-induced anti-proliferative activity. After the mechanism study, the data showed that KNG-I-322 induced a dramatic down-regulation of GRP78 expression, which was significantly inhibited by verapamil and completely diminished by the knockdown of P-gp. The protein profile analysis of detergent resistant membranes showed that upon the stimulation by KNG-I-322, the level of P-gp expression in non-raft fractions was dramatically increased and, concomitantly, the GRP78 expression was significantly decreased. Taken together, the data suggest that KNG-I-322 induces anticancer activity in Hep3B/VIN cells through a direct interaction with P-gp, leading to the inhibition of mTOR pathways and the induction of GRP78 down-regulation. The data support that KNG-I-322 is a selective anticancer agent against P-gp-overexpressed other than P-gp-deficient cancer cells.

Antitumor agents. 284. New desmosdumotin B analogues with bicyclic B-ring as cytotoxic and antitubulin agents

We previously reported that the biological activity of analogues of desmosdumotin B (1) was dramatically changed depending on the B-ring system. A naphthalene B-ring analogue 3 exerted potent in vitro activity against a diverse panel of human tumor cell lines with GI(50) values of 0.8-2.1 μM. In contrast, 1 analogues with a phenyl B-ring showed unique selective activity against P-glycoprotein (P-gp) overexpressing multidrug resistant cell line. We have now prepared and evaluated 1 analogues with bicyclic or tricyclic aromatic B-ring systems as in vitro inhibitors of human cancer cell line proliferation. Among all synthesized derivatives, 21 with a benzo[b]thiophenyl B-ring was highly active, with GI(50) values of 0.06-0.16 μM, and this activity was not influenced by overexpression of P-gp. Furthermore, 21 inhibited tubulin assembly in vitro with an IC(50) value of 2.0 μM and colchicine binding by 78% as well as cellular microtubule polymerization and spindle formation.

Antitumor Agents 291 Expanded B-Ring Modification Study of 6,8,8-Triethyl Desmosdumotin B Analogues as Multidrug-Resistance Selective Agents

Drug usefulnessis frequently obstructed by the incidence of the multidrug resistance (MDR) phenotype and severe adverse effects. Exploiting collateral sensitive(CS)agents (in this case also called MDR-selective agents), which selectively target only MDR cells, is an emerging and novel approach to overcome MDR in cancer treatment. In prior studies, we found that 4'-methyl-6,6,8-triethyldesmosdumotin B (4'-Me-TEDB, 2) is an MDR-selective synthetic flavonoid with significant in vitro anticancer activity against a MDR cell line (KB-Vin) but without activity against the parent cells (KB) as well as other non-MDR tumor cells. Our recent results suggest the absolute MDR-selectivity varies depending on the cell-line system. In order to explore this further and to better understand the critical pharmacophores, we have synthesized nine novel analogues of 2, which contain heteroaromatic as well ascycloalkyl B-rings. The new compounds were evaluated for cytotoxicity to explore the effect of B-ring modifications on MDR-selectivity. All analogues, except 7, 9 and 10, were identified as significant MDR-selective compounds. This observation solidifies the importance of the 5-hydroxy-6,8,8-trialkyl-4H-chromene-4,7(8H)-dione skeleton (AC-ring system) for the pharmacological activity and establishes the B-ring as less critical for the broader spectrum MDR-selectivity. Notably, 3-furanyl (3)and 2-thiophenyl (6)analogues displayed substantial MDR-selectivity with KB/KB-Vin ratios of >12 and 16, respectively. Furthermore, 3 and 6 also exhibited MDR-selectivity in a second set of paired cell lines, the MDR/non-MDR hepatoma-cell system. Interestingly, a cyclohexyl analogue (11) showed moderate inhibition of A549, DU145, and PC-3 cell growth, while the other compounds were inactive. These new findings are discussed in terms of current understanding of mechanism and structure-activity relationship (SAR) of our novel MDR-selective flavonoids.

Karavilagenin C derivatives as antimalarials

Karavilagenin C (1), a cucurbitane-type triterpenoid, previously isolated from the aerial parts of Momordica balsamina, was acylated with different alkanoyl, aroyl and cinnamoyl chlorides/anydrides, yielding ten new mono or diesters, karavoates F (7) and H-P (8-16). Furthermore, the new compound cucurbalsaminol C (17) was isolated from the same plant. Their structures were assigned by spectroscopic methods, including 2D NMR experiments. Compounds 1 and 17 and the acyl derivatives 8-16 along with other five esters (2-6, karavoates A-E), previously prepared from 1, were evaluated for their in vitro antimalarial activity against the chloroquine-sensitive (3D7) and the chloroquine-resistant (Dd2) strains of Plasmodium falciparum. Compound 1 exhibited a moderate activity and 17 was inactive. However, a remarkable antiplasmodial activity was observed for most of karavilagenin C alkanoyl and monoaroyl/cynamoyl derivatives. Karavoates B, D, E, I, and M were the most active, displaying IC(50) values similar to those found for chloroquine, particularly against the resistant strain (IC(50) <0.6μM). Structure-activity relationships (SAR) are discussed. Moreover, the preliminary toxicity toward human cells of compounds 1-17 was also evaluated in breast cancer cell line (MCF-7). Most of the esters showed no toxicity, displaying, in general, much higher selectivity index values than those obtained for the parent compound.

Synthetic lethal hubs associated with vincristine resistant neuroblastoma

Chemotherapy of cancer experiences a number of shortcomings including development of drug resistance. This fact also holds true for neuroblastoma utilizing chemotherapeutics as vincristine. We performed a comparative analysis of molecular and cellular mechanisms associated with vincristine resistance utilizing cell line as well as human tissue data. Differential gene expression analysis revealed molecular features, processes and pathways afflicted with drug resistance mechanisms in general, and specifically with vincristine significantly involving actin associated features. However, specific mode of resistance as well as underlying genotype of parental, vincristine sensitive cells apparently exhibited significant heterogeneity. No consensus profile for vincristine resistance could be derived, but resistance-associated changes on the level of individual neuroblastoma cell lines as well as individual patient profiles became clearly evident. Based on these prerequisites we utilized the concept of synthetic lethality aimed at identifying hub proteins which when inhibited promise to induce cell death due to a synthetic lethal interaction with down-regulated, chemoresistance associated features. Our screening procedure identified synthetic lethal hub proteins afflicted with actin associated processes holding synthetic lethal interactions to down-regulated features individually found in all chemoresistant cell lines tested, therefore promising an improved therapeutic window. Verification of such synthetic lethal hub candidates in human neuroblastoma tissue expression profiles indicated the feasibility of this screening approach for addressing vincristine resistance in neuroblastoma.

Antitumor agents. 280. Multidrug resistance-selective desmosdumotin B analogues

6,6,8-Triethyldesmosdumotin B (2) was discovered as a MDR-selective flavonoid with significant in vitro anticancer activity against a multidrug resistant (MDR) cell line (KB-VIN) but without activity against the parent cells (KB). Additional 2 analogues were synthesized and evaluated to determine the effect of B-ring modifications on MDR-selectivity. Analogues with a B-ring Me (3) or Et (4) group had substantially increased MDR selectivity. Three new disubstituted analogues, 35, 37, and 49, also had high collateral sensitivity (CS) indices of 273, 250, and 100, respectively. Furthermore, 2-4 also displayed MDR selectivity in an MDR hepatoma-cell system. While 2-4 showed either no or very weak inhibition of cellular P-glycoprotein (P-gp) activity, they either activated or inhibited the actions of the first generation P-gp inhibitors verapamil or cyclosporin, respectively.