Piperlongumine induces inhibition of the ubiquitin-proteasome system in cancer cells.

Piperlongumine, a natural product from the plant Piperlongum, has demonstrated selective cytotoxicity to tumor cells and to show anti-tumor activity in animal models [1]. Cytotoxicity of piperlongumine has been attributed to increase in reactive oxygen species (ROS) in cancer cells. We here report that piperlongumine is an inhibitor of the ubiquitin-proteasome system (UPS). Exposure of tumor cells to piperlongumine resulted in accumulation of a reporter substrate known to be rapidly degraded by the proteasome, and of accumulation of ubiquitin conjugated proteins. However, no inhibition of 20S proteolytic activity or 19S deubiquitinating activity was observed at concentrations inducing cytotoxicity. Consistent with previous reports, piperlongumine induced strong ROS activation which correlated closely with UPS inhibition and cytotoxicity. Proteasomal blocking could not be mimicked by agents that induce oxidative stress. Our results suggest that the anti-cancer activity of piperlongumine involves inhibition of the UPS at a pre-proteasomal step, prior to deubiquitination of malfolded protein substrates at the proteasome, and that the previously reported induction of ROS is a consequence of this inhibition.

Gambogic acid is cytotoxic to cancer cells through inhibition of the ubiquitin-proteasome system.

Gambogic acid (GA), displays cytotoxicity towards a wide variety of tumor cells and has been shown to affect many important cell-signaling pathways. In the present work, we investigated the mechanism of action of GA by analysis of drug-induced changes in gene expression profiles and identified GA and the derivative dihydro GA as possible inhibitors of the ubiquitin-proteasome system (UPS). Both GA and dihydro GA inhibited proteasome function in cells resulting in the accumulation of polyubiquitin complexes. In vitro experiments showed that both GA and dihydro GA inhibited 20S chymotrypsin activity and the inhibitory effects of GA and dihydro GA on proteasome function corresponded with apoptosis induction and cell death. In conclusion, our results show that GA and dihydro GA exert their cytotoxic activity through inhibition of the UPS, specifically by acting as inhibitors of the chymotrypsin activity of the 20S proteasome.

Screening for phenotype selective activity in multidrug resistant cells identifies a novel tubulin active agent insensitive to common forms of cancer drug resistance.

BACKGROUND: Drug resistance is a common cause of treatment failure in cancer patients and encompasses a multitude of different mechanisms. The aim of the present study was to identify drugs effective on multidrug resistant cells. METHODS: The RPMI 8226 myeloma cell line and its multidrug resistant subline 8226/Dox40 was screened for cytotoxicity in response to 3,000 chemically diverse compounds using a fluorometric cytotoxicity assay (FMCA). Follow-up profiling was subsequently performed using various cellular and biochemical assays. RESULTS: One compound, designated VLX40, demonstrated a higher activity against 8226/Dox40 cells compared to its parental counterpart. VLX40 induced delayed cell death with apoptotic features. Mechanistic exploration was performed using gene expression analysis of drug exposed tumor cells to generate a drug-specific signature. Strong connections to tubulin inhibitors and microtubule cytoskeleton were retrieved. The mechanistic hypothesis of VLX40 acting as a tubulin inhibitor was confirmed by direct measurements of interaction with tubulin polymerization using a biochemical assay and supported by demonstration of G2/M cell cycle arrest. When tested against a broad panel of primary cultures of patient tumor cells (PCPTC) representing different forms of leukemia and solid tumors, VLX40 displayed high activity against both myeloid and lymphoid leukemias in contrast to the reference compound vincristine to which myeloid blast cells are often insensitive. Significant in vivo activity was confirmed in myeloid U-937 cells implanted subcutaneously in mice using the hollow fiber model. CONCLUSIONS: The results indicate that VLX40 may be a useful prototype for development of novel tubulin active agents that are insensitive to common mechanisms of cancer drug resistance.

Stat1 activation attenuates IL-6 induced Stat3 activity but does not alter apoptosis sensitivity in multiple myeloma.

BACKGROUND: Multiple myeloma (MM) is at present an incurable malignancy, characterized by apoptosis-resistant tumor cells. Interferon (IFN) treatment sensitizes MM cells to Fas-induced apoptosis and is associated with an increased activation of Signal transducer and activator of transcription (Stat)1. The role of Stat1 in MM has not been elucidated, but Stat1 has in several studies been ascribed a pro-apoptotic role. Conversely, IL-6 induction of Stat3 is known to confer resistance to apoptosis in MM. METHODS: To delineate the role of Stat1 in IFN mediated sensitization to apoptosis, sub-lines of the U-266-1970 MM cell line with a stable expression of the active mutant Stat1C were utilized. The influence of Stat1C constitutive transcriptional activation on endogenous Stat3 expression and activation, and the expression of apoptosis-related genes were analyzed. To determine whether Stat1 alone would be an important determinant in sensitizing MM cells to apoptosis, the U-266-1970-Stat1C cell line and control cells were exposed to high throughput compound screening (HTS). RESULTS: To explore the role of Stat1 in IFN mediated apoptosis sensitization of MM, we established sublines of the MM cell line U-266-1970 constitutively expressing the active mutant Stat1C. We found that constitutive nuclear localization and transcriptional activity of Stat1 was associated with an attenuation of IL-6-induced Stat3 activation and up-regulation of mRNA for the pro-apoptotic Bcl-2 protein family genes Harakiri, the short form of Mcl-1 and Noxa. However, Stat1 activation alone was not sufficient to sensitize cells to Fas-induced apoptosis. In a screening of > 3000 compounds including bortezomib, dexamethasone, etoposide, suberoylanilide hydroxamic acid (SAHA), geldanamycin (17-AAG), doxorubicin and thalidomide, we found that the drug response and IC50 in cells constitutively expressing active Stat1 was mainly unaltered. CONCLUSION: We conclude that Stat1 alters IL-6 induced Stat3 activity and the expression of pro-apoptotic genes. However, this shift alone is not sufficient to alter apoptosis sensitivity in MM cells, suggesting that Stat1 independent pathways are operative in IFN mediated apoptosis sensitization.

High content screening for inhibitors of protein interactions and post-translational modifications in primary cells by proximity ligation.

The cost of developing new drugs is a major obstacle for pharmaceutical companies and academia with many drugs identified in the drug discovery process failing approval for clinical use due to lack of intended effect or because of severe side effects. Since the early 1990 s, high throughput screening of drug compounds has increased enormously in capacity but has not resulted in a higher success rate of the identified drugs. Thus, there is a need for methods that can identify biologically relevant compounds and more accurately predict in vivo effects early in the drug discovery process. To address this, we developed a proximity ligation-based assay for high content screening of drug effects on signaling pathways. As a proof of concept, we used the assay to screen through a library of previously identified kinase inhibitors, including six clinically used tyrosine kinase inhibitors, to identify compounds that inhibited the platelet-derived growth factor (PDGF) receptor beta signaling pathway in stimulated primary human fibroblasts. Thirteen of the 80 compounds were identified as hits, and the dose responses of these compounds were measured. The assay exhibited a very high Z' factor (0.71) and signal to noise ratio (11.7), demonstrating excellent ability to identify compounds interfering with the specific signaling event. A comparison with regular immunofluorescence detection of phosphorylated PDGF receptor demonstrated a far superior ability by the in situ proximity ligation assay to reveal inhibition of receptor phosphorylation. In addition, inhibitor-induced perturbation of protein-protein interactions of the PDGF signaling pathway could be quantified, further demonstrating the usefulness of the assay in drug discovery.

Novel activity of acriflavine against colorectal cancer tumor cells.

A high-throughput screen of the cytotoxic activity of 2000 molecules from a commercial library in three human colon cancer cell lines and two normal cell types identified the acridine acriflavin to be a colorectal cancer (CRC) active drug. Acriflavine was active in cell spheroids, indicating good drug penetration and activity against hypoxic cells. In a validation step based on primary cultures of patient tumor cells, acriflavine was found to be more active against CRC than ovarian cancer and chronic lymphocytic leukemia. This contrasted to the activity pattern of the CRC active standard drugs 5-fluorouracil, irinotecan and oxaliplatin. Mechanistic studies indicated acriflavine to be a dual topoisomerase I and II inhibitor. In conclusion, the strategy used seems promising for identification of new diagnosis-specific cancer drugs.

Induction of the lysosomal apoptosis pathway by inhibitors of the ubiquitin-proteasome system.

The lysosomal apoptosis pathway is a potentially interesting therapeutic target. Since apoptosis involving the lysosomal pathway has been described to involve cathepsins, we screened a drug library for agents that induce cathepsin-dependent apoptosis. Using pharmacological inhibitors and siRNA, we identified 2 structurally related agents (NSC687852 and NSC638646) that induced cathepsin D-dependent caspase-cleavage activity in human breast cancer cells. Both agents were found to induce the mitochondrial apoptosis pathway. NSC687852 and NSC638646 were found to inhibit the activity of ubiquitin isopeptidases and to induce the accumulation of high-molecular-mass ubiquitins in cells. We show that 3 other inhibitors of the proteasome degradation pathway induce lysosomal membrane permeabilization (LMP) and that cathepsin-D siRNA inhibits apoptosis induced by these agents. We conclude that a screen for cathepsin-dependent apoptosis-inducing agents resulted in the identification of ubiquitin isopeptidase inhibitors and that proteasome inhibitors with different mechanisms of action induce LMP and cathepsin D-dependent apoptosis.

Cytotoxic effects of cardiac glycosides in colon cancer cells, alone and in combination with standard chemotherapeutic drugs.

Cardiac glycosides have been reported to exhibit cytotoxic activity against several different cancer types, but studies against colorectal cancer are lacking. In a screening procedure aimed at identifying natural products with activity against colon cancer, several cardiac glycosides were shown to be of interest, and five of these were further evaluated in different colorectal cancer cell lines and primary cells from patients. Convallatoxin (1), oleandrin (4), and proscillaridin A (5) were identified as the most potent compounds (submicromolar IC50 values), and digitoxin (2) and digoxin (3), which are used in cardiac disease, exhibited somewhat lower activity (IC50 values 0.27-4.1 microM). Selected cardiac glycosides were tested in combination with four clinically relevant cytotoxic drugs (5-fluorouracil, oxaliplatin, cisplatin, irinotecan). The combination of 2 and oxaliplatin exhibited synergism including the otherwise highly drug-resistant HT29 cell line. A ChemGPS-NP application comparing modes of action of anticancer drugs identified cardiac glycosides as a separate cluster. These findings demonstrate that such substances may exhibit significant activity against colorectal cancer cell lines, by mechanisms disparate from currently used anticancer drugs, but at concentrations generally considered not achievable in patient plasma.

Image-based screening for the identification of novel proteasome inhibitors.

The proteasome is a new, interesting target in cancer drug therapy, and the proteasome inhibitor bortezomib has shown an effect in myeloma patients. It is of interest to efficiently discover and evaluate new proteasome inhibitors. The authors describe the development of an image-based screening assay for the identification of compounds with proteasome-inhibiting activity. The stably transfected human embryo kidney cell line HEK 293 ZsGreen Proteasome Sensor Cell Line expressing the ZsProSensor-1 fusion protein was used for screening and evaluation of proteasome inhibitors. Inhibition of the proteasome leads to accumulation of the green fluorescent protein ZsGreen, which is measured in the ArrayScan High Content Screening system, in which cell morphology is studied simultaneously. When screening the LOPAC(1280) substance library, several compounds with effect on the proteasome were found; among the hits were disulfiram and ammonium pyrrolidinedithiocarbamate (PDTC). Cytotoxic analysis of disulfiram and PDTC showed that the compounds induced cytotoxicity in the myeloma cell line RPMI 8226. The average Z' value for the assay was 0.66. The results indicate that the assay rapidly identifies new proteasome-inhibiting substances, and it will be further used as a tool for image-based screening of other chemically diverse compound libraries.

Pharmacological profiling of disulfiram using human tumor cell lines and human tumor cells from patients.

The thiocarbamate drug disulfiram has been used for decades in the treatment of alcohol abuse. Disulfiram induces apoptosis in a number of tumor cell lines and was recently by us proposed to act as a 26S proteasome inhibitor. In this work we characterized disulfiram in vitro with regard to tumor-type specificity, possible mechanisms of action and drug resistance and cell death in human tumor cell lines and in 78 samples of tumor cells from patients using the fluorometric microculture cytotoxicity assay and the automated fluorescence-imaging microscope ArrayScan((R)). Disulfiram induced cytotoxicity in a biphasic pattern in both cell lines and patient tumor cells. Disulfiram induced apoptosis as measured by cell membrane permeability, nuclear fragmentation/condensation and caspase-3/7 activation using high content screening assays. For many of the cell lines tested disulfiram was active in sub-micromolar concentrations. When comparing the logIC(50) patterns with other cytotoxic agents, disulfiram showed low correlation (R<0.5) with all drugs except lactacystin (R=0.69), a known proteasome inhibitor, indicating that the two substances may share mechanistic pathways. Disulfiram was more active in hematological than in solid tumor samples, but substantial activity was observed in carcinomas of the ovary and the breast and in non-small cell lung cancer. Disulfiram also displayed higher cytotoxic effect in cells from chronic lymphocytic leukemia than in normal lymphocytes (p<0.05), which may indicate some tumor selectivity. These results together with large clinical experience and relatively mild side effects encourage clinical studies of disulfiram as an anti-cancer agent.

Evaluation of the antitumor activity of NOV202, a novel microtubule targeting and vascular disrupting agent.

PURPOSE: Overall, ~65% of patients diagnosed with advanced ovarian cancer (OC) will relapse after primary surgery and adjuvant first-line platinum- and taxane-based chemotherapy. Significant improvements in the treatment of OC are expected from the development of novel compounds having combined cytotoxic and antiangiogenic properties that make them effective on refractory tumors. METHODS: Permeability of NOV202 was determined with Caco-2 monolayer assay. The compound's pharmacokinetic profile and plasma:brain distribution were assessed in male C57Bl/6 mice. The compound's impacts on tubulin, microtubules and cell cycle were investigated by using in vitro tubulin polymerization assay, cell-based immunofluorescence and live cell microscopy. The IC50 concentrations of NOV202 were assessed in a panel of eight cancer cell lines. Impact of the compound on vascular tube formation was determined using the StemKit and Chick chorioallantoic membrane assays. The in vivo efficacy of the compound was analyzed with an OC xenograft mouse model. RESULTS: NOV202 was found to suppress cancer cell proliferation at low nanomolar concentrations (IC50 2.3-12.0 nM) and showed equal efficacy between OC cell line A2780 (IC50 2.4 nM) and its multidrug-resistant subline A2780/Adr (IC50 2.3 nM). Mechanistically, NOV202 targeted tubulin polymerization in vitro in a dose-dependent manner and in cells induced an M phase arrest. In vivo, NOV202 caused a dose-dependent reduction of tumor mass in an A2780 xenograft model, which at the highest dose (40 mg/kg) was comparable to the effect of paclitaxel (24 mg/kg). Interestingly, NOV202 exhibited vascular disrupting properties that were similar to the effects of Combretastatin A4. CONCLUSION: NOV202 is a novel tubulin and vascular targeting agent that shows strong anticancer efficacy in cells and OC xenograft models. The finding that the compound induced significantly more cell death in Pgp/MDR1 overexpressing OC cells compared to vincristine and paclitaxel warrants further development of the compound as a new therapy for OC patients with treatment refractory tumors and/or relapsing disease.

Phenotype-based screening of mechanistically annotated compounds in combination with gene expression and pathway analysis identifies candidate drug targets in a human squamous carcinoma cell model.

The squamous cell carcinoma HeLa cell line and an epithelial cell line hTERT-RPE with a nonmalignant phenotype were interrogated for HeLa cell selectivity in response to 1267 annotated compounds representing 56 pharmacological classes. Selective cytotoxic activity was observed for 14 of these compounds dominated by cyclic adenosine monophosphate (cAMP) selective phosphodiesterase (PDE) inhibitors, which tended to span a representation of the chemical descriptor space of the library. The PDE inhibitors induced delayed cell death with features compatible with classical apoptosis. The PDE inhibitors were largely inactive when tested against a cell line panel consisting of hematological and nonsquamous epithelial phenotypes. In a genome-wide DNA microarray analysis, PDE3A and PDE2A were found to be significantly increased in HeLa cells compared to the other cell lines. The pathway analysis software PathwayAssist was subsequently used to extract a list of proteins and small molecules retrieved from Medline abstracts associated with the hit compounds. The resulting list consisted of major parts of the cAMP-protein kinase A pathway linking to ERK, P38, and AKT. This molecular network may provide a basis for further exploitation of novel candidate targets for the treatment of squamous cell carcinoma.

Screening of an annotated compound library for drug activity in a resistant myeloma cell line.

PURPOSE: Resistance to anticancer drugs is a major problem in chemotherapy. In order to identify drugs with selective cytotoxic activity in drug-resistant cancer cells, the annotated compound library LOPAC1280, containing compounds from 56 pharmacological classes, was screened in the myeloma cell line RPMI 8226 and its doxorubicin-resistant subline 8226/Dox40. METHODS: Cell survival was measured by the Fluorometric Microculture Cytotoxicity Assay. RESULTS: Selective cytotoxic activity in 8226/Dox40 was obtained for 33 compounds, with the most pronounced difference observed for the glucocorticoids. A microarray analysis of the cells showed a difference in mRNA-expression for the glucocorticoid receptor suggesting potential mechanisms for the difference in glucocorticoid sensitivity. In the presence of the glucocorticoid-receptor antagonist RU486, the sensitivity to the glucocorticoids was reduced and a similar effect level in RPMI 8226 and 8226/Dox40 was achieved. CONCLUSION: In conclusion, screening of mechanistically annotated compounds on drug-resistant cancer cells can identify compounds with selective activity and provide a basis for the development of novel treatments of drug-resistant malignancies.

Identification and evaluation of potential anti-cancer drugs on human neuroendocrine tumor cell lines.

UNLABELLED: The aim of this study was to investigate drug sensitivity in neuroendocrine tumor cell lines. MATERIALS AND METHODS: In vitro drug sensitivity screening was performed using the fluorometric microculture cytotoxicity assay in one human pancreatic carcinoid and two human bronchial carcinoid cell lines. In addition, a normal human retinal pigment epithelial cell line was used for comparison. A total of 18 drugs with different mechanisms of action were tested. RESULTS: The most active agents were brefeldin A, emetine, bortezomib and idarubicin, having IC5o values <1 microM in all four cell lines. In addition, the three tumor cell lines showed sensitivity for sanguinarine, Bayll-7085, mitoxantrone, doxorubicin, beta-lapachone, NSC 95397 and CGP-74514A. CONCLUSION: The cell lines were sensitive for several drugs acting in different ways, covering a broad spectrum of mechanisms of action. Some of these compounds may possibly be used in clinical trials and show therapeutic effect in patients with neuroendocrine tumors.

AKN-028 induces cell cycle arrest, downregulation of Myc associated genes and dose dependent reduction of tyrosine kinase activity in acute myeloid leukemia.

AKN-028 is a novel tyrosine kinase inhibitor with preclinical activity in acute myeloid leukemia (AML), presently undergoing investigation in a phase I/II study. It is a potent inhibitor of the FMS-like kinase 3 (FLT3) but shows in vitro activity in a wide range of AML samples. In the present study, we have characterized the effects of AKN-028 on AML cells in more detail. AKN-028 induced a dose-dependent G0/1 arrest in AML cell line MV4-11. Treatment with AKN-028 caused significantly altered gene expression in all AML cell types tested (430 downregulated, 280 upregulated transcripts). Subsequent gene set enrichment analysis revealed enrichment of genes associated with the proto-oncogene and cell cycle regulator c-Myc among the downregulated genes in both AKN-028 and midostaurin treated cells. Kinase activity profiling in AML cell lines and primary AML samples showed that tyrosine kinase activity, but not serine/threonine kinase activity, was inhibited by AKN-028 in a dose dependent manner in all samples tested, reaching approximately the same level of kinase activity. Cells sensitive to AKN-028 showed a higher overall tyrosine kinase activity than more resistant ones, whereas serine/threonine kinase activity was similar for all primary AML samples. In summary, AKN-028 induces cell cycle arrest in AML cells, downregulates Myc-associated genes and affect several signaling pathways. AML cells with high global tyrosine kinase activity seem to be more sensitive to the cytotoxic effect of AKN-028 in vitro.

Induction of mitochondrial dysfunction as a strategy for targeting tumour cells in metabolically compromised microenvironments.

Abnormal vascularization of solid tumours results in the development of microenvironments deprived of oxygen and nutrients that harbour slowly growing and metabolically stressed cells. Such cells display enhanced resistance to standard chemotherapeutic agents and repopulate tumours after therapy. Here we identify the small molecule VLX600 as a drug that is preferentially active against quiescent cells in colon cancer 3-D microtissues. The anticancer activity is associated with reduced mitochondrial respiration, leading to bioenergetic catastrophe and tumour cell death. VLX600 shows enhanced cytotoxic activity under conditions of nutrient starvation. Importantly, VLX600 displays tumour growth inhibition in vivo. Our findings suggest that tumour cells in metabolically compromised microenvironments have a limited ability to respond to decreased mitochondrial function, and suggest a strategy for targeting the quiescent populations of tumour cells for improved cancer treatment.

Screening for cytotoxic compounds in poor-prognostic chronic lymphocytic leukemia.

BACKGROUND/AIM: For chronic lymphocytic leukemia (CLL) patients with poor-prognostic genomic aberrations the therapeutic options are limited. We used the Spectrum Collection library to identify compounds with anti-leukemia activity in high-risk CLL. MATERIALS AND METHODS: We identified substances with equal high cytotoxic activity in vitro in samples from poor-prognostic CLL (11q-/17p-, n=3) as compared to those from favourable-prognostic CLL (13q-, n=3). Cell survival was measured by fluorometric microculture cytotoxicity assay. RESULTS: Out of 2,000 compounds, 65 had a similar effect in both prognostic groups. Fifteen compounds were selected for dose-response experiments in 16 additional CLL samples. Of these compounds, 12 continued to have similar cytotoxicity between prognostic subgroups. Additional experiments demonstrated that in CLL cells with 11q or 17p deletion, 5-azacytidine induced apoptosis in a dose-dependent manner and lipoprotein lipase expression was reduced following orlistat treatment. CONCLUSION: Using primary cultures of cells from high-risk CLL patients for compound screening is a feasible approach and that 5-azacytidine and orlistat exemplify substances that exhibit cytotoxicity in poor-risk CLL.

Identification of agents that induce apoptosis of multicellular tumour spheroids: enrichment for mitotic inhibitors with hydrophobic properties.

Cell-based anticancer drug screening generally utilizes rapidly proliferating tumour cells grown as monolayer cultures. Hit compounds from such screens are not necessarily effective on hypoxic and slowly proliferating cells in 3-D tumour tissue. The aim of this study was to examine the potential usefulness of 3-D cultured tumour cells for anticancer drug screening. We used colon carcinoma multicellular spheroids containing hypoxic and quiescent cells in core areas for this purpose. Three libraries (∼11 000 compounds) were screened using antiproliferative activity and/or apoptosis as end-points. Screening of monolayer and spheroid cultures was found to identify different sets of hit compounds. Spheroid screening enriched for hydrophobic compounds: median XLogP values of 4.3 and 4.4 were observed for the hits in two independent screening campaigns. Mechanistic analysis revealed that the majority of spheroid screening hits were microtubuli inhibitors. One of these inhibitors was examined in detail and found to be effective against non-dividing cells in the hypoxic centres of spheroids. Spheroid screening represents a conceptually new strategy for anticancer drug discovery. Our findings have implications for drug library design and hit selection in projects aimed to develop drugs for the treatment of solid tumours.

Phenotype-based drug screening in primary ovarian carcinoma cultures identifies intracellular iron depletion as a promising strategy for cancer treatment.

Primary cultures of patient tumor cells (PCPTC) have been used for prediction of diagnosis-specific activity and individual patient response to anticancer drugs, but have not been utilized as a model for identification of novel drugs in high throughput screening. In the present study, ovarian carcinoma cells from three patients were tested in response to a library of 3000 chemically diverse compounds. Eight hits were retrieved after counter screening using normal epithelial cells, and one of the two structurally related hit compounds was selected for further preclinical evaluation. This compound, designated VLX 50, demonstrated a broad spectrum of activity when tested in a panel of PCPTCs representing different forms of leukemia and solid tumors and displayed a high tumor to normal cell activity. VLX 50 induced delayed cell death with some features of classical apoptosis. Significant in vivo activity was confirmed on primary cultures of human ovarian carcinoma cells in mice using the hollow fiber model. Mechanistic exploration was performed using gene expression analysis of drug exposed tumor cells to generate a drug-specific signature. This query signature was analyzed using the Gene Set Enrichment Analysis and the Connectivity Map database. Strong connections to hypoxia inducible factor 1 and iron chelators were retrieved. The mechanistic hypothesis of intracellular iron depletion leading to hypoxia signaling was confirmed by a series of experiments. The results indicate the feasibility of using PCPTC for cancer drug screening and that intracellular iron depletion could be a potentially important strategy for cancer therapy.

Rapamycin shows anticancer activity in primary chronic lymphocytic leukemia cells in vitro, as single agent and in drug combination.

The mammalian target of rapamycin inhibitor rapamycin and its analogues show promising anticancer activity in various experimental tumor models and are presently evaluated in clinical trials. We, here, evaluated the in vitro activity of rapamycin with regard to tumor-type specificity and possible mechanisms of drug resistance in 97 tumor cell samples from patients and in a resistance-based cell line panel, using the fluorometric microculture cytotoxicity assay. Rapamycin was dose-dependently cytotoxic in patient tumor cells and in cell lines. In primary cells, rapamycin was more active in hematological than in solid tumor samples, with chronic lymphocytic leukemia (CLL) and acute lymphocytic leukemia being the most sensitive tumor types. Considerable inter-individual differences in sensitivity were apparent among CLL samples, but no difference was observed between IGHV mutated and unmutated CLL samples, whereas a tendency to lower rapamycin sensitivity was indicated for samples displaying poor-prognostic genomic markers. Combination experiments in CLL cells indicated that rapamycin acted synergistically with vincristine, cisplatin, chlorambucil and taxotere. These results and the clinically-experienced good tolerance to rapamycin analogues encourage clinical studies of rapamycin in CLL treatment as single agent but also in combination with, e.g., vincristine and chlorambucil.