Synergistic Effect of SN-38 in Combination with Cetuximab on Angiogenesis and Cancer Cell Invasion

BACKGROUND

The combination of irinotecan, a topoisomerase I inhibitor with cetuximab, an antibody against epidermal growth factor receptor, produces synergistic and beneficial effects in patients with irinotecan-refractory colorectal cancer. Our hypothesis was that synergistic effects could be due to anti-angiogenesis and anti-invasion, but not to cytotoxicity.

MATERIALS AND METHODS

Cytotoxicity was assessed by viability test and flow cytometry. Anti-angiogenesis, anti-invasion were studied by the endothelial cell capillary-like network formation and transmigration through an extracellular matrix. Protein kinase B (PKB, frequently cited as AKT), and extracellular signal-regulated kinases (ERK) activation was assayed by cell-based enzyme-linked immunosorbent assay (ELISA).

RESULTS

Combinations of SN-38 (the active of irinotecan) and cetuximab did not induce any synergistic cytotoxicity confirmed by viability test and cell-cycle analyses. Interestingly, their combination produced synergistic anti-angiogenesis and anti-invasion activities revealed by endothelial cell capillary-like network formation and cell invasion tests. Subsequently, their combination attenuated either expression or phosphorylation of AKT and ERK1/2 using cell-based ELISA.

CONCLUSION

SN-38/cetuximab combination has synergistic anti-angiogenesis and anti-invasion activities mediated by down-regulation of phosphatidylinositol-3-kinases/AKT and mitogen-activated protein kinase/ERK pathways.

Association of circulating VEGF-A levels with outcome in patients with vascular sarcomas receiving sorafenib (Sor): Exploratory analysis from AngioNext study

10525

Background: We have carried out a stratified phase II study of Sor in pts with advanced angiosarcoma (AA, n=32), malignant solitary tumour (SFT, n=4) & epithelioid hemangioendothelioma (EE, n=13). We report here the correlative analysis of predictive value of circulating pro/anti-angiogenetic biomarkers. Methods: Using ELISA method (R&D SYSTEMS) Circulating biomarkers (VEGF-A [pg/mL], Thrombospondin-1 (TSP1) [µg/mL], Stem Cell Factor (SCF) [pg/mL], Placental growth factor (PlGF) [pg/mL], VEGF-C [pg/mL] & E-selectin [ng/mL]) have been measured before Sor treatment & after 7 days. We analyze the correlation with histological subtypes, presence of metastases, best response and occurrence of hemorrhage and Gr3-4 arterial hypertension. Results: VEGF-A (mean value 475 vs 541 pg/mL, p=0.002), TSP1 (16 vs 24 µg/mL, p=0.0002), PlGF (20.9 vs 40.7 pg/mL, p=0.0001) significantly increased during the treatment. Sor treatment did not affect the levels of SCF, VEGF-C & E-selectin. The distributions of all biomarkers were similar across the histological subtypes, whatever the presence of metastasis, the occurrence of hemorrhage or arterial hypertension. 2 biomarkers were associated with better outcome:VEGF-A & PlGF. Best objective response and non-progression at 180 days were associated with low level of VEGF-A at baseline (p=0.04 and p= 0,03 respectively). There was a correlation between circulating level of VEGF-A & time to progression (TTP) (r=-0.47, p=0.001). Best objective response and non-progression at 180 days were not associated with baseline level of PlIGF (p=0.34 and 0.07), but there was a correlation between circulating level of PlIGF at baseline and TTP (-0.31, p=0.02). Conclusions: In pts with vascular sarcomas receiving Sor, we have observed a significant decrease in circulating level of VEGF-A. Low level of VEGF-A at baseline (<500 pg/mL) was significantly associated with better outcome, especially best objective response rate, non-progression at 180 days and time to progression. Clinical trial information: 2007-004651-10.

Abstract 3799: Polyamine Transport System (PTS) activity and hijacking in cancer cells: New option in Head and Neck tumors treatment with the polyamine-containing drug candidate F14512

The Polyamine Transport System, although not clearly identified at the molecular level in eukaryotic organisms, was found over activated in many types of cancer cells, such as leukemia, prostate, melanoma and NSCLC. Polyamines are implicated in many biological functions, and the need for polyamines in tumor cells, conveyed by the PTS, is crucial. New therapeutic strategies consist to use this transport system to deliver a cytotoxic agent specifically into cancer cells. Head and neck cancer remains the 6th common cancer with a very poor survival rate indicating the crucial need for new targeted strategies. For this study, we used 4 Head and Neck (H & N) cancer cell lines representative of various localizations: CAL 33 and CAL 27 from base of the tongue, Fadu from the pharynx and SQ20B from the larynx. Here, using a polyamine-coupled fluorescent probe, we show that the PTS is active in all head and neck cancer cell lines regardless the tumor localization. In these models, flow cytometry demonstrated that the PTS incorporates quickly, massively and specifically the probe into cancer cells. Confocal microscopy observations revealed that the spermine probe accumulates into the cell nuclei, the site of action of F14512 which is a potent topoisomerase II inhibitor. Considering this property, we evaluated the potential of the F14512 (Pierre Fabre laboratories, France) in these H & N cancer cell lines. F14512 contains a PTS-recognized spermine side chain attached to an epipodophyllotoxin moiety targeting topoisomerase II. We found that F14512 presents a much higher cytotoxicity than etoposide in the 4 cell lines. Competition assays showed that this effect is dependent of the PTS activity and confirmed the targeted action of F14512 against cells with active PTS. The high efficiency of F14512 in the head and neck cancer cell lines is reported here for the first time and may be of interest for the future development of this novel drug candidate, currently in phase 1 clinical trial in leukemia. Studies are in progress, using fresh tumor biopsies from patients with head and neck cancer, to analyze the PTS status of the tumors using the specific spermine-containing fluorescent probe and to evaluate the activity of F14512.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3799. doi:1538-7445.AM2012-3799

Synthesis of chromeno[3,4-b]indoles as Lamellarin D analogues: a novel DYRK1A inhibitor class

A library of substituted chromeno[3,4-b]indoles was developed as Lamellarin isosters. Synthesis was achieved from indoles after a four-step pathway sequence involving C-3 iodination, a Suzuki cross-coupling reaction, and a one pot deprotection/lactonisation step. Twenty final compounds were tested in order to determine their activity against topoisomerase I and kinases, the two major biological activities of Lamellarins. One newly synthesized derivative exhibited a strong topoisomerase activity comparable to reference compounds such as campthotecin and Lamellarin with only a weak kinase inhibition. Two other lead compounds were identified as new nanomolar DYRK1A inhibitors and several other drugs affected the kinases in the sub-micromolar range. These results will enable us to use the chromeno[3,4-b]indole as a pharmacophore to develop potent treatments for neurological or oncological disorders in which DYRK1A is fully involved.

Abstract 3266: Cilengitide targets more efficiently pediatric than adult glioma cells in vitro through cell detachment and anoikis induction

Despite the development of numerous clinical trials in pediatric cancers, the prognosis for children with high grade gliomas (HGG) remains poor, indicating a critical need for new treatments. Cilengitide (EMD 121974, Merck KgaA, Germany), a new compound in early phase clinical trial, is a selective antagonist of αvβ3 and αvβ5 integrins, known to be involved in tumor growth, angiogenesis and metastasis development. A previous hopeful study shown that Cilengitide is able to control the growth of tumor from adult glioblastoma U87MG xenografts through blood vessel growth inhibition [1]. The aim of our study was to evaluate, the direct efficiency of Cilengitide on pediatric glioma cell lines and to determine its mechanism of action in these specific glioma cells.

For this study, we used a wide tumor cell panel including three high grade (SF188, KNS42, UW479) and two low grade (Res259, Res186) pediatric glioma cell lines compared to the prototypic adult HGG cell line U87MG. Flow cytometry analysis revealed various αVβ3 expression levels but no correlation with the tumor grade. As expected, the pediatric cell line UW479, which doesn't express αVβ3, presents no sensitivity to Cilengitide. All other pediatric cell lines, expressing αVβ3, are sensitive to Cilengitide. Interestingly, U87MG adult cells, which express a high αVβ3 protein level, are very slightly altered by Cilengitide. Thus we have demonstrated a higher cytotoxicity of Cilengitide in pediatric than in adult glioma cells. In addition, the presence of the target appears necessary but not sufficient for Cilengitide action. Cilengitide is a selective antagonist of integrins implicated in cell adhesion. Cell detachment assay have shown a rapid and dose dependant Cilengitide action in pediatric cell lines expressing αVβ3 as well as in adult cells which detach from their matrix after only 2h of treatment. However, this detachment is translated in a Cilengitide cytotoxicity in pediatric cell lines but not in adult cells that are able to growth in spite of the strong cell detachment induced by Cilengitide. Interestingly, growth kinetics in non adherent conditions, revealed that pediatric glioma cells were sensitive to anoikis and so died after Cilengitide-induced cell detachment. In contrast, adult glioma cells are able to form cells clusters and to resist to anoikis and thus to Cilengitide-induced detachment.

In conclusion, Cilengitide action on glioma cells appears dependent of αVβ3 expression and sensitivity to anoikis, two properties found in the majority of pediatric cell lines. Thus Cilengitide is able to target directly and efficiently pediatric glioma cells in vitro. Future in vivo experiments should confirm these promising results in pediatric glioma orthotopic xenografts models.

[1] Yamada et al., Effect of the angiogenesis inhibitor Cilengitide (EMD 121974) on glioblastoma growth in nude mice.

Neurosurgery, 2006.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3266. doi:10.1158/1538-7445.AM2011-3266

Differential apoptosis-like cell death in amastigote and trypomastigote forms from Trypanosoma cruzi-infected heart cells in vitro

Apoptosis, type-I of programmed cell death (PCD-I), is not restricted to multicellular organisms since many apoptotic features have been described in different trypanosomatids, including Trypanosoma cruzi. Our present aim was to monitor, by different morphological markers, the occurrence of apoptosis-like death in amastigotes and trypomastigotes of T.cruzi (Y strain) during the infection of heart culture cells. We documented the differential occurrence of PCD-I in amastigotes and trypomastigotes, with distinct death rates noticed between these two parasite-distinct forms. Fluorescence microscopy and flow cytometry analysis using different hall markers of apoptosis (phosphatidylserine exposure, collapse of mitochondrial membrane potential and DNA fragmentation) showed that amastigotes present higher levels of apoptosis-like cell death as compared to trypomastigotes. It is possible that the higher levels of PCD-I in these highly multiplicative forms may contribute to the control of the parasite burden within the host cells. On the other hand, the apoptosis-like occurrence in the infective but non-proliferative stage of the parasite (trypomastigotes) may play a role in parasite evasion mechanisms as suggested for other parasites.

Abstract A84: Role of Akt pathway in the response of cetuximab in a resistant head and neck cell line

The Epidermal Growth Factor Receptor (EGFR) is a central regulator of cell proliferation and differentiation in human cancers and is frequently overexpressed in many types of tumors. Cetuximab is a monoclonal antibody designed to block the EGFR activation, to induce the internalization of the receptor and to reduce the signaling pathway. However many tumors exert resistance to EGFR inhibitors. Accumulating evidence indicates that the response of cancer cells to cetuximab is a complex process that can be affected by multiple intrinsic and extrinsic resistance mechanisms. The aim of this study was to investigate the mechanisms of resistance to cetuximab treatment in vitro. We investigated the EGFR pathway and the receptor internalization after cetuximab treatment, alone or combined, and we compared results in CAL33 (HNSCC) and A431 (epidermoïd cancer) cell lines which overexpressed EGFR and present different sensitivities to cetuximab and to EGF (EGFR natural ligand).

Cetuximab induced a high growth inhibition and a high inhibition of ERK and AKT phosphorylation in A431. By contrast, cetuximab induced a lower growth inhibition, an ERK phosphorylation inhibition but any inhibition of the AKT pathway. The cetuximab sensitivity of these cell lines was thus different and the difference would be based on the AKT pathway. To verify this hypothesis, we used an EGFR tyrosine kinase inhibitor and an AKT inhibitor. Results have shown that the AKT pathway can be inhibited in CAL33 cell line by an EGFR tyrosine kinase inhibitor and an AKT inhibitor. While the cetuximab induced a strong growth inhibition in A431, this growth inhibition was weakly increased by the combination. In contrast, cetuximab induced a weak growth inhibition in CAL33 while the growth inhibition was much stronger with the combination than in A431. According to the central role of the EGFR internalization in the AKT pathway activation, we investigated the EGFR internalization role in these models. Preliminary results showed significant differences between these two cell lines. In conclusion, this study had shown some explanations to the limited efficacy to cetuximab in CAL33. Firstly, we have shown a persistent activation of AKT in CAL33 which might prevent the antitumor effect of cetuximab. Secondly, we have shown that the EGFR internalization and signal transduction mediation were different in CAL33 and A431 and this difference might contribute to the response to cetuximab. The AKT pathway appears as a central element in the cetuximab sensitivity in these models and the combination of cetuximab with an AKT inhibitor could be a good therapeutic option in HNSCC

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A84.

A universal formula based on cystatin C to perform individual dosing of carboplatin in normal weight, underweight, and obese patients

2527

Background: It has recently been shown that it is possible to improve the prediction of carboplatin clearance by adding plasma cystatin C level (cysC), an endogenous marker of glomerular filtration rate, to the other patient characteristics routinely used for carboplatin individual dosing, namely serum creatinine (Scr), body weight (BW), age, and sex. This multi-center pharmacokinetic study was performed to evaluate prospectively the benefit of using cysC for carboplatin individual dosing. Methods: The 357 patients included in the study were receiving carboplatin as part of established protocols. A population pharmacokinetic analysis was performed using the NONMEM program. Seven covariates were studied: Scr, cysC, age, sex, BW, ideal body weight, and lean body mass. Results: The best covariate equation was: carboplatin clearance (mL/min) = 105. (Scr/75)-0.433. (cysC/1.00)-0.290 . (BW/65)+0.547 . (age/56)-0.351 . 0.855sex, with Scr in μmol/L, cysC in mg/L, BW in kg, age in years, and sex = 0 for male. Using an alternative weight descriptor (ideal body weight or lean body mass) did not improve the prediction. This final covariate model was validated by bootstrap analysis. The bias (mean percentage error) and imprecision (mean absolute percentage error) were +2% and 15% respectively on the total population, and were of a similar magnitude in each of the three subgroups of patients defined according to their body mass index. Conclusions: For the first time, a unique formula is proposed for carboplatin individual dosing to patients which is shown to be equally valid for underweight, normal weight, and obese patients.

No significant financial relationships to disclose.

Biological and clinical prognostic factors in patients with advanced non-small-cell cancer (NSCLC) treated by erlotinib: Preliminary results of the ERMETIC cohort

8079

Background: Although it has been suggested that results of EGFR-IHC, EGFR-FISH and EGFR and KRAS mutations are predictive of EGFR-TKIs efficacy, they are not used in clinical practice. ERMETIC is a French NCI granted prospective study aiming to facilitate implementation of these biomarkers in France. Methods: After a preliminary phase of validation in 16 French centers, these biomarkers were studied in available tumor specimens collected from all consecutive NSCLC patients (pts) treated by erlotinib, for the first time. Patients were followed-up until progression or death. Demographic, clinical, pathological characteristics and biomarkers were studied by Cox model as predictors of progression free survival (PFS) and overall survival (OS). Results: 493 of the 530 enrolled pts between 02/07 and 03/08 are included in this analysis, among whom at least 357 (72%) tumor specimens were collected. 88% were Caucasians, 32% females; median age was 62 (33–93); 15% were never smoker, 67% former and 15% active. 18% were PS0, 51% PS1 and 31% PS2–3. 92% pts were metastatic and 66% had adenocarcinoma (ADC). Erlotinib (150 mg in 98% of pts) was given as first-, second- and third-or-more-line in 10, 45, 42% of pts, respectively. 94% of pts had received previous platin-based chemotherapy. With a 11.5 months (mo.) median of follow-up, PFS was 2.4 mo. and OS 5.6 mo., with a 30% 1-year OS. Clinical factors independently associated with shorter OS were: PS1 and PS2–3 (HR=1.79 and 4.3, p<.0001); histology other than ADC and squamous cell (HR=1.44 and 1.03, p=.05); 2 sites of metastasis and more (HR=1.60 and 1.82, p<.0001) and former and active smoking (HR=1.49 and 2.12, p=.002). In multivariate analysis, line of treatment was not prognostic. Conclusions: The study confirmed in a large cohort of advanced NSCLC treated by erlotinib the independent value of several prognostic factors. Prognostic values of EGFR-IHC, EGFR-FISH and EGFR and KRAS mutations will be reported at the meeting.

[Table: see text]

Design, synthesis and biological evaluation of new oligopyrrole carboxamides linked with tricyclic DNA-intercalators as potential DNA ligands or topoisomerase inhibitors

In the context of the design and synthesis of minor groove binding and intercalating DNA ligands some new oligopyrrole carboxamides were synthesized. These hybrid molecules (combilexins) possess a variable and conformatively flexible spacer at the N-terminal end. As intercalating tricyclic systems acridone, acridine, anthraquinones and in a special case iminostilbene terminate the N-terminal end of the pyrrole chain. The cytotoxicity was examined by the NCI antitumor screening, furthermore, biophysical as well as biochemical studies were performed in order to get some information about the DNA binding properties and topoisomerase inhibition effect of this new series of molecules.

Synthesis and anticancer activity of new pyrrolocarbazoles and pyrrolo-β-carbolines

'Bended' 1, 3 or 'linear' 2 pyrrolidino-fused (aza)carbazoles were prepared and screened towards a few cancer-related targets. Whereas 'bended' derivatives 1 and 3 proved to be weakly toxic, several members of the 'linear' family strongly interact with DNA, especially derivative 28a.

In vitro measurement of enzymatic markers as a tool to detect mouse cardiomyocytes injury

Primary cultures of cardiomyocytes represent a useful model for analyzing cardiac cell biology as well as pathogenesis of several cardiovascular disorders. Our aim was to standardize protocols for determining the damage of cardiac cells cultured in vitro by measuring the creatine kinase and its cardiac isotype and lactate dehydrogenase activities in the supernatants of mice cardiomyocytes submitted to different protocols of cell lysis. Our data showed that due to its higher specificity, the cardiac isotype creatine kinase was the most sensitive as compared to the others studied enzymatic markers, and can be used to monitor and evaluate cardiac damage in in vitro assays.

Phenyl substitution of furamidine markedly potentiates its anti-parasitic activity against Trypanosoma cruzi and Leishmania amazonensis

Furamidine (DB75) and related unfused aromatic diamidines have proven useful for the treatment of parasitic infections. These compounds were primarily developed to combat infections by Pneumocystis carinii and African trypanosomes but they are also active against other parasites. Here we have investigated the in vitro effects of DB75 and its phenyl-substituted analog DB569 on two kinetoplastid haemoflagellates Trypanosomatidae: Trypanosoma cruzi and Leishmania (L) amazonensis. The phenyl-amidine compound DB569 has equivalent DNA binding properties compared to DB75 but it was selected on the basis of its distinct tumor cell distribution properties. We found that DB569 is significantly more potent than DB75 at reducing the proliferation of the parasites, using either isolated parasites in cultures or with cardiomyocyte and macrophage host cells. DB569 is effective towards the intracellular forms of T. cruzi (IC(50) in the low-micromolar range) and it exhibits trypanocidal dose-dependent effects against trypomastigote forms of T. cruzi parasites obtained from the Y strain and Dm28c clone, which belong to two different biodemes. Fluorescence microscopy experiments indicated that both diamidines were mostly localized in the nucleus of the mammalian host cells and within the nuclei and kinetoplast of the parasites. Electron microscopy studies showed that the treatment of the parasites with DB75 and DB569 induces important alterations of the parasite nucleus and kinetoplast, at sites where their DNA target is localized. Altogether, the data suggest that the phenyl-substituted furamidine analogue DB569 is a potential new candidate for the treatment of the Chagas' disease and Leishmaniasis.

Host and parasite apoptosis following Trypanosoma cruzi infection in in vitro and in vivo models

The mechanism of cell death which occurs during Chagas' cardiopathy is disputed. To address this issue we analyzed the molecular pathways implicated in the death of cardiomyocytes during T. cruzi invasion and found that they undergo apoptosis during both in vitro and in vivo infections. However, the death rates and onset were related to the parasite stocks belonging to different biodemes, which can be correlated to the different histological inflammation findings that have already been reported. Our in vitro data provide additional support for this hypothesis since higher levels and earlier apoptosis induction were noted during the interaction with the Dm28c (type I) as compared to the Y and CL stocks (type II). Modifications of the surface carbohydrates of the infected cardiomyocytes were observed and these molecular events may be acting as "eat me" tags for their final engulfment by macrophages and/or other non-professional phagocytes. The analysis of other host cell types showed that the in vitro infection of fibroblasts did not result in host apoptosis even when a highly infective stock was used. Conversely, infected macrophages undergo apoptosis but at a higher degree than cardiomyocytes. Apoptotic intracellular parasites were observed to varied extents depending on the T. cruzi stock, which was related to the parasite invasion and proliferation. In summary, our results show that during T. cruzi infection, the extent of apoptosis varies according to the host cell type and the parasite stocks. The apoptosis of both host and T. cruzi can contribute to the silent spreading and persistence of the parasite without triggering an exacerbated inflammatory response.

Apoptosis induced by the homocamptothecin anticancer drug BN80915 in HL-60 cells

The homocamptothecin (hCPT) derivative BN80915 containing a seven-membered lactone ring represents one of the most potent topoisomerase I inhibitors described. This anticancer agent, currently undergoing phase I clinical trials, has been shown to produce a greater number of DNA strand breaks than conventional camptothecins with a six-membered lactone ring. To shed light on the mechanism of action of hCPT at the cellular level, we compared the effects of BN80915 and the classic camptothecin SN-38, the active metabolite of irinotecan, on HL-60 human promyelocytic cancer cells. A variety of biochemical events, at both the mitochondrial and the nuclear levels, were characterized to determine how and to what extent the hCPT derivative can induce apoptotic cell death. The use of cytometry, Western blot analysis, confocal microscopy, and different colorimetric assays enabled us to demonstrate that BN80915 is a potent inducer of apoptosis in HL-60 cells. This induction of apoptosis is associated with cell cycle changes, a marked decrease of intracellular pH, activation of caspase-3 and -8, DNA fragmentation, and externalization of phosphatidylserine lipids but no significant changes of the mitochondrial membrane potential or the expression of Bcl-2. The interconnections between these different events are discussed. Collectively, the results indicate that the superior activity expressed at the topoisomerase I level leads to a more pronounced induction of apoptosis by BN80915 compared with SN-38. The study identifies and delineates signaling factors involved in BN80915-induced apoptosis in HL-60 cells.

DNA topoisomerase II inhibition by peroxisomicine A(1) and its radical metabolite induces apoptotic cell death of HL-60 and HL-60/MX2 human leukemia cells

Peroxisomicine A(1) (T-514) is a dimeric anthracenone first isolated from the plant Karwinskia humboldtiana. The compound presents a high and selective toxicity toward liver and skin cell cultures and is currently the subject of preclinical studies as an antitumor drug. To date, the molecular basis for its diverse biological effects remains poorly understood. To elucidate its mechanism of action, we studied its interaction with DNA and its effects on human DNA topoisomerases. Practically no interaction with DNA was detected. Peroxisomicine was found to inhibit topoisomerase II but not topoisomerase I. DNA relaxation and decatenation assays indicated that the drug interferes with the catalytic activity of topoisomerase II but does not stimulate DNA cleavage, in contrast to conventional topoisomerase poisons such as etoposide. Two human leukemia cell lines sensitive or resistant to mitoxantrone were used to assess the cytotoxicity of the toxin and its effect on the cell cycle. In both cases, peroxisomicine treatment was associated with a loss of cells from every phase of the cell cycle and was accompanied by a large increase in the sub-G1 region which is characteristic of apoptotic cells. The cell cycle changes were more pronounced with the sensitive HL-60 cells than with the resistant HL-60/MX2 cells (with reduced topoisomerase II activity), in agreement with the cytotoxicity measurements. Treatment of HL-60 cells with T-514 stimulated the cleavage of the poly(ADP-ribose) polymerase by intracellular proteases such as caspase-3. The cytometry and Western blot analyses reveal that peroxisomicine induces apoptosis in leukemia cells. In addition, we characterized a catabolite of peroxisomicine, named T-510R, in the form of a highly stable radical metabolite. The electron spin resonance and mass spectrometry data are consistent with the formation of an anionic semiquinonic radical. The oxidized product T-510R inhibits topoisomerase II with a reduced efficiency compared to the parent toxin and was found to be about 3-4 times less toxic to both the sensitive and resistant leukemia cell lines than T-514. Collectively, the results suggest that topoisomerase II inhibition plays a role in the cytotoxicity of the plant toxin peroxisomicine. Inhibition of topoisomerase II may serve as an inducing signal triggering the apoptotic cell death of leukemia cells exposed to the toxin. The dihydroxyanthracenone unit may represent a useful chemotype for the preparation of topoisomerase II-targeted anticancer agents.

Topoisomerase I-DNA covalent complexes in human colorectal cancer xenografts with different p53 and microsatellite instability status: relation with their sensitivity to CTP-11

The topoisomerase I poison CPT-11 has proved efficient for the treatment of untreated metastatic colorectal cancers (CRC) and those refractory to fluoropyrimidines. However, the interpatient variability is important. A previous in vitro study suggested that measurements of the level of topoisomerase I-DNA intermediates trapped by camptothecin may be useful to estimate the chemosensitivity of colon carcinoma cell lines. To verify this hypothesis, we developed an immuno-assay to detect covalent topoisomerase I-DNA complexes in a series of human colorectal cancers xenografted in nude mice. Six human CRCs were selected for their distinctive p53 and microsatellite instability (MSI) status. Tumour lysates, prepared from mice untreated or treated with CPT-11, were fractionated onto CsCl gradients to separate free and DNA-bound topoisomerase I by centrifugation. Interestingly, significant levels of DNA-topoisomerase I complexes were detected in the tumours most responsive to the treatment with CPT-11, irrespective of their MSI and p53 phenotypes. Our in vivo study fully agrees with the predictions from the in vitro data indicating that evaluation of topoisomerase I-DNA complexes would be useful to predict the response of CRC to a treatment with CPT-11.

Cytotoxicity and cell cycle effects of the plant alkaloids cryptolepine and neocryptolepine: relation to drug-induced apoptosis

Cryptolepine and neocryptolepine are two indoloquinoline derivatives isolated from the roots of the african plant Cryptolepis sanguinolenta. These two alkaloids, which only differ by the respective orientation of their indole and quinoline rings, display potent cytotoxic activities against tumour cells and present antibacterial and antiparasitic properties. Our previous molecular studies indicated that these two natural products intercalate into DNA and interfere with the catalytic activity of human topoisomerase II. Here we have extended the study of their mechanism of action at the cellular level. Murine and human leukemia cells were used to evaluate the cytotoxicity of the drugs and their effects on the cell cycle were measured by flow cytometry. Cryptolepine, and to a lesser extent neocryptolepine, provoke a massive accumulation of P388 murine leukemia cells in the G2/M phase. With HL-60 human leukemia cells, the treatment with cryptolepine leads to the appearance of a hypo-diploid DNA content peak (sub-G1) characteristic of the apoptotic cell population. With both P388 and HL-60 cells, cryptolepine proved about four times more toxic than its isomer. But the use of the HL-60/MX2 cell line resistant to the anticancer drug mitoxantrone suggests that topoisomerase II may not represent the essential cellular target for the alkaloids, which are both only two times less toxic to the resistant HL-60/MX2 cells compared to the parental cells. The capacity of the drugs to induce apoptosis of HL-60 human leukemia cells was examined by complementary biochemical techniques. Western blotting analysis revealed that cryptolepine, but not neocryptolepine, induces cleavage of poly(ADP-ribose) polymerase but both alkaloids induce the release of cytochrome c from the mitochondria. The cleavage of poly(ADP-ribose) polymerase observed with cryptolepine correlates with the appearance of a marked sub-G1 peak in the cell cycle experiments. The proteolytic activity of Asp-Glu-Val-Asp- or Ile-Glu-Thr-Asp-caspases was found to be enhanced much more strongly with cryptolepine than with its isomer, as expected from their different cytotoxic potential. Despite the activation of the caspase cascade, we did not detect internucleosomal cleavage of DNA in the HL-60 cells treated with the alkaloids. Altogether, the results shed light on the mechanism of action of these two plant alkaloids.

Inhibition of topoisomerase II by the marine alkaloid ascididemin and induction of apoptosis in leukemia cells

Ascididemin (ASC) is a pentacyclic DNA-intercalating agent isolated from the Mediterranean ascidian Cystodytes dellechiajei. This marine alkaloid exhibits marked cytotoxic activities against a range of tumor cells, but its mechanism of action remains poorly understood. We investigated the effects of ASC on DNA cleavage by human topoisomerases I and II. Relaxation assays using supercoiled DNA showed that ASC stimulated double-stranded cleavage of DNA by topoisomerase II, but exerted only a very weak effect on topoisomerase I. ASC is a conventional topoisomerase II poison that significantly promoted DNA cleavage, essentially at sites having a C on the 3' side of the cleaved bond (-1 position), as observed with etoposide. The stimulation of DNA cleavage by topoisomerase I in the presence of ASC was considerably weaker than that observed with camptothecin. Cytotoxicity measurements showed that ASC was even less toxic to P388 leukemia cells than to P388CPT5 cells resistant to camptothecin. In addition, the marine alkaloid was found to be equally toxic to HL-60 leukemia cells sensitive or resistant to mitoxantrone. It is therefore unlikely that topoisomerases are the main cellular targets for ASC. This alkaloid was found to strongly induce apoptosis in HL-60 and P388 leukemia cells. Cell cycle analysis showed that ASC treatment was associated with a loss of cells in the G1 phase accompanied with a large increase in the sub-G1 region. Cleavage experiments with poly(ADP-ribose) polymerase (PARP) revealed that caspase-3 was a mediator of the apoptotic pathway induced by ASC. The DNA of ASC-treated cells was severely fragmented. Collectively, these findings indicate that ASC is a potent inducer of apoptosis in leukemia cells.

Apoptotic response of HL-60 human leukemia cells to the antitumor drug TAS-103

TAS-103 is a DNA intercalating indeno-quinoline derivative that stimulates DNA cleavage by topoisomerases. This synthetic drug has a broad spectrum of antitumor activity against many human solid tumor xenografts and is currently undergoing clinical trials. We investigated the induction of apoptosis in human promyelocytic leukemia cells treated with TAS-103. The treatment of proliferating human leukemia cells for 24 h with various concentrations of the drug induces significant variations in the mitochondrial transmembrane potential (delta(psi)mt) measured by flow cytometry using the fluorochromes 3,3-dihexyloxacarbocyanine iodide, Mitotracker Red, and tetrachloro-tetraethylbenzimidazolcarbocyanine iodide. The collapse of delta(psi)mt is accompanied by a marked decrease of the intracellular pH. Cleavage experiments with the substrates N-acetyl-Asp-Glu-Val-Asp-pNA, poly(ADP-ribose) polymerase, and pro-caspase-3 reveal unambiguously that caspase-3 is a key mediator of the apoptotic pathway induced by TAS-103. Caspase-8 is also cleaved, and the bcl-2 oncoprotein is underexpressed. Drug-induced internucleosomal DNA fragmentation and the externalization of phosphatidylserine residues in the outer leaflet of the plasma membrane were also characterized. The cell cycle perturbations produced by TAS-103 can be connected with the changes in deltapsi(mt). At low concentrations (2-25 nM), the drug induces a marked G2 arrest and concomitantly provokes an increase in the potential of mitochondrial membranes. In contrast, treatment of the HL-60 cells with higher drug concentrations (50 nM to 1 microM) triggers massive apoptosis and a collapse of deltaP(mt) that is a signature for the opening of the mitochondrial permeability transition pores. The discovery of a correlation between the G2 arrest and changes in mitochondrial membrane potential provides an important mechanistic insight into the action of TAS-103.

Relationship between cell cycle changes and variations of the mitochondrial membrane potential induced by etoposide

Etoposide, a clinically useful anticancer drug, is a potent inhibitor of topoisomerase II. The DNA strand breaks caused by this epipodophyllotoxin lead to apoptotic death of tumor cells. Flow cytometry was used to investigate the relationship between the effects of the drug on the cell cycle of human leukemia HL-60 cells and the variations of the mitochondrial transmembrane potential (DeltaPsi(mt)). Three cationic fluorescent probes, DiOC(6), JC-1, and TMRM, were used to measure drug-induced changes of DeltaPsi(mt). In all three cases, we found that the arrest in the G2/M phase of the cells treated with 0.5 microM etoposide is associated with an increase in the potential of mitochondrial membranes whereas treatment with a tenfold higher drug concentration trigger massive apoptosis and a collapse of DeltaPsi(mt). DNA fragmentation (TUNEL assay) and externalization of phosphatidylserine residues in the outer leaflet of the plasma membrane (annexin V binding) were measured to characterize the apoptotic cell population.

Homocamptothecin, an E-ring-modified camptothecin analogue, generates new topoisomerase I-mediated DNA breaks

Homocamptothecin (hCPT) contains a seven-membered beta-hydroxylactone in place of the conventional six-membered alpha-hydroxylactone ring found in camptothecin and its tumor active analogues, including topotecan and irinotecan. The homologation of the lactone E-ring reinforces the stability of the lactone, thus reducing considerably its conversion into a carboxylate form which is inactive. We have recently shown that hCPT is much more active than the parent compound against a variety of tumor cells in vitro and in xenograft models, suggesting that a highly reactive lactone is not essential for topoisomerase I-mediated anticancer activity [Lesueur-Ginot et al. (1999) Cancer Res. 59, 2939-2943]. In the present study, we provide further evidence that hCPT has superior topoisomerase I inhibition capacities to CPT. In particular, we show that replacement of the camptothecin lactone E-ring with a homologous seven-membered lactone ring changes the sequence-specificity of the drug-induced DNA cleavage by topoisomerase I. Both CPT and hCPT stimulate the cleavage by topoisomerase I at T( downward arrow)G sites, but in addition, hCPT stabilizes cleavage at specific sites containing the sequence AAC( downward arrow)G. At low drug concentrations, the cleavage at the T( downward arrow)G sites and at the hCPT-specific C( downward arrow)G sites is more pronounced and more stable with hCPT than with CPT. The in vitro data were confirmed in cells. Higher levels of protein-DNA complexes were detected in P388 leukemia cells treated with hCPT than those treated with CPT. Immunoblotting experiments revealed that endogenous topoisomerase I was efficiently trapped onto DNA by hCPT in cells. Finally, the use of a leukemia cell line resistant to CPT provided evidence that topoisomerase I is involved in the cytotoxicity of hCPT. Altogether, the results show that the beta-hydroxylactone ring of hCPT plays an important and positive role in the poisoning of topoisomerase I. An explanation is proposed to account for such remarkable changes in the sequence specificity of topoisomerase I cleavage consequent to the modification of the lactone. The study sheds new light on the importance of the lactone ring of camptothecins for the stabilization of topoisomerase I-DNA complexes.