Cytotoxic Mode Of Action Research Articles

Cytotoxic Activity of LCS-1 is not Only due to Inhibition of SOD1.

The cytotoxic activity of the pyridazin-3-one derivative LCS-1 was previously suggested to be due to the inhibition of superoxide dismutase 1 (SOD1). However, no direct evidence was provided that LCS-1 inhibits SOD1 within cells. In this study, we investigated the cytotoxic activity of LCS-1 against bloodstream forms of Trypanosoma brucei, a protozoan parasite that does not express copper/zinc-containing SOD1, but an iron-containing superoxide dismutase (FeSOD). At 250 µM, LCS-1 did not inhibit the activity of FeSOD in cell lysates of bloodstream forms of T. brucei, confirming that the compound is a specific inhibitor of SOD1. However, LCS-1 displayed substantial trypanocidal activity with a minimum inhibitory concentration of 10 µM and a half-maximal effective concentration of 1.36 µM, indicating that the cytotoxic action of the compound cannot solely be due to inhibition of SOD1. The results of this study is an important finding as it shows that LCS-1 has more than one cytotoxic mode of action.

The biogenic amines putrescine and cadaverine show in vitro cytotoxicity at concentrations that can be found in foods

Putrescine and cadaverine are among the most common biogenic amines (BA) in foods, but it is advisable that their accumulation be avoided. Present knowledge about their toxicity is, however, limited; further research is needed if qualitative and quantitative risk assessments for foods are to be conducted. The present work describes a real-time analysis of the cytotoxicity of putrescine and cadaverine on intestinal cell cultures. Both BA were cytotoxic at concentrations found in BA-rich foods, although the cytotoxicity threshold for cadaverine was twice that of putrescine. Their mode of cytotoxic action was similar, with both BA causing cell necrosis; they did not induce apoptosis. The present results may help in establishing legal limits for both putrescine and cadaverine in food.

Oncolysis with DTT-205 and DTT-304 generates immunological memory in cured animals

Oncolytic peptides and peptidomimetics are being optimized for the treatment of cancer by selecting agents with high cytotoxic potential to kill a maximum of tumor cells as well as the capacity to trigger anticancer immune responses and hence to achieve long-term effects beyond therapeutic discontinuation. Here, we report on the characterization of two novel oncolytic peptides, DTT-205 and DTT-304 that both selectively enrich in the lysosomal compartment of cancer cells yet differ to some extent in their cytotoxic mode of action. While DTT-304 can trigger the aggregation of RIP3 in ripoptosomes, coupled to the phosphorylation of MLKL by RIP3, DTT-205 fails to activate RIP3. Accordingly, knockout of either RIP3 or MLKL caused partial resistance against cell killing by DTT-304 but not DTT-205. In contrast, both agents shared common features in other aspects of pro-death signaling in the sense that their cytotoxic effects were strongly inhibited by both serum and antioxidants, partially reduced by lysosomal inhibition with bafilomycin A1 or double knockout of Bax and Bak, yet totally refractory to caspase inhibition. Both DTT-304 and DTT-205 caused the exposure of calreticulin at the cell surface, as well as the release of HMGB1 from the cells. Mice bearing established subcutaneous cancers could be cured by local injection of DTT-205 or DTT-304, and this effect depended on T lymphocytes, as it led to the establishment of a long-term memory response against tumor-associated antigens. Thus, mice that had been cured from cancer by the administration of DTT compounds were refractory against rechallenge with the same cancer type several months after the disappearance of the primary lesion. In summary, DTT-205 and DTT-304 both have the capacity to induce immunotherapeutic oncolysis.

Cytotoxic Profiling of Endogenous Metabolites Relevant to Chronic Fatigue Immune Dysfunction Syndrome (CFIDS) on p53 Variant Human Colon Carcinoma Cell Lines.

Chemoprophylatic strategies against development of multifactorial diseases utilize compounds to block the multistep events in chronic inflammation and carcinogenesis. The successful chemopreventative candidate must therefore selectively inhibit growth of transformed cells and be administered frequently to confer maximal protection with minimal side effects. In addition to synthetic and exogenous natural compounds, endogenous metabolites represent another class of compounds that exhibit anticarcinogenic and anti-inflammatory properties contributing to proper cell function. To assess the effectiveness of these compounds warrants an understanding of their cytotoxic mode of action. In this study, p53 variant human colon carcinoma cell lines were chronically exposed to varying concentrations of the endogenous metabolites—phenyl acetate, ursodeoxycholate, and tauroursodeoxycholate—to determine the role of p53-induced cytotoxicity, with p53 mutant and deficient cell lines representing precancerous lesions. Cytotoxicity was assessed using clonogenic assays, and macroscopic colony counts were used to quantify cell survival. The results demonstrate that the bile acids, ursodeoxycholate and tauroursodeoxycholate, exhibit selective cytotoxicity toward nonfunctional p53 cell lines suggesting a p53-mediated role in inhibition of cell clonogenicity and potential chemopreventative properties. Although each compound displays this described effect, the tauroursodeoxycholate demonstrates high significance suggesting it might have practical uses in vivo.

Abstract B012: A novel in vitro approach elucidates a new mode of cytotoxic action of bispecific antibodies on hematologic malignancies

Abstract Objectives: Bispecific antibodies (BsAbs) act through the formation of an immunologic synapse between T-cells (CD3) and a tumor-associated surface antigen (TAA) leading to T-cell activation and serial lysis of tumor cells. The aim of the present study is to explore the mechanism of action (MOA) and the in vitro effect of BsAbs on hematologic samples with the PharmaFlow platform. Methods: Fresh whole bone marrow (BM) and peripheral blood (PB) from 41 samples from 3 different hematologic diseases (31 AML, 3 ALL, and 7 CLL) and two AML cell lines were tested with CD3-CD123 (for AML patients and cell lines) or CD3-CD19 (for ALL and CLL patients) BsAbs in the PharmaFlow platform, an innovative proprietary method that uses flow cytometry (FCM) to efficiently count the number of tumor cells killed by activated T-cells. We analyzed the populations of leukemic cells, activated T-cells, and residual normal cells. Additional key parameters were also used to explore the MOA after BsAb exposure at different time incubations (24h-144h), such as the effective E:T ratio (the number of T cells that kill a number of leukemic cells), real basal E:T ratio, tumor antigen expression, T-cell expansion, and expression of immune checkpoint proteins on target and effector cells before and after cell culture. For some experiments, fluorescence-activated cell sorting (FACS) was performed to evaluate T-cell cytotoxicity after BsAb exposure. Results: Most of the samples demonstrated T-cell activation and effective lysis of tumor cells after BsAb exposure independent of TAA expression and in a dose-response manner. Once sorted, these T cells could kill tumor cells in the absence of BsAb, as well as tumor cells that did not express the TAA target. Interestingly, these activated T cells selectively killed tumor cells with low cytotoxicity in residual normal cells from the same patients. Moreover, differential T-cell cytotoxicity was observed between samples. We observed samples with leukemic resistance or no T-cell activity (especially in CLL with CD3-CD19), as well as others with higher T-cell cytotoxicity and minimal number of activated T cells (especially in AML with CD3-CD123). The integration of all the predictive parameters (E:T ratios, Tumor-Specific Antigen (TSA) expression, etc.) allowed us to generate an in vitro response model and select samples with higher T-cell cytotoxicity after the BsAb exposure. Conclusion: Our findings are consistent with a model where, in addition to the standard MOA inducing tumor cells lysis by proximity, BsAbs can highly enrich cytotoxic clonal T-cell subsets with TSA and induce strong activation and proliferation of T cells capable of killing tumor cells in an effective and selective manner. This differential in vitro T-cell cytotoxicity effect between patients could allow us to select better candidates for adoptive antitumor immunotherapy with BsAbs. Citation Format: Daniel Primo, Pilar Hernandez, Julian Gorrochategui, Maria L. Vicente, David Martinez-Cuadron, Pau Montesinos, Joaquin Mártinez-López, Joan Ballesteros. A novel in vitro approach elucidates a new mode of cytotoxic action of bispecific antibodies on hematologic malignancies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B012.

Cytotoxic Profiling of Plant Secondary Metabolites on P53 Variant Human Colon Carcinoma Cell Lines

Chemoprevention strategies employ the use of compounds to inhibit the initiation, promotion, and progression phases of carcinogenesis. The successful chemopreventative candidate must therefore (1) selectively inhibit growth of transformed cells and (2) be administered on a frequent basis to confer maximal protection. Phytochemicals are a subclass of bioactive plant secondary metabolites that exhibit antioxidative, anticarcinogenic, and anti-inflammatory properties contributing to proper cell function. To assess the effectiveness of these compounds warrants an understanding of their cytotoxic mode of action. In this study, p53 variant human colon carcinoma cell lines were chronically exposed to varying concentrations of the phytochemicals—curcumin, andrographolide, and d-limonene—to determine the role of p53-induced cytotoxicity, with p53-mutant and p53-deficient cell lines representing precancerous lesions. Cytotoxicity was assessed using clonogenic assays and macroscopic colony counts were used to quantify cell survival. The results demonstrate that each phytochemical exhibits selective cytotoxicity toward nonfunctional p53 cell lines, suggesting a p53-mediated role in inhibition of cell clonogenicity and potential chemopreventative properties. Although each compound displays this described effect, only the d-limonene demonstrates considerable chemoprotection, suggesting it might have practical implications in vivo.

Six coordination compounds: mode of cytotoxic action and biological evaluation

This study describes the biological and anticancer properties of followed coordination compounds. IR spectra, magnetic properties, thermal analyses and crystal structures of six cyanido-complexes derivatives with [M II (CN) 4 ] 2- (M II = Ni and Pd) and [Co(CN) 6 ] 3- anions and N - bishydeten ( N,N -bis(2-hydroxyethyl)ethylenediamine) as a capping ligand have been previously reported. Here, we investigated these complexes denoted as[Ni( N-bishydeten )Ni(CN) 4 ] ( C1 ), [Zn 2 ( N-bishydeten ) 2 Ni(CN) 4 ] ( C2 ), [Ni( N-bishydeten )Pd(CN) 4 ] ( C3 ), [Cd( N-bishydeten ) 2 ][Pd(CN) 4 ] ( C4 ), [Ni 2 ( N-bishydeten ) 2 Co(CN) 6 ].3H 2 O ( C5 ) and K[Cd( N-bishydeten )Co(CN) 6 ].1.5H 2 O ( C6 ), which were tested for their anti-proliferative activity against human cervical cancer (HeLa), human colon cancer (HT29), rat glioma (C6) and African green monkey kidney (Vero) cell lines. The DNA/BSA binding affinities of these compounds were also elucidated by spectroscopic titrations, displacement experiments and electrophoresis measurements. Studies on cancerous cells revealed that C1 , C2 , C4 and C6 exhibited significant antitumor activity and inhibited tumor progression in testing cell lines and showed high solubility in the solvent. Absorbance and emission spectra data results revealed that the complexes interact with the DNA via groove binding mode of interaction. Overall, these compounds have been found to demonstrate effective anti-proliferative activity against the cancer cell lines, indicating that they are a potent candidate for preclinical or clinical study.

EFFECT OF DIPHTHERIA TOXIN T-DOMAIN ON ENDOSOMAL pH.

A key step in the mode of cytotoxic action of diphtheria toxin (DT) is the transfer of its catalytic domain (Cd) from endosomes into the cytosol. The main activity in this process is performed by the transport domain (Td), but the molecular mechanism of its action remains unknown. We have previously shown that Td can have some influence on the endosomal transport of DT The aim of this work was to study the effect of diphtheria toxin on the toxin compartmentalization in the intracellular transporting pathway and endosomal pH. We used recombinant fragments of DT which differed only by the presence of Td in their structure, fused with fluorescent proteins. It was shown that the toxin fragment with Td moved slower by the pathway early-late endosomes-lysosomes, and had a slightly different pattern of colocalization with endosomal markers than DT fragment without Td. In addition, endosomes containing DT fragments with Td had a constant pH of about 6.5 from the 10th to 50th minute of observation, for the same time endosomes containing DT fragments without Td demonstrated a decrease in pH from 6.3 to 5.5. These results indicate that Td inhibits acidification of endosomal medium. One of possible explanations for this may be the effect of the ion channel formed by the T-domain on the process of the endosomal acidification. This property of Td may not only inhibit maturation of endosomes but also inhibit activation of endosomal pH-dependent proteases, and this promotes successful transport of Cd into the cell cytosol.

In Vitro and In Vivo Experiments on Electrochemotherapy for Bladder Cancer

Electrochemotherapy is widely performed to treat solid tumors but experience with bladder cancer is limited. We investigated mitomycin C and cisplatin administered with electrochemotherapy for bladder cancer in vitro and in vivo. The human bladder cancer cell line SW780 was used. Cells were treated with electroporation, drug alone or electroporation plus increasing concentrations of drug (mitomycin C 0.001 to 2,000 μM or cisplatin 1.56 to 300 μM). Electrochemotherapy parameters were 8 pulses of 1.2 kV/cm for 99 microseconds at 1 Hz. We investigated survival and apoptosis, the latter evaluated by caspase activity. NMRI-Fox1nu nude mice were inoculated subcutaneously and randomized to 1) electrochemotherapy plus NaCl, 2) NaCl alone, 3) electrochemotherapy plus drug or 4) drug alone (mitomycin C 5 mM or cisplatin 250 μM). Tumors were measured 3 times per week. A similar experiment was done to assess necrosis by histology at days 2 and 6. In vitro mitomycin C cytotoxicity and caspase activity was unaffected by electrochemotherapy (p = 0.9057 and 0.53, respectively). However, electrochemotherapy with cisplatin caused 6.6-fold increased cytotoxicity and higher caspase activity (p <0.0001 and <0.001, respectively). In vivo electrochemotherapy plus mitomycin C resulted in tumor volume reduction (p <0.0005). The survival rate in mice that received electrochemotherapy plus mitomycin C and mitomycin C alone was greater than in controls (p = 0.0004). The tumor response rate was 100% for electrochemotherapy plus mitomycin C, 53% for mitomycin C alone, 14% for electrochemotherapy plus NaCl and 0% for NaCl alone. In vivo electrochemotherapy plus cisplatin was associated with slower tumor growth over other combinations as well as significantly higher survival (p = 0.0005 and 0.0003, respectively). The tumor response rate was 47% for electrochemotherapy plus cisplatin, 0% for cisplatin alone, 0% for electrochemotherapy plus NaCl and 8% for NaCl alone In vivo electrochemotherapy with mitomycin C or cisplatin was more effective than chemotherapy alone in a bladder cancer tumor model, opening new perspectives in bladder cancer therapy.

Elegaphenone and 7-epi-clusianone, the major cytotoxic constituents of Hypericum elegans

Two cytotoxic constituents, namely elegaphenone and 7-epi-clusianone, were isolated from the aerial parts of Hypericum elegans Stepan ex Willd. Elegaphenone was identified as (E)-(2-(3,7-dimethylocta-2,6-dienyloxy)-4,6-dihydroxyphenyl)(phenyl)methanone by means of spectral evidence. Both compounds showed prominent cytotoxicity on HD-MY-Z, K-562 and KE-37 tumour cell lines. The IC50 values for elegaphenone were 15.9 (HD-MY-Z), 13.9 (K-562) and 16.9 (KE-37) µmol while those of 7-epi-clusianone were 9.8 (HD-MY-Z), 11.8 (K-562) and 13.6 (KE-37) µmol. The established oligonucleosomal deoxyribonucleic acid (DNA) fragmentation of genomic DNA following short-term (6 h) or long-term (24 h) exposure to the tested compounds clearly indicates that the induction of apoptotic cell death is an important component for their cytotoxic mode of action.

The impact of S- and G2-checkpoint response on the fidelity of G1-arrest by cisplatin and its comparison to a non-cross-resistant platinum(IV) analog

Objective Cisplatin is a DNA-damaging antitumor agent that is highly effective in treating ovarian cancer. It activates the p53/p21 pathway for its cytotoxic mode of action, but it does not induce p21-dependent cell cycle arrest in G1. Therefore, we investigated this paradox, and used the model analog DAP as a positive control for p21-dependent G1-arrest. Methods Studies were conducted in p53-proficient ovarian A2780 tumor cells to examine Cdk activity, cell cycle distribution and DNA damage signaling after cisplatin or DAP in combination with the mitotic inhibitor nocodazole. Results Cisplatin consistently induced transient S-phase arrest by inhibiting Cdk2/cyclin A complex in S-phase at 12 h and then a durable G2/M-arrest by inhibiting Cdc2/cyclin B complex at 12–18 h. These inhibitions were associated with Chk1 and Chk2 activation and resultant increase in inhibitory tyrosine phosphorylation of Cdk2 and Cdc2. Cisplatin also potently inhibited G1-phase Cdk4/cyclin D1 and Cdk2/cyclin E activities at ~ 18 h. In agreement, exposure of cisplatin-treated A2780, HCT-116 p53−/− and HCT-116 p21−/− tumor cells to nocodazole revealed limited G1-arrest that was dependent on p53 and p21. In contrast, the durable G1-arrest by DAP, which failed to activate Chk1 and Chk2, was unaffected by nocodazole. Conclusions Cisplatin induced G1-arrest, but at an attenuated level. This was primarily due to orchestration of Cdk inhibition in S-phase first, then in G2, and finally in G1 that effectively blocked cells in G2 and prevented cells from progressing and arresting in G1. These studies demonstrate that cisplatin unequivocally activates G1-checkpoint response, but the fidelity of G1-arrest is compromised by Chk1/2 activation and checkpoint response in S- and G2/M-phase.

Abstract 176: A characteristic set of microRNAs are deregulated in pre-neoplastic liver exposed to chemical carcinogens

Abstract This study aimed to identify microRNAs (miRNAs) that were deregulated in pre-neoplastic liver exposed to chemical hepatocarcinogens. Expressions of miRNAs are dysregulated in liver diseases, including hepatocellular carcinomas (HCC). However, the hepatic miRNA profiles at the early stages of liver carcinogenesis, which may be important in cancer initiation and progression, have not so far been thoroughly examined. Male rats were dietary exposed to the maximum tolerated dose of eight compounds, six carcinogens and two non-carcinogens, for a period of 90 days. The selected carcinogens were a diverse group including a genotoxic carcinogen, one carcinogen with a cytotoxic mode of action, a PPARa agonist, a CAR activator, and two chemicals with unknown carcinogenic mechanisms. The non carcinogens were one CAR activator and a chemical with no liver effects. MiRNA microarrays were then used to profile the animals’ liver It was found that at 90 days all tested carcinogens resulted in altered expression of liver miRNAs. It is predicted that some miRNAs altered by a given carcinogen will be chemical-specific and irrelevant to the process of carcinogenesis. Conversely, miRNAs that were deregulated by several carcinogens are more likely to be involved in liver carcinogenesis. A set of ten miRNAs that were deregulated by two or more of the tested carcinogens were identified. These included the miR-200a/200b/429 cluster (induced by five carcinogens), miR-34a (induced by four carcinogens), and the miR-182/96 cluster (induced by four carcinogens). The biggest induction of the miR-200a/200b/429 and miR-182/96 clusters was observed in the livers of animals treated with the CAR activator phenobarbital (PB). To uncover the temporal and dose pattern of the deregulation of these clusters their hepatic expression was assessed in animals treated with one carcinogenic and two non-carcinogenic doses of PB for 1,3,7, and 14 days. A progressive pattern of deregulation was found, with both clusters showing the greatest induction at 90 days, a smaller induction at 14 days, and little or no deregulation at earlier timepoints. Moreover, it was found that induction of the miR-200a/200b/429 and miR-182/96 clusters was specific to the carcinogenic dose of PB. Therefore, the effect of PB on these clusters is both time and dose dependent. Induction of miR-200b was confirmed by qPCR and immunoblotting of its predicted target proteins zeb1 and zeb2. In conclusion, the diverse chemical carcinogens tested here result in the deregulation of a characteristic set of miRNAs in pre-neoplastic liver. These characteristic set did not include a number of miRNAs that have previously been reported to be deregulated in later stages of HCC development (e.g. miR-122, miR-21, miR-221 and let-7). It is possible that the miRNAs identified here are involved in the earliest stages of chemical hepatocarcinogenesis. 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 176. doi:10.1158/1538-7445.AM2011-176

Effect of Variants at the 78th Residue on Neocarzinostatin Chromophore Release

Neocarzinostatin is a potent antitumor antibiotics chromoprotein complex. It consists of a labile enediyne chromophore and a protective carrier protein. Release of the bioactive chromophore is the initial key step in its cytotoxic mode of action. We showed that F78 of the carrier protein plays an important role in gating the release. To further study the structural role of 78th side chain in the release mechanism, we made variants at the 78th residue using a PCR-based in vitro mutagenesis method. The identity and purity of the expressed protein variants were confirmed by SDS-PAGE, UV, HPLC, and MS spectroscopy. Oxygenation was applied to promote formation of disulfide bonds when necessary, and the integrity of disulfide linkages was verified by a designed iodoacetamide-based test. Thermal denaturation study and CD spectroscopic analysis showed that stability and backbone conformation of these variants were conserved as compared to the wild-type protein. Two-dimensional 15N-1H HSQC NMR study suggested that the majority of residues around the binding cleft were not disturbed. Reconstitution of the enediyne chromophore into these protein variants showed efficient binding and the binding structure appeared to be similar to that of the native neocarzinostatin. Once the backbone structural effect was excluded, side chain structural effect on kinetic release was studied by monitoring fluorescence changes on samples containing glutathione and the reconstituted variants. The results showed that, except aromatic side chains, the basic variants increased the release rate more significantly than the acidic ones. Depended upon the length, steric hindrance, or hydrophobicity, the variants with aliphatic side chain also increased the release rate at different extent.

Mode of action of the novel phenazine anticancer agents XR11576 and XR5944

The substituted phenazines XR11576 and XR5944 were originally described as dual topoisomerase-I/II poisons. Subsequent reports, however, indicated that the association of their cytotoxicity with cellular topoisomerases was not clear. We set out to study this further using human tumour cell lines, PEO1 ovarian cancer, MDA-MB-231 breast cancer and variants with acquired resistance to VP-16 and XR11576: PEO1VPR, MB-231VPR, MB-231-11576R and camptothecin: PEO1CamR. Cytotoxicity testing [3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl tetrazolium bromide assay], DNA-protein crosslink formation, cell cycle analysis (flow cytometry) for DNA content, apoptosis (flow cytometry) for Annexin V and Western blotting for apoptotic factors. Cytotoxicity testing showed potent cytotoxicity with no cross-resistance to XR11576 or XR5944 in VP-16 or camptothecin-resistant lines. Importantly, we have shown for the first time that the activities of XR5944 and XR11576 are similar as MB-231-11576R cells were resistant to both agents and to a similar extent. XR5944 showed the greatest, albeit slower, interaction with DNA with high levels of DNA-protein crosslinks. Levels of apoptosis in XR5944-treated cells were significantly less than those in VP-16 or XR11576 treatments, suggestive of a more cytostatic rather than cytotoxic mode of action. Interestingly, XR5944 failed to give rise to a G2/M blockade, in contrast to VP-16 or XR11576. XR5944 and XR11576, in line with a dual topoisomerase-I/II-directed mechanism of action, retain potent activity in tumour cells with acquired resistance to VP-16 and camptothecin. Although these agents appear to behave differently from each other according to experimental conditions, this study suggests a substantial overlap in their mechanism(s) of action.

Mode of Action in Relevance of Rodent Liver Tumors to Human Cancer Risk

Hazard identification and risk assessment paradigms depend on the presumption of the similarity of rodents to humans, yet species specific responses, and the extrapolation of high-dose effects to low-dose exposures can affect the estimation of human risk from rodent data. As a consequence, a human relevance framework concept was developed by the International Programme on Chemical Safety (IPCS) and International Life Sciences Institute (ILSI) Risk Science Institute (RSI) with the central tenet being the identification of a mode of action (MOA). To perform a MOA analysis, the key biochemical, cellular, and molecular events need to first be established, and the temporal and dose-dependent concordance of each of the key events in the MOA can then be determined. The key events can be used to bridge species and dose for a given MOA. The next step in the MOA analysis is the assessment of biological plausibility for determining the relevance of the specified MOA in an animal model for human cancer risk based on kinetic and dynamic parameters. Using the framework approach, a MOA in animals could not be defined for metal overload. The MOA for phenobarbital (PB)-like P450 inducers was determined to be unlikely in humans after kinetic and dynamic factors were considered. In contrast, after these factors were considered with reference to estrogen, the conclusion was drawn that estrogen-induced tumors were plausible in humans. Finally, it was concluded that the induction of rodent liver tumors by porphyrogenic compounds followed a cytotoxic MOA, and that liver tumors formed as a result of sustained cytotoxicity and regenerative proliferation are considered relevant for evaluating human cancer risk if appropriate metabolism occurs in the animal models and in humans.

A Novel, Evolutionarily Conserved Protein Phosphatase Complex Involved in Cisplatin Sensitivity

Using a combination of tandem affinity purification tagging and mass spectrometry, we characterized a novel, evolutionarily conserved protein phosphatase 4 (PP4)-containing complex (PP4cs, protein phosphatase 4, cisplatin-sensitive complex) that plays a critical role in the eukaryotic DNA damage response. PP4cs is comprised of the catalytic subunit PP4C; a known regulatory subunit, PP4R2; and a novel protein that we termed PP4R3. The Saccharomyces cerevisiae PP4R3 ortholog Psy2 was identified previously in a screen for sensitivity to the DNA-damaging agent and anticancer drug cisplatin. We demonstrated that deletion of any of the PP4cs complex orthologs in S. cerevisiae elicited cisplatin hypersensitivity. Furthermore human PP4R3 complemented the yeast psy2 deletion, and Drosophila melanogaster lacking functional PP4R3 (flfl) exhibited cisplatin hypersensitivity, suggesting a highly conserved role for PP4cs in DNA damage repair. Finally we found that PP4R3 may target PP4cs to the DNA damage repair machinery at least in part via an interaction with Rad53 (CHK2).

Cisplatin: mode of cytotoxic action and molecular basis of resistance.

Cisplatin is one of the most potent antitumor agents known, displaying clinical activity against a wide variety of solid tumors. Its cytotoxic mode of action is mediated by its interaction with DNA to form DNA adducts, primarily intrastrand crosslink adducts, which activate several signal transduction pathways, including those involving ATR, p53, p73, and MAPK, and culminate in the activation of apoptosis. DNA damage-mediated apoptotic signals, however, can be attenuated, and the resistance that ensues is a major limitation of cisplatin-based chemotherapy. The mechanisms responsible for cisplatin resistance are several, and contribute to the multifactorial nature of the problem. Resistance mechanisms that limit the extent of DNA damage include reduced drug uptake, increased drug inactivation, and increased DNA adduct repair. Origins of these pharmacologic-based mechanisms, however, are at the molecular level. Mechanisms that inhibit propagation of the DNA damage signal to the apoptotic machinery include loss of damage recognition, overexpression of HER-2/neu, activation of the PI3-K/Akt (also known as PI3-K/PKB) pathway, loss of p53 function, overexpression of antiapoptotic bcl-2, and interference in caspase activation. The molecular signature defining the resistant phenotype varies between tumors, and the number of resistance mechanisms activated in response to selection pressures dictates the overall extent of cisplatin resistance.

Cytotoxic and cytostatic activity of erlangerins from Commiphora erlangeriana

The resin of Commiphora erlangeriana is known to be poisonous to humans and animals and has traditionally been used as an arrow poison. Since recent phytochemical studies on this plant material has identified four major lignans (named, Erlangerins A to D) that closely relate to the structure of podophyllotoxin, it was hypothesised that the well known poisoning effect of the resin could in part be due to direct toxicity to mammalian cells. Hence, the toxicity of Erlangerins was studied by measuring the viability of two human (HeLa and EAhy926) and two murine (L929 and RAW 264.7) cell lines. As assessed by the MTT assay, the effect of Erlangerin C and D closely follow the activity profile of podophyllotoxin: they induced a concentration-dependent cytotoxicity in the murine macrophage cells (RAW 264.7) and a cytostatic effect in HeLa, EAhy926 and L929 cells. In contrast, Erlangerins A and B suppressed cell viability at relatively higher concentrations (EC 50 values higher than 3 μM as compared with nM concentration range for Erlangerins C and D and podophyllotoxin) and their activity appears to be consistent with a cytotoxic mode of action in all cell lines studied. The structure–activity-relationship established from the study is discussed.

Relationship between cytocidal activity and glutathione-S-transferase inhibition using doxorubicin coupled to stereoisomers of glutathione with different substrate specificity.

To determine the cytotoxic mode of action of a glutathione (GSH)--doxorubicin (DXR) conjugate, which exhibited potent cytotoxicity against various multidrug-resistant as well as DXR-sensitive cell lines, the molecular interaction between covalent GSH--DXR conjugates and glutathione-S-transferase (GST), a possible molecular target of the conjugates, was investigated. The following four GSH molecules with stereoisomeric forms were prepared: L-Glu--L-Cys--Gly (LL-GSH), D-Glu--L-Cys--Gly (DL-GSH), L-Glu--D-Cys--Gly (LD-GSH) and D-Glu--D-Cys--Gly (DD-GSH). The enzymic activity of GST against each GSH stereoisomer was 88, 38, 8 and 4 nmol/mg/min, respectively, suggesting that the L-form cysteine residue in the molecule was an important substrate of GST. Addition of DXR conjugated with each isomer (10 microM) to a GSH-containing GST assay mixture inhibited the GST activity to 32% for LL-GSH--XR, 16% for DL-GSH-DXR and 61% for LD-GSH-DXR as compared with the solvent control. Moreover, IC50 values for these conjugates were 30, 20 and 250 nM, respectively. The cytocidal activity of each conjugate corresponded to the substrate specificity of GST activity for the GSH isomer. These conjugates bound to the GST molecule, and the binding ability was 0.746, 0.627 and 0.462 mol/mol of GST for LL-GSH--XR, DL-GSH-DXR and LD-GSH--XR, respectively. These findings suggested that GSH--DXR interacted with the substrate-binding site of the GST molecule and inhibition of GST activity exhibited potent cytotoxicity.

Cytotoxicity of two novel cisplatin analogues, (CPA)2Pt[DOLYM] and (DACH)Pt[DOLYM], to human cancer cellsin vitro

Despite the impressive antitumor activity of cisplatin, two major limitations of the drug, that is severe side effects and drug-resistance of cancer cells, make its use difficult for cancer therapy. These limitations have resulted in a great deal of effort having been expended into structural modifications of cisplatin. In this study, we tested two novel cisplatin analogues, (CPA)2Pt [DOLYM] (COMP-I) and (DACH)Pt[DOLYM] (COMP-II), for the mode of cytotoxic action against human tumor cells comparing with cisplatin and carboplatin in vitro. These two novel analogues had considerable cytotoxic activities against five kinds of human solid tumor cells, and especially COMP-II was more effective on HCT15 colon cancer cells than other compounds. In addition, COMP-II had cytostatic activity at low concentrations (10-0.3 microgram/ml), but other compounds revealed little effect on tumor growth at the low concentration.