Phase I and clinical pharmacological evaluation of aphidicolin glycinate

Ion Mobility-Derived Collision Cross-Sections Add Extra Capability in Distinguishing Isomers and Compounds with Similar Retention Times: The Case of Aphidicolanes

The hyphenation of ion mobility spectrometry with high-resolution mass spectrometry has been widely used in the characterization of various metabolites. Nevertheless, such a powerful tool remains largely unexplored in natural products research, possibly mainly due to the lack of available compounds. To evaluate the ability of collision cross-sections (CCSs) in characterizing compounds, especially isomeric natural products, here we measured and compared the traveling-wave IMS-derived nitrogen CCS values for 75 marine-derived aphidicolanes. We established a CCS database for these compounds which contained 227 CCS values of different adducts. When comparing the CCS differences, 36 of 57 pairs (over 60%) of chromatographically neighboring compounds showed a ΔCCS over 2%. What is more, 64 of 104 isomeric pairs (over 60%) of aphidicolanes can be distinguished by their CCS values, and 13 of 18 pairs (over 70%) of chromatographically indistinguishable isomers can be differentiated from the mobility dimension. Our results strongly supported CCS as an important parameter with good orthogonality and complementarity with retention time. CCS is expected to play an important role in distinguishing complex and diverse marine natural products.

Nuclear localization of endothelial nitric oxide synthase and nitric oxide production attenuates aphidicolin-induced endothelial cell death

Aphidicolin represses DNA replication by inhibiting DNA polymerase α and δ, which leads to cell cycle arrest and cell damage. Nitric oxide (NO) generated by endothelial NO synthase (eNOS) plays an essential role in maintenance of endothelial integrity including endothelial cell (EC) survival. Previously, we reported that aphidicolin increases NO production in bovine aortic ECs (BAECs). However, the role of aphidicolin-induced NO on EC viability and its molecular mechanism remain to be elucidated. Treatment with 20 μM aphidicolin for 24 h reduced BAEC viability by ~40%, which was accompanied by increased NO production, phosphorylation of eNOS at Ser1179 (p-eNOS-Ser¹¹⁷⁹), and eNOS protein expression. The aphidicolin-increased eNOS expression and p-eNOS-Ser¹¹⁷⁹ were not altered by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM), a cell permeable and specific intracellular Ca²⁺ chelator. Co-treatment with 2-phenyl-4, 4, 5, 5,-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), an NO scavenger, or Nω-Nitro-l-arginine methyl ester hydrochloride (l-NAME), a NOS inhibitor, exacerbated aphidicolin-stimulated BAEC death. Knockdown of eNOS gene expression using siRNA aggravated aphidicolin-induced BAEC death. However, exogenous NO donors including S-nitroso-l-glutathione (GSNO) or diethylenetriamine NONOate (DETA NO) had no effect on aphidicolin-decreased BAEC viability and aggravated BAEC viability at higher doses. Interestingly, aphidicolin accumulated eNOS protein in the active form, p-eNOS-Ser¹¹⁷⁹, in the nucleus. When cells were ectopically transfected with a wild-type (WT)-eNOS gene, aphidicolin induced significant localization of the protein product in the nucleus. Additionally, aphidicolin-elicited cell death was significantly reversed in WT-eNOS gene-transfected BAECs. Furthermore, overexpression of the eNOS gene containing nuclear localization signal (NLS) but not nuclear export signal (NES) significantly attenuated aphidicolin-induced BAEC death. When G2A-eNOS mutant lacking myristoylation at Gly2 was transfected, its intracellular distribution became diffuse and included the nucleus. Finally, expression of N-myristoyltransferase 2 (NMT2) but not NMT1 significantly decreased in aphidicolin-treated BAECs. Taken together, our results suggest that aphidicolin attenuates BAEC death in part by increasing nuclear eNOS localization and NO production.

Activation of ATM/Akt/CREB/eNOS Signaling Axis by Aphidicolin Increases NO Production and Vessel Relaxation in Endothelial Cells and Rat Aortas

Although DNA damage responses (DDRs) are reported to be involved in nitric oxide (NO) production in response to genotoxic stresses, the precise mechanism of DDR-mediated NO production has not been fully understood. Using a genotoxic agent aphidicolin, we investigated how DDRs regulate NO production in bovine aortic endothelial cells. Prolonged (over 24 h) treatment with aphidicolin increased NO production and endothelial NO synthase (eNOS) protein expression, which was accompanied by increased eNOS dimer/monomer ratio, tetrahydrobiopterin levels, and eNOS mRNA expression. A promoter assay using 5'-serially deleted eNOS promoters revealed that Tax-responsive element site, located at -962 to -873 of the eNOS promoter, was responsible for aphidicolin-stimulated eNOS gene expression. Aphidicolin increased CREB activity and ectopic expression of dominantnegative inhibitor of CREB, A-CREB, repressed the stimulatory effects of aphidicolin on eNOS gene expression and its promoter activity. Co-treatment with LY294002 decreased the aphidicolin-stimulated increase in p-CREB-Ser133 level, eNOS expression, and NO production. Furthermore, ectopic expression of dominant-negative Akt construct attenuated aphidicolin-stimulated NO production. Aphidicolin increased p-ATM-Ser1981 and the knockdown of ATM using siRNA attenuated all stimulatory effects of aphidicolin on p-Akt-Ser473, p-CREB-Ser133, eNOS expression, and NO production. Additionally, these stimulatory effects of aphidicolin were similarly observed in human umbilical vein endothelial cells. Lastly, aphidicolin increased acetylcholine-induced vessel relaxation in rat aortas, which was accompanied by increased p-ATM-Ser1981, p-Akt-Ser473, p-CREB-Ser133, and eNOS expression. In conclusion, our results demonstrate that in response to aphidicolin, activation of ATM/Akt/CREB/eNOS signaling cascade mediates increase of NO production and vessel relaxation in endothelial cells and rat aortas.

CHK1 Inhibition Is Synthetically Lethal with Loss of B-Family DNA Polymerase Function in Human Lung and Colorectal Cancer Cells

Checkpoint kinase 1 (CHK1) is a key mediator of the DNA damage response that regulates cell-cycle progression, DNA damage repair, and DNA replication. Small-molecule CHK1 inhibitors sensitize cancer cells to genotoxic agents and have shown single-agent preclinical activity in cancers with high levels of replication stress. However, the underlying genetic determinants of CHK1 inhibitor sensitivity remain unclear. We used the developmental clinical drug SRA737 in an unbiased large-scale siRNA screen to identify novel mediators of CHK1 inhibitor sensitivity and uncover potential combination therapies and biomarkers for patient selection. We identified subunits of the B-family of DNA polymerases (POLA1, POLE, and POLE2) whose silencing sensitized the human A549 non-small cell lung cancer (NSCLC) and SW620 colorectal cancer cell lines to SRA737. B-family polymerases were validated using multiple siRNAs in a panel of NSCLC and colorectal cancer cell lines. Replication stress, DNA damage, and apoptosis were increased in human cancer cells following depletion of the B-family DNA polymerases combined with SRA737 treatment. Moreover, pharmacologic blockade of B-family DNA polymerases using aphidicolin or CD437 combined with CHK1 inhibitors led to synergistic inhibition of cancer cell proliferation. Furthermore, low levels of POLA1, POLE, and POLE2 protein expression in NSCLC and colorectal cancer cells correlated with single-agent CHK1 inhibitor sensitivity and may constitute biomarkers of this phenotype. These findings provide a potential basis for combining CHK1 and B-family polymerase inhibitors in cancer therapy. SIGNIFICANCE: These findings demonstrate how the therapeutic benefit of CHK1 inhibitors may potentially be enhanced and could have implications for patient selection and future development of new combination therapies.

Aphidicolin Chemistry of the Deep-Sea-Derived Fungus Botryotinia fuckeliana MCCC 3A00494

Aphidicolin, a potent DNA polymerase α inhibitor, has been explored in clinical trials for the treatment of cancer. So far, about 300 modified aphidicolins have been discovered. However, none have shown a stronger effect. Herein, we report 71 new (aphidicolins A1-A71, 1-71) and eight known (72-79) aphidicolin congeners from Botryotinia fuckeliana MCCC 3A00494, a fungus isolated from the western Pacific Ocean (-5572 m). The structures of 1-71 were determined through extensive spectroscopic analysis, X-ray crystallography, chemical derivatization, modified Mosher's method, and the ECD exciton chirality method. Compounds 54-57 and 58-64 are novel 6/6/5/6/5 pentacyclic aphidicolins featuring tetrahydrofuran and dihydrofuran rings, respectively, while compounds 65-71 are rare noraphidicolins. Aphidicolin A8 (8) significantly induced apoptosis in T24 (IC₅₀ = 2.5 μM) and HL-60 (IC₅₀ = 6.1 μM) cancer cells by causing DNA damage. By docking its structure to the human DNA polymerase α binding pocket, 8 was found to form tight intermolecular contacts, elaborating aphidicolin A8 as a potently cytotoxic lead compound.

Genome-wide mapping and profiling of γH2AX binding hotspots in response to different replication stress inducers

BACKGROUND

Replication stress (RS) gives rise to DNA damage that threatens genome stability. RS can originate from different sources that stall replication by diverse mechanisms. However, the mechanism underlying how different types of RS contribute to genome instability is unclear, in part due to the poor understanding of the distribution and characteristics of damage sites induced by different RS mechanisms.

RESULTS

We use ChIP-seq to map γH2AX binding sites genome-wide caused by aphidicolin (APH), hydroxyurea (HU), and methyl methanesulfonate (MMS) treatments in human lymphocyte cells. Mapping of γH2AX ChIP-seq reveals that APH, HU, and MMS treatments induce non-random γH2AX chromatin binding at discrete regions, suggesting that there are γH2AX binding hotspots in the genome. Characterization of the distribution and sequence/epigenetic features of γH2AX binding sites reveals that the three treatments induce γH2AX binding at largely non-overlapping regions, suggesting that RS may cause damage at specific genomic loci in a manner dependent on the fork stalling mechanism. Nonetheless, γH2AX binding sites induced by the three treatments share common features including compact chromatin, coinciding with larger-than-average genes, and depletion of CpG islands and transcription start sites. Moreover, we observe significant enrichment of SINEs in γH2AX sites in all treatments, indicating that SINEs may be a common barrier for replication polymerases.

CONCLUSIONS

Our results identify the location and common features of genome instability hotspots induced by different types of RS, and help in deciphering the mechanisms underlying RS-induced genetic diseases and carcinogenesis.

Discovery of a novel DNA polymerase inhibitor and characterization of its antiproliferative properties

Chromosomal duplication is targeted by various chemotherapeutic agents for the treatment of cancer. However, there is no specific inhibitor of DNA polymerases that is viable for cancer management. Through structure-based in silico screening of the ZINC database, we identified a specific inhibitor of DNA polymerase δ. The discovered inhibitor, Zelpolib, is projected to bind to the active site of Pol δ when it is actively engaged in DNA replication through interactions with DNA template and primer. Zelpolib shows robust inhibition of Pol δ activity in reconstituted DNA replication assays. Under cellular conditions, Zelpolib is taken up readily by cancer cells and inhibits DNA replication in assays to assess global DNA synthesis or single-molecule bases by DNA fiber fluorography. In addition, we show that Zelpolib displays superior antiproliferative properties to methotrexate, 5-flourouracil, and cisplatin in triple-negative breast cancer cell line, pancreatic cancer cell line and platinum-resistant pancreatic cancer cell line. Pol δ is not only involved in DNA replication, it is also a key component in many DNA repair pathways. Pol δ is the key enzyme responsible for D-loop extension during homologous recombination. Indeed, Zelpolib shows robust inhibition of homologous recombination repair of DNA double-strand breaks and induces "BRCAness" in HR-proficient cancer cells and enhances their sensitivity to PARP inhibitors.

Targeting DNA repair with aphidicolin sensitizes primary chronic lymphocytic leukemia cells to purine analogs

Purine analogs are among the most effective chemotherapeutic drugs for the treatment of chronic lymphocytic leukemia (CLL). However, chemoresistance and toxicity limit their clinical use. Here, we report that the DNA polymerase inhibitor aphidicolin, which displayed negligible cytotoxicity as a single agent in primary CLL cells, markedly synergizes with fludarabine and cladribine via enhanced apoptosis. Importantly, synergy was recorded regardless of CLL prognostic markers. At the molecular level, aphidicolin enhanced purine analog-induced phosphorylation of p53 and accumulation of γH2AX, consistent with increase in DNA damage. In addition, aphidicolin delayed γH2AX disappearance that arises after removal of purine analogs, suggesting that aphidicolin causes an increase in DNA damage by impeding DNA damage repair. Similarly, aphidicolin inhibited UV-induced DNA repair known to occur primarily through the nucleotide excision repair (NER) pathway. Finally, we showed that fludarabine induced nuclear import of XPA, an indispensable factor for NER, and that XPA silencing sensitized cell lines to undergo apoptosis in response to fludarabine. Together, our data indicate that aphidicolin potentiates the cytotoxicity of purine analogs by inhibiting a DNA repair pathway that involves DNA polymerases, most likely NER, and provide a rationale for manipulating it to therapeutic advantage.

Common Chemical Inductors of Replication Stress: Focus on Cell-Based Studies

DNA replication is a highly demanding process regarding the energy and material supply and must be precisely regulated, involving multiple cellular feedbacks. The slowing down or stalling of DNA synthesis and/or replication forks is referred to as replication stress (RS). Owing to the complexity and requirements of replication, a plethora of factors may interfere and challenge the genome stability, cell survival or affect the whole organism. This review outlines chemical compounds that are known inducers of RS and commonly used in laboratory research. These compounds act on replication by direct interaction with DNA causing DNA crosslinks and bulky lesions (cisplatin), chemical interference with the metabolism of deoxyribonucleotide triphosphates (hydroxyurea), direct inhibition of the activity of replicative DNA polymerases (aphidicolin) and interference with enzymes dealing with topological DNA stress (camptothecin, etoposide). As a variety of mechanisms can induce RS, the responses of mammalian cells also vary. Here, we review the activity and mechanism of action of these compounds based on recent knowledge, accompanied by examples of induced phenotypes, cellular readouts and commonly used doses.

A Nonclonogenic Cytotoxicity Assay Using Primary Cultures of Patient Tumor Cells for Anticancer Drug Screening

Primary cultures of cells from patients with chronic lymphocytic leukemia (CLL) and ovarian carcinoma (Ovca) were compared with the renal carcinoma ACHN and the lymphocytic CCRF-CEM human tumor cell lines in response to 63 toxic or nontoxic compounds. The experiments were conducted at 1, 10, and 100 μ/ml in 96-well microtiter plates for an assay time of 72 h. The plates were analyzed by the fully automated Dynatech Immuno Assay System (DIAS) using Alamar blue as a fluorometric/colorimetric indicator of metabolic activity. Drug response data were reported as the area under the tumor cell survival-concentration curve (AUC). Noncytotoxic compounds were classified as inactive by all cell systems. According to the AUC, CCRF-CEM and CLL cells were the most sensitive, followed by ACHN, and then Ovca. Many of the clinically active drugs were detected by all cell systems. However, the sensitivity pattern differed considerably between the cell types as judged from correlation analysis, and a higher proportion of clinically inactive drugs were scored as active by the cell lines compared with the primary cultures. Ovca showed the highest ratio of clinically solid tumor active/clinically inactive agents followed by CLL. The results indicate that primary cultures of human tumor cells may be a useful and valuable model for anticancer drug screening.

The antitumor toxin CD437 is a direct inhibitor of DNA polymerase α

CD437 is a retinoid-like small molecule that selectively induces apoptosis in cancer cells, but not in normal cells, through an unknown mechanism. We used a forward-genetic strategy to discover mutations in POLA1 that coincide with CD437 resistance (POLA1(R)). Introduction of one of these mutations into cancer cells by CRISPR-Cas9 genome editing conferred CD437 resistance, demonstrating causality. POLA1 encodes DNA polymerase α, the enzyme responsible for initiating DNA synthesis during the S phase of the cell cycle. CD437 inhibits DNA replication in cells and recombinant POLA1 activity in vitro. Both effects are abrogated by the identified POLA1 mutations, supporting POLA1 as the direct antitumor target of CD437. In addition, we detected an increase in the total fluorescence intensity and anisotropy of CD437 in the presence of increasing concentrations of POLA1 that is consistent with a direct binding interaction. The discovery of POLA1 as the direct anticancer target for CD437 has the potential to catalyze the development of CD437 into an anticancer therapeutic.

Toward a Cancer Drug of Fungal Origin

Although fungi produce highly structurally diverse metabolites, many of which have served as excellent sources of pharmaceuticals, no fungi-derived agent has been approved as a cancer drug so far. This is despite a tremendous amount of research being aimed at the identification of fungal metabolites with promising anticancer activities. This review discusses the results of clinical testing of fungal metabolites and their synthetic derivatives, with the goal to evaluate how far we are from an approved cancer drug of fungal origin. Also, because in vivo studies in animal models are predictive of the efficacy and toxicity of a given compound in a clinical situation, literature describing animal cancer testing of compounds of fungal origin is reviewed as well. Agents showing the potential to advance to clinical trials are also identified. Finally, the technological challenges involved in the exploitation of fungal biodiversity and procurement of sufficient quantities of clinical candidates are discussed, and potential solutions that could be pursued by researchers are highlighted.

Exploiting replicative stress to treat cancer

DNA replication in cancer cells is accompanied by stalling and collapse of the replication fork and signalling in response to DNA damage and/or premature mitosis; these processes are collectively known as 'replicative stress'. Progress is being made to increase our understanding of the mechanisms that govern replicative stress, thus providing ample opportunities to enhance replicative stress for therapeutic purposes. Rather than trying to halt cell cycle progression, cancer therapeutics could aim to increase replicative stress by further loosening the checkpoints that remain available to cancer cells and ultimately inducing the catastrophic failure of proliferative machineries. In this Review, we outline current and future approaches to achieve this, emphasizing the combination of conventional chemotherapy with targeted approaches.

Structural basis for inhibition of DNA replication by aphidicolin

Natural tetracyclic diterpenoid aphidicolin is a potent and specific inhibitor of B-family DNA polymerases, haltering replication and possessing a strong antimitotic activity in human cancer cell lines. Clinical trials revealed limitations of aphidicolin as an antitumor drug because of its low solubility and fast clearance from human plasma. The absence of structural information hampered the improvement of aphidicolin-like inhibitors: more than 50 modifications have been generated so far, but all have lost the inhibitory and antitumor properties. Here we report the crystal structure of the catalytic core of human DNA polymerase α (Pol α) in the ternary complex with an RNA-primed DNA template and aphidicolin. The inhibitor blocks binding of dCTP by docking at the Pol α active site and by rotating the template guanine. The structure provides a plausible mechanism for the selectivity of aphidicolin incorporation opposite template guanine and explains why previous modifications of aphidicolin failed to improve its affinity for Pol α. With new structural information, aphidicolin becomes an attractive lead compound for the design of novel derivatives with enhanced inhibitory properties for B-family DNA polymerases.

Novel therapeutic agents in ovarian cancer

AIMS

Epithelial ovarian cancer is responsible for 4% of all cancer deaths in women, and the five-year overall survival of patients with advanced disease is 30-40%. Treatment currently comprises a combination of surgery and chemotherapy with carboplatin and paclitaxel. The main reason for treatment failure is that the majority of patients present with advanced disease, and current drugs are unable to effect a cure even in chemosensitive patients. This article systematically reviews novel therapeutic strategies that have been evaluated in patients with ovarian cancer in the last 5 years.

METHODS

Pubmed (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed) and American Society of Clinical Oncology Annual conference abstracts were searched using the terms "(phase I OR phase II OR phase III OR phase 1 OR phase 2 OR phase 3) AND (ovary OR ovarian) AND (cancer OR carcinoma)" from January 2000 to May 2005 to identify studies for potential inclusion in this review. Reviews of novel therapies in ovarian cancer were also used to identify additional clinical trials.

FINDINGS

A wide range of therapeutic strategies are currently being evaluated in ovarian cancer. These include novel cytotoxics, small molecule inhibitors, monoclonal antibodies, gene therapy and immuno-therapy strategies. The rationale for the development of these agents includes enhancement of efficacy by targeting novel biological pathways, re-sensitisation to existing drugs, simplification of drug administration and/or reduction of drug-toxicity.

CONCLUSIONS

Current developments have the potential to result in substantial improvements in the outlook for women with ovarian cancer.

Identification of compounds that enhance the anti-lymphoma activity of rituximab using flow cytometric high-content screening

In this report, we describe a new flow cytometry technique termed flow cytometric high-content screening (FC-HCS) which involves semi-automated processing and analysis of multiparameter flow cytometry samples. As a first test of the FC-HCS technique, we used it to screen a 2000-compound library, called the National Cancer Institute (NCI) Diversity Set, to identify agents that would enhance the anti-lymphoma activity of the therapeutic monoclonal antibody rituximab. FC-HCS identified 15 compounds from the Diversity Set that significantly enhanced the ability of rituximab to inhibit cell cycle progression and induce apoptosis in lymphoma cells. The validity of the screening results was confirmed for several compounds using additional assays of cell proliferation, apoptosis and cell growth. The FC-HCS technique was relatively simple and reliable and could process up to 1000 samples/day on a single flow cytometer. The FC-HCS technique may be useful for a variety of applications including drug discovery, immunologic monitoring of patients, functional genomics studies and tissue engineering efforts.

Ovarian cancer: strategies for overcoming resistance to chemotherapy

Ovarian cancer is responsible for 4% of deaths from cancer in women. Treatment comprises a combination of surgery and chemotherapy, but patients typically experience disease relapse within 2 years of the initial treatment. Further treatment can extend survival, although relapse eventually occurs again. A better understanding of the mechanisms that underlie this drug resistance should allow treatment to be optimized, so that substantial improvements in the outlook for women with this disease can be achieved.

The use of the MTT assay to study drug resistance in fresh tumour samples

We have found the short-term MTT assay to be a simple, reproducible chemosensitivity technique, suitable for use throughout the time course of disease. We now have 12 years' experience of using this method in a variety of tumour types, both haematological and solid malignancies. Tumour cells are isolated from bone marrow, malignant effusions or solid biopsies and subjected to drug exposure for 48-96 h. Cell survival is measured by re-incubation in MTT for 4 h. We have found a significant correlation of in vitro results with in vivo outcome for acute myeloid leukaemia (AML) and for ovarian cancer (both p < 0.0001) with an assay sensitivity of 98% for AML and 81% for ovarian cancer. Furthermore, the 5-year survival of ovarian cancer patients treated with a drug found sensitive in vitro is significantly higher than that for patients treated with a drug found resistant in vitro (p = 0.033). We have correlated assay results with drug resistance markers. For example, expression of the newly described half transporter BCRP is related to daunorubicin resistance (p < 0.05). The MTT assay is also suitable for screening for modulation of drug resistance. We have found that the DNA polymerase inhibitor aphidicolin markedly increases in vitro sensitivity to the platinum drugs in ovarian cancer and cytosine arabinoside in AML in the majority of patients. The greatest effect was seen for patients deemed resistant in vitro to these agents. We have identified novel drug combinations which demonstrate significant synergism using this methodology and have also used it to study the emergence of drug resistance in cell line models with a view to its prevention. In conclusion, we have found the MTT assay to be a simple, repeatable, adaptable technique which produces accurate information to help the clinician select suitable treatment for individual cases.

Treatment of drug-resistant human neuroblastoma cells with cyclodextrin inclusion complexes of aphidicolin

Treatment failure in most neuroblastoma (NB) patients is related to primary and/or acquired resistance to conventional chemotherapeutic agents. Aphidicolin (APH), a tetracyclic diterpene, exhibits specific cytotoxic action against NB cells. The purpose of this study was to compare antitumoral efficacy of APH in parental NB cell lines and cell subclones that exhibit drug resistance to vincristine (VCR), doxorubicin (DOX) and cisplatin. Due to poor solubility of APH in water, gamma-cyclodextrin (gamma-CD) inclusion complexes of APH were used for systemic treatment of xenotransplanted parental and VCR-resistant UKF-NB-3 tumours. APH and its gamma-CD inclusion complexes inhibited growth of parental and drug-resistant NB cells at equimolar doses in vitro. Growth of VCR-sensitive and -resistant NB tumors was inhibited at equal doses in a dose-dependent fashion in vivo. These results indicate that the specific cytotoxic activity of APH against NB cells in vitro and in vivo is independent of cellular mechanisms facilitating drug resistance to conventional chemotherapeutic drugs. Hence, taking into account our previous findings that APH acts synergistically with VCR and DOX, APH might be an additive tool for the therapy of NB and is suitable for evaluation in clinical studies of NB treatment protocols.

Circumvention of ara-C resistance by aphidicolin in blast cells from patients with AML

Treatment failure in AML is often attributed to P-glycoprotein-associated multidrug resistance. However, the importance of increased DNA repair in resistant cells is becoming more apparent. In order to investigate the ability of the DNA repair inhibitor aphidicolin to modulate drug resistance, we continually exposed blasts cells, isolated from 22 patients with AML, to a variety of agents +/- 15 microM aphidicolin for 48 hours. Cell survival was measured using the MTT assay. Overall, there was no significant effect of aphidicolin on sensitivity to daunorubicin, doxorubicin, etoposide or fludarabine. However, there was a marked increase in sensitivity to ara-C with a median 4.75-fold increase overall (range 0.8-80-fold;P< 0.005). The effect of aphidicolin was significantly greater in blast cells found resistant in vitro to ara-C (8.9-fold compared to 2.12-fold, P< 0.01). This observation was further validated by the correlation between ara-C LC(50)and extent of modulation effect (P< 0.05). Cells isolated from 10 cord blood samples were also tested in order to establish the haematological toxicity of combining ara-C and aphidicolin. The therapeutic index (LC(50)normal cells/tumour cells) for ara-C + aphidicolin was higher than that for ara-C alone suggesting no increased myelotoxicity for the combination. Increased cytotoxicity without increased haematotoxicity makes the combination of ara-C plus aphidicolin ideal for inclusion in future clinical trials.

Cytotoxicity of aphidicolin and its derivatives against neuroblastoma cells in vitro: synergism with doxorubicin and vincristine

Disseminated neuroblastoma diseases are still indicated by a poor outcome despite treatment regimens including radiation therapy and high-dose chemotherapy with stem cell rescue. Therefore, new substances and treatment regimens are of interest. Aphidicolin (APH), a tetracyclic diterpene antibiotic produced by Cephalosporium aphidicola, has a specific toxicity for neuroblastoma cells. Furthermore, it was shown to enhance the effects of X-ray radiation and chemotherapy on malignant cells. To find new substances, 20 APH derivatives were tested for their anti-neuroblastoma efficacy in vitro in UKF-NB-2 cells. Five derivatives had antitumoral activity in neuroblastoma cells. A relationship between the structure and the antitumoral efficacy showed that the hydroxyl groups at C-3 and C-18 are essential for the antitumoral effects. Furthermore, antitumoral effects of APH in combination with doxorubicin and vincristine, both part of commonly used treatment regimens for disseminated neuroblastoma diseases, were tested in the neuroblastoma cell line UKF-NB-2. APH was found to act synergistically with vincristine and synergistically to additive with doxorubicin depending on the molecular ratio of the substances in combination. This may offer the chance to use APH and its derivatives as additional tools in the treatment of neuroblastomas.

Mismatch repair and drug responses in cancer

Defects in mismatch repair contribute to development of approximately 15% of colon cancers and to origination of endometrial, gastric and other cancers. Tumors with defects in mismatch repair exhibit marked resistance to alkylators and a variety of anticancer agents that modify DNA to create substrates for the mismatch repair system. These altered drug responses appear to derive from requirements for mismatch repair proteins in signalling apoptosis, altered cell cycle checkpoint behaviour and/or loss of mismatch repair dependent toxicity arising from futile repair cycling. Altered repair mechanisms for mismatched substrates in mismatch repair defective tumors provide both challenges for development of tumor-phenotype-screening methodologies to assure appropriate therapy is administered for these cancers and foci for development of new therapy approaches that capitalize on modified drug responses in mismatch repair- defective cells. Copyright 1999 Harcourt Publishers Ltd.

Aphidicolin markedly increases the in vitro sensitivity to ara-C of blast cells from patients with AML

Drug resistant cells often have an increased capacity to repair their DNA after damage by cytotoxic agents. Aphidicolin can inhibit this DNA repair. We describe a study of the effect of aphidicolin to modulate the sensitivity to cytotoxic drugs of blast cells from 13 patients with AML, 11 with de novo disease on presentation and 2 secondary to MDS. Three patients had relapsed following previous therapy and samples were received from 1 patient both on presentation and relapse. Blast cells were exposed to anthracyclines, antimetabolites or etoposide +/- aphidicolin (15 microM) for 48 hours. The MTT assay was used to measure cell survival and the LC50 (concentration of drug required for 50% cell kill) was calculated. Overall, there was a significant increase in sensitivity to ara-C on co-incubation with aphidicolin in 12/14 samples (p = 0.007). The median increase in sensitivity was 3.88-fold (range 1.26- to 80-fold). Interestingly, when patients were grouped according to in vitro sensitivity to ara-C, cells from resistant patients demonstrated the greatest increase in sensitivity (median 14-fold compared to 2-fold for the sensitive group, p = 0.02). Despite the documented evidence for altered DNA repair as a mechanism of resistance to the topoisomerase II inhibitors, we found no significant increase in sensitivity to daunorubicin, doxorubicin or etoposide on co-incubation with aphidicolin. Nevertheless, we believe the unparalleled modulation of ara-C warrants further investigation.

Aphidicolin markedly increases the platinum sensitivity of cells from primary ovarian tumours

Enhanced DNA repair has been observed in cisplatin-resistant ovarian cancer cell lines. This resistance can be modulated, on co-incubation with aphidicolin in established cell lines and animal tumour models, by inhibiting DNA polymerases. We describe a study of the in vitro modulation effect of aphidicolin on cisplatin and carboplatin using fresh cells harvested from biopsy samples or ascitic fluids from 25 patients with ovarian adenocarcinoma. The MTT assay was used to measure cell survival after drug exposure. Aphidicolin (up to 30 microM) showed no cytotoxicity when tested alone. Forty-seven comparisons were made between drug with and without aphidicolin, and 37 (79%) cases demonstrated a significant increase in sensitivity to the platinum agents on co-incubation. Overall, there was a median 10-fold (range 1.64- to 58.5-fold) increase in sensitivity. When patients were grouped according to in vitro sensitivity to platinum, aphidicolin had a significantly greater effect in the "resistant' group, causing a median 13.5-fold increase in sensitivity compared with 2.4-fold in the "sensitive' group. Furthermore, a positive correlation between the LC50 for platinum and the corresponding fold increase in sensitivity suggests that aphidicolin overcomes platinum resistance in fresh cells from primary tumours. These results encourage the further development of this interesting compound.

High-dose chemotherapy with autologous stem cell support in advanced ovarian cancer

Since 1981, over 300 patients reported with advanced or refractory ovarian cancer have been treated with high-dose chemotherapy supported by autologous bone marrow or peripheral blood stem cell transplantation. Partial or complete clinical response has been reported in 54-100% of the cases, but the median duration of the response in the majority of patients has been only a few months. It is obvious from the available data that high-dose regimens supported by autologous stem cell transplantation (ASCT) are not capable of inducing long-term survival in patients with heavy tumour burden or chemoresistant ovarian cancer. Recent reports on nearly 100 patients have described results of the use of high-dose chemotherapy as first-line treatment for patients with optimally debulked disease or negative second-look laparotomy. Response rates and survival have been better when compared to historical controls, but the efficacy of this treatment modality in inducing durable remission has not been tested in randomized trials. Most of the ongoing trials presented briefly in this review have been designed to evaluate the potential of high-dose therapy as first-line treatment in preventing the development of resistant tumour clones and recurrence. The role of sequential high-dose chemotherapy with ASCT as a part of primary treatment or as salvage therapy for chemosensitive recurrent disease is also under investigation.

Clinical drug resistance: the role of factors other than P-glycoprotein

Clinical drug resistance is a major obstacle to successful chemotherapy for cancer. When it occurs, resistance to a wide range of agents is noted. This clinical observation should not be confused with so-called "multidrug resistance," which is a laboratory-based phenomenon, whereby cross-resistance in experimental models to structurally unrelated compounds is seen and is due to increased expression of P-glycoprotein (PGP). In the majority of cases of clinical drug resistance in solid tumors it is likely that other factors will play a major role. These other factors can be defined as pharmacologic or cellular. Pharmacologic factors are those that prevent an adequate degree of tumor cell exposure and include considerations of dose and schedule of drug, and also of drug metabolism, which may relate to concomitant medication and to genetic variations. Clinical maneuvers to circumvent drug resistance by increasing dose are so far of unproven value. Cellular factors are those that apply at the tumor cell itself, and it is probable that multiple mechanisms exist. These include considerations of drug uptake, activation/inactivation, and changes in target enzymes and in DNA repair processes. After DNA damage has occurred, a key determinant of the sensitivity of the tumor cell is its ability to undergo apoptosis. It is conceivable that failure to engage this process is a key factor in resistance to a number of drug classes, although there is little clinical evidence to support this at present. However, the genetic controls for the process of apoptosis are now being unraveled, and if this notion proves correct, the possibility will exist for the design of more rational means of circumventing drug resistance to a wide range of agents. In the meantime, strategies that should be pursued further in order to overcome this key clinical problem include further exploration of alternating or sequential drug schedules and using new non-cross-resistant agents, such as taxoids.

Promising new therapies in the treatment of advanced ovarian cancer

Advanced stage ovarian cancer is the most lethal gynecologic cancer. Despite initial response rates of 60-80% with platinum-based chemotherapy, more than 75% of women with this malignancy die of complications associated with this disease. There is a pressing need to find new chemotherapeutic agents for patients with advanced ovarian cancer. Phase II studies have identified paclitaxel as the most active drug in ovarian cancer since the introduction of cisplatin in the 1970s. Phase III studies will define the role of paclitaxel as initial therapy. Camptothecins (topotecan, CPT-11, 9-amino-camptothecin) inhibit topoisomerase I. CPT-11 and topotecan have shown activity in Phase II trials. Gemcitabine, a pyrimidine antimetabolite, has shown activity in Phase II trials. Other promising drugs (docetaxel, treosulfan) are under investigation. Modulation of drug resistance is being explored in Phase I/II studies. Clinical trials have been initiated with buthionine-sulfoximine, an inhibitor of glutathione biosynthesis, which decreases the ability of resistant cells to inactivate platinum compounds and alkylating agents. Cyclosporin has been shown to increase cisplatin cytotoxicity. Phase I trials have demonstrated the feasibility of combining cyclosporin and cisplatin. Phase II trials of cyclosporin analogs (PSC 833) and paclitaxel in refractory ovarian cancer are ongoing. Promising leads in drug development should provide new therapies for patients with ovarian cancer. Further research in the modulation of drug resistance may identify new mechanisms or strategies with which to prevent the emergence of drug resistance.

CRC/EORTC/NCI Joint Formulation Working Party: experiences in the formulation of investigational cytotoxic drugs

The pharmaceutical formulation of a new anti-tumour agent has often been perceived as the bottleneck in anti-cancer drug development. In order to increase the speed of this essential development step, the Cancer Research Campaign (CRC), the European Organization for Research and Treatment of Cancer (EORTC) and the National Cancer Institute (NCI) agreed in 1987 to form the Joint Formulation Working Party (JFWP). The main goal of the JFWP is to facilitate the rapid progress of a new drug through pharmaceutical developmental to preclinical toxicology and subsequently to phase I clinical trial. Under the auspices of the JFWP around 50 new agents have been developed or are currently in development. In this report we present our formulation experiences since the establishment of the JFWP with a selected number of agents: aphidicolin glycinate, bryostatin 1, carmethizole, carzelesin, combretastatin A4, dabis maleate, disulphonated aluminium phthalocyanine, E.O.9, 4-hydroxyanisole, pancratistatin, rhizoxin, Springer pro-drug, SRI 62-834, temozolomide, trimelamol and V489. The approaches used and problems presented may be of general interest to scientists in related fields and those considering submitting agents for development.

Modulation of resistance to cisplatin by amphotericin B and aphidicolin in human larynx carcinoma cells

The aim of this study was to examine whether resistance to cisplatin [cis-diamminedichloroplatinum (II)] (CDDP) could be overcome by amphotericin B, cyclosporin A and aphidicolin in two sublines of human larynx carcinoma HEp2 cells. The sensitivity of parental and cisplatin-resistant CA3 and CK2 cells to amphotericin B, cyclosporin A and aphidicolin, and also the effects of these drugs (given in maximal nontoxic concentrations) on cisplatin sensitivity were determined by clonogenic survival assay. CA3 and CK2 cells were sensitive to amphotericin B, and resistant to cyclosporin A and aphidicolin, compared with their parental cells. Amphotericin B increased cisplatin toxicity 2-fold in CA3 cells and 2.7-fold in CK2 cells, while it had no effect in parental HEp2 cells. Cyclosporin A did not influence the sensitivity of examined cells to cisplatin. The sensitizing effect of aphidicolin was more obvious in cisplatin-resistant cells. Cisplatin toxicity was increased by aphidicolin: 1.5-fold in HEp2 cells, 2-fold in CA3 cells, and 1.9-fold in CK2 cells. Therefore, the resistance to cisplatin in human larynx carcinoma CA3 and CK2 cells can be partially reversed by amphotericin B and aphidicolin.

Potentiation of chlorambucil toxicity in B-CLL lymphocytes using the DNA synthesis inhibitors aphidicolin and 1-beta-D-arabinofuranosylcytosine

Previous studies in our laboratory have identified enhanced cross-link repair as a primary mechanism of resistance to nitrogen mustards in B-cell chronic lymphocytic leukemia (B-CLL). To evaluate the therapeutic potential for modulation of DNA repair by aphidicolin and 1-beta-D-arabinofuranosylcytosine (ara-C), we examined the interaction between these two agents and chlorambucil in lymphocytes from untreated and treated-resistant B-CLL patients. We found that both aphidicolin and ara-C displayed synergy with chlorambucil over a range of inhibitor concentrations. This synergy was not restricted to the resistant samples. Our results indicate that these combinations can enhance the potency of chlorambucil in a clinically relevant model and should be considered for further preclinical and, eventually, clinical trials.

Future directions for paclitaxel (TAXOL) in the treatment of ovarian cancer

Ovarian cancer is the leading cause of death from gynecologic cancers in women in the United States. Although response rates to primary platinum-based therapy are high, relapse is common. The overall survival of patients with advanced-stage disease has not changed dramatically over the last 20 years, even with the advent of effective combination chemotherapy. Paclitaxel (TAXOL) is a new chemotherapeutic agent that shows promise for the treatment of patients with advanced disease as well as those with recurrent cisplatin-refractory disease. Overall response rates have been encouraging, although there are few pathological complete remissions. In contrast to other treatments, many patients achieve prolonged stable disease over multiple cycles of therapy. However, the optimal use of paclitaxel has not yet been determined. Current studies aim to evaluate dose intensity, administration schedule, multidrug regimens, sequence of drug administration, and combinations with radiation therapy. Future studies may address adjuvant therapy of early-stage disease. Results of these trials will clarify the role of paclitaxel in the primary treatment of ovarian cancer.

Alkylating agents and immunotoxins exert synergistic cytotoxic activity against ovarian cancer cells. Mechanism of action

Alkylating agents can be administered in high dosage to patients with ovarian cancer using autologous bone marrow support, but drug-resistant tumor cells can still persist. Immunotoxins provide reagents that might eliminate drug resistant cells. In the present study, concurrent treatment with alkylators and immunotoxins proved superior to treatment with each agent alone. Toxin immunoconjugates prepared from different monoclonal antibodies and recombinant ricin A chain (rRTA) inhibited clonogenic growth of ovarian cancer cell lines in limiting dilution assays. When alkylating agents and toxin conjugates were used in combination, the addition of the immunotoxins to cisplatin, or to cisplatin and thiotepa, produced synergistic cytotoxic activity against the OVCA 432 and OVCAR III cell lines. Studies performed to clarify the mechanism of action showed that cisplatin and thiotepa had no influence on internalization and binding of the 317G5-rRTA immunotoxin. Intracellular uptake of [195m]Pt-cisplatin was not affected by the immunoconjugate and thiotepa. The combination of the 317G5-rRTA and thiotepa, as well as 317G5-rRTA alone, increased [195m]Pt cisplatin-DNA adduct levels. The immunotoxin alone and in combination with the alkylators decreased intracellular glutathione levels and reduced glutathione-S-transferase activity. Repair of DNA damage induced by the combination of alkylators and 317G5-rRTA was significantly reduced when compared to repair after damage with alkylators alone. These findings suggest that immunotoxins affect levels and activity of enzymes required for the prevention and repair of alkylator damage.

Mass spectrometric identification and analysis of some aphidicolin metabolites in cancer patients

We used gas chromatography/mass spectrometry to identify and quantitate some aphidicolin metabolites in plasma and urine of patients receiving aphidicolin-17-glycinate. The major metabolite found in plasma was 3-ketoaphidicolin, present at about one thent the concentrations of aphidicolin. 3-Ketoaphidicolin undergoes dehydroxymethylation to give 18-nor-3-ketoaphidicolin. This metabolite was also found in plasma and its concentration reached a maximum of 1% of aphidicolin. Small amounts of aphidicolin and 3-ketoaphidicolin were found free in urine but almost all of the drug was conjugated to glucuronic acid, as shown by mass spectrometric analysis of urine extracts and by enzymatic digestion with beta-glucoronidase followed by gas chromatographic/mass spectrometric analysis.

Mechanisms and modulation of resistance to chemotherapy in ovarian cancer

Chemotherapy for advanced ovarian cancer remains suboptimal. Despite the improvements in objective response rates realized with cisplatin-based combination chemotherapeutic regimens, most patients still die of refractory cancer. Drug resistance has emerged as the single most important determinant of treatment outcome. Laboratory studies have provided substantial insights into the cellular mechanisms of resistance to the commonly used chemotherapeutic agents. Decreased drug accumulation, metabolic drug inactivation, and repair or tolerance to drug-induced cellular injury all contribute to resistance at the cellular level. Identification of these mechanisms has facilitated the development of specific treatment strategies, many of which are in or nearing clinical trials. These strategies include dose intensification, inhibition of P-glycoprotein function, inhibition of cellular glutathione synthesis, and inhibition of cellular DNA repair. The initial results from clinical trials that use these strategies provide reasonable grounds for optimism. In addition, efforts to identify new drugs with activity against resistant cells continue. One such drug, taxol, has significant activity in tumors refractory to conventional therapy. These approaches offer hope that intensive laboratory and clinical efforts ultimately will translate into real improvements in the efficacy of chemotherapy for ovarian cancer.

Aphidicolin selectively kills neuroblastoma cells in vitro

Aphidicolin is a tetracyclic diterpene antibiotic which is known to inhibit the growth of eucaryotic cells by reversible binding to DNA polymerase alpha without significant effect on cell viability in most common human cell lines. We observed that aphidicolin at a concentration of 5 x 10(-7) M kills all cells of four human neuroblastoma cell lines. In contrast, viability of normal human embryonal cells and of human continuous cell lines including HeLa, H9, A549 and Caco-2 was influenced only moderately by aphidicolin. In addition, neuroblastoma cells were killed after treatment with 5 x 10(-7) M aphidicolin in cocultures with normal embryonal cells which continued to proliferate after removal of aphidicolin. These results show that aphidicolin provides an agent which selectively kills neuroblastoma cells in vitro.

The impact of pharmacokinetically guided dose escalation strategies in phase I clinical trials: critical evaluation and recommendations for future studies

Phase I studies requiring multiple dose escalation steps have led to the development of pharmacokinetically guided dose escalation (PGDE) strategies to expedite the conduct of early clinical trials. This article critically reviews PGDE strategies for a number of new anticancer agents including amphethinile, brequinar sodium, iodo-doxorubicin, the anthrapyrazoles (DuP 941, DuP 942 and DuP 937), rhizoxin, and aphidicolin glycinate. The benefits and problems associated with PGDE are examined. Recommendations are made for the optimal deployment of pharmacological information in future phase I studies.

Do anticancer agents reach the tumor target in the human brain?

The development of effective chemotherapy for tumors of the central nervous system (CNS) is complicated in that the blood-brain barrier (BBB) hampers the penetration of most drugs into the brain and cerebrospinal fluid (CSF). This review summarizes the main reports on the distribution to CNS tumors and peritumoral normal brain of antitumor agents such as epipodophyllotoxins, cis-diamminedichloroplatinum(II), some nitrosoureas, bleomycin, vinblastine, and other clinically used antitumor agents as well as that of some experimental compounds with specific physicochemical properties. Drug levels were measured at surgical resection or in autopsy samples taken from patients who presented with different primary brain tumors or with brain metastases from extracerebral tumors. The observations made in each study were summarized in some detail, and the main points were then evaluated comparatively so as to highlight common aspects in the pharmacokinetic patterns of antitumor agents in human CNS tumors. Independently of their physicochemical properties, most antitumor agents appear to accumulate to a greater extent and to persist longer in intracerebral tumors than in the normal peritumoral brain. From in vitro cytotoxicity assays, it appears that epipodophyllotoxins, platinum compounds, bleomycin, and nitrosoureas reach potentially active therapeutic concentrations at the tumor target. However, all drugs have difficulty in reaching brain tissue adjacent to the tumor, as the intact BBB hampers their penetration. Plasma and CSF drug concentrations usually give little useful indication of the absolute quantity of drugs in brain tumors. To obtain a clear understanding of the CNS distribution of antitumor agents, one must determine whether the compound being measured is actually responsible for the observed activity and must consider the role of metabolites in the effect of the parent drug.