Synthesis and biological evaluation of novel analogues of the pan class I phosphatidylinositol 3-kinase (PI3K) inhibitor 2-(difluoromethyl)-1-[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474)

A structure-activity relationship (SAR) study of the pan class I PI 3-kinase inhibitor 2-(difluoromethyl)-1-[4,6-di(4-morpholinyl)-1,3,5-triazin-2-yl]-1H-benzimidazole (ZSTK474) identified substitution at the 4 and 6 positions of the benzimidazole ring as having significant effects on the potency of substituted derivatives. The 6-amino-4-methoxy analogue displayed a greater than 1000-fold potency enhancement over the corresponding 6-aza-4-methoxy analogue against all three class Ia PI 3-kinase enzymes (p110α, p110β, and p110δ) and also displayed significant potency against two mutant forms of the p110α isoform (H1047R and E545K). This compound was also evaluated in vivo against a U87MG human glioblastoma tumor xenograft model in Rag1(-/-) mice, and at a dose of 50 mg/kg given by ip injection at a qd × 10 dosing schedule it dramatically reduced cancer growth by 81% compared to untreated controls.

Comparison of the effects of the PI3K/mTOR inhibitors NVP-BEZ235 and GSK2126458 on tamoxifen-resistant breast cancer cells

BACKGROUND

Treatment with anti-estrogens or aromatase inhibitors is commonly used for patients with estrogen receptor-positive (ER+) breast cancers; however resistant disease develops almost inevitably, requiring a choice of secondary therapy. One possibility is to use inhibitors of the PI3K/mTOR pathway and several candidate drugs are in development. We examined the in vitro effects of two inhibitors of the PI3K/mTOR pathway on resistant MCF-7 cells.

METHODS

We cultured MCF-7 cells for prolonged periods either in the presence of the anti-estrogen tamoxifen (3 sub-lines) or in estrogen free medium (2 sub-lines) to mimic the effects of clinical treatment. We then analyzed the effects of two dual PI3K/mTOR phosphoinositide-3-kinase inhibitors, NVP-BEZ235 and GSK2126458, on the growth and signaling pathways of these MCF-7 sub-lines. The functional status of the PI3K, mTOR and ERK pathways was analyzed by measuring phosphorylation of AKT, p70S6K, rpS6 and ERK.

RESULTS

The derived sub-lines showed increased resistance to tamoxifen but none exhibited concomitantly increased sensitivity to the PI3K inhibitors. NVP-BEZ235 and GSK2126458 acted mainly by induction of cell cycle arrest, particularly in G1-phase, rather than by induction of apoptosis. The lines varied considerably in their utilization of the AKT, p70S6K and ERK pathways. NVP-BEZ235 and GSK2126458 inhibited AKT signaling but NVP-BEZ235 showed greater effects than GSK2126458 on p70S6K and rpS6 signaling with effects resembling those of rapamycin.

CONCLUSION

Increased resistance to tamoxifen in these MCF-7 sub-lines is not associated with hypersensitivity to PI3K inhibitors. While both drugs inhibited AKT signaling, NVP-BEZ235 resembled rapamycin in inhibiting the mTOR pathway.

Temporal aspects of the action of ASA404 (vadimezan; DMXAA)

IMPORTANCE OF THE FIELD

Tumor vascular disrupting agents (tumor VDAs) act by selective induction of tumor vascular failure. While their action is distinct from that of antiangiogenic agents, their clinical potential is likely to reside in improving the efficacy of combination therapy.

AREAS COVERED IN THIS REVIEW

This review describes the preclinical development, clinical trial and mode of action of ASA404, a flavonoid class tumor VDA. This class has a unique dual action, simultaneously disrupting vascular endothelial function and stimulating innate tumor immunity. This review covers the early development of ASA404, through to Phase III trial.

WHAT THE READER WILL GAIN

The reader will gain insight into the sequence of ASA404-induced changes in tumor tissue. Early events include increased vascular permeability, increased endothelial apoptosis and decreased blood flow, while later effects include the induction of serotonin, tumor necrosis factor, other cytokines and chemokines, and nitric oxide. This cascade of events induces sustained reduction of tumor blood flow, induction of tumor hypoxia and increased inflammatory responses. The reader will also gain an appreciation of how the potentiation of radiation and chemotherapeutic effects by ASA404 in murine tumors shaped the development of combination clinical trials.

TAKE HOME MESSAGE

Although there are species differences in ASA404 activity, many features of its action in mice translate to human studies. The future of ASA404 as an effective clinical agent will rely on the development of an appreciation of its ability to optimize the complex interaction between tumor vasculature and tumor immunity during therapy.

ASA404: a tumor vascular-disrupting agent with broad potential for cancer therapy

ASA404 (5,6-dimethylxanthenone-4-acetic acid) was developed as an analogue of flavone acetic acid and found to induce hemorrhagic necrosis of experimental tumors. ASA404 simultaneously targets at least two cell types - vascular endothelial cells and macrophages - within the tumor microenvironment. In murine tumors, ASA404 induces coordinated decreases in tumor blood flow, increases in vascular permeability and increases in vascular endothelial apoptosis, all occurring within 1 h of administration. Over a slightly longer time scale, ASA404 induces an increase in tumor concentrations of TNF and a number of other cytokines. Phase I clinical trials confirmed its vascular effects in humans and Phase II trials demonstrated its activity in combination with the cytotoxic agents carboplatin and paclitaxel. While the molecular target of its action is not yet identified, current results suggest that ASA404 has the potential to augment the antitumor effects of other agents in cancer treatment. Studies of changes in tumor tissue following treatment with ASA404 either alone or combined and other agents will provide new insights into the dynamics of the tumor microenvironment.

The paradox of cancer cell apoptosis

Resistance to apoptosis is an accepted property of human cancer cells and resistance to cancer therapy is often considered to involve increased resistance to apoptosis. However, comparison of the potential doubling times of human tumour cells with the volume doubling times of the tumour from which they are derived implies a high rate of apoptosis. For at least some cancer types, increased proliferation rate and correspondingly increased apoptosis is associated with a poor prognosis. How can resistance to apoptosis and apoptosis be reconciled? One possible resolution of this paradox is that at least two tumour cell populations are involved, a smaller, more rapidly growing population with self-renewal properties and resistance to apoptosis, and a larger, more slowly growing population that is susceptible to apoptosis. The progeny of smaller population thus maintains the larger population. This review describes the evidence for such a model and its implications for cancer therapy.

MCF-7 breast cancer cells selected for tamoxifen resistance acquire new phenotypes differing in DNA content, phospho-HER2 and PAX2 expression, and rapamycin sensitivity

Patients with estrogen receptor-positive (ER+) breast cancers are often treated with aromatase inhibitors or by antiestrogens such as tamoxifen to prevent disease recurrence. Resistant tumors nevertheless develop and it is commonly assumed that they arise by the induction of mutations. However, it is also possible that resistant tumors grow from preexisting variant populations within the original tumor. We have investigated this possibility in the case of the MCF-7 breast cancer cell line. The line was cultured for a prolonged period either in the presence of tamoxifen to block the action of oestrogen or in the absence of estrogen to mimic the action of oophorectomy or treatment with aromatase inhibitors. Both treatments led to growth inhibition followed by eventual outgrowth of sub-lines. Five of these sub-lines were developed and characterized for sensitivity to tamoxifen and to the antibiotic rapamycin, expression of HE R2 and PAX2, and phosphorylation of Akt, p70S6K, 4E-BP1, rpS6, EGFR1, Erk and HE R2. All six lines were ER+ and could be divided into four phenotypes distinguished by cell volume, DNA content (ploidy) and cell cycle time. In two cases, selection with tamoxifen and selection in the absence of estrogen produced similar phenotypes. Rapamycin resistance was a feature of the sub-lines developed under estrogen deprivation and was associated with loss of active phospho-HE R2 and acquisition of PAX2 expression. The results support the conclusion that the MCF-7 cell line is heterogeneous and that the selection conditions allow the growth of pre-existing phenotypes.

Pharmacokinetics and pharmacodynamics of chlorambucil delivered in long-circulating nanoemulsion

Chlorambucil was incorporated into a nanoemulsion modified with poly(ethylene glycol) to improve its pharmacokinetics and tissue distribution, and thus enhance its therapeutic efficacy. A long-circulating nanoemulsion (LNE) was prepared using soybean oil, egg lecithin, cholesterol and PEG(2000)DSPE. The LNE had an oil droplet size <200 nm with a surface charge of -32.2 to -35.6 mV. Approximately, 97% of the chlorambucil was encapsulated in the LNE. Intravenous (i.v.) administration of the chlorambucil LNE to C57 B/6 mice showed improved pharmacokinetic parameters with 1.4-fold higher area under the plasma concentration-time curve (AUC) and 1.3-fold longer half-life compared to a non-PEG-modified nanoemulsion, and 2.7-fold higher AUC and 7.6-fold longer half-life compared to chlorambucil solution. Tissue distribution studies after i.v. administration with LNE showed a considerable decrease in drug uptake in the reticulo-endothelial system containing organs compared to non-PEG-modified nanoemulsion. Additionally, the chlorambucil delivered in LNE significantly enhanced therapeutic efficacy in the subcutaneous colon-38 adenocarcinoma tumor mouse model with no apparent increase in toxicity. This study suggests that LNE could produce remarkably improved pharmacokinetic profile and therapeutic efficacy of chlorambucil compared to non-PEG-modified nanoemulsion and solution.

In vivo and in vitro assessment of the action of SN 28049, a benzonaphthyridine derivative targeting topoisomerase II, on the murine Colon 38 carcinoma

AIM

SN 28049 (N-[2-(dimethylamino)ethyl]-2,6-dimethyl-1-oxo-1,2-dihydrobenzo[b]-1,6-naphthyridine-4-carboxamide) is a new DNA binding drug that targets topoisomerase II. SN 28049 is curative against the murine Colon 38 adenocarcinoma (CT38) while etoposide, another topoisomerase II-directed drug, shows minimal activity; we investigated the basis for this difference in vivo and in vitro.

METHODS

Colon 38 tumours were grown in C57Bl mice and in immunodeficient mice. Tumour sections were examined by staining and TUNEL assays. A new cell line (Co-38P) derived from the in vivo tumour was developed and responses were analysed using flow cytometry.

RESULTS

Both SN 28049 and etoposide induced similar tumour histological changes, reducing mitotic index and increasing apoptotic index 8 h after administration. At later times however, SN 28049-treated tumours showed further progressive morphological changes while etoposide-treated tumours reverted to their original growth characteristics. The effects of SN 28049 on tumour growth were delayed and attenuated when Colon 38 tumours were grown in immunodeficient mice. SN 28049 and etoposide both induced dose-dependent increases of γ-phosphorylation of histone H2AX and cell cycle perturbation of the Co-38P cell line, indicative of DNA damage, although SN 28049 had 30-fold higher activity. Following 1-hour drug exposure of Co-38P cells, SN 28049 was more effective that etoposide in inducing persistent cycle arrest for the same degree of DNA damage.

CONCLUSION

The superior antitumour activity of SN 28049 may result from its ability to induce long term cycle arrest. Host immune responses contribute to the curative activity of SN 28049 and this could result from the induction of cycle arrest.

Action of SN 28049, a new DNA binding topoisomerase II-directed antitumour drug: comparison with doxorubicin and etoposide

AIM

We have examined the cellular action of SN 28049 (N-[2-(dimethylamino)ethyl]-2,6-dimethyl-1-oxo-1,2-dihydrobenzo[b]-1,6-naphthyridine-4-carboxamide), a DNA binding drug with curative activity against the Colon 38 transplantable murine carcinoma, on human tumour cells. Its action has been compared with that of two topoisomerase II-targetted drugs, etoposide and doxorubicin.

METHODS

The NZM3 melanoma and HCT116 colon carcinoma cell lines, each expressing wild-type p53, were cultured and responses were compared by flow cytometry, electrophoresis, microscopy, and growth of tumour xenografts.

RESULTS

Responses of NZM3 cells to all three drugs, as measured by histone H2AX γ-phosphorylation, induction of the p53 pathway and cell cycle arrest, were comparable and typical of those of topoisomerase II poisons. Xenografts of NZM3 cells responded to SN 28049 with a tumour growth delay of 16 days. In contrast, HCT116 cells had an attenuated DNA damage response to the drugs and SN 28049 had no in vivo activity, consistent with low topoisomerase II activity. However, SN 28049 inhibited HCT116 cell growth in vitro and activated the p53 pathway to induce a state with G(2)/M-phase DNA content, low mitotic index and a high proportion of binucleate cells. Treated cells expressed cyclin E and the senescence marker β-galactosidase but showed low expression of cyclin B and survivin. In comparison, etoposide caused little p53 expression or cycle arrest, and doxorubicin had an intermediate effect.

CONCLUSION

The action of SN 28049 in NZM3 cells is typical of a topoisomerase II poison, but the low topoisomerase IIα activity of HCT116 cells allowed the detection of a second antiproliferative action of SN 28049 in which cells undergo post-mitotic cycle arrest and induction of p53.

Analysis of radiation-induced changes to human melanoma cultures using a mathematical model

OBJECTIVES

Tumour cells respond to ionizing radiation by cycle arrest, cell death or repair and possible regrowth. We have developed a dynamic mathematical model of the cell cycle to incorporate transition probabilities for entry into DNA replication and mitosis. In this study, we used the model to analyse effects of radiation on cultures of five human melanoma cell lines.

MATERIALS AND METHODS

Cell lines were irradiated (9 Gy) prior to further culture and harvesting at multiple points up to 96 h later. Cells were fixed, stained with propidium iodide and analysed for G(1)-, S- and G(2)/M-phase cells by flow cytometry. Data for all time points were fitted to a mathematical model. To provide unique solutions, cultures were grown in the presence and absence of the mitotic poison paclitaxel, added to prevent cell division.

RESULTS

The model demonstrated that irradiation at 9 Gy induced G(2)-phase arrest in all lines for at least 96 h. Two cell lines with wild-type p53 status additionally exhibited G(1)-phase arrest with recovery over 15 h, as well as evidence of cell loss. Resumption of cycling of surviving cells, as indicated by increases in G(1)/S and G(2)/M-phase transitions, was broadly comparable with results of clonogenic assays.

CONCLUSIONS

The results, combined with existing data from clonogenic survival assays, support the hypothesis that a dominant effect of radiation in these melanoma lines is the induction of long-term cell cycle arrest.

Abstract 1660: The antitumor action of ASA404 (vadimezan; DMXAA); potential involvement of vascular endothelial growth factor (VEGF)

ASA404, a Tumor-Vascular Disrupting Agent developed at the Auckland Cancer Society Research Centre, is currently undergoing two Phase III clinical trials (Novartis) against non-small cell lung cancer. Its early effects on tumor tissue include increased tumor vascular permeability, increased tumor endothelial apoptosis, decreased tumor blood flow and increased tumor hypoxia. The basis for the selective effect of ASA404 on tumor versus normal vasculature is not clear. We hypothesized that VEGF, produced in tumor tissue in responses to hypoxia and other stresses, played a possible role in this selectivity. We compared the actions of ASA404 and VEGF in vitro and also examined the potential for induction of tumor VEGF by ASA404 treatment in vivo. We grew human umbilical vein endothelial cells (HUVEC) on Matrigel and compared in vitro responses to ASA404 and human VEGF using time-lapse microscopy and image analysis. Both ASA404 (30 and 300 µM; 9.9 and 99 µg/ml) and VEGF (50 ng/ml) stimulated the initial formation of 2-dimensional tubular networks on Matrigel. Both ASA404 and VEGF induced actin stress fiber formation in cultured HUVEC's, consistent with increased migration. To determine if there was an in vivo relationship between ASA404 and VEGF expression, mice with Rif-1 tumors (7-9 mm diameter) were treated i.p. with ASA404 (70 µmol/kg; 21 mg/kg) and mRNA was extracted from tumor tissue 4 hours later. VEGF-A mRNA abundance was measured by co-amplification with endogenous 18S mRNA as standard. VEGF-A relative abundance rose from 0.6 ± 0.1 to 1.3 ± 0.1 (p &lt; 0.01). VEGF-B and VEGF-D concentrations also increased but the changes were not significant. The results are consistent with the following model: hypoxia and other stresses in tumor tissue lead to synthesis by both tumor and host cells of VEGF, which increases vascular permeability and partially compromises tumor blood flow. The administration of ASA404 leads to increased tumor vascular permeability and vascular endothelial apoptosis, possibly by a pathway involving p38 kinase, ceramide synthesis and actin stress fiber formation. The resultant decrease in tumor blood flow further increases hypoxia, potentially providing a positive feedback loop by stimulating further VEGF production. These effects, in combination with other cytokines such as tumor necrosis factor that are induced by ASA404, lead to tumor vascular failure. Thus, endogenous VEGF production by tumor tissue may contribute to the antitumor selectivity of ASA404. Further studies addressing the modulatory role of VEGF in the action of ASA404 are warranted.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1660.

Abstract 4443: Molecular mechanistic study of ASA404 (vadimezan)-induced endothelial cell death

The tumor vasculature is a key component in maintaining tumor growth and regulating the tumor microenvironment through the supply of nutrients and oxygen. ASA404 (vadimezan, 5,6-dimethylxanthenone-4-acetic acid) is a flavonoid, non-tubulin-binding Tumor-Vascular Disrupting Agent (Tumor-VDA) that induces breakdown of the established tumor vasculature through the induction of tumor endothelial cell apoptosis, resulting in the inhibition of tumor blood supply, leading to tumor ischemia and extensive necrosis of the tumor core. In contrast to antiangiogenics, ASA404 has limited effects on angiogenesis at the tumor periphery and so the remaining peripheral tumor rim represents a source of viable cells for regrowth of the tumor upon cessation of ASA404 treatment. Preclinical and clinical studies have highlighted the effectiveness of combining chemotherapies, especially taxanes, with ASA404 for marked tumor inhibition. Currently, the benefits of adding ASA404 to the standard of care in first- and second-line NSCLC indications are being evaluated in the ATTRACT-1 and ATTRACT-2 Phase III trials respectively. Despite the prior clinical efficacy observed with ASA404, the detailed molecular mechanism by which ASA404 selectively targets the tumor vasculature still remains to be defined. To investigate this, we initiated studies to explore the function of ASA404 both in vitro using human endothelial cells (HUVEC) and in ASA404-treated MDA-MB-231 breast cancer xenografts examined ex vivo. Consistent with previous reports, we observed inhibition of HUVEC cell proliferation along with rapid cell death in vitro as assessed using FACS analysis and newly demonstrated the rapid induction of caspase 3 assessed by immunocytochemistry. Treatment of the MDA-MB-231 breast xenografts in vivo with ASA404 resulted in significant tumor growth inhibition, tumor necrosis and elevation of the hypoxia marker carbonic anhydrase 9 (CAIX). The nature of ASA404-induced endothelial cell death is likely to be due to apoptosis since rapid caspase 3 cleavage was observed in the endothelial cells both in vitro and in vivo. In addition, the mitochondrial potential of cultured HUVEC cells in vitro was found to be seriously compromised after 1-2 hours treatment with ASA404 as monitored by Mito-tracker staining. This was accompanied by cytochrome C release into the cytosol as evidenced by immunohistochemical co-localization of cytochrome C and mitochondria markers. We also detected a moderate, but concentration-dependent increase in ceramide levels in HUVEC cells following 2 hours of ASA404 treatment, which may serve as a link to the observed mitochondrial depolarization phenotype. Lastly, a pooled-based shRNA screen was also conducted to search for genes that could modulate ASA404 activity in HUVEC cells. Some of the candidates identified may provide new insights into the pathways that are critical in the ASA404 mechanism of action.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4443.

The ubiquitin-proteasome system is inhibited by p53 protein expression in human ovarian cancer cells

The ubiquitin-proteasome system (UPS) and autophagy provide major cellular pathways for protein degradation. Since the p53 pathway controls autophagy, we investigated whether p53 regulates UPS in ovarian tumour cell lines. A reporter cell line (SKOV3-EGFPu) was established to measure UPS function against a constant genetic background. Transient expression of either wild type or mutant p53 in SKOV3-EGFPu cells reduced UPS activity as compared to vector control. These results, together with those from endogenous p53 expression in seven ovarian cancer cell lines, suggest that expression of both wild-type and mutant p53 protein impairs UPS function. Thus, p53 expression may regulate protein homeostasis by down-regulating UPS function in response to cellular stress.

Multidrug resistance in cancer

It is becoming increasingly clear that the proliferation of human tumours is driven by a small proportion of cells, termed tumour stem cells, which have the properties of self-renewal. On analogy with stem cells for normal tissues, there are likely to be multiple mechanisms, involving both intrinsic cellular properties and microenvironmental factors, which enable tumour stem cells to resist potentially genotoxic agents. Intrinsic properties include maintenance of cells in a predominantly non-cycling state, expression of transport proteins such as P-glycoprotein, protection from induced apoptosis or other forms of cell death, and limitation of diffusion of potential cytotoxins from the bloodstream. In addition, tumour stem cells are likely to contain multiple genetic changes that will potentially activate host immune mechanisms, which are designed to respond to such changes, and the methods by which tumours suppress such mechanisms are of great relevance to drug resistance. A number of methods of overcoming intrinsic multidrug resistance of tumours have been developed but methods for overcoming tumour resistance mediated by host cells are still at an early stage and require further research.

Cell cycle times of short-term cultures of brain cancers as predictors of survival

Tumour cytokinetics estimated in vivo as potential doubling times (T_pot values) have been found to range in a variety of human cancers from 2 days to several weeks and are often related to clinical outcome. We have previously developed a method to estimate culture cycle times of short-term cultures of surgical material for several tumour types and found, surprisingly, that their range was similar to that reported for T_pot values. As T_pot is recognised as important prognostic variable in cancer, we wished to determine whether culture cycle times had clinical significance. Brain tumour material obtained at surgery from 70 patients with glioblastoma, medulloblastoma, astrocytoma, oligodendroglioma and metastatic melanoma was cultured for 7 days on 96-well plates, coated with agarose to prevent proliferation of fibroblasts. Culture cycle times were estimated from relative ³H-thymidine incorporation in the presence and absence of cell division. Patients were divided into two groups on the basis of culture cycle times of ⩽10 days and >10 days and patient survival was compared. For patients with brain cancers of all types, median survival for the ⩽10-day and >10-day groups were 5.1 and 12.5 months, respectively (P=0.0009). For 42 patients with glioblastoma, the corresponding values were 6.5 and 9.0 months, respectively (P=0.03). Lower grade gliomas had longer median culture cycle times (16 days) than those of medulloblastomas (9.9 days), glioblastomas (9.8 days) or melanomas (6.7 days). We conclude that culture cycle times determined using short-term cultures of surgical material from brain tumours correlate with patient survival. Tumour cells thus appear to preserve important cytokinetic characteristics when transferred to culture.

Development and validation of a liquid chromatography-mass spectrometry (LC-MS) assay for the determination of the anti-cancer agent N-[2-(dimethylamino)ethyl]-2,6-dimethyl-1-oxo-1,2-dihydrobenzo[b]-1,6-naphthyridine-4-carboxamide (SN 28049)

N-[2-(Dimethylamino)ethyl]-2,6-dimethyl-1-oxo-1,2-dihydrobenzo[b]-1,6-naphthyridine-4-carboxamide (SN 28049) is a potent topoisomerase II poison being developed to treat solid tumours. A reliable and sensitive LC-MS method has been developed and validated for the determination of SN 28049 in plasma using a structurally similar internal standard. This method had acceptable intra- and inter-assay accuracy (95-105%) and precision (R.S.D.<6.5%) over the range 0.062-2.5 microM (using a 100 microl sample), and had a lower limit of quantitation of 0.062 microM. Both aqueous and plasma solutions of SN 28049 were stable during short-term (24h at room temperature or 4 degrees C) and long-term storage (8 months at -80 degrees C), and following freezing and thawing (three cycles). The method was applied to study the pharmacokinetics of SN 28049 in mice after iv administration (8.9 mg/kg; n=3 mice per time point). The maximum plasma concentration achieved was 1.22+/-0.05 microM, and concentrations were measurable up to 12h post-administration. A bi-exponential concentration-time curve was observed with an elimination half-life of 2.3+/-0.2h (mean+/-S.E.), a volume of distribution of 34.5+/-2.2l/kg, and a plasma clearance of 12+/-0.5l/h/kg.

Enhancement of the action of the antivascular drug 5,6-dimethylxanthenone-4-acetic acid (DMXAA; ASA404) by non-steroidal anti-inflammatory drugs

AIM

5,6-Dimethylxanthenone-4-acetic acid (DMXAA) (ASA404), a low molecular weight antivascular drug currently in clinical trial, acts both directly on the tumour vascular endothelium and indirectly through the induction of inflammatory cytokines and other vasoactive molecules from macrophages and other host cells. We wished to determine whether co-administration of non-steroidal anti-inflammatory drugs (NSAIDs) could modulate the antivascular effects of DMXAA in mice.

METHODS

The effects of diclofenac, salicylate, ibuprofen, celecoxib and rofecoxib on the antitumour response to DMXAA were compared using growth delay assays of Colon 38 adenocarcinomas in C57Bl mice. Concentrations of DMXAA in mice were measured by high performance liquid chromatography.

RESULTS

Administration of DMXAA alone (25 mg/kg i.p.) or of NSAIDs alone induced small tumour growth delays from 2 to 7 days. Co-administration of each of the NSAIDs augmented DMXAA effects with tumour growth delays from 4.5 to >20 days. The possibility of a pharmacokinetic interaction was investigated using diclofenac and it was found that diclofenac did not affect DMXAA pharmacokinetics.

CONCLUSIONS

NSAIDs increase the antitumour activity of DMXAA in a murine tumour model. The effects are consistent with hypothesis that NSAIDs antagonises some of the protective effects of prostaglandins released in response to vascular injury. Co-administration of NSAIDs with DMXAA might be considered as a possible strategy for use in combination cancer therapy.

Tumor stem cell niches: a new functional framework for the action of anticancer drugs

Newer treatments of advanced human cancer increasingly rely on combinations of drugs that have quite different actions yet unexpectedly potentiate each other's effects. Recent research in stem cell biology suggests a model for tumors in which tumor growth is governed by the generation of cells from tumor cell niches rather than from the population as a whole. Each niche contains a population of tumor stem cells supported by a closely associated vascular bed comprising mesenchyme-derived cells and an extracellular matrix. Division of tumor stem cells is asymmetric in the sense that some daughter cells are always retained within the niche while others leave the niche to proliferate further and eventually die. One important potential difference between normal and tumor stem cell niches is that while most normal stem cells are in a non-proliferating or G(0)-state, tumor stem cells are continuously in cycle. Combinations of cytotoxic drugs and antagonists of survival factors to reduce the stem cell population may require the addition of vascular disrupting agents to compromise the function of the tumor cell niche. As well as providing opportunities for new drug discovery, this model of tumor growth also presents challenges as to how the contributions of individual drugs in a combination might be assessed in individual patients.