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Experimental Therapeutics, Preclinical Pharmacology |
1 Department of Internal Medicine and Molecular Therapeutics, Course of Medical Oncology, University of Tokushima Graduate School, Tokushima, Japan; and 2 Cancer and Infection Bioscience Department, AstraZeneca, Alderley Park, Macclesfield, Cheshire, United Kingdom
| ABSTRACT |
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Experimental Design: Human lung adenocarcinoma PC14PE6 cells were injected into the tail vein of nude mice. Five weeks after injection animals were treated with ZD6126 (200 mg/kg i.p.), cisplatin (6 mg/kg i.v.), or a combination of the two agents. The animals were sacrificed 24 hours later, and the extent of lung metastases and the presence of apoptotic cells were assessed.
Results: Histologic analysis revealed that the ZD6126/cisplatin combination resulted in a 2 to 4-fold increase in the total number of tumor-associated apoptotic cells compared with either treatment alone. ZD6126 alone induced apoptosis of tumor-associated endothelial cells in tumors, and the extent of apoptosis was increased 2-fold in combination with cisplatin. The lung weight was significantly reduced, and the number of metastatic nodules significantly was lower in the combined treatment group than in the control group.
Conclusions: These data suggest that the antimetastatic effect of the vascular-targeting agent ZD6126 can be increased by use in combination with cisplatin, which increases the incidence of endothelial cell apoptosis.
| INTRODUCTION |
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In contrast to antiangiogenic approaches, vascular-targeting agents disrupt existing tumor vasculature leading to vessel occlusion and arrested blood flow. As a large number of tumor cells depend on a relatively small number of blood vessels, interruption of the vascular supply could have significant antitumor effects. Tumor blood vessels differ significantly from those found in normal tissues and contain a chaotic network of tortuous thin-walled vessels with a significant degree of neovasculature and a relatively high proportion of proliferating endothelial cells (2 , 11) . Vascular-targeting agents exploit the distinctive features of the tumor vasculature to irreversibly arrest blood flow in tumors (12) . The resulting ischemia leads to a rapid cascade of secondary tumor cell death and the destruction of central areas of the tumor, which are normally most resistant to conventional therapies (13, 14, 15) . ZD6126 is a novel vascular-targeting agent that is rapidly converted by serum phosphatases to ZD6126 phenol (N-acetylcolchinol), which disrupts the tubulin cytoskeleton of endothelial cells, resulting in conformational changes in immature, but not mature, endothelial cells (16) . A single dose of ZD6126 has been shown to induce hemorrhage and necrosis in established lung metastases and lead to apoptosis of endothelial cells in tumor tissue but not in the normal lung parenchyma (17) .
Despite the potent antivascular effects of ZD6126, a narrow rim of viable tumor cells is often observed at the periphery of the tumor after treatment (16) . The residual tumor tissue can rapidly reestablish the tumor mass, but because of its high proliferation rate, it is still a good target for conventional anticancer therapies. Combined treatment of ZD6126 with the cytotoxic agents paclitaxel or cisplatin or irradiation has demonstrated additional growth delay in tumor xenografts. However, the mechanism of action by which these combined therapies cause an enhanced antitumor effect has yet to be established (16 , 18 , 19) .
The aim of this study was to assess the benefit of combining the vascular-targeting agent ZD6126 and platinum-based chemotherapy, cisplatin, on lung cancer metastasis. An immunohistochemical technique was used to determine the effect of ZD6126 and cisplatin on tumor cells and endothelial cells in the metastatic PC14PE6 lung tumor model.
| MATERIALS AND METHODS |
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Reagents.
ZD6126 was provided by AstraZeneca (Cheshire, United Kingdom) and cisplatin by Bristol-Myers Squibb (Princeton, NJ).
Animals.
Male, 5-week old, athymic BALB/c nude mice were obtained from CLEA (Tokyo, Japan) and maintained under specific pathogen-free conditions throughout the study. Experiments were performed according to guidelines set out by the University of Tokushima School of Medicine.
Experimental Metastasis In vivo.
For production of lung metastases, human lung adenocarcinoma PC14PE6 cells (1 x 106) were suspended in 0.2 mL of PBS and injected into the tail vein of nude mice (20)
. For evaluation of apoptosis of tumor and endothelial cells, animals received cisplatin (6 mg/kg i.v.), ZD6126 (200 mg/kg i.p.), or a combination of the two agents (in which case ZD6126 was administered 1 hour after cisplatin therapy) 5 weeks after PC14PE6 cell injection. After 24 hours (day 35), the mice were sacrificed and the lungs removed for histologic analysis. For evaluation of antimetastatic effect, animals (n = 5, each group) received cisplatin (6 mg/kg i.v.) once on day 14, ZD6126 (100 mg/kg i.p.) daily from day 14 to day 34, or a combination of the two agents. Doses of cisplatin and ZD6126 were chosen based on the results of our preliminary experiments and previous report (17)
, respectively. The mice were sacrificed on day 35. The lungs were removed and fixed in Bouins solution for 24 hours. The number of metastatic lesions > 0.5 mm in diameter in the major organs was counted macroscopically.
Histology and Immunohistochemistry.
Tissue from the excised lungs was cut into 5-mm slices and placed into either buffered 10% formalin solution or OCT compound (Sakura Finetechnical Co., Tokyo, Japan) to be snap-frozen in liquid nitrogen for immunohistochemical analysis. Terminal deoxynucleotidyl transferase-mediated nick end labeling staining was performed using the Apoptosis Detection System (Promega, Madison, WI). Briefly, the frozen tissue sections (9-µm thick) were fixed with PBS containing 4% formalin. The slides were washed with PBS and permeabilized with 20 µg/mL proteinase K. The samples were then equilibrated, and DNA strand breaks were labeled with fluorescein-12-dUTP by adding nucleotide mix and terminal deoxynucleotidyl transferase enzyme. The reaction was stopped with saline sodium citrate, and the localized green fluorescence of apoptotic cells was detected by fluorescence microscopy. Double staining for endothelial cells and the presence of apoptosis was performed on frozen tissue sections with an anti-CD-31 monoclonal antibody (PharMingen, San Diego, CA) and Texas Red-conjugated secondary antibody (Vector Laboratories, Burlingame, CA), followed by terminal deoxynucleotidyl transferase-mediated nick end labeling staining.
Cell Proliferation Assay.
Cell proliferation was measured by the MTT dye reduction method (21)
. Briefly, 2 x 103 PC14PE6 cells/100 µL were plated into 96-well plates in medium and incubated at 37°C. After 24 hours, 100 µL of media containing increasing concentrations of ZD6126 were added and incubated for 2 hours. Cells were then washed and fresh media containing 1 µg/mL cisplatin added and incubated for 70 hours. After this, 50 µL of stock MTT solution (2 mg/mL; Sigma, St. Louis, MO) were added and incubated for 2 hours. The medium containing MTT solution was removed and the residual dark blue crystals dissolved in 100 µL of DMSO. Absorbance was measured using an MTP-120 microplate reader (Corona Electric, Ibaraki, Japan) at test and reference wavelengths of 550 and 630 nm, respectively.
Statistical Analysis.
In vivo data were analyzed by the Mann-Whitney U test; in vitro data were analyzed by the Students t test (two-tailed).
| RESULTS |
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| DISCUSSION |
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Use of vascular-targeting therapy to block the blood supply from established vessels to tumor cells is a promising means of controlling metastasis (3) . Although a potent vascular-damaging effect is observed after treatment with ZD6126, the antitumor effect is not complete as a narrow rim of viable tumor cells often remains at the periphery (16) . This is thought to occur as a result of nutrients and oxygen being obtained by diffusion from the surrounding normal vasculature. Although residual tumor tissue can rapidly reestablish the tumor, its high proliferation rate represents a suitable target for conventional anticancer therapies. Recently, Siemann and Rojiani reported that the antitumor effect of ZD6126 was enhanced by combining its use with irradiation (18) or with cisplatin (19) in s.c. inoculated rodent sarcoma (KHT) and human renal (Caki-1) tumor models. In addition, Davis et al. (16) reported that the delay in growth of s.c. inoculated human pharynx squamous cell carcinoma (FaDu) cells was increased by combining paclitaxel with ZD6126. Although combining ZD6126 with other anticancer treatments to theoretically target different cell components appears to be beneficial, the mechanism of action by which this enhanced antitumor effect is achieved has not been ascertained. The present study has shown that the chemotherapeutic agent, cisplatin, sensitizes tumor-associated endothelial cells to apoptosis by ZD6126, thereby causing additional inhibition of lung metastasis. Although cisplatin should have a direct cytotoxic effect against tumor cells, the rate of apoptosis of tumor cells was very low even 24 hours after a single injection of 6 mg/kg cisplatin and did not reduce the number of lung metastases. These data suggest that a vascular-targeting effect may be the major antitumor mechanism of action of the ZD6126/cisplatin combination in this model. The methods used do not differentiate between apoptotic and necrotic cells, and therefore, whether the effect is due to apoptosis or necrosis is unclear. Combining ZD6126 with cisplatin treatment increased the number of apoptotic endothelial cells when compared with ZD6126 therapy alone but only in the tumors and not in normal lung parenchyma, whereas cisplatin alone did not induce endothelial cell apoptosis. The detailed mechanism of action of this combined effect will need to be clarified in future studies.
From a clinical point of view, the timing and sequencing of antivascular agents and chemotherapeutic drugs are considerable. In general, marked enhancements in antitumor activities were demonstrated when the vascular targeting agents were delivered within a few hours after chemotherapy (18 , 22) . These seem to be reasonable because administration of vascular-targeting agents after chemotherapy may not affect delivery of chemotherapeutic drugs to tumors but rather cause trapping of the drugs in the tumors. Therefore, we administered ZD6126 1 hour after cisplatin treatment in the in vivo experiments. On the other hand, in the in vitro experiments, we treated endothelial cells with ZD6126 for 1 hour and then with cisplatin for 72 hours. ZD6126 is known to be rapidly cleared from plasma (18) and presumably cause trapping of the cisplatin in the tumors (18) . To mimic these pharmacodynamics, we chose this experimental condition for in vitro experiments.
This model appears to be important in evaluating the efficacy of anticancer agents in vivo. Tumor cells exhibit heterogeneous biological and metastatic properties, with the outcome of metastasis being dependent on the properties of both tumor cells and host factors (23, 24, 25) . Therefore, the host organ microenvironment surrounding the tumor should always be considered when treating cancer. The characteristics of metastases in different organs may differ; for example, we have previously shown that the antimetastatic effect of both macrophage colony-stimulating factor (26) and an inhibitor of matrix metalloproteinase (27) are specific to particular organs, although both macrophage colony-stimulating factor and matrix metalloproteinase inhibitors dramatically block the growth of s.c. inoculated tumor cells. For this reason, metastatic models should be used in preference to s.c. xenograft models for evaluating the therapeutic potential of new anticancer agents.
In summary, this study has shown that administration of ZD6126 therapy alone could induce apoptosis of tumor-associated endothelial cells but not endothelial cells in normal parenchyma, resulting in hemorrhage and necrosis of established lung metastases. In addition, combined use of ZD6126 with a clinically available anticancer agent, cisplatin, increased apoptosis of tumor-associated endothelial cells and enhanced the antimetastatic effect. Although additional evaluations of the therapeutic potential are necessary, combined use of vascular-targeting agents with conventional chemotherapy may provide a promising therapeutic option for treating patients with advanced lung cancer and metastasis.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Seiji Yano, Department of Internal Medicine and Molecular Therapeutics, Course of Medical Oncology, The University of Tokushima School of Medicine, Tokushima, 3-18-15 Kuramoto-cho, Tokushima Graduate 770-8503, Japan. Phone: 81-88-633-7127; Fax: 81-88-633-2134, E-mail: manae{at}clin.med.tokushima-u.ac.jp
Received 6/16/04; revised 8/ 2/04; accepted 8/ 9/04.
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