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Experimental Therapeutics, Preclinical Pharmacology |
Departments of Internal Medicine and Molecular Therapeutics [S. Y., H. Z., M. H., T. M., H. G., S. S.] and Environmental and Molecular Pathology [H. U.], Course of Bioregulation and Medical Oncology, University of Tokushima School of Medicine, Tokushima, Japan
| ABSTRACT |
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Experimental Design: Human small cell lung cancer (SBC-5) cells i.v. inoculated into natural killer cell-depleted severe combined immunodeficient mice metastasized to multiple organs, including the lungs, liver, kidneys, lymph nodes, and bone. SBC-5-bearing mice were treated with YM529 and/or VP-16 and sacrificed 5 weeks after tumor cell inoculation. Bone metastasis was assessed by X-ray photographs, and visceral metastasis was evaluated macroscopically. The number of osteoclasts in the bone lesions was examined by tartrate-resistant acid phosphatase staining.
Results: Monotherapy with YM529 suppressed the production of bone metastases, but not visceral metastasis. Histological analyses revealed that the number of osteoclasts in bone lesions was lower in YM526-treated mice, compared with control mice. VP-16 inhibited both bone metastasis and visceral (lung and liver) metastasis. However, neither YM529 alone nor VP-16 alone significantly prolonged the survival of SBC-5-bearing mice. Combined use of YM529 and VP-16 further inhibited the production of bone metastasis and significantly prolonged survival.
Conclusions: Combined therapy with bisphosphonate and chemotherapy may be useful for small cell lung cancer patients with multiple organ metastases including bone metastasis.
| INTRODUCTION |
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Bisphosphonates are potent inhibitors of osteoclastic bone resorption and have been widely used in the treatment of hypercalcemia (3) . In addition, several bisphosphonate products, for example, clodronate (4) , etidronate (5) , alendronate (6) , ibandronate (7) , pamidronate (8) , and zoledronic acid (9) , have been developed and used in the treatment of cancer patients with bone metastases. These bisphosphonates, however, could improve the quality of life but could not improve the survival of advanced cancer patients with bone metastases (10) .
We recently established a model of multiple-organ metastasis with a small cell lung cancer cell line, SBC-5, in NK3 cell-depleted SCID mice (11) . In this model, SBC-5 cells metastasize into multiple organs, such as the lung, liver, kidneys, systemic lymph nodes, and bone, resembling characteristics of small cell lung cancer in humans. We found that a new bisphosphonate, minodronate (YM529), inhibited osteolytic bone metastasis via the inhibition of bone resorption; however, it could not prolong the survival of tumor-bearing mice because of visceral metastasis (12) .
In the present study, we examined the therapeutic efficacy of YM529 combined with etoposide (VP-16), which is commonly used for small cell lung cancer patients, in terms of metastasis formation and survival.
| MATERIALS AND METHODS |
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Reagents.
Antimouse IL-2 receptor ß chain monoclonal antibody, TM-ß1 (IgG2b), was supplied by Drs. M. Miyasaka and T. Tanaka (Osaka University, Osaka, Japan; Ref. 13
). Minodronate (YM529) was provided by Yamanouchi Pharmaceutical Co. (Ibaragi, Japan; Ref. 14
). Etoposide (VP-16) was obtained from Nippon Kayaku Co. (Tokyo, Japan).
Animals.
Male S EB-17/Icr-scid mice, age 68 weeks, were obtained from CLEA (Osaka, Japan) and maintained under specific pathogen-free conditions throughout the experiment. Experiments were performed according to the guidelines of our university.
In Vitro Effect of YM529 on Proliferation of SBC-5 Cells.
SBC-5 cells at 80% confluence were harvested, plated into 96-well tissue culture plates (5000 cells/100 µl/well), and incubated for 24 h at 37°C in 5% CO2. Then, various concentrations of YM529 were added to the cultures. After a 72-h incubation at 37°C, 50 µl of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide stock solution (2 mg/ml) were added to each well, and the cells were further incubated for 2 h at 37°C (15)
. Then, the culture media were removed, and 100 µl of DMSO were added to dissolve the dark blue crystals. Absorbance was measured with a MTP-32 Microplate Reader (Corona Electric, Ibaragi, Japan) at test and reference wavelengths of 550 and 630 nm, respectively.
Effect of YM529 on Production of PTHrP and VEGF of SBC-5 Cells.
SBC-5 cells at 80% confluence were harvested, plated into 6-well tissue culture plates (1 x 105 cells/2 ml/well), and incubated for 24 h at 37°C and 5% CO2. Then, the cultures were washed, and various concentrations of YM529 were added. After a 72-h incubation at 37°C, culture supernatants were collected, and the concentrations of PTHrP and VEGF were determined using radioimmunoassay (Otsuka Assay, Tokushima, Japan) and ELISA (R&D Systems, Minneapolis, MN), respectively.
Model of Multiple-Organ Metastasis by SBC-5 Cells and Antimetastatic Effect of YM529.
To facilitate the metastasis of SBC-5 small cell lung cancer cells, NK cells were depleted in SCID mice (16)
. For NK cell depletion, TM-ß1 monoclonal antibody (300 µg/300 µl PBS/mouse) was injected i.p. into SCID mice 2 days before tumor cell inoculation. SBC-5 cells at the subconfluent condition were harvested and washed with Ca2+- and Mg2+-free PBS (CMF-PBS). Cell viability was determined by the trypan blue exclusion test, and only single cell suspensions of >90% viability were used. SBC-5 cells (1 x 106 cells/300 µl) were injected into the lateral tail vein of mice on day 0. At the indicated periods, tumor-bearing mice were treated with i.v. administration of YM529 (0.2 µg) on day 7 and/or VP-16 (200 µg) on days 2, 3, 9, and 10, based on the previous report (17)
.
Five weeks after the tumor cell inoculation, the mice were anesthetized by i.p. injection of pentobarbital (0.5 mg/body), and X-ray photographs of the mice were taken to evaluate bone metastasis. Then, mice were sacrificed by cutting the subclavian artery, and all major organs were removed. The lungs were fixed in Bouins solution for 24 h. The number of metastatic lesions larger than 0.5 mm in diameter in the major organs was counted macroscopically.
For evaluatation of survival, tumor-bearing mice were sacrificed when the mice became moribund.
Histology and Immunohistochemistry.
The hind limbs of the mice were taken and fixed in 10% formalin. The bone specimens were decalcified in 10% EDTA solution for 1 week and then embedded in paraffin. Tissue sections (4-µm thick) were processed. For detection of osteoclasts, TRAP staining was performed using a Sigma Diagnostics Acid Phosphatase Kit (Sigma Diagnostics, St. Louis, MO). The number of TRAP-positive osteoclasts at the tumor-bone interface was counted under a microscope in five random fields at x200 magnification. Sections (4-µm thick) of formalin-fixed, paraffin-embedded tumors were also stained with H&E for routine histological examination.
Statistical Analysis.
The Mann-Whitney U test was used to determine the significance of difference in the number of multiorgan (bone, liver, lungs, kidneys, and lymph nodes) metastases between YM529 and/or VP-16-treated groups and the untreated group. The significance of differences in the number of TRAP-positive cells and the survival rate of the mice were analyzed by Students t test (two-tailed) and the log-rank test, respectively. P < 0.05 was considered significant in all experiments.
| RESULTS |
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| DISCUSSION |
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A large number of studies have reported the inhibitory effects of bisphosphonates against bone metastasis; however, the survival benefit of bisphosphonates is still controversial in experimental animal models (18, 19, 20) . In the present study, treatment with YM529 alone could not significantly prolong survival, although it inhibited bone metastasis remarkably, suggesting that selective inhibition of bone metastasis is not sufficient for prolonging survival. Our results are consistent with most of the clinical studies showing that bisphosphonates can improve the quality of the life but not the survival of advanced cancer patients with bone metastases (21 , 22) . On the other hand, a chemotherapeutic agent, VP-16, suppressed metastasis to some visceral organs (the liver and lungs). It also suppressed bone metastasis in our protocol, but with much less efficiency than YM529. In addition, VP-16 alone could not prolong survival. Because the mice with severe bone metastases became paralyzed and could not obtain enough food or water, the poor prognosis of the mice treated with VP-16 alone may be due not only to the progression of visceral metastasis but also to restricted quality of life because of bone metastasis. Collectively, the control of both bone metastasis and visceral metastases by multiple modalities may be necessary to prolong the survival of cancer patients with multiple organ metastases. To further prolong survival, experiments to evaluate the effect of continuing treatment with YM529 and VP-16 are ongoing.
It is well accepted that bone destruction caused by bone metastasis is mediated by various factors produced or induced by tumor cells and/or microenvironments that stimulate the formation and activation of osteoclasts, the normal bone-resorbing cells (23
, 24)
. Several factors, including IL-1, IL-6, receptor activator of nuclear factor
B (RANK) ligand, macrophage inflammatory protein-1
, and PTHrP, have been implicated as factors that enhance osteoclast formation and bone destruction in malignant diseases (23)
. Of these factors, PTHrP has been reported to indirectly activate osteoclasts by induction or stimulation of RANK ligand expression by osteoblasts and hence augment bone resorption (23)
. We demonstrated previously that SBC-5 cells overexpressed PTHrP, and treatment with anti-PTHrP neutralizing antibody inhibited the development of bone metastasis by SBC-5 cells in NK cell-depleted SCID mice, indicating that PTHrP is responsible for the production of bone metastasis by SBC-5 cells (25)
.
A topoisomerase II inhibitor, VP-16, is commonly used for patients with small cell lung cancer. We reported previously (17) that 200 µg (8 mg/kg) of VP-16 i.v. injected twice a week for 2 weeks was feasible and efficiently inhibited production of multiple organ metastases by SBC-3 (human small cell lung cancer cell line) in NK cell-depleted SCID mice. Therefore, we used the same protocol in the present study. Although it is not clear why lymph node or kidney metastasis was not inhibited by VP-16 with this protocol, it is possible that the dose of VP-16 was not sufficient to suppress metastasis to these two organs.
Recent studies (26 , 27) have reported that some bisphosphonates (pamidronate and zoledronate) directly induced apoptosis or had cytostatic effects against cancer cells at relatively high concentrations. Moreover, YM529 was also reported to cause apoptosis of myeloma cell lines (28) . Therefore, one possible explanation for the therapeutic effect of YM529 on bone metastasis was due to direct cytotoxicity. However, this is not the case, because YM529 did not have a direct cytotoxic effect and did not enhance the cytotoxicity of VP-16 against SBC-5 cells at physiologically achievable concentrations (<1 µg/ml; Ref. 12 ). Furthermore, YM529 did not affect the production of PTHrP or VEGF, even in combination with VP-16. Therefore, the augmented therapeutic efficiency of YM529 with VP-16 against bone metastasis may be predominantly due to inhibition of osteoclast generation and/or function by YM529 plus suppression of cell proliferation by VP-16 in the bone lesions.
In conclusion, use of bisphosphonate (YM529) alone inhibited bone metastasis but was not sufficient to prolong the survival of mice with bone metastasis and visceral dissemination. Combined use of YM529 with VP-16 to suppress visceral metastasis successfully suppressed multiorgan metastases and prolonged survival. Therefore, a combined modality with bisphosphonate and chemotherapy may be useful for lung cancer patients who have bone metastasis and visceral dissemination.
| FOOTNOTES |
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This work was supported in part by a Grant-in-aid for Cancer Research from the Ministry of Education, Science, Sports and Culture of Japan.
1 Both authors contributed equally to this work. ![]()
2 To whom requests for reprints should be addressed, at Department of Internal Medicine and Molecular Therapeutics, Course of Bioregulation and Medical Oncology, University of Tokushima School of Medicine, Kuramoto-cho 3-18-15, Tokushima 770-8503, Japan. Phone: 81-88-633-9274; Fax: 81-88-633-2134; E-mail: manae{at}clin.med.tokushima-u.ac.jp ![]()
3 The abbreviations used are: NK, natural killer; SCID, severe combined immunodeficient; PTHrP, parathyroid hormone-related protein; VEGF, vascular endothelial growth factor; TRAP, tartrate-resistant acid phosphatase; IL, interleukin. ![]()
4 Miki, T., Yano, S., Hanibuchi, M., Kanematsu, T., Muguruma, H., and Sone, S. Parathyroid hormone-related protein (PTHrP) is responsible for production of bone metastasis, but not visceral metastasis, by human small cell lung cancer SBC-5 cells in natural killer cell-depleted SCID mice. Int. J. Cancer, in press, 2003. ![]()
Received 4/16/03; revised 7/18/03; accepted 7/22/03.
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