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1Department of Cell Biology and Anatomy, University of Miami, School of Medicine and Sylvester Comprehensive Cancer Center, Miami, Florida; 2Hematology/Oncology Section, Department of Medicine, University of Miami School of Medicine, Miami, Florida; 3Department of Bioimmunotherapy, The University of Texas M. D. Anderson Cancer Center, Houston, Texas; 4Department of Radiation Oncology, University of Miami, School of Medicine and Sylvester Comprehensive Cancer Center, Miami, Florida; 5Division of Biostatistics, University of Miami, School of Medicine and Sylvester Comprehensive Cancer Center, Miami, Florida; and 6Threshold Pharmaceuticals, South San Francisco, California
| ABSTRACT |
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| Introduction |
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For decades, screening and selection of anticancer agents were based on the more rapid division of tumor cells relative to normal cells. As a result, radiation therapy and most chemotherapy currently used to treat cancer target the rapidly dividing cells of a tumor as well as the most rapidly dividing normal cells that reside in the bone marrow, gastrointestinal regions, and hair follicles. Thus, the main selectivity of these treatments is not between malignant and normal cells but between rapidly and slowly dividing cells. Solid tumors, however, contain regions of slowly proliferating cells and because most normal cells are either resting in G0 or are dividing slowly, one of the most difficult populations to selectively target are the slow-growing malignant cells. A distinguishing feature between slow-growing tumor and normal cells is that the microenvironment of the former is hypoxic, which contributes to reduced growth rate and drug resistance (6, 7, 8) . Under hypoxia, tumor cells must metabolize glucose anaerobically to generate ATP, thereby providing a window of selectivity that can be exploited with inhibitors of glycolysis (9, 10, 11) .
In a recent series of papers using three in vitro models of simulated hypoxia (9, 10, 11) , we have shown that cells under hypoxic conditions are more sensitive than cells under aerobic conditions to agents that inhibit glycolysis, such as 2-deoxy-D-glucose (2-DG) and oxamate. The basis for this increased sensitivity is that a tumor cell growing under low or no oxygen must rely primarily on glycolysis to produce ATP. Thus, when challenged with a glycolytic inhibitor, ATP synthesis is shut off, and the cell succumbs to this treatment. However, in the presence of oxygen, a cell can use alternative sources of energy, such as fats and proteins, to synthesize ATP and hence survive inhibition of glycolysis. Because the slowly proliferating tumor population can be selectively killed with glycolytic inhibitors, combining such agents with chemotherapeutic drugs, which target the rapidly dividing aerobic cells, should raise the overall efficacies of these treatments (9, 10, 11) . Here, we have designed experiments to test this strategy in vivo using 2-DG in combination with either Adriamycin (ADR) or paclitaxel in two different human tumor types, osteosarcoma and non-small cell lung cancer (NSCLC), growing in nude mice.
| Materials and Methods |
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2-DG + Paclitaxel in Human MV522 Lung Tumor Carcinoma Cells in Nude Mice.
Nude female mice, approximately 56 weeks of age and weighing approximately 20 g, received s.c. implant by trocar with fragments of human MV522 lung tumor carcinoma harvested from s.c. tumors growing in nude mice hosts. When tumors were approximately 70 mm3 in size (11 days later), the animals were pair-matched into five groups (10 mice/group) as follows: vehicle-treated control; paclitaxel alone; paclitaxel + 2-DG (500 mg/kg); paclitaxel + 2-DG (1000 mg/kg); and paclitaxel + 2-DG (2000 mg/kg). An aqueous solution of 2-DG was delivered twice daily by oral gavage. Doses of 500 and 1000 mg/kg were given for the duration of the experiment, whereas the 2000 mg/kg 2-DG dose was given twice daily for only 10 days. Paclitaxel was given by i.p. injection at 16 mg/kg once a day for 5 days, beginning 5 days after the first day of 2-DG treatment. Mice were weighed, and tumor measurements were taken by caliper twice weekly. Tumor measurements were converted to tumor volume using the formula W x L2/2. Mice were killed when their tumor volume exceeded 1000 mm3. At sacrifice, mice were weighed, and tumors were excised and weighed.
Statistical Analysis.
For the osteosarcoma studies, linear regression was used to determine the rate of the log of tumor growth over time for each treatment using the following equation: Loge(tumor volume + 1) =
i + ßi(time) +
ik, where i = 1,2,3,4 indicates treatment group, and k is an index for each mouse (k = 1, ... , ni). Faster growth of tumor is represented by larger slopes (ß) in the regression equation, which in turn represent greater rates of disease progression. This model was used to estimate the number of days required to reach specific tumor volumes of 400, 800, and 1200 mm3. Although the mice in some of the treatment groups survived for more than 4 weeks, the analyses of tumor growth rates were truncated at the time when the control animals began to reach the maximum tumor volume allowed. Multiple comparisons were made between the various treatment groups to determine whether the rates of tumor growth, i.e., slopes, differed across treatment groups.
For the NSCLC study, the time for each individual tumor to grow to 600 mm3 was determined, and a one-way ANOVA repeated measures test was done to determine whether there was a significant difference in time for any of the groups to reach a tumor size of 600 mm3. When a significant difference was found (P < 0.050, a Dunnetts post hoc comparison was performed to determine whether the combined treatment was significantly different from paclitaxel alone.
Data are plotted for each group up until the day when no animals have been sacrificed due to tumor burden. For animals that died prematurely, tumor volumes were set to 1000 mm3 for all subsequent time points.
| Results |
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When the time to achieve tumor volumes of 400, 800, and 1200 mm3 was estimated by linear regression in this experiment, the longest delay in tumor growth was again found in the group of animals treated with ADR + 2-DG for all three tumor volumes (Table 2)
. Specifically, this group of animals required approximately 20 days for tumor volume to reach 400 mm3, whereas this volume was reached in approximately 2 days with control, 3 days with 2-DG alone, and 4 days with ADR alone. Similar advantageous results for the combination treatment of 2-DG + ADR were observed when time to reach 800 and 1200 mm3 tumor volumes were estimated. It should be noted that this second experiment was a more stringent test of the effect of the various treatment regimens because larger tumors are typically more difficult to treat. The data from both studies are consistent in demonstrating that beginning with small or large tumors, there is significant advantage in combination treatment with 2-DG + ADR as compared with treatment with either agent alone. In addition to slowing tumor growth, cures were found or significant tumor shrinkage was observed only in the ADR + 2-DG group.
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| Discussion |
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One of these factors is that chemotherapeutic agents including paclitaxel and ADR display antiangiogenic and or anti-HIF activity (12, 13, 14) . Thus, tumors treated with these agents may become more hypoxic, leading to enhanced efficacy of 2-DG. Moreover, if anti-HIF agents reduce the overexpression of glycolytic enzymes, then theoretically, this should reduce the amount of glycolytic inhibitor necessary to shut down glycolysis. Therefore, the multiple effects that anti-HIF agents such as 2-methoxyestradiol (15) have on tumor metabolism in blocking both angiogenesis and reducing the expression of glycolytic enzymes would theoretically increase the effectiveness of 2-DG or other inhibitors of glycolysis.
Another factor that may be contributing to the results presented here is based on reports that the combination of 2-DG and cisplatin is more effective than either agent alone when applied to various cell lines that are rapidly proliferating in vitro (16) . One of the explanations for this effect is that cells treated with drugs like cisplatin, which are known to cause DNA damage, cannot repair lesions as readily when their ATP levels are reduced as a result of glycolytic inhibition (16) . Similar in vitro synergism has been observed with the combination of 2-DG and ADR in MCF-7 cells (17) , however, other studies have reported antagonistic effects (18) . Thus, further experimentation will be required to determine whether the increased effectiveness of the combination therapy presented here is due to 2-DG preventing cells from repairing damage caused by ADR or paclitaxel.
An alternative explanation for how 2-DG increases the activity of chemotherapeutic agents is based on the fact that the p-glycoprotein effluxing pumps require ATP for their activity (19) . If ATP concentrations are reduced, as has been reported to occur when cells are treated in vitro with 2-DG (16) , the pumps will cease to function, and drug accumulation should increase intracellularly, thereby killing the cell. Thus, treating such resistant tumors with a combination of glycolytic inhibitor and any of the numerous anticancer agents known to be recognized by this mechanism (19 , 20) should provide clinical benefit. Although we reported previously that the osteosarcoma cell line used here expressed low levels of multidrug resistance-related protein (MRP), and the MDR1 protein was undetectable, it remains possible that other effluxing pumps may be present in this cell line as well as in the NSCLC cell line used in these experiments.
In summary, regardless of the underlying mechanisms, it is demonstrated here that 2-DG does indeed increase the efficacy of standard chemotherapeutic drugs when applied to human tumors in vivo. We believe that this strategy of inhibiting glycolysis will have broad clinical benefit as an adjunct to cancer therapies. Our approach should be particularly applicable to antiangiogenic treatment and to the new agents tested against HIF, both of which should make the tumor more hypoxic and therefore more sensitive to inhibitors of glycolysis. In conclusion, we anticipate that inhibitors of glycolysis could have widespread application for a variety of tumor types not only in combination with existing anticancer agents but also with emerging new treatments that target the metabolic state of a tumor.
| 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: Theodore J. Lampidis, University of Miami School of Medicine, P.O. Box 016960, Department of Cell Biology and Anatomy (R-124), Miami, Florida 33101. Phone: (305) 243-4846; Fax: (305) 243-3414; E-mail, tlampidi{at}med.miami.edu
Received 10/21/03. Accepted 11/13/03.
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