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Advances in Brief |
Department of Radiation Oncology, Henry Ford Health System, Detroit, Michigan 48202 [Y. S. L., S. L. B., J. H. K.] and Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota Medical School, Minneapolis, Minnesota 55455 [R. J. G., C. W. S.]
| ABSTRACT |
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| Introduction |
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It has been documented that arsenic can induce apoptosis in leukemic cells by activating apoptotic pathways (5 , 6) . However, the primary target of arsenic-induced cytotoxicity remains unknown. Trivalent arsenicals are regarded primarily as sulfhydryl reagents at the biochemical level. As2O3 inhibits many enzymes by reacting with biological ligands containing available sulfhydryl groups. Among them, arsenic targets vicinal thiol groups in tubulin with a high specificity (7) . We postulated that As2O3 may have an antivascular effect on solid tumors because tubulin has been shown to be a primary target for other antitumor and antivascular agents, such as colchicine (8) , flavone acetic acid (9) , and combretastatin-A4 (10) . We report the first in vivo observation demonstrating the antivascular effect of As2O3 in well-established murine solid tumors.
| Materials and Methods |
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As2O3 purchased from Sigma Chemical (St. Louis, MO) was dissolved in H2O by continuous stirring. As2O3 was kept at 4°C as a stock solution. Dilution was made with 0.9% NaCl to a final concentration of 0.5 mg/ml As2O3. Dextrose was added to a final concentration of 5% to minimize acute arsenic toxicity (12) .
Gross Morphology and Histopathology
Gross morphological changes were followed up to 48 h after a single injection of As2O3 (10 mg/kg i.p.). At selected time points, tumors were excised and histopathological examinations were performed. Tumor tissues from different groups were fixed in 10% neutral formalin and processed in paraffin wax. Sections (4 µm thick) were stained with H&E and assessed microscopically to investigate the temporal effect of As2O3.
Inhibition of Tumor Growth
Mice, typically five per group, with mean tumor diameters of 12.0 ± 1.0 mm were used in treatment or control studies. As2O3 was administered i.p. in doses of 10 mg/kg once a week for the duration of the experiment, starting 13 days after tumor inoculation. The same volume of normal saline with 5% dextrose was given to the control group. Tumor volume was assessed three times a week. Tumors were measured with a caliper, and tumor volumes were calculated assuming cylindrical growth using the formula a/2 x b/2 x h x
, where a, b, and h are the minor and major dimensions and height, respectively. No attempt was made to correct the tumor volume measurement for the area of central necrosis.
Blood Perfusion Measurement
Two methods, technetium clearance and rubidium uptake, were used to assess the antivascular effect of a single injection of As2O3 (10 mg/kg i.p.). Both methods have been described in detail elsewhere (13
, 14)
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99mTc Clearance.
Clearance measurements were performed as described previously (13)
. Briefly, mice were anesthetized with a 60 mg/kg i.p. injection of sodium pentobarbital and were immobilized using a jig that did not restrict blood flow to the tissue of interest. Technetium bound as sodium pertechnetate, Na99mTcO4 (typical activity, 6 µCi/ml), was injected directly into the central area of the intradermal tumor 10 min after the pentobarbital injection. A total volume of 10 µl was deposited, using a 29-gauge needle. Radioactivity was measured and recorded every 10 s, 200 times, using a multichannel scintillation counter (AccuSpec; Canberra Industries, Meriden, CT) interfaced with a personal computer. Raw data were corrected for the physical decay of technetium (6 h) and fit to a single exponential decay constant. No attempt to measure the partition coefficient was made, and the results are reported as clearance rate, not blood perfusion. The clearance rate was measured in each mouse before and 2, 6, and 24 h after arsenic treatment. Six tumor bearing mice with 12.0 ± 1.0 mm mean diameter tumors were used to study the effect of As2O3 on tumor clearance rate at each time point. Control experiments included clearance rate measurements in normal skin after As2O3 and also in untreated control tumors.
86Rb Uptake.
Changes in blood perfusion were measured in mouse tissue, using the rubidium uptake method (14)
. The rubidium studies were performed using s.c. Meth-A tumors. The tumor site was not critical to the results because we obtained similar results with rubidium-uptake studies using intradermal tumors at selected time point. In all experiments, 5 µCi of 86RbCl in a 0.1-ml volume were injected through a tail vein after anesthesia with 100 mg/kg ketamine and 10 mg/kg xylazine, and mice were sacrificed by cervical dislocation 60 s after injection. The tumors, kidney, skin, and muscle were excised and counted in a well-counter (1282 Comugamma Cs; Pharmacia LKB Nuclear Inc., Gaithersburg, MD). The ratio of radioactive counts from the tissue of interest to the counts in the standard 86Rb solution equivalent to the total 86Rb activity injected multiplied by 100 gives the percentage of cardiac output to the tissue of interest. In this study, we did not attempt to measure absolute blood flow; relative blood perfusion values are presented. 86Rb uptake was measured before and 2, 6, and 24 h after arsenic treatment. At least 10 tumor-bearing mice were used for each time point. The results are reported as the percentage of 86Rb uptake per gram of tissue and plotted as a percentage of the control tissue.
| Results |
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Gross Morphological and Histopathological Changes.
Fig. 1A
illustrates gross tumor morphology before and after a single injection of the drug. As2O3 induced a pronounced central necrosis in the tumor, which was observed within 4 h and steadily progressed until at least 48 h. More than 70% of the tumor tissue underwent central necrosis, and the necrotic area formed a crust that became stony hard by 48 h. The tumor tissue under the crust was malleable. The peripheral zone of tumor tissue appeared unaffected. Tumor regrowth appeared to be initiated from the peripheral zone.
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Tumor Growth Delay.
Fig. 2
shows changes in the tumor volume as a function of time after a weekly administration of the drug. Immediately after the drug was administered, the tumor ceased to grow for
2 days and then resumed growing. The regrowth was always seen from the periphery of the tumor. After administration of a second dose of the drug, tumor growth stopped for
2 days, after which the tumor continued growing. A similar growth inhibitory pattern was observed after a third dose of the drug.
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89 min, and this clearance rate did not change with subsequent injections even with seven repeat injections in the same area (data not shown). In contrast, the rate of technetium clearance was markedly reduced by the administration of As2O3 and remained low at 24 h, consistent with morphological observations. Technetium clearance was 4.94 ± 1.53 h-1 before treatment and 2.57 ± 1.70, 0.58 ± 0.35, and 0.18 ± 0.09 h-1 at 2, 6, and 24 h after treatment, respectively. The calculated percentages relative to the control tumor were 52.1 ± 35.5, 11.6 ± 7.1, and 3.9 ± 1.8% of the control at 2, 6, and 24 h, respectively.
Uptake.
Rubidium uptake in Meth-A tumors showed that a significant decrease in tumor blood perfusion occurred at 2 and 6 h after As2O3 injection and that tumor blood perfusion had recovered by 24 h. Rubidium uptake was 1.50 ± 0.21%/g tissue before treatment, and it decreased to 0.20 ± 0.07, 0.30 ± 0.08, and 1.0 ± 0.20%/g tissue at 2, 6, and 24 h after treatment, respectively. The calculated percentage relative to control tumor blood perfusion was 16.4 ± 5.0, 20.5 ± 5.2, and 64.3 ± 13.3% of the control at 2, 6, and 24 h respectively. The data of 2 and 6 h after the drug treatment were similar to the trends for the clearance rate. The uptake data at 24 h were inconsistent with both technetium clearance and morphological observation (see "Discussion"). Similar trends in uptake were observed in two other tumor lines studied (FSaII in C3H mice and SCK in A/J mice; data not shown).
Fig. 4
illustrates the effect of As2O3 (10 mg/kg) on rubidium uptake in the kidney, skin, and leg muscle measured at the same time as the measurement in the tumor. Blood perfusion in the skin and muscle was not changed significantly compared with control values. The maximum reduction of blood perfusion in normal tissues was that of the kidney at 2 h after As2O3 (60.0 ± 5.5% of control). The reduction in blood perfusion reduction to the tumor was much greater than that in the normal tissues studied.
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| Discussion |
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Two different methods of blood perfusion measurements gave complementary information. Technetium clearance, which allows multiple measurements in the same tumor, demonstrated a decreased clearance rate with increasing time after As2O3 treatment. This finding corresponded well with the morphological findings. Rubidium uptake, a single measurement in different animals, yielded decreased blood perfusion relative to controls up to 6 h after As2O3 treatment, with some recovery at 24 h. Further analysis of the perfusion data at the 24-h time point showed that perfusion in half of the group of tumors (n = 6 of 14) remained low (mean ± SE, 24.4 ± 10.9% of the control tumor) and that perfusion in the other tumors (n = 8) recovered to near control values (94.2 ± 46.1%). Separate uptake studies with intradermal Meth-A tumors showed results similar to the results for the s.c. tumors. When rubidium activity was assessed from the central and peripheral parts of the tumor separately, the central part of the tumors had a lower uptake than the tumor periphery (37.6 ± 25.1% versus 82.2 ± 53.3%). These data indicate that the discrepancy between technetium clearance and rubidium uptake could be explained by the rubidium activity in the peripheral zone of the tumor. Regardless of the 24-h data, As2O3 treatment causes a rapid, selective shut down of tumor blood perfusion and induces massive necrosis in the central part of the tumor.
Several other antitumor agents, including combretastatin A-4 (10) , colchicine (8) , flavone acetic acid (9) and hydralazine (15) , have been shown to have their primary effects on tumor vasculature. Their effects are characterized by massive vascular shut down in the central region of the tumor. Flavone acetic acid, hydralazine, and colchicine are either ineffective or toxic in species other than mice. Combretastatin A-4 and new flavone acetic acid derivatives have shown promise in overcoming these past problems. Similarly, As2O3 has been used in humans for many years, and preliminary studies on rats in this laboratory indicate that its antitumor effects may not be species specific.4
The underlying mechanisms of vascular shut down and central necrosis are not easily explained by our current knowledge of the known biological effects of As2O3. However, several possible scenarios are being entertained to illustrate this phenomenon. One possible mechanism is that As2O3 can induce selective endothelial cell injury, resulting in vascular shut down to tumor tissue. This mechanism is based on the observation that As2O3 is a noncompetitive inhibitor of GTP binding to tubulin (7) . Combretastatin A-4 affects normal microtuble function and has been shown to have a greater toxicity toward endothelial cells compared with tumor cells in vitro and induces selective vascular shut down in P22 rat carcinosarcoma in vivo (10) . Another tubulin-binding agent, colchicine, produced hemorrhagic necrosis in experimental tumors (8) .
Another possible scenario is tumor necrosis factor-mediated vascular shut down, because the gross observation after As2O3 treatment is similar to that seen with endotoxin-induced necrosis. The evidence is strengthened by the observation that cultured human keratinocytes overexpress tumor necrosis factor mRNA after arsenic exposure (16) .
Apoptosis or/and necrosis represents a third possible scenario. As2O3 triggers apoptosis in APL cells by degrading promyelocytic leukemia/retinoic acid receptor-
fusion protein (6
, 17)
, but it also induces apoptosis in other hematological malignancies and several solid tumor cells in vitro (7
, 18
, 19)
at a lower concentration (0.55 µM). At higher concentrations (>5 µM), As2O3 causes acute necrosis in various cell lines (7)
. In this scenario, we can speculate massive apoptosis or/and necrosis in the central area of the tumor after As2O3 exposure.
The decrease in blood perfusion in the kidney and tumors observed with rubidium uptake suggests a systemic effect of As2O3. Changes in blood pressure measured using plethysmography on the tail revealed that the systolic pressure dropped to 50% of the original pressure 1 h after As2O3 injection (data not shown), remained low for
1 h, and recovered gradually to near normal levels by 6 h. As2O3-induced systemic hypotension in the presence of high tumor interstitial fluid pressure, especially in the central area of the tumor (20)
, may be considered a contributing factor in inducing a vascular shut down.
We currently are investigating these possible scenarios, but most of our preliminary data do not unequivocally confirm any of the possible scenarios. Nonetheless, the present findings that As2O3 induces a rapid tumor mass reduction with a viable, presumably well-oxygenated peripheral zone of the tumor may provide an opportunity to combine this approach with standard cancer therapy that includes radiation and cytotoxic agents.
| FOOTNOTES |
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1 This work was supported in part by NIH Grants NIH-CA59542, NIH-CA44114, and DAMD17-99-9215. ![]()
2 To whom requests for reprints should be addressed, at Department of Radiation Oncology, Henry Ford Health System, Detroit, MI 48202 Fax: (313) 916-3235; E-mail: younglew{at}yahoo.com ![]()
3 The abbreviations used are: APL, acute promyelocytic leukemia; As2O3, arsenic trioxide; Meth-A, methylcholanthrene-induced fibrosarcoma. ![]()
4 Y. S. Lew et al., unpublished results. ![]()
Received 9/13/99. Accepted 10/29/99.
| REFERENCES |
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/PML proteins. Blood, 88: 1052-1061, 1996.
protein in acute promyelocytic leukemia cells. J. Natl Cancer Inst., 90: 124-133, 1998.
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