[CANCER RESEARCH 52. 5635-5640. October 15. 1992]

Chemoprevention of Colon Carcinogenesis by the Synthetic OrganoseleniumCompound 1,4-Phenylenebis(methylene)selenocyanate1

Bandaru S. Reddy,2 Abraham Rivenson, Nalini Kulkarni, Pramod Upadhyaya, and Karam EI-Bayoumy

Divisions of Nutritional Carcinogenesis ¡B.S. R., N. K.], Experimental Pathology and Toxicólo®-¡A.R.], and Chemical Carcinogenesis ¡P.U., K. E-B.J, American

Health Foundation, Valhalla, New York 10595

ABSTRACT

The chemopreventive effect of 40% and 80% maximum tolerated dose(MTD) levels of l,4-phenylenebis(methylene)selenocyanate (/;-\S( )

administered in the diet during the initiation phase (2 weeks before,during, and up to 3 days after carcinogen administration) and the post-

initiation phase (3 days after carcinogen treatment until termination) ofazoxymethane (AOM)-induced colon carcinogenesis »as studied inmale F344 rats. The MTD of p-\SC was determined in male F344 rats

and found to be 50 ppm. Beginning at 5 weeks of age, all animals weredivided into various experimental groups (42 rats/group) and fed thehigh-fat semipurified diet or diets containing 20 (40% MTD) and 40(80% MTD) ppm p-XSC. At 7 weeks of age, all animals (30 rats/group)except the vehicle-treated groups (12 rats/group) were administered s.c.

injections of AOM (15 mg/kg body weight/week for 2 weeks). Threedays after the second injection of AOM or vehicle (normal saline),groups of animals fed thep-XSC diets and control diet were transferred,respectively, to control diet and p-XSC diets and continued on these

diets until the termination of the study. All animals were necropsiedduring the 36th week after AOM treatment. Colonie mucosa! prosta-glandin I and selenium-dependent glutathione peroxidase were measured in animals fed the control and />-\S< ' diets at the termination of

the study. The results indicate that 40 ppm p-XSC administered during

the initiation phase significantly inhibited the colon tumor incidence(percentage of animals with tumors). Dietary p-XSC administered at 20

and 40 ppm levels during the initiation phase significantly inhibitedcolon tumor multiplicity (tumors/animal and tumors/tumor-bearing an

imal). Colon tumor incidence and multiplicity were significantly reducedin groups fed 20 and 40 ppm p-\SC diets at the postinitiation phase ofcarcinogenesis. Colonie mucosa! selenium-dependent glutathione peroxidase activity was increased, and prostaglandin 1'_.was reduced in ani

mals fed the p-\SC diet compared to animals fed the control diet.Whereas the precise mechanisms ofp-XSC-induced inhibition of colon

carcinogenesis remain to be elucidated, it is likely that the effect duringthe initiation and postinitiation phases may be due to alteration incarcinogen metabolism and to modulation of prostaglandin synthesisand selenium-dependent glutathione peroxidase activity.

INTRODUCTION

Epidemiological studies indicate an increased incidence ofcolorectal cancer in humans in geographic regions where selenium is deficient (1-3). Case-control studies suggest an inhibitory effect of selenium on cancer mortality (4, 5). Prospectivestudies also appear to provide a clear reflection of the association between the levels of selenium and colon cancer (4, 5).Beginning with the work of Clayton and Baumann (6), a seriesof experiments were conducted in laboratory animals whichdemonstrate that selenium supplementation in the diet or in

Received 4/2/92; accepted 8/10/92.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.

1Supported by USPHS Grants CAI7613 and CA46589 from the NationalCancer Institute. This paper is fourteenth in the series "Selenium in Chemoprevention of Carcinogenesis."

2 To whom requests for reprints should be addressed at the Division of Nutritional Carcinogenesis. American Health Foundation. One Dana Road. Valhalla.NY 10595.

drinking water inhibits initiation and/or postinitiation stages ofpancreas, liver, colon, and mammary gland carcinogenesis(7-12).

Although inorganic selenium compounds have been shown toinhibit colon carcinogenesis, their toxicity is of concern (13).Since human exposure to selenium occurs primarily throughingestion of the organic form, which occurs predominantly inthe form of selenomethionine in cereals, grains, and vegetables,investigators have focused on the inhibitory effect of organicforms of selenium. Ip and White (14) found no significant differences in the tumor-inhibiting potential of the inorganic (e.g.,selenate, selenite, selenium dioxide) and organic (e.g., selenomethionine and selenocysteine) selenium compounds in DMBA3-

induced mammary carcinogenesis. Thompson et al. (15) alsoshowed that selenite was more potent and less toxic than selenomethionine in DMBA-induced mammary tumorigenesis during the postinitiation stage. In fact, selenomethionine at 6 ppm(as selenium) caused liver damage in rats. Milner and Hsu (16)reported that the inorganic selenium compounds were moreeffective than either selenomethionine or selenocysteine. It isapparent from these studies that the naturally occurring orga-noselenium compounds do not provide an advantage over inorganic selenium compounds as protective agents in carcinogenesis.

During the last decade it was our goal to develop novel synthetic organoselenium compounds with optimal chemopreventive potency and yet with low toxicity (17). Experiments conducted in our laboratory indicate that synthetic organoseleniumcompound, BSC, but not its sulfur analogue, benzyl thiocyan-ate, inhibited AOM-induced colon carcinogenesis when administered during the initiation or postinitiation stage of carcinogenesis, whereas the inorganic form of selenium, sodiumselenite, inhibited colon carcinogenesis only during the postinitiation stage (18). Although BSC was less toxic than Na2SeO1on the basis of its acute 50% lethal dosage (BSC, 125 mg/kgbody weight; Na2SeO,, 35 mg/kg body weight), animals fedboth agents gained less weight and consumed less food compared to those on the control diet, suggesting that these agentsexerted a systemic toxic effect (18). Because of these side effects, additional organoselenium compounds were synthesizedin an effort to design more effective chemopreventive agentswith minimal toxic side effects. Based on mechanistic and metabolism studies (19), we modified the structure of BSC todevelop a more effective and less toxic chemopreventive agent.Among these novel compounds, p-XSC (Fig. 1) was less toxicthan BSC and Na2SeO3, based on 50% lethal dosage and sub-chronic studies (50% lethal dosage of p-XSC, 1000 mg/kg bodyweight; Ref. 20).

Selenium is an important component of GSH-Px, the biochemical function of which is to protect cells from oxidative

'The abbreviations used are: DMBA, 7,12-dimethylben/[a]anthracene; BSC,ben/ylselenocyanate; AOM. azoxymethane; GSH-Px, selenium-dependent glutathione peroxidase; PGE2, prostaglandin E2; MTD. maximum tolerated dose;MAM. methjlazoxj methanol; p-XSC. 1,4-phen\lenebis(methylene)sclenocyanate.

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INHIBITION OF COLON CANCER BY ORGANOSELENIUM

CHoSeCN

CHoSeCNFig. 1. Chemical structure of p-XSC.

damage (21, 22). Indication that a dietary excess of selenium inthe form of organoselenium might increase the GSH-Px activity in the colon is provided by our previous study with BSC (23).For this reason, it is possible that the chemopreventive effectsof p-XSC might be mediated, in part, through an alteration inGSH-Px activity in the colon. There are reports that indicateexcessive prostaglandin production, particularly of the E series,in several malignant tissues in both human and experimentalmodels (24, 25). GSH-Px has been shown to be involved in theregulation of fatty acid hydroperoxides during the productionof prostaglandins (26), indicating the underlying relationshipbetween the prostaglandins and GSH-Px.

Accordingly, the present study was designed to investigatethe efficacy of p-XSC in AOM-induced colon carcinogenesisduring the initiation and postinitiation stages in male F344 ratsfed the high-fat diet. Biochemical studies to determine themechanistic basis for chemoprevention which include coloniemucosal PGE2 and GSH-Px were conducted. The rationale forthe high-fat content was to simulate a western-style diet; also, ahigh-fat diet containing corn oil has been shown to elevatecolonie mucosal PGE2 levels as compared to a low-fat diet (27).

MATERIALS AND METHODS

Animals, Diets, and Carcinogen. Weanling male inbred F344 ratswere purchased from Charles River Breeding Laboratories (Kingston,NY) and AOM (CAS:25843-45-2) from Ash-Stevens (Detroit, MI). Allingredients of the semipurified diet were purchased from Dyets, Inc.(Bethlehem, PA) and stored at 4°Cprior to formulation of the diets.

Animals obtained at weaning were quarantined for 10 days prior totransferring them to a holding room which was maintained under controlled environmental conditions of a 12-h light-dark cycle, 50% humidity, and 22°C.Animals received proper care and maintenance inaccordance with the institutional guidelines. The high-fat control (modified AIN-76A) and experimental diets were prepared in our laboratoryonce weekly and stored in a cold room in air-tight plastic containersfilled with nitrogen (Table 1; Ref. 28).

p-XSC was synthesized from the corresponding a,a'-dibromo-p-xy-lene and KSeCN as described (28). Briefly, a solution of a,a'-dibromo-

p-xylene in acetone was added dropwise to a suspension of KSeCN inacetone. The acetone was evaporated, and the residue was dissolved inCH2C12. The solution was washed with water followed by drying inMgSCX,. The product was crystalized from hexane:benzene (3:1). Thepurity of p-XSC was >99.9°/obased on high-performance liquid chro-matography analysis. p-XSC had a retention time of 29-30 min usinga C|8 reverse-phase column and a linear gradient from 20% CH,OH-H2O to 100% CH,OH in 60 min at a flow rate of 1 ml/min.

The stability of p-XSC in the diet at room temperature was confirmed by high-performance liquid chromatography analysis (20).p-XSC was extracted with ethyl acetate. The extract was dried withMgSO4 and evaporated to dryness. A known volume of tetrahydrofuranwas added to the residue, and an aliquot was analyzed by high-performance liquid chromatography for p-XSC as described above. The recovery of p-XSC was more than 97% under the bioassay feeding con

ditions.Determination of Maximum Tolerated Dose of p-XSC. The aim of

this study was to determine the MTD of p-XSC, which is defined as thehighest dose that causes no more than 10% body weight reduction (ascompared to those fed the control diet) and does not induce mortalityor external clinical signs of toxicity that would be predicted to shortenthe natural life span of the animal. At 35 days of age, groups of maleF344 rats (10/group) were fed the high-fat control diet (Table 1) andhigh-fat diets containing 20, 40, 50, 75, and 100 ppm p-XSC. All

animals were examined daily for any symptoms of toxicity. Bodyweights were recorded twice weekly for 10 weeks. After 10 weeks on theexperimental diets, all animals were sacrificed, and tissues were examined grossly under the dissection microscope for any abnormalities thatcan be attributed to toxicity of the test agents. The body weights ofanimals fed the diets containing 20, 40, and 50 ppm of p-XSC were

comparable to those fed the control diet. Animals fed 75 and 100 ppmof p-XSC showed a decline in body weight by about 12 and 20%,

respectively. The results of this study thus suggest that the MTD ofp-XSC is 50 ppm.

Experimental Procedure. Experiments were designed to study theefficacy of two levels (80% and 40% MTD) of p-XSC administered inthe high-fat diet during the initiation and postinitiation phases ofAOM-induced intestinal carcinogenesis in male F344 rats. The doselevels, respectively representing 80% and 40% MTD, were 40 and 20ppm. The high-fat control diet contained about 0.1 ppm selenium asNa2SeO-i. The experimental 20 and 40 ppm p-XSC diets containedabout 10 and 20 ppm selenium, respectively.

Male F344 rats received at weaning were quarantined for 10 days. At5 weeks of age, all animals were divided at random into various experimental groups (42 animals/group) using formal randomized methods(Table 2) and were fed the control or one of the experimental dietscontaining p-XSC (Table 1). At 7 weeks of age, all animals (30/group)except the vehicle-treated received a s.c. injection of AOM at a doselevel of 15 mg/kg body weight/week for 2 weeks. Vehicle-treated groups(12 animals/group) received an equal volume of normal saline. Threedays after the second injection of AOM or normal saline, groups ofanimals receiving the experimental diets containing 20 and 40 ppmp-XSC were transferred to the control diet and continued on this dietuntil termination of the experiment (initiation stage). Additional subgroups fed the control diet were transferred to the experimental dietscontaining 20 and 40 ppm p-XSC and continued on these diets until thetermination of the study (postinitiation stage). The 3-day delay beforefeeding the experimental diets was meant to ensure complete metabolism and excretion of AOM. All animals were fed their respective experimental diets and continued on this regimen until the termination ofthe experiment. Body weights were recorded every 2 weeks until the

Table 1 Percentage composition of experimental semipurifieddietsDiet

ingredientsCaseinD,L-MethionineCornstarchDextroseCorn

oilAlphacelMineral

mix,AIN-76AVitaminmix,AIN-76ACholine

bitartratep-XSC*Control

diet"23.500.3532.908.3023.525.904.111.180.240Experimentaldiets23.500.3532.908.3023.525.904.111.180.240.004

or 0.002" Adopted from American Institute of Nutrition Reference Diet (AIN-76A) with

the modification of source of carbohydrate (27).*p-XSC was added to the diet at the expense of cornstarch.

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INHIBITION OF COLON CANCER BY ORGANOSELENIUM

Table 2 Experimental designInitiation"Dietary

groupsAOM-treated

Control diet80% MTD, p-XSC40% MTD,p-XSCSaline-treated

Control diet80% MTD, p-XSC40% MTD. p-XSCNo.

of animals/group30

30301212

12Levels

ofp-XSC0

20 ppm40ppm0

20 ppm40 ppmPostinitiationANo.

of animals/group30

303012

1212Levels

ofp-XSC0

20 ppm40ppm0

20 ppm40 ppm

" Animals were fed the diets containing 0, 20, and 40 ppm p-XSC 2 weeksbefore, during, and until 3 days after the carcinogen or saline treatment and thentransferred to control diet.

'" Animals were fed the control diet 2 weeks before, during, and until 3 days aftercarcinogen or vehicle treatment, and then groups in 40 and 80% MTD, weretransferred to their respective p-XSC diets and fed these diets until termination ofthe experiment. One group continued on the control diet.

16th week and then every 4-6 weeks until the termination of the experiment. Food intake was measured during the 12th week on experimental diets. The experiment was terminated during the 36th weekafter the AOM treatment. Animals that were dying or moribund werenecropsied. Surviving animals were necropsied as scheduled. All organs, including the intestines, were examined grossly under the dissection microscope. Intestines were fixed in 10% neutral buffered formalin, embedded in paraffin blocks, and processed by routine histológica!methods with the use of hematoxylin and eosin stains. The histológica!criteria used for intestinal tumor classification were as described previously (29, 30).

Analysis of l'(,l , and GSH-Px. Animals intended for PGE2 andGSH-Px analysis were sacrificed by CO2 euthanasia. Colons were rapidly removed, cut open longitudinally, and rinsed with ice-cold normalsaline to free them of all contents. They were laid flat on a glass plate,and the mucosal layer was scraped with a microscope slide. The sampleswere quickly frozen in liquid nitrogen and stored at -80°C. The sam

ples were weighed and homogenized with polytron in 0.1 MTris-HCIbuffer (pH 7.4).

For the assay of PGE2, indomethacin (10 Mg/rnl) was added to thescrapings to inhibit further synthesis of prostaglandins. Ice-cold meth-anol was added to the homogenate and mixed vigorously. After acidification to pH 3.5-4 with 1% formic acid, the aqueous phase was mixedwith an equal volume of chloroform. After vigorous agitation, the sam

ple was centrifuged at 900 x g. The bottom chloroform layer wasremoved and then evaporated to dryness under nitrogen. The driedextract was resuspended in the assay buffer. The radioimmunoassay wasperformed by using a commercially available 12M-labeledPGE2 kit (31).To 100 ti\ of the sample, 100 nl of PGE2 (I25I) tracer and rabbit anti-PGE2 were added and mixed well. After overnight incubation at 4°C,1

ml of cold precipitating agent containing 16% polyethylene glycol wasadded and allowed to stand for 30 min. Then the samples were centrifuged at 2000 x g for 30 min. The supernatant was removed and thepellet was counted for radioactivity in a gamma counter. A standardcurve was also prepared using different concentrations of PGE2.

For the analysis of GSH-Px, colonie mucosal scrapings as preparedabove were homogenized in three volumes of homogenizing buffer (pH7.8) containing 0.1 Mphosphate buffer and 0.1 Mdithiothreitol in 20%glycerol. Samples were centrifuged at 105,000 x g for l h to separatethe cytosol fraction. GSH-Px was determined spectrophotometricallyaccording to a modification of the enxyme-coupled assay procedure(32-34). Assay was carried in a reaction mixture containing 50 miviphosphate buffer (pH 7.0), 1 mm EDTA, 1 mw NaN,, 0.2 HIMNADPH,1 enzyme unit/ml of glutathione reducÃ-ase,and 1 imi glutathione (reduced form) in a total volume of 1 ml. To 0.8 ml of the above reactionmixture, 0.1 ml of cytosol fraction was added and incubated at 37°Cfor

5 min. Then the reaction mixture was transferred to a cuvette, and thereaction was initiated by the addition of 0.1 ml of 0.25 imi II.()..Absorbance was recorded at 340 nm for 5 min, and the activity wasexpressed as nmol NADPH oxidized/min/mg protein.

Statistical Analysis. The data of tumor incidence were analyzed statistically by the x2 method and Fisher's exact test. The significantdifferences between the means were tested by using Student's t test as

well as analysis of variance. The analyses were carried out on a VAX11/750 computer using the SAS package.

RESULTS

General Observations. Food intake measured at 12 weeks onexperimental diets indicated no significant differences in foodconsumption between the groups fed the control and p-XSCdiets. On the average, the daily food consumption of animals inthe control diet group, 20 ppm p-XSC group, and 40 ppmp-XSC group was about 14.2, 13.9, and 13.8 g, respectively.

Table 3 summarizes the body weights of animals fed thecontrol and experimental diets. Using analysis of variance, it

Table 3 Body weights of male F344 rats fed p-XSC and treated with AOM

Body weights (g) of animals on experimental diets atweek"Dietary

treatment(n)AOM-treated

Control diet(30)*Initiation

stage20 ppm p-XSC (30)40 ppm p-XSC(30)Postinitiation

stage20 ppm p-XSC (30)40 ppm p-XSC(30)Saline-treated

Control diet (12)080

±6'-*80

±780±580

±681±579

±7'2146

±9rf142

±10139±9148

±9149 ±10147

±12'8278

±IS*277

±16276 ±16267

±17268 ±18292

±15'16364

±21''366

±20364 ±20354

±21348 ±22355

±28'24433

±26*436

±27437 ±26424

±25416 ±29446

±28'36465

±28*469

±30468 ±26464

±25462 ±26486

±32'

Initiation stage20 ppm p-XSC (12)40 ppm p-XSC(12)Postinitiation

stage20 ppm p-XSC (12)40 ppm p-XSC (12)79

±679±779

±678±7141

±12139 ±12148

±14147 ±12289

±19281 ±18278

±17280 ±19373

±26363 ±26357

±25361 ±26442

±30434±31422

±29431 ±454484

±31480 ±32476

±29475 ±30

0 At 5 weeks of age. groups of animals were fed the control or one of the experimental diets containing p-XSC. This period is denoted as 0. At 7 weeks of age whichis denoted as week 2. all animals except the vehicle-treated received s.c. injections of AOM.

'' No. of animals in each group is shown in parenthesis.r Mean ±SD.d' " Differences among the dietary groups in AOM-treated groups and in saline-treated groups are not significant; P > 0.05.

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INHIBITION OF COLON CANCER BY ORGANOSELENIUM

was found that the differences in body weights from week 0 toweek 36 among the dietary groups (AOM-treated and saline-treated animals) were not significant (P > 0.05). AOM-treatedanimals weighed slightly less than those treated with saline inall dietary groups.

Table 4 shows the AOM-induced colon tumor incidence (percentage of animals with adenocarcinomas) and multiplicity(adenocarcinomas/animal and/tumor-bearing animals). In thepresent investigation, colon tumor classification was based onstudies of Elwell and McConnell (30) and Pozharisski (29).According to this classification, more than 90% of colon tumors in the present study were adenocarcinomas and the restwere adenomas. There were no differences in the incidence ofadenomas among the dietary groups. Therefore, Table 4 summarizes only the results of adenocarcinomas. There was noevidence of tumors in animals not treated with AOM and fedp-XSC. The incidence of colon adenocarcinomas was significantly inhibited in animals fed 40 ppm p-XSC during the stageof initiation. The diet containing 20 ppm p-XSC also decreasedthe colon tumor incidence during the stage of initiation, but thedifferences did not reach a statistical significance (P > 0.05).Colon tumor multiplicity (tumors/animal and tumors/tumor-bearing animal) was significantly inhibited in animals fed 20 and40 ppm during the stage of initiation. Feeding of 20 and 40 ppmp-XSC during the postinitiation stage significantly suppressedcolon tumor incidence and multiplicity.

Table 5 summarizes PGE2 and GSH-Px levels in the colonmucosa. The results indicate that 40 ppm p-XSC in the dietsignificantly decreased the colonie mucosa! PGE2. Also, dietaryp-XSC at 20 and 40 ppm feeding significantly increased (about3-fold) the colonie mucosa! GSH-Px activity.

DISCUSSION

The purpose of the current study was to investigate the efficacy of dietary organoselenium, p-XSC, when fed during theinitiation or postinitiation stage of AOM-induced colon car-cinogenesis in F344 rats. The results of the present study are ofconsiderable interest because to our knowledge, this is the firstreport showing that p-XSC can effectively reduce colon carcino-genesis when administered during the initiation or postinitia-tion phase. There are limited reports on the chemopreventiveeffects of p-XSC in laboratory animal models. Recently, El-Bayoumy et al. (35) and Ip et al. (36) demonstrated that dietaryp-XSC significantly suppressed DMBA-induced mammary car-cinogenesis during the initiation and/or postinitiation phase.

In the current study, the use of p-XSC permitted feeding of 20ppm of selenium (about 5 times the amount of selenium thatcan be fed to rats daily in the form of Na2SeO3) without retardation of body weight and other toxic effects of selenium. In ourearlier study, BSC at a level of 10 ppm as selenium and

Table 5 Effect of dietary- p-\'SC on colonie mucosa! PCE2 andselenium-dependent glutalhione peroxidase activity in AOM-treated rats

Dietary regimen (n —¿�6)Control

diet20 ppm p-XSC40 ppm p-XSCPGE213

±6.4"'*

11 ±2.99 ±4.6''GSH

Px102±54

INHIBITION OK COLON CANCER BY ORGANOSELENIUM

In the present study, dietary supplementation of p-XSC at 40ppm significantly suppressed colonie mucosal PGE2 levels.Prostaglandins are known to be one of the regulators of tumorgrowth and spread (24, 25, 40). Increased prostaglandin production by tumors has been associated with aggressive tumorprogression (41, 42). Furthermore, the suppression of immuneresponses in patients and laboratory animals bearing tumorsmay be due to tumor-derived factors such as PGE2 (43-45).Recent studies have shown that the administration of in-

domethacin or piroxicam, inhibitors of prostaglandin synthesis,suppresses colon carcinogenesis in rats (46). It has been shownthat Se-GSH-Px may be involved in the regulation of fatty acidhydroperoxides generated during the production of prostaglan-dins, leukotrienes, and related compounds via cyclooxygenaseand lipoxygenase pathways (26). Whether this may partiallyaccount for the inhibitory action of p-XSC during the postinitiation phase of colon carcinogenesis remains to be determined.

Another possible mechanism of tumor inhibition by syntheticorganoselenium may be explained on the basis of pharmacok-inetics and tissue distribution. Sohn et al. (47), who examinedthe excretion and tissue distribution of BSC, an analogue ofp-XSC, reported that selenium in animals fed BSC is excretedvery slowly and is retained in the organs for a much longerperiod compared to rats given Na2SeO> The persistence oftotal selenium in the colon with BSC suggests that BSC or itsselenium-containing metabolites may be bound to tissue components, probably proteins, which may influence the normalcellular functions such as rate of cell turnover, enzyme activities, and degradation and/or synthesis of various enzymes.Whether the prolonged retention of selenium in colons of ratsfed BSC (or p-XSC) is associated with tumor inhibition is yet tobe investigated.

In conclusion, the results of this study demonstrate that dietary p-XSC inhibits colon carcinogenesis at both the initiationand postinitiation phases. The inhibitory effect of p-XSC onAOM-induced colon carcinogenesis during the initiation phasemay be partly explained by its ability to alter the metabolicactivation of carcinogen. Although the exact mechanisms involved for its protective effect during the postinitiation phase,however, are not clearly understood at present, the results fromcolonie GSH-Px and PGE2 should provide a stimulus to designadditional studies to investigate the mechanism of colon tumorinhibition of p-XSC.

ACKNOWLEDGMENTS

We thank Donna Virgil for preparation of the manuscript and JeffRigotty for expert technical assistance.

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1992;52:5635-5640. Cancer Res Bandaru S. Reddy, Abraham Rivenson, Nalini Kulkarni, et al. 1,4-Phenylenebis(methylene)selenocyanateOrganoselenium Compound Chemoprevention of Colon Carcinogenesis by the Synthetic

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