Role of phase 2 enzyme induction in chemoprotection by dithiolethiones

One of the major mechanisms of protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by carcinogens is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases (GSTs), UDP-glucuronosyl transferases, and quinone reductases. Animal studies indicate that induction of phase 2 enzymes is a sufficient condition for obtaining chemoprevention and can be achieved by administering any of a diverse array of naturally-occurring and synthetic chemopreventive agents. Indeed, monitoring of enzyme induction has led to the recognition or isolation of novel, potent chemopreventive agents such as 1,2-dithiole-3-thiones, terpenoids and the isothiocyanate sulforaphane. For example, oltipraz, a substituted 1,2-dithiole-3-thione originally developed as an antischistosomal agent, possesses chemopreventive activity against different classes of carcinogens targeting multiple organs. Mechanistic studies in rodent models for chemoprevention of aflatoxin B(1) (AFB(1))-induced hepatocarcinogenesis by oltipraz indicates that increased expression of phase 2 genes is of central importance, although inhibition of phase 1 activation of AFB(1) can also contribute to protection. Exposure of rodents to 1,2-dithiole-3-thiones triggers nuclear accumulation of the transcription factor Nrf2 and its enhanced binding to the "antioxidant response element" (ARE), leading to transcriptional activation of a score of genes involved in carcinogen detoxication and attenuation of oxidative stress. Nrf2-deficient mice fail to induce many of these genes in response to dithiolethiones; moreover, basal expression of these genes is typically repressed. To test the hypothesis that enzyme induction is a useful strategy for chemoprevention in humans, three key elements are necessary: a candidate agent, an at-risk population and modulatable intermediate endpoints. Towards this end, a placebo-controlled, double blind clinical trial of oltipraz was conducted in residents of Qidong, PR China who are exposed to dietary aflatoxins and who are at high risk for the development of liver cancer. Oltipraz significantly enhanced excretion of a phase 2 product, aflatoxin-mercapturic acid, a derivative of the aflatoxin-glutathione conjugate, in the urine of study participants administered 125 mg oltipraz by mouth daily. Administration of 500 mg oltipraz once a week led to a significant reduction in the excretion of the primary oxidative metabolite of AFB(1), AFM(1), when measured shortly after drug administration. While this study highlighted the general feasibility of inducing phase 2 enzymes in humans, a longer term intervention is addressing whether protective alterations in aflatoxin metabolism can be sustained for extended periods of time in this high-risk population.

Sensitivity to carcinogenesis is increased and chemoprotective efficacy of enzyme inducers is lost in nrf2 transcription factor-deficient mice

Induction of phase 2 enzymes, which neutralize reactive electrophiles and act as indirect antioxidants, appears to be an effective means for achieving protection against a variety of carcinogens in animals and humans. Transcriptional control of the expression of these enzymes is mediated, at least in part, through the antioxidant response element (ARE) found in the regulatory regions of their genes. The transcription factor Nrf2, which binds to the ARE, appears to be essential for the induction of prototypical phase 2 enzymes such as glutathione S-transferases (GSTs) and NAD(P)H:quinone oxidoreductase (NQO1). Constitutive hepatic and gastric activities of GST and NQO1 were reduced by 50-80% in nrf2-deficient mice compared with wild-type mice. Moreover, the 2- to 5-fold induction of these enzymes in wild-type mice by the chemoprotective agent oltipraz, which is currently in clinical trials, was almost completely abrogated in the nrf2-deficient mice. In parallel with the enzymatic changes, nrf2-deficient mice had a significantly higher burden of gastric neoplasia after treatment with benzo[a]pyrene than did wild-type mice. Oltipraz significantly reduced multiplicity of gastric neoplasia in wild-type mice by 55%, but had no effect on tumor burden in nrf2-deficient mice. Thus, Nrf2 plays a central role in the regulation of constitutive and inducible expression of phase 2 enzymes in vivo and dramatically influences susceptibility to carcinogenesis. Moreover, the total loss of anticarcinogenic efficacy of oltipraz in the nrf2-disrupted mice highlights the prime importance of elevated phase 2 gene expression in chemoprotection by this and similar enzyme inducers.

Conceptually new 20-epi-22-oxa sulfone analogues of the hormone 1alpha,25-dihydroxyvitamin D(3): synthesis and biological evaluation

New C,D-ring side-chain-modified sulfone 4a, with natural 1alpha, 3beta-hydroxyl groups but lacking the 25-hydroxyl group characteristic of the natural hormone 1alpha,25-dihydroxyvitamin D(3) (1), has been prepared and characterized. Novel synthetic features include: (1) chemoselective oxidation of only a primary silyl ether in a primary-secondary bis-silyl ether intermediate and (2) smooth reductive etherification without interference by a neighboring sulfonyl group. Sulfone 4a, but not its 1beta, 3alpha-diastereomer 4b, is powerfully antiproliferative and transcriptionally active in vitro but desirably noncalcemic in vivo. Although sulfone 4a, designed to resemble Leo Pharmaceutical Co.'s KH-1060 (3), is recognized by catabolic enzymes, the selective biological profile of sulfone 4a is likely not due to its metabolites that are formed in only minor amounts.

Conceptually new deltanoids (vitamin D analogs) inhibit multistage skin tumorigenesis

Development of vitamin D analogs (deltanoids) as chemopreventive agents requires separation of desirable antiproliferative and pro-differentiating activities from the undesirable calcemic activity also found in the hormone calcitriol (1 alpha, 25-dihydroxyvitamin D(3)). Therefore, several conceptually new deltanoids were synthesized with modifications to the 1alpha- and/or 25-hydroxyl groups, positions traditionally considered essential for stimulating biological responses. In this study, 1 beta-hydroxymethyl-3-epi-25-hydroxyvitamin D(3), a non-calcemic CH(2) homolog of the natural hormone with antiproliferative activity in vitro, was ineffective as an inhibitor of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced induction of ornithine decarboxylase activity in mouse epidermis. However, a hybrid analog incorporating not only the calcemia-ablating 1 beta-hydroxymethyl alteration, but potentiating C,D ring 16-unsaturation and side chain 24,24-fluorination and 26, 27-homologation was found to be as effective as calcitriol. Several non-calcemic 24- or 25-t-butyl sulfones, some containing side chain fluorination but all lacking the 25-hydroxyl group, were also shown to be active in this assay. Three sulfones and the 1 beta-hydroxymethyl hybrid were evaluated as inhibitors of multistage carcinogenesis in mouse skin. Female CD-1 mice were initiated with a single dose of 7,12-dimethylbenz[a]anthracene and then promoted twice weekly for 20 weeks with TPA. Deltanoids were applied topically 30 min before TPA. Unlike calcitriol, none of the atypical deltanoids affected body weight gain in these animals. Minimal effects on urinary calcium excretion were observed following chronic treatment with these analogs. All deltanoids inhibited the incidence and multiplicity of papilloma formation, with the hybrid analog showing the greatest efficacy. With this deltanoid, tumor incidence was significantly reduced by 28% and tumor multiplicity by 63%. These results, coupled with the rich chemical diversity available in side chain sulfur-containing deltanoids, particularly when combined with A ring modifications such as 1 beta-hydroxylalkyl groups, provide important new advances in the fundamental understanding of chemical structure-biological activity relationships as well as more potent and safe vitamin D analogs for cancer chemoprevention and other medicinal uses.

Predictive value of molecular dosimetry: individual versus group effects of oltipraz on aflatoxin-albumin adducts and risk of liver cancer

Studies in animals and humans have established serum aflatoxin-albumin adducts as biomarkers of exposure to aflatoxin B1 (AFB1), a food-borne hepatocarcinogen. To assess the utility of measurements of aflatoxin-albumin adducts to predict risk of hepatocellular carcinoma (HCC), 123 male F344 rats were dosed with 20 microg of AFB1 daily for 5 weeks after randomization into three groups: no intervention; delayed-transient (500 ppm of oltipraz, weeks 2 and 3 relative to AFB1); or persistent (500 ppm oltipraz, weeks -1 to 5). Serial blood samples were collected from each animal at weekly intervals throughout aflatoxin B1 exposure and assayed for levels of aflatoxin-albumin by radioimmune assay. Area under the curve (AUC) values for aflatoxin-albumin adducts decreased 20 and 39% in the delayed-transient and persistent oltipraz intervention groups, respectively, as compared to no intervention. Similarly, the total incidence of HCC dropped from 83 to 60% (P = 0.03) and 48% (P < 0.01) in these groups. Tumor multiplicity was also reduced in the two oltipraz intervention groups, whereas time to HCC was increased. Mononuclear cell leukemia, a common neoplasm in F344 rats, was seen in 39% of the control animals, whereas the two oltipraz interventions reduced incidence to 18% (P = 0.05) and 13% (P = 0.01), respectively. Overall, a significant association was seen between biomarker AUC and risk of HCC (P = 0.01). However, when the predictive value of aflatoxin-albumin adducts was assessed within treatment groups, there was no association between AUC and risk of HCC (P = 0.56). Thus, aflatoxin-albumin adducts can be useful for monitoring population-based changes induced by interventions, such as in chemoprevention trials, but have limited utility in identifying individuals destined to develop HCC. As a consequence, the use of this biomarker in quantitative risk assessment should be pursued cautiously.

Retrieval of organs for transplantation: experience of the Australian National Liver Transplantation Unit

The Australian National Liver Transplantation Unit carried out 251 multiple organ-retrieval procedures between January 1986 and December 1994: 53% were in the Sydney metropolitan area, 22% in country New South Wales and the Australian Capital Territory, 21% interstate and 4% in New Zealand; mean total procurement times were, respectively, 4 hours 57 minutes, 6 hours 58 minutes, 9 hours 20 minutes and 12 hours 20 minutes. Donor operation times steadily decreased over the nine years. One-year patient and graft survival rates were 77.1% and 70.3%, respectively; primary non-function occurred in only one case. The importance of coordination of donor organ procurement and efficient organ retrieval to the success of transplantation programs is discussed.

Biliary strictures after liver transplantation: clinical picture, correlates and outcomes

We retrospectively examined 154 adults to ascertain the frequency, site of and pre-disposing factors for biliary strictures after liver transplantation, as well as their management and clinical outcome. Twenty patients (12.5%) were identified with biliary strictures; 16 were non-anastomotic and four were anastomotic strictures. The median time from transplantation to stricture diagnosis was 17 weeks (range 3-366). Of the 16 non-anastomotic strictures, six were intrahepatic, eight hilar and two extrahepatic (donor bile duct). A control group (n = 32) of patients transplanted immediately before and after index cases was used to examine for correlates in patients with non-anastomotic strictures. At the time of diagnosis in the non-anastomotic index cases, there was a higher incidence of: (i) biliary sludge (63 vs 0%; P < 0.001); and (ii) clinical cholangitis (75 vs 0%; P < 0.001) compared with controls. Primary sclerosing cholangitis was more often the diagnosis in index patients with non-anastomotic strictures compared with controls (31 vs 9%; P < 0.05). There were no differences between index patients and controls (non-anastomotic group) in ABO blood group non-identity, cold allograft ischaemia time, use of OKT3 (murine monoclonal antibody to CD3) and hepatic artery thrombosis. Of 15 patients treated with balloon dilatation, seven required stent insertion although none have required surgery. As determined by liver function tests, there was evidence of persisting graft dysfunction in index patients compared with controls (SAP 381 vs 112 U/L, P < 0.001; GGT 529 vs 80 U/L, P < 0.001), but there was no difference in survival during a median follow-up time of 16 months (range: 3-48 months) from stricture diagnosis. In conclusion, biliary strictures tend to occur within 6 months of transplantation and are an important cause of ongoing graft dysfunction. Non-anastomotic strictures were more common in patients requiring transplantation for primary sclerosing cholangitis.

Levels of aflatoxin-albumin biomarkers in rat plasma are modulated by both long-term and transient interventions with oltipraz

The validation process for biomarkers to be used for monitoring the efficacy of preventive interventions in humans includes assessments of whether levels of the biomarker can be modulated in experimental models. From this perspective, the influence of two intervention protocols with the chemopreventive agent oltipraz on rates of formation and disappearance of aflatoxin-albumin adducts has been evaluated in rats chronically exposed to aflatoxin B1. Male F344 rats were treated daily with 20 micrograms aflatoxin B1, p.o. for 35 days. The first strategy employed a standard, long-term intervention in which basal AIN-76A diet was supplemented with 0.05% oltipraz beginning one week before AFB1 treatment and continuing throughout the period of carcinogen exposure. In this setting, treatment with oltipraz reduced the rate of formation of aflatoxin-albumin adducts such that steady-state levels were lowered by > 50% from control values of 400 pmol aflatoxin adducts/mg albumin. The time-course for reaching respective steady-state levels was unchanged, with or without oltipraz intervention. The second intervention strategy utilized a delayed, transient protocol in which oltipraz was fed for 2 weeks beginning 1 week after AFB1 dosing began and ending 2 weeks before AFB1 dosing was completed. This second strategy, which models clinical interventions in chronically exposed individuals, produced a steady decline in aflatoxin-albumin adduct levels that approached a 50% reduction by the end of the AFB1 exposure period. Development of smooth curve functions allowed for the estimation of the ratio of effects between the non-intervention and intervention groups as well as the simultaneous 90% confidence intervals for the aflatoxin-albumin adduct levels. These analyses indicated that long-term intervention with oltipraz produced a statistically significant reduction in levels of the aflatoxin-albumin biomarker at all times throughout aflatoxin exposure. By contrast, a statistically significant decrease in biomarker levels was not seen in the delayed, transient intervention protocol until the ninth day of the intervention. However, once achieved, significant differences from the control group were maintained for the remainder of the aflatoxin exposure period. These changes in aflatoxin biomarker levels are consistent with the cancer chemopreventive outcomes of these intervention protocols in rats. Collectively, these results support the utility of measuring this biomarker as a means for assessing the efficacy or chemopreventive interventions in individuals at high risk for aflatoxin exposure and development of hepatocellular carcinoma.

Quinone methide mediates in vitro induction of ornithine decarboxylase by the tumor promoter butylated hydroxytoluene hydroperoxide

Metabolism of the skin tumor promoter butylated hydroxy-toluene hydroperoxide (2,6-di-tert-butyl-4-hydroperoxyl-4-methyl-2,5-cyclohexadienone; BHTOOH) to reactive intermediates is required for tumor promotion by this compound. In particular, an electrophilic quinone methide is known to mediate both in vivo tumor promotion as well as in vitro cytotoxicity by BHTOOH. In the present study, the role of this reactive intermediate in the induction of ornithine decarboxylase (ODC), a gene strongly associated with tumor promotion, was investigated in cultured keratinocytes. BHTOOH stimulates a time-dependent increase in ODC enzyme activity, paralleled by ODC mRNA induction, suggesting transcriptional regulation of ODC by BHTOOH. Depletion of intracellular glutathione caused a 5-fold potentiation of keratinocyte sensitivity to BHTOOH. Concordantly, ODC induction by BHTOOH could be completely inhibited by soluble thiol compounds. These results suggest that ODC induction is mediated by a thiol-reactive metabolite of BHTOOH. The iron-specific chelator desferal blocked ODC induction by BHTOOH, indicating that formation of this intermediate is iron-dependent. Substitution of the 4-methyl group of BHTOOH with alkyl groups of incrementally larger size is known to reduce accordingly quinone methide production; comparative study of these BHTOOH analogs demonstrated a corresponding loss of potency for ODC induction, indicating that BHT-quinone methide mediates the in vitro induction of ODC by BHTOOH. Finally, kinase inhibitor studies suggested a role for protein kinase C in the induction of ODC by BHTOOH. Taken together, these results provide insight into the cellular mechanisms through which the reactive electrophile BHT-quinone methide can mediate alterations in gene expression, such as occur in tumor promotion in vivo.

Analysis of factors contributing to success in a program of micromanipulation-assisted fertilization

OBJECTIVE

To determine factors important to clinical success in micromanipulation-assisted in vitro fertilization (IVF).

DESIGN

Procedures invoked in two separate series of micromanipulation-assisted IVF cycles, one unsuccessful (series I) and the other successful (series II), were compared in an effort to identify changes that led to clinical success.

SETTING

University-based IVF clinic.

PATIENTS

In both IVF series involving micromanipulation, patients consisted of infertile couples who fit any of five categories of male-factor related infertility. The female patients underwent controlled hyperstimulation for oocyte retrieval and the oocytes were inseminated normally or were subjected either to partial zona dissection or subzonal sperm insertion to assist fertilization. Results in all groups were compared between the two patient series.

RESULTS

In the successful series II, a noticeable improvement in fertilization rate and embryo quality was observed compared with series I. A significant increase in the percentage of patients reaching embryo transfer, the pregnancy rate per transfer, and the pregnancy rate per retrieval were noted in series II; a 25% ongoing pregnancy rate per retrieval was observed overall in this successful group, with "ongoing" defined as manifestation of at least a fetal sac on ultrasound with no detectable problems. Patients with a mixed transfer of embryos derived from manipulated and normally inseminated oocytes had a 75% rate of pregnancy per transfer in series II. Differences between the two series could not be attributed to patient selection or biases in selection of oocytes relegated to micromanipulation. However, oocyte handling, micromanipulation, and culture protocols differed significantly between the two series in that temperature and pH of oocytes was better controlled, and micromanipulation time was minimized in series II.

CONCLUSION

Success in micromanipulation depends on maintenance of the oocyte in a stable and supportive environment throughout the micromanipulation procedure. It is also important to minimize trauma to the eggs by performing micromanipulation rapidly and with minimal distortion of the egg. Patients with a poor fertilization rate in standard IVF may experience a substantial increase in the likelihood of pregnancy when micromanipulation-assisted fertilization is performed on some eggs.

Role of the benzoyloxyl radical in DNA damage mediated by benzoyl peroxide

Benzoyl peroxide (BzPO) is both a tumor promoter and progressor in mouse skin; however, BzPO is neither an initiator nor a complete carcinogen in this tissue. Although not mutagenic, BzPO has been observed to produce strand breaks in DNA of exposed cells. These actions are presumed to be mediated by free-radical derivatives of BzPO. Previous studies suggested that the metabolism of BzPO in keratinocytes proceeds via the initial cleavage of the peroxide bond, yielding benzoyloxy radicals which, in turn, can either fragment to form phenyl radicals and carbon dioxide or abstract H atoms from biomolecules to yield benzoic acid. Benzoic acid is the major stable metabolite of BzPO produced by keratinocytes. In the present study we have investigated the role of BzPO and its metabolites in the generation of strand scissions in a cell-free system using phi X-174 plasmid DNA. In this system BzPO produced DNA damage that was dose-dependent over a concentration range of 0.1-1 mM and required the presence of copper but not other transition metals. By contrast, benoic acid did not produce DNA damage in this system, either in the presence or in the absence of copper. The inclusion of spin trapping agents, such as N-tert-butyl-alpha-phenylnitrone (PBN), 3,5-dibromo-4-nitrosobenzenesulfonate, and nitrosobenzene, in incubations was found to significantly reduce the extent of DNA damage generated via the copper-mediated activation of BzPO. Electron paramagnetic resonance spectroscopy studies suggested that the primary radical trapped by PBN following copper-mediated decomposition of BzPO was the benzoyloxy radical.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of protection against aflatoxin tumorigenicity in rats fed 5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione (oltipraz) and related 1,2-dithiol-3-thiones and 1,2-dithiol-3-ones

1,2-Dithiol-3-thiones, reported constituents of cruciferous vegetables, are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, and chemoprotective activities. The effects of dietary administration of a substituted 1,2-dithiol-3-thione, oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione], a potent antischistosomal agent, on aflatoxin B1 (AFB1) metabolism, DNA adduct formation, and hepatic tumorigenesis were examined in male F344 rats. Rats were fed graded doses of oltipraz (0.01-0.1%) for 4 wk. During the second and third wk of oltipraz feeding rats were gavaged with 250 micrograms of AFB1/kg five times a wk. Rats were finally restored to control diet 1 wk after cessation of AFB1 dosing. At 4 months focal areas of hepatocellular alteration were identified and quantitated by staining sections of liver for gamma-glutamyl transpeptidase activity. Treatment with oltipraz at all doses reduced by greater than 90% the volume of liver occupied by gamma-glutamyl transpeptidase-positive foci. Levels of AFB1 bound to hepatic DNA were reduced between 40 and 80% in animals fed increasing doses of dietary oltipraz (0.01-0.1%) for 1 wk prior to a single exposure to AFB1. Feeding of the higher levels of oltipraz led to marked increases in the specific activity of glutathione S-transferases, presumably serving to facilitate the detoxication of the ultimate electrophilic form of AFB1, the 8,9-oxide. At low dietary concentrations of oltipraz (0.01%), the only inductive effects seen were on the activities of selected cytochrome P-450 monooxygenases. Therefore, the protection afforded by oltipraz may be due to both the enhancement of electrophile detoxication pathways as well as modified oxidative metabolism of AFB1. In in vitro metabolism studies with hepatic post-mitochondrial supernatant, low-dose oltipraz pretreatment facilitated the oxidative production of aflatoxins P1 and Q1, but not M1, from AFB1. High-dose (0.1%) oltipraz pretreatment enhanced the primary metabolism of AFB1 to aflatoxins P1, M1, and Q1 as well as the formation of chloroform-insoluble metabolites. Feeding studies with a series of 1,2-dithiol-3-thione and 1,2-dithiol-3-one derivatives of oltipraz demonstrated that the inductive activity for cytochrome P-450-dependent monooxygenases and electrophile detoxication enzymes, such as glutathione S-transferases, could be readily separated by minor modifications of the 1,2-dithiol-3-thione structure. The unsubstituted 1,2-dithiol-3-thione nucleus strongly induced electrophile detoxication enzymes, but not the monooxygenases, and was the most effective inhibitor of the binding of AFB1 to hepatic DNA in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Carcinogenicity of the antischistosomal nitrofuran trans-5-amino-3-[2-(5-nitro-2-furyl)vinyl]-1,2,4-oxadiazole

The antischistosomal and antitrypanosomal drug trans-5-amino-3-[2-(5-nitro-2-furyl)vinyl]-1,2,4-oxadiazole [(SQ18506) CAS: 28754-68-9; (E)-amino-3-(2-(5-nitro-2-furyl)-vinyl)-1,2,4-oxadiazole] was carcinogenic for both male and female CD-1 mice when it was administered either in the diet or by gastric intubation. Dose-dependent increases in tumors of the forestomach and lymphatic tissues were observed in all groups receiving SQ18506 including mice infected with Schistosoma mansoni. The predominant tumor observed was squamous cell carcinoma of the forestomach. The presence or absence of schistosome infection did not appear to alter the incidence or distribution of tumors at comparable doses of SQ18506. The incidence of bladder tumors was positively correlated with the dose in gastric intubation studies and inversely correlated with the dose in dietary studies. The carcinogen N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide (CAS: 24554-26-5) was fed to male and female CD-1 mice in the diet as a positive control. The predominant tumor observed in these groups was transitional cell carcinoma of the bladder. These data indicate that SQ18506 is unsuitable for use in the treatment of parasitic diseases.

Effects of multiple putative anticarcinogens on the carcinogenicity of trans-5-amino-3-[2-(5-nitro-2-furyl)vinyl]-1,2,4-oxadiazole

In an attempt to dissociate the chemotherapeutic from the carcinogenic properties of the antischistosomal and antitrypanosomal nitrovinylfuran trans-5-amino-3-[2-(5-nitro-2-furyl)vinyl]1,2,4-oxadiazole [(SQ18506) CAS: 28754-68-9; (E)-5-amino-3-(2-(5-nitro-2-furyl)vinyl)-1,2,4-oxadiazole], potential inhibitors of carcinogenesis were administered to female outbred CD-1 mice before and during exposure to SQ18506. The compounds tested were ascorbic acid, etretinate, butylated hydroxyanisole (BHA), cysteamine hydrochloride, cysteine hydrochloride, dimercaprol, disulfiram, 1,4-dithiothreitol, reduced glutathione, and spermidine phosphate. The primary types of tumors observed were squamous cell carcinomas of the stomach and thymic and nonthymic lymphomas. BHA reduced the incidence of malignant tumors to control levels, whereas cysteine hydrochloride, spermidine phosphate, and disulfiram reduced the incidence of chemically induced tumors by 42, 34, and 32%, respectively. Although cysteamine hydrochloride and disulfiram had no or only a modest effect on the overall incidence of tumors, the data suggested possible tissue-specific anticarcinogenic properties for these agents. Of the 8 antioxidants tested, only 1 had marked anticarcinogenic properties against SQ18506. These data indicate that antioxidant properties alone cannot account for the anticarcinogenic activity of the compounds tested. Coadministration of the anticarcinogen BHA with SQ18506 also blocked the chemotherapeutic effects of this agent on female CD-1 mice infected with Schistosoma mansoni.