Richard C. Moon

Rotenoids mediate potent cancer chemopreventive activity through transcriptional regulation of ornithine decarboxylase

For the discovery of new cancer chemopreventive agents, we have studied the potential of plant extracts to inhibit phorbol ester-induced ornithine decarboxylase (ODC) activity in cell culture. Four active rotenoids were obtained from the African plant Mundulea sericea (Leguminosae). These isolates were highly potent when evaluated for inhibition of chemically induced preneoplastic lesions in mammary organ culture and inhibition of papillomas in the two-stage mouse skin model, and they appear to function by a unique mechanism at the level of ODC messenger RNA expression. Based on our findings, rotenoids can be regarded as promising new chemopreventive or anticancer agents.

Cancer chemopreventive activity and metabolism of isoliquiritigenin, a compound found in licorice

Isoliquiritigenin (2′,4′,4-trihydroxychalcone; ILG), a chalcone found in licorice root and many other plants, has shown potential chemopreventive activity through induction of phase II enzymes such as quinone reductase-1 in murine hepatoma cells. In this study, the in vivo metabolism of ILG was investigated in rats. In addition, ILG glucuronides and ILG-glutathione adducts were observed in human hepatocytes and in livers from rats treated with ILG. ILG glucuronides were detected in both plasma and rat liver tissues. In addition, in a full-term cancer chemoprevention study conducted with 7,12-dimethylbenz(a)anthracene–treated female Sprague-Dawley rats, dietary administration of ILG slightly increased tumor latency but had a negative effect on the incidence of mammary tumors starting at ∼65 days after 7,12-dimethylbenz(a)anthracene administration. Further, no significant induction of phase II enzymes was found in mammary glands, which is consistent with the low level of ILG observed in these tissues. However, ILG significantly induced quinone reductase-1 activity in the colon, and glutathione as well as glutathione S-transferase in the liver. Analysis of mRNA expression in tissues of rats treated with ILG supported these findings. These results suggest that ILG should be tested for chemopreventive efficacy in nonmammary models of cancer. Cancer Prev Res; 3(2); 221–32

Dietary retinoids and carotenoids in rodent models of mammary tumorigenesis

In this review of the scientific literature the relationship between retinoids, carotenoids, and mammary carcinogenesis is examined. Several retinoids have shown promise as chemopreventive agents against chemically induced mammary carcinogenesis in mice and especially in rats. The most promising retinoids are retinyl acetate (RA) and N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide). In rats, dietary administration of these retinoids reduced tumor incidence and multiplicity, and increased the latency of DMBA or MNU-induced mammary cancers. In mice, 4-HPR reduced the number of hyperplastic alveolar nodules and the number of tumors in MTV- and MTV+ mice, respectively. Among retinoids, 4-HPR is at present the most promising analogue, due to its ability to concentrate in the mammary gland. The combination of 4-HPR with tamoxifen not only is more effective in suppressing breast cancer than either agent alone, but also inhibits the appearance of subsequent cancers following the surgical removal of the first tumor. These studies suggest that retinoids, like tamoxifen, may be applicable to the prevention of contralateral breast cancer in women who underwent breast cancer surgery. It is also becoming evident that differentiation therapy and chemoprevention can become attractive alternative approaches to intensive cytotoxic chemotherapy. The role of carotenoids in the prevention of mammary carcinogenesis, however, is ambiguous. Poor absorption and low levels of carotenoids that reach the target tissues complicate interpretation of data in rodent models of mammary carcinogenesis. Very few animal studies are presently available in which purified carotenoids were found effective against mammary carcinogenesis. These results do not justify undertaking clinical evaluation of individual carotenoids against breast cancer at this time.

Vitamin A, Retinoids and Breast Cancer

Both epidemiologic and experimental evidence are highly suggestive of an inverse relationship between vitamin A status and cancer induction. However, protection by vitamin A and retinoids has not been a universal finding, nor are the compounds equally effective in inhibiting cancer induction at all organ sites. Although the experimental evidence for a protective effect of retinoids is especially strong for mammary cancer, the epidemiologic evidence is less convincing. Chronic pharmacologic administration of retinoids may be limited by their potential toxicity; such potential toxicity is particularly important in light of the expectation that, in order to achieve effective anticarcinogenesis in a clinical setting, administration of retinoids may be required at relatively high doses for extended periods. Clinical trials to determine the efficacy of several retinoids as cancer preventive agents are only in the early stages. Meanwhile, experimentation to develop synthetic vitamin A analogs with increased activity, increased target organ specificity, and reduced toxicity must continue. The question of increasing anticarcinogenic activity by increasing vitamin A intake in populations whose vitamin A status is normal is more difficult to assess. The vast majority of epidemiologic studies indicate that groups with a relatively high intake of carotenoids are at a reduced risk for cancer in several target tissues. However, in populations whose vitamin A status is not deficient, increases in vitamin A and carotene intake are not well correlated with increases in serum vitamin A.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitation of preinvasive neoplastic progression in animal models of chemical carcinogenesis

An assay method that precisely quantitates the cellular and tissue changes associated with early, preinvasive neoplasia is much needed as a surrogate endpoint biomarker (SEB) in clinical trials to predict the potential efficacy of chemopreventive agents in bringing about cancer incidence reduction. Quantification of histological changes at the tissue level are potentially powerful SEBs since these visually apparent changes are common in all neoplastic development, regardless of tissue type or neoplastic cause. Currently, subjective inspection of the histological appearance of sectioned and stained material, or “grading,” by experienced pathologists is used to evaluate neoplastic progression. This has well‐known limitations of reproducibility, accuracy, and resolution of grading scale. Since neoplastic changes are visually apparent and morphologic in nature, quantification by image analysis is a measurement modality of choice.

Cancer chemoprevention by retinoids : animal models

Publisher Summary Chemoprevention of cancer can be achieved either by eliminating the causative agent or by administering compounds that prevent or reduce the occurrence of cancer. Experimentally, the efficacy of a potential chemopreventive agent can be evaluated in animal models for carcinogenesis. Several target organ-specific model systems are available to study the effect of agents such as retinoids on the inhibition of carcinogenesis. A number of tumor models have been utilized for determining the chemopreventive efficacy of retinoids, and, in general, these agents have shown chemopreventive activity against carcinogen-induced skin, urinary bladder, tracheobronchial, pancreas, and mammary tumors. Because the majority of the retinoid chemoprevention studies conducted in the laboratory employ the rat mammary carcinogenesis model, this chapter focuses on retinoids and mammary carcinogenesis. Generally, a single treatment with the carcinogen is sufficient to induce tumors in female rats; these tumors resemble human breast cancers histopathologically. The two carcinogens used by most investigators are 7,12-dimethylbenz[a]anthracene (DMBA) and Nmethyl-N-nitrosourea (MNU). There are certain advantages and disadvantages in using either of the carcinogens. MNU is a direct acting agent. DMBA, on the other hand, is an aromatic hydrocarbon that requires metabolism to an active carcinogen component.

Preclinical Animal Models for the Development of Cancer Chemoprevention Drugs

Preclinical cell culture and animal efficacy testing models are currently used to identify, assess, and prioritize chemical agents and natural products with the aim of preventing human cancer. If little is known about a potential agent, the first step is a sequential series of short-term in vitro prescreens of mechanistic, biochemical assays. These assays provide quantitative data to help establish an early indication of chemopreventive efficacy and to assist in prioritizing agents for further evaluation in longer-term in vitro transformation bioassays and whole animal models. Promising chemical agents or combinations of agents that work through different inhibitory mechanisms are subsequently tested in well-established chemically induced or spontaneous animal tumor/cancer models, which typically include models of the colon, lung, bladder, mammary, prostate, and skin. These animal bioassays afford a strategic framework for evaluating agents according to defined criteria, and not only provide evidence of agent efficacy, but also serve to generate valuable dose-response, toxicity, and pharmacokinetic data required prior to Phase I clinical safety testing. Based on preclinical efficacy and toxicity screening studies, only the most successful agents considered to have potential as human chemopreventives will progress into clinical chemoprevention trials.

Dietary Administration of Asimina triloba. (Paw Paw) Extract Increases Tumor Latency in N.-Methyl-N.-nitrosourea–Treated Rats

Abstract The paw paw tree, Asimina triloba. (L.) Dunal (Annonaceae), contains more than 50 bioactive components, primarily annonaceous acetogenins. Some therapeutic activities have been associated with this material, but the potential to mediate a cancer chemopreventive effect has not been reported. In this study, a standardized extract from the twigs, in which bullatacin, asimicin, and trilobacin represent the most potent and major bioactive acetogenins, was tested in the N.-methyl-N.-nitrosourea–induced mammary carcinogenesis model. With Sprague-Dawley rats given a diet containing paw paw extract (1250 and 2500 mg/kg diet; based on maximum tolerated dose studies), mammary tumor latency was increased from 55 to 66 days. However, mammary tumor incidence and multiplicity were not affected by extract consumption.

Identification of degradation product of deguelin and its stability using liquid chromatography and electrospray/ mass spectrometry

Abstract The degradation product and its stability of deguelin, a naturally occurring rotenoid and potential cancer chemopreventive agent, was evaluated in several vehicles using liquid chromatography‐electrospray mass spectrometry. Deguelin was prepared as a 2 mg/mL solution in acetonitrile, corn oil, yohimbine oil, Tween‐80, Cremophor‐EL, liposomes, and phospholipid micelles, and chemical stability was monitored for 28 days using HPLC‐UV and HPLC‐MS systems coupled to C18 reverse phase columns. Deguelin was stable in acetonitrile, corn oil, and yohimbine oil for 28 days when stored at 4°C. However, it rapidly decomposed in liposomes, Cremophor‐EL, Tween 80, and phospholipid micelles, with daily decay rates of 0.39%, 0.86%, 1.93%, and 2.82%, respectively. The degradation product was determined by physical and spectral data to be tephrosin after isolation by semi‐preparative HPLC. Tephrosin was produced by deguelin auto‐oxidation, and this was likely accelerated by endogenous peroxides found in Cremophor‐EL, Tween‐80, and phospholipid preparations.

Chemoprevention of DMBA-induced Mammary Carcinogenesis: Relationship Between Induction of Phase II Enzymes, Effects on DMBA-induced Hemoglobin Adducts and Decreases in Mammary Tumor Multiplicity

Abstract The chemopreventive potential of 1,2-dithiole-3-thione, ethoxyquin, butylated hydroxanisole (BHA) and β-naphthoflavone (BNF) was investigated in the 7,12-dimethylbenz(a)anthracene (DMBA) rat mammary tumor model. Female Sprague-Dawley rats received a single dose of DMBA (15 mg) at 50 days of age. Chemopreventive agents were administered via the diet, beginning one week prior to DMBA dosing and continuing until one week following DMBA. Tumor latency, multiplicity and incidence were not affected by BHA (2.5 or 5.0 g/kg diet), but administration of low or high doses of β-napthflavone (1.65 and 3.3 g/kg diet), or high doses of ethoxyquin (5.0 g/kg diet) or 1,2-dithiole-3-thione (0.6 g/kg diet), effectively inhibited all parameters of tumorigenesis. It is presumed that many of these agents which decrease the tumorigenicity act by altering the metabolism of the carcinogen resulting in diminished numbers of reactive ultimate carcinogens. We examined both for the induction of enzymes that might alter carcinogen metabolism as well as alterations in levels of DMBA adducts to hemoglobin. A strong correlation was observed between the effect of these agents on carcinogenesis and the induction of phase I1 enzymes (quinone reductase and glutathione S-transferase) and glutathione in the mammary gland. We also examined the effects of BNF, 1.2- dithiol-3-thione and BHA on formation of hemoglobin adducts by DMBA. Similarly to the results on carcinogenesis the relative efficacy of compounds in inhibiting DMBA adduct formation and DMBA induced mammary tumorigenesis was BNF > 1,2 Dithiolthione >> BHA. These results support the hypothesis that these agents work by altering metabolism of DMBA to reactive intermediates but further offer the possibility that modulation of hemoglobin adducts may serve as a surrogate marker of efficacy.