Miscellaneous Agents For Chemoprevention

1,2-Dithiol-3-thiones, reported constituents of cruciferous vegetables, are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, and chemoprotective activities. In this context, oltipraz, which was originally developed as an antischistosomal agent, was found to protect against chemically induced carcinogens in the lung, stomach, colon, and urinary bladder in animals.33 Its utility as a cancer chemopreventive agent is thought to depend on the induction of enzymes involved in Phase II xenobiotic detoxification.34 Polycyclic aromatic hydrocarbons, N-nitrosamines, and other compounds produce electrophilic carcinogenic metabolites. Thus, the fungal toxic secondary metabolite aflatoxin B1 (AFB1) may contaminate food and by epoxydation of a furane double bond by a P450 cytochrome (Fig. 13.5) give carcinogenic compounds that are inactivated by glutathione addition catalyzed by glutathione S (GSH)-transferase (see Section 2.1 of Chapter 11). Oltipraz stimulates this enzyme35 and can be effective in patients with histories of Schistosoma haematobium bladder infections that are at increased risk for developing bladder cancer (Fig. 13.6).36

Sulforophane is another component of cruciferous vegetables (e.g., broccoli) that has been proposed as a chemoprotector. This compound bears an unusual isothiocyanate function and is generated from its precursor glucoraphanin by the myrosinase enzyme on damage to the plant (e.g., from chewing), as shown in Fig. 13.7. Similarly to the previously mentioned oltipraz, sulforophane is an inducer of Phase II metabolizing enzymes and is being clinically studied as a chemopreventive agent in several cancers.37

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