Role of Instability in the Action of Indole-3-carbinol (I3C)

Assessment of the oral use of indole-3-carbinol (I3C) as a chemoprotective compound has not sufficiently considered the chemical instability of I3C. This review addresses the question of whether I3C is directly active in its own right or only serves as a precursor, with all of the biological responses coming from reaction products arising in culture media and in the presence of stomach acid. Because of the rapid conversion of I3C into its dimer. diindolylmethane (DIM), and trimers very little circulating I3C is present following oral use to effect a biological response. Reports of toxicity associated with oral use of I3C relate to unfavorable enzyme induction, which can be attributed to non-DIM reaction products. Because DIM provides a predictable, safer response than the mélange of compounds derived from I3C DIM should be regarded as the chemoprotective compound of choice. In studies going back to early work by Wattenberg, it was shown that compounds present in cruciferous vegetables had potent anticancer activity (1). In crucifers, active phytochemicals are derived from precursor glucosinolates in plant cells through the action of myrosinase enzymes. Chemical analysis showed that the activity could be accounted for by two groups of metabolites: the indole derivatives originating from glucobrassinin and the various isothiocyanates including phenethylisothiocyanate from gluconasturtun, and sulforaphanes derived from glucorafanin. The indoles are unique in that they have already been used therapeutically and will be the focus of this review. Most attention has been given to indole-3-carbinol (I3C) as an estrogen-modulating cancer chemopreventive, despite in vivo evidence of ovarian toxicity (2) and tumor promotion (3, 4). Less attention has been directed to 3,3’-diindolylmethane (DIM), a stable product, which is formed from I3C both in vitro and in vivo. While the idea of obtaining these compounds from the consumption of cruciferous vegetables (e.g. broccoli, cabbage, cauliflower and mustard greens) seemed attractive, the concentration of these compounds present in plants is actually highly variable depending on factors such as the seed strain, myrosinase enzyme activity, rainfall, soil and amount of sunlight. As a result, vegetable or seed extracts are not a practical way to get the daily amounts of these compounds required to be effective. Consequently, most investigators have turned to semisynthetic sources of cruciferous indole compounds, particularly I3C and DIM. Role of Instability in the Action of Indole-3-carbinol (I3C) The initial compound released from cruciferous glucobrassinin by the action of the enzyme myrosinase is I3C, a compound that has proved to be highly unstable both in vitro and in vivo (5). As a result, one cannot be certain whether I3C itself or one of the condensation products arising from I3C is the active agent. Nevertheless, because of its initial greater availability, most early studies were carried out with I3C without any concern for its stability either as an additive in cell culture studies or orally in animal and human studies. In animal studies, it was shown to have activity against breast tumor formation (6, 7), against endometrial tumors (8), and most recently against prostate cancer growth (9). In cell culture studies and in human studies, I3C was shown to increase 2-hydroxylation of estradiol and decrease 16α-hydroxylation (10, 11). This change correlated with its antitumor activity. Subsequently, investigators using isolated DIM showed that this single 441 Correspondence to: H. Leon Bradlow, David and Alice Jurist Institute for Medical Research, Hackensack University Medical Center Hackensack, NJ, 07601 and Department of Obstetrics and Gynecology, University of Medicine and Dentistry of New Jersey, Newark, NJ, U.S.A. e-mail [email protected]