Kava, a tonic for relieving the irrational development of natural preventive agents.

The last two decades have witnessed explosive growth in the study of natural and other cancer chemopreventive agents (1, 2). Extensive preclinical data have been generated for natural agents, and many (such as green tea, curcumin, phenyl isothiocyanate, indole-3-carbinol, silibinin, lycopene, genistein, selenium, and vitamins A, E, and D) are currently in different phases of clinical testing (3). The definitive clinical prevention trials of natural agents completed thus far have been largely negative, suggesting that detailed mechanistic and efficacy studies are necessary to supplement the epidemiologic data before clinically testing novel natural compounds (3). Therefore, investigators increasingly are studying the mechanisms of cancer chemopreventive agents (natural or synthetic, including molecular targeted) to substantiate their potential efficacy. A substantial body of work showing a broad spectrum of natural-agent mechanisms has raised important issues. For example, what is a relevant, achievable dose in vivo for targeting relevant pathways or targets (versus the potentially high, unachievable doses studied in vitro)? Which of the multiple mechanisms are potential causes of toxicity? A multiplicity of mechanisms certainly could enhance natural agent effects, but it is important also to try to identify specific relevant or predominant mechanism(s) that are critical for preventive activity in specific carcinogenic systems. Besides giving scientific credibility, mechanistic insight will facilitate clinical development by elucidating key pathway(s) and target(s) to be monitored in dose-finding early-phase clinical trials (4). It also helps in selecting patient populations based on appropriate prevention settings and potential sensitivity to the preventive and/or toxic effects of the agent. Understanding relevant mechanisms also helps in developing more specifically targeted analogues with potentially less toxicity, greater preventive activity, and less variability in formulation, which is important for assuring the desired effects of an intervention. The substantial data reported thus far on mechanisms of chemopreventive agents are just the tip of an iceberg of mostly unknown mechanisms involving about 20,000 protein-coding human genes and the epigenetic machinery that contributes to the regulation of gene expression. Most advances in understanding the mechanisms of natural agent actions have been confined, until recently, to cell culture studies. Now, however, various animal models (e.g., knockout, knock-in, or transgenic mice) are frequently used to establish the in vivo mechanistic aspects of natural agents. Furthermore, the use of “omic” approaches (genomic, proteomic, metabolomic, etc.) in chemoprevention has helped in speedily measuring altered expression of thousands of genes in response to phytochemicals (plant-derived chemical compounds) and promises to further crystallize natural-agent mechanisms. Silibinin, a constituent of milk thistle, can help in illustrating current mechanistic study of natural agent mechanisms. Milk thistle extracts have been used for centuries as a medicament for hepatobiliary diseases and during the last few decades in clinical testing for treating acute mushroom poisoning, hepatic cirrhosis, and acute viral hepatitis (5, 6). Milk thistle extract, also labeled silymarin or silibinin, is now marketed as a nutritional supplement to promote healthy liver function (5). In the 1970s, we reported the first evidence of the cancer preventive activity of silymarin/silibinin in a series of in vivo studies using mouse skin cancer models (7, 8). Studies of the last 15 years in different in vitro and in vivo models have established the mechanistic details of silibinin cancer preventive effects in various epithelial cancers, including prostate, lung, bladder, colorectal, kidney, oral, skin, renal, breast, ovarian, and tongue cancers (6, 7, 9, 10). These mechanistic studies showed that silibinin treatment inhibits unchecked cellular proliferation in cancer cells by targeting the cell cycle through modulation of various cell cycle regulators (11). This activity includes strongly inhibiting constitutive as well as growth factor– induced receptor tyrosine signaling and inhibiting androgen receptor and signal transducer and activator of transcription signaling (9). The growth of cancer cells is almost always accompanied by the loss of apoptotic response, and silibinin treatment has been shown to induce apoptosis in many cancer cell lines and in tumor tissues via modulation of expression of various Bcl2 and inhibitor of apoptosis family members through inhibition of nuclear factor κB and with or without the activation of various caspases (7, 9). Silibinin treatment has been shown to target the expression of various proangiogenic factors (e.g., vascular endothelial growth factor, basic fibroblast growth factor, and inducible nitric oxide synthase), thereby affording strong antiangiogenic efficacy (7, 9). Overall, these studies suggest pleiotropic mechanisms for silibinin anticancer activity; more recent studies, however, showed that inhibition of epidermal growth factor receptor activation is necessary and sufficient for the anticancer effects of silibinin (12). This places silibinin in the class of receptor tyrosine kinase inhibitors, which have undergone extensive clinical testing for cancer control (13). Another good example of a mechanistically evaluated natural agent with chemopreventive potential is the phytochemical deguelin. Deguelin has several relevant mechanisms Authors' Affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, University of Colorado Denver, Denver, Colorado Received 08/21/2008; accepted 09/29/2008. Grant support: National Cancer Institute R01 grants CA102514, CA112304, CA113876, and CA116636. Requests for reprints: Rajesh Agarwal, Department of Pharmaceutical Sciences, School of Pharmacy, University of Colorado Denver, 4200 East 9th Street, Box C238, Denver, CO 80262. Phone: 303-315-1381, Fax: 303-3156281; E-mail: [email protected]. ©2008 American Association for Cancer Research. doi:10.1158/1940-6207.CAPR-08-0172