John M. Pezzuto,
Ching J. Chang,
Bruce A. Craig,
Mark S. Cushman,
William Fenical,
Harry H.S. Fong,
Andrew Mesecar,
Richard C. Moon,
Richard B. van Breeman
University of Hawaii, Hilo Hilo, HI
Purdue University
West Lafayette, IN,
Scripps Institution of Oceanography
La Jolla, CA
University of Illinois at Chicago
Chicago, IL
Cancer may be prevented or delayed through the use of naturally occurring chemicals, and with support from the U.S. National Cancer Institute, we are discovering ways to prevent this life-threatening disease. In spite of intense effort, the survival rate following the appearance of malignant metastatic cancer is generally poor. An unconventional approach, which is now showing promise, is chemoprevention. Unlike the treatment of existing tumors, cancer chemoprevention targets the process of carcinogenesis itself, and the desired outcome is the prevention or delay of the disease.
In an ideal world, cancer chemoprevention would work as well as vaccines for the prevention of human ailments. Although this has yet to be accomplished, proof-of-principle has been established by seminal clinical trials conducted for the prevention of breast cancer by tamoxifen, and more recently with tamoxifen relatives such as raloxifene, and a separate class of aromatase inhibitors. Celebrex® is used to reduce the burden of colorectal polyps, the precursor of colon cancer, in familial adenomatous polyposis. Agents such as finasteride have also shown promise for the prevention of prostate cancer. Nonetheless, the pipeline of new cancer chemopreventive drugs is thin. FDA approval for cancer prevention is cumbersome and pharmaceutical companies are not heavily committed to this area.
Plants Yield Up Their Secrets
Through serendipity, epidemiological observations and research, dietary phytochemicals such as sulforaphane and phenethyl isothiocyanate (cruciferous vegetables), epigallocatechin-3-gallate (green tea), curcumin (turmeric), sulfur-containing compounds and selenium (the genus Allium), and lycopene (tomatoes) are considered promising for cancer prevention. Based on this information, until recently, we have focused on terrestrial plants for the discovery of novel chemopreventive agents.
Using an approach called 'activity-guided fractionation' which utilizes a battery of state-of-the-art in vitro assays either developed by us or pre-selected for their known correlation to cancer, we monitor the natural product purification process so as to isolate the most active agents in their pure form. Once purified, the structures of the molecules are determined using advanced NMR, mass spectrometry and X-ray crystallographic methods.
Already, about 200 active substances have been discovered, from a variety of structural classes, such as alkaloids, flavonoids, coumarins, triperpenoids, and with anolides. Some of the compounds have shown promise for clinical trials, such as the rotenoid, deguelin.
One of our most notable discoveries is the structurally simple chemical known as resveratrol. Resveratrol is a common constituent of grapes and grape products, and the highest concentrations are often found in red wine. As discovered by our group, resveratrol can mediate cancer chemopreventive activity. Stimulated by our original report, resveratrol is now the subject of nearly 2,000 manuscripts, and it has been entered into clinical trials for the prevention of colon cancer.
Along the way, we investigated the absorption and metabolism of resveratrol and, through crystallographic analysis, observed its interactions within the arachidonic acid binding site of the enzyme cyclooxygenase (COX) which is the target of non-steroidal, anti-inflammatory drugs (see Figure 1). New biological targets have been developed, as well as sophisticated new techniques involving LS/MS/MS and ultrafiltration-mass spectrometry and, most recently, macromolecular X-ray crystallographic assisted deconvolution of natural product mixtures.
It logically follows that synthetic organic chemistry is an integral component of the program, and some semi-synthetic compounds such as 4´-bromoflavone and oxomate (a relative to the chemopreventive molecule found in cruciferous vegetables, sulforaphane) have shown promise. Recently, after testing thousands of inactive extracts and compounds for interaction with the retinoid X receptor (RXR), the synthetic substance with code name AM6-36 was finally uncovered as a superior lead.
Promise from the Sea
Although edible and nonedible terrestrial plants have yielded interesting leads, a new and exciting area of research involves exploring the biodiversity provided by microbes of the marine environment. Already, promising leads have been discovered for cancer therapy and entered into clinical trials. And now we are concentrating on the discovery of marine microorganism-based cancer chemopreventive agents.
Since carcinogenesis is a multistage process, a comprehensive battery of novel assays has been devised to monitor inhibition of cancer initiation, promotion and progression. Some recent examples include interaction with RXR or Keap1, and inhibition of quinone reductase 2 or NF-kB. Chemical compounds showing activity in these assays should be capable of inducing enzymes that inactivate carcinogens, or cellular responses such as differentiation and apoptosis. These types of responses are biomarkers for cancer chemoprevention.
Following this approach, as illustrated in Figure 2, many active compounds have been identified with a fascinating array of chemical structures. As would be the case with resveratrol, compounds appearing at the heart of this Venn diagram are multifunctional agents, and some of these leads are now being tested for efficacy in animal models.
Clearly, chemoprevention is a viable strategy in the fight against cancer. The current armamentarium of agents has resulted largely from epidemiological observations, off-shoots of cancer therapeutic agents, or agents that were used for other therapeutic indications. With concerted effort involving a range of expertise, new natural product chemopreventive agents with clinical potential are being uncovered using a systematic approach of drug discovery.
(This work is supported by program project P01 CA48112 awarded by the U.S. National Cancer Institute.)
Figure 1. Macromolecular X-ray crystallography is an integral part of our cancer chemopreventive discovery efforts. The active site of COX1 in complex with resveratrol is shown and the structure has been a valuable tool for the structure-based design of novel resveratrol analogs.
Figure 2. Venn diagram illustrating marine nature products that have cancer chemopreventive potential, as well as the type of biological responses mediated by the isolates.
