A large number of mechanisms have been identified through which resveratrol may exert cancer chemopreventive and cancer chemotherapeutic effects. Mechanisms of resveratrol anti-cancer activity may include antioxidant activity and radical scavenging; anti-inflammatory effects; modulation of the activity of carcinogen-metabolizing enzymes; inhibition of cell proliferation; induction of apoptosis; inhibition of angiogenesis; chemosensitization; and inhibition of tumor metastasis [7, 18]. However, resveratrol exhibits low oral bioavailability and undergoes rapid first-pass metabolism; glucuronide and sulfate conjugates are the major resveratrol species found in plasma [41, 44]. Pharmacokinetic studies in human volunteers have led to questions concerning whether high oral doses of resveratrol can generate plasma levels of parent drug that are necessary to achieve the chemopreventive and other desirable activities that have been reported in experimental model systems [6]. However, two very recent references reported data suggesting that biologically active concentrations of resveratrol and/or metabolites may be achievable in human subjects on chronic dosing [8, 22].
The relatively poor bioavailability and rapid metabolism of resveratrol are not uncommon among polyphenols. By contrast, methylated polyphenols appear to demonstrate substantially greater intestinal absorption and enhanced hepatic stability [42]. Pterostilbene (4-[(E)-2-(3,5-dime-thoxyphenyl)ethenyl]phenol; Fig. 1), a naturally occurring dimethylether analog of resveratrol, is a phytoalexin that, like resveratrol, is generated by plants in response to microbial infestation or exposure to ultraviolet light [21]. Pterostilbene has also been reported to possess cancer chemopreventive activity and other resveratrol-like health benefits [10, 24, 27–29, 36, 38].
In consideration of the potential limitations that bio-availability and metabolism may impose on the pharmacodynamic activity of resveratrol, congeners and analogs of resveratrol are attracting increasing attention as possible agents for cancer chemoprevention. The present studies were performed to determine the absolute and relative bioavailability of equimolar oral doses of resveratrol and pterostilbene in rats and to characterize their qualitative and quantitative in vivo metabolic profiles.
Experimental Methods
Animals and animal husbandry
All animal care and use in this study were performed in accordance with standards set forth in the Guide for Care and Use of Laboratory Animals (National Research Council, 1996), by the U.S. Department of Agriculture through the Animal Welfare Act (7 USC 2131, 1985), and Animal Welfare Standards incorporated in Title 9, Part 3 of the Code of Federal Regulations, 1991.
Male CD ([Crl:CD(SD) IGS BR]; Charles River Laboratories, Portage, MI) rats were received at 6–7 weeks of age and were held in quarantine for 1 week prior to use in the study. During the quarantine period, rats were observed daily for survival and general health status. Prior to randomization into experimental groups, each animal underwent a detailed physical examination to demonstrate its suitability for use as a test animal.
Throughout the study, rats were housed individually in stainless steel cages in a windowless room that was maintained within a temperature range of approximately 18–23°C and a humidity range of approximately 50–80%. Fluorescent lighting in the animal room was provided on a daily cycle of 12 h of light followed by 12 h of darkness. At all times during the quarantine and dosing periods, rats were permitted free access to Certified Rodent Diet #5002 (PMI Nutrition International, Brentwood, MO) and City of Chicago drinking water (administered by an automatic watering system).
Test articles and dosing formulations
Resveratrol (Sochinaz SA, Vionnaz, Switzerland) and pterostilbene (H&Y International Group Ltd., Hangzhou, China) were provided by the Division of Cancer Prevention, National Cancer Institute. The purity of each agent was >99%, as determined by HPLC.
Dosing formulations of resveratrol and pterostilbene were prepared for intravenous administration in a vehicle of DMSO:PEG-300 (15:85; v/v)]; a dosing volume of 2.5 mL/kg body weight was used for intravenous injections. Oral dosing formulations were prepared in a vehicle of 0.5% (w/v) aqueous methylcellulose containing 0.2% (w/v) Tween 80; a dosing volume of 10 mL/kg body weight was used for gavage administration. All vehicle components were purchased from Sigma–Aldrich (St. Louis, MO).
Study design and conduct
At the end of the quarantine period, rats were randomly assigned to one of ten dosing groups using a computerized body weight stratification procedure that produced similar group mean body weight values. Body weights for the animals assigned to the study ranged from 201 to 241 g. The study design is summarized in Table 1.