Repeat Dose Study of the Cancer Chemopreventive Agent Resveratrol in Healthy Volunteers: Safety, Pharmacokinetics, and Effect on the Insulin-like Growth Factor Axis

In this report, we describe the pharmacokinetics of resveratrol after repeated oral administration of high doses and define potential pharmacodynamic end points pertinent to optimize future long-term intervention studies of resveratrol. Doses of up to 5 g given daily for 29 days were safe, although the two highest doses used here (2.5 and 5 g) caused gastrointestinal symptoms of mild to moderate severity. On the basis of these findings, we would tentatively recommend that in future intervention studies of resveratrol the daily dose should perhaps not exceed 1.0 g. The highest dose generated circulating peak levels of the parent agent which approached concentrations reported to cause pharmacologic activity in cells in vitro (24). Circulating levels of its major metabolites, resveratrol-3-O-sulfate, resveratrol-4′-O-glucuronide, and resveratrol-3-O-glucuronide, were much higher, in the case of the sulfate, the highest dose yielded a mean Cmax of 18.3 μmol/L. These results are important in light of the suspicion that resveratrol metabolites may contribute to the pharmacologic activity of the parent agent (2). Experimental evidence to support this notion is scarce, but recent publications suggests that resveratrol sulfate conjugates could induce quinone reductase and inhibit cyclooxygenase enzymes, nitric oxide production, and NFκB induction in cells in vitro (25, 26). It is not known whether resveratrol metabolites can engage estrogenic effects, a property which the parent agent is suspected to possess (27), although this notion has been disputed (28). Although the Cmax and AUC values described here for resveratrol and its metabolites after multiple resveratrol doses are similar to those reported previously after single dose resveratrol at levels identical to those used here (9), there are subtle differences (Fig. 4). In the case of the 0.5 g dose, the Cmax values for resveratrol-3-O-sulfate and the two monoglucuronides after repeat resveratrol were 50% to 60% of those after a single dose, consistent with multiple administration of resveratrol at this dose causing inhibition of its metabolic conjugation or augmentation of metabolite elimination. In contrast, after the 5 g dose, the Cmax values for parent resveratrol and the two resveratrol glucuronides after multiple dosing were approximately double those after single dose resveratrol, indicative of accumulation of parent and glucuronides. After multiple administration of the 5 g dose, the AUC values for resveratrol and its metabolites were higher than those observed after single dosing; however, these differences did not reach significance levels (results not shown). Although the design of the study does not allow delineation of steady state, it is conceivable that steady state was achieved. Given that the time to steady state is three to five half-lives, and the half-life of resveratrol administered once daily was 4.8 to 9.7 hours, approximately 15 hours to 2 days would be required to attain steady-state. It needs to be stressed that the dosing regime was not optimized in this study, and a shorter dosing interval might have been used to increase the steady state concentrations and maintain levels within a narrower range. Likewise, sustained release formulations of resveratrol might possess pharmacokinetic properties superior to those of the caplet formulation used in this study. However, it is pertinent to point out that in a recent phase 1 study in colorectal cancer patients who ingested 0.5 or 1.0 g of the same resveratrol formulation used here daily for 7 days, resveratrol was still present at concentrations of between 8.3 and 674 nmol/g tissue in surgically resected colon tissue beyond ∼6 hours after the last dose (29). This means that for the prevention of colorectal malignancies by resveratrol, the once daily dosing schedule used in the present investigation might well be sufficient. The results suggest that repeated consumption of resveratrol may decrease circulating levels of IGF-I and IGFBP-3. These observations render IGF proteins potential biomarkers of pharmacologic activity of resveratrol in humans. The reduction of IGF-I and IGFBP-3 by resveratrol did not follow a linear dose-response relationship at the dose range tested here. Although the effect of resveratrol was not significant at the lowest (0.5 g) or highest (5.0 g) doses, significant decreases were observed at the medium dose of 2.5 g. Unconventional dose-pharmacologic response relationships have been described for resveratrol before. For example, in a murine model of aorta repair in vivo, resveratrol at 10 mg/kg increased both endothelial nitric oxide synthase expression in injured arteries and the number of endothelial progenitor cells in the circulation, although such responses were not elicited by 50 mg/kg (30). It needs to be emphasized that the current study only hints at the possibility that high-dose resveratrol could depress circulating levels of IGF proteins, and these observations need to be corroborated in long-term studies with larger numbers of participants. If IGF protein modulation is indeed found to be a genuine property of resveratrol, its consumption for years—rather than weeks—may profoundly affect IGF axis signaling. The findings reported here for resveratrol have to be interpreted in the light of the importance of the IGF system for the development of malignancies. High levels of IGF-I have been causally associated with risk of several cancers (16), so that the ability to decrease IGF-I, which we have shown here may be achieved in humans by resveratrol, constitutes an anticarcinogenic mechanism. Intervention with 9-cis-retinoic acid for 3 months decreased circulating IGF-I in former smokers (20). Reduction in IGF-I is often the corollary of elevation of IGFBP-3 concentrations, which sequester IGF-I and decrease its bioavailability and thus its interaction with IGF receptors by which it engages mitogenic and antiapoptotic actions. The results presented here show that exposure to resveratrol did not elevate IGFBP-3 levels in humans, rather there was some reduction. It is difficult to interpret this finding in terms of contribution to the mechanisms by which resveratrol may exert chemoprevention. Circulating levels of IGFBP-3 are now thought to be directly associated with an increased risk of common cancers, albeit associations are modest and vary between sites (31, 32). On the basis of these insights, one may argue that the resveratrol-induced decrease in circulating IGFBP-3, like the decrease in IGF-I, may constitute an anticarcinogenic event.