Forging a modern generation of polyphenol‐based therapeutics

Polyphenols are micronutrients produced as secondary metabolites by dietary plants (Scalbert, 1993). Although these compounds display poor bioavailability (only a proportion of ingested amounts are absorbed and excretion is rapid), and complex pharmacodynamics and metabolism (Manach et al., 2005), they present therapeutic properties. A substantial body of evidence (epidemiological studies, animal studies and human clinical trials) indicate that polyphenols reduce a range of pathologies associated with cardiovascular disease including thrombosis (Navarro-Nunez et al., 2008), atherosclerosis (Chiva-Blanch et al., 2012) and inflammation (Rieder et al., 2012), as well as displaying anti-cancer (Gali et al., 1991) and neuroprotective (Gatson et al., 2013) properties. The activities of these compounds is achieved via a range of mechanisms including their well-characterized antioxidant effects (Pignatelli et al., 2006), inhibition of intracellular kinase activity (Wright et al., 2010), binding to cell surface receptors (Jacobson et al., 2002) and disrupting the integrity of cell plasma membranes (Pawlikowska-Pawlega et al., 2007). Epidemiological and clinical studies demonstrating the therapeutic potential and benefits of polyphenols, do not definitively describe the true importance of these compounds. Dietary supplementation may elicit beneficial physiological effects, but these are often achieved at supraphysiological concentrations, which have been demonstrated to be toxic (Mennen et al., 2005). Some studies take into account confounding effects of age, sex and lifestyle on polyphenol effects, but complete information on how these may disrupt the impact of these compounds on health is also lacking. Therefore, at present, methodology for therapeutic application of these compounds is not entirely clear, but what is clear, is that dietary supplementation is not enough. An understanding of the significance of individual functional groups on the inhibitory activity/potency of these compounds may allow more potent and selective analogues to be made using polyphenols as a basis. Information from structure-activity studies has allowed the construction of more selective analogues, for example quercetin-3-O-amino acid esters were more selective for Src tyrosine kinase than EGF receptor kinase (Huang et al., 2009). One strategy is to use these analogues together with dietary supplementation to reinforce and accelerate the actions of parent compound metabolites generated in vivo. Rieder et al. (2012) suggested an alternative strategy involving an unmodified polyphenol (resveratrol) as viable treatment for lung injury. The authors demonstrated that the phyto-oestrogen/phytoallexin, resveratrol (trans-3,5,4′-trihydroxystilbene) was effective for the treatment of staphylococcal enterotoxin B (SEB)-induced lung injury. Resveratrol achieved this by decreasing SEB-induced pulmonary vascular permeability, inhibiting inflammation through blocking cytokine production and infiltration of monocytes, by increasing caspase 8-dependent apoptosis of T-cells and by up-regulating myeloid-derived suppressor cells that inhibit SEB-mediated T-cell activation. Mechanisms included up-regulation of sirtuin (silent mating type information regulation 2 homologue) 1 (SIRT1) and down-regulation of NF-κB activation in inflammatory cells. In addition, to affecting the expression and activity of these signalling proteins that regulate inflammation and apoptosis, resveratrol modulated the functions of a number of cells (endothelial cells, neutrophils, monocytes). Collectively, the activities of the stilbene were complementary and resulted in the single effect of reducing SEB-mediated injury. The dose (100 mg·kg−1) of resveratrol administered was supraphysiological. The lack of toxicity was not surprising, however, as oral absorption of resveratrol is 75% of the ingested amount and considerably less than 1% of this is bioavailable (Walle, 2011). Major metabolites of resveratrol (glucuronides, sulphates and dihydroresveratrol conjugates) may have exerted the observed effects, or deconjugation by β-glucuronidase and sulphatase may have allowed specific tissue accumulation of resveratrol (Walle, 2011). The anti-inflammatory properties of resveratrol present this small molecule as a potentially important therapeutic agent against a number of degenerative conditions (e.g. coronary artery disease, brain injury) that manifest acute inflammation as a major pathological factor. A key recent study described the preclinical application of a grape extract containing 8 mg total resveratrol to a group of coronary artery disease patients, and demonstrated an increase in anti-inflammatory factors (Tome-Carneiro et al., 2013). Importantly, this study did not report any adverse side effects due to high levels of resveratrol, indicating that the rapid clearance rates or poor absorbance of this polyphenol may be beneficial. This is in line with previous human studies that indicate that resveratrol is well-tolerated and does not induce marked toxicity (Cottart et al., 2010). Interestingly, Rieder et al. (2012) demonstrated that resveratrol was selective for apoptosis of cells as the stilbene induced apoptosis of SEB-activated T-cells, but protected endothelial cells from SEB-mediated apoptosis, indicating complex mechanistic actions for the stilbene. A number of supporting studies demonstrate that resveratrol selectively induces apoptosis of malignant cancer cells (Dhandayuthapani et al., 2013; Quoc Trung et al., 2013), and in healthy cells, apoptosis is attenuated by this compound (Liu et al., 2013). Similar to the anti-inflammatory activities of resveratrol, the underlying mechanisms for pro- and anti-apoptotic activities of this compound involve down-regulation of the expression and attenuation of the function of a number of kinases and housekeeping proteins. The structural features that influence mechanistic activities of resveratrol may allow important information to be gained to inform the synthesis of analogues selective for individual cellular process, regardless of multiple molecular targets. Novel analogues of resveratrol include HS-1793 (Kim et al., 2012). At a pharmacological dose (1.3–20 μM), HS-1793 was demonstrated to exert a cytotoxic effect on murine breast cancer cells that resulted in apoptosis. Network/systems pharmacology may allow the development of methodology to direct the clinical use of resveratrol, through the use of enhanced pharmacodynamic models that analyse regulatory networks involved in drug action (Iyengar et al., 2012). These can account for multiple targets of a drug as well as the effects of genomic, epigenomic and posttranslational changes on drug efficacy, and therefore, may allow the development of resveratrol or resveratrol analogues into precision therapy. This approach will be possible with more complete clarification of the physiological effects and underlying mechanisms exerted by these compounds. The body of evidence for the application of resveratrol as drug therapy described by the Rieder et al. (2012) study left several questions unanswered. Resveratrol reduced capillary permeability after SEB infection by protecting lung endothelial cells – how was this achieved? Was this due to the anti-apoptotic properties of this compound and/or the reported ability of resveratrol to reinforce the integrity of tight junctions (Lin et al., 2010)? The main transport system for polyphenols is blood, and blood vessel barrier function must be tightly regulated. How does resveratrol affect uninjured blood vessel walls – is resveratrol inactive in healthy, homeostatic cellular systems? Moreover, in vivo, would the functions of resveratrol be limited to anti-inflammatory and anti- or pro-apoptotic effects? The anti-inflammatory properties of resveratrol may render this compound as an immunosuppressant. Resveratrol was previously reported to function as an immunosuppressant by decreasing the expression of CD28 and CD80, through augmentation of the production of IL-10 (Sharma et al., 2007). This property may confound the beneficial effects exerted by this compound. The therapeutic effects of resveratrol may be directed through local delivery of the stilbene using biomaterial devices. This strategy has already been investigated using nanomaterial devices that allow controlled release of polyphenols (including resveratrol), concentrating the compound at predetermined amounts over specified time periods in the physiological region of interest. Hydrogel matrices with intrinsic porosity, particularly biocompatible alginate gels (Wright et al., 2012), are suitable for the release of polyphenols to localized areas (Figure 1). A recent study described a method for coupling resveratrol through a hydrolysable covalent bond to the carboxylic acid groups in porous poly-e-caprolactone surface grafted with acrylic acid, and used this construct for in vivo bone regeneration (Li et al., 2011). Furthermore, sodium deoxycholate elastic liposomes loaded with resveratrol were shown to be stable when delivered subcutaneously (Cadena et al., 2013). These novel delivery systems may allow greater understanding of the long-term effects of resveratrol. Figure 1 Biomaterial scaffolds for the delivery of polyphenols. Alginate hydrogels (A) are versatile, biocompatible matrices comprising cross-linked chains of mannuronic and guluronic acid (A: inset) that can be applied as medical devices for the controlled release ...