Recent advances in the discovery of flavonoids and analogs with high-affinity binding to P-glycoprotein responsible for cancer cell multidrug resistance

Flavonoid-membrane interactions: possible consequences for biological effects of some polyphenolic compounds

Flavonoids are found ubiquitously in higher plants and constitute an important component of the majority of people daily diets. The biological activities of flavonoids cover a very broad spectrum, from anticancer and antibacterial activities through to inhibition of bone resorption. In the present paper, the interactions between flavonoids and lipid bilayers as well as biological membranes and their components are reviewed, with special emphasis on the structure-activity relationships and mechanisms underlying the biological activity of flavonoids.

Effects of alkyl gallates on P-glycoprotein function

In this study, we examined the effects of the food antioxidants, alkyl gallates, on the function of P-glycoprotein (P-gp) and elucidated the importance of alkyl chains and gallic acid moieties on the activity of P-gp. We examined the effects of three alkyl (n-butyl, n-octyl and n-dodecyl) gallates and their related compounds on the cellular accumulation and efflux of rhodamine 123 and daunorubicin in P-gp overexpressing KB-C2 cells. Alkyl gallates increased the cellular accumulation of these P-gp substrates dependent on their alkyl chain lengths by inhibiting the efflux of the substrates. n-Dodecylresorcinol also increased the accumulation, but its effect was less than that of n-dodecyl gallate. However, either lauric acid or n-dodecyl-beta-d-maltoside, which does not have a phenol group, did not increase the accumulation. The results indicated that both the gallic acid moiety and a long alkyl chain play important roles in the modification of P-gp function. The cytotoxicity of daunorubicin was recovered in the presence of alkyl gallates possibly due to their inhibition of P-gp function.

Structure activity relationships and quantitative structure activity relationships for the flavonoid-mediated inhibition of breast cancer resistance protein

Breast cancer resistance protein (BCRP) is a newly identified ABC transporter, which plays an important role in drug disposition and represents an additional mechanism for the development of MDR. Flavonoids, a major class of natural compounds widely present in foods and herbal products, have been shown to be BCRP inhibitors. The objective of the present study was to elucidate the SAR and derive a QSAR model for flavonoid-BCRP interaction. The EC(50) values for increasing mitoxantrone accumulation in MCF-7 MX100 cells for 25 flavonoids, from five flavonoid subclasses, were determined in this study or obtained from our previous publication [Zhang S, Yang X, Morris ME. Combined effects of multiple flavonoids on breast cancer resistance protein (ABCG2)-mediated transport. Pharm Res 2004;21(7):1263-73], and ranged from 0.07+/-0.02 microM to 183+/-21.7 microM. We found that the presence of a 2,3-double bond in ring C, ring B attached at position 2, hydroxylation at position 5, lack of hydroxylation at position 3 and hydrophobic substitution at positions 6, 7, 8 or 4', are important structural properties important for potent flavonoid-BCRP interaction. These structural requirements are similar but not identical to those for potent flavonoid-NBD2 (P-glycoprotein) interaction, indicating that inhibition of BCRP by flavonoids may involve, in part, the binding of flavonoids with the NBD of BCRP. In addition, a QSAR model consisting three structural descriptors was constructed, and both internally and externally validated, suggesting the model could be used to quantitatively predict BCRP inhibition activity of flavonoids. These findings should be useful for predicting BCRP inhibition activity of other untested flavonoids and for guiding the synthesis of potent BCRP inhibitors for potential clinical application.

Anticancer multidrug resistance mediated by MRP1: recent advances in the discovery of reversal agents

Multidrug resistance protein 1 (MRP1) belongs to the ATP-binding cassette (ABC) transporter family. It is able to transport a broad range of anticancer drugs through cellular membranes, thus limiting their antiproliferative action. Since its discovery in 1992, MRP1 has been the most studied among MRP proteins, which now count nine members. Besides the biological work, which targets structure elucidation, binding sites location, and mode of action, most efforts have been focused on finding molecules which act as MRP1 inhibitors. In this review, we attempt to summarize and highlight studies dealing with modulators of MRP1-mediated multidrug resistance (MDR), which have been accomplished in the last 5 years. The reported MRP1 inhibitors are discussed according to their chemical class. Finally, we try to bring information on structure-activity relationship (SAR) aspects and how modulators might interact with MRP1. This study may facilitate the rational design of future modulators of MDR.

Modulation of P-glycoprotein function and reversal of multidrug resistance by (-)-epigallocatechin gallate in human cancer cells

Multidrug resistance (MDR) is a major obstacle in the chemotherapeutic treatment of many human cancers. In this study, the reversal of P-glycoprotein (P-gp) mediated multidrug resistance by (-)-epigallocatechin gallate (EGCG) and its molecular mechanism were investigated. A three-dimensional model of carboxyl-terminal nucleotide binding domain (NBD2) from P-gp was built by homology modeling. The structural model of the complex indicates that EGCG was tightly bound to the ATP-binding site of NBD2. EGCG modulated the function of P-gp and increased the intracellular accumulation of chemotherapeutic agent doxorubicin (DOX) in drug-resistant KB-A1 cells. When KB-A1 cells were exposed to 10 microg/ml DOX combined with 10, 30, 50 microM EGCG for 4 h, the intracellular concentrations of DOX were increased 1.5, 1.9, 2.3 times, respectively compared with DOX alone treatment. In vitro EGCG potentiated the cytotoxicity of DOX to drug-resistant KB-A1 cells. In KB-A1 cell xenograft model, EGCG could also enhance the efficacy of DOX and increased the DOX concentration in the resistant tumors. Thus, these results suggest that EGCG modulated the function of P-gp and reversed P-gp mediated multidrug resistance in human cancer cells.

Quantitative structure activity relationship studies on the flavonoid mediated inhibition of multidrug resistance proteins 1 and 2

In the present study, the effects of a large series of flavonoids on multidrug resistance proteins (MRPs) were studied in MRP1 and MRP2 transfected MDCKII cells. The results were used to define the structural requirements of flavonoids necessary for potent inhibition of MRP1- and MRP2-mediated calcein transport in a cellular model. Several of the methoxylated flavonoids are among the best MRP1 inhibitors (IC(50) values, ranging between 2.7 and 14.3 microM) followed by robinetin, myricetin and quercetin (IC(50) values ranging between 13.6 and 21.8 microM). Regarding inhibition of MRP2 activity especially robinetin and myricetin appeared to be good inhibitors (IC(50) values of 15.0 and 22.2 microM, respectively). Kinetic characterization revealed that the two transporters differ marginally in the apparent K(m) for the substrate calcein. For one flavonoid, robinetin, the kinetics of inhibition were studied in more detail and revealed competitive inhibition with respect to calcein, with apparent inhibition constants of 5.0 microM for MRP1 and 8.5 microM for MRP2. For inhibition of MRP1, a quantitative structure activity relationship (QSAR) was obtained that indicates three structural characteristics to be of major importance for MRP1 inhibition by flavonoids: the total number of methoxylated moieties, the total number of hydroxyl groups and the dihedral angle between the B- and C-ring. Regarding MRP2 mediated calcein efflux inhibition, only the presence of a flavonol B-ring pyrogallol group seems to be an important structural characteristic. Overall, this study provides insight in the structural characteristics involved in MRP inhibition and explores the differences between inhibitors of these two transporters, MRP1 and MRP2. Ultimately, MRP2 displays higher selectivity for flavonoid type inhibition than MRP1.

A One-Step Synthesis of 2-Alkyl-5-hydroxychromones and 3-Alkoyl-2-alkyl-5-hydroxychromones

2-Alkyl-5-hydroxychromones (2-alkyl-5-hydroxy-4-oxo-4H-1-benzopyran) and 3-alkoyl-2-alkyl-5-hydroxychromones (3-alkoyl-2-alkyl-5-hydroxy-4-oxo-4H-1-benzopyran) were prepared in one-step and one pot reaction by condensation of 2',6'-dihydroxyacetophenone with an alkoyl chloride in the presence of K2CO3.

Health promotion by flavonoids, tocopherols, tocotrienols, and other phenols: direct or indirect effects? Antioxidant or not?

Foods and beverages rich in phenolic compounds, especially flavonoids, have often been associated with decreased risk of developing several diseases. However, it remains unclear whether this protective effect is attributable to the phenols or to other agents in the diet. Alleged health-promoting effects of flavonoids are usually attributed to their powerful antioxidant activities, but evidence for in vivo antioxidant effects of flavonoids is confusing and equivocal. This may be because maximal plasma concentrations, even after extensive flavonoid intake, may be low (insufficient to exert significant systemic antioxidant effects) and because flavonoid metabolites tend to have decreased antioxidant activity. Reports of substantial increases in plasma total antioxidant activity after flavonoid intake must be interpreted with caution; findings may be attributable to changes in urate concentrations. However, phenols might exert direct effects within the gastrointestinal tract, because of the high concentrations present. These effects could include binding of prooxidant iron, scavenging of reactive nitrogen, chlorine, and oxygen species, and perhaps inhibition of cyclooxygenases and lipoxygenases. Our measurements of flavonoids and other phenols in human fecal water are consistent with this concept. We argue that tocopherols and tocotrienols may also exert direct beneficial effects in the gastrointestinal tract and that their return to the gastrointestinal tract by the liver through the bile may be physiologically advantageous.

Increased anti-P-glycoprotein activity of baicalein by alkylation on the A ring

The aqueous extract of Scutellariae baicalensis Georgi has inhibitory activity against P-gp 170, a multiple drug resistant gene product. Baicalein, one of the major flavones, was found to be responsible for this activity. The hydroxyl groups of the A ring of baicalein were systematically alkylated in order to assess the effect of such modifications on the activity against P-gp 170. The impact of the baicalein modifications on activity against the growth of a human nasopharyngeal cancer cell line KB and its P-gp 170 overexpressing cell line KB/MDR were also examined. The results indicate that alkylation of R5 of baicalein does not have a major impact on the interaction with P-gp 170, whereas alkylation of R6 or R7 alone or both, could enhance the interaction of baicalein with P-gp 170 as well as the amount of intracellular accumulation of vinblastine, a surrogate marker for the activity of P-gp 170 pump of KB/MDR cells. In this case, the optimal linear alkyl functionality is a propyl side chain. These modifications could also alter the activity of compounds inhibiting cell growth. Among the different compounds synthesized, the most potent molecule against P-gp 170 is 5-methoxy-6,7-dipropyloxyflavone (23). Its inhibitory activity against P-gp 170 is approximately 40 times better, based on EC50 (concentration of the compound enhancing 50% of the intracellular vinblastine accumulation in the KB/MDR cells) and 3 times higher, based on Amax (the intracellular vinblastine accumulation of the KB/MDR cells caused by the compound) as compared to baicalein. Compound 23 is also a more selective inhibitor than baicalein against P-gp 170, because its cytotoxicity is less than that observed for baicalein. The growth inhibitory IC50 of compound 23 against KB and KB/MDR cells are about the same, suggesting that compound 23 is unlikely to be a substrate of P-gp 170 pump. Acetylation of R6, R7 or both could also decrease EC50 and increase Amax. Acetylated compounds are more toxic than baicalein, and their potency against cell growth is compromised by the presence of P-gp 170, suggesting that these compounds are substrates of P-gp 170. Benzylation of R6 or R7 but not both also enhanced anti-P-gp170 activity and potency against cell growth; however, the presence of P-gp 170 in cells did not have an impact on their sensitivity to these molecules, suggesting that the benzylated compounds are inhibitors but not substrates of P-gp 170, and perhaps have a different mechanism of action. In conclusion, the substitutions of R6 and R7 hydroxyl groups by alkoxy groups, acetoxy groups, or benzyloxy groups could yield compounds with different modes of action against P-gp 170 with different mechanisms of action against cell growth.

Herbal modulation of P-glycoprotein

P-glycoprotein (Pgp) is a 170 kDa phosphorylated glycoprotein encoded by human MDR1 gene. It is responsible for the systemic disposition of numerous structurally and pharmacologically unrelated lipophilic and amphipathic drugs, carcinogens, toxins, and other xenobiotics in many organs, such as the intestine, liver, kidney, and brain. Like cytochrome P450s (CYP3A4), Pgp is vulnerable to inhibition, activation, or induction by herbal constituents. This was demonstrated by using an ATPase assay, purified Pgp protein or intact Pgp-expressing cells, and proper probe substrates and inhibitors. Curcumin, ginsenosides, piperine, some catechins from green tea, and silymarin from milk thistle were found to be inhibitors of Pgp, while some catechins from green tea increased Pgp-mediated drug transport by heterotropic allosteric mechanism, and St. John's wort induced the intestinal expression of Pgp in vitro and in vivo. Some components (e.g., bergamottin and quercetin) from grapefruit juice were reported to modulate Pgp activity. Many of these herbal constituents, in particular flavonoids, were reported to modulate Pgp by directly interacting with the vicinal ATP-binding site, the steroid-binding site, or the substrate-binding site. Some herbal constituents (e.g., hyperforin and kava) were shown to activate pregnane X receptor, an orphan nuclear receptor acting as a key regulator of MDR1 and many other genes. The inhibition of Pgp by herbal constituents may provide a novel approach for reversing multidrug resistance in tumor cells, whereas the stimulation of Pgp expression or activity has implication for chemoprotective enhancement by herbal medicines. Certain natural flavonols (e.g., kaempferol, quercetin, and galangin) are potent stimulators of the Pgp-mediated efflux of 7,12-dimethylbenz(a)-anthracene (a carcinogen). The modulation of Pgp activity and expression by these herb constituents may result in altered absorption and bioavailability of drugs that are Pgp substrates. This is exemplified by increased oral bioavailability of phenytoin and rifampin by piperine and decreased bioavailability of indinavir, tacrolimus, cyclosporine, digoxin, and fexofenadine by coadministered St. John's wort. However, many of these drugs are also substrates of CYP3A4. Thus, the modulation of intestinal Pgp and CYP3A4 represents an important mechanism for many clinically important herb-drug interactions. Further studies are needed to explore the relative role of Pgp and CYP3A4 modulation by herbs and the mechanism for the interplay of these two important proteins in herb-drug interactions.

Antimitotic Activity of 5-Hydroxy-7-methoxy-2-phenyl-4-quinolones

We report the synthesis of 5-hydroxy-7-methoxy-2-phenyl-4-quinolones and their biological activity as antitumor agents. These molecules were initially evaluated for their ability to induce cell cycle arrest in the G2/M phase. Compounds that showed significant G2/M cell cycle arrest were tested for antiproliferative activity using both the MTT assay and the NCI in vitro 60 cell line human tumor screen. The 5-hydroxy-7-methoxy-2-phenyl-4-quinolone (3a) and 2-(3-fluorophenyl)-5-hydroxy-7-methoxy-4-quinolone (3f) were the most active in the cell cycle arrest test whereas 3f was found to be the most active in the MTT assay. In terms of structural requirements, we found that the presence of a 5-hydroxyl group, a 7-methoxy group, and an unsubstituted N1 were essential for the antimitotic activity. In accordance with the literature, a fluoro group at the 3'- or 2'-position and a methoxy or a chloro group at the 3'-position were found to be highly advantageous for both the cell cycle arrest and the antiproliferative activities.

Structural requirements for the flavonoid-mediated modulation of glutathione S-transferase P1-1 and GS-X pump activity in MCF7 breast cancer cells

The objective of this study was to investigate the structural requirements necessary for inhibition of glutathione S-transferase P1-1 (GSTP1-1) and GS-X pump (MRP1 and MRP2) activity by structurally related flavonoids, in GSTP1-1 transfected MCF7 cells (pMTG5). The results reveal that GSTP1-1 activity in MCF7 pMTG5 cells can be inhibited by some flavonoids. Especially galangin was able to inhibit almost all cellular GSTP1-1 activity upon exposure of the cells to a concentration of 25microM. Other flavonoids like kaempferol, eriodictyol and quercetin showed a moderate GSTP1-1 inhibitory potential. For GSTP1-1 inhibition, no specific structural requirements necessary for potent inhibition could be defined. Most flavonoids appeared to be potent GS-X transport inhibitors with IC(50) values ranging between 0.8 and 8microM. Luteolin and quercetin were the strongest inhibitors with IC(50) values of 0.8 and 1.3microM, respectively. Flavonoids without a C2-C3 double bond like eriodictyol, taxifolin and catechin did not inhibit GS-X pump activity. The results of this study demonstrate that the structural features necessary for high potency GS-X pump inhibition by flavonoids are (1) the presence of hydroxyl groups, especially two of them generating the 3',4'-catechol moiety; and (2) a planar molecule due to the presence of a C2-C3 double bond. Other factors, like lipophilicity and the total number of hydroxyl groups do not seem to be dominating the flavonoid-mediated GS-X pump inhibition. To identify the GS-X pump responsible for the DNP-SG efflux in MCF7 cells, the effects of three characteristic flavonoids quercetin, flavone and taxifolin on MRP1 and MRP2 activity were studied using transfected MDCKII cells. All three flavonoids as well as the typical MRP inhibitor (MK571) affected MRP1-mediated transport activity in a similar way as observed in the MCF7 cells. In addition, the most potent GS-X pump inhibitor in the MCF7 cells, quercetin, did not affect MRP2-mediated transport activity. These observations clearly indicate that the GS-X pump activity in the MCF7 cells is likely to be the result of flavonoid-mediated inhibition of MRP1 and not MRP2. Altogether, the present study reveals that a major site for flavonoid interaction with GSH-dependent toxicokinetics is the GS-X pump MRP1 rather than the conjugating GSTP1-1 activity itself. Of the flavonoids shown to be most active especially quercetin is frequently marketed in functional food supplements. Given the physiological levels expected to be reached upon supplement intake, the IC(50) values of the present study point at possible flavonoid-drug and/or flavonoid-xenobiotic interactions especially regarding transport processes involved in toxicokinetics.

Flavonoid permeability across an in situ model of the blood-brain barrier

Understanding mechanisms associated with flavonoid neuroprotection is complicated by the lack of information on their ability to enter the CNS. This study examined naringenin and quercetin permeability across the blood-brain barrier (BBB), using in vitro (ECV304/C6 coculture) and in situ (rat) models. We report measurable permeabilities (P(app)) for both flavonoids across the in vitro BBB model, consistent with their lipophilicity. Both flavonoids showed measurable in situ BBB permeability. The rates of uptake (K(in)) into the right cerebral hemisphere were 0.145 and 0.019 ml min(-1) g(-1) for naringenin and quercetin, respectively. Quercetin K(in) was comparable to that of colchicine (0.006 ml min(-1) g(-1)), a substrate for P-glycoprotein (P-gp). Preadministration of the P-gp inhibitor PSC833 or GF120918 (10 mg/kg body wt) significantly increased colchicine K(in), but only GF120918 (able to inhibit breast cancer resistance protein, BCRP) affected K(in) for quercetin. Naringenin K(in) was not affected. The influence of efflux transporters on flavonoid permeability at the BBB was further studied using MDCK-MDR1 and immortalized rat brain endothelial cells (RBE4). Colchicine, quercetin, and naringenin all showed measurable accumulation (distribution volume, V(d) (microl/mg protein)) in both cell types. The V(d) for colchicine increased significantly in both cell lines following coincubation with either PSC833 (25 microM) or GF120918 (25 microM). Both inhibitors also caused an increase in naringenin V(d); by contrast only GF120918 coincubation significantly increased quercetin V(d). In conclusion, the results demonstrate that flavonoids are able to traverse the BBB in vivo. However, the permeability of certain flavonoids in vivo is influenced by their lipophilicity and interactions with efflux transporters.

Modulation of P-glycoprotein-mediated multidrug resistance by flavonoid derivatives and analogues

Flavonoid derivatives were synthesized and tested for their ability to modulate P-glycoprotein (Pgp)-mediated multidrug resistance (MDR) in vitro. These compounds belong to various flavonoid subclasses, namely: chromones, azaisoflavones, and aurones. Among the investigated compounds, three showed potent reversing activity. 2-(4-methylpiperazin-1-ylcarbonyl)-5-hydroxychromone (4a), 5,7-dimethoxy-3-phenyl-4-quinolone (5), and 4,6-dimethoxyaurone (6) potentiated daunorubicin cytotoxicity on resistant K562 cells. They were also able to increase the intracellular accumulation of rhodamine-123, a fluorescent molecule which acts as a probe of P-glycoprotein-mediated MDR. This suggests that these compounds act, at least in part, by inhibiting P-glycoprotein activity. The most active compound, 5-hydroxy-2-(4-methylpiperazin-1-ylcarbonyl)chromone (4a) was found to be a powerful reversal agent, more potent than cyclosporin A, used as the reference molecule. No effect was observed on MRP transport nor on cell proliferation. Little apoptosis was induced on K562S cells with 4a compared to K562R, probably due to the extrusion of the compound by Pgp.