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Ethynylflavones, Highly Potent, and Selective Inhibitors of Cytochrome P450 1A1.
Chem. Res. Toxicol.
PUBLISHED: 07-18-2014
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The flavone backbone is a well-known pharmacophore present in a number of substrates and inhibitors of various P450 enzymes. In order to find highly potent and novel P450 family I enzyme inhibitors, an acetylene group was incorporated into six different positions of flavone. The introduction of an acetylene group at certain locations of the flavone backbone lead to time-dependent inhibitors of P450 1A1. 3'-Ethynylflavone, 4'-ethynylflavone, 6-ethynylflavone, and 7-ethynylflavone (KI values of 0.035-0.056 ?M) show strong time-dependent inhibition of P450 1A1, while 5-ethynylflavone (KI value of 0.51 ?M) is a moderate time-dependent inhibitor of this enzyme. Meanwhile, 4'-ethynylflavone and 6-ethynylflavone are highly selective inhibitors toward this enzyme. Especially, 6-ethynylflavone possesses a Ki value of 0.035 ?M for P450 1A1 177- and 15-fold lower than those for P450s 1A2 and 1B1, respectively. The docking postures observed in the computational simulations show that the orientation of the acetylene group determines its capability to react with P450s 1A1 and 1A2. Meanwhile, conformational analysis indicates that the shape of an inhibitor determines its inhibitory selectivity toward these enzymes.
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3-Ketone-4,6-diene ceramide analogs exclusively induce apoptosis in chemo-resistant cancer cells.
Bioorg. Med. Chem.
PUBLISHED: 01-08-2014
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Multidrug-resistance is a major cause of cancer chemotherapy failure in clinical treatment. Evidence shows that multidrug-resistant cancer cells are as sensitive as corresponding regular cancer cells under the exposure to anticancer ceramide analogs. In this work we designed five new ceramide analogs with different backbones, in order to test the hypothesis that extending the conjugated system in ceramide analogs would lead to an increase of their anticancer activity and selectivity towards resistant cancer cells. The analogs with the 3-ketone-4,6-diene backbone show the highest apoptosis-inducing efficacy. The most potent compound, analog 406, possesses higher pro-apoptotic activity in chemo-resistant cell lines MCF-7TN-R and NCI/ADR-RES than the corresponding chemo-sensitive cell lines MCF-7 and OVCAR-8, respectively. However, this compound shows the same potency in inhibiting the growth of another pair of chemo-sensitive and chemo-resistant cancer cells, MCF-7 and MCF-7/Dox. Mechanism investigations indicate that analog 406 can induce apoptosis in chemo-resistant cancer cells through the mitochondrial pathway. Cellular glucosylceramide synthase assay shows that analog 406 does not interrupt glucosylceramide synthase in chemo-resistant cancer cell NCI/ADR-RES. These findings suggest that due to certain intrinsic properties, ceramide analogs' pro-apoptotic activity is not disrupted by the normal drug-resistance mechanisms, leading to their potential use for overcoming cancer multidrug-resistance.
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Cytochrome p450 family 1 inhibitors and structure-activity relationships.
Molecules
PUBLISHED: 09-22-2013
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With the widespread use of O-alkoxyresorufin dealkylation assays since the 1990s, thousands of inhibitors of cytochrome P450 family 1 enzymes (P450s 1A1, 1A2, and 1B1) have been identified and studied. Generally, planar polycyclic molecules such as polycyclic aromatic hydrocarbons, stilbenoids, and flavonoids are considered to potentially be effective inhibitors of these enzymes, however, the details of the structure-activity relationships and selectivity of these inhibitors are still ambiguous. In this review, we thoroughly discuss the selectivity of many representative P450 family 1 inhibitors reported in the past 20 years through a meta-analysis.
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A review of ceramide analogs as potential anticancer agents.
Future Med Chem
PUBLISHED: 08-08-2013
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Ceramide serves as a central mediator in sphingolipid metabolism and signaling pathways, regulating many fundamental cellular responses. It is referred to as a tumor suppressor lipid, since it powerfully potentiates signaling events that drive apoptosis, cell cycle arrest, and autophagic responses. In the typical cancer cell, ceramide levels and signaling are usually suppressed by overexpression of ceramide-metabolizing enzymes or downregulation of ceramide-generating enzymes. However, chemotherapeutic drugs as well as radiotherapy increase intracellular ceramide levels, while exogenously treating cancer cells with short-chain ceramides leads to anticancer effects. All evidence currently points to the fact that the upregulation of ceramide levels is a promising anticancer strategy. In this review, we exhibit many anticancer ceramide analogs as downstream receptor agonists and ceramide-metabolizing enzyme inhibitors.
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Pyranoflavones: a group of small-molecule probes for exploring the active site cavities of cytochrome P450 enzymes 1A1, 1A2, and 1B1.
J. Med. Chem.
PUBLISHED: 05-02-2013
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Selective inhibition of P450 enzymes is the key to block the conversion of environmental procarcinogens to their carcinogenic metabolites in both animals and humans. To discover highly potent and selective inhibitors of P450s 1A1, 1A2, and 1B1, as well as to investigate active site cavities of these enzymes, 14 novel flavone derivatives were prepared as chemical probes. Fluorimetric enzyme inhibition assays were used to determine the inhibitory activities of these probes toward P450s 1A1, 1A2, 1B1, 2A6, and 2B1. A highly selective P450 1B1 inhibitor 5-hydroxy-4-propargyloxyflavone (5H4FPE) was discovered. Some tested compounds also showed selectivity between P450s 1A1 and 1A2. ?-Naphthoflavone-like and 5-hydroxyflavone derivatives preferentially inhibited P450 1A2, while ?-naphthoflavone-like flavone derivatives showed selective inhibition of P450 1A1. On the basis of structural analysis, the active site cavity models of P450 enzymes 1A1 and 1A2 were generated, demonstrating a planar long strip cavity and a planar triangular cavity, respectively.
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Binding of Diverse Environmental Chemicals with Human Cytochromes P450 2A13, 2A6, and 1B1 and Enzyme Inhibition.
Chem. Res. Toxicol.
PUBLISHED: 03-13-2013
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A total of 68 chemicals including derivatives of naphthalene, phenanthrene, fluoranthene, pyrene, biphenyl, and flavone were examined for their abilities to interact with human P450s 2A13 and 2A6. Fifty-one of these 68 chemicals induced stronger Type I binding spectra (iron low- to high-spin state shift) with P450 2A13 than those seen with P450 2A6, i.e., the spectral binding intensities (?Amax/Ks ratio) determined with these chemicals were always higher for P450 2A13. In addition, benzo[c]phenanthrene, fluoranthene, 2,3-dihydroxy-2,3-dihydrofluoranthene, pyrene, 1-hydroxypyrene, 1-nitropyrene, 1-acetylpyrene, 2-acetylpyrene, 2,5,2,5-tetrachlorobiphenyl, 7-hydroxyflavone, chrysin, and galangin were found to induce a Type I spectral change only with P450 2A13. Coumarin 7-hydroxylation, catalyzed by P450 2A13, was strongly inhibited by 2-methoxy-5,7-dihydroxyflavone, 2-ethynylnaphthalene, 2-methoxyflavone, 2-naphththalene propargyl ether, acenaphthene, acenaphthylene, naphthalene, 1-acetylpyrene, flavanone, chrysin, 3-ethynylphenanthrene, flavone, and 7-hydroxyflavone; these chemicals induced Type I spectral changes with low Ks values. On the basis of the intensities of the spectral changes and inhibition of P450 2A13, we classified the 68 chemicals into eight groups based on the order of affinities for these chemicals and inhibition of P450 2A13. The metabolism of chemicals by P450 2A13 during the assays explained why some of the chemicals that bound well were poor inhibitors of P450 2A13. Finally, we compared the 68 chemicals for their abilities to induce Type I spectral changes of P450 2A13 with the Reverse Type I binding spectra observed with P450 1B1: 45 chemicals interacted with both P450s 2A13 and 1B1, indicating that the two enzymes have some similarty of structural features regarding these chemicals. Molecular docking analyses suggest similarities at the active sites of these P450 enzymes. These results indicate that P450 2A13, as well as Family 1 P450 enzymes, is able to catalyze many detoxication and activation reactions with chemicals of environmental interest.
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Potent mechanism-based inactivation of cytochrome P450 2B4 by 9-ethynylphenanthrene: implications for allosteric modulation of cytochrome P450 catalysis.
Biochemistry
PUBLISHED: 01-04-2013
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The mechanism-based inactivation of cytochrome P450 2B4 (CYP2B4) by 9-ethynylphenanthrene (9EP) has been investigated. The partition ratio and k(inact) are 0.2 and 0.25 min(-1), respectively. Intriguingly, the inactivation exhibits sigmoidal kinetics with a Hill coefficient of 2.5 and an S(50) of 4.5 ?M indicative of homotropic cooperativity. Enzyme inactivation led to an increase in mass of the apo-CYP2B4 by 218 Da as determined by electrospray ionization liquid chromatography and mass spectrometry, consistent with covalent protein modification. The modified CYP2B4 was purified to homogeneity and its structure determined by X-ray crystallography. The structure showed that 9EP is covalently attached to O? of Thr 302 via an ester bond, which is consistent with the increased mass of the protein. The presence of the bulky phenanthrenyl ring resulted in inward rotations of Phe 297 and Phe 206, leading to a compact active site. Thus, binding of another molecule of 9EP in the active site is prohibited. However, results from the quenching of 9EP fluorescence by unmodified or 9EP-modified CYP2B4 revealed at least two binding sites with distinct affinities, with the low-affinity site being the catalytic site and the high-affinity site on the protein periphery. Computer-aided docking and molecular dynamics simulations with one or two ligands bound revealed that the high-affinity site is situated at the entrance of a substrate access channel surrounded by the F helix, ?1-?2 loop, and ?4 loop and functions as an allosteric site to enhance the efficiency of activation of the acetylenic group of 9EP and subsequent covalent modification of Thr 302.
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Structure-function relationships of inhibition of human cytochromes P450 1A1, 1A2, 1B1, 2C9, and 3A4 by 33 flavonoid derivatives.
Chem. Res. Toxicol.
PUBLISHED: 11-09-2010
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Structure-function relationships for the inhibition of human cytochrome P450s (P450s) 1A1, 1A2, 1B1, 2C9, and 3A4 by 33 flavonoid derivatives were studied. Thirty-two of the 33 flavonoids tested produced reverse type I binding spectra with P450 1B1, and the potencies of binding were correlated with the abilities to inhibit 7-ethoxyresorufin O-deethylation activity. The presence of a hydroxyl group in flavones, for example, 3-, 5-, and 7-monohydroxy- and 5,7-dihydroxyflavone, decreased the 50% inhibition concentration (IC50) of P450 1B1 from 0.6 ?M to 0.09, 0.21, 0.25, and 0.27 ?M, respectively, and 3,5,7-trihydroxyflavone (galangin) was the most potent, with an IC50 of 0.003 ?M. The introduction of a 4-methoxy- or 3,4-dimethoxy group into 5,7-dihydroxyflavone yielded other active inhibitors of P450 1B1 with IC50 values of 0.014 and 0.019 ?M, respectively. The above hydroxyl and/or methoxy groups in flavone molecules also increased the inhibition activity with P450 1A1 but not always toward P450 1A2, where 3-, 5-, or 7-hydroxyflavone and 4-methoxy-5,7-dihydroxyflavone were less inhibitory than flavone itself. P450 2C9 was more inhibited by 7-hydroxy-, 5,7-dihydroxy-, and 3,5,7-trihydroxyflavones than by flavone but was weakly inhibited by 3- and 5-hydroxyflavone. Flavone and several other flavonoids produced type I binding spectra with P450 3A4, but such binding was not always related to the inhibitiory activities toward P450 3A4. These results indicate that there are different mechanisms of inhibition for P450s 1A1, 1A2, 1B1, 2C9, and 3A4 by various flavonoid derivatives and that the number and position of hydroxyl and/or methoxy groups highly influence the inhibitory actions of flavonoids toward these enzymes. Molecular docking studies suggest that there are different mechanisms involved in the interaction of various flavonoids with the active site of P450s, thus causing differences in inhibition of these P450 catalytic activities by flavonoids.
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Ethynyl and Propynylpyrene Inhibitors of Cytochrome P450.
J Chem Crystallogr
PUBLISHED: 05-18-2010
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The single-crystal X-ray structures and in vivo activities of three aryl acetylenic inhibitors of cytochromes P450 1A1, 1A2, 2A6, and 2B1 have been determined and are reported herein. These are 1-ethynylpyrene, 1-propy-nylpyrene, and 4-propynylpyrene. To investigate electronic influences on the mechanism of enzyme inhibition, the experimental electron density distribution of 1-ethynylpy-rene has been determined using low-temperature X-ray diffraction measurements, and the resulting net atomic charges compared with various theoretical calculations. A total of 82,390 reflections were measured with Mo K? radiation to a (sin?/?)(max) = 0.985 Å(-1). Averaging symmetry equivalent reflections yielded 8,889 unique reflections. A least squares refinement procedure was used in which multipole parameters were added to describe the distortions of the atomic electron distributions from spherical symmetry. A map of the model electron density distribution of 1-ethynylpyrene was obtained. Net atomic charges calculated from refined monopole population parameters yielded charges that showed that the terminal acetylenic carbon atom (C18) is more negative than the internal carbon (C17). Net atomic charges calculated by ab initio, density functional theory, and semi-empirical methods are consistent with this trend suggesting that the terminal acetylenic carbon atom is more likely to be the site of oxidation. This is consistent with the inhibition mechanism pathway that results in the formation of a reactive ketene intermediate. This is also consistent with assay results that determined that 1-ethynylpyrene acts as a mechanism-based inhibitor of P450s 1A1 and 1A2 and as a reversible inhibitor of P450 2B1. Crystallographic data: 1-ethynylpyrene, C(18)H(10), P2(1)/c, a = 14.571(2) Å, b = 3.9094(5) Å, c = 20.242(3) Å, ? = 105.042(2)°, V = 1,113.5(2) Å(3); 1-propynylpyrene, C(19)H(12), P2(1)/n, a = 8.970(2) Å, b = 10.136(1) Å, c = 14.080(3) Å, ? = 99.77(2)°, V = 1,261.5(4) Å(3); 4-propynylpyrene, C(19)H(12), Pbca, a = 9.904(1) Å, b = 13.174(2) Å, c = 19.401(1) Å, V = 2,531.4(5) Å(3).
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Novel anti-viability ceramide analogs: design, synthesis, and structure-activity relationship studies of substituted (S)-2-(benzylideneamino)-3-hydroxy-N-tetradecylpropanamides.
Bioorg. Med. Chem.
PUBLISHED: 05-14-2010
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A group of novel L-serinamides, substituted (S)-2-(benzylideneamino)-3-hydroxy-N-tetradecylpropanamides (3a-o) and substituted (S)-2-(benzylamino)-3-hydroxy-N-tetradecyl propanamides (4c, 4i, 4l, and 4o), were synthesized as potential anti-tumor lead compounds. In vitro cell viability assay results indicate treatment with 3a-o compounds resulted in significant inhibition of cell viability in the chemoresistant breast cancer cell line, MCF-7TN-R. Compounds 3i and 3l, both ortho-substituted analogs, show the greatest efficacy with IC50 values of 10.3 microM and 12.5 microM, respectively. The SAR analysis indicate that the imine functional group of 3a-o is critical for the cellular anti-viability effect, and the partial atomic charge (PAC) value of imine C atom is a valuable structural parameter for predicting the activity of these ceramide analogs.
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Novel D: -erythro N-octanoyl sphingosine analogs as chemo- and endocrine-resistant breast cancer therapeutics.
Cancer Chemother. Pharmacol.
PUBLISHED: 02-14-2010
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Resistance to endocrine and chemotherapies remains the primary cause of breast cancer treatment failure. We have synthesized four novel D: -erythro N-octanoyl sphingosine analogs and catalogued their activity in drug-sensitive (MCF-7), endocrine-resistant (MDA-MB-231) and chemoresistant (MCF-7TN-R) breast cancer cells.
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In silico studies of polyaromatic hydrocarbon inhibitors of cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1.
Chem. Res. Toxicol.
PUBLISHED: 01-19-2010
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A computational study was undertaken to understand the nature of binding and the structural features that play a significant role in the binding of arylacetylene molecules to cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1. Nine polycyclic arylacetylenes determined to be mechanism-based P450 enzyme inhibitors were studied. The lack of polar substituents in these compounds causes them to be incapable of hydrogen bonding to the polar protein residues. The four P450 enzymes of interest all have phenylalanine residues in the binding pocket for potential pi-pi interactions with the aromatic rings of the inhibitors. The inhibition potency of these arylacetylenes toward P450s 1A1 and 2B1 showed a dependence on the proximity of the inhibitors triple bond to the prosthetic heme Fe of the enzyme. In P450 enzyme 1A2, the inhibitors potency showed more dependence on the pi-pi interactions of the inhibitors ring systems with the phenylalanine residues of the protein, with the proximity of the inhibitor triple bond to the heme Fe weighing in as the second most important factor. The results suggest that maximizing the pi-pi interactions with phenylalanine residues in the binding pocket and optimum proximity of the acetylene moiety to the heme Fe will provide for a substantial increase in the potency of the polyaromatic hydrocarbon mechanism-based inhibitors. A fine balance of these two aspects of binding coupled with attention to supplementing hydrophobic interactions could address potency and selectivity issues for these inhibitors.
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Design, synthesis, and biological activity of a family of novel ceramide analogues in chemoresistant breast cancer cells.
J. Med. Chem.
PUBLISHED: 08-22-2009
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Resistance to chemotherapy and endocrine therapy is a major cause of breast cancer treatment failure. We have synthesized six novel analogues using C8-ceramide as the lead analogue and studied their effect on hormone therapy resistant (MDA-MB-231) and chemoresistant (MCF-7TN-R) breast cancer cells. Pharmacologic intervention using these ceramide analogues inhibited clonogenic survival and induced apoptosis, with one analogue being more effective than C8-ceramide. Our results show ceramide-based therapy has therapeutic potential in treating drug resistant breast cancer.
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Reverse type I binding spectra of human cytochrome P450 1B1 induced by flavonoid, stilbene, pyrene, naphthalene, phenanthrene, and biphenyl derivatives that inhibit catalytic activity: a structure-function relationship study.
Chem. Res. Toxicol.
PUBLISHED: 07-01-2009
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Fifty-one chemicals including derivatives of 16 flavonoids, three stilbenes, six pyrenes, seven naphthalenes, seven phenanthrenes, 10 biphenyls, 17beta-estradiol, and estrone were examined for their abilities to induce reverse type I binding spectra with human cytochrome P450 (P450) 1B1 and to inhibit 7-ethoxyresorufin O-deethylation (EROD) activities catalyzed by P450 1B1. Forty-nine chemicals showed reverse type I spectra with P450 1B1, and we found that 3,5,7-trihydroxyflavone, 3,4-dimethoxy-5,7-dihydroxyflavone, 4-methoxy-5,7-dihydroxyflavone, alpha- and beta-naphthoflavones, 2,4,3,5-tetramethoxystilbene, pyrene, and several acetylenic pyrenes and phenanthrenes were strong inducers of the spectra and also potent inhibitors of EROD activities catalyzed by P450 1B1. The spectral dissociation constant (K(s)) and the magnitude of the binding (DeltaA(max)/K(s)) of 49 chemicals were correlated with the inhibition potencies of EROD activities by these chemicals [correlation coefficients (r) of 0.72 and 0.74, respectively]. The K(s) and DeltaA(max)/K(s) values were more correlated with IC(50) values when compared in a group of derivatives of flavonoids, stilbenes, and estrogens (r = 0.81 and 0.88, respectively) or a group of derivatives of pyrenes, naphthalenes, phenanthrenes, and biphenyls (r = 0.88 and 0.91, respectively). Among 14 flavonoids examined, 3,5,7-trihydroxyflavone and 4-methoxy- and 3,4-dimethoxy-5,7-dihydroxyflavone were more active than flavone in interacting with P450 1B1, but the respective 7,8-dihydroxyflavones were less active. Pyrene itself was highly active in interacting with P450 1B1, but its binding was slightly decreased when substituted with acetylenic groups. In contrast, substitution of naphthalene with methyl and ethyl propargyl ethers led to more interaction with P450 1B1 than with naphthalene itself. Similarly, substitution on phenanthrene or biphenyl with acetylenic groups and propargyl ethers increased affinities to P450 1B1. These results suggest that the reverse type I binding of chemicals to P450 1B1 may determine how they interact with and inhibit the catalytic activity of the enzyme. Substitutions on the compounds with various acetylenic groups and propargyl ethers cause an increase or decrease of their affinities to P450 1B1, depending on the parent compound used.
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Development of flavone propargyl ethers as potent and selective inhibitors of cytochrome P450 enzymes 1A1 and 1A2.
Drug Metab Lett
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Naturally occurring flavonoids are known to be metabolized by several cytochrome P450 enzymes including P450s 1A1, 1A2, 1B1, 2C9, 3A4, and 3A5. In general flavonoids can act as substrates, inducers, and/or inhibitors of P450 enzymes. The position of the substituents on the flavone backbone has been shown to impact the biological activity against P450 enzymes. To explore the effect of a propargyl ether substitution on flavones and flavanones, 2´-flavone propargyl ether (2´-PF), 3´-flavone propargyl ether (3´-PF), 4´-flavone propargyl ether (4´-PF), 5-flavone propargyl ether (5-PF), 6-flavone propargyl ether (6-PF), 7-flavone propargyl ether (7-PF), 6-flavanone propargyl ether (6-PFN), and 7- flavanone propargyl ether (7-PFN) were synthesized. All of the newly synthesized compounds and the parent hydroxy flavones were tested for both direct inhibition and mechanism-based inhibition of cytochrome P450 enzymes 1A1, 1A2, 2A6, and 2B1. The flavone propargyl ether derivatives were found to be more potent inhibitors of P450s 1A1 and 1A2. None of the flavones and flavanones in our study showed any inhibition of P450 2A6. Only 2´-PF and 6-PFN inhibited P450 2B1. 3´-PF showed direct inhibition of P450 1A1 with the highest observed potency of 0.02 µM, in addition to its ability to cause mechanism-based inhibition with KI and kinactivation values of 0.24 µM and 0.09 min-1 for this enzyme. 7- Hydroxy flavone also exhibited mechanism-based inhibition of P450 1A1 with KI and kinactivation values of 2.43 µM and 0.115 min-1. Docking studies and QSAR studies on P450 enzymes 1A1 and 1A2 were performed which revealed important insights into the nature of binding of these molecules and provided us with good QSAR models that can be used to design new flavone derivatives.
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Insights on cytochrome p450 enzymes and inhibitors obtained through QSAR studies.
Molecules
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The cytochrome P450 (CYP) superfamily of heme enzymes play an important role in the metabolism of a large number of endogenous and exogenous compounds, including most of the drugs currently on the market. Inhibitors of CYP enzymes have important roles in the treatment of several disease conditions such as numerous cancers and fungal infections in addition to their critical role in drug-drug interactions. Structure activity relationships (SAR), and three-dimensional quantitative structure activity relationships (3D-QSAR) represent important tools in understanding the interactions of the inhibitors with the active sites of the CYP enzymes. A comprehensive account of the QSAR studies on the major human CYPs 1A1, 1A2, 1B1, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4 and a few other CYPs are detailed in this review which will provide us with an insight into the individual/common characteristics of the active sites of these enzymes and the enzyme-inhibitor interactions.
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7-Ethynylcoumarins: selective inhibitors of human cytochrome P450s 1A1 and 1A2.
Chem. Res. Toxicol.
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To discover new selective mechanism-based P450 inhibitors, eight 7-ethynylcoumarin derivatives were prepared through a facile two-step synthetic route. Cytochrome P450 activity assays indicated that introduction of functional groups in the backbone of coumarin could enhance the inhibition activities toward P450s 1A1 and 1A2, providing good selectivity against P450s 2A6 and 2B1. The most potent product 7-ethynyl-3,4,8-trimethylcoumarin (7ETMC) showed IC(50) values of 0.46 ?M and 0.50 ?M for P450s 1A1 and 1A2 in the first six minutes, respectively, and did not show any inhibition activity for P450s 2A6 and 2B1 even at the dose of 50 ?M. All of the inhibitors except 7-ethynyl-3-methyl-4-phenylcoumarin (7E3M4PC) showed mechanism-based inhibition of P450s 1A1 and 1A2. In order to explain this mechanistic difference in inhibitory activities, X-ray crystallography data were used to study the difference in conformation between 7E3M4PC and the other compounds studied. Docking simulations indicated that the binding orientations and affinities resulted in different behaviors of the inhibitors on P450 1A2. Specifically, 7E3M4PC with its two-plane structure fits into the P450 1A2s active site cavity with an orientation leading to no reactive binding, causing it to act as a competitive inhibitor.
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