Synthetic inhibitors of cytochrome P-450 2A6: inhibitory activity, difference spectra, mechanism of inhibition, and protein cocrystallization

Inhibitory Effect of Two Carbonic Anhydrases Inhibitors on the Activity of Major Cytochrome P450 Enzymes

BACKGROUND AND OBJECTIVES

Both AW-9A (coumarin derivative) and WES-1 (sulfonamide derivative) were designed and synthesized as potential selective carbonic anhydrase inhibitors and were tested for anticancer activity. This study was undertaken to investigate their potential inhibitory effects on the major human cytochrome P450 (CYP) drug-metabolizing enzymes.

METHODS

Specific CYP probe substrates and validated analytical methods were used to measure the activity of the tested CYP enzymes. Furthermore, in silico simulations were conducted to understand how AW-9A and WES-1 bind to CYP2A6 at a molecular level. Molecular docking experiments were performed using the high-resolution X-ray structure, Protein Data Bank (PDB) ID: 2FDV for CYP2A6.

RESULTS

CYP2E1-catalyzed chlorzoxazone-6'-hydroxylation was strongly inhibited by AW-9A and WES-1 with IC50 values of 0.084 µM and 0.101 µM, respectively. CYP2A6-catalyzed coumarin-7'-hydroxylation was moderately inhibited by AW-9A (IC50 = 4.2 µM). CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 enzymes were weakly or negligibly inhibited by both agents. Docking studies suggest elevated potential to block the catalytic activity of CYP2A6.

CONCLUSIONS

These findings point to the feasibility of utilizing these agents as promising chemopreventive agents (owing to inhibition of CYP2E1), and AW-9A as a smoking cessation aid (owing to inhibition of CYP2A6). Additional in-vivo studies should be conducted to examine the impact of CYP2A6 and CYP2E1 inhibition on drug interactions with probe substrates of these enzymes.

Cytochrome P450 Enzymes as Drug Targets in Human Disease

Although the mention of cytochrome P450 (P450) inhibition usually brings to mind unwanted variability in pharmacokinetics, in several cases P450s are good targets for inhibition. These P450s are essential, but in certain disease states, it is desirable to reduce the concentrations of their products. Most of the attention to date has been with human P450s 5A1, 11A1, 11B1, 11B2, 17A1, 19A1, and 51A1. In some of those cases, there are multiple drugs in use, e.g., exemestane, letrozole, and anastrozole with P450 19A1, the steroid aromatase target in breast cancer. There are also several targets that are less developed, e.g., P450s 2A6, 8B1, 4A11, 24A1, 26A1, and 26B1. SIGNIFICANCE STATEMENT: The selective inhibition of certain cytochrome P450s that have major physiological functions has been shown to be very efficacious in certain human diseases. In several cases, the search for better drugs continues.

Identification of New Substrates and Inhibitors of Human CYP2A7

CYP2A7 is one of the most understudied human cytochrome P450 enzymes and its contributions to either drug metabolism or endogenous biosynthesis pathways are not understood, as its only known enzymatic activities are the conversions of two proluciferin probe substrates. In addition, the CYP2A7 gene contains four single-nucleotide polymorphisms (SNPs) that cause missense mutations and have minor allele frequencies (MAFs) above 0.5. This means that the resulting amino acid changes occur in the majority of humans. In a previous study, we employed the reference standard sequence (called CYP2A7*1 in P450 nomenclature). For the present study, we created another CYP2A7 sequence that contains all four amino acid changes (Cys311, Glu169, Gly479, and Arg274) and labeled it CYP2A7-WT. Thus, it was the aim of this study to identify new substrates and inhibitors of CYP2A7 and to compare the properties of CYP2A7-WT with CYP2A7*1. We found several new proluciferin probe substrates for both enzyme variants (we also performed in silico studies to understand the activity difference between CYP2A7-WT and CYP2A7*1 on specific substrates), and we show that while they do not act on the standard CYP2A6 substrates nicotine, coumarin, or 7-ethoxycoumarin, both can hydroxylate diclofenac (as can CYP2A6). Moreover, we found ketoconazole, 1-benzylimidazole, and letrozole to be CYP2A7 inhibitors.

1-Benzylimidazole attenuates the stemness of breast cancer cells through partially targeting CYP4Z1

Cytochrome P450 (CYP) 4Z1 (CYP4Z1) has recently garnered much interest as its expression predicts a poor prognosis and as a oncogene in breast cancer, and overexpressed in other many cancers. We previously showed that CYP4Z1 acts as a promoter of cancer stem cells (CSCs) to facilitate the occurrence and development of breast cancer. Here, RNA sequencing found that 1-benzylimidazole (1-Benzy) held a preferable correlation with breast cancer and suppressed the expression of CSC makers. Further functional experiments, including mammary spheroid formation, wound-healing, transwell-invasion, detection of tumor initiation, and metastatic ability, showed that 1-Benzy suppressed the stemness and metastasis of breast cancer cells. Additionally, we further demonstrated that CYP4Z1 is necessary for 1-Benzy-mediated suppression on breast cancer stemness and 1-Benzy exerted a weaker effect in breast cancer cells with CYP4Z1 knockdown. Taken together, our data suggest that 1-Benzy might be a potential drug suppressing breast cancer stemness via targeting CYP4Z1.

Discovery of thiophen-2-ylmethylene bis-dimedone derivatives as novel WRN inhibitors for treating cancers with microsatellite instability

Microsatellite instability (MSI) is a hypermutable condition caused by DNA mismatch repair system defects, contributing to the development of various cancer types. Recent research has identified Werner syndrome ATP-dependent helicase (WRN) as a promising synthetic lethal target for MSI cancers. Herein, we report the first discovery of thiophen-2-ylmethylene bis-dimedone derivatives as novel WRN inhibitors for MSI cancer therapy. Initial computational analysis and biological evaluation identified a new scaffold for a WRN inhibitor. Subsequent SAR study led to the discovery of a highly potent WRN inhibitor. Furthermore, we demonstrated that the optimal compound induced DNA damage and apoptotic cell death in MSI cancer cells by inhibiting WRN. This study provides a new pharmacophore for WRN inhibitors, emphasizing their therapeutic potential for MSI cancers.

Synthesis and biological evaluation of coumarin derivatives as selective CYP2A6 inhibitors

Cytochrome P450 2A6 (CYP2A6) inhibitors are expected to be suitable as smoking cessation aids and for cancer prevention. Because the typical coumarin-based CYP2A6 inhibitor methoxsalen also inhibits CYP3A4, unintended drug-drug interactions are still a concern. Therefore, the development of selective CYP2A6 inhibitors is desirable. In this study, we synthesized coumarin-based molecules, determined the IC₅₀ values for CYP2A6 inhibition, verified the possibility of mechanism-based inhibition, and compared the selectivity for CYP2A6 versus CYP3A4. The results demonstrated that we developed CYP2A6 inhibitors that were more potent and selective than methoxsalen.

3D-Frameworks and 2D-networks of lanthanide coordination polymers with 3-pyridylpyrazole: photophysical and magnetic properties

A series of 15 lanthanide-containing coordination polymers, both 3D- and 2D-networks, as well as complexes of Ln-trichlorides with 3-(3-pyridyl)pyrazole (3-PyPzH), were synthesized. A large structural diversity is observed depending on the ligand content: 3∞[Ln(3-PyPzH)Cl₃], Ln = Eu and Gd, of sra topology, 2∞[Sm(3-PyPzH)Cl₃], 2∞[Ln₂(3-PyPzH)₃Cl₆]·2solv, Ln = Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Er³⁺, solv = Tol and MeCN, of sql topology and 2∞[Ln(3-PyPzH₂)Cl₄], Ln = La and Nd, of hcb topology with salt like complexes of the formula [(3-PyPzH₂)][Ln(3-PyPzH)₂Cl₄], Ln = Eu, Tb, Dy and Ho. The products were characterized by single-crystal and powder X-ray diffraction, high-temperature X-ray diffraction, differential thermal analysis and thermogravimetry (DTA/TG) combined with mass spectrometry, differential scanning calorimetry (DSC), IR-spectroscopy, UV-visible spectrophotometry, photoluminescence spectroscopy, and magnetic susceptibility. Absorption spectroscopy shows ion-specific 4f-4f transitions that can be assigned to Sm³⁺, Eu³⁺, Dy³⁺, Ho³⁺ and Er³⁺ in a wide range from the UV-VIS to NIR region. An excellent antenna effect through ligand-metal energy transfer was observed in 2∞[Tb₂(3-PyPzH)₃Cl₆]·2solv, leading to high efficiency of the luminescence indicated by a quantum yield up to 76%. Direct current magnetic susceptibility studies reveal the absence of interatomic interaction for Dy³⁺ and Er³⁺ and weak ferromagnetic interaction for Ho³⁺. Thermal analysis shows good stability up to 365 °C for 2∞[Ho₂(3-PyPzH)₃Cl₆]·2MeCN.

Protein-Ligand Identification and In Vitro Inhibitory Effects of Cathine on 11 Major Human Drug Metabolizing Cytochrome P450s

Cathine is the stable form of cathinone, the major active compound found in khat (Catha edulis Forsk) plant. Khat was found to inhibit major phase I drug metabolizing cytochrome P450 (CYP) enzyme activities in vitro and in vivo. With the upsurge of khat consumption and the potential use of cathine to combat obesity, efforts should be channelled into understanding potential cathine-drug interactions, which have been rather limited. The present study aimed to assess CYPs activity and inhibition by cathine in a high-throughput in vitro fluorescence-based enzyme assay and molecular docking analysis to identify how cathine interacts within various CYPs' active sites. The half maximal inhibitory concentration (IC50) values of cathine determined for CYP2A6 and CYP3A4 were 80 and 90 μM, while CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP2J2 and CYP3A5 showed no significant inhibition. Furthermore, in Ki analysis, the Lineweaver-Burk plots depicted non-competitive mixed inhibition of cathine on both CYP2A6 and CYP3A4 with Ki value of 63 and 100 μM, respectively. Cathine showed negligible time-dependent inhibition on CYPs. Further, molecular docking studies showed that cathine was bound to CYP2A6 via hydrophobic, hydrogen and π-stacking interactions and formed hydrophobic and hydrogen bonds with active site residues in CYP3A4. Both molecular docking prediction and in vitro outcome are in agreement, granting more detailed insights for predicting CYPs metabolism besides the possible cathine-drug interactions. Cathine-drug interactions may occur with concomitant consumption of khat or cathine-containing products with medications metabolized by CYP2A6 and CYP3A4.

Roles of cytochrome P450 enzymes in pharmacology and toxicology: Past, present, and future

The development of the cytochrome P450 (P450) field has been remarkable in the areas of pharmacology and toxicology, particularly in drug development. Today it is possible to use the knowledge base and relatively straightforward assays to make intelligent predictions about drug disposition prior to human dosing. Much is known about the structures, regulation, chemistry of catalysis, and the substrate and inhibitor specificity of human P450s. Many aspects of drug-drug interactions and side effects can be understood in terms of P450s. This knowledge has also been useful in pharmacy practice, as well as in the pharmaceutical industry and medical practice. However, there are still basic and practical questions to address regarding P450s and their roles in pharmacology and toxicology. Another aspect is the discovery of drugs that inhibit P450 to treat diseases.

Atom-economic Approach to the Synthesis of α-(Hetero)aryl-substituted Furan Derivatives from Biomass

An atom-economic ring construction approach to the synthesis of α-(hetero)arylfurans based on renewable furanic platform chemicals has been developed. Corresponding compounds have been prepared in good to excellent yields via [2+2+2] and [4+2] cycloaddition reactions using metal-catalyzed or photoredox protocols. Easily available HMF-based 2-hydroxymethyl-5-ethynylfuran and 2-hydroxymethyl-5-cyanofuran were used as starting materials. A synthetic route with an improved carbon economy factor has been implemented to achieve sustainability aim. The possible application of arylfurans as molecular conductors has been investigated by DFT calculations, which revealed excellent charge transfer properties. As a future perspective, integration of biomass processing strategy into manufacturing of molecular electronics was pointed out to achieve the aim of sustainability.

Inhibition of Cytochrome P450 Enzymes by Drugs-Molecular Basis and Practical Applications

Drug-drug interactions are a major cause of hospitalization and deaths related to drug use. A large fraction of these is due to inhibition of enzymes involved in drug metabolism and transport, particularly cytochrome P450 (P450) enzymes. Understanding basic mechanisms of enzyme inhibition is important, particularly in terms of reversibility and the use of the appropriate parameters. In addition to drug-drug interactions, issues have involved interactions of drugs with foods and natural products related to P450 enzymes. Predicting drug-drug interactions is a major effort in drug development in the pharmaceutical industry and regulatory agencies. With appropriate in vitro experiments, it is possible to stratify clinical drug-drug interaction studies. A better understanding of drug interactions and training of physicians and pharmacists has developed. Finally, some P450s have been the targets of drugs in some cancers and other disease states.

Roles of cytochrome P450 2A6 in the oxidation of flavone, 4'-hydroxyflavone, and 4'-, 3'-, and 2'-methoxyflavones by human liver microsomes

Nine forms of recombinant cytochrome P450 (P450 or CYP) enzymes were used to study roles of individual P450 enzymes in the oxidation of flavone and some other flavonoids, 4'-hydroxyflavone and 4'-, 3'-, and 2'-methoxyflavones, by human liver microsomes using LC-MS/MS analysis.As has been reported previously , 4'-, 3'-, and 2'-methoxyflavones were preferentially O-demethylated by human liver P450 enzymes to form 4'-, 3'-, and 2'-hydroxylated flavones and also 3',4'-dihydroxyflavone from the former two substrates.In comparisons of product formation by oxidation of these methoxylated flavones, CYP2A6 was found to be a major enzyme catalysing flavone 4'- and 3'-hydroxylations by human liver microsomes but did not play significant roles in 2'-hydroxylation of flavone, O-demethylations of three methoxylated flavones, and the oxidation of 4'-hydroxyflavone to 3',4'-dihydroxyflavone.The effects of anti-CYP2A6 IgG and chemical P450 inhibitors suggested that different P450 enzymes, as well as CYP2A6, catalysed oxidation of these flavonoids at different positions by liver microsomes.These studies suggest that CYP2A6 catalyses flavone 4'- and 3'-hydroxylations in human liver microsomes and that other P450 enzymes have different roles in oxidizing these flavonoids.

Identification of CYP 2A6 inhibitors in an effort to mitigate the harmful effects of the phytochemical nicotine

AIM

In this study, our goal was to study the inhibition of nicotine metabolism by P450 2A6, as a means for reduction in tobacco use and consequently the prevention of smoking-related cancers. Nicotine, a phytochemical, is an addictive stimulant, responsible for the tobacco-dependence in smokers. Many of the other phytochemicals in tobacco, including polycyclic aromatic hydrocarbons, N-nitrosamines, and aromatic amines, are potent systemic carcinogens. Tobacco smoking causes about one of every five deaths in the United States annually. Nicotine plasma concentration is maintained by the smokers' smoking behavior within a small range. Nicotine is metabolized by cytochrome P450s 2A6 and 2A13 to cotinine. This metabolism causes a decrease in nicotine plasma levels, which in turn leads to increased tobacco smoking, and increased exposure to the tobacco carcinogens.

METHODS

Using the phytochemical nicotine as a lead structure, and taking its interactions with the P450 2A6 binding pocket into consideration, new pyridine derivatives were designed and synthesized as potential selective mechanism-based inhibitors for this enzyme.

RESULTS

The design and synthesis of two series of novel pyridine-based compounds, with varying substituents and substitution locations on the pyridine ring, as well as their inhibitory activities on cytochrome P450 2A6 and their interactions with its active site are discussed here. Substitutions at position 3 of the pyridine ring with an imidazole or propargyl ether containing group showed the most optimal interactions with the P4502A6 active site.

CONCLUSION

The pyridine compounds with an imidazole or propargyl ether containing substituent on position 3 were found to be promising lead compounds for further development. Hydrogen-bonding interactions were determined to be crucial for effective binding of these molecules within the P450 2A6 active site.

Decision Making in Structure-Based Drug Discovery: Visual Inspection of Docking Results

Molecular docking is a computational method widely used in drug discovery. Due to the inherent inaccuracies of molecular docking, visual inspection of binding modes is a crucial routine in the decision making process of computational medicinal chemists. Despite its apparent importance for medicinal chemistry projects, guidelines for the visual docking pose assessment have been hardly discussed in the literature. Here, we review the medicinal chemistry literature with the aim of identifying consistent principles for visual inspection, highlighting cases of its successful application, and discussing its limitations. In this context, we conducted a survey reaching experts in both academia and the pharmaceutical industry, which also included a challenge to distinguish native from incorrect poses. We were able to collect 93 expert opinions that offer valuable insights into visually supported decision-making processes. This perspective shall motivate discussions among experienced computational medicinal chemists and guide young scientists new to the field to stratify their compounds.

Discovery of 4,4'-Dipyridylsulfide Analogs as "Switchable Electrophiles" for Covalent Inhibition

Electrophilic heterocycles offer attractive features as covalent fragments for inhibitor and probe development. A focused library of heterocycles for which protonation can enhance reactivity (called "switchable electrophiles") is screened for inhibition of the proposed drug target dimethylarginine dimethylaminohydrolase (DDAH). Several novel covalent fragments are identified: 4-chloroquinoline, 4-bromopyridazine, and 4,4-dipyridylsulfide. Mechanistic studies of DDAH inactivation by 4,4-dipyridylsulfide reveal selective covalent S-pyridinylation of the active-site Cys through catalysis by a neighboring Asp residue. Inactivation (k_inact/K_I = 0.33 M^-1 s^-1) proceeds with release of 4-thiopyridone (0.78 equiv), and structure-activity relationships reveal that the leaving group pK_a can be modulated to tune reactivity. The use of a "switchable electrophile" strategy helps impart selectivity, even to fragment-sized modifiers. Identification of 4,4-dipyridylsulfide analogs as inactivators offers an easily tunable covalent fragment with multiple derivatization sites on both the leaving and staying groups.

Liquid chromatography-tandem mass spectrometry analysis of oxidation of 2'-, 3'-, 4'- and 6-hydroxyflavanones by human cytochrome P450 enzymes

2'-Hydroxyflavanone (2'OHFva), 3'OHFva, 4'OHFva, and 6OHFva, the major oxidative products of flavanone by human cytochrome P450 (P450, CYP) enzymes, were studied in regard to further oxidation by human CYP1A1, 1A2, 1B1.1, 1B1.3, and 2A6. The products formed were analyzed with LC-MS/MS and characterized by their positive ion fragmentations on mass spectrometry. Several di-hydroxylated flavanone (diOHFva) and di-hydroxylated flavone (diOHFvo) products, detected by analyzing parent ions at m/z 257 and 255, respectively, were found following incubation of these four hydroxylated flavanones with P450s. The m/z 257 products were produced at higher levels than the latter with four substrates examined. The structures of the m/z 257 products were characterized by LC-MS/MS product ion spectra, and the results suggest that 3'OHFva and 4'OHFva are further oxidized mainly at B-ring by P450s while 6OHFva oxidation was at A-ring. Different diOHFvo products (m/z 255) were also characterized by LC-MS/MS, and the results suggested that most of these diOHFvo products were formed through oxidation or desaturation of the diOHFva products (m/z 257) by P450s. Only when 4'OHFva (m/z 241) was used as a substrate, formation of 4'OHFvo (m/z 239) was detected, indicating that diOHFvo might also be formed through oxidation of 4'OHFvo by P450s. Finally, our results indicated that CYP1 family enzymes were more active than CYP2A6 in catalyzing the oxidation of these four hydroxylated flavanones, and these findings were supported by molecular docking studies of these chemicals with active sites of P450 enzymes.

Combined Linear Interaction Energy and Alchemical Solvation Free-Energy Approach for Protein-Binding Affinity Computation

Calculating free energies of binding (ΔG_bind) between ligands and their target protein is of major interest to drug discovery and safety, yet it is still associated with several challenges and difficulties. Linear interaction energy (LIE) is an efficient in silico method for ΔG_bind computation. LIE models can be trained and used to directly calculate binding affinities from interaction energies involving ligands in the bound and unbound states only, and LIE can be combined with statistical weighting to calculate ΔG_bind for flexible proteins that may bind their ligands in multiple orientations. Here, we investigate if LIE predictions can be effectively improved by explicitly including the entropy of (de)solvation into our free-energy calculations. For that purpose, we combine LIE calculations for the protein-ligand-bound state with explicit free-energy perturbation to rigorously compute the unbound ligand's solvation free energy. We show that for 28 Cytochrome P450 2A6 (CYP2A6) ligands, coupling LIE with alchemical solvation free-energy calculation helps to improve obtained correlation between computed and reference (experimental) binding data.

Structure-activity relationship and in vitro inhibition of human cytochrome CYP2A6 and CYP2A13 by flavonoids

Cigarette smoking is one of the major risk factors of various diseases including respiratory diseases and lung cancer. While the liver-specific CYP2A6 is associated with the nicotine clearance and smoking addiction, the metabolic activation of the tobacco-specific nitrosamine by lung-specific CYP2A13 can lead to lung tumorigenesis.It has been reported that inhibition of CYP2A6 and CYP2A13 enzymes by flavonoids constituents could be an aids in smoking cessation. This study demonstrates the inhibition activity of kaempferol and myricetin and the structure-function relationship of these two flavonoids and previously isolated flavonoids from Vernonia cinerea and Pluchea indica against both enzymes.Kaempferol could inhibit CYP2A6 with K_ic value of 1.77 ± 0.47 µM while inhibit CYP2A13 with K_ic value of 0.12 ± 0.01 µM. Myricetin could inhibit CYP2A6 with K_ic value of 4.06 ± 0.52 µM while inhibit CYP2A13 with K_ic value of 1.88 ± 0.03 µM.Molecular docking indicated that CYP2A13 enzyme has strong hydrophobic interaction with ring B of flavonoids compared to CYP2A6 enzyme. The presence of the hydroxyl group at C3 position of ring C and the hydroxyl group at C5' of ring B affected inhibitory activity on both enzymes.

Conformational Response of N-Terminally Truncated Cytochrome P450 3A4 to Ligand Binding in Solution

Human cytochrome P450 3A4 (CYP3A4) is a membrane-associated monooxygenase that is responsible for metabolizing >50% of the pharmaceuticals in the current market, so studying its chemical mechanism and structural changes upon ligand binding will help provide deeper insights into drug metabolism and further drug development. The best-characterized cytochrome P450 is a bacterial form, P450cam, which undergoes significant conformational changes upon binding substrate and its redox partner, putidaredoxin. In contrast, most crystal structures of CYP3A4 with or without ligands have shown few changes, although allosteric effects and multiple-substrate binding in solution are well-documented. In this study, we use double electron-electron resonance (DEER) to measure distances between spatially separated spin-labels on CYP3A4 and molecular dynamics to interpret the DEER data. These methods were applied to a soluble N-terminally truncated CYP3A4 form, and the results show that there are few changes in the average structure upon binding ketoconazole, ritonavir, or midazolam. However, binding of midazolam, but not ketoconazole or ritonavir, resulted in a significant change in the motion and/or disorder in the F/G helix region near the substrate binding pocket. These results suggest that soluble CYP3A4 behaves in a unique way in response to inhibitor and substrate binding.

Inhibition of human cytochrome P450 2A6 by 7-hydroxycoumarin analogues: Analysis of the structure-activity relationship and isoform selectivity

Compared with coumarin, 7-hydroxycoumarin could serve as a better hit for developing CYP2A6 inhibitors. In this study, a series of 7-hydroxycoumarin and its structural analogues were collected to study their structure-activity relationship (SAR) and isoform selectivity for inhibiting CYP2A6. All tested coumarins except a C4 phenyl derivative (11) showed higher inhibitory activities for CYP2A6 over the other CYP isoforms, including CYP1A2, CYP2D6, CYP2E1, CYP3A4, CYP2C8, and CYP2C9. Of these coumarins, 6,7-dihydroxycoumarin (1) and 7,8-dihydroxycoumarin (9) were found to be potent inhibitors of CYP2A6 with IC₅₀/K_i value of 0.39/0.25 and 4.61/3.02 μM, respectively, compared to methoxalen as positive control (IC₅₀/K_i = 0.43/0.26 μM). In contrast, other coumarins showed low or decreased CYP2A6-inhibiting activities. SAR analysis showed that hydroxy groups might be important for CYP2A6 inhibition, and the rank order of sites for hydroxy substitution was C6 > C7 > C8. In addition, either hydrophobic or hydrophilic substituents introduced into C4, C6 and C8 led to a reduction in CYP2A6-inhibiting activity, and the degree of influence was dependent on the size and electrical charge of substituents. Furthermore, inhibition kinetic analysis and docking simulations demonstrated that the 8-O-glucosylated coumarin derivative (17) exhibited noncompetitive inhibition against CYP2A6, while competitive inhibition patterns were noted for the other tested coumarins. The mechanisms underlying the inhibitors binding to CYP2A6 were further investigated by molecular docking study. The findings presented herein are very helpful for developing highly selective and more potent CYP2A6 inhibitors.

Discovery of (5-Phenylfuran-2-yl)methanamine Derivatives as New Human Sirtuin 2 Inhibitors

Human sirtuin 2 (SIRT2), a member of the sirtuin family, has been considered as a promising drug target in cancer, neurodegenerative diseases, type II diabetes, and bacterial infections. Thus, SIRT2 inhibitors have been involved in effective treatment strategies for related diseases. Using previously established fluorescence-based assays for SIRT2 activity tests, the authors screened their in-house database and identified a compound, 4-(5-((3-(quinolin-5-yl)ureido)methyl)furan-2-yl)benzoic acid (20), which displayed 63 ± 5% and 35 ± 3% inhibition against SIRT2 at 100 μM and 10 μM, respectively. The structure-activity relationship (SAR) analyses of a series of synthesized (5-phenylfuran-2-yl)methanamine derivatives led to the identification of a potent compound 25 with an IC50 value of 2.47 μM, which is more potent than AGK2 (IC50 = 17.75 μM). Meanwhile, 25 likely possesses better water solubility (cLogP = 1.63 and cLogS = -3.63). Finally, the molecular docking analyses indicated that 25 fitted well with the induced hydrophobic pocket of SIRT2.

Oxidation of Flavone, 5-Hydroxyflavone, and 5,7-Dihydroxyflavone to Mono-, Di-, and Tri-Hydroxyflavones by Human Cytochrome P450 Enzymes

Biologically active plant flavonoids, including 5,7-dihydroxyflavone (57diOHF, chrysin), 4',5,7-trihydroxyflavone (4'57triOHF, apigenin), and 5,6,7-trihydroxyflavone (567triOHF, baicalein), have important pharmacological and toxicological significance, e.g., antiallergic, anti-inflammatory, antioxidative, antimicrobial, and antitumorgenic properties. In order to better understand the metabolism of these flavonoids in humans, we examined the oxidation of flavone, 5-hydroxyflavone (5OHF), and 57diOHF to various products by human cytochrome P450 (P450 or CYP) and liver microsomal enzymes. Individual human P450s and liver microsomes oxidized flavone to 6-hydroxyflavone, small amounts of 5OHF, and 11 other monohydroxylated products at different rates and also produced several dihydroxylated products (including 57diOHF and 7,8-dihydroxyflavone) from flavone. We also found that 5OHF was oxidized by several P450 enzymes and human liver microsomes to 57diOHF and further to 567triOHF, but the turnover rates in these reactions were low. Interestingly, both CYP1B1.1 and 1B1.3 converted 57diOHF to 567triOHF at turnover rates (on the basis of P450 contents) of >3.0 min^-1, and CYP1A1 and 1A2 produced 567triOHF at rates of 0.51 and 0.72 min^-1, respectively. CYP2A13 and 2A6 catalyzed the oxidation of 57diOHF to 4'57triOHF at rates of 0.7 and 0.1 min^-1, respectively. Our present results show that different P450s have individual roles in oxidizing these phytochemical flavonoids and that these reactions may cause changes in their biological and toxicological properties in mammals.

Site-specific oxidation of flavanone and flavone by cytochrome P450 2A6 in human liver microsomes

The roles of human cytochrome P450 (P450 or CYP) 2A6 in the oxidation of flavanone [(2R)- and (2S)-enantiomers] and flavone were studied in human liver microsomes and recombinant human P450 enzymes. CYP2A6 was highly active in oxidizing flavanone to form flavone, 2'-hydroxy-, 4'-, and 6-hydroxyflavanones and in oxidizing flavone to form mono- and di-hydroxylated products, such as mono-hydroxy flavones M6, M7, and M11 and di-hydroxy flavones M3, M4, and M5. Liver microsomes prepared from human sample HH2, defective in coumarin 7-hydroxylation activity, were very inefficient in forming 2'-hydroxyflavanone from flavanone and a mono-hydroxylated product, M6, from flavone. Coumarin and anti-CYP2A6 antibodies strongly inhibited the formation of these metabolites in microsomes prepared from liver samples HH47 and 54, which were active in coumarin oxidation activities. Molecular docking analysis showed that the C2'-position of (2R)-flavanone (3.8 Å) was closer to the iron center of CYP2A6 than the C6-position (10 Å), while distances from C2' and C6 of (2S)-flavanone to the CYP2A6 were 6.91 Å and 5.42 Å, respectively. These results suggest that CYP2A6 catalyzes site-specific oxidation of (racemic) flavanone and also flavone in human liver microsomes. CYP1A2 and CYP2B6 were also found to play significant roles in some of the oxidations of these flavonoids by human liver microsomes.

Identification of the 4-Position of 3-Alkynyl and 3-Heteroaromatic Substituted Pyridine Methanamines as a Key Modification Site Eliciting Increased Potency and Enhanced Selectivity for Cytochrome P-450 2A6 Inhibition

Cigarette smoking causes nearly one in every five deaths in the United States. The development of a specific inhibitor of cytochrome P450 2A6 (CYP2A6), the major nicotine-metabolizing enzyme in humans, which could be prescribed for the cessation of cigarette smoking, has been undertaken. To further refine the structure activity relationship of CYP2A6, previously synthesized 3-alkynyl and 3-heteroaromatic substituted pyridine methanamines were used as lead compounds. Isosteric pyridine replacement and appendage of all available positions around the pyridine ring with a methyl group was performed to identify a modification that would increase CYP2A6 inhibition potency, which led to 4g (IC₅₀ = 0.055 mM) as a primary analogue. Potent compounds were evaluated for CYP selectivity, human liver microsomal half-life, and compliance with the rules of five. Top compounds (i.e., 6i, IC₅₀ = 0.017 mM, >120 min half-life) are poised for further development as treatments for smoking and tobacco use cessation.

Cytochrome P450 2A6 and other human P450 enzymes in the oxidation of flavone and flavanone

1. We previously reported that flavone and flavanone interact spectrally with cytochrome P450 (P450 or CYP) 2A6 and 2A13 and other human P450s and inhibit catalytic activities of these P450 enzymes. In this study, we studied abilities of CYP1A1, 1A2, 1B1, 2A6, 2A13, 2C9 and 3A4 to oxidize flavone and flavanone. 2. Human P450s oxidized flavone to 6- and 5-hydroxylated flavones, seven uncharacterized mono-hydroxylated flavones, and five di-hydroxylated flavones. CYP2A6 was most active in forming 6-hydroxy- and 5-hydroxyflavones and several mono- and di-hydroxylated products. 3. CYP2A6 was also very active in catalyzing flavanone to form 2'- and 6-hydroxyflavanones, the major products, at turnover rates of 4.8 min^-1 and 1.3 min^-1, respectively. Other flavanone metabolites were 4'-, 3'- and 7-hydroxyflavanone, three uncharacterized mono-hydroxylated flavanones and five mono-hydroxylated flavones, including 6-hydroxyflavone. CYP2A6 catalyzed flavanone to produce flavone at a turnover rate of 0.72 min^-1 that was ∼3-fold higher than that catalyzed by CYP2A13 (0.29 min^-1). 4. These results indicate that CYP2A6 and other human P450s have important roles in metabolizing flavone and flavanone, two unsubstituted flavonoids, present in dietary foods. Chemical mechanisms of P450-catalyzed desaturation of flavanone to form flavone are discussed.

Systematic Structure-Activity Relationship (SAR) Exploration of Diarylmethane Backbone and Discovery of A Highly Potent Novel Uric Acid Transporter 1 (URAT1) Inhibitor

In order to systematically explore and better understand the structure-activity relationship (SAR) of a diarylmethane backbone in the design of potent uric acid transporter 1 (URAT1) inhibitors, 33 compounds (1a-1x and 1ha-1hi) were designed and synthesized, and their in vitro URAT1 inhibitory activities (IC₅₀) were determined. The three-round systematic SAR exploration led to the discovery of a highly potent novel URAT1 inhibitor, 1h, which was 200- and 8-fold more potent than parent lesinurad and benzbromarone, respectively (IC₅₀ = 0.035 μM against human URAT1 for 1h vs. 7.18 μM and 0.28 μM for lesinurad and benzbromarone, respectively). Compound 1h is the most potent URAT1 inhibitor discovered in our laboratories so far and also comparable to the most potent ones currently under development in clinical trials. The present study demonstrates that the diarylmethane backbone represents a very promising molecular scaffold for the design of potent URAT1 inhibitors.

Nicotine Component of Cigarette Smoke Extract (CSE) Decreases the Cytotoxicity of CSE in BEAS-2B Cells Stably Expressing Human Cytochrome P450 2A13

Cytochrome P450 2A13 (CYP2A13), an extrahepatic enzyme mainly expressed in the human respiratory system, has been reported to mediate the metabolism and toxicity of cigarette smoke. We previously found that nicotine inhibited 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism by CYP2A13, but its influence on other components of cigarette smoke remains unclear. The nicotine component of cigarette smoke extract (CSE) was separated, purified, and identified using high-performance liquid chromatography (HPLC) and ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS), splitting CSE into a nicotine section (CSE-N) and nicotine-free section (CSE-O). Cell viability and apoptosis by Cell Counting Kit-8 (CCK-8) and flow cytometry assays were conducted on immortalized human bronchial epithelial (BEAS-2B) cells stably expressing CYP2A13 (B-2A13) or vector (B-V), respectively. Interestingly, CSE and CSE-O were toxic to BEAS-2B cells whereas CSE-N showed less cytotoxicity. CSE-O was more toxic to B-2A13 cells than to B-V cells (IC₅₀ of 2.49% vs. 7.06%), which was flatted by 8-methoxypsoralen (8-MOP), a CYP inhibitor. CSE-O rather than CSE or CSE-N increased apoptosis of B-2A13 cells rather than B-V cells. Accordingly, compared to CSE-N and CSE, CSE-O significantly changed the expression of three pairs of pro- and anti-apoptotic proteins, Bcl-2 Associated X Protein/B cell lymphoma-2 (Bax/Bcl-2), Cleaved Poly (Adenosine Diphosphate-Ribose) Polymerase/Poly (Adenosine Diphosphate-Ribose) Polymerase (C-PARP/PARP), and C-caspase-3/caspase-3, in B-2A13 cells. In addition, recombination of CSE-N and CSE-O (CSE-O/N) showed similar cytotoxicity and apoptosis to the original CSE. These results demonstrate that the nicotine component decreases the metabolic activation of CYP2A13 to CSE and aids in understanding the critical role of CYP2A13 in human respiratory diseases caused by cigarette smoking.

Intersection of the Roles of Cytochrome P450 Enzymes with Xenobiotic and Endogenous Substrates: Relevance to Toxicity and Drug Interactions

Today much is known about cytochrome P450 (P450) enzymes and their catalytic specificity, but the range of reactions catalyzed by each still continues to surprise. Historically, P450s had been considered to be involved in either the metabolism of xenobiotics or endogenous chemicals, in the former case playing a generally protective role and in the latter case a defined physiological role. However, the line of demarcation is sometimes blurred. It is difficult to be completely specific in drug design, and some P450s involved in the metabolism of steroids and vitamins can be off-targets. In a number of cases, drugs have been developed that act on some of those P450s as primary targets, e.g., steroid aromatase inhibitors. Several of the P450s involved in the metabolism of endogenous substrates are less specific than once thought and oxidize several related structures. Some of the P450s that primarily oxidize endogenous chemicals have been shown to oxidize xenobiotic chemicals, even in a bioactivation mode.

Menthol Smokers: Metabolomic Profiling and Smoking Behavior

BACKGROUND

The use of menthol in cigarettes and marketing is under consideration for regulation by the FDA. However, the effects of menthol on smoking behavior and carcinogen exposure have been inconclusive. We previously reported metabolomic profiling for cigarette smokers, and novelly identified a menthol-glucuronide (MG) as the most significant metabolite directly related to smoking. Here, MG is studied in relation to smoking behavior and metabolomic profiles.

METHODS

This is a cross-sectional study of 105 smokers who smoked two cigarettes in the laboratory one hour apart. Blood nicotine, MG, and exhaled carbon monoxide (CO) boosts were determined (the difference before and after smoking). Spearman correlation, χ2, and ANCOVA adjusted for gender, race, and cotinine levels for menthol smokers assessed the relationship of MG boost, smoking behavior, and metabolic profiles. Multivariate metabolite characterization using supervised partial least squares-discriminant analysis (PLS-DA) was carried out for the classification of metabolomics profiles.

RESULTS

MG boost was positively correlated with CO boost, nicotine boost, average puff volume, puff duration, and total smoke exposure. Classification using PLS-DA, MG was the top metabolite discriminating metabolome of menthol versus nonmenthol smokers. Among menthol smokers, 42 metabolites were significantly correlated with MG boost, which linked to cellular functions, such as of cell death, survival, and movement.

CONCLUSIONS

Plasma MG boost is a new smoking behavior biomarker that may provide novel information over self-reported use of menthol cigarettes by integrating different smoking measures for understanding smoking behavior and harm of menthol cigarettes.

IMPACT

These results provide insight into the biological effect of menthol smoking. Cancer Epidemiol Biomarkers Prev; 26(1); 51-60. ©2016 AACR.

Oxidation of Acenaphthene and Acenaphthylene by Human Cytochrome P450 Enzymes

Acenaphthene and acenaphthylene, two known environmental polycyclic aromatic hydrocarbon (PAH)pollutants, were incubated at 50 μM concentrations in a standard reaction mixture with human P450s 2A6, 2A13, 1B1,1A2, 2C9, and 3A4, and the oxidation products were determined using HPLC and LC-MS. HPLC analysis showed that P450 2A6 converted acenaphthene and acenaphthylene to several mono- and dioxygenated products. LC-MS analysis of acenaphthene oxidation by P450s indicated the formation of1-acenaphthenol as a major product, with turnover rates of 6.7,4.5, and 3.6 nmol product formed/min/nmol P450 for P4502A6, 2A13, and 1B1, respectively. Acenaphthylene oxidation by P450 2A6 showed the formation of 1,2-epoxyacenaphthene as a major product (4.4 nmol epoxide formed/min/nmol P450) and also several mono- and dioxygenated products.P450 2A13, 1B1, 1A2, 2C9, and 3A4 formed 1,2-epoxyacenaphthene at rates of 0.18, 5.3 2.4, 0.16, and 3.8 nmol/min/nmol P450, respectively. 1-Acenaphthenol, which induced Type I binding spectra with P450 2A13, was further oxidized by P450 2A13 but not P450 2A6. 1,2-Epoxyacenaphthene induced Type I binding spectra with P450 2A6 and 2A13 (K(s) 1.8 and 0.16 μM,respectively) and was also oxidized to several oxidation products by these P450s. Molecular docking analysis suggested different orientations of acenaphthene, acenaphthylene, 1-acenaphthenol, and 1,2-epoxyacenaphthene in their interactions with P450 2A6a nd 2A13. Neither of these four PAHs induced umu gene expression in a Salmonella typhimurium NM tester strain. These results suggest, for the first time, that acenaphthene and acenaphthylene are oxidized by human P450s 2A6 and 2A13 and other P450s to form several mono- and dioxygenated products. The results are of use in considering the biological and toxicological significance of these environmental PAHs in humans.

Inhibitory effects and oxidation of 6-methylcoumarin, 7-methylcoumarin and 7-formylcoumarin via human CYP2A6 and its mouse and pig orthologous enzymes

1. Information about the metabolism of compounds is essential in drug discovery and development, risk assessment of chemicals and further development of predictive methods. 2. In vitro and in silico methods were applied to evaluate the metabolic and inhibitory properties of 6-methylcoumarin, 7-methylcoumarin and 7-formylcoumarin with human CYP2A6, mouse CYP2A5 and pig CYP2A19. 3. 6-Methylcoumarin was oxidized to fluorescent 7-hydroxy-6-methylcoumarin by CYP2A6 (Km: 0.64-0.91 µM; Vmax: 0.81-0.89 min(-1)) and by CYP2A5 and CYP2A19. The reaction was almost completely inhibited at 10 µM 7-methylcoumarin in liver microsomes of human and mouse, but in pig only 40% inhibition was obtained with the anti-CYP2A5 antibody or with methoxsalen and pilocarpine. 7-Methylcoumarin was a mechanism-based inhibitor for CYP2A6, but not for the mouse and pig enzymes. 7-Formylcoumarin was a mechanism-based inhibitor for CYP2As of all species. 4. Docking and molecular dynamics simulations of 6-methylcoumarin and 7-methylcoumarin in the active sites of CYP2A6 and CYP2A5 demonstrated a favorable orientation of the 7-position of 6-methylcoumarin towards the heme moiety. Several orientations of 7-methylcoumarin were possible in CYP2A6 and CYP2A5. 5. These results indicate that the active site of CYP2A6 has unique interaction properties for ligands and differs in this respect from CYP2A5 and CYP2A19.

An efficient continuous flow approach to furnish furan-based biaryls

Suzuki cross-couplings of 5-formyl-2-furanylboronic acid with activated or neutral aryl bromides were performed under continuous flow conditions in the presence of (Bu)4N(+)F(-) and the immobilised t-butyl based palladium catalyst CatCart™ FC1032™. Deactivated aryl bromides and activated aryl chlorides were cross-coupled with 5-formyl-2-furanylboronic in the presence of (Bu)4N(+)OAc(-) using the bis-triphenylphosphine CatCart™ PdCl2(PPh3)2-DVB. Initial evidence indicates the latter method may serve as a universal approach to conduct Suzuki cross-couplings with the protocol successfully employed in the synthesis of the current gold standard Hedgehog pathway inhibitor LDE225.

Rational design of novel CYP2A6 inhibitors

Inhibition of CYP2A6-mediated nicotine metabolism can reduce cigarette smoking. We sought potent and selective CYP2A6 inhibitors to be used as leads for drugs useful in smoking reduction therapy, by evaluating CYP2A6 inhibitory effect of novel formyl, alkyl amine or carbonitrile substituted aromatic core structures. The most potent CYP2A6 inhibitors were thienopyridine-2-carbaldehyde, benzothienophene-3-ylmethanamine, benzofuran-5-carbaldehyde and indole-5-carbaldehyde, with IC50 values below 0.5 μM for coumarin 7-hydroxylation. Nicotine oxidation was effectively inhibited in vitro by two alkyl amine compounds and benzofuran-5-carbonitrile. Some of these molecules could serve as potential lead molecules when designing CYP2A6 inhibitory drugs for smoking reduction therapy.

Inhibition of human cytochrome P450 2E1 and 2A6 by aldehydes: structure and activity relationships

The purpose of this study was to probe active site structure and dynamics of human cytochrome P4502E1 and P4502A6 using a series of related short chain fatty aldehydes. Binding efficiency of the aldehydes was monitored via their ability to inhibit the binding and activation of the probe substrates p-nitrophenol (2E1) and coumarin (2A6). Oxidation of the aldehydes was observed in reactions with individually expressed 2E1, but not 2A6, suggesting alternate binding modes. For saturated aldehydes the optimum chain length for inhibition of 2E1 was 9 carbons (KI=7.8 ± 0.3 μM), whereas for 2A6 heptanal was most potent (KI=15.8 ± 1.1 μM). A double bond in the 2-position of the aldehyde significantly decreased the observed KI relative to the corresponding saturated compound in most cases. A clear difference in the effect of the double bond was observed between the two isoforms. With 2E1, the double bond appeared to remove steric constraints on aldehyde binding with KI values for the 5-12 carbon compounds ranging between 2.6 ± 0.1 μM and 12.8 ± 0.5 μM, whereas steric effects remained the dominant factor in the binding of the unsaturated aldehydes to 2A6 (observed KI values between 7.0 ± 0.5 μM and >1000 μM). The aldehyde function was essential for effective inhibition, as the corresponding carboxylic acids had very little effect on enzyme activity over the same range of concentrations, and branching at the 3-position of the aldehydes increased the corresponding KI value in all cases examined. The results suggest that a conjugated π-system may be a key structural determinant in the binding of these compounds to both enzymes, and may also be an important feature for the expansion of the active site volume in 2E1.

Binding free energies for nicotine analogs inhibiting cytochrome P450 2A6 by a combined use of molecular dynamics simulations and QM/MM-PBSA calculations

Molecular dynamics (MD) simulations and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations have been performed to explore the dynamic behaviors of cytochrome P450 2A6 (CYP2A6) binding with nicotine analogs (that are typical inhibitors) and to calculate their binding free energies in combination with Poisson-Boltzmann surface area (PBSA) calculations. The combined MD simulations and QM/MM-PBSA calculations reveal that the most important structural parameters affecting the CYP2A6-inhibitor binding affinity are two crucial internuclear distances, that is, the distance between the heme iron atom of CYP2A6 and the coordinating atom of the inhibitor, and the hydrogen-bonding distance between the N297 side chain of CYP2A6 and the pyridine nitrogen of the inhibitor. The combined MD simulations and QM/MM-PBSA calculations have led to dynamic CYP2A6-inhibitor binding structures that are consistent with the observed dynamic behaviors and structural features of CYP2A6-inhibitor binding, and led to the binding free energies that are in good agreement with the experimentally-derived binding free energies. The agreement between the calculated binding free energies and the experimentally-derived binding free energies suggests that the combined MD and QM/MM-PBSA approach may be used as a valuable tool to accurately predict the CYP2A6-inhibitor binding affinities in future computational design of new, potent and selective CYP2A6 inhibitors.

Electrochemical detection of human cytochrome P450 2A6 inhibition: a step toward reducing dependence on smoking

Inhibition of human cytochrome P450 2A6 has been demonstrated to play an important role in nicotine metabolism and consequent smoking habits. Here, the "molecular Lego" approach was used to achieve the first reported electrochemical signal of human CYP2A6 and to improve its catalytic efficiency on electrode surfaces. The enzyme was fused at the genetic level to flavodoxin from Desulfovibrio vulgaris (FLD) to create the chimeric CYP2A6-FLD. Electrochemical characterization by cyclic voltammetry shows clearly defined redox transitions of the haem domain in both CYP2A6 and CYP2A6-FLD. Electrocatalysis experiments using coumarin as substrate followed by fluorimetric quantification of the product were performed with immobilized CYP2A6 and CYP2A6-FLD. Comparison of the kinetic parameters showed that coumarin catalysis was carried out with a higher efficiency by the immobilized CYP2A6-FLD, with a calculated kcat value significantly higher (P < 0.005) than that of CYP2A6, whereas the affinity for the substrate (KM) remained unaltered. The chimeric system was also successfully used to demonstrate the inhibition of the electrochemical activity of the immobilized CYP2A6-FLD, toward both coumarin and nicotine substrates, by tranylcypromine, a potent and selective CYP2A6 inhibitor. This work shows that CYP2A6 turnover efficiency is improved when the protein is linked to the FLD redox module, and this strategy can be utilized for the development of new clinically relevant biotechnological approaches suitable for deciphering the metabolic implications of CYP2A6 polymorphism and for the screening of CYP2A6 substrates and inhibitors.

TRAM-34, a putatively selective blocker of intermediate-conductance, calcium-activated potassium channels, inhibits cytochrome P450 activity

TRAM-34, a clotrimazole analog characterized as a potent and selective inhibitor of intermediate-conductance, calcium-activated K⁺ (IK_Ca) channels, has been used extensively in vitro and in vivo to study the biological roles of these channels. The major advantage of TRAM-34 over clotrimazole is the reported lack of inhibition of the former drug on cytochrome P450 (CYP) activity. CYPs, a large family of heme-containing oxidases, play essential roles in endogenous signaling and metabolic pathways, as well as in xenobiotic metabolism. However, previously published work has only characterized the effects of TRAM-34 on a single CYP isoform. To test the hypothesis that TRAM-34 may inhibit some CYP isoforms, the effects of this compound were presently studied on the activities of four rat and five human CYP isoforms. TRAM-34 inhibited recombinant rat CYP2B1, CYP2C6 and CYP2C11 and human CYP2B6, CYP2C19 and CYP3A4 with IC₅₀ values ranging from 0.9 µM to 12.6 µM, but had no inhibitory effects (up to 80 µM) on recombinant rat CYP1A2, human CYP1A2, or human CYP19A1. TRAM-34 also had both stimulatory and inhibitory effects on human CYP3A4 activity, depending on the substrate used. These results show that low micromolar concentrations of TRAM-34 can inhibit several rat and human CYP isoforms, and suggest caution in the use of high concentrations of this drug as a selective IK_Ca channel blocker. In addition, in vivo use of TRAM-34 could lead to CYP-related drug-drug interactions.

Structural diversity of eukaryotic membrane cytochrome p450s

X-ray crystal structures are available for 29 eukaryotic microsomal, chloroplast, or mitochondrial cytochrome P450s, including two non-monooxygenase P450s. These structures provide a basis for understanding structure-function relations that underlie their distinct catalytic activities. Moreover, structural plasticity has been characterized for individual P450s that aids in understanding substrate binding in P450s that mediate drug clearance.