More potent inhibition of human CYP2A6 than mouse CYP2A5 enzyme activities by derivatives of phenylethylamine and benzaldehyde

Non-nicotine constituents in cigarette smoke extract enhance nicotine addiction through monoamine oxidase A inhibition

Tobacco addiction has been largely attributed to nicotine, a component in tobacco leaves and smoke. However, extensive evidence suggests that some non-nicotine components of smoke should not be overlooked when considering tobacco dependence. Yet, their individual effect and synergistic effect on nicotine reinforcement remain poorly understood. The study herein focused on the role of non-nicotine constituents in promoting the effects of nicotine and their independent reinforcing effects. Denicotinized cigarettes were prepared by chemical extracting of cut tobacco, and the cigarette smoke extracts (CSE, used as a proxy for non-nicotine ingredients) were obtained by machine-smoking the cigarettes and DMSO extraction. The compositions of harmful components, nicotine, and other minor alkaloids in both cut tobacco and the CSE of experimental denicotinized cigarettes were examined by GC-MS, and compared with 3R4F reference cigarettes. individually and in synergy with nicotine were determined by conditioned place preference (CPP), dopamine (DA) level detection, the open field test (OFT), and the elevated plus maze (EPM). Finally, the potential enhancement mechanism of non-nicotinic constituents was investigated by nicotine metabolism and monoamine oxidase A (MAOA) activity inhibition in the striatum of mice and human recombinant MAOA. Thenicotine content in smoke from the experimental denicotinized cigarettes (under ISO machine-smoking conditions) was reduced by 95.1% and retained most minor alkaloids, relative to the 3R4F reference cigarettes. It was found that non-nicotine constituents increased acute locomotor activities. This was especially pronounced for DA levels in NAc and CPP scores, decreased the time in center zone. There were no differences in these metrics with DNC group when compared to the NS group. Non-nicotine constituents alone did not show reinforcing effects in CPP or striatum DA levels in mice. However, in the presence of nicotine, non-nicotine constituents further increased the reinforcing effects. Furthermore, non-nicotine constituents may enhance nicotine's reinforcing effects by inhibiting striatum MAOA activity rather than affecting nicotine metabolism or total striatum DA content in mice. These findings expand our knowledge of the effect on smoking reinforcement of non-nicotine constituents found in tobacco products.

Impact of E-Cigarette Liquid Flavoring Agents on Activity of Microsomal Recombinant CYP2A6, the Primary Nicotine-Metabolizing Enzyme

Nicotine is the primary psychoactive chemical in both traditional and electronic cigarettes (e-cigarettes). Nicotine levels in both traditional cigarettes and e-cigarettes are an important concern for public health. Nicotine exposure due to e-cigarette use is of importance primarily due to the addictive potential of nicotine, but there is also concern for nicotine poisoning in e-cigarette users. Nicotine concentrations in e-liquids vary widely. Additionally, there is significant genetic variability in the rate of metabolism of nicotine due to polymorphisms of CYP2A6, the enzyme responsible for the metabolism of approximately 80% of nicotine. Recent studies have shown CYP2A6 activity is also reduced by aromatic aldehydes such as those added to e-liquids as flavoring agents, which may increase nicotine serum concentrations. However, the impacts of flavored e-liquids on CYP2A6 activity are unknown. In this study, we investigated the impact of three flavored e-liquids on microsomal recombinant CYP2A6. Microsomal recombinant CYP2A6 was challenged at e-liquid concentrations ranging up to 0.125% (v/v) and monitored for metabolic activity using a probe molecule approach. Two e-liquids exhibited dose-dependent inhibition of CYP2A6 activity. Mass spectrometry was conducted to identify flavoring agents in flavored e-liquids that inhibited CYP2A6. Microsomal recombinant CYP2A6 was subsequently exposed to flavoring agents at concentrations ranging from 0.03 μM to 500 μM. Cinnamaldehyde and benzaldehyde were found to be the most potent inhibitors of microsomal CYP2A6 of the flavoring agents tested, with identified IC₅₀ values of 1.1 μM and 3.0 μM, respectively. These data indicate certain aromatic aldehyde flavoring agents are potent inhibitors of CYP2A6, which may reduce nicotine metabolism in vivo. These findings indicate an urgent need to evaluate the effects of flavoring agents in e-cigarette liquids on the pharmacokinetics of nicotine in vivo.

The Clinical Pharmacokinetics of Amphetamines Utilized in the Treatment of Attention-Deficit/Hyperactivity Disorder

Amphetamine (AMP), an indirectly acting psychostimulant approved for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children, adolescents, and adults, is among the most long-standing therapeutic agents in all of clinical psychopharmacology. This review focuses on AMP absorption, metabolism, and elimination brought to bear on comparative pharmacokinetics in its various formulations. A comprehensive search of the published literature was conducted using MEDLINE (PubMed) and Google Scholar databases through April 2017 to retrieve all pertinent in vitro and human studies for review and synthesis. Additionally, Food and Drug Administration (FDA) databases were accessed for otherwise unavailable data when possible. Initially available as racemic (dl)-AMP, this drug was later supplanted by enantiopure (d)-AMPH or enantioenriched (75:25 dl)-AMP formulations; although racemic AMP returned as an approved drug to treat ADHD in 2014. Presently, there are several immediate-release (IR) formulations available, including d-AMP, dl-AMP, and mixed amphetamine salts, which are neither racemic nor the pure d-enantiomer (i.e., a 3:1 mixture of d-AMP and l-AMP). Furthermore, new modified-release AMP formulations, including an oral suspension and an orally disintegrating tablet, are now available. A lysine-bonded prodrug form of d-AMP also serves as a treatment option. Oral AMP is rapidly absorbed, with high absolute bioavailability, followed by extensive metabolism involving multiple enzymes. Some metabolic pathways exhibit stereoselective biotransformations favoring the l-isomer substrate. Drug exposure exhibits dose-proportional pharmacokinetics. Body weight is a fundamental determinant of differences in observed AMP plasma concentrations. IR formulations typically provide a T_max from 2 to 3 hours. In replicated studies, children exhibit a shorter plasma T_1/2 (∼7 hours) relative to adults (∼10 to 12 hours). There are few documented pharmacokinetic drug interactions of clinical significance beyond influences of drug-induced alteration of urinary pH. The array of AMP formulations addressed in this review offer flexibility in dosing, drug onset, and offset to assist in individualized pharmacotherapy of ADHD.

Lisdexamfetamine Dimesylate: Prodrug Delivery, Amphetamine Exposure and Duration of Efficacy

Lisdexamfetamine dimesylate (LDX) is a long-acting d-amphetamine prodrug used to treat attention-deficit/hyperactivity disorder (ADHD) in children, adolescents and adults. LDX is hydrolysed in the blood to yield d-amphetamine, and the pharmacokinetic profile of d-amphetamine following oral administration of LDX has a lower maximum plasma concentration (C_max), extended time to C_max (T_max) and lower inter- and intra-individual variability in exposure compared with the pharmacokinetic profile of an equivalent dose of immediate-release (IR) d-amphetamine. The therapeutic action of LDX extends to at least 13 h post-dose in children and 14 h post-dose in adults, longer than that reported for any other long-acting formulation. Drug-liking scores for LDX are lower than for an equivalent dose of IR d-amphetamine, which may result from the reduced euphorigenic potential associated with its pharmacokinetic profile. These pharmacokinetic and pharmacodynamic characteristics of LDX may be beneficial in the management of symptoms in children, adolescents and adults with ADHD.

Celery extract inhibits mouse CYP2A5 and human CYP2A6 activities via different mechanisms

Human cytochrome P450 (CYP) 2A6 participates in the metabolism of nicotine and precarcinogens, thus the deliberate inhibition of CYP2A6 may reduce cigarette consumption and therefore reduce the risk of developing the types of cancer associated with smoking. The inhibitory effects and mechanisms of celery (Apium graveolens) extract on mouse CYP2A5 and human CYP2A6 activity remain unclear. These effects were investigated in mouse and human liver microsomes using coumarin 7-hydroxylation in a probe reaction. Celery extract reduced CYP2A5 and CYP2A6 activities in vitro in a dose-dependent manner. In vivo experiments also showed that celery extract markedly decreased CYP2A5 activity. The inhibition of celery extract on CYP2A5 was time- and nicotinamide adenine dinucleotide phosphate (NADPH)-independent, and was markedly reduced by ultracentrifugation. Additionally, the inhibition of celery extract on CYP2A6 was time and NADPH-dependent. Levels of inhibition were characterized by a K_i, the measure of the tightness of bonds between the enzyme and its inhibitor, of 266.4 µg/ml for CYP2A5, and a K_i of 1,018 µg/ml and K_inact of 0.3/min for CYP2A6. K_inact is the maximal rate of enzyme inactivation at a saturating concentration of inhibitor. The coumarin derivative 5-methoxypsoralen present in celery extract did not solely to the inhibition of CYP2A5/6 activity. In conclusion, celery extract inhibited the levels of mouse CYP2A5 and human CYP2A6 activity via different mechanisms: Mixed competitive inhibition for CYP2A5 and mechanism-based inhibition for CYP2A6.

An Inventory of Methods for the Assessment of Additive Increased Addictiveness of Tobacco Products

BACKGROUND

Cigarettes and other forms of tobacco contain the addictive drug nicotine. Other components, either naturally occurring in tobacco or additives that are intentionally added during the manufacturing process, may add to the addictiveness of tobacco products. As such, these components can make cigarette smokers more easily and heavily dependent.Efforts to regulate tobacco product dependence are emerging globally. Additives that increase tobacco dependence will be prohibited under the new European Tobacco Product Directive.

OBJECTIVE

This article provides guidelines and recommendations for developing a regulatory strategy for assessment of increase in tobacco dependence due to additives. Relevant scientific literature is summarized and criteria and experimental studies that can define increased dependence of tobacco products are described.

CONCLUSIONS

Natural tobacco smoke is a very complex matrix of components, therefore analysis of the contribution of an additive or a combination of additives to the level of dependence on this product is challenging. We propose to combine different type of studies analyzing overall tobacco product dependence potential and the functioning of additives in relation to nicotine. By using a combination of techniques, changes associated with nicotine dependence such as behavioral, physiological, and neurochemical alterations can be examined to provide sufficient information.Research needs and knowledge gaps will be discussed and recommendations will be made to translate current knowledge into legislation. As such, this article aids in implementation of the Tobacco Product Directive, as well as help enable regulators and researchers worldwide to develop standards to reduce dependence on tobacco products.

IMPLICATIONS

This article provides an overall view on how to assess tobacco product constituents for their potential contribution to use and dependence. It provides guidelines that help enable regulators worldwide to develop standards to reduce dependence on tobacco products and guide researches to set research priorities on this topic.

Applying human and pig hepatic in vitro experiments for sulfur mustard study: screening and identification of metabolites by liquid chromatography/tandem mass spectrometry

RATIONALE

Sulfur mustard is a chemical warfare agent (CWA) with high toxicity and complex metabolism. This study aimed at identification of new metabolic biomarkers for sulfur mustard using in in vitro exposures and various mass spectrometric techniques.

METHODS

Human and pig liver subcellular fractions were used as biocatalysts. Metabolites were screened by liquid chromatography and tandem mass spectrometry (LC/MS/MS) using positive electrospray ionization (ESI). For structural identification, product ion scans (MS/MS, MS(3) ) and accurate mass measurements using liquid chromatography/time-of-flight mass spectrometry (LC/TOFMS) were acquired.

RESULTS

Sulfur mustard is metabolized in vitro by S-oxidation and glutathione (GSH) conjugations. One S-oxidized metabolite, bis(2-chloroethyl) sulfoxide (m/z 175), was formed in both species only when liver microsomes were present in incubations, and it was the main metabolite if GSH was not added into the reaction mixture. However, conjugation with GSH was found to be a spontaneous reaction in physiological pH and buffered solution. Three GSH conjugates of sulfur mustard were detected and identified, among which two were novel; 2-((2-(S-glutathionyl)ethyl)thio)ethanol (m/z 412) and 2-((2-(S-glutathionyl)ethyl)thio)ethyl phosphate (m/z 492).

CONCLUSIONS

To our knowledge, this was the first time that S-oxidized metabolites and GSH conjugates of sulfur mustard have been detected and identified from human samples in vitro by LC/MS/MS. The usefulness of the GSH conjugates to serve as biomarkers for sulfur mustard exposure in human samples requires further studies.

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.

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.

Preclinical pharmacology of FL442, a novel nonsteroidal androgen receptor modulator

The preclinical profiles of two most potent compounds of our recently published cycloalkane[d]isoxazole pharmacophore-based androgen receptor (AR) modulators, FL442 (4-(3a,4,5,6,7,7a-hexahydro-benzo[d]isoxazol-3-yl)-2-(trifluoromethyl)benzonitrile) and its nitro analog FL425 (3-(4-nitro-3-(trifluoromethyl)phenyl)-3a,4,5,6,7,7a-hexahydrobenzo[d]isoxazole), were explored to evaluate their druggability for the treatment of AR dependent prostate cancer. The studies revealed that both compounds are selective to AR over other closely related steroid hormone receptors and that FL442 exhibits equal inhibition efficiency towards the androgen-responsive LNCaP prostate cancer cell line as the most widely used antiandrogen bicalutamide and the more recently discovered enzalutamide. Notably, FL442 maintains antiandrogenic activity with enzalutamide-activated AR mutant F876L. In contrast to bicalutamide, FL442 does not stimulate the VCaP prostate cancer cells which express elevated levels of the AR. Distribution analyses showed that [(14)CN]FL442 accumulates strongly in the mouse prostate. In spite of its low plasma concentration obtained by intraperitoneal administration, FL442 significantly inhibited LNCaP xenograft tumor growth. These findings provide a preclinical proof for FL442 as a promising AR targeted candidate for a further optimization.

Benzylmorpholine analogs as selective inhibitors of lung cytochrome P450 2A13 for the chemoprevention of lung cancer in tobacco users

PURPOSE

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), one of the most prevalent and procarcinogenic compounds in tobacco, is bioactivated by respiratory cytochrome P450 (CYP) 2A13, forming DNA adducts and initiating lung cancer. CYP2A13 inhibition offers a novel strategy for chemoprevention of tobacco-associated lung cancer.

METHODS

Twenty-four analogs of a 4-benzylmorpholine scaffold identified by high throughput screening were evaluated for binding and inhibition of both functional human CYP2A enzymes, CYP2A13 and the 94%-identical hepatic CYP2A6, whose inhibition is undesirable. Thus, selectivity is a major challenge in compound design.

RESULTS

A key feature resulting in CYP2A13-selective binding and inhibition was substitution at the benzyl ortho position, with three analogs being >25-fold selective for CYP2A13 over CYP2A6.

CONCLUSIONS

Two such analogs were negative for genetic and hERG toxicities and metabolically stable in human lung microsomes, but displayed rapid metabolism in human liver and in mouse and rat lung and liver microsomes, likely due to CYP2B-mediated degradation. A specialized knockout mouse mimicking the human lung demonstrates compound persistence in lung and provides an appropriate test model. Compound delivered by inhalation may be effective in the lung but rapidly cleared otherwise, limiting systemic exposure.

Interactions of inhibitor molecules with the human CYP2E1 enzyme active site

CYP2E1 is an important enzyme oxidizing ethanol as well as several drugs and other xenobiotics in the human liver. We determined the inhibition potency of structurally diverse compounds against human CYP2E1, and analyzed their interactions with the enzyme active site by molecular docking and 3D-QSAR approaches. The IC(50) values for the tested compounds varied from 1.4 μM for γ-undecanolactone to over 46 mM for glycerol. This data set was used to create a comparative molecular field analysis (CoMFA) model. The most important interactions for binding of inhibitors were identified by docking and key features for inhibitors were characterized via the COMFA model. Since the active site of CYP2E1 is flexible, long chain lactones and alkyl alcohols fitted best into the larger open form while the other compounds fitted better in the smaller closed form of the active site. Electrostatic interactions near the Thr(303) residue proved to be important for inhibition of the enzyme activity. Thus, docking analysis and the predictive CoMFA model proved to be efficient tools for revealing interactions between inhibiting compounds and CYP2E1. These approaches can be used to analyze CYP2E1-mediated metabolism and drug interactions in the development of new chemical entities.

Identification of novel CYP2A6 inhibitors by virtual screening

The human CYP2A6 enzyme metabolises several xenobiotics including nicotine, the addictive component in tobacco. Reduced activity of CYP2A6, either for genetic reasons or by administering inhibitors of CYP2A6, reduces tobacco smoking. The aim was to design novel inhibitors of CYP2A6 using 3D-QSAR analysis combined with virtual screening. A 3D-QSAR model was utilised to identify the most important features of the inhibitors, and this knowledge was used to design inhibitors for CYP2A6. Chemical database screening yielded several potent inhibitor candidates such as alkylamine derivatives (compound no. 5, IC(50)=0.1 μM) and 1-benzothiophene-3-carbaldehyde that can be used as lead compounds in the development of drugs for smoking reduction therapy.

Thiomers: Inhibition of cytochrome P450 activity

The aim of the present study was to investigate the potential of different thiolated polymers (thiomers) on the catalytic activity of CYP450s on one hand and to explore new inhibitors for CYP activity on the other hand. Several thiolated polymers including poly(acrylic acid)-cysteine (PAA-cysteine), chitosan-thioglycolic acid (chitosan-TGA), and thiolated PEG-g-PEI copolymer along with brij 35, myrj 52 and the well-established CYPP450 inhibitor verapamil were screened for their CYP3A4 and CYP2A6 inhibitory activity, and their IC(50) values were determined. Both enzyme inhibition assays were performed in 96-well microtiter plates. 7-Benzyloxy-4-(trifluoromethyl)-coumarin (BFC) and 7-hydroxycoumarin (7-HC) were used as fluorescent substrates in order to determine CYP3A4 and CYP2A6 catalytic activity, respectively. All investigated compounds inhibited CYP3A4 as well as CYP2A6 activity. All tested (thiolated) polymers were found to be more potent inhibitors of CYP3A4 than of CYP2A6 catalytic activity. Apart from verapamil that is a known CYP3A4 inhibitor, brij 35 and myrj 52 were explored as potent inhibitors of CYP3A4 and CYP2A6 catalytic activity. Among the tested polymers, the rank order for CYP3A4 inhibition was PAA-cysteine (100 kDa)>brij 35>thiolated PEG-g-PEI copolymer (16 kDa)>myrj 52>PAA (100 kDa)>PAA-cysteine (450 kDa)>verapamil>PAA (450 kDa)>chitosan-TGA (150 kDa)>chitosan (150 kDa). On the other hand, the rank order of CYP2A6 inhibition was brij 35>PAA-cysteine (100kDa)>chitosan-TGA (150 kDa)>PAA (100 kDa)>thiolated PEG-g-PEI copolymer (16 kDa)>PAA-cysteine (450 kDa)>chitosan (150 kDa)>verapamil>PAA (450 kDa)>myrj 52. Thus, this study suggests that (thiolated) polymers display a promising potential to inhibit cytochrome P450s activity and might turn out to be potentially valuable tools for improving the oral bioavailability of actively secreted compounds by avoiding intestinal metabolism.

Nicotine metabolism and urinary elimination in mouse: in vitro and in vivo

This study aimed at elucidating the in vivo metabolism of nicotine both with and without inhibitors of nicotine metabolism. Second, the role of mouse CYP2A5 in nicotine oxidation in vitro was studied as such information is needed to assess whether the mouse is a suitable model for studying chemical inhibitors of the human CYP2A6. The oxidation of nicotine to cotinine was measured and the ability of various inhibitors to modify this reaction was determined. Nicotine and various inhibitors were co-administered to CD2F1 mice, and nicotine and urinary levels of nicotine and four metabolites were determined. In mouse liver microsomes anti-CYP2A5 antibody and known chemical inhibitors of the CYP2A5 enzyme blocked cotinine formation by 85-100%, depending on the pre-treatment of the mice. The amount of trans-3-hydroxycotine was five times higher than cotinine N-oxide, and ten times higher than nicotine N-1-oxide and cotinine. Methoxsalen, an irreversible inhibitor of CYP2A5, significantly reduced the metabolic elimination of nicotine in vivo, but the reversible inhibitors had no effect. It is concluded that the metabolism of nicotine in mouse is very similar to that in man and, therefore, that the mouse is a suitable model for testing novel chemical inhibitors of human CYP2A6.

Selegiline is a mechanism-based inactivator of CYP2A6 inhibiting nicotine metabolism in humans and mice

Selegiline (l-deprenyl) is in clinical treatment trials as a potential smoking cessation drug. We investigated the affect of selegiline and its metabolites on nicotine metabolism. In mice, selegiline was a potent inhibitor of nicotine metabolism in hepatic microsomes and cDNA-expressed CYP2A5; the selegiline metabolites desmethylselegiline, l-methamphetamine, and l-amphetamine, also inhibited nicotine metabolism. Pretreatment with selegiline and desmethylselegiline increased inhibition (IC(50)) in microsomes by 3.3- and 6.1-fold, respectively. In mice in vivo, selegiline increased AUC (90.7 +/- 5.8 versus 57.4 +/- 5.3 ng/h/ml, p < 0.05), decreased clearance (4.6 +/- 0.4 versus 7.3 +/- 0.3 ml/min, p < 0.05), and increased elimination half-life (12.5 +/- 6.3 versus 6.6 +/- 1.4 min, p < 0.05) of nicotine. In vitro, selegiline was a potent inhibitor of human nicotine metabolism in hepatic microsomes and cDNA-expressed CYP2A6; desmethylselegiline and l-amphetamine also inhibited nicotine metabolism. Selegiline preincubation increased inhibition in microsomes (3.7-fold) and CYP2A6 (14.8-fold); the K(i) for CYP2A6 was 4.2 muM. Selegiline dose- and time-dependently inhibited nicotine metabolism by CYP2A6 (K(i) = 15.6 +/- 2.7 muM; k(inact) = 0.34 +/- 0.04 min(-1)), and the inhibition was irreversible in the presence of NADPH, indicating that it is a mechanism-based inhibitor of CYP2A6. Thus, inhibition of mouse nicotine metabolism by selegiline was competitive in vitro and significantly increased plasma nicotine in vivo. In humans, where selegiline is both a competitive and mechanism-based inhibitor, it is likely to have even greater effects on in vivo nicotine metabolism. Our findings suggest that an additional potential mechanism of selegiline in smoking cessation is through inhibition of nicotine metabolism.

Identification of inhibitors of the nicotine metabolising CYP2A6 enzyme--an in silico approach

The compulsive nature of tobacco use is attributable to nicotine addiction. Nicotine is eliminated by metabolism through the cytochrome P450 2A6 (CYP2A6) enzyme in liver. Inhibition of CYP2A6 by chemical compounds may represent a potential supplement to anti-smoking therapy. The purpose of this study was to rationally design potent inhibitors of CYP2A6. 3D-QSAR models were constructed to find out which structural characteristics are important for inhibition potency. Specifically located hydrophobic and hydrogen donor features were found to affect inhibition potency. These features were used in virtual screening of over 60,000 compounds in the Maybridge chemical database. A total of 22 candidate molecules were selected and tested for inhibition potency. Four of these were potent and selective CYP2A6 inhibitors with IC(50) values lower than 1 muM. They represent novel structures of CYP2A6 inhibitors, especially N1-(4-fluorophenyl)cyclopropane-1-carboxamide. This compound can be used as a lead in the design of CYP2A6 inhibitor drugs to combat nicotine addiction.

Characterization of humanized liver from chimeric mice using coumarin as a human CYP2A6 and mouse CYP2A5 probe

Coumarin 7-hydroxylation (COH), which is catalyzed almost solely by human CYP2A6 and mouse CYP2A5, shows large differences in activity (humans>>mice) and inhibitor specificity between mice and humans. To differentiate human and mouse liver functions of chimeric mice (CM1, CM2 and CM3) prepared with hepatocytes from 3 donors, the microsomal COH activities were measured with and without benzaldehyde and undecanoic gamma-lactone as a specific inhibitor of human CYP2A6 and mice CYP2A5, respectively. The replacement % to human hepatocytes designated as replacement index (RI) was calculated from human specific cytokeratin 8/18 expression in the liver section. The COH activities correlated well with RIs in CM2 (R(2)=0.98) and CM3 (R(2)=0.94), except CM1 whose genotype of donor is CYP2A6*4/*4. However, the COH activities expressed as % of donor activities were not always coincident with RIs, and the inhibition pattern of CM2 and CM3 was human-type after RI exceeded approximately 50%. Subsequently, our attempts to use % of COH activities or inhibition patterns as an accurate functional replacement index were unsuccessful. Since the detection of human CYP2A6 protein in the liver and the steep increase of human albumin (hAlb) levels in the blood were begun from almost RI=50% similarly to the changes of inhibition pattern, RI=50% is the turning point for chimeric mice to have humanized liver function.

Plasmodium berghei (ANKA): infection induces CYP2A5 and 2E1 while depressing other CYP isoforms in the mouse liver

It has been reported that malaria infection impairs hepatic drug clearance and causes a down-regulation of CYP-mediated monooxygenase activities in rodents and humans. In the present study, we investigated the effects of Plasmodium berghei infection on the activity of liver monooxygenases in female DBA/2 and C57BL/6 mice. In both mouse strains, P. berghei infection decreased activities mediated by CYP1A (EROD: DBA/2 65.3%, C57BL/6 44.7%) and 2B (BROD: DBA/2 64.3%, C57BL/6 49.8%) subfamily isoforms and increased activities mediated by 2A5 (COH: DBA/2 182.4%, C57BL/6 148.5%) and 2E1 (PNPH: DBA/2 177.8%, C57BL/6 128.5%) isoforms as compared to non-infected controls. Since malaria infection also produced an increase in ALT (273.1%) and AST (354.1%) activities in the blood serum, our findings are consistent with the view that CYP2A5 activity is induced by liver injury. An almost generalized depression of CYP-mediated activities has been found with numerous infections and inflammatory stimuli but an induction of CYP2A5 had been previously noted only in some viral hepatitis and trematode (liver fluke) infections.

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

Quantitative Structure−Activity Relationship Analysis of Inhibitors of the Nicotine Metabolizing CYP2A6 Enzyme

The purpose of this study was to develop screening and in silico modeling methods to obtain accurate information on the active center of CYP2A6, a nicotine oxidizing enzyme. The inhibitory potencies of 26 naphthalene and 16 non-naphthalene derivatives were determined for human CYP2A6 and mouse CYP2A5 enzymes. Several comparative molecular field analysis (CoMFA) models were developed to find out what types of steric and electrostatic properties are required for potent inhibitors. The IC(50) values of the tested compounds varied from 0.55 to 35 400 microM for CYP2A6 and from 1 to 1500 microM for CYP2A5. The generated CoMFA models were able to accurately predict the inhibition potencies of an external test set of chemicals. Potent and specific inhibitors of the CYP2A6 enzyme can be used in the future to increase nicotine bioavailability and thus make oral nicotine administration feasible in smoking cessation therapy.