Cytochrome P450 enzymes in the brain: emerging evidence of biological significance

Cytochrome P450 (CYP) enzymes are responsible for the metabolism of many exogenous and endogenous compounds. CYPs are abundant in the liver and are also expressed in many extra-hepatic tissues including the brain. Although total CYP levels in the brain are much lower than in the liver, brain CYPs are concentrated near drug targets in specific regions and cell types, and can potentially have a considerable impact on local metabolism. Individual differences in brain CYP metabolism, due to inducers, inhibitors or genetic variation, can influence sensitivity and response to centrally acting drugs. Brain CYPs may also play a role in modulating brain activity, behavior, susceptibility to central nervous system diseases and treatment outcomes. This review highlights recent progress that has been made in understanding the functional significance of CYPs in the brain.     

Drug Metabolism within the Brain Changes Drug Response: Selective Manipulation of Brain CYP2B Alters Propofol Effects

Drug-metabolizing cytochrome P450 (CYPs) enzymes are expressed in the liver, as well as in extrahepatic tissues such as the brain. Here we show for the first time that drug metabolism by a CYP within the brain, illustrated using CYP2B and the anesthetic propofol (2, 6-diisopropylphenol, Diprivan), can meaningfully alter the pharmacological response to a CNS acting drug. CYP2B is expressed in the brains of animals and humans, and this CYP isoform is able to metabolize centrally acting substrates such as propofol, ecstasy, and serotonin. Rats were given intracerebroventricularly (i.c.v.) injections of vehicle, C8-xanthate, or 8-methoxypsoralen (CYP2B mechanism-based inhibitors) and then tested for sleep time following propofol (80 mg/kg intraperitoneally). Both inhibitors significantly increased sleep-time (1.8- to 2-fold) and brain propofol levels, while having no effect on plasma propofol levels. Seven days of nicotine treatment can induce the expression of brain, but not hepatic, CYP2B, and this induction reduced propofol sleep times by 2.5-fold. This reduction was reversed in a dose-dependent manner by i.c.v. injections of inhibitor. Sleep times correlated with brain (r=0.76, P=0.0009), but not plasma (r=0.24, P=0.39) propofol concentrations. Inhibitor treatments increased brain, but not plasma, propofol levels, and had no effect on hepatic enzyme activity. These data indicate that brain CYP2B can metabolize neuroactive substrates (eg, propofol) and can alter their pharmacological response. This has wider implications for localized CYP-mediated metabolism of drugs, neurotransmitters, and neurotoxins within the brain by this highly variable enzyme family and other CYP subfamilies expressed in the brain.     

Phenobarbital increases monkey in vivo nicotine disposition and induces liver and brain CYP2B6 protein

1. CYP2B6 is a drug-metabolizing enzyme expressed in the liver and brain that can metabolize bupropion (Zyban), a smoking cessation drug), activate tobacco-smoke nitrosamines, and inactivate nicotine. Hepatic CYP2B6 is induced by phenobarbital and induction may affect in vivo nicotine disposition, while brain CYP2B6 induction may affect local levels of centrally acting substrates. We investigated the effect of chronic phenobarbital treatment on induction of in vivo nicotine disposition and CYP2B6 expression in the liver and brain of African Green (Vervet) monkeys. 2. Monkeys were split into two groups (n=6 each) and given oral saccharin daily for 22 days; one group was supplemented with 20 mg kg(-1) phenobarbital. Monkeys were given a 0.1 mg kg(-1) nicotine dose subcutaneously before and after treatment. 3. Phenobarbital treatment resulted in a significant, 56%, decrease (P=0.04) in the maximum nicotine plasma concentration and a 46% decrease (P=0.003) in the area under the concentration-time curve. Phenobarbital also increased hepatic CYP2B6 protein expression. In monkey brain, significant induction (P<0.05) of CYP2B6 protein levels was observed in all regions tested (caudate, putamen, hippocampus, cerebellum, brain stem and frontal cortex) ranging from 2-fold to 150-fold. CYP2B6 expression was induced in specific cells, such as frontal cortical pyramidal cells and thalamic neurons. 4. In conclusion, chronic phenobarbital treatment in monkeys resulted in increased in vivo nicotine disposition, and induced hepatic and brain CYP2B6 protein levels and cellular expression. This induction may alter the metabolism of CYP2B6 substrates including peripherally acting drugs such as cyclophosphamide and centrally acting drugs such as bupropion, ecstasy and phencyclidine.     

Regional and cellular distribution of CYP2E1 in monkey brain and its induction by chronic nicotine

CYP2E1 is expressed in liver and extrahepatic tissues, including brain. It metabolizes ethanol and other drugs and toxins, such as acetaminophen, chlorzoxazone and tobacco-derived nitrosamines. Hepatic CYP2E1 is inducible by nicotine, and cigarette smoke accelerates chlorzoxazone metabolism. Smokers have higher levels of brain CYP2E1 expression than non-smokers, but the specific regions and cell types which have the higher expression differ from nicotine-induced rat brain. We therefore investigated the expression and distribution of brain CYP2E1 in a non-human primate, the African green monkey, and determined the effect of nicotine treatment. CYP2E1 levels varied among saline-treated monkey brain regions (P < 0.01) and expression was cell-type specific. Chronic nicotine treatment induced CYP2E1 expression in the frontal cortex (1.5-fold, P < 0.05) and cerebellum (1.5-fold, P < 0.01), specifically in cortical pyramidal neurons and cerebellar Purkinje cells but no change was seen in temporal cortex (P = 0.20), hippocampus (P = 0.29), putamen (P = 0.26) and thalamus (P = 0.08). CYP2E1 expression pattern in monkey brain following chronic nicotine treatment is similar to that in smokers, suggesting that nicotine may be the primary component in cigarette smoke that induces CYP2E1. Increased CYP2E1 in brain may contribute to oxidative stress and alter localized metabolism, and resulting pharmacology, of centrally acting drugs metabolized by CYP2E1.     

Induction of the drug metabolizing enzyme CYP2D in monkey brain by chronic nicotine treatment

Cytochrome P450 (CYP) 2D6, an enzyme found in the liver and the brain, is involved in the metabolism of numerous centrally acting drugs (e.g. antidepressants, neuroleptics, opiates), endogenous neurochemicals (e.g. catecholamines) and in the inactivation of neurotoxins (e.g. pesticides, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)). Although CYP2D6 is essentially an uninducible enzyme in the liver, we show that smokers have higher CYP2D6 in the brain, especially in the basal ganglia. In order to determine whether nicotine, a component of cigarette smoke, could increase brain CYP2D, African Green monkeys were treated chronically with nicotine (0.05 mg/kg for 2 days, then 0.15 mg/kg for 2 days followed by 0.3 mg/kg for 18 days s.c., b.i.d.). Monkeys treated with nicotine showed significant induction of CYP2D in brain when compared to saline-treated animals as detected by western blotting and immunocytochemistry. No changes in liver CYP2D were observed in nicotine-treated monkeys. Induction was observed in various brain regions including those affected in Parkinson's disease (PD) such as substantia nigra (3-fold, p = 0.01), putamen (2.1-fold, p = 0.001) and brainstem (2.4-fold, p = 0.001), with the caudate nucleus approaching significance (1.6-fold, p = 0.07). Immunocytochemistry revealed that the expression of CYP2D in both saline- and nicotine-treated monkeys is cell-specific particularly in the cerebellum, frontal cortex and hippocampus. These results suggest that monkey brain expresses CYP2D, which is induced in specific cells and brain regions upon chronic nicotine treatment. Smokers, or those using nicotine treatment, may have higher levels of brain CYP2D6 that may result in altered localized CNS drug metabolism and inactivation of neurotoxins.     

Cytochrome P450‐mediated metabolism in the human gut wall

Objective Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug‐metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation. Key findings Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine. Summary This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP‐mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.     

黄酮类化合物对细胞色素P450 CYP1，2E1，3A4和19的影响

黄酮类化合物广泛存在于蔬菜、坚果、水果和饮料及中草药中，可诱导或抑制多种细胞色素P450的活性。本篇综述主要集中回顾黄酮类化合物对于细胞色素P450 CYP1，2E1，3A4和19的影响。归纳总结了该类物质抑制和诱导细胞色素P450的多种机制，如刺激特定的受体、稳定相关mRNA等。并总结了该类物质对细胞色素P450的影响体内和体外水平的研究结果并非总是一致的原因，如体内外的浓度的差异、基因和其他环境因素的影响。由于黄酮类化合物可通过影响细胞色素P450的活性影响药物代谢从而导致药物不良反应和药物相互作用，因此在将该类物质与其他药物合用时应高度重视。     

Effect of prenatal exposure of deltamethrin on the ontogeny of xenobiotic metabolizing cytochrome P450s in the brain and liver of offsprings

Prenatal exposure to low doses (0.25 or 0.5 or 1.0 mg/kg, p.o.) of deltamethrin, a type II pyrethroid insecticide, to pregnant dams from gestation days 5 to 21 (GD5-21) produced dose-dependent alterations in the ontogeny of xenobiotic metabolizing cytochrome P450 (CYP) isoforms in brain and liver of the offsprings. RT-PCR analysis revealed dose-dependent increase in the mRNA expression of cerebral and hepatic CYP1A1, 1A2, 2B1, 2B2, and 2E1 isoenzymes in the offsprings exposed prenatally to deltamethrin. Similar increase in the activity of the marker enzymes of these CYP isoforms has indicated that placental transfer of the pyrethroid, a mixed type of CYP inducer, even at these low doses may be sufficient to induce the CYPs in brain and liver of the offsprings. Our data have further revealed persistence in the increase in expression of xenobiotics metabolizing CYPs up to adulthood in brain and liver of the exposed offsprings, suggesting the potential of deltamethrin to imprint the expression of CYPs in brain and liver of the offsprings following its in utero exposure. Furthermore, though the levels of CYPs were several fold lower in brain, almost equal magnitude of induction in cerebral and hepatic CYPs has further suggested that brain CYPs are responsive to the induction by environmental chemicals. The present data indicating alterations in the expression of xenobiotic metabolizing CYPs during development following prenatal exposure to deltamethrin may be of significance as these CYP enzymes are not only involved in the neurobehavioral toxicity of deltamethrin but have a role in regulating the levels of ligands that modulate growth, differentiation, and neuroendocrine functions.     

Involvement of enzymes other than CYPs in the oxidative metabolism of xenobiotics

Although the majority of oxidative metabolic reactions are mediated by the CYP superfamily of enzymes, non-CYP-mediated oxidative reactions can play an important role in the metabolism of xenobiotics. The (major) oxidative enzymes, other than CYPs, involved in the metabolism of drugs and other xenobiotics are: the flavin-containing monooxygenases, the molybdenum hydroxylases (aldehyde oxidase and xanthine oxidase), the prostaglandin H synthase, the lipoxygenases, the amine oxidases (monoamine, polyamine, diamine and semicarbazide-sensitive amine oxidases) and the alcohol and aldehyde dehydrogenases. In a similar manner to CYPs, these oxidative enzymes can also produce therapeutically active metabolites and reactive/toxic metabolites, modulate the efficacy of therapeutically active drugs or contribute to detoxification. Many of them have been shown to be important in endobiotic metabolism, and, consequently, interactions between drugs and endogenous compounds might occur when they are involved in drug metabolism. In general, most non-CYP oxidative enzymes appear to be noninducible or much less inducible than the CYP system, although some of them may be as inducible as some CYPs. Some of these oxidative enzymes exhibit polymorphic expression, as do some CYPs. It is possible that the contribution of non-CYP oxidative enzymes to the overall metabolism of xenobiotics is underestimated, as most investigations of drug metabolism in discovery and lead optimisation are performed using in vitro test systems optimised for CYP activity.     

Role of CYP pharmacogenetics and drug-drug interactions in the efficacy and safety of atypical and other antipsychotic agents

Cytochrome P450 (CYP) drug oxidases play a pivotal role in the elimination of antipsychotic agents, and therefore influence the toxicity and efficacy of these drugs. Factors that affect CYP function and expression have a major impact on treatment outcomes with antipsychotic agents. In particular, aspects of CYP pharmacogenetics, and the processes of CYP induction and inhibition all influence in-vivo rates of drug elimination. Certain CYPs that mediate the oxidation of antipsychotic drugs exhibit genetic variants that may influence in-vivo activity. Thus, single nucleotide polymorphisms (SNPs) in CYP genes have been shown to encode enzymes that have decreased drug oxidation capacity. Additionally, psychopharmacotherapy has the potential for drug-drug inhibitory interactions involving CYPs, as well as drug-mediated CYP induction. Literature evidence supports a role for CYP1A2 in the clearance of the atypical antipsychotics clozapine and olanzapine; CYP1A2 is inducible by certain drugs and environmental chemicals. Recent studies have suggested that specific CYP1A2 variants possessing individual SNPs, and possibly also SNP combinations (haplotypes), in the 5'-regulatory regions may respond differently to inducing chemicals. CYP2D6 is an important catalyst of the oxidation of chlorpromazine, thioridazine, risperidone and haloperidol. Certain CYP2D6 allelic variants that encode enzymes with decreased drug oxidation capacity are more common in particular ethnic groups, which may lead to adverse effects with standard doses of psychoactive drugs. Thus, genotyping may be useful for dose optimization with certain psychoactive drugs that are substrates for CYP2D6. However, genotyping for inducible CYPs is unlikely to be sufficient to direct therapy with all antipsychotic agents. In-vivo CYP phenotyping with cocktails of drug substrates may assist at the commencement of therapy, but this approach could be complicated by pharmacokinetic interactions if applied when an antipsychotic drug regimen is ongoing.     

Prediction of inter-individual variability on the pharmacokinetics of CYP2C8 substrates in human

Inter-individual variability in pharmacokinetics can lead to unexpected side effects and treatment failure, and is therefore an important factor in drug development. CYP2C8 is a major drug-metabolizing enzyme known to be involved in the metabolism of over 100 drugs. In this study, we predicted the inter-individual variability in AUC/Dose of CYP2C8 substrates in healthy volunteers using the Monte Carlo simulation. Inter-individual variability in the hepatic intrinsic clearance of CYP2C8 substrates (CL_int,h,2C8) was estimated from the inter-individual variability in pharmacokinetics of pioglitazone, which is a major CYP2C8 substrate. The coefficient of variation (CV) of CL_int,h,2C8 was estimated to be 40%. Using this value, the CVs of AUC/Dose of other major CYP2C8 substrates, rosiglitazone and amodiaquine, were predicted to validate the estimated CV of CL_int,h,2C8. As a result, the reported CVs of both substrates were within the 2.5–97.5 percentile range of the predicted CVs. Furthermore, the CVs of AUC/Dose of the CYP2C8 substrates loperamide and chloroquine, which are affected by renal clearance, were also successfully predicted. Combining this value with previously reported CVs of other CYPs, we were able to successfully predict the inter-individual variability in pharmacokinetics of various drugs in clinical.     

Smoking,alcoholism and genetic polymorphisms alter CYP2B6 levels in human brain

CYP2B6 metabolizes drugs such as nicotine and bupropion, and many toxins and carcinogens. Nicotine induces CYP2B1 in rat brain and in humans polymorphic variation in CYP2B6 affects smoking cessation rates. The aim of this study was to compare CYP2B6 expression in brains of human smokers and non-smokers and alcoholics and non-alcoholics (n=26). CYP2B6 expression was brain region-specific, and was observed in both neurons and astrocytes. CYP2B6 levels were higher in brains of smokers and alcoholics, particularly in cerebellar Purkinje cells and hippocampal pyramidal neurons, cells known to be damaged in alcoholics. Significantly more (p<0.05) CYP2B6 protein was seen in four brain regions of smoking alcoholics compared to non-smoking non-alcoholics: hippocampus (5.8-fold), caudate nucleus (3.3-fold), putamen (3.0-fold) and cerebellar hemisphere (1.6-fold). The genetic variant C1459T (R487C) has been associated with reduced hepatic enzyme levels, stability and activity. Preliminary genotyping of this small sample (n=24) suggested that individuals with the CC genotype had higher brain CYP2B6 than those with the CT or TT genotype. Higher brain CYP2B6 activity in smokers and alcoholics may cause altered sensitivity to centrally acting drugs, increased susceptibility to neurotoxins and carcinogenic xenobiotics and contribute to central tolerance to nicotine.     

Interplay of drug metabolizing CYP450 enzymes and ABC transporters in the blood-brain barrier

The recent identification of drug-metabolizing enzymes cytochrome P450 (CYP) in the human blood-brain barrier (BBB) raises the question of whether these enzymes act in concert with ATP-binding cassette (ABC) transporters to limit the brain distributions of drugs. We recently demonstrated several CYP genes in freshly isolated human brain microvessels; the main isoforms expressed were CYP1B1 and CYP2U1. Many studies using different experimental approaches have revealed that P-glycoprotein (P-gp, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and the multidrug resistance-associated protein 4 (MRP4, ABCC4) are the main ABC transporters in the human BBB. The first part of this review covers recent studies on the expression, regulation and function of CYP450 and ABC transporters in the rodent and human BBBs. The second part focuses on the possible interplay between some CYPs and certain ABC transporters at the BBB, which makes it a determining element of brain drug concentrations and thus of the effects of centrally acting drugs.     

CYP2E1 induction leads to oxidative stress and cytotoxicity in glutathione-depleted cerebellar granule neurons

Increasing evidence suggests that brain cytochrome P450 (CYP) can contribute to the in situ metabolism of xenobiotics. In the liver, some xenobiotics can be metabolized by CYPs into more reactive products that can damage hepatocytes and induce cell death. In addition, normal CYP activity may produce reactive oxygen species (ROS) that contribute to cell damage through oxidative mechanisms. CYP2E1 is a CYP isoform that can generate ROS leading to cytotoxicity in multiple tissue types. The aim of this study was to determine whether CYP2E1 induction may lead to significant brain cell impairment. Immunological analysis revealed that exposure of primary cerebellar granule neuronal cultures to the CYP inducer isoniazid, increased CYP2E1 expression. In the presence of buthionine sulfoximine, an agent that reduces glutathione levels, isoniazid treatment also resulted in reactive oxygen species (ROS) production, DNA oxidation and cell death. These effects were attenuated by simultaneous exposure to diallyl sulfide, a CYP2E1 inhibitor, or to a mimetic of superoxide dismutase/catalase, (Euka). These results suggest that in cases of reduced antioxidant levels, the induction of brain CYP2E1 could represent a risk of in situ neuronal damage.     

Monitoring expression of cytochrome P450 genes during postischemic rat liver reperfusion using DNA microarrays

We investigated the expression of 50 cytochrome P450 (CYP) genes in the postischemic reperfused rat liver using a DNA microarray. Thirteen CYPs showed absent expression in all experiments and 2 CYPs were induced by pentobarbital, which was used as an anesthetic. Eight of the remaining 35 CYPs showed significantly decreased expression following ischemia-reperfusion. Monitoring CYP expression may be a powerful approach for elucidation of pathways regulating drug metabolism that may be involved in postischemic reperfusion liver injury.