In vitro metabolism of imipramine by brain microsomes: effects of inhibitors and exogenous cytochrome P450 reductase

The potential impact of CYP and UGT drug-metabolizing enzymes on brain target site drug exposure

Drug metabolism is one of the critical determinants of drug disposition throughout the body. While traditionally associated with the liver, recent research has unveiled the presence and functional significance of drug-metabolizing enzymes (DMEs) within the brain. Specifically, cytochrome P-450 enzymes (CYPs) and UDP-glucuronosyltransferases (UGTs) enzymes have emerged as key players in drug biotransformation within the central nervous system (CNS). This comprehensive review explores the cellular and subcellular distribution of CYPs and UGTs within the CNS, emphasizing regional expression and contrasting profiles between the liver and brain, humans and rats. Moreover, we discuss the impact of species and sex differences on CYPs and UGTs within the CNS. This review also provides an overview of methodologies for identifying and quantifying enzyme activities in the brain. Additionally, we present factors influencing CYPs and UGTs activities in the brain, including genetic polymorphisms, physiological variables, pathophysiological conditions, and environmental factors. Examples of CYP- and UGT-mediated drug metabolism within the brain are presented at the end, illustrating the pivotal role of these enzymes in drug therapy and potential toxicity. In conclusion, this review enhances our understanding of drug metabolism's significance in the brain, with a specific focus on CYPs and UGTs. Insights into the expression, activity, and influential factors of these enzymes within the CNS have crucial implications for drug development, the design of safe drug treatment strategies, and the comprehension of drug actions within the CNS. To that end, CNS pharmacokinetic (PK) models can be improved to further advance drug development and personalized therapy.

Efficient brain uptake and distribution of an expanded CAG RNA inhibitor DB213 via intranasal administration

DB213 is an expanded CAG RNA inhibitor targeting polyglutamine diseases. This current study aims to investigate biopharmaceutic characteristics of DB213 as well as its brain uptake and distribution in C57 wild type mice, R6/2 Huntington's disease mice and Sprague-Dawley (SD) rats via intranasal administration. The biopharmaceutic characteristics of DB213 were investigated in vitro using Calu-3/MDCK/HEK293 cell lines and brain slices for its membrane transport, equilibrium dialysis for its plasma protein/brain tissue bindings and liver/brain microsomes incubation for its enzyme kinetics profiles. In vivo study of DB213 brain distribution was conducted in rats via intravenous and intranasal routes at 50 mg/kg followed by its brain uptake evaluation in mice at 25 mg/kg via intranasal route. In vitro membrane transport studies found that DB213 not only had a limited passive diffusion with a P_{app (a→b)} value of 1.75 × 10^-6 cm/s in Calu-3 cell monolayer model but also was substrate of MRP2, MRP3, and amino acid transporter. Furthermore, DB213 demonstrated higher binding towards brain homogenate (80%) than plasma (10%) with limited metabolism in liver and brain. After intranasal administration of DB213, both olfactory bulb and trigeminal nerve served as its entry points to reach brain as demonstrated in rats while efficient brain uptake was observed in mice. In summary, limited nasal epithelium permeability and MRP2/MRP3 mediated efflux transport of DB213 could be overcome by its influx transport via amino acid transporter and minimal liver and brain metabolism, which further contribute to its rapid brain uptake and distribution in mice and rats.

The effect of psychotropic drugs on cytochrome P450 2D (CYP2D) in rat brain

The aim of the study was to investigate the influence of selected antidepressants and neuroleptics on the protein level and activity of cytochrome P450 2D (CYP2D) in rat brain. The obtained results showed that imipramine, fluoxetine, nefazodone, thioridazine and perazine, added to brain microsomes of control rats, inhibited CYP2D activity to a lower extent (K(i)=255-485μM) than when added to liver microsomes (K(i)=1-45μM), which may result from their stronger affinity for liver CYP2D2 (K(i)=2.7 and 1.25μM for imipramine and fluoxetine, respectively) than for brain CYP2D4 (K(i)=25 and 10μM for imipramine and fluoxetine, respectively), as well as from their high non-specific binding in brain microsomes. Two-week treatment with fluoxetine evoked decreases in the level and activity of CYP2D in the striatum and the nucleus accumbens. In contrast, fluoxetine increased CYP2D expression in the cerebellum, while nefazodone considerably enhanced the activity (but not the protein level) of CYP2D in the truncus cerebri. Imipramine and mirtazapine (active in the liver) did not affect brain CYP2D. Chronic thioridazine decreased CYP2D activity in the substantia nigra and nucleus accumbens, but significantly increased that activity in the striatum and cerebellum. Clozapine significantly enhanced CYP2D activity in the truncus cerebri. In conclusion, psychotropics influence CYP2D in the brain, but their effect is different than in the liver and depends on the cerebral structure. The observed psychotropics-brain CYP2D interactions may be important for the metabolism of neurosteroids and monoaminergic neurotransmitters, and for the local biotransformation of drugs.

Cytochrome P4503A: evidence for mRNA expression and catalytic activity in rat brain

Studies initiated to investigate the presence of cytochrome P4503A (CYP3A) isoenzymes in brain revealed constitutive mRNA and protein expression of CYP3A1 in rat brain. Western blotting studies showed that pretreatment with CYP3A inducer such as pregnenolone-16alpha -carbonitrile (PCN) significantly increased the cross reactivity comigrating with hepatic CYP3A1 and CYP3A2 in rat brain microsomes. RT-PCR studies have also shown increase in mRNA expression of CYP3A1 following pretreatment of rats with PCN. The ability of rat brain microsomes to catalyze the demethylation of erythromycin, known to be mediated by CYP3A isoenzymes in liver and significant increase in the activity of erythromycin demethylase (EMD) following pretreatment with dexamethasone or PCN have indicated that CYP3A isoenzymes expressed in brain are functionally active. Kinetic studies revealed that increase in the enzyme activity following pretreatment with PCN resulted in increase in the apparent affinity (Km) and Vmax of the reaction. Similarities in the inhibition of the constitutive and inducible brain and liver EMD activity following in vitro addition of ketoconazole, a inhibitor specific for CYP3A catalysed reactions and anti-CYP3A have further indicated that like in liver, CYP3A isoenzymes catalyse the activity of EMD in rat brain. Data also revealed regional differences in the activity of EMD in the brain. Relatively higher constitutive as well as inducible mRNA expression of CYP3A1 in hypothalamus and hippocampus, the brain regions responsive to steroid hormones have suggested that CYP3A isoenzymes may not only be involved in the process of detoxication mechanism but also in the metabolism of endogenous substrates in brain.

CYP3A4, CYP2C9 and CYP2B6 expression and ifosfamide turnover in breast cancer tissue microsomes

Ifosfamide is a prodrug that requires bioactivation by cytochrome P450 for antitumour activity. Up to now, little is known, to what extent in addition to the liver the ifosfamide metabolism may occur intratumorally. For this purpose, we investigated the expression of CYP3A4, CYP2C9 and CYP2B6 in breast cancer tissue using Western Blotting. Ifosfamide turnover was determined by detection of metabolites of the ifosfamide 4-hydroxylation and N-dechloroethylation in tumour microsomal incubations using HPLC/UV and LC/MS. The results demonstrate that all mammary tumours (n=11) reveal CYP3A4 expression; contents varied from 0.5 to 63 pmol mg(protein)(-1). CYP2C9 (n=9) was present in all tested breast tumour samples, too, while CYP2B6 (n=10) protein could not be detected. All measured breast cancer microsomes (n=4) showed an ifosfamide N-dechloroethylation capacity in the range from 0.04 to 0.21 pmol min(-1) mg(protein)(-1), while metabolites of the 4-hydroxylation could not be determined. In conclusion, the detected presence of CYP3A4 and CYP2C9 in breast tumours offers the possibility of intratumoral turnover of ifosfamide. For the first time in the literature, we could demonstrate a turnover of ifosfamide by microsomal preparations from human breast cancer tissue. A calculated modulation of intratumoral ifosfamide turnover could considerably influence its therapeutic efficiency.

Constitutive expression and localization of the major drug metabolizing enzyme, cytochrome P4502D in human brain

Cytochrome P4502D6, an important isoform of cytochrome P450, mediates the metabolism of several psychoactive drugs in liver. Quantitatively, liver is the major drug metabolizing organ, however metabolism of drugs in brain could modulate pharmacological and pharmacodynamic effects of psychoactive drugs at their site of action and explain some of the variation typically seen in patient population. We have measured cytochrome P450 content and examined constitutive expression of CYP2D mRNA and protein in human brain regions by reverse transcription polymerase chain reaction, Northern and immunoblotting and localized it by in situ hybridization and immunohistochemistry. CYP2D mRNA was expressed constitutively in neurons of cerebral cortex, Purkinje and granule cell layers of cerebellum, reticular neurons of midbrain and pyramidal neurons of CA1, CA2 and CA3 subfields of hippocampus. Immunoblot studies demonstrated the presence of cytochrome P4502D protein in cortex, cerebellum, midbrain, striatum and thalamus of human brain. Immunohistochemical localization showed the predominant presence of cytochrome P4502D not only in neuronal soma but also in dendrites of Purkinje and cortical neurons. These studies demonstrate constitutive expression of cytochrome P4502D in neuronal cell population in human brain, indicating its possible role in metabolism of psychoactive drugs directly at or near their site of action, in neurons, in human brain.

Subchronic fluoxetine treatment induces a transient potentiation of amphetamine-induced hyperlocomotion: possible pharmacokinetic interaction

The results of the present study show that 5 days of systemic treatment with fluoxetine (5 mg/kg) resulted in an augmented locomotor response to amphetamine (0.5 mg/kg). This augmented response to amphetamine was observed 24 and 48 h, but not 5 days, after the cessation of fluoxetine treatment. Subchronic fluoxetine treatment also produced an increase in the brain concentration of amphetamine when rats were challenged with amphetamine 48 h, but not 5 days, after the cessation of fluoxetine treatment. Thus, the effect of subchronic fluoxetine in augmenting amphetamine-induced hyperactivity was consistent with the effect of subchronic fluoxetine in augmenting the amphetamine concentration in the brain. This pattern of results indicates that subchronic fluoxetine potentiates the response to amphetamine within a limited time-window, and that this potentiating effect is likely to be due to the reduced metabolism of amphetamine via the inhibition of cytochrome P450 by fluoxetine and/or its metabolite norfluoxetine.

Cytochrome P-450 activities in human and rat brain microsomes

The role of cytochrome P450 in the metabolism of dextromethorphan, amitriptyline, midazolam, S-mephenytoin, citalopram, fluoxetine and sertraline was investigated in rat and human brain microsomes. Depending on the parameters, the limit of quantification using gas chromatography-mass spectrometry methods was between 1.6 and 20 pmol per incubation, which generally contained 1500 microg protein. Amitriptyline was shown to be demethylated to nortriptyline by both rat and human microsomes. Inhibition studies using ketoconazole, furafylline, sulfaphenazole, omeprazole and quinidine suggested that CYP3A4 is the isoform responsible for this reaction whereas CYP1A2, CYP2C9, CYP2C19 and CYP2D6 do not seem to be involved. This result was confirmed by using a monoclonal antibody against CYP3A4. Dextromethorphan was metabolized to dextrorphan in rat brain microsomes and was inhibited by quinidine and by a polyclonal antibody against CYP2D6. Only the addition of exogenous reductase allowed the measurement of this activity in human brain microsomes. Metabolites of the other substrates could not be detected, possibly due to an insufficiently sensitive method. It is concluded that cytochrome P450 activity in the brain is very low, but that psychotropic drugs could undergo a local cerebral metabolism which could have pharmacological and/or toxicological consequences.

Isolation of partially purified P450 2D18 and characterization of activity toward the tricyclic antidepressants imipramine and desipramine

Previous reports have shown that rat brain microsomes are capable of metabolizing tricyclic antidepressants such as imipramine. Subsequent studies have shown that the protein products of several clones isolated from rat brain cDNA libraries are capable of metabolizing imipramine to both its active metabolite, desipramine, and its inactive hydroxylated metabolites. We report here the overexpression and partial purification of P450 2D18 using the baculovirus expression system and the incorporation of a C-terminal [His]4 tag. P450 2D18 was partially purified to a specific content of 4.8 nmol/mg protein and shown to be electrophoretically pure. The apparent KM values for P450 2D18 toward imipramine and desipramine were 374 and 314 microM, respectively. While apparent KM values were similar, P450 2D18 was shown to have a fivefold increased Vmax (2.2 nmol/min/nmol P450) for imipramine compared to desipramine (0.44 nmol/min/nmol P450), suggesting a primary involvement in the activation of imipramine to desipramine. We also examined the effect of the CYP2D6 inhibitor quinidine, the CYP3A inhibitor ketoconazole, and the dopamine reuptake inhibitor GBR-12935 for their ability to inhibit P450 2D18-mediated metabolism of imipramine. These results, when compared with previous studies using rat brain microsomes, suggest that P450 2D18 may play an important role in the conversion of imipramine to its active metabolite desipramine in the rat brain.

Relative quantities of catalytically active CYP 2C9 and 2C19 in human liver microsomes: application of the relative activity factor approach

The relative catalytic activities of CYP2C9 and CYP2C19 in human liver microsomes has been determined using the approach of relative activity factors (RAFs). Tolbutamide methylhydroxylation and S-mephenytoin 4'-hydroxylation were used as measures of CYP2C9 and CYP2C19 activity, respectively. The kinetics of these reactions were studied in human liver microsomes, in microsomes from human lymphoblastoid cells, and in insect cells expressing CYP2C9 and CYP2C19. RAFs were calculated as the ratio of Vmax (reaction velocity at saturating substrate concentrations) in human liver microsomes of the isoform-specific index reaction divided by the Vmax of the reaction catalyzed by the cDNA expressed isoform. RAFs were also determined for SUPERMIX, a commercially available mixture of cDNA expressed human drug metabolizing CYPs formulated to achieve a balance of enzyme activities similar to that found in human liver microsomes. Lymphoblast RAF2C9 in human liver microsomes ranged from 54 to 145 pmol CYP/mg protein (mean value: 87), while a value of 251 pmol CYP/mg protein was obtained for SUPERMIX. Insect cell RAF2C9 in human liver microsomes ranged from 1.6 to 143 pmol CYP/mg protein (mean value: 49), while a value of 201 pmol CYP/mg protein was obtained for SUPERMIX. Both lymphoblast and insect cell RAF2C19 in human liver microsomes ranged from 4 to 45 pmol CYP/mg protein (mean values: 29 and 28, respectively), while a value of 29 pmol CYP/mg protein was obtained for SUPERMIX. The nature of the cDNA expression system used had no effect on the kinetic parameters of CYP2C9 as a tolbutamide methylhydroxylase, or of CYP2C19 as a S-mephenytoin hydroxylase. However insect cell expressed CYP2C19 (which includes oxidoreductase) had substantially greater activity as a tolbutamide methylhydroxylase when compared to lymphoblast expressed CYP2C19. The ratio of mean lymphoblast-determined RAF2C9 to RAF2C19 in human livers was 3.0 (range 1.6-17.9; n = 10), while this ratio for SUPERMIX was 8.6. The ratio of mean insect cell-determined RAF2C9 to RAF2C19 in human livers was 1.7 (range 0.04-16.2; n = 10), while this ratio for SUPERMIX was 7.0. Neither ratio is in agreement with the 20:1 ratio of immunoquantified levels of CYP2C9 and 2C19 in human liver microsomes reported in previous studies. SUPERMIX may contain catalytically active CYP2C9 in levels higher than those in human liver microsomes.

Identification of cytochrome P450 1A1 in human brain

We have isolated cDNA fragments that were originated from P450 1A1 in human brain by polymerase chain reaction (PCR) analysis. Their size matched up to what we expected based on the reported P450 1A1 mRNA sequence of human breast carcinoma cell line MCF-7. To confirm proper PCR products, they were subcloned and their nucleotide sequences were determined. This result showed that PCR products were actually derived from P450 1A1 mRNA in human brain. We demonstrated for the first time that human brain contains a P450 1A1-dependent metabolism system which seems to be important for understanding the metabolism of endogenous and exogenous compounds.

Potential metabolic bioactivation pathways involving cyclic tertiary amines and azaarenes

A major theme explored in this review is the MAO-and cytochrome P450-catalyzed alpha-carbon oxidations of selected cyclic tertiary amines to give iminium metabolites that undergo further chemical modifications to form known or potentially toxic products. The most dramatic illustration of this type of bioactivation process is the conversion of the parkinsonian-inducing neurotoxin MPTP (23) by brain MAO-B to the iminium (dihydropyridinium) metabolite 24 which is oxidized further to the pyridinium species MPP+ (25). The selective destruction of nigrostriatal neurons by MPP+ is dependent on a unique sequence of events (transport into the nerve terminals by the dopamine transporter, localization in the inner mitochondrial membrane by electromotive forces, and inhibition of complex I of the mitochondrial electron transport chain) that, fortunately, are unlikely to be encountered with many substances. A second example of a well-documented metabolic bioactivation sequence involves the highly toxic pyrrolizidine alkaloids (102). These compounds undergo cytochrome P450-catalyzed alpha-carbon oxidation which converts the 3-pyrrolinyl moiety present in the parent alkaloids into a pyrrolyl-containing metabolite (105). The presence of labile functional groups results in the spontaneous conversion of 105 to reactive electrophilic products (106 and 108) that undergo Michael addition reactions with nucleophiles on biomacromolecules leading to a variety of toxic outcomes. Less clearly defined are the potential contributions to neurodegenerative processes that may be mediated by low-level, long term exposure to less potent toxins. Examples of potential proneurotoxins are the endogenously formed tetrahydroisoquinolines (such as 40-50) and tetrahydro-beta-carbolines (such as 54) that may be biotransformed to neurotoxic isoquinolinium (such as 51) and beta-carbolinium (such as 52) species in the brain. A similar argument can be made for 4-piperidinols (compounds that are at the same oxidation state as the tetrahydropyridines) which may be metabolized via iminium intermediates to amino enols that spontaneously convert to dihydropyridinium species and hence to pyridinium metabolites (67-->68-->69-->70-->71, Scheme 10). This type of reaction sequence has been well documented with the parkinsonian-inducing neuroleptic agent haloperidol (72) which is metabolized in humans, baboons, and rodents to the pyridinium species HPP+ (75), a potent inhibitor of mitochondrial respiration. Finally, an appreciation of the alpha-carbon oxidations of fully reduced azacycles such as (S)-nicotine (61) and phencyclidine (82) to chemically reactive metabolites that form covalent adducts with proteins, including the enzymes that are responsible for their formation, may prove of toxicological importance when attempting to account for the effects of chronic abuse of these potent drugs.1

Regulation of CYP3A9 gene expression by estrogen and catalytic studies using cytochrome P450 3A9 expressed in Escherichia coli

Sexual dimorphism in the expression of CYP3A9, a novel form of CYP3A from rat brain, is shown for the first time in rat brain as well as in rat liver. CYP3A9 expression is female specific in rat liver as judged by its 10-fold higher expression in females than in males. CYP3A9 gene expression was inducible by estrogen treatment both in male and in female rats. Ovariectomy of adult female rats elicited a drastic reduction on the mRNA level of CYP3A9 which could be fully restored by estrogen replacement. These results suggest that estrogen may play an important role in the female-specific expression of the CYP3A9 gene. P450 3A9 recombinant protein was expressed in Escherichia coli by means of the pCWOri+ expression vector and the MALLLAVF amino terminal sequence modification. This construct gave a high level of expression (130 nmol P450 3A9/liter culture) and the recombinant protein of the modified P450 3A9 was purified to electrophoretic homogeneity with a specific content of 10.1 nmol P450/mg protein from solubilized fractions through two chromatographic steps. The purified P450 3A9 protein was active in the metabolism of imipramine, erythromycin, benzphetamine, and ethylmorphine as well as 17beta-estradiol in a reconstituted system containing lipid and rat NADPH-P450 reductase. Of special interest is the finding that P450 3A9 can catalyze the formation of desipramine with a turnover number of 4.9 nmol/min/nmol P450, suggesting the possible involvement of this isoform in the metabolism of imipramine in brain. Optimal reconstitution conditions for P450 3A9 activities required a lipid mixture (1:1:1 mixture of L-alpha-dilauroyl phosphatidylcholine, L-alpha-dioleoyl phosphatidylcholine, and phosphatidylserine) and GSH.