Cytochrome P450 expression and regulation in the brain

The mechanisms of interactions of psychotropic drugs with liver and brain cytochrome P450 and their significance for drug effect and drug-drug interactions

Cytochrome P450 (CYP) plays an important role in psychopharmacology. While liver CYP enzymes are responsible for the biotransformation of psychotropic drugs, brain CYP enzymes are involved in the local metabolism of these drugs and endogenous neuroactive substances, such as neurosteroids, and in alternative pathways of neurotransmitter biosynthesis including dopamine and serotonin. Recent studies have revealed a relation between the brain nervous system and cytochrome P450, indicating that CYP enzymes metabolize endogenous neuroactive substances in the brain, while the brain nervous system is engaged in the central neuroendocrine and neuroimmune regulation of cytochrome P450 in the liver. Therefore, the effect of neuroactive drugs on cytochrome P450 should be investigated not only in vitro, but also at in vivo conditions, since only in vivo all mechanisms of drug-enzyme interaction can be observed, including neuroendocrine and neuroimmune modulation. Psychotropic drugs can potentially affect cytochrome P450 via a number of mechanisms operating at the level of the nervous, hormonal and immune systems, and the liver. Their effect on cytochrome P450 in the brain is often different than in the liver and region-dependent. Since psychotropic drugs can affect cytochrome P450 both in the liver and brain, they can modify their own pharmacological effect at both pharmacokinetic and pharmacodynamic level. The article describes the mechanisms by which psychotropic drugs can change the expression/activity of cytochrome P450 in the liver and brain, and discusses the significance of those mechanisms for drug action and drug-drug interactions. Moreover, the brain CYP2D6 is considered as a potential target for psychotropics.

CYP450 2D6 and 2C19 genotypes in ADHD: not related with treatment resistance but with over-representation of 2C19 ultra-metabolizers

OBJECTIVES

Cytochrome P450 (CYP450) is a major enzyme system involved in drug metabolism as well as regulation of brain function. Although individual variability in CYP enzymes have been studied in terms of personality traits and treatment effects, no study up to now evaluated CYP polymorphisms in children with attention deficit/hyperactivity disorder (ADHD). We aimed to define the genetic profiles of CYP2D6 and CYP2C19 relevant alleles in children with ADHD according to treatment status and compare the frequencies according to past results.

METHODS

Three hundred and seventeen patients with ADHD-Combined Presentation were enrolled; symptom severity was evaluated by parents and clinicians while adverse effects of previous treatments were evaluated with parent and child reports. Reverse blotting on strip assays was used for genotyping and descriptive and bivariate analyses were conducted. A p-value was set at 0.05 (two-tailed).

RESULTS

Children were divided into treatment-naïve (n=194, 61.2%) and treatment-resistant (n=123, 38.8%) groups. Within the whole sample PM, EM and UM status according to 2D6 were 3.8% (n=12), 94.3% (n=299) and 21.9% (n=6); respectively. PM, IM, EM and UM status according to 2C19 were 2.5% (n=8), 19.8% (n=63), 48.6% (n=154) and 29.0% (n=92), respectively. No relationship with treatment resistance, comorbidity or gender could be found. Importantly, CYP2C19 UMs were significantly more frequent in ADHD patients compared to previous studies in the general population.

CONCLUSIONS

CYPs may be a rewarding avenue of research to elucidate the etiology and treatment of patients with ADHD.

Radiosynthesis and in Vivo and ex Vivo Evaluation of Isomeric [11C]methoxy Analogs of Nimesulide as Brain Cyclooxygenase-2-Targeted Imaging Agents

Our previous studies identified that nimesulide analogs which bear a methoxy substituent at the para-position of the phenyl ring could be potential radiotracer candidates for detecting disorders related to cyclooxygenase-2 (COX-2) expression and activity in vivo using positron emission tomography (PET) in the brain. The present study was conducted to evaluate the in vivo characteristics of ¹¹C-labeled para-methoxy nimesulide ([¹¹C]1d) as a brain COX-2-targeted imaging agent compared to other isomeric methoxy analogs of nimesulide ([¹¹C]1b and [¹¹C]1c). [¹¹C]1b-d were synthesized with reasonable yield and purity by the methylation of the O-desmethyl precursor with [¹¹C]methyl triflate in the presence of NaOH at room temperature. We performed in vivo biodistribution analysis, brain PET imaging, ex vivo autoradiography, and metabolite analysis in mice. The uptake of [¹¹C]1b-d was lower in the brain than in other tissues, including in the blood, and both [¹¹C]1c and [¹¹C]1d were rapidly metabolized. However, [¹¹C]1d showed a small, but significant, specific signal and heterogeneous distribution in the brain. In vivo evaluation suggested that [¹¹C]1d might correlate with COX-2 expression in the brain. Given its instability in vivo, [¹¹C]1d seems unsuitable as a brain-COX-2 radioimaging agent. Further structural refinement of these radiotracers is necessary to enhance their uptake in the brain and to achieve sufficient metabolic stability.

Cytochrome P450 2D (CYP2D) enzyme dysfunction associated with aging and serotonin deficiency in the brain and liver of female Dark Agouti rats

Among the enzymes that support brain metabolism, cytochrome P450 (CYP) enzymes occupy an important place. These enzymes catalyze the biotransformation pathways of neuroactive endogenous substrates (neurosteroids, neurotransmitters) and are necessary for the detoxification processes. The aim of the present study was to assess changes in the CYP2D activity and protein level during the aging process and as a result of serotonin deficiency in the female brain. The CYP2D activity was measured in brain and liver microsomes of Dark Agouti wild type (WT) female rats (mature 15-week-old and senescent 18-month-old rats) and in tryptophan hydroxylase 2 (TPH2)-deficient senescent female rats. The CYP2D activity in mature WT Dark Agouti females was independent of the changing phases of the estrous cycle. In senescent WT females rats, the CYP2D activity and protein level were decreased in the cerebral cortex, hippocampus, cerebellum and liver, but increased in the brain stem. In the other examined structures (frontal cortex, hypothalamus, thalamus, striatum), the enzyme activity did not change. In aging TPH2-deficient females, the CYP2D activity and protein levels were decreased in the frontal cortex, hypothalamus and brain stem (activity only), remaining unchanged in other brain structures and liver, relative to senescent WT females. In summary, the aging process and TPH2 deficit affect the CYP2D activity and protein level in female rats, which may have a negative impact on the compensatory capacity of CYP2D in the synthesis of serotonin and dopamine in cerebral structures involved in cognitive and emotional functions. In the liver, the CYP2D-catalyzed drug metabolism may be diminished in elderly females. The results in female rats are compared with those obtained previously in males. It is concluded that aging and serotonin deficiency exert sex-dependent effects on brain CYP2D, which seem to be less favorable in females concerning CYP2D-mediated neurotransmitter synthesis, but beneficial regarding slower neurosteroid metabolism.

Long-Term Treatment with Atypical Antipsychotic Iloperidone Modulates Cytochrome P450 2D (CYP2D) Expression and Activity in the Liver and Brain via Different Mechanisms

CYP2D enzymes engage in the synthesis of endogenous neuroactive substances (dopamine, serotonin) and in the metabolism of neurosteroids. The present work investigates the effect of iloperidone on CYP2D enzyme expression and activity in rat brains and livers. Iloperidone exerted a weak direct inhibitory effect on CYP2D activity in vitro in the liver and brain microsomes (Ki = 11.5 μM and Ki = 462 μM, respectively). However, a two-week treatment with iloperidone (1 mg/kg ip.) produced a significant decrease in the activity of liver CYP2D, which correlated positively with the reduced CYP2D1, CYP2D2 and CYP2D4 protein and mRNA levels. Like in the liver, iloperidone reduced CYP2D activity and protein levels in the frontal cortex and cerebellum but enhanced these levels in the nucleus accumbens, striatum and substantia nigra. Chronic iloperidone did not change the brain CYP2D4 mRNA levels, except in the striatum, where they were significantly increased. In conclusion, by affecting CYP2D activity in the brain, iloperidone may modify its pharmacological effect, via influencing the rate of dopamine and serotonin synthesis or the metabolism of neurosteroids. By elevating the CYP2D expression/activity in the substantia nigra and striatum (i.e., in the dopaminergic nigrostriatal pathway), iloperidone may attenuate extrapyramidal symptoms, while by decreasing the CYP2D activity and metabolism of neurosteroiods in the frontal cortex and cerebellum, iloperidone can have beneficial effects in the treatment of schizophrenia. In the liver, pharmacokinetic interactions involving chronic iloperidone and CYP2D substrates are likely to occur.

Chronic treatment with asenapine affects cytochrome P450 2D (CYP2D) in rat brain and liver. Pharmacological aspects

Neuroleptics have to be used for a long time to produce a therapeutic effect. Cytochrome P450 2D (CYP2D) enzymes mediate alternative pathways of neurotransmitter synthesis (i.e. tyramine hydroxylation to dopamine and 5-methoxytryptamine O-demethylation to serotonin), and metabolism of neurosteroids. The aim of our present study was to examine the influence of chronic treatment with the new atypical neuroleptic asenapine on CYP2D in rat brain. In parallel, liver CYP2D was investigated for comparison. Asenapine added in vitro to microsomes of control rats competitively, but weakly inhibited the activity of CYP2D (brain: Ki = 385 μM; liver: Ki = 36 μM). However, prolonged administration of asenapine (0.3 mg/kg sc. for 2 weeks) significantly diminished the activity and protein level of CYP2D in the frontal cortex, nucleus accumbens, hippocampus and cerebellum, but did not affect the enzyme in the hypothalamus, brain stem, substantia nigra and the remainder of the brain. In contrast, asenapine enhanced the enzyme activity and protein level in the striatum. In the liver, chronically administered asenapine reduced the activity and protein level of CYP2D, and the CYP2D1 mRNA level. In conclusion, prolonged administration of asenapine alters the CYP2D expression in the brain structures and in the liver. Through affecting the CYP2D activity in the brain, asenapine may modify its pharmacological effect. By increasing the CYP2D expression/activity in the striatum, asenapine may accelerate the synthesis of dopamine (via tyramine hydroxylation) and serotonin (via 5-methoxytryptamine O-demethylation), and thus alleviate extrapyramidal symptoms. By reducing the CYP2D expression/activity in other brain structures asenapine may diminish the 21-hydroxylation of neurosteroids and thus have a beneficial influence on the symptoms of schizophrenia. In the liver, by reducing the CYP2D activity, asenapine may slow the biotransformation of concomitantly administered CYP2D substrates (drugs) during continuous treatment of schizophrenia or bipolar disorders.

Role of Vitamin D in Cognitive Dysfunction: New Molecular Concepts and Discrepancies between Animal and Human Findings

PURPOSE OF REVIEW

increasing evidence suggests that besides the several metabolic, endocrine, and immune functions of 1alpha,25-dihydroxyvitamin D (1,25(OH)2D), the neuronal effects of 1,25(OH)2D should also be considered an essential contributor to the development of cognition in the early years and its maintenance in aging. The developmental disabilities induced by vitamin D deficiency (VDD) include neurological disorders (e.g., attention deficit hyperactivity disorder, autism spectrum disorder, schizophrenia) characterized by cognitive dysfunction. On the other hand, VDD has frequently been associated with dementia of aging and neurodegenerative diseases (e.g., Alzheimer's, Parkinson's disease).

RECENT FINDINGS

various cells (i.e., neurons, astrocytes, and microglia) within the central nervous system (CNS) express vitamin D receptors (VDR). Moreover, some of them are capable of synthesizing and catabolizing 1,25(OH)2D via 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1) and 25-hydroxyvitamin D 24-hydroxylase (CYP24A1) enzymes, respectively. Both 1,25(OH)2D and 25-hydroxyvitamin D were determined from different areas of the brain and their uneven distribution suggests that vitamin D signaling might have a paracrine or autocrine nature in the CNS. Although both cholecalciferol and 25-hydroxyvitamin D pass the blood-brain barrier, the influence of supplementation has not yet demonstrated to have a direct impact on neuronal functions. So, this review summarizes the existing evidence for the action of vitamin D on cognitive function in animal models and humans and discusses the possible pitfalls of therapeutic clinical translation.

A Comparison of Calcium Aggregation and Ultracentrifugation Methods for the Preparation of Rat Brain Microsomes for Drug Metabolism Studies

Preparation of brain microsomes by the calcium chloride aggregation method has been suggested as an alternative to the ultracentrifugation method. However, the effects of the calcium chloride concentration on the quality of the microsomal fractions are not known. Brain microsomes were prepared from the adult rat brains using the high-speed ultracentrifugation and low-speed calcium chloride (10-100 mM) aggregation methods (n = 5-6 per group). The microsomal protein yield (spectrometry), the cytochrome P450 reductase (CPR) activity (spectrometry), and the monooxygenase activities (UPLC-MS/MS) of CYP2D and CYP2E1 were determined in the obtained fractions. Increasing the concentrations of calcium chloride progressively increased the protein yield of the low-speed microsomal fractions. However, the increased yield was associated with a significant decrease in the activities of CPR, CYP2D, and CYP2E1. Additionally, the CYP2D and CYP2E1 activities were significantly correlated with the CPR activities of the fractions. In conclusion, when an ultracentrifuge is available, preparation of brain microsomes by the ultracentrifugation method might be preferable. However, the calcium aggregation method at a calcium chloride concentration of 10 mM is an acceptable alternative to the ultracentrifuge method.

The role of efflux transporters and metabolizing enzymes in brain and peripheral organs to explain drug-resistant epilepsy

Drug‐resistant epilepsy has been explained by different mechanisms. The most accepted one involves overexpression of multidrug transporters proteins at the blood brain barrier and brain metabolizing enzymes. This hypothesis is one of the main pharmacokinetic reasons that lead to the lack of response of some antiseizure drug substrates of these transporters and enzymes due to their limited entrance into the brain and limited stay at the sites of actions. Although uncontrolled seizures can be the cause of the overexpression, some antiseizure medications themselves can cause such overexpression leading to treatment failure and thus refractoriness. However, it has to be taken into account that the inductive effect of some drugs such as carbamazepine or phenytoin not only impacts on the brain but also on the rest of the body with different intensity, influencing the amount of drug available for the central nervous system. Such induction is not only local drug concentration but also time dependent. In the case of valproic acid, the deficient disposition of ammonia due to a malfunction of the urea cycle, which would have its origin in an intrinsic deficiency of L‐carnitine levels in the patient or by its depletion caused by the action of this antiseizure drug, could lead to drug‐resistant epilepsy. Many efforts have been made to change this situation. In order to name some, the administration of once‐daily dosing of phenytoin or the coadministration of carnitine with valproic acid would be preferable to avoid iatrogenic refractoriness. Another could be the use of an adjuvant drug that down‐regulates the expression of transporters. In this case, the use of cannabidiol with antiseizure properties itself and able to diminish the overexpression of these transporters in the brain could be a novel therapy in order to allow penetration of other antiseizure medications into the brain.

The Reduction of the Combined Effects of Aflatoxin and Ochratoxin A in Piglet Livers and Kidneys by Dietary Antioxidants

The purpose of this study was to investigate the combined effects of aflatoxin B1 and ochratoxin A on protein expression and catalytic activities of CYP1A2, CYP2E1, CYP3A29 and GSTA1 and the preventive effect of dietary byproduct antioxidants administration against these mycotoxin damage. Three experimental groups (E1, E2, E3) and one control group (C) of piglets after weaning (TOPIGS-40 hybrid) were fed with experimental diets for 30 days. A basal diet containing normal compound feed for starter piglets was used as a control treatment and free of mycotoxin. The experimental groups were fed as follows: E1—basal diet plus a mixture (1:1) of two byproducts (grapeseed and sea buckthorn meal), E2—the basal diet experimentally contaminated with mycotoxins (479 ppb OTA and 62ppb AFB1) and E3—basal diet containing 5% of the mixture (1:1) of grapeseed and sea buckthorn meal and contaminated with the mix of OTA and AFB1. After 4 weeks, the animals were slaughtered, and tissue samples were taken from liver and kidney in order to perform microsomal fraction isolation, followed by protein expression and enzymatic analyses. The protein expressions of CYP2E1 and CYP3A29 were up-regulated in an insignificant manner in liver, whereas in kidney, those of CYP1A2, CYP2E1 and CYP3A29 were down-regulated. The enzymatic activities of CYP1A2, CYP2E1 and CYP3A29 decreased in liver, in a significant manner, whereas in kidney, these increased significantly. The co-presence of the two mycotoxins and the mixture of grape seed and sea buckthorn meal generated a tendency to return to the control values, which suggest that grapeseed and sea buckthorn meal waste represent a promising source in counteracting the harmful effect of ochratoxin A and aflatoxin B.

UPLC-MS/MS analysis of the Michaelis-Menten kinetics of CYP3A-mediated midazolam 1'- and 4-hydroxylation in rat brain microsomes

Midazolam (MDZ) is a short-acting benzodiazepine with rapid onset of action, which is metabolized by CYP3A isoenzymes to two hydroxylated metabolites, 1'-hydroxymidazolam and 4-hydroxymidazolam. The drug is also commonly used as a marker of CYP3A activity in the liver microsomes. However, the kinetics of CYP3A-mediated hydroxylation of MDZ in the brain, which contains much lower CYP content than the liver, have not been reported. In this study, UPLC-MS/MS and metabolic incubation methods were developed and validated for simultaneous measurement of low concentrations of both hydroxylated metabolites of MDZ in brain microsomes. Different concentrations of MDZ (1-500 µM) were incubated with rat brain microsomes (6.25 µg) and NADPH over a period of 10 min. After precipitation of the microsomal proteins with acetonitrile, which contained individual isotope-labeled internal standards for each metabolite, the analytes were separated on a C₁₈ UPLC column and detected by a tandem mass spectrometer. Accurate quantitation of MDZ metabolism in the brain microsomes presented several challenges unique to this tissue, which were resolved. The optimized method showed validation results in accordance with the FDA acceptance criteria, with a linearity ranging from 1 to 100 nM and a lower limit of quantitation of 0.4 pg on the column for each of the two metabolites. The method was successfully used to determine the Michaelis-Menten (MM) kinetics of MDZ 1'- and 4-hydroxylase activities in rat brain microsomes (n = 5) for the first time. The 4-hydroxylated metabolite had 2.4 fold higher maximum velocity (p < 0.01) and 1.9 fold higher (p < 0.05) MM constant values than the 1'-hydroxylated metabolite. However, intrinsic clearance values of the two metabolites were similar. The optimized analytical and metabolic incubation methods reported here may be used to study the effects of various pathophysiological and pharmacological factors on the CYP3A-mediated metabolism of MDZ in the brain.

Benzo[a]pyrene impairs the migratory pattern of human gonadotropin-releasing-hormone-secreting neuroblasts

Benzo[a]pyrene (BaP) is a widespread pollutant that can act as an endocrine disrupting compound (EDC) and interferes with reproductive function. The central regulatory network of the reproductive system is mediated by gonadotropin-releasing hormone (GnRH) neurons, which originate in the olfactory placode and, during ontogenesis, migrate into the hypothalamus. Given the importance of the migratory process for GnRH neuron maturation, we investigated the effect of BaP (10 μM for 24 h) on GnRH neuroblasts isolated from the human fetal olfactory epithelium (FNCB4). BaP exposure significantly reduced the mRNA level of genes implicated in FNCB4 cell migration and affected their migratory ability. Our findings demonstrate that BaP may interfere with the central neuronal network controlling human reproduction affecting GnRH neuron maturation.