Differential metabolism of alprazolam by liver and brain cytochrome (P4503A) to pharmacologically active metabolite

Designer benzodiazepine rat pharmacokinetics: A comparison of alprazolam, flualprazolam and flubromazolam

Designer benzodiazepines, including flualprazolam and flubromazolam, are clandestinely produced to circumvent federal regulations. Although flualprazolam and flubromazolam are structurally similar to alprazolam, they do not have an approved medical indication. Flualprazolam differs from alprazolam by the addition of a single fluorine atom. Whereas, flubromazolam differs by the addition of a single fluorine atom and substitution of a bromine for a chlorine atom. The pharmacokinetics of these designer compounds have not been extensively evaluated. In the present study, we evaluated flualprazolam and flubromazolam in a rat model and compared the pharmacokinetics of both compounds to alprazolam. Twelve male, Sprague-Dawley rats were given a 2 mg/kg subcutaneous dose of alprazolam, flualprazolam and flubromazolam and plasma pharmacokinetic parameters were evaluated. Both compounds displayed significant two-fold increases in volume of distribution and clearance. Additionally, flualprazolam displayed a significant increase in half-life leading to a nearly double half-life when compared to alprazolam. The findings of this study demonstrate that fluorination of the alprazolam pharmacophore increases pharmacokinetic parameters including half-life and volume of distribution. The increase in these parameters for flualprazolam and flubromazolam leads to an overall increased exposure in the body and a potential for greater toxicity than alprazolam.

Differential expression and activities of cytochrome P450 3A in the rat brain microsomes and mitochondria

Midazolam (MDZ), a benzodiazepine derivative, is metabolized to 1'- and 4-hydroxylated metabolites (1'-OH-MDZ and 4-OH-MDZ, respectively) by cytochrome P450 3A (CYP3A). The purpose of this study was to investigate the CYP3A-mediated hydroxylation of MDZ in the rat brain mitochondria (MT). Brain microsomes (MC) and MT fractions were prepared from rats (n = 8) using differential and density gradient centrifugations, and the purity of the fractions was evaluated using VDAC1 and calreticulin as markers of MT and MC, respectively. The formation rates of 1'-OH-MDZ and 4-OH-MDZ in the rat brain MC and MT samples were determined using an LC-MS/MS method after validation. Subsequently, Michaelis-Menten kinetics of 1'- and 4-hydroxylation of MDZ were estimated. Western blot (WB) analysis was used to determine the protein expression of CYP3A in the rat brain MC and MT. The MC fractions had 5.93% ± 3.01% mitochondrial impurity, and the MT fractions had 19.3% ± 7.8% microsomal impurity (mean ± SD). The maximum velocity (V_max ) values of the formation of the hydroxylated metabolites in the brain MT were 2.4-9-fold higher than those in MC. Further, the V_max values of 4-OH-MDZ in both MC and MT fractions were substantially higher than those of 1'-OH-MDZ. The WB analysis showed that the intensity of the CYP3A immunoreactive band in MT was more than twofold higher than that in MC. It is concluded that compared with MC, rat brain MT contains substantial CYP3A, which may affect the pharmacology or toxicology of centrally acting xenobiotic and endogenous substrates of this enzyme.

Flualprazolam distribution in postmortem samples

The constant emergence of novel psychoactive substances is troubling to both public health officials and legislators. Additionally, sufficient data collection for each new compound can take months up to years. Flualprazolam, a triazolobenzodiazepine, quickly garnered attention as a sedative drug that likely expresses adverse reactions similarly to alprazolam. This study focuses on the distribution of flualprazolam in multiple common postmortem matrices. Central blood, vitreous humor, liver homogenate, brain homogenate, gastric contents, and urine samples from death investigation cases were quantitated when available. Samples were screened with liquid chromatography quadrupole time-of-flight with limit of detection set at 4 ng/ml and quantitated on liquid chromatography tandem mass spectrometry, with concentration range from 4 to 256 ng/ml. From August 2018 to September 2020, 24 central blood samples were quantitated for flualprazolam. Central bloods of 22 cases had concentrations above the limit of quantitation. The average flualprazolam central blood concentration was 16.3 ng/ml with a median of 9.95 ng/ml (4.24-48.0). Additional analyses for unconjugated flualprazolam were performed on at a total of 15 urine samples ( x ¯  = 14.4, 4.07-36.1 ng/ml), 23 brain homogenates ( x ¯  = 23.2, 3.99-69.3 ng/g), 23 liver homogenates ( x ¯  = 50.7, 13.6-156 ng/g), five vitreous humor samples ( x ¯  = 7.70, 4.03-12 ng/ml), and 12 gastric contents samples ( x ¯  = 0.36, 0.02-2.51 mg). The cause of death for 13 of the 24 cases listed flualprazolam as a contributing factor of death.

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.

The role of sex, age and genetic polymorphisms of CYP enzymes on the pharmacokinetics of anticholinergic drugs

There is evidence that use of drugs with anticholinergic properties increases the risk of cognitive impairment, and increased exposure to these drugs potentiates this risk. Anticholinergic drugs are commonly used even with associated risk of adverse events. Aging, sex, and genetic polymorphisms of cytochrome P450 (CYP) enzymes are associated with alterations in pharmacokinetic processes, which increase drug exposure and may further increase the risk of adverse drug events. Due to the increasing burden of cognitive impairment in our aging population and the future of personalized medicine, the objective of this review was to provide a critical clinical perspective on age, sex, and CYP genetic polymorphisms and their role in the metabolism and exposure to anticholinergic drugs. Age-related changes that may increase anticholinergic drug exposure include pseudocapillarization of liver sinusoidal endothelial cells, an approximate 3.5% decline in CYP content for each decade of life, and a reduction in kidney function. Sex-related differences that may be influenced by anticholinergic drug exposure include women having delayed gastric and colonic emptying, higher gastric pH, reduced catechol-O-methyl transferase activity, reduced glucuronidation, and reduced renal clearance and men having larger stomachs which may affect medication absorption. The overlay of poor metabolism phenotypes for CYP2D6 and CYP2C19 may further modify anticholinergic drug exposure in a significant proportion of the population. These factors help explain findings of clinical trials that show older adults and specifically older women achieve higher plasma concentrations of anticholinergic drugs and that poor metabolizers of CYP2D6 experience increased drug exposure. Despite this knowledge neither age, sex nor CYP phenotype are routinely considered when making decisions about the use or dosing of anticholinergic medications. Future study of anticholinergic medication needs to account for age, sex and CYP polymorphisms so that we may better approach personalized medicine for optimal outcomes and avoidance of medication-related cognitive impairment.

Cytochrome P450 expression and regulation in the brain

The regulation of brain cytochrome P450 enzymes (CYPs) is different compared with respective hepatic enzymes. This may result from anatomical bases and physiological functions of the two organs. The brain is composed of a variety of functional structures built of different interconnected cell types endowed with specific receptors that receive various neuronal signals from other brain regions. Those signals activate transcription factors or alter functioning of enzyme proteins. Moreover, the blood-brain barrier (BBB) does not allow free penetration of all substances from the periphery into the brain. Differences in neurotransmitter signaling, availability to endogenous and exogenous active substances, and levels of transcription factors between neuronal and hepatic cells lead to differentiated expression and susceptibility to the regulation of CYP genes in the brain and liver. Herein, we briefly describe the CYP enzymes of CYP1-3 families, their distribution in the brain, and discuss brain-specific regulation of CYP genes. In parallel, a comparison to liver CYP regulation is presented. CYP enzymes play an essential role in maintaining the levels of bioactive molecules within normal ranges. These enzymes modulate the metabolism of endogenous neurochemicals, such as neurosteroids, dopamine, serotonin, melatonin, anandamide, and exogenous substances, including psychotropics, drugs of abuse, neurotoxins, and carcinogens. The role of these enzymes is not restricted to xenobiotic-induced neurotoxicity, but they are also involved in brain physiology. Therefore, it is crucial to recognize the function and regulation of CYP enzymes in the brain to build a foundation for future medicine and neuroprotection and for personalized treatment of brain diseases.

An ex Vivo Model for Evaluating Blood-Brain Barrier Permeability, Efflux, and Drug Metabolism

The metabolism of drugs in the brain is difficult to study in most species because of enzymatic instability in vitro and interference from peripheral metabolism in vivo. A locust ex vivo model that combines brain barrier penetration, efflux, metabolism, and analysis of the unbound fraction in intact brains was evaluated using known drugs. Clozapine was analyzed, and its major metabolites, clozapine N-oxide (CNO) and N-desmethylclozapine (NDMC), were identified and quantified. The back-transformation of CNO into clozapine observed in humans was also observed in locusts. In addition, risperidone, citalopram, fluoxetine, and haloperidol were studied, and one preselected metabolite for each drug was analyzed, identified, and quantified. Metabolite identification studies of clozapine and midazolam showed that the locust brain was highly metabolically active, and 18 and 14 metabolites, respectively, were identified. The unbound drug fraction of clozapine, NDMC, carbamazepine, and risperidone was analyzed. In addition, coadministration of drugs with verapamil or fluvoxamine was performed to evaluate drug-drug interactions in all setups. All findings correlated well with the data in the literature for mammals except for the stated fact that CNO is a highly blood-brain barrier permeant compound. Overall, the experiments indicated that invertebrates might be useful for screening of blood-brain barrier permeation, efflux, metabolism, and analysis of the unbound fraction of drugs in the brain in early drug discovery.

Differential expression of human cytochrome P450 enzymes from the CYP3A subfamily in the brains of alcoholic subjects and drug-free controls

Cytochrome P450 enzymes are responsible for the metabolism of most commonly used drugs. Among these enzymes, CYP3A forms mediate the clearance of around 40-50% of drugs and may also play roles in the biotransformation of endogenous compounds. CYP3A forms are expressed both in the liver and extrahepatically. However, little is known about the expression of CYP3A proteins in specific regions of the human brain. In this study, form-selective antibodies raised to CYP3A4 and CYP3A5 were used to characterize the expression of these forms in the human brain. Both CYP3A4 and CYP3A5 immunoreactivity were found to varying extents in the microsomal fractions of cortex, hippocampus, basal ganglia, amygdala, and cerebellum. However, only CYP3A4 expression was observed in the mitochondrial fractions of these brain regions. N-terminal sequencing confirmed the principal antigen detected by the anti-CYP3A4 antibody in cortical microsomes to be CYP3A4. Immunohistochemical analysis revealed that CYP3A4 and CYP3A5 expression was primarily localized in the soma and axonal hillock of neurons and varied according to cell type and cell layer within brain regions. Finally, analysis of the frontal cortex of chronic alcohol abusers revealed elevated expression of CYP3A4 in microsomal but not mitochondrial fractions; CYP3A5 expression was unchanged. The site-specific expression of CYP3A4 and CYP3A5 in the human brain may have implications for the role of these enzymes in both normal brain physiology and the response to drugs.

Cytochrome P450-mediated metabolism in brain: functional roles and their implications

INTRODUCTION

Cytochromes P450 (P450) and associated monooxygenases are a family of heme proteins involved in metabolism of endogenous compounds (arachidonic acid, eicosanoids and prostaglandins) as also xenobiotics including drugs and environmental chemicals. Liver is the major organ involved in P450-mediated metabolism and hepatic enzymes have been characterized. Extrahepatic organs, such as lung, kidney and brain have the capability for biotransformation through P450 enzymes. Brain, including human brain, expresses P450 enzymes that metabolize xenobiotics and endogenous compounds.

AREAS COVERED

An overview of P450-mediated metabolism in brain is presented focusing on distinct differences seen in expression of P450 enzymes, generation of unique P450 enzymes in brain through alternate splicing and their consequences in terms of metabolism of psychoactive drugs and inflammatory prompts, such as leukotrienes, thus modulating inflammatory response.

EXPERT OPINION

The brain possesses unique P450s that metabolize drugs and endogenous compounds through pathways that are markedly different from that seen in liver indicating that extrapolation directly from liver to brain is not appropriate. It is therefore necessary to characterize the unique brain P450s and their ability to metabolize xenobiotics and endogenous compounds to better understand the functions of this important class of enzymes in brain, especially human brain.

Targeting nevirapine delivery across human brain microvascular endothelial cells using transferrin-grafted poly(lactide-co-glycolide) nanoparticles

Aims: Poly(lactide-co-glycolide) (PLGA) nanoparticles (NPs) were grafted with transferrin (Tf) to enhance the transport of nevirapine (NVP) across human brain microvascular endothelial cells (HBMECs). Methods: NVP-loaded PLGA NPs with surface-grafting Tf (Tf/NVP–PLGA NPs) were incubated with HBMECs and immunochemical staining characterized Tf receptors (TfRs). Results: The polydispersity index of Tf/NVP–PLGA NPs was lower than 0.008. The entrapment efficiency of NVP and loading efficiency of Tf was 20–75% and 15–80%, respectively. Tf slightly retarded the release of NVP from PLGA. Dioctadecyldimethylammonium bromide (DODAB)-stabilized Tf/NVP–PLGA NPs reduced the viability of HBMECs to 70–75%. The secretion of TNF-α was inhibited by Tf and stimulated by DODAB. The permeability of NVP across HBMECs reached maxima at 67% DODAB and 0.1–0.2% Tf. An increase in the concentration of Tf enhanced the uptake of Tf/NVP–PLGA NPs via a TfR-mediated mechanism. Conclusion: Tf/NVP–PLGA NPs are efficacious carriers in targeting delivery across HBMECs for viral therapy.

In vitro and in vivo oxidative metabolism and glucuronidation of anastrozole

AIMS

Little information is available regarding the metabolic routes of anastrozole and the specific enzymes involved. We characterized anastrozole oxidative and conjugation metabolism in vitro and in vivo.

METHODS

A sensitive LC-MS/MS method was developed to measure anastrozole and its metabolites in vitro and in vivo. Anastrozole metabolism was characterized using human liver microsomes (HLMs), expressed cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs).

RESULTS

Hydroxyanastrozole and anastrozole glucuronide were identified as the main oxidative and conjugated metabolites of anastrozole in vitro, respectively. Formation of hydroxyanastrozole from anastrozole was markedly inhibited by CYP3A selective chemical inhibitors (by >90%) and significantly correlated with CYP3A activity in a panel of HLMs (r= 0.96, P= 0.0005) and mainly catalyzed by expressed CYP3A4 and CYP3A5. The K(m) values obtained from HLMs were also close to those from CYP3A4 and CYP3A5. Formation of anastrozole glucuronide in a bank of HLMs was correlated strongly with imipramine N-glucuronide, a marker of UGT1A4 (r= 0.72, P < 0.0001), while expressed UGT1A4 catalyzed its formation at the highest rate. Hydroxyanastrozole (mainly as a glucuronide) and anastrozole were quantified in plasma of breast cancer patients taking anastrozole (1 mg day⁻¹); anastrozole glucuronide was less apparent.

CONCLUSION

Anastrozole is oxidized to hydroxyanastrozole mainly by CYP3A4 (and to some extent by CYP3A5 and CYP2C8). Once formed, this metabolite undergoes glucuronidation. Variable activity of CYP3A4 (and probably UGT1A4), possibly due to genetic polymorphisms and drug interactions, may alter anastrozole disposition and its effects in vivo.

Statins and neuroprotection: a prescription to move the field forward

There is growing interest in the use of statins, HMG-CoA reductase inhibitors, for treating specific neurodegenerative diseases (e.g., cerebrovascular disease, Parkinson's disease, Alzheimer's disease, multiple sclerosis) and possibly traumatic brain injury. Neither is there a consensus on the efficacy of statins in treating the aforementioned diseases nor are the mechanisms of the purported statin-induced neuroprotection well-understood. Part of the support for statin-induced neuroprotection comes from studies using animal models and cell culture. Important information has resulted from that work but there continues to be a lack of progress on basic issues pertaining to statins and brain that impedes advancement in understanding how statins alter brain function. For example, there are scant data on the pharmacokinetics of lipophilic and hydrophilic statins in brain, statin-induced neuroprotection versus cell death, and statins and brain isoprenoids. The purpose of this mini-review will be to examine those aforementioned issues and to identify directions of future research.

Drug metabolism in human brain: high levels of cytochrome P4503A43 in brain and metabolism of anti-anxiety drug alprazolam to its active metabolite

Cytochrome P450 (P450) is a super-family of drug metabolizing enzymes. P450 enzymes have dual function; they can metabolize drugs to pharmacologically inactive metabolites facilitating their excretion or biotransform them to pharmacologically active metabolites which may have longer half-life than the parent drug. The variable pharmacological response to psychoactive drugs typically seen in population groups is often not accountable by considering dissimilarities in hepatic metabolism. Metabolism in brain specific nuclei may play a role in pharmacological modulation of drugs acting on the CNS and help explain some of the diverse response to these drugs seen in patient population. P450 enzymes are also present in brain where drug metabolism can take place and modify therapeutic action of drugs at the site of action. We have earlier demonstrated an intrinsic difference in the biotransformation of alprazolam (ALP) in brain and liver, relatively more α-hydroxy alprazolam (α-OHALP) is formed in brain as compared to liver. In the present study we show that recombinant CYP3A43 metabolizes ALP to both α-OHALP and 4-hydroxy alprazolam (4-OHALP) while CYP3A4 metabolizes ALP predominantly to its inactive metabolite, 4-OHALP. The expression of CYP3A43 mRNA in human brain samples correlates with formation of relatively higher levels of α-OH ALP indicating that individuals who express higher levels of CYP3A43 in the brain would generate larger amounts of α-OHALP. Further, the expression of CYP3A43 was relatively higher in brain as compared to liver across different ethnic populations. Since CYP3A enzymes play a prominent role in the metabolism of drugs, the higher expression of CYP3A43 would generate metabolite profile of drugs differentially in human brain and thus impact the pharmacodynamics of psychoactive drugs at the site of action.

An alternatively spliced cytochrome P4501A1 in human brain fails to bioactivate polycyclic aromatic hydrocarbons to DNA-reactive metabolites

CYP1A1, a cytochrome P450 enzyme, metabolizes polycyclic aromatic hydrocarbons to genotoxic metabolite(s) that bind to DNA and initiate carcinogenesis. RT-PCR amplification of the complete open reading frame of CYP1A1 generated an amplicon of 1593 bp having deletion of 87 bp of exon-6 that translated into functional P450 enzyme. Unlike wild type CYP1A1, exon 6 del CYP1A1 did not metabolize polycyclic aromatic hydrocarbons such as, benzo(a)pyrene to genotoxic, ultimate carcinogens that form DNA adducts. Exon 6 del CYP1A1 metabolized ethoxyresorufin (the classical substrate for CYP1A1) less efficiently compared with wild type CYP1A1 while pentoxy and benzyloxyresorufin (classical substrates for CYP2B) were dealkylated more efficiently. In silico docking showed alteration of the substrate access channel in exon 6 del CYP1A1 such that benzo(a)pyrene does not bind in any orientation that would permit the formation of carcinogenic metabolites. Genotyping revealed that the splice variant was not generated due to differences in genomic DNA sequence and the variant was present only in brain but not in liver, kidney, lung, or heart from the same individual. We provide evidence that unique P450 enzymes, generated by alternate splicing in a histiospecific manner can modify genotoxic potential of carcinogens such as benzo(a)pyrene by altering their biotransformation pathway.

Unique cytochromes P450 in human brain: implication in disease pathogenesis

Cytochromes P450 is a family of heme proteins that metabolize xenobiotics including drugs. Unique human brain cytochrome P450 enzymes metabolize xenobiotics including drugs to active/inactive metabolites through biotransformation pathways that are different from the well-characterized ones in liver. We have identified an alternate spliced functional transcript of CYP2D7 containing partial inclusion of intron 6 in human brain but not in liver or kidney from the same individual. Genotyping revealed the presence of the frame-shift mutation 138delT only in those subjects who expressed the brain variant CYP2D7, which metabolizes codeine exclusively to morphine unlike hepatic CYP2D6 that metabolizes codeine to nor codeine and morphine. CYP1A1 bioactivates polycyclic aromatic hydrocarbons to reactive DNA binding metabolites and initiates carcinogenesis. We have identified a unique splice variant of CYP1A1 having deletion of 87 bp of exon 6 which is present in human brain but not in liver of the same individual. We present evidence for the existence of biotransformation pathways in human brain that are dissimilar from known pathways in liver. Identification and characterization of novel CNS-specific P450 enzymes generated by alternate splicing of known genes or as yet unidentified genes may help predict consequences of exposure to xenobiotics including pesticides in the brain.

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.

Pharmaceutical development and clinical effectiveness of a novel gel technology for transdermal drug delivery

Transdermal gels are designed to deliver sustained drug amounts, resulting in systemically consistent levels. They represent an improvement compared with transdermal delivery by patches because they offer more dosage flexibility, less irritation potential and a better cosmetic appearance. Advanced Transdermal Delivery (ATD) gel technology was developed in order to provide enhanced passive skin permeation of various active drugs for the treatment of many conditions, including hypogonadism, female sexual dysfunction, postmenopausal symptoms, overactive bladder and anxiety. The technology consists of a combination of solvent systems and permeation enhancers enabling systemic drug delivery, and is covered by many patents. Pharmaceutical development of formulations based on the technology allowed optimisation of physicochemical parameters (rheological profile, pH) as well as skin permeation properties (type and concentration of permeation enhancers, thermodynamic activity of the drug). This gel technology has demonstrated to be efficient for many drugs, as shown in the preclinical and clinical pharmacokinetic studies presented in this technology evaluation.

Presence of splice variant forms of cytochrome P4502D1 in rat brain but not in liver

Cytochromes P450 (P450), a family of heme-containing proteins, is involved in the oxidative metabolism of both foreign and endogenous compounds. Although liver is quantitatively the major organ involved in the metabolism of most xenobiotics, there is increasing evidence that these enzymes are present in extrahepatic tissues, such as lung, kidney, brain, etc and they may contribute to the in situ metabolism of xenobiotics in these organs. The possible relationship between genetic polymorphism seen in P4502D6 and incidence of neurodegenerative diseases, such as Parkinson's disease, has prompted the characterization of P4502D enzymes in rat brain. In the present study, we demonstrate that P4502D1 (the rat homologue of human P4502D6) is constitutively expressed in rat brain and the mRNA and protein are localized predominantly in neuronal cell population in the olfactory bulb, cortex, cerebellum, and hippocampus. An alternate spliced transcript of CYP2D1 having exon 3 deletion was detected in rat brain but not in liver. Deletion of exon 3 causes frame shift and generates a stop codon at 391 bp relative to the start codon ATG leading to premature termination of translation. Thus, Northern blotting and in situ hybridization represent contributions from functional transcripts and alternate spliced variants that do not translate into functional protein. Further, the splice variant having partial inclusion of intron 6 detected in human brain was not detected in rat brain indicating that alternate spliced gene products of P450 enzymes are generated in species-specific and tissue-specific manner.

Potential role of cerebral cytochrome P450 in clinical pharmacokinetics: modulation by endogenous compounds

Cytochrome P450 (CYP) enzymes catalyse phase I metabolic reactions of psychotropic drugs. The main isoenzymes responsible for this biotransformation are CYP1A2, CYP2D6, CYP3A and those of the subfamily CYP2C. Although these enzymes are present in the human brain, their specific role in this tissue remains unclear. However, because CYP enzymatic activities have been reported in the human brain and because brain microsomes have been shown to metabolise the same probe substrates used to assess specific hepatic CYP activities and substrates of known hepatic CYPs, local drug metabolism is believed to be likely. There are also indications that CYP2D6 is involved in the metabolism of endogenous substrates in the brain. This, along with the fact that several neurotransmitters modulate CYP enzyme activities in human liver microsomes, indicates that CYP enzymes present in brain could be under various regulatory mechanisms and that those mechanisms could influence drug pharmacokinetics and, hence, drug response. In this paper we review the presence of CYP1A2, CYP2C9, CYP2D6 and CYP3A in brain, as well as the possible existence of local brain metabolism, and discuss the putative implications of endogenous modulation of these isoenzymes by neurotransmitters.

A Frameshift Mutation and Alternate Splicing in Human Brain Generate a Functional Form of the Pseudogene Cytochrome P4502D7 That Demethylates Codeine to Morphine

A frameshift mutation 138delT generates an open reading frame in the pseudogene, cytochrome P4502D7 (CYP2D7), and an alternate spliced functional transcript of CYP2D7 containing partial inclusion of intron 6 was identified in human brain but not in liver or kidney from the same individual. mRNA and protein of the brain variant CYP2D7 were detected in 6 of 12 human autopsy brains. Genotyping revealed the presence of the frameshift mutation 138delT only in those human subjects who expressed the brain variant CYP2D7. Genomic DNA analysis in normal volunteers revealed the presence of functional CYP2D7 in 4 of 8 individuals. In liver, the major organ involved in drug metabolism, a minor metabolic pathway mediated by CYP2D6 metabolizes codeine (pro-drug) to morphine (active drug), whereas norcodeine is the major metabolite. In contrast, when expressed in Neuro2a cells, brain variant CYP2D7 metabolized codeine to morphine with greater efficiency compared with the corresponding activity in cells expressing CYP2D6. Morphine binds to micro-opioid receptors in certain regions of the central nervous system, such as periaqueductal gray, and produces pain relief. The brain variant CYP2D7 and micro-opioid receptor colocalize in neurons of the periaqueductal gray area in human brain, indicating that metabolism of codeine to morphine could occur at the site of opioid action. Histio-specific isoforms of P450 generated by alternate splicing, which mediate selective metabolism of pro-drugs within tissues, particularly the brain, to generate active drugs may play an important role in drug action and provide newer insights into the genetics of metabolism.

Transgenic mouse models of human CYP3A4 gene regulation

CYP3A4, the predominant but variably expressed cytochrome P450 of adult human liver, is subject to multifaceted constitutive regulation as well as transcriptional induction by a variety of structurally unrelated xenobiotics. Using transient transfections in HepG2 cells, we previously demonstrated the existence of a potent xenobiotic-responsive enhancer module located between - 7.2 and - 7.8 kilobases upstream of the CYP3A4 transcription start site. Induction is mediated by interaction of transcription factor binding sites in the XREM with the nuclear receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR). To determine the in vivo relevance of these findings and to establish a mouse model of human CYP3A4 regulation, we have generated transgenic mice carrying constructs comprising the upstream regulatory region of the human CYP3A4 gene linked to the lacZ reporter gene. Constitutive expression was observed in a developmental, tissue- and cell-specific fashion that mirrors the human situation. In addition, robust hepatic and intestinal induction with a range of reagents known to activate PXR and/or CAR (e.g., dexamethasone, pregnenolone 16alpha-carbonitrile, and phenobarbital) was observed. However, no expression or induction was apparent with a construct lacking upstream sequences beyond - 3.2 kilobases. Histochemical staining for beta-galactosidase activity revealed that dose-dependent increases in transgene levels were associated with a zonal expansion of lacZ expressing hepatocytes, suggesting that xenobiotic induction of CYP3A genes operates primarily through the recruitment of more cells committed to expression. In summary, CYP3A4/lacZ transgenic mice provide an in vivo model for the study of the molecular mechanisms involved in the regulation of a significant human drug metabolizing enzyme.