The preparation of microsomes

Effects of species of origin and mode of induction of microsomes on carbamazepine-induced cell toxicity

Standardization and validation of in vitro drug metabolism is essential for pre-clinical drug development as well as for in vitro toxicity assays including the lymphocyte toxicity assay (LTA) and the in vitro platelet toxicity assay (iPTA). Use of isolated liver microsomes (MIC) in in vitro testing has been utilized for a long time; however, the effect of species of origin and induction agents on the metabolic capacities of MIC is not adequately evaluated. In this study we investigated the impact of species of origin and induction agent on the capacity of MICs to bioactivate carbamazepine (CBZ) using cytotoxicity as a gross endpoint to measure the levels of cytotoxic metabolites generated by each type of MICs. Jurkat E6.1 cell line was used and MICs from human, rat, mouse, minipig and rabbit origin as well as rat MICs that is either non-induced or induced by phenobarbitone (PHB), dexamethasone (DEXA), 3-methylcholanthrene (3MC), clofibrate (CLOF) and isoniazid (INH) were investigated. MICs from minipig and rat MICs induced with 3MC exhibited the highest capacity to produce cytotoxic metabolites of CBZ. These findings will help optimize and standardize in vitro toxicity assays and provide guidance to pre-clinical investigation of drugs.

Ion Trap LC/MS reveals the generation of reactive intermediates in acalabrutinib metabolism: phase I metabolic profiling and bioactivation pathways elucidation

Acalabrutinib (CALQUENCE; ACB) is a Bruton tyrosine kinase inhibitor (BTKI) used to treat mantle cell lymphoma, small lymphocytic lymphoma (SLL), and chronic lymphocytic leukemia (CLL). On 21 November 2019, ACB was approved by the U.S. FDA for the use as a single therapy for the treatment of CLL/SLL. In silico studies were first done to propose vulnerable sites of metabolism and reactivity pathways by StarDrop software and Xenosite online software; respectively. ACB metabolites and stable adducts were characterized in vitro from rat liver microsomes (RLMs) using Ion Trap LC/MS. Generation of reactive intermediates (RIs) in the in vitro metabolism of ACB was investigated using glutathione, potassium cyanide, and methoxylamine as trapping nucleophiles for the RIs including iminopyridinone, iminium, and aldehyde, respectively, to form stable adducts that can be identified and characterized by Ion Trap LC/MS. Five phase I metabolites, seven 6-iminopyridin-3(6H)-one and five aldehyde RIs of ACB were identified. Based on literature reviews, the generation of RIs of ACB, and the subsequent drug-induced organ toxicity (DIOT) reactions may provide an explanation of ACB ADRs. Additional drug discovery investigations can be performed to facilitate the creation of novel medications with improved safety characteristics.

Evaluation of Cytotoxic, Necrotic, Apoptotic, and Autophagic Effects of Methamphetamine and 3,4-Methylenedioxymethamphetamine on U-87 MG (Glial) and B104-1-1 (Neuronal) Cell Lines

Methamphetamine (MA) and 3,4-methylenedioxymethamphetamine (MDMA) are empathogen (entactogen) psychoactive designer drugs which are mainly used for recreational purposes. Both MA and MDMA are central nervous system stimulants which are classified as monoamine neurotransmitter reuptake inhibitors. They have strong cytotoxic effects on dopaminergic and serotonergic neurons. Neurotoxicities of MA and MDMA by glial activation have been shown. The present work has investigated and measured cytotoxic, necrotic and apoptotic, and autophagic effects of MA and MDMA on U-87 MG (glial) and B104-1-1 (neuronal) cell lines by janus green, ethidium bromide/acridine orange, and monodansylcadaverine/propidium iodide staining to evaluate and compare their effects on glial and neuronal cells, respectively. The results of the present work showed that: (1) MDMA induced more potent mitochondrial toxicity, stronger necrotic and autophagic effects than MA in both B104-1-1 (neuronal) and U-87 MG (glial) cell lines; (2) although MDMA induced stronger apoptotic effect than MA on U-87 MG cell line, it had equal apoptotic effect on B104-1-1 cell line with MA; and (3) MDMA induced more potent mitochondrial toxicity, stronger necrotic, apoptotic, and autophagic effects than MA in B104-1-1 cell line than U-87 MG cell line.

Successful and Unsuccessful Prediction of Human Hepatic Clearance for Lead Optimization

Development of new chemical entities is costly, time-consuming, and has a low success rate. Accurate prediction of pharmacokinetic properties is critical to progress compounds with favorable drug-like characteristics in lead optimization. Of particular importance is the prediction of hepatic clearance, which determines drug exposure and contributes to projection of dose, half-life, and bioavailability. The most commonly employed methodology to predict hepatic clearance is termed in vitro to in vivo extrapolation (IVIVE) that involves measuring drug metabolism in vitro, scaling-up this in vitro intrinsic clearance to a prediction of in vivo intrinsic clearance by reconciling the enzymatic content between the incubation and an average human liver, and applying a model of hepatic disposition to account for limitations of protein binding and blood flow to predict in vivo clearance. This manuscript reviews common in vitro techniques used to predict hepatic clearance as well as current challenges and recent theoretical advancements in IVIVE.

Identification and characterization of in vitro, in vivo, and reactive metabolites of tandutinib using liquid chromatography ion trap mass spectrometry

Tandutinib (TND) is a novel, oral small molecule designed for treating acute myeloid leukemia (AML) by inhibiting type III receptor tyrosine kinases. This study reports the use of in silico, in vivo, and in vitro methods to investigate the metabolism and possible metabolic bioactivation of TND. First, in silico metabolism of TND was assessed using the WhichP450™ module of the StarDrop® software to determine labile sites of metabolism in the TND chemical structure. Second, the XenoSite reactivity model, a web-based metabolism prediction software, was used to determine probable bioactive centers. Based on the in silico outcomes, a list of predicted metabolites and reactive intermediates were prepared. Third, in vitro and in vivo experiments were performed. In vitro TND metabolites were generated through incubation of TND with rat liver microsomes (RLMs). Another incubation of TND with RLMs was separately performed in the presence of GSH and KCN to check for the generation of reactive intermediates (soft and hard electrophiles). In vitro phase II metabolism was assessed by incubation of TND with isolated perfused rat hepatocytes. In vivo metabolism was investigated by oral gavage of TND (37 mg kg-1) in Sprague Dawley rats. Five in vitro phase I metabolites, one in vitro phase II and five reactive iminium intermediates (cyano adducts), six in vivo phase I, and one in vivo phase II metabolites of TND were characterized. The in vitro and in vivo metabolic pathways involved were O-dealkylation, α-hydroxylation, α-carbonyl formation, reduction, glucuronide, and sulfate conjugation. No GSH conjugate or its catabolic products were detected either in vitro or in vivo. Two cyclic tertiary rings of TND (piperazine and piperidine) were metabolically bioactivated to generate reactive iminium intermediates forming cyano adducts with KCN. The formed reactive intermediates may be the reason behind TND toxicity. In silico toxicological studies were performed for TND and its related (in vitro and in vivo) metabolites were evaluated using the DEREK software tool.

Development of a method to determine cytochrome P450 1A2, 2C9, 2D6 and 3A4 activity sheep hepatic microsomes

INTRODUCTION

Ex vivo studies of human fetal hepatic drug metabolism are uncommon as it requires access to functional liver tissue and therefore raises practical and ethical concerns. Large animal models provide an alternative opportunity to study changes in cytochrome P450 (CYP) activity in the mother and fetus during pregnancy. We aimed to develop methods to determine the activity of CYP1A2, CYP2C9, CYP2D6 and CYP3A4 in sheep hepatic microsomes.

METHODS

We identified optimal conditions to determine the activity of CYP1A2 (using the probe drug phenacetin), CYP2C9 (diclofenac), CYP2D6 (dextromethorphan) and CYP3A4 (midazolam) by varying techniques for microsome extraction, probe drug concentration, incubation time and microsome concentration. The specificity of each probe drug was assessed by determining the rate of metabolism when specific CYP enzyme inhibitors were included in the reaction.

RESULTS

The optimum incubation time and probe drug concentration was six hours with 5 μM phenacetin (CYP1A2), four hours with 10 μM diclofenac (CYP2C9), 30 min with 1 μM of midazolam (CYP3A4) and 10 min with 1 μM dextromethorphan (CYP2D6). For both CYP2D6 and CYP3A4 reactions required 20 μg of microsomal protein, whereas for CYP1A2 and CYP2C9, reactions required 40 μg of microsomal protein. Metabolism of phenacetin, dextromethorphan and midazolam was reduced by specific enzyme inhibitors, but the specific CYP2C9 inhibitor sulfaphenazole did not substantially inhibit diclofenac metabolism.

DISCUSSION

This study identifies the optimal conditions for determining CYP activity in maternal sheep hepatic microsomes. In doing so, we have developed a standardised protocol for assessment of microsomal activity of CYP3A4, CYP1A2 and CYP2D6, but we were unable to optimise conditions for assessment of CYP2C9. This approach can be applied to investigate the impact of pregnancy complications on maternal and fetal hepatic drug metabolism.

In silico and in vitro metabolism of ribociclib: a mass spectrometric approach to bioactivation pathway elucidation and metabolite profiling

Ribociclib (RBC, Kisqali®) is a highly selective CDK4/6 inhibitor that has been approved for breast cancer therapy. Initially, prediction of susceptible sites of metabolism and reactivity pathways were performed by the StarDrop WhichP450™ module and the Xenosite web predictor tool, respectively. Later, in vitro metabolites and adducts of RBC were characterized from rat liver microsomes using LC-MS/MS. Subsequently, in silico data was used as a guide for the in vitro work. Finally, in silico toxicity assessment of RBC metabolites was carried out using DEREK software and structural modification was proposed to reduce their side effects and to validate the bioactivation pathway theory using the StarDrop DEREK module. In vitro phase I metabolic profiling of RBC was performed utilizing rat liver microsomes (RLMs). Generation of reactive metabolites was investigated using potassium cyanide (KCN) as a trapping nucleophile for the transient and reactive iminium intermediates to form a stable cyano adduct that can be identified and characterized using mass spectrometry. Nine phase I metabolites and one cyano adduct of RBC were characterized. The proposed metabolic pathways involved in generation of these metabolites are hydroxylation, oxidation and reduction. The reactive intermediate generation mechanism of RBC may provide an explanation of its adverse reactions. Aryl piperazine is considered a structural alert for toxicity as proposed by the DEREK report. We propose that the generation of only one reactive metabolite of RBC in a very small concentration is due to the decreased reactivity of the piperazine ring compared to previous reports of similar drugs. Docking analysis was performed for RBC and its proposed derivatives at the active site of the human CDK6 enzyme. Methyl-RBC exhibited the best ADMET and docking analysis and fewer side effects compared to RBC and fluoro-RBC. Further drug discovery studies can be conducted taking into account this concept allowing the development of new drugs with enhanced safety profiles that were confirmed by using StarDrop software. To the best of our knowledge, this is the first literature report of RBCin vitro metabolic profiling and structural characterization and toxicological properties of the generated metabolites.

Nine phase I metabolites and one product of KCN trapping of RBC were characterized. Aryl piperazine is considered a structural alert for toxicity as proposed by the DEREK report. Methyl-RBC exhibited less toxicity and more binding affinity to CDK6.

Detection and characterization of olmutinib reactive metabolites by LC-MS/MS: Elucidation of bioactivation pathways

Olmutinib (Olita™) is an orally bioavailable third generation epidermal growth factor receptor tyrosine kinase inhibitor. Olmutinib was approved in South Korea in May 2016 for the treatment of patients suffering from locally advanced or metastatic epidermal growth factor receptor T790M mutation-positive non-small cell lung cancer. Reactive olmutinib intermediates may be responsible for the severe side effects associated with the treatment. However, literature review revealed no previous reports on the structural identification of reactive olmutinib metabolites. In this work, the formation of reactive olmutinib metabolites in rat liver microsomes was investigated. Methoxylamine, glutathione, and potassium cyanide were used as capturing agents for aldehyde, iminoquinones, and iminium intermediates, respectively. The stable complexes formed were identified using liquid chromatography-tandem mass spectrometry. The major phase I metabolic pathway observed in vitro was hydroxylation of the piperazine ring. Seven potential reactive intermediates were characterized, including three iminium ions, three iminoquinones, and one aldehyde. Based on the findings, various bioactivation pathways were postulated. Hence, identifying the reactive intermediates of olmutinib that may be the cause of severe side effects can provide new insights, leading to improved treatments for patients.

Identification of reactive intermediate formation and bioactivation pathways in Abemaciclib metabolism by LC-MS/MS: in vitro metabolic investigation

Abemaciclib (Verzenio®) is approved as a tyrosine kinase inhibitor (TKI) for breast cancer treatment. In this study, in vitro phase I metabolic profiling of Abemaciclib (ABC) was done using rat liver microsomes (RLMs). We checked the formation of reactive intermediates in ABC metabolism using RLMs in the presence of potassium cyanide (KCN) that was used as a capturing agent for iminium reactive intermediates forming a stable complex that can be characterized by LC-MS/MS. Nine in vitro phase I metabolites and three cyano adducts were identified. The metabolic reactions involved in the formation of these metabolites and adducts are reduction, oxidation, hydroxylation and cyanide addition. The bioactivation pathway was also proposed. Knowing the electrodeficient bioactive centre in ABC structure helped in making targeted modifications to improve its safety and retain its efficacy. Blocking or isosteric replacement of α-carbon to the tertiary nitrogen atoms of piperazine ring can aid in reducing toxic side effects of ABC. No previous articles were found about in vitro metabolic profiling for ABC or structural identification of the formed reactive metabolites for ABC.

Reactive intermediates and bioactivation pathways characterization of avitinib by LC-MS/MS: In vitro metabolic investigation

Avitinib (AC0010) is a third generation inhibitor of the EGFR (epidermal growth factor receptor) that was permitted parallel phase I clinical trials in the US and in 2014. It is estimated to enter in market within two years. In the current study, eight in vitro metabolites were detected and their chemical structures were postulated. The main in vitro phase-I metabolic reaction was N-oxidation in piperazine moiety. The generation of reactive metabolites in avitinib metabolism was investigated using rat liver microsomes while adding capturing agents, viz potassium cyanide for reactive iminium intermediates, GSH for iminoquinones and methoxylamine for aldehyde forming stable adducts which are identifiable by LC-MS/MS. Ten reactive intermediates (four iminoquinones, three iminium and three aldehydes) were characterized. The three capturing agents used resulted in proposing four different bioactivation pathways. Upon literature examination, no former articles were found for avitinib metabolism including the produced reactive metabolites.

Characterization of reactive intermediates formation in dacomitinib metabolism and bioactivation pathways elucidation by LC-MS/MS: in vitro phase I metabolic investigation

Dacomitinib (DCB) is a second generation irreversible tyrosine kinase inhibitor (TKI) that is claimed to overcome the disadvantages of the resistance developed by the first line epidermal growth factor receptor (EGFR) TKIs. In the current study, metabolites of phase I for DCB were systematically explored. DCB reactive metabolites were also investigated in rat liver microsomes in presence of potassium cyanide or methoxylamine that were employed as capturing agents for iminium reactive intermediates and aldehyde, respectively, to form stable complexes which can be detected by LC-MS/MS. As a result, four in vitro phase I metabolites were observed with major pathway of piperidine ring hydroxylation. Additionally, two potentially reactive intermediates, one aldehyde and one iminium ions were characterized. Two different pathways of bioactivation were ultimately proposed.

Identification and characterization of in vivo, in vitro and reactive metabolites of vandetanib using LC-ESI-MS/MS

Vandetanib (Caprelsa tablets, VNT) is an orally inhibitor of vascular endothelial growth factor receptor 2. The current research reports the characterization and identification of in vitro, in vivo and reactive intermediates of VNT. In vitro metabolites of VNT were performed by incubation with rat liver microsomes (RLMs). Extraction of vandetanib and its in vitro metabolites from the incubation mixtures were done by protein precipitation. In vivo metabolism was done by giving one oral dose of vandetanib (30.8 mg/kg) to Sprague Dawley rats in metabolic cages by using oral gavage. Urine was gathered then filtered at certain time intervals (0, 6, 12, 18, 24, 48, 72, 96 and 120 h) from vandetanib dosing. A similar volume of ACN was added to each collected urine sample. Both layers (organic and aqueous) were injected into liquid chromatography electro spray ionization tandem mass spectrometry (LC-ESI-MS/MS) to detect in vivo vandetanib metabolites. N-methyl piperidine ring of vandetanib is considered a cyclic tertiary amine that undergoes metabolism forming iminium intermediates that are very reactive toward nucleophilic macromolecules. Incubation of vandetanib with RLMs in the presence of 1.0 mM KCN was made to check reactive metabolites as it is usually responsible for noticeable idiosyncratic toxicities including phototoxicity and QT interval prolongation. Four in vivo phase I, one in vivo phase II metabolites, six in vitro phase I metabolites and four cyano conjugates of vandetanib were detected by LC-MS/MS. In vitro and in vivo phase I metabolic reactions were N-oxide formation, N-demethylation, α-carbonyl formation and α-hydroxylation. In vivo phase II metabolic reaction was direct conjugation of vandetanib with glucuronic acid. All metabolic reactions occurred in N-methyl piperidine of vandetanib which causes toxicity and instability of vandetanib.

A reliable and stable method for the determination of foretinib in human plasma by LC-MS/MS: Application to metabolic stability investigation and excretion rate

Foretinib (GSK1363089) is a multiple receptor tyrosine kinases inhibitor. In this study, a reliable, fast liquid chromatography-tandem mass spectrometric method was described for assaying foretinib in plasma, urine, and rat liver microsome samples. Simple extraction procedure by protein preciptation with acetonitrile was implemented for foretinib and brigatinib (internal standard) analysis. Chromatographic resolution of analytes was achieved on C₁₈ column with the help of isocratic mobile phase. The binary mobile phase consisted of 60% ammonium formate (10 mM, pH 4.2) and 40% acetonitrile at a flow rate of 0.25 mL/min. Run time was 3 min, and both foretinib and brigatinib were eluted within 0.74 and 1.95 min; they were detected in positive ion mode utilizing multiple reactions monitoring mode. Linearity of the proposed method ranged from 5 to 500 ng/mL (r²≥ 0.9993) in the human plasma. Lower limit of quantification and detection were 6.0 and 1.8 ng/mL, respectively. Intraday and interday precision and accuracy were 0.16 to 1.67 % and -2.39 to -0.52 %. In vitro half-life and intrinsic clearance were 24.93 min and 6.56 mL/min/kg, respectively. Literature review showed that no previous studies have been proposed for the analytical quantification of foretinib in human plasma or its metabolic stability. The established method was also applied to estimate the rate of foretinib excretion in rat urine. The developed method can be used for foretinib pharmacokinetic applications.

LC-MS/MS reveals the formation of iminium and quinone methide reactive intermediates in entrectinib metabolism: In vivo and in vitro metabolic investigation

Entrectinib (RXDX-101) is orally available inhibitor of the tyrosine kinases including tropomyosin receptor kinases (Trk) A-C, C-ros oncogene 1 (ROS1) and anaplastic lymphoma kinase (ALK), with potential antineoplastic activity. Entrectinib (ENB) granted breakthrough designation by FDA for NTRK + Solid tumors. In vitro metabolism of ENB generates quinone methide and iminium reactive intermediates that were captured by potassium cyanide and GSH, respectively forming stable conjugates that were characterized by LC-MS/MS. Seven in vitro ENB metabolites were identified through four metabolic reactions including hydroxylation, N-dealkylation, N-oxidation and reduction. Furthermore, four reactive intermediates including two quinone methide and two iminium ions were detected and the bioactivation mechanisms were supposed. In vivo metabolism of ENB was done by giving single oral dose (35.2 mg/kg) to Sprague Dawley rats. In vivo metabolism generates five phase I metabolites similar to in vitro metabolism except no metabolic reactions were identified on indazole ring. One phase II metabolite was characterized in in vivo metabolism of ENB resulted from glucuronidation of hydroxyl metabolite of ENB. Reporting these data for ENB is very crucial in the development stage. Reviewing literatures revealed the absence of previous articles have been done for the ENB in vitro or in vivo metabolism study or structural characterization of the formed reactive intermediates.

Investigation of metabolic degradation of new ALK inhibitor: Entrectinib by LC-MS/MS

Entrectinib (ENC) is a potent orally available anaplastic lymphoma kinase (ALK) inhibitor. In 10 July 2017, biotechnology company (Ignyta) announced that granted orphan drug designation approval was given by the FDA to ENC for "treatment of NTRK fusion-positive solid tumors". A validated LC-MS/MS methodology was developed for ENC quantification in human plasma matrix. The supposed method characterized by high speed, specificity and sensitivity. This established method was applied for metabolic degradation assessment of ENC. Reversed stationary phase (C₁₈ column) and elution mobile phase (48% 10 mM ammonium formate in H₂O (pH: 4.2 adjusted by adding few drops of formic acid): 52% ACN) were utilized for chromatographic resolution of ENC and lapatinib as internal standard (IS). Total elution time, flow rate and injection volume were 4 min., 0.25 mL/min., and 5 μL, respectively. Electrospray ionization source was used for ions generation, while positive multiple reactions monitoring (MRM) mode was used for ion analysis. The data of calibration curve of ENC in human plasma was linear in the range of 5-500 ng/mL with correlation coefficient (r²) >0.999. LOQ and LOD for ENC were 2.17 ng/mL and 0.71 ng/mL, respectively. Inter-day and intra-day precision and accuracy were 97.52 to 101.83%, and 0.38 to 1.32%, respectively. Intrinsic clearance (Clint) and in vitro half-life (t_1/2) were equal to 15.67 mL/min/kg and 9.1 min, respectively. To our knowledge, this is considered the first method for ENC quantification in human plasma and its metabolic degradation assessment.

LC-MS/MS reveals the formation of reactive ortho-quinone and iminium intermediates in saracatinib metabolism: Phase I metabolic profiling

Saracatinib (AZD-0530) is a drug under clinical trials that developed by AstraZeneca. It is considered a dual kinase inhibitor, with selective actions as a Src inhibitor and a Bcr-Abl tyrosine-kinase inhibitor. Saracatinib chemical structure contains N-methyl piperazine group and 1,3 benzodioxole group. N-methyl piperazine group that can be bioactivated to form iminium intermediates which can be captured by KCN. 1,3-Benzodioxole group can be bioactivated to form ortho-quinone intermediate that can be conjugated with GSH. The formed conjugates are stable and can be identified using LC-MS/MS. In our current work, we are trying to give insight into the reasons that may be responsible for saracatinib side effects. Using LC-MS/MS, in vitro metabolic pathways were investigated for saracatinib in rat liver microsomes. Ten saracatinib phase I metabolites were characterized and the metabolic pathways were found to be hydroxylation, oxidation, reduction, dealkylation, N-oxidation and ether cleavage. Also, four potential reactive intermediates (three cyanide adducts and one GSH conjugate) were identified and the bioactivation mechanisms were explained. The existence of these four reactive metabolites may be the main reason for observed saracatinib side effects in clinical trials. Literature review showed no previous articles have been proposed the detailed structural identification of the formed reactive metabolites.

Investigation of metabolic stability of the novel ALK inhibitor brigatinib by liquid chromatography tandem mass spectrometry

Brigatinib (BGB) belongs to a class of drugs called ALK inhibitor. On April 28, 2017, BGB has been approved by U.S. FDA for use in metastatic ALK-positive NSCLC. A fast, specific, sensitive and validated LC-MS/MS method was developed for the quantification of BGB in human plasma matrix. This method was applied successfully to study metabolic stability of BGB. Reversed phase (C18 column) and isocratic binary mobile phase (55% 0.1% formic acid: 45% ACN) were used for chromatographic separation of BGB and ponatinib (IS). The flow rate, total run time and injection volume were fixed at 0.2 mL/min, 4 min, 5 μL respectively. ESI source was utilized for ions formation, while multiple reaction monitoring (MRM) mode was used for ion analysis. In human plasma matrix, the Linearity range of the calibration curve was 5-500 ng/mL (r² ≥ 0.9982). LOQ and LOD were found to be 1.89 and 5.72 ng/mL. The precision and accuracy for the intra-day and inter-day were 0.45 to 1.85% and 97.37 to 104.85%. In vitro half-life (t_1/2) and intrinsic clearance (CL_int) were equal to 12.0 min and 13.1 ± 0.15 mL/min/kg respectively. The quantification of BGB in human plasma or its metabolic stability has not been studied as seen in literature review.

LC-ESI-MS/MS reveals the formation of reactive intermediates in brigatinib metabolism: elucidation of bioactivation pathways

Four phase I BGB metabolites and three cyano adducts for BGB were detected using LC-MS/MS. The piperidine ring was found to be responsible for BGB bioactivation and the bioactivation pathways are proposed.

Investigation of the metabolic stability of olmutinib by validated LC-MS/MS: quantification in human plasma

Olmutinib (OTB, Olita™) is an orally available third-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR TKI). It was developed by Boehringer Ingelheim and Hanmi Pharmaceutical Co. Ltd for the cure of non-small cell lung cancer (NSCLC). In May 2016, OTB was approved in South Korea for the treatment of patients suffering from metastatic or locally advanced EGFR T790M mutation-positive NSCLC. A LC-MS/MS methodology was validated for OTB quantification in human plasma. An extended application for this validated LC-MS/MS is OTB metabolic stability evaluation. Chromatographic separation of OTB and ponatinib (PNT, IS) was attained using a reversed phase with isocratic elution. The linearity of the developed LC-MS/MS method ranged from 5.00 to 500.00 ng mL⁻¹ with r² ≥ 0.9999 in human plasma. LOD and LOQ were 1.12 and 3.39 ng mL⁻¹, respectively. The intra-day and inter-day precision and accuracy were 1.17 to 2.75% and 97.86 to 101.48%, respectively. The intrinsic clearance (CL_int) was 2.71 mL min⁻¹ kg⁻¹ and the in vitro half-life (t_1/2) was 48.80 min. A review of the literature revealed that there are no previous articles about the quantification of OTB in human plasma using LC-MS/MS or its metabolic stability assessment.

Olmutinib (OTB, Olita™) is an orally available third-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR TKI).

LC-MS/MS method for the quantification of masitinib in RLMs matrix and rat urine: application to metabolic stability and excretion rate

Masitinib (MST) is a selective tyrosine kinase inhibitor. Validated liquid chromatography tandem mass spectrometric method (LC–MS/MS) was developed for the quantification of MST in rat liver microsomes (RLMs) matrix. The developed method was applied to metabolic stability and excretion rate studies. Reversed phase liquid chromatography was used for resolution of MST and bosutinib (IS) using C₁₈ (50 mm × 2.1 mm, 1.8 μm). Binary solvent system consisted of 35% solvent A (0.1% formic acid in H₂O, pH: 3.2) and 65% solvent B (acetonitrile) used as mobile phase at flow rate of 0.25 mL with a total run time of 5 min. Injection volume was 5 µL. Generation of ions was done in positive ESI source and quantification of MST and IS were done using MRM mode. The developed method showed a linearity in the range of 5–200 ng/mL (r² ≥ 0.9992) with LOQ and LOD of 0.25 and 0.76 ng/mL in RLMs. The intra- and inter-day precision and accuracy ranged from 0.95 to 1.49 and − 5.22 to 1.13%, respectively in RLMs. Rate of disappearance of MST during incubation with RLMs was almost linear allover incubation time. In vitro t_1/2 was 50.38 min and CL_in was 3.11 ± 0.2. The developed method was applied also to measure the rate of masitinib excretion in rat urine. The method can used for further pharmacokinetic studies of MST.

Liquid chromatography tandem mass spectrometry method for the quantification of vandetanib in human plasma and rat liver microsomes matrices: metabolic stability investigation

Vandetanib (VNT) is a new oral tyrosine kinase inhibitor that acts mainly by inhibiting vascular endothelial growth factor receptor (VEGFR). Fast, specific, sensitive and validated LC–MS/MS was established for the determination of VNT in two various matrices including rat liver microsomes (RLMs) and human plasma. This method was applied in metabolic stability investigation of VNT. Resolution of two analytes was performed using C18 column and isocratic mobile phase composed of binary system of 10 mM ammonium formate (pH 4.1) and acetonitrile in a ratio of 1:1. The flow rate was set at 0.25 mL/min and total run time was 4 min with injection volume of 5 µL. Ions were generated by ESI source and analyzed by multiple reaction monitoring mode (basis for quantification) in the Agilent 6410 QqQ analyzer. The linearity of the established method ranged from 5 to 500 ng/mL (r² ≥ 0.9996) in human plasma and RLMs. LOQ and LOD were 2.48 and 7.52 ng/mL, and 2.14 and 6.49 in human plasma and RLMs matrices. The intra-day and inter-day precision and accuracy were 0.66–2.66% and 95.05–99.17% in human plasma matrix while in RLMs matrix, ranged from 0.97 to 3.08% and 95.8 to 100.09%, respectively. In vitro half-life was 39.85 min and intrinsic clearance was 3.92 ± 0.28 mL/min/kg.

LC-MS/MS reveals the formation of aldehydes and iminium reactive intermediates in foretinib metabolism: phase I metabolic profiling

Using LC-MS/MS, six phase I foretinib metabolites in addition to four potential reactive metabolites, two aldehydes and two iminium ions, were detected and the bioactivation pathways were proposed.

Identification and characterization of in vitro phase I and reactive metabolites of masitinib using a LC-MS/MS method: bioactivation pathway elucidation

In vitro phase I metabolic reactions & cyano conjugates of masitinib.

Biotransformation of bromhexine by Cunninghamella elegans, C. echinulata and C. blakesleeana

Fungi is a well-known model used to study drug metabolism and its production in in vitro condition. We aim to screen the most efficient strain of Cunninghamella sp. among C. elegans, C. echinulata and C. blakesleeana for bromhexine metabolites production. We characterized the metabolites produced using various analytical tools and compared them with mammalian metabolites in Rat liver microsomes (RLM). The metabolites were collected by two-stage fermentation of bromhexine with different strains of Cunninghamella sp. followed by extraction. Analysis was done by thin layer chromatography, high performance thin layer chromatography, Fourier transform infrared spectroscopy, high performance liquid chromatography and Liquid chromatography–mass spectrometry. The role of Cytochrome P3A4 (CYP3A4) enzymes in bromhexine metabolism was studied. Fungal incubates were spiked with reference standard – clarithromycin to confirm the role of CYP3A4 enzyme in bromhexine metabolism. Three metabolites appeared at 4.7, 5.5 and 6.4 min retention time in HPLC. Metabolites produced by C. elegans and RLM were concluded to be similar based on their retention time, peak area and peak response of 30.05%, 21.06%, 1.34%, and 47.66% of three metabolites and bromhexine in HPLC. The role of CYP3A4 enzyme in metabolism of bromhexine and the presence of these enzymes in Cunninghamella species was confirmed due to absence of peaks at 4.7, 5.4 and 6.7 min when RLM were incubated with a CYP3A4 enzyme inhibitor – clarithromycin.

Validated LC-MS/MS Method for the Quantification of Ponatinib in Plasma: Application to Metabolic Stability

In the current work, a rapid, specific, sensitive and validated liquid chromatography tandem mass-spectrometric method was developed for the quantification of ponatinib (PNT) in human plasma and rat liver microsomes (RLMs) with its application to metabolic stability. Chromatographic separation of PNT and vandetanib (IS) were accomplished on Agilent eclipse plus C18 analytical column (50 mm × 2.1 mm, 1.8 μm particle size) maintained at 21±2°C. Flow rate was 0.25 mLmin^-1 with run time of 4 min. Mobile phase consisted of solvent A (10 mM ammonium formate, pH adjusted to 4.1 with formic acid) and solvent B (acetonitrile). Ions were generated by electrospray (ESI) and multiple reaction monitoring (MRM) was used as basis for quantification. The results revealed a linear calibration curve in the range of 5–400 ngmL^-1 (r² ≥ 0.9998) with lower limit of quantification (LOQ) and lower limit of detection (LOD) of 4.66 and 1.53 ngmL^-1 in plasma, 4.19 and 1.38 ngmL^-1 in RLMs. The intra- and inter-day precision and accuracy in plasma ranged from1.06 to 2.54% and -1.48 to -0.17, respectively. Whereas in RLMs ranged from 0.97 to 2.31% and -1.65 to -0.3%. The developed procedure was applied for quantification of PNT in human plasma and RLMs for study metabolic stability of PNT. PNT disappeared rapidly in the 1st 10 minutes of RLM incubation and the disappearance plateaued out for the rest of the incubation. In vitro half-life (t_1/2) was 6.26 min and intrinsic clearance (CL_in) was 15.182± 0.477.

Detection and characterization of ponatinib reactive metabolites by liquid chromatography tandem mass spectrometry and elucidation of bioactivation pathways

This study involved the identification and characterization of ponatinib by LC-MS/MS. Additionally ponatinib reactive metabolites were also investigated using KCN as a trapping agent, showing the formation of four reactive cyano adducts.

Fermented wheat powder induces the antioxidant and detoxifying system in primary rat hepatocytes

Many plants exhibit antioxidant properties which may be useful in the prevention of oxidative stress reactions, such as those mediated by the formation of free radical species in different pathological situations. In recent years a number of studies have shown that whole grain products in particular have strong antioxidant activity. Primary cultures of rat hepatocytes were used to investigate whether and how a fermented powder of wheat (Lisosan G) is able to modulate antioxidant and detoxifying enzymes, and whether or not it can activate Nrf2 transcription factor or inhibit NF-kB activation. All of the antioxidant and detoxifying enzymes studied were significantly up-regulated by 0.7 mg/ml Lisosan G treatment. In particular,

NAD(P)H

quinone oxidoreductase and heme oxygenase-1 were induced, although to different degrees, at the transcriptional, protein and/or activity levels by the treatment. As for the Nrf2 transcription factor, a partial translocation of its protein from the cytosol to the nucleus after 1 h of Lisosan G treatment was revealed by immunoblotting. Lisosan G was also observed to decrease H2O2-induced toxicity Taken together, these results show that this powder of wheat is an effective inducer of ARE/Nrf2-regulated antioxidant and detoxifying genes and has the potential to inhibit the translocation of NF-kB into the nucleus.

Structural influence of isothiocyanates on expression of cytochrome P450, phase II enzymes, and activation of Nrf2 in primary rat hepatocytes

Primary cultures of rat hepatocytes were used to investigate whether and how eight isothiocynates (ITCs) with different chemical structures (the aromatic benzyl, 4-hydroxybenzyl, phenethyl isothiocyanates and the aliphatic allyl, napin, iberin, raphasatin isothiocyanates and sulforaphane) derived from hydrolyzed glucosinolates, were able to modulate cytochrome P450 (CYP) and antioxidant/detoxifying enzymes and to activate the Nrf2 transcription factor. The aromatic ITCs at 40 μM markedly increased the transcription of CYP1A1 and 1A2 mRNA and increased the associated ethoxyresorufin O-deethylase (EROD) activity after 24 h of treatment. By contrast, the aliphatic ITCs (40 μM) decreased CYP1A1 and 1A2 transcription, together with the corresponding EROD activity. The same treatment also caused a striking and similar transcriptional repression of CYP3A2, and the corresponding benzyloxyquinoline debenzylase activity in response to all the ITCs tested. In the same culture conditions, most of the antioxidant/detoxifying enzymes were significantly up-regulated by 40μM ITCs. In particular, NAD(P)H:quinone oxidoreductase and heme oxygenase-1 were induced, although to different levels, at transcriptional, protein and/or activity levels by all the ITCs. However, glutathione S-transferase activity was not induced by the allyl, benzyl, and 4-hydroxybenzyl ITCs, glutathione reductase activity was not induced by benzyl, and 4-hydroxybenzyl ITCs and catalase activity was not induced by allyl ITC. As for the Nrf2 transcription factor, a partial translocation of its protein from the cytosol to the nucleus was revealed by immunoblotting after 1h of treatment for all the ITCs tested. The ability of ITCs to induce the antioxidant and phase II enzymes did not appear to be affected by their hydrophilicity or other structural factors. Taken together, these results show that these ITCs are effective inducers of ARE/Nrf2-regulated antioxidant/detoxifying genes and have the potential to inhibit, at least in rat liver, the bioactivation of carcinogens dependent on CYP3A2 catalysis.

Metabolic studies on haloperidol and its tetrahydropyridinyl dehydration product (HPTP) in C57BL/6 mouse brain preparations

The neuroleptic agent haloperidol (HP) and its tetrahydropyridinyl dehydration product HPTP are biotransformed by humans, baboons and rodents to the HP pyridinium (HPP(+)) and reduced HP pyridinium (RHPP(+)) species, potential neurotoxic metabolites that have been detected in the brain. HPP(+), however, does not pass the mouse blood-brain barrier since it is not detected in the brain following systemic administration. We report here that C57BL/6 mouse brain preparations catalyze the oxidation of HP and HPTP to HPP(+). The initial rate of HPP(+) formation from HPTP by whole brain homogenates was estimated to be approximately 20 times faster than that observed with HP as substrate. HPTP also was converted to HPP(+) by mouse brain microsomal preparations and brain slices. These results suggest that the presence of HPP(+) in the C57BL/6 mouse brain following systemic administration of HPTP may be due primarily to its in situ metabolism to HPP(+). Attempts to identify the catalyst responsible for these biotransformations, however, have not been successful.

Inactivation of phenobarbital-inducible rabbit-liver microsomal cytochrome P-450 by allylisopropylacetamide: impact on electron transfer

Application of a single dose of allylisopropylacetamide (AIA) to phenobarbital-pretreated rabbits resulted in partial destruction of the heme moiety of liver microsomal cytochrome P-450. A minor fraction of chromophore loss was accounted for by heme-derived product(s) covalently attached to microsomal proteins. Interestingly, cytochrome P-450 appeared to have undergone significant drug-mediated alkylation of the apohemoprotein. The modified species was purified to apparent homogeneity and shown to arise from AIA-induced blockage of about 2 histidines in the cytochrome P-450LM2 molecule located close to the heme edge. AIA administration to the animals caused inhibition of hexobarbital-promoted electron flow from NADPH-cytochrome P-450 reductase to phenobarbital-inducible ferricytochrome P-450 both in microsomal particles and reconstituted systems. The impaired interaction between the proteins was shown not to originate from decreased capacity to bind each other but more likely to be due to some defect in a step subsequent to complex formation. In contrast, treatment with the porphyrogenic agent did not affect microsomal electron transmission from cytochrome b5 to the ferric monooxygenase. However, when the intermediate carrier was to donate reducing equivalents to the ferrous oxycytochrome in the presence of benzphetamine, there was a pronounced deceleration of the electron flux observable. These findings were interpreted to mean that there exist multiple reductase- and cytochrome-b5-binding domains in phenobarbital-inducible cytochrome P-450, some of which seem to be common to the two redox proteins. This sheds interesting light on the molecular organization of the catalytic electron transfer complexes.

Evidence of the existence of structurally distinct hepatic and pulmonary forms of microsomal flavin-containing monooxygenase in the rabbit

The flavin-containing monooxygenase has been purified from rabbit liver and lung microsomes. SDS-PAGE analysis shows that both enzyme forms migrate as a single band with an apparent Mr of 59000. The NH2-terminus of both forms is blocked. The liver oxidase contains a lower percentage of glutamine/glutamate and a greater amount of phenylalanine than does the lung flavoprotein. Polyclonal antibodies to a 14-amino-acid peptide obtained after CNBr cleavage of the liver oxidase cross-react with the microsomal and purified liver enzyme, but do not recognize the lung oxidase. HPLC profiles of tryptic digests of the liver and lung enzymes exhibit different patterns. Sequence alignment of selected peptides from the liver and lung oxidases reveals aberrant residues within homologous segments. These findings are interpreted to mean that both enzymes represent distinct gene products.

Some properties of an inhibitory cytochrome P-450 metabolic-intermediate complex existing in a spin-state equilibrium

Incubation, in the presence of NADPH/O2, of the type II compound revenast with a partially solubilized, phenobarbital-induced rat liver microsomal cytochrome P-450 system results in the formation of a difference spectrum exhibiting a Soret band at 407 nm and a trough at 423 nm. Experiments with N2 and metyrapone suggest this spectral perturbation to originate from binding to the hemoprotein of a metabolic intermediate derived from the amine substrate. The percentage of pigment complexed can be assessed to be about 7%, with half-maximal complexation occurring at a revenast concentration of 125 microM. Adduct formation is inhibitory to the N-hydroxylation of 4-chloroaniline and N-demethylation of N,N-dimethylaniline; kinetic analysis suggests a competitive interaction at the catalytic site of the reactive revenast derivative with the tertiary arylamine. Inhibition of the two monooxygenation reactions is reversible, indicating weak heme bonding of the active intermediate. This behaviour differs from that of most other product adducts so far examined. The aberrant functional properties of the metabolic adduct appear to reflect complex molecular organization of this compound, as is also evidenced by the unique position of the Soret region absorption maximum at 407 nm; this spectral data hints at the presence of a mixture of high- and low-spin forms. Indeed, chemical reduction or oxidation of the product complex reveals the existence of a low-spin component hidden in the metabolic spectrum. Model studies suggest that the product adduct possibly arises from N-oxidation of the revenast molecule.

The functional role of cytochrome b5 reincorporated into hepatic microsomal fractions

Incorporation of detergent-solubilized cytochrome b5 into phenobarbital-induced rabbit liver microsomal fractions decelerates hexobarbital-dependent reduction of ferric cytochrome P-450; this is accompanied by retardation of NADPH utilization and H2O2 formation in the assay media. Integration of manganese-substituted cytochrome b5 into the microsomal preparations fails to affect these parameters. Analysis of the cytochrome P-450 reduction kinetics in the presence of increasing amounts of cytochrome b5 reveals a gradual augmentation of the amplitude of slow-phase electron transfer at the expense of the relative contribution of the fast phase; finally, a slow, apparently monophasic reaction persists. This defect in enzymatic reduction is not due to detergent effects and also does not seem to reflect cytochrome b5-induced perturbation of anchoring of NADPH-cytochrome c(P-450) reductase to cytochrome P-450. Experiments with the highly purified cytochrome P-450 isozyme LM2, in which amino acid residue(s) close to the heme edge had undergone suicidal inactivation through covalent attachment of chloramphenicol metabolite(s) do not exclude the possibility that cytochrome b5 and reductase might compete for a common electron transmission site on the terminal acceptor. Hence, the inhibitory action of cytochrome b5 on the reduction of ferric cytochrome P-450 is tentatively attributed to partial substitution of the former pigment for reductase in direct transport of the first electron to the monooxygenase.

Influence of N,N-dimethylaniline on the association of phenobarbital-induced cytochrome P-450 and NADPH-cytochrome c(P-450) reductase in a reconstituted rabbit liver microsomal enzyme system

N,N-Dimethylaniline when added to reaction mixtures provokes deviation from Michaelis-Menten law of the interaction kinetics of NADPH-cytochrome c(P-450) reductase (NADPH:ferrihaemoprotein oxidoreductase, EC 1.6.2.4) with highly purified phenobarbital-induced rabbit liver microsomal cytochrome P-450 (P-450LM2). This phenomenon is not associated with the low-to-high spin transition in the iron-coordination sphere of the haemoprotein, as elicited by the arylamine. Substrate-triggered departure from linearity of the kinetics is abolished by inclusion into the assay media of p-chloromercuribenzoate, hinting at a vital role in the process of thiols. Similarly, the parabolic progress curve (nH = 1.7) is transformed to a straight line (nH = 1.01) when the N-terminal reductase-binding domain in the P-450LM2 molecule is selectively blocked through covalent attachment of fluorescein isothiocyanate (FITC); such a modification does not alter the affinity of the haemoprotein for the amine substrate. Steady-state fluorescence polarization measurements reveal that N,N-dimethylaniline perturbs the motional properties of the fluorophore-bearing reductase-binding region, suggesting the induction of a conformational change. Summarizing these results, the data possibly indicate N,N-dimethylaniline-induced cooperativity in the association of reductase with P-450LM2.

Regulative mechanisms in NADH- and NADPH-supported N-oxidation of 4-chloroaniline catalyzed by cytochrome b5-enriched rabbit liver microsomal fractions

Incorporation into rabbit liver microsomal membranes of detergent-solubilized cytochrome b5 stimulates NADH-supported electron flow to ferric cytochrome P-450, but impairs NADPH-dependent reduction of the pigment such as to make the rates of both reactions equivalent; yet, in the enriched preparations NADPH-driven N-oxidation of 4-chloroaniline proceeds at considerably higher rate than does the NADH-supported process. Analysis of transfer of the second electron to oxyferrous cytochrome P-450, as assessed by measuring substrate-induced reoxidation of ferrous cytochrome b5, reveals faster flow with NADH than with NADPH as the source of reducing equivalents. Quantification of the pools of cytochrome P-450 active in attack on the amine substrate in the presence of either reduced pyridine nucleotide, as well as measurements of maximum arylamine turnover suggest that the cofactor-dependent discrepancy in N-oxidase activity reflects differences in the rates of breakdown of the intermediary enzyme complexes. The NADH- and NADPH-supported pathway of N-oxidation in the cytochrome b5-supplemented microsomal fractions thus probably involves distinct forms of cytochrome P-450. Alternatively, functional linkage of the cofactor-specific electron-transfer chains to a single cytochrome P-450 species might yield aggregates of differing conformational state and catalytic capacity. The latter concept receives support from experiments with individually reconstituted enzyme systems.

Formation and disposition of nitrosochloramphenicol in rat liver

It has been suggested that in the chloramphenicol-induced aplastic anemia nitrosochloramphenicol may be involved as a toxic intermediate. We found that aminochloramphenicol, which reportedly is formed from chloramphenicol by intestinal bacteria, is N-oxygenated by liver microsomes of untreated rats with apparent Km = 0.4 mM and Vmax = 0.28 nmole/min/mg protein. These values are in close agreement with those reported for aniline N-oxygenation. Reductive reactions, however, eliminate the N-oxygenation products at markedly higher rates. As judged from hemoglobin-free single-pass liver perfusion experiments, N-hydroxy-chloramphenicol is reduced at rates faster than 300 nmole/min/g liver wet, and nitrosochloramphenicol is eliminated at rates faster than 1.5 mumole/min/g liver. At least two NADPH- and two NADH-dependent cytosolic enzymes are responsible for nitrosochloramphenicol reduction. Determination of the kinetic parameters of these enzymes by stop-flow analysis revealed the contribution of enzymes, one of it being alcohol dehydrogenase, with Michaelis constants in the micromolar range. Despite this high reducing capacity, about 10% of nitrosochloramphenicol reacted with GSH under formation of glutathionesulfinamidochloramphenicol and GSSG released from the liver into bile and venous effluent. At high nitrosochloramphenicol load these reactions led to glutathione depletion of the liver, caused membrane damage, and impaired bile production. At low nitrosochloramphenicol load, i.e. below 0.5 mumole/min/g, no relevant nitrosochloramphenicol passed the liver. These data together with the previously reported reactions of nitrosochloramphenicol within human blood suggest that nitrosochloramphenicol, if formed at all in the intestine or liver, is rather unlikely to be transferred to the critical target.

The role of lipid peroxidation in the N-oxidation of 4-chloroaniline

Irradiation with u.v. light of aerobic aqueous media containing both rabbit liver microsomal fraction and 4-chloroaniline results in N-oxidation of the arylamine. The reaction is severely blocked by exhaustive extraction with organic solvents of the microsomal membranes to remove lipids. Further, scavengers of OH. and O2.-impair the photochemical process. These findings suggest that the observed phenomenon may be closely associated with light-induced lipid peroxidation. Indeed, N-oxidation of 4-chloroaniline is fully preserved when either phospholipid liposomes or dispersed linoleic acid substitute for intact microsomal fraction. Co-oxidation of the amine substrate occurs during iron/ascorbate-promoted lipid peroxidation also, but H2O2 or free OH. radicals do not appear to be involved. Cumene hydroperoxide-sustained rabbit liver microsomal turnover of the amine generates N-oxy product via O2-dependent and -independent pathways; propagation of lipid peroxidation is presumed to govern the former route. Lipid hydroperoxides, either exogenously added to rabbit liver microsomal suspensions or enzymically formed from arachidonic acid in ram seminal-vesicle microsomal preparations, support N-oxidation of 4-chloroaniline. The significance, in arylamine activation, of lipid peroxidation in certain extrahepatic tissues exhibiting but low mono-oxygenase activity is discussed.

On the function of cytochrome b5 in the cytochrome P-450-dependent oxygenase system

Complex formation between the phenobarbital-inducible form of rabbit liver microsomal cytochrome P-450 incorporated into phosphatidylcholine and detergent-solubilized cytochrome b5 is associated with a low-to-high spin transition of the former pigment. It is concluded that the proteins combine in a 1:1 molar ratio. CD spectral analysis in the far uv region reveals that interaction of the cytochromes results in a conformational change of one or both hemoproteins. Such a cytochrome b5-induced structural alteration of the reconstituted enzyme system is accompanied by an increase in affinity of 4-chloroaniline for cytochrome P-450, as measured in terms of cumene hydroperoxide-supported N-oxidation of the arylamine; the maximum velocity of the catalytic process remains unchanged. Similarly, incorporation into the assay media of cytochrome b5 decreases the apparent Kd values of both the amine substrate and the oxygen donor, as determined by optical titration. Stopped-flow spectrophotometric studies on the influence of cytochrome b5 on the kinetics of binding to cytochrome P-450 of 4-chloroaniline and/or cumene hydroperoxide show that the rates of formation and decay of the adducts change as the molar ratio of cytochrome b5 to cytochrome P-450 varies. Moreover, cytochrome b5 modifies the activation energies required for production of the substrate-bound oxy complex. These findings suggest that cytochrome b5, apart from its well-known role as an electron carrier, might exert an effector function in the cytochrome P-450 system.

Comparative studies on the cumene hydroperoxide- and NADPH-supported N-oxidation of 4-chloroaniline by cytochrome P-450

The present study confirms that cytochrome P-450 can act as a catalyst in the cumene hydroperoxide-supported N-oxidation of 4-chloroaniline. Analogous to the NADPH/O2-driven N-oxidation process, product dissociation is likely to limit the overall rate of cytochrome P-450 cycling also in the peroxidatic pathway. The oxy complexes involved in either metabolic route differ with respect to stability, spectral properties and need for thiolate-mediated resonance stabilization. With the organic hydroperoxide, the metabolic profile is shifted from the preponderant production of N-(4-chlorophenyl)hydroxylamine to the formation of 1-chloro-4-nitrobenzene. This finding suggests that the peroxide-sustained N-oxidation mechanism differs in several ways from that functional in the NADPH/O2-dependent oxenoid reaction. Thus one-electron oxidation, triggered by homolytic cleavage of the oxygen donor, is proposed as the mechanism of peroxidatic transformation of 4-chloroaniline.

Interaction of cyclophosphamide metabolites with membrane proteins: an in vitro study with rabbit liver microsomes and human red blood cells. Effect of thiols

Cyclophosphamide metabolites have been generated in vitro by microsomal oxidation of cyclophosphamide and their binding to rabbit liver microsomes and to intact human red blood cells has been investigated. Reactions with proteins of membrane and cytoplasm were detected by SDS polyacrylamide gel electrophoresis. The protein bands were analysed for incorporation of radioactivity. The following results were obtained. (1) Preferential binding of acrolein to microsomes and erythrocytes, with only little binding of metabolites containing the chloroethyl moiety. (2) Reduction of acrolein binding by the thiol compounds glutathione, 2,3-dimercaptopropane-1-sulfonate and mercaptoethane sulfonate. (3) In microsomes: formation of protein polymerisation products and incorporation of radioactivity. (4) In red blood cells: cross-linking of membrane proteins and formation of globin dimerization products in the cytoplasm.

Interaction of ligands with cytochrome P-450. On the 442 nm spectral species generated during the oxidative metabolism of pyridine

When added to aerobic rabbit liver microsomal fractions fortified with an NADPH-generating system, pyridine initially produces a type II difference spectrum such as is observed with other aromatic amines. There is a time-dependent conversion of this perturbation into a new spectral species characterized by an absorbance maximum at 442 nm and a minor peak at 389 nm. Experiments with inhibitors of the cytochrome P-450-dependent electron-transport chain suggest that these species originate from binding to the haemoprotein of metabolic intermediate(s) derived from the amine substrate. Analysis of the incubation mixtures by t.l.c., high-pressure liquid chromatography, u.v.- and mass-spectrometry reveals the presence of a single metabolite arising from cytochrome P-450-catalysed oxidation of the heteroaromatic tertiary amine, which was identified as pyridine N-oxide, obviously accounting for adduct formation. This view is supported by comparative studies on the spectral changes generated by exogenous amine oxide with NADPH-reduced cytochrome P-450. Moreover, dithiothreitol, a potent N-oxidase inhibitor, strongly suppresses development of the 442 nm and 389 nm complexes. The ability of forming low-spin adducts with ferrous cytochrome P-450 absorbing around 440 nm appears to be an inherent property of different types of N-oxides. Considering the dipole character of the N+-O- function, a co-ordinate iron-oxygen bond is proposed to be formed in these complexes.

Some features of nucleo-cytoplasmic RNA transport from isolated nuclei

Messenger RNA is released preferentially from isolated rat liver nuclei in the presence of the ATP-generating system and cytosol. The release is suppressed by spermidine, while cytoplasmic RNase inhibitor was ineffective and PCMB like some other thiol-blocking agents inhibitory. Cytoplasmic SOD added to the system strongly suppressed RNA release. A similar effect could be obtained by anaerobiosis due to addition of SMP. In both cases the inhibition is reversed by cyanide. In contrast to normal liver where the generation of superoxide radicals takes place almost exclusively in microsomes and is coupled with the oxidation of NADPH, in mouse ascites hepatoma 22a the generation of superoxide radicals occurs mainly in the nuclear envelope and is coupled wih the oxidation of both NADPH and NADH and inhibited by cyanide.

Studies on the cytochrome P-450 product complexes formed during the metabolism of N,N-dimethylaniline

The oxidative metabolism of N,N-dimethylaniline by partially solubilized cytochrome P-450 from rabbit liver was found to be associated with the formation of a 424- and 448-nm product adduct of the hemoprotein. From the effects of temperature, hydrogen ion concentration, n-octylamine, extraction of the enzyme preparations with organic solvents and pretreatment of the animals with inducers of drug metabolism on both the formation of the spectral species and the enzymic C- and N-oxidation of N,N-dimethylaniline it is concluded that the 424-nm spectral change is generated from an intermediate in the C-oxidation reaction, whereas formation of the 448-nm spectral perturbation is the result of binding to cytochrome P-450 of a metabolite arising from N-oxidation of the arylamine; N-dealkylation of the parent amine is not a obligatory intermediary step in 448-nm complex formation. The 448-nm ferrohemochrome is supposed to be formed through coordination of the N-oxidized intermediate via the oxygen atom. This type of interaction appears to require considerably stronger thermal activation as compared with the 424-nm complex. The 448-nm product adduct of cytochrome P-450 is unstable in the ferric state or in the presence of sodium dithionite.

Comparative studies on the N-oxidation of aniline and N,N-dimethylaniline by rabbit liver microsomes

1. Rate studies of N-oxidation of aniline and N,N-dimethylaniline by rabbit liver microsomal preparations were performed at different pH values. The apparent pKs of the free functional groups were 7-2 and 6-9, respectively, at 26 degrees. The apparent heats of ionization of these groups varied from 26-8 to 31-8 kJ mol-1. 2. Photo-oxidation of the microsomal mixed function oxidase resulted in rapid loss of N-oxygenating activity. The enzyme was markedly protected from inactivation by the presence of aniline or N,N-dimethylaniline. The apparent KD values for protection were close to the Km and KS values for the individual arylamines. The pH profiles of the initial rates of photo-inactivation resembled the titration curves of groups with an apparent pKa between 6-0 and 6-2. 3. The N-oxidase was strongly inhibited by diethyl pyrocarbonate at pH 6-2. 3. The N-oxidase was strongly inhibited by diethyl pyrocarbonate at pH 6-0. Catalytic capacity was partially restored by treatment with neutral hydroxylamine. Pyridine protected the enzyme from acylation. 4. A close relationship exists between the N-hydroxylation of aniline and the N-oxide formation from N,N-dimethylaniline with respect to sensitivity to photo-oxidation, reactivity to protective substrates and susceptibility to carbethoxylation.